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1 SEPARATION AND PURIFICATION REVIEWS 2 Vol. 32, No. 2, pp. 121–213, 2003
3 Extractive Distillation: A Review
4 Zhigang Lei, Chengyue Li,* and Biaohua Chen
5 The Key Laboratory of Science and Technology of Controllable 6 Chemical Reactions, Ministry of Education, Beijing University of 7 Chemical Technology, Beijing, China 8
9 CONTENTS
10 ABSTRACT ...... 122 11 1. INTRODUCTION ...... 123 12 2. PROCESS OF EXTRACTIVE DISTILLATION ...... 127 13 2.1. Column Sequence...... 127 14 2.2. Combination with Other Separation Processes...... 132 15 2.3. Tray Configuration ...... 134 16 2.4. Operation Policy ...... 136 17 3. SOLVENT OF EXTRACTIVE DISTILLATION ...... 140 18 3.1. Extractive Distillation with Solid Salt ...... 141
*Correspondence: Chengyue Li, The Key Laboratory of Science and Technology of Controllable Chemical Reactions, Ministry of Education, Beijing University of Chemical Technology, Box 35, Beijing, 100029, China; Fax: (+8610)-64436787; E-mail: [email protected].
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DOI: 10.1081/SPM-120026627 1542-2119 (Print); 1542-2127 (Online) Copyright D 2003 by Marcel Dekker, Inc. www.dekker.com 中国科技论文在线 http://www.paper.edu.cn
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19 3.2. Extractive Distillation with Liquid Solvent...... 145 20 3.3. Extractive Distillation with the Combination of Liquid 21 Solvent and Solid Salt ...... 153 22 3.4. Extractive Distillation with Ionic Liquid ...... 156 23 4. EXPERIMENT TECHNIQUE OF 24 EXTRACTIVE DISTILLATION ...... 160 25 4.1. Direct Method ...... 161 26 4.2. Gas-Liquid Chromatography Method ...... 162 27 4.3. Ebulliometric Method...... 163 28 4.4. Inert Gas Stripping and Gas Chromatography Method...... 164 29 5. CAMD OF EXTRACTIVE DISTILLATION...... 166 30 5.1. CAMD for Screening Solvents ...... 166 31 5.2. Other Methods for Screening Solvents...... 173 32 6. THEORY OF EXTRACTIVE DISTILLATION...... 177 33 6.1. Prausnitz and Anderson Theory...... 177 34 6.2. Scaled Particle Theory ...... 181 35 7. MATHEMATIC MODEL OF 36 EXTRACTIVE DISTILLATION ...... 183 37 7.1. EQ Stage Model ...... 183 38 7.2. NEQ Stage Model ...... 193 39 8. COMPARISON OF EXTRACTIVE DISTILLATION AND 40 ADSORPTION DISTILLATION ...... 194 41 8.1. Definition of Adsorption Distillation ...... 194 42 8.2. Process Experiment ...... 195 43 8.3. VLE Experiment ...... 198 44 9. CONCLUDING REMARKS ...... 201 45 REFERENCES ...... 203
46 ABSTRACT
47 Extractive distillation is more and more commonly applied in industry, 48 and becomes an important separation method in chemical engineering. 49 This paper provides an in-depth review for extractive distillation. Sepa- 50 ration sequence of the columns, combination with other separation pro- 51 cesses, tray configuration and operation policy are included in process of 52 extractive distillation. Since the solvent plays an important role in the 53 design of extractive distillation, such conventional and novel separating 中国科技论文在线 http://www.paper.edu.cn
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54 agents as solid salt, liquid solvent, the combination of liquid solvent and 55 solid salt, and ionic liquid are concerned. The prominent characteristics 56 of extractive distillation is that one new solvent with high boiling-point, 57 i.e. separating agent, is added to the components to be separated, so as to 58 increase their relative volatility. Selection of a suitable solvent is fun- 59 damental to ensure an effective and economical design. CAMD as a 60 useful tool is applied for screening the solvents and thus reducing the 61 experimental work. Theories from molecular thermodynamics, which 62 can interpret the microscale mechanism of selecting the solvents, are 63 also collected. To accurately describe the extractive distillation process, 64 mathematical model is necessary. There are two types of mathematical 65 models to simulate extractive distillation process, i.e. equilibrium(EQ) 66 stage model and non-equilibrium(NEQ) stage model. The EQ stage 67 model as an old method is widely used, but the NEQ stage model should 68 be pay more attention. In the end comparison of extractive distillation 69 and adsorption distillation, both of which belong to special distillation 70 suitable for separating close boiling point or azeotropic components, is 71 done, and it is shown that extractive distillation is more advantageous.
AQ1
72 1. INTRODUCTION
73 Extractive distillation is commonly applied in industry, and is be- 74 coming a more and more important separation method in petrochemical 75 engineering. The product scale in industrial equipment is diverse, from 76 several kilotons(column diameter about 0.5m) to hundred kilotons(column 77 diameter about 2.5m) per year. It is mainly used in the following cases 78 (Berg, 1983; Duan, 1978; Ewanchyna and Ambridge, 1958; Hafslund, 1969; 79 Hilal et al., 2002). One application is separating hydrocarbons with close 80 boiling point, such as C4, C5, C6 mixtures and so on, the other is the 81 separation of mixtures which exhibit an azeotrope, such as alcohol/water, 82 acetic acid/water, acetone/methanol, methanol/ methyl acetate, ethanol/ethyl 83 acetate, acetone/ethyl ether and so on. 84 In extractive distillation, an additional solvent, i.e. separating agent is 85 used to alter the relative volatility of the components to be separated. In this 86 way, it is possible to obtain one pure component at the top of one column 87 and the other, together with the solvent at the bottom, which may be sep- 88 arated easily in a secondary distillation column, due to a high boiling point 89 of the solvent. The solvent doesn’t need to be vaporized in the extractive 90 distillation process. However, in azeotropic distillation, which is also often 91 used for the separation of close boiling point or azeotropic mixtures, both 92 the solvent and components must be vaporized into the top of an azeotropic 93 distillation column. Moreover, the amount of azeotropic solvent is usually 中国科技论文在线 http://www.paper.edu.cn
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94 large, which leads to large energy consumption compared to extractive 95 distillation. For this reason, extractive distillation is more attractive than 96 azeotropic distillation (Sucksmith, 1982). In recent years, an interesting 97 special distillation method, i.e. adsorption distillation, has been proposed 98 and in this work is compared with extractive distillation (See chapter 8). 99 The ease of separation of a given mixture with key components i and j 100 is given by the relative volatility:
0 yi=xi giPi aij ¼ ¼ 0 ð1Þ yj=xj gjPj
101102 where x is molar fraction in the liquid phase, y is molar fraction in the 0 103 vapor phase, g is the activity coefficient, and Pi is the pure component 104 vapor pressure. 105 The solvent is introduced to change the relative volatility as far away 0 0 106 from one as possible. Since the ratio of Pi /Pj is constant for small tem- 107 perature changes, the only way that the relative volatility is affected is by 108 introducing a solvent which changes the ratio gi /gj. This ratio, in the 109 presence of the solvent, is called selectivity Sij: !
gi Sij ¼ ð2Þ gj s
110111 In some cases a significant change in operating pressure, and hence tem- 112 perature, changes aij enough to eliminate an azeotrope. 113 Besides altering the relative volatility, the solvent should also be 114 easily separated from the distillation products, that is, high boiling point 115 difference between the solvent and the components to be separated is 116 desirable. Other criteria, e.g. corrosion, prices, sources, etc. should also be 117 taken into consideration. However, the relative volatility(which is consis- 118 tent with selectivity) is the most important. When solvents are ranked in 119 the order of relative volatility(or selectivity), the solvent with the highest 120 relative volatility is always considered to be the most promising solvent 121 for a given separation task. This may indicate that, from the viewpoint of 122 economic consideration, the use of the solvent with the highest relative 123 volatility(or selectivity) will always give the lowest total annual cost(TAC) 124 of the extractive distillation process (Momoh, 1991). The economic eva- 125 luations were carried out by using many different solvents for separating 126 three different binary mixtures: 2-methyl-butene/isoprene(Mixture A), n- 127 butane/trans-2-butene (Mixture B), and n-hexane/benzene (Mixture C). For 128 each case a complete design and costing of the process was done by using 129 cost estimating computer programs. Figures 1, 2, and 3 show, respectively, F1--F3 中国科技论文在线 http://www.paper.edu.cn
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Figure 1. The effect of solvent selectivity on the total annual cost of an extractive distillation operation (2-methyl-1-butene/isoprene); Adapted from Momoh (1991).
Figure 2. The effect of solvent selectivity on the total annual cost of an extractive distillation operation (n-butane/trans-2-butene); Adapted from Momoh (1991). 中国科技论文在线 http://www.paper.edu.cn
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Figure 3. The effect of solvent selectivity on the total annual cost of an extractive distillation operation (n-hexane/benzene); Adapted from Momoh (1991).
130 the ranking of solvent selectivity at infinite dilution with the TAC of 131 extractive distillation process for separating 2-methyl-butene/isoprene, 132 n-butane/trans-2-butene, and n-hexane/benzene mixtures for the various 133 potential solvents. 134 It can be seen from Figures 1, 2, and 3 that as the solvent selectivity 135 increases, in general, the total annual cost of the extractive distillation 136 decreases. However, as selectivity increases to a high value, the matching of 137 selectivity with the TAC is not perfect, which suggests how good or accurate 138 the use of selectivity as a basis for screening solvents. Anyway, it indicates 139 that the solvent with the highest selectivity has a potential as the best solvent. 140 Berg (1969) classified the liquid solvents into five groups according to 141 their potential for forming hydrogen bonds. However, it is found by much 142 experience that Class I and Class II, which are a highly hydrogen bonded 143 liquids, are successful solvents in most cases. 144 Class I: Liquids capable of forming three dimensional networks of 145 strong hydrogen bonds, e.g. water, glycol, glycerol, amino alcohols, hy- 146 droxylamine, hydroxyacids, polyphenols, amides, etc. Compounds such as 147 nitromethane and acetonitrile also form three dimensional hydrogen bond 148 networks, but the bonds are much weaker than those involving OH and NH 149 groups. Therefore, these types of compounds are placed in Class II. 150 Class II: Other liquids composed of molecules containing both active 151 hydrogen atoms and donor atoms(oxygen, nitrogen and fluorine), e.g. 中国科技论文在线 http://www.paper.edu.cn
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Table 1. Examples of the single liquid solvents commonly used in the extrac- T1.1 tive distillation.
T1.2 No. Components to be separated Solvents as the separating agents
T1.3 1 Alcohol(ethanol, isopropanol, Ethylene glycol tert-butanol) and water T1.4 2 Acetic acid and water Tributylamine T1.5 3 Acetone and methanol Water, ethylene glycol T1.6 4 Methanol/ methyl acetate Water T1.7 5 Propylene and propane ACN 6 C4 hydrocarbons Acetone, ACN(acetonitrile), DMF(N,N-dimethylformamide), NMP(N-methyl-2-pyrrolidone), T1.8 NFM(N-formylmorpholine) T1.9 7 Alcohol(ethanol, isopropanol) DMF and water T1.10 8 C5 hydrocarbons DMF 9 Aromatics and non-aromatics DMF, NMP, NFM
152 alcohols, acids, phenols, primary and secondary amines, oximes, nitro- 153 compounds with alpha-hydrogen atoms, nitriles with alpha-hydrogen atoms, 154 ammonia, hydrazine, hydrogen fluoride, hydrogen cyanide, etc. 155 Table 1 illustrates some examples of the single liquid solvents com- T1 156 monly used in the extractive distillation (Min et al., 2002; Shealy et al., 157 1987; Wu et al., 1988), and proves the idea of Berg to be reliable. 158 For simplification, it is noted herein that the words ‘‘separating agents’’ 159 and ‘‘solvents’’ are confused and have the same meaning in what follows.
160 2. PROCESS OF EXTRACTIVE DISTILLATION
161 In this section, process of extractive distillation is discussed from four 162 aspects: column sequence, combination with other separation processes, 163 tray configuration and operation policy, which is indispensable in the 164 process design.
165 2.1. Column Sequence
166 In general, for a two-component system the extractive distillation 167 process is made up of two columns, i.e. an extractive distillation column 168 and a solvent recovery column. The two column process is diagrammed in 中国科技论文在线 http://www.paper.edu.cn
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169 Figure 4 where components A and B to be separated are respectively F4 170 obtained from the top of two columns, and the solvent S is recovered in the 171 solvent recovery column and recycled (Chen at al., 2003; Lei et al., 2002a). 172 For a multi-component system, the arrangement of column sequence 173 may be complicated, but is developed from the above two column process. 174 An interesting example is for the separation of C4 mixture (Bannister and 175 Buck, 1969; Barba et al., 1978; Coogler, 1967; King and Mondria, 1967; 176 Klein and Weitz, 1968; Takao, 1966, 1979). 177 Among C4 mixture(butane, butene-1, trans-butylene-2, cis-butylene-2, 178 1,3-butadiene and vinylacetylene(VAC)), 1,3-butadiene is a basic organic 179 raw material. In industry, 1,3-butadiene is usually separated from C4 180 mixtures by extractive distillation with the solvent acetronitile(ACN). On 181 the basis of the two column process, the extractive distillation process for 182 separating 1,3-butadiene is designed. Accordingly, the process designed is 183 in the following sequence: extractive distillation (column 1) – solvent re- 184 covery (column 2) – extractive distillation (column 3) – solvent recovery 185 (column 4), and is illustrated in Figure 5. In this case, butane and butene F5 186 are regarded as one part and used for fuel. 187 In Figure 5 the feedstock flows into the extractive distillation column 188 1. The solvent S enters the top sections of columns 1 and 3. The mixtures 189 of butane and butene are escaped from the top of column 1 while VAC 190 from the top of column 4. The product, 1,3-butadiene, is obtained from the 191 top of column 3. The solvent S is recovered in columns 2 and 4 and 192 recycled. It can be seen that column 2 is unnecessary and can be eliminated
Figure 4. The two column process for extractive distillation. 1. Extractive distil- lation column; 2. Solvent recovery column; 3. Heat exchanger. 中国科技论文在线 http://www.paper.edu.cn
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Figure 5. The extractive distillation process designed for separating C4 mixture.
193 because the solvent is still needed in the following column 3 and need not 194 be separated. At the same time, much energy is consumed because buta- 195 diene-1,3 goes through the unnecessarily repeated evaporation and con- 196 densation. The current process for separating C4 mixture with ACN is 197 shown in Figure 6. F6 198 The product, 1,3-butadiene is obtained from the top of column 2. Al- 199 kyne hydrocarbons mainly containing VAC and drawn out through stream 200 SL1, are removed from flash column 3. But SL2 stream is also returned 201 into column 2. This is to say that column 2 and column 3 are thermally
Figure 6. The current process of extractive distillation with ACN method for separating C4 mixture; Adapted from Lei et al. (2002b). 中国科技论文在线 http://www.paper.edu.cn
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Figure 7. The optimum process of extractive distillation with ACN method for separating C4 mixture. 1. The first extractive distillation column; 2. The second extractive distillation column; 3. Flash column; Adapted from Lei et al. (2002b).
202 coupled. However, there exists an obvious disadvantage in the current ex- 203 tractive distillation process, that is, the liquid load is very high in the lower 204 section of column 2, which has negative influence on the tray efficiency 205 and decreasing production capacity of 1,3-butadiene. It is the current case 206 that the demand for 1,3-butadiene goes up gradually year by year. Moreover, 207 the thermal coupling between columns 2 and 3 leads to the difficulty in 208 operation and control. 209 To solve this problem, a new optimization process was put forward as 210 shown in Figure 7. The vapor stream SV is drawn from the lower section of F7 211 the first extractive distillation column 1 into the second extractive distil- 212 lation column 2. SL1 mainly containing ACN and alkyne hydrocarbons is 213 flashed in column 3, and the solvent coming from SL2 is recycled. It can be 214 seen that the thermal coupling in the optimization process is eliminated and 215 operation and control would be more convenient. 216 By simulation it is found that the liquid load in the lower section of the 217 second extractive distillation column 2 is effectively decreased from 218 52,000kg/hr to 12,000kg/hr, about 77.0% of the original process, under the 219 same production capacity of 1,3-butadiene 5200kg/hr (Lei et al., 2002b). In 220 other words, this demonstrates that it is possible to farther increase the yield 221 of butadiene on the basis of old equipment. 222 On the other hand, the comparison of heat duties on reboilers QR and 223 condensers QC for Figure 5 to Figure 7 is done and listed in Table 2, where T2 中国科技论文在线 http://www.paper.edu.cn
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T2.1 Table 2. The comparison of heat duties of three processes.
The figure 5 The figure 6 The figure 7 T2.2 process process process
T2.3 No. QC QR QC QR QC QR T2.4 Column1 1161.5 2936.3 1161.5 2936.3 1163.6 4693.0 T2.5 Column 2 1183.3 1984.7 1122.6 2545.1 1122.3 1178.0 T2.6 Column 3 1122.0 1728.1 0.0 0.0 0.0 0.0 T2.7 Total duty 3466.8 6649.1 2284.1 5481.4 2285.9 5871.0
224 the heat duties on reboilers and condensers are neglected in the column of 225 separating VAC and solvent due to the very much smaller values than those 226 of other columns. 227 It can be seen from Table 2 that heat duties on reboilers QR and 228 condensers QC are the greatest in the Figure 5 process, and those are 229 approximately equal in the Figures 6 and 7 processes, where 1,3-butadiene 230 goes through the minimum number of evaporation and condensation. 231 Therefore, it indicates that the Figure 7 process is the best from the aspects 232 of equipment investment, energy consumption and production capacity. 233 However, in the past we only paid more attention on extracting 1,3- 234 butadiene from C4 mixture. Indeed 1,3-butadiene is an important raw 235 material, which is used for the preparation of synthetic rubber, e.g. the 236 copolymers of butadiene and styrene. But it is a pity that the residual C4 237 mixture has always been burned as fuel. Until recently, many plants require 238 the residual C4 mixture to be utilized more reasonably. Among the residua, 239 n-butene(i.e. the mixture of 1-butene, trans-2-butene and cis-2-butene), is 240 able to be used as one monomer of synthesizing polymers and one reactant 241 of producing acetone by reacting with water. Isobutene can be selectively 242 combined with methanol to make methyl tert-butyl ether(MTBE) (Eldridge, 243 1993; Ren et al., 1998; Safarik and Eldridge, 1998; Wang et al., 1999). 244 Butane is still used as fuel. Therefore, the residual C4 mixture can be used 245 in many respects, not only as fuel. Only are the useful C4 components 246 separated from mixtures, the reasonable utilization is possible. 247 C4 mixture can be divided into four parts: butane(n-butane and isobu- 248 tane), butene(n-butene, isobutene), 1,3-butadiene and VAC. N-butene and 249 isobutene are regarded as one part, which is based on the consideration that 250 isobutene is easy to be removed from n-butene by reacting with methanol. 251 Those four parts have different fluidity of electron cloud, the sequence being 252 butane < butene < 1,3-butadiene < VAC. Therefore, the interaction force be- 253 tween the polar solvent and C4 mixture is different, and the corresponding 中国科技论文在线 http://www.paper.edu.cn
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Figure 8. A new process for separating butane, butene, 1,3-butadiene and VAC.
254 order is that butane < butene < 1,3-butadiene < VAC. Thus C4 mixture can 255 be separated in series by extractive distillation. 256 In terms of the above optimization process of separating 1,3-butadiene 257 from C4 mixtures, it is straightforward to have a new process for separating 258 butane, butene,1,3-butadiene and VAC in series, which is diagrammed in 259 Figure 8. F8 260 It is evident that the new process has the advantages in equipment 261 investment, energy consumption and production capacity. That is to say, for 262 the very complicated separation of C4 mixture, we can still deduce the most 263 optimum process step by step from the simplest two column process of 264 extractive distillation as shown in Figure 4.
265 2.2. Combination with Other Separation Processes
266 As is said in the 1st chapter, extractive distillation is used more often 267 than azeotropic distillation because of low energy consumption and flexible 268 selection of the possible solvents. However, anything has its defects, and 269 extractive distillation is not exceptive. For instance, comparing with azeo- 270 tropic distillation, extractive distillation can’t obtain a very high purity of 271 product because the solvent coming from the bottom of solvent recovery 272 column more or less contains a little impurity which influences the sepa- 273 ration effect. Moreover, comparing with liquid–liquid extraction, extractive 274 distillation consumes much more energy. So it is advisable to combine 275 extractive distillation with another separation process such as azeotropic 276 distillation, liquid–liquid extraction and so on. 中国科技论文在线 http://www.paper.edu.cn
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277 2.2.1. Combination of Extractive Distillation and 278 Azeotropic Distillation
279 Extractive distillation can’t obtain a very high purity of product, but 280 consumes less energy; however, azeotropic distillation can obtain a very 281 high purity of product, but consumes much more energy. Therefore, a 282 separation method, i.e. the combination of extractive distillation and 283 azeotropic distillation is proposed (Lei et al., 1999), which eliminates the 284 disadvantages of both extractive distillation and azeotropic distillation, and 285 retains the advantages of them. This method has been used for the sepa- 286 ration of 2-propanol(IPA) and water while the high purity of 2-propanol 287 over 99.8%wt is required. The process of this method is diagrammed in 288 Figure 9, where column 1, column 2, column 3 and column 4 are extractive F9 289 distillation column, solvent recovery column(separating agent S1 for ex- 290 tractive distillation), azeotropic distillation column and solvent recovery 291 column(separating agent S2 for azeotropic distillation), respectively. 292 This process is designed according to the sequence, firstly extractive 293 distillation which ensures low energy consumption, and then azeotropic dis- 294 tillation which ensures high purity of 2-propanol as product. But this sequence 295 can’t be in reverse. Otherwise, high purity of 2-propanol can’t be obtained. 296 In addition, the energy consumption of this process is greater than that 297 of extractive distillation, but less than that of extractive distillation. So it is
Figure 9. The process of the combination of extractive distillation and azeo- tropic distillation. 中国科技论文在线 http://www.paper.edu.cn
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298 concluded that this process is especially suitable for separations requiring 299 high purity of product. For instance, in the case of separating acetic acid 300 and water, the concentration of acetic acid in water below 20ppm is re- 301 quired in industry.
302 2.2.2. Combination of Extractive Distillation and 303 Liquid–Liquid Extraction
304 This method has been sucessfully used for the recovery of benzene and 305 toluene from pyrolysis hydrogenation gasoline fraction (Tian et al., 2001a; 306 Wang et al., 2002). The mixture of benzene and toluene is firstly separated 307 into two parts, i.e. one mainly containng benzene and the other mainly 308 containing toluene. Then the rich-benzene mixture is dealt with by extractive 309 distillation, while the rich-toluene mixture is dealt with by liquid–liquid 310 extraction. The solvent used in these two processes is the same, sulfolane. 311 The reason why extractive distillation is selected for the separation of 312 the rich-benzene mixture is that its boiling-point is lower than that of rich- 313 toluene mixture, and thus the boiling-point difference between the rich- 314 benzene mixture and the solvent is great enough, which facilitates the 315 solvent recovery. However, the boiling-point difference between the rich- 316 toluene mixture and the solvent is relatively small, which may not accord 317 with the solvent criteria. Therefore, it is wise to select liquid–liquid ex- 318 traction for the separation of the rich-toluene mixture. 319 The practical result from the Aromatics Plant of Yangzi Petrochemical 320 Company with a capacity of 360kt a year showed that the the freezing point 321 of benzene obtained by this method was 5.48°C, the sulfur content in 322 benzene was less than 0.5 mg/g, and the non-aromatics content in toluene 323 was less than 1000 mg/g. The recovery yield of benzene and toluene was 324 99.9% and 99.1% respectively. This proves that the combination of ex- 325 tractive distillation and liquid–liquid extraction is effective for the recovery 326 of benzene and toluene from pyrolysis hydrogenation gasoline fraction.
327 2.3. Tray Configuration
328 One prominent characteristics of extractive distillation is that the sol- 329 vent ratio (namely the mass ratio of solvent and feed) is very high, gen- 330 erally 5–8, which restricts increasing capacity. Accordingly, the liquid load 331 is very large in the extractive distillation column, but the vapor load is 332 relatively small. So when we design the extractive distillation column, more 333 attention should be paid on the channels of passing liquid phase. 334 In most cases, either tray column or packed column can be adopted. 335 However, if the extractive distillation column is operated under the middle 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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336 or even high pressure such as the separation of C3, C4 and C5 mixtures, or 337 if the components to be separated are easy to polymerize such as the 338 separation of C5 mixture, then it is better to choose tray column. Herein, 339 two types of plate trays used in the extractive distillation column are in- 340 troduced (Department of Chemical Engineering, 1975; Duan et al., 1997; 341 Research group on trays, 1977; Lei et al., 2002b; Li et al., 2001; Li and 342 Zhang, 2001; Peng et al., 1996; Song et al., 2001). 343 Under the middle or even high pressure, the plate trays, especially 344 double overflow trays, are generally used as internal fittings. Both double 345 overflow valve trays and double overflow slant-hole trays have ever been 346 adopted. However, it is reported that, if double overflow valve trays are 347 replaced by double overflow slant-hole trays in a column for separating 348 propane and propylene, the feeds to be treated are raised above 50% with a 349 tray efficiency similar to or higher than that of the valve trays and an 350 energy save of 10% by decreasing pressure drop to about 1/3 of the original. 351 Slant-hole trays, very excellent and extensively applied in the industry, 352 has opposite stagger arrangement of slant-holes, which causes rational 353 flowing of vapor and liquid phases, level blowing of vapor, permission of a 354 large vapor speed, no mutual interference, steady liquid level and high 355 efficiency of trays. On the basis of studying and analyzing the columns with 356 multiple downcomer (MD) trays, a new-type multi-overflow compound 357 slant-hole trays were invented, which adopt the downcomer similar to MD 358 trays. The number of downcomers used is not too many, but only two. The 359 downcomer has the feature of simple structure, longer flowing distance of 360 liquid, higher capacity of column and high efficiency of trays. The con- 361 figuration of the double overflow slant-hole trays is illustrated in the ref- 362 erence (Lei et al., 2002b). In terms of the vapor and liquid load of the 363 extractive distillation column, the tray parameters are obtained by a com- 364 puter program for tray design. Of course, it should be mentioned that the 365 designed values are within the range of normal operation condition. 366 In the case that the components to be separated are easy to polymerize, 367 the big-hole sieve trays are desirable. For instances, for the separation of 368 the C5 mixture, the key components being pentene and isoprene, it is well- 369 known that the polymerization reactions among unsaturated hydrocarbons 370 take place, and influence the normal operation of the column while the 371 conventional valve trays are employed. However, in this case the big-hole 372 sieve trays with the hole diameter of 10–15mm are effective because the 373 holes with large diameter prevent the trays from jam due to the formation 374 of polymer. In addition, this type of trays eliminates the gradient of liquid 375 layer and causes the rational flowing of vapor and liquid phases on the tray, 376 with the help of directed holes arranged for decreasing the radial mix and 377 bubble promoter installed in the outlet of the downcomer. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
136 Lei, Li, and Chen
Figure 10. The gradient of liquid layer on the traditional trays.
378 Figures 10 and 11 illustrate the gradient of liquid layer on the tradi- F10--F11 379 tional trays and the improved gradient of liquid layer on the big-hole sieve 380 trays, respectively. Figure 12 shows the flowing direction of liquid phases F12 381 on the big-hole sieve trays. This manifests that the flowing of liquid phases 382 on the big-hole sieve tray is indeed rational.
383 2.4. Operation Policy
384 We know that the simplest extractive distillation process is made up of 385 two columns, i.e. an extractive distillation column and a solvent recovery 386 column. Herein, we call it two column process. However, at some times(not 387 often), only one column, which is either as extractive distillation column or
Figure 11. The gradient of liquid layer on the big-hole trays. 中国科技论文在线 http://www.paper.edu.cn
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Figure 12. The flowing direction of liquid phases on the big-hole trays. (Go to www.dekker.com to view this figure in color.)
388 as solvent recovery column, is employed in the extractive distillation pro- 389 cess. Herein, we call it single column process.
390 2.4.1. Two Column Process
391 For extractive distillation, two column process is often adopted. The 392 two column process can be operated either in the batch way or in the 393 continual way. In the continual way, two columns are operated at the same 394 time, while in the batch way, there is still one column left no run. 395 Frankly speaking, we like the continual way rather than the batch way 396 because the batch operation is tedious and the production efficiency is very 397 low. However, the reason why the batch way is carried out comes from the 398 solvent recovery column. Due to the very high boiling point of the solvent, 399 the solvent recovery column has to be operated at vacuum pressure in order 400 to decrease the temperature of the bottom and thus prevent the solvent from 401 decomposition. Moreover, at some time there may be a strict requirement 402 for the purity of the recycled solvent. All these lead to the difficulty of the 403 continual operation in the solvent recovery column. But a strategy to solve 404 them is by decreasing the boiling point of the solvent and canceling the 405 vacuum system(See the 3rd chapter). 406 However, in the fine chemical engineering, the batch way is often used. 407 Although it is complicated, the amount of treated feed is yet not many. 中国科技论文在线 http://www.paper.edu.cn
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408 2.4.2. Single Column Process
409 For the single column process, there are two situations: batch mode and 410 semi-continuous mode (Bai et al., 2001; Cui et al., 2002; Lang et al., 1994; 411 Milani, 1999; Mujtaba, 1999; Safrit et al., 1995). In the batch mode, the 412 solvent is charged in the reboiler with the feed mixture at the beginning of 413 operation. Therefore, it limits the amount of feed mixture which can be 414 processed as the reboiler has a limited capacity. This increases the number 415 of batches to be processed in a campaign mode operation. In this mode, 416 finding the optimum feed charge to solvent ratio is an important factor to 417 maximize the productivity. 418 In the semi-continuous mode, the solvent is fed to the column in a 419 semi-continuous fashion at some point of the column. There can be two 420 strategies in this mode of operation as far as charging of the initial feed 421 mixture is concerned.
422 1. Full charge. In this strategy the feed mixture is charged in the re- 423 boiler to its maximum capacity at the beginning of operation. For a 424 given condenser vapor load, if the reflux ratio and the solvent feed 425 rate are not carefully controlled, the column will be flooded. 426 2. Fractional charge. ‘‘Full charge’’ is not a suitable option to 427 achieve the required product specifications for many azeotropic 428 mixtures in a given sized column, operating even at the maximum 429 reflux ratio, because the amount of solvent required in such 430 separations is more than those that the column can accommodate 431 without flooding. Mujtaba, (1999) proposed a fractional feed 432 charge strategy for this type of mixture. In this strategy the feed 433 mixture is charged to a certain fraction of the maximum capacity. 434 The column can operate at a reflux ratio greater than maximum 435 reflux ratio for some period until the reboiler level reaches to its 436 maximum capacity.
437 However, it can be easily deduced that for batch mode and semi- 438 continuous mode, the latter is more promising because this mode can deal 439 with much more products in the same column. A complete batch extractive 440 distillation of semi-continuous mode consists of the following steps:
441 1. Operation under total reflux without solvent feeding. 442 2. Operation under total reflux with solvent feeding. For decreasing 443 the concentration of the less volatile component(LVC) in the dis- 444 tillate, this step is not absolutely necessary because the first part of 中国科技论文在线 http://www.paper.edu.cn
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445 the distillate containing too much of the LVC can be recycled to 446 the next charge. 447 3. Operation under finite reflux ratio with solvent feeding(production 448 of the more volatile component(MVC)). 449 4. Operation under finite reflux ratio without solvent-feeding(separa- 450 tion of the LVC from the solvent). Before the start of the pro- 451 duction of LVC an off-cut can be taken.
452 It should be noted that the holdup on the reboiler must not exceed the 453 maximum capacity at any time within the entire operation period to avoid 454 column flooding. 455 Many researchers focus on exploring the semi-continuous mode in such 456 aspects as simulations, improving equipment with external middle vessel 457 and so on, among which batch extractive distillation with external idle 458 vessel is an interesting topic. The typical process and apparatus are dia- 459 grammed in Figure 13. F13 460 In one example, the packed column with a 38mm diameter was com- 461 posed of three parts, i.e. scrubbing section, rectifying section and stripping 462 section. The scrubbing section was packed to a height of 150mm with 463 2.5 Â 2.5 Dixon rings, and the rectifying and stripping were, respectively, 464 packed to a height of 500mm with 2.5 Â 2.5 Dixon rings. The external
Figure 13. Single column process with an external middle vessel. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
140 Lei, Li, and Chen
465 middle vessel is a 2000ml agitated flat bottom flask. The materials to be 466 separated were charged to the middle vessel. The liquid flow from the 467 middle vessel to the stripping section was controlled by an electric-mag- 468 netic device. The column was operated under the following steps:
469 1. Operation under total reflux without solvent feeding; 470 2. Operation under total reflux with solvent feeding; 471 3. Operation under finite reflux ratio and total reboiler ratio with 472 solvent feeding(production of the more volatile component from 473 the top of the column, while no production of solvent from 474 the bottom); 475 4. Operation under finite reflux ratio and finite reboiler ratio with 476 solvent feeding(production of more volatile component from the 477 top and solvent from the bottom of the column simultaneously); 478 5. Operation under finite reflux ratio and finite reboiler ratio without 479 solvent feeding (production of slop cut or less volatile component 480 from the top while solvent from the bottom); 481 6. When the less volatile component or solvent in the middle vessel 482 reaches the required purity or there is only a little liquid in the 483 middle vessel, the operation is stopped.
484 The operation and simulation of this process, i.e. extractive distillation 485 in a batch column with a middle vessel, is very complex, but it has many 486 advantages such as flexibility, multi-component separation in one column 487 and so on. It is especially suitable for the batch extractive distillation with 488 large solvent ratio. However, when the boiling point of solvent is very high 489 while the boiling point of the components to be separated are low, it is 490 inconvenient to use the column, since the reboiler needs a high temperature 491 heat resource.
492 3. SOLVENT OF EXTRACTIVE DISTILLATION
493 The two factors that influence the extractive distillation process are 494 separation process and solvents(or separation agents). Assuming that the 495 separation process is determined, the task is to select the basic solvent with 496 high separation ability. When a basic solvent is found, this solvent should 497 be further optimized to improve the separation ability and to decrease the 498 solvent ratio and liquid load of the extractive distillation column. 499 The solvent is the core of extractive distillation. It is well-known that 500 selection of the most suitable solvent plays an important role in the eco- 501 nomical design of extractive distillation. Excellent solvents should at least 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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502 decrease the solvent ratio and the liquid load of the extractive distillation 503 column, and make the operation easily implemented. Up to date, there are 504 four kinds of solvents used in extractive distillation, i.e. solid salt, liquid 505 solvent, the combination of liquid solvent and solid salt, and ionic liquid, 506 which are discussed in detail later.
507 3.1. Extractive Distillation with Solid Salt
508 3.1.1. The Definition of Extractive Distillation with Solid Salt
509 In certain systems where solubility permits, it is feasible to use a solid 510 salt dissolved into the liquid phase, rather than a liquid additive, as the 511 separating agent for extractive distillation. The extractive distillation pro- 512 cess in which the solid salt is used as the separating agent is called ex- 513 tractive distillation with solid salt. However, herein, the ionic liquids are 514 not included in the solid salts although they are a kind of salts. Ionic liquid 515 as a special separating agent will be discussed in what follows. 516 The so-called ‘‘salt effect in vapor-liquid equilibrium(VLE)’’ refers to 517 the ability of a solid salt which has been dissolved into a liquid phase 518 consisting of two or more volatile components to alter the composition of 519 the equilibrium vapor without itself being present in the vapor. The feed 520 component in which the equilibrium vapor is enhanced is said to have been 521 ‘‘salted out’’ by the salt, while the other feed component is ‘‘salted in.’’ 522 This phenomenon can be described by the following equation which is 523 known as Setschenow equation and expresses the solubility of a non- 524 electrolyte in a solid salt solution with low salt concentration (Johnson and 525 Furter, 1960). c log 0 ¼ k c ð3Þ c s s
526527 In the above equation, c0 is the solubility in pure solvent, c the solubility in 528 a salt solution of concentration cs, and ks is the salting coefficient which has 529 a characteristic value for a given salt-non-electrolyte pair. A positive value 530 of ks corresponds to salting out(c0 > c); if ks is negative, salting in is ob- 531 served (c0 < c).
532 3.1.2. The Process of Extractive Distillation with Salt
533 The process of extractive distillation with salt is somewhat different 534 from the process shown in Figure 4 in the 2nd chapter, in that the salt is not 535 recovered by means of distillation. Anyway, any extractive distillation 536 process can be taken on as consisting of one extractive distillation column 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
142 Lei, Li, and Chen
537 and one solvent recovery equipment, sometimes maybe not distillation 538 column (Barba et al., 1985; Furter, 1992; Furter, 1993). 539 Figure 14 demonstrates a typical flowsheet for salt-effect extractive F14 540 distillation. The solid salt, which must be soluble to some extent in both 541 feed components, is fed at the top of the column by dissolving it at a steady 542 state into the boiling reflux just prior to entering the column. The solid salt, 543 being nonvolatile, flows entirely downward in the column, residing solely 544 in the liquid phase. Therefore, no scrubbing section is required above the 545 situation of feeding separating agent to strip agent from the overhead 546 product. Recovery of the salt from the bottom product for recycle is by 547 either full or partial drying, rather than by the subsequent distillation op- 548 eration for recovering the liquid separating agents. 549 Several variations on the Figure 14 process are possible. For example, 550 an azeotrope-containing system could be separated by first taking it almost 551 to the azeotrope by ordinary distillation without adding any separating 552 agent, then across the former azeotrope point by extractive distillation with 553 solid salt present, usually containing a little components to be separated,
Figure 14. One flowsheet of extractive distillation with salt. 1-feed stream; 2-extractive distillation column; 3-salt recovery equipment; 4-bottom product; 5-the salt recovered; 6-reflux tank; 7-overhead product. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
Extractive Distillation 143
Figure 15. One flowsheet of extractive distillation with salt. 1-feed stream; 2-extractive distillation column; 3-evaporation tank; 4-bottom product; 5-salt solution; 6-reflux tank; 7-overhead product.
554 and then to final purity by further distillation without solid salt. The solid 555 salt is generally recovered by evaporation. 556 In this case, the flowsheet of extractive distillation with salt is shown in 557 Figure 15. One advantage of this process is that the salt isn’t difficult to be F15 558 recovered, only by evaporation, and thus the process is convenient to operate.
559 3.1.3. Case Studies
560 Separation of Ethanol and Water 561 Ethanol is a basic chemical material and solvent used in the production 562 of many chemicals and intermediates. Especially in the recent year, ethanol 563 is paid more attention because it is an excellent alternative fuel and has a 564 virtually limitless potential for growth. However, ethanol are usually diluted 565 and required to be separated from water. It is known that ethanol forms 566 azeotrope with water, and can not be extracted to a high concentration from 567 the aqueous solutions by ordinary distillation methods. 中国科技论文在线 http://www.paper.edu.cn
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568 A process known as HIAG(Holz Industrie Acetien Geselleschaft), li- 569 censed by DEGUSSA and based on patents registered by Adolph Gorhan, 570 employed extractive distillation using a 70/30 mixture of potassium and 571 sodium acetates as the separating agent, and produced above 99.8% ethanol 572 completely free of separating agent directly from the top of the column 573 (Furter, 1992; Furter, 1993). 574 The separation of ethanol and water is the most important application 575 of extractive distillation with solid salt. The influence of various salts on 576 the relative volatility of ethanol and water was investigated by Duan et al. 577 (Duan et al., 1980; Lei et al., 1982; Zhang et al., 1984), and the results are 578 listed in Table 3, where the volume ratio of the azeotropic ethanol-water T3 579 solution and the separating agent is 1.0, and the concentration of salt is 580 0.2g(salt)/ml(solvent). 581 It can be seen from Table 3 that the higher the valence of metal ion is, 582 the more obvious the salt effect is. That is to say, AlCl3 > CaCl2 > NaCl2; 583 Al(NO3)3 > Cu(NO3)2 > KNO3. Besides, the effects of acidic roots are dif- À À À 584 ferent, in an order of Ac >Cl >NO 3.
585 Separation of Isopropanol and Water 586 Azeotropic distillation with benzene as the entrainer is also a conven- 587 tional process for the isopropanol-water separation. The Ishikawajima- 588 Harima Heavy industries (IHI) company in Japan has developed a process 589 for isopropanol production from aqueous solution using the solid salt, cal- 590 cium chloride, as the azeotrope-breaking agent. In a 7300 tonnes per year
Table 3. The influence of various solid salts and liquid solvents on the relative T3.1 volatility of ethanol and water.
T3.2 No. Separating agent Relative volatility
T3.3 1 No. 1.01 T3.4 2 Ethylene glycol 1.85 T3.5 3 Calcium chloride saturated 3.13 T3.6 4 Potassium acetate 4.05 T3.7 5 Ethylene glycol+NaCl 2.31 T3.8 6 Ethylene glycol+CaCl2 2.56 T3.9 7 Ethylene glycol+SrCl2 2.60 T3.10 8 Ethylene glycol+AlCl3 4.15 T3.11 9 Ethylene glycol+KNO3 1.90 T3.12 10 Ethylene glycol+Cu(NO3)2 2.35 T3.13 11 Ethylene glycol+Al(NO3)3 2.87 T3.14 12 Ethylene glycol+CH3COOK 2.40 13 Ethylene glycol+K2CO3 2.60 中国科技论文在线 http://www.paper.edu.cn
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591 plant, IHI reported a capital cost of only 56%, and an energy requirement of 592 only 45%, of those for the benzene process (Furter, 1992; Furter, 1993).
593 Separation of Nitric Acid and Water 594 A process in North America using extractive distillation by salt effect 595 is the production of nitric acid from aqueous solution using a solid salt, 596 magnesium nitrate as the separating agent. Hercules Inc. has been operating 597 such plants since 1957, and reports lower capital costs and lower overall 598 operating costs than for the conventional extractive distillation process 599 which uses a liquid solvent, sulfuric acid, as the separating agent (Furter, 600 1992; Furter, 1993).
601 3.1.4. The Advantages and Disadvantages of Extractive 602 Distillation with Solid Salt
603 In systems where solubility considerations permit their use, solid salt 604 as the separating agents can have major advantages. The ions of a solid salt 605 are typically capable of causing much large effects than the molecules of a 606 liquid agent, both in the strength of attractive forces they can exert on feed 607 component molecules, and in the degree of selectivity exerted. This means 608 that the salt has a good separation ability and in the extractive distillation 609 process, the solvent ratio is much smaller than that of the liquid sol- 610 vent(discussed in the next section), which leads to high production capacity 611 and low energy consumption. Moreover, since solid salt is not volatile, it 612 can’t be entrained into the product. At the same time, no salt vapor is 613 inhaled by operators. 614 However, it is a pity that in industrial operation, when solid salt is 615 used, dissolution, reuse and transport of salt is quite a problem. The con- 616 current jam and erosion limit the industrial value of extractive distillation 617 with solid salt. That is why the technique, extractive distillation with solid 618 salt, is not widely used in the industry.
619 3.2. Extractive Distillation with Liquid Solvent
620 3.2.1. The Definition of Extractive Distillation with 621 Liquid Solvent
622 Like the extractive distillation with solid salt, in certain systems where 623 solubility permits, it is feasible to use a liquid solvent dissolved into the 624 liquid phase, rather than salt, as the separating agent for extractive distilla- 625 tion. Therefore, the extractive distillation process in which the liquid solvent 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
146 Lei, Li, and Chen
626 is used as the separating agent is called extractive distillation with liquid 627 solvent. However, herein, the ionic liquids are not included in the liquid 628 solvents although they are in the liquid phase from room temperature to 629 higher temperature. The contents about ionic liquid as a special separating 630 agent will be discussed later.
631 3.2.2. The Process of Extractive Distillation with Liquid Solvent
632 See Figure 4 in the 2nd chapter(the process of extractive distillation 633 with liquid solvent).
634 3.2.3. Case Studies
635 Reactive Extractive Distillation 636 There are many examples for extractive distillation with a single liquid 637 solvent as the separating agent, as shown in Table 1 in the 1st chapter. Herein, 638 this example is concerned with the separation of acetic acid and water by 639 reactive extractive distillation in which chemical reaction is involved. 640 Acetic acid is an important raw material in the chemical industries. But 641 in the production of acetic acid, it often exists with much water. Because a 642 high-purity of acetic acid is needed in industry, the problem of separating 643 acetic acid and water is an urgent thing. By now, there are three methods 644 commonly used for this separation, i.e. ordinary distillation, azetropic dis- 645 tillation and extractive distillation (Berg, 1987; Berg, 1992; Dil et al., 1993; 646 Golob et al., 1981; Helsel, 1977; Ma and Sun, 1997). Although ordinary 647 distillation is simple and easy to be operated, its energy consumption is 648 large and a lot of trays are required. The number of trays for azeotropic 649 distillation is fewer than that for ordinary distillation. But the amount of 650 azeotropic agent is great, which leads to much energy consumption because 651 the azeotropic agents must be vaporized in the column. However, in the 652 extractive distillation process the separating agents are not vaporized and 653 thus the energy consumption is relatively little. Therefore, extractive dis- 654 tillation is an attractive method for separating acetic acid and water, and has 655 been studied by Berg, (1987, 1992). 656 The reported separating agents in extractive distillation are sulfolane, 657 adiponitrile, pelargonic acid, heptanoic acid, isophorone, neodecanoic acid, 658 acetophenone, nitrobenzene and so on. It is evident that the interaction 659 between acetic acid or water and these separating agents is mainly physical 660 force including the van der Waals bonding and hydrogen bonding. 661 However, a new term, complex extractive distillation, has been put for- 662 ward, and a single solvent, tributylamine, is selected as the separating agent. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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663 If we select the solvent, tributylamine(b.p. 213.5°C), as the separating 664 agent, then the following reversible chemical reaction may take place:
þ À HAc þ R3N À! À R3NH Á OOCCH3
+ À 665666 where HAc, R3N and R3NH Á OOCCH3 represent acetic acid, tributylamine 667 and the salt formed by reaction, respectively. This reaction may be reversible 668 because weak acid(acetic acid) and weak base(tributylamine) are used as 669 reactants. That is, for the extractive distillation process the forward reaction 670 occurs in the extractive distillation column and the reverse reaction occurs in 671 the solvent recovery column. Therefore, this new separation method is dif- 672 ferent from traditional extractive distillation with liquid solvent, and based 673 on the reversible chemical interaction between weak acid (acetic acid) and 674 weak base (separating agent). So we call it reactive extractive distillation. + À 675 A new substance, R3NH Á OOCCH3, is produced in this reaction, 676 which can be verified by infrared spectra(IR) technique. It can be seen from 677 the IR diagrams that a new characteristic peak in the range of 1550cmÀ1 to 678 1600cmÀ1 appears in the mixture of acetic acid and tributylamine, and is 679 assigned to the carboxylic-salt function group, —COOÀ. This indicates that 680 chemical reaction between HAc and R3N indeed takes place. 681 On the other hand, this chemical reaction is reversible, which can be 682 verified by mass chromatogram(MS) technique. Two peaks denoting acetic 683 acid and tributylamine respectively, can be found for the mixture of acetic 684 acid 10%wt and tributylamine 90%wt. 685 So it indicates that HAc, R3N and the product produced by them can 686 all be detected by the combination of IR and MS techniques. In general, the 687 reaction rate between weak acid and weak base is very quick. So the 688 chemical reaction between HAc and R3N may be reversible. The further 689 proof of reversible reaction is supported by calculating the chemical 690 equilibrium constant. 691 In order to verify the effect of tributylamine as the separating agent, the 692 vapor liquid equilibrium(VLE) was measured by experiment. Figure 16 F16 693 shows the equilibrium data for the ternary system of water(1) + acetic 694 acid(2) + tributylamine(3), plotted on a tributylamin free basis. It may be 695 observed that the solvent, tributylamin, enhances the relative volatility of 696 water to acetic acid in such a way that the composition of the more volatile 697 component(water) is higher in the liquid than in the vapor phase. The reason 698 may be that the interaction forces between acetic acid and tributylamin 699 molecules are stronger than those between water and tributylamin molecules 700 because in the former reversible chemical reaction takes place. That is, water 701 would be obtained as the overhead product in the extractive distillation col- 702 umn, being acetic acid and the solvent, tributylamin, the bottom product. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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Figure 16. VLE curves on the solvent free basis for the ternary system of water(1) + acetic acid(2) + tributylamine(3) at 101.33 kPa. ^—solvent/feed volume ratio 2:1; .—solvent/feed volume ratio 1:1; 4—no solvent.
703 By analyzing the reaction system, it is found that the new group –NH 704 is formed and the old group –OH is disappeared during the reaction. This 705 reaction is exothermic and the heat generated can be obtained from the 706 reference (Ma and Sun, 1997), i.e. À2.17kJÁmolÀ1. In addition, the ioni- 707 zation constant PKa of acetic acid and tributylamine at 25°C can be found 708 from the reference (Chen, 1997), i.e. 4.76 and 10.87 respectively. There- 709 fore, the chemical equilibrium constant at 25°C can be deduced, and thus 710 the relation of chemical equilibrium constant with temperature is expressed 711 by the following equation. 2 Csalt 261:01 Kp ¼ ¼ exp À4:6284 þ ð4Þ CHAcCR3N T
712713 where T is the absolute temperature(K), and C is the mole concen- 714 tration(kmol/m3). 715 It is found that chemical equilibrium constant is small, about 0.02 716 under the operation condition of the extractive distillation column. This 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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717 means that this reaction is reversible, and the chemical interaction between 718 acetic acid and tributylamine is weak. 719 In terms of the mechanism of reactive extractive distillation, the fol- 720 lowing criteria should be satisfied to ensure that this method can 721 be implemented.
722 1. The chemical reaction is reversible. The reaction product (here as + À 723 R3NH Á OOCCH3) is used as carrier to carry the separated 724 materials back and forth. 725 2. One of the reactants(here as acetic acid) is a low boiling-point 726 component. It can be easily removed by distillation so that the 727 separating agent(here as tributylamine) is able to be regenerated 728 and recycled. 729 3. There are no other side reactions between the separating agent and 730 the component to be separated. Otherwise, the separation process 731 will be complicated and some extra equipment may be added, 732 which results in no economy of this technique.
733 It is obvious that the system consisting of water, acetic acid and tri- 734 butylamine, meets these requirements. So it seems to be advisable to sep- 735 arate water and acetic acid by complex extractive distillation. However, it 736 should be cautious that one solvent with a tri-amine R3N, not di-amine 737 R2NH or mono-amine RNH2, can be selected as the separating agent be- 738 cause the reaction between solvent with two or single amine groups and 739 acetic acid is irreversible, and some amides will be produced.
740 The Mixture of Liquid Solvents 741 Compared with the single solvent, the mixture of the liquid solvents as 742 the separating agents is complicated and interesting. However, in most cases, 743 the number of the liquid solvents in the mixture is just two. According to the 744 aims of adding one solvent to another, it can be divided into two categories: 745 increasing separation ability and decreasing the boiling point of the mixture.
746 Increasing Separation Ability. As we know, selection of the most 747 suitable solvent plays an important role in the economical design of ex- 748 tractive distillation. Moreover, the most important factor in selecting the 749 solvents is relative volatility. Increasing separation ability of the solvent 750 means increasing the relative volatility of the components to be separated. 751 That is to say, when one basic solvent is given, it is better to find the additive 752 to make into a mixture to improve the relative volatility, and thus to decrease 753 the solvent ratio and liquid load of the extractive distillation column. 754 It has been verified by many examples that adding a little solvent 755 (additive) to one basic solvent will increase the separation ability. 中国科技论文在线 http://www.paper.edu.cn
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756 For example, adding small amounts of water has improved the sepa- 757 ration ability of acetonitrile(ACN) in separating C4 mixture (Chemical En- 758 gineering Department of Zhejiang University, 1973; Lu, 1977). Some results 759 are listed in Table 4, where the subscript (1–13) represents n-butane, iso- T4 760 butane, isobutene, 1-butene, trans-2-butene, propadiene, cis-2-butene, 1,3- 761 butadiene, 1,2-butadiene, methyl acetylene, butyne-1, butyne-2 and vinyla- 762 cetylene(VAC) respectively, and the numbers, 100%, 80%,70% are the 763 solvent weight fraction in the mixture of the solvent and C4. 764 It can be seen from Table 1 that adding a little water to ACN indeed 765 increases the relative volatilities of a18, a28, a38, a48, a58, a68 and a78 in 766 which 1,3-butadiene is as the heavy component, and decreases the relative 767 volatilities of a98, a10,8, a11,8, a12,8 and a13,8 in which 1,3-butadiene is as the 768 light component. So the mixture of water and ACN can enhance the sepa- 769 ration ability. However, ACN is blended with water to separate C4 mixture 770 with the defect that ACN is prone to hydrolyze, which leads to equipment 771 corrosion and operation difficulty. From this viewpoint, it is thought that 772 water is not the best additive. Which additive is the best? This question will 773 be answered in the 5th chapter. 774 Other examples in which the second additive is added to improve the 775 separation ability of the original solvent are illustrated in Tables 5 and T5 776 6 for separating cyclopentane(1)/2,2-dimethylbutane(2) and n-pentane(1)/ T6 777 1-pentene(2), respectively. In Table 5, NMP, CHOL and NMEP stand 778 for N-methyl-2-pyrrolidone, cyclohexanol and N-(b-mercaptoethyl)-2-pyr- 779 rolidone, respectively (Brown and Lee, 1990; Cui et al., 2001; Lee and 780 Brown, 1990).
T4.1 Table 4. The relative volatility of C4 mixture to 1,3-butadiene at 50°C.
T4.2 ACN ACN + 5 %wt water ACN + 10%wt water
T4.3 100% 80% 70% 100% 80% 70% 100% 80% 70%
T4.4 a18 3.01 2.63 2.42 3.46 2.94 2.66 3.64 3.11 2.75 T4.5 a28 4.19 3.66 3.37 4.95 4.11 3.72 5.40 4.35 3.82 T4.6 a38 1.92 1.79 1.72 2.01 1.84 1.75 2.09 1.89 1.78 T4.7 a48 1.92 1.79 1.72 2.01 1.84 1.75 2.09 1.89 1.78 T4.8 a58 1.59 1.48 1.42 1.59 1.54 1.46 1.78 1.58 1.49 T4.9 a68 2.09 2.14 2.17 1.97 2.04 2.08 1.88 1.97 2.02 T4.10 a78 1.45 1.35 1.30 1.48 1.36 1.30 1.51 1.37 1.30 T4.11 a88 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 T4.12 a98 0.731 0.720 0.712 0.750 0.733 0.722 0.755 0.738 0.728 T4.13 a10,8 1.00 1.09 1.16 0.947 1.060 1.130 0.897 1.050 1.120 T4.14 a11,8 0.481 0.499 0.508 0.456 0.478 0.490 0.435 0.462 0.476 T4.15 a12,8 0.296 0.303 0.308 0.278 0.288 0.293 0.267 0.279 0.287 T4.16 a13,8 0.389 0.413 0.430 0.364 0.400 0.414 0.344 0.379 0.403 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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Table 5. Relative volatility a12 of the mixture of the solvents for separating T5.1 cyclopentane(1) and 2,2-dimethylbutane(2).
T5.2 No. Solvent/feed mass ratio Mixture of the solvents a12 T5.3 1 3:1 NMEP 1.28 T5.4 2 3:1 A: NMEP+CHOL 1.40 T5.5 3 3:1 B: NMEP+CHOL 1.42 T5.6 4 3:1 C: NMEP+NMP 1.32 T5.7 5 5:1 NMEP 1.48 T5.8 6 5:1 CHOL 1.32 T5.9 7 5:1 NMP 1.37 T5.10 8 5:1 A: NMEP+CHOL 1.64 T5.11 9 5:1 B: NMEP+CHOL 1.57 T5.12 10 5:1 C: NMEP+NMP 1.54 T5.13 11 7:1 NMEP 1.71 T5.14 12 7:1 CHOL 1.26 T5.15 13 7:1 NMP 1.33 T5.16 14 7:1 A: NMEP+CHOL 1.70 T5.17 15 7:1 B: NMEP+CHOL 1.64 T5.18 16 7:1 C: NMEP+NMP 1.74
Adapted from Cui et al. (2001).
781 Decreasing Boiling Point. In general, the solvent has a much 782 higher boiling point than the components to be separated in order to ensure 783 the complete recovery of the solvent. It indicates that at the bottom of the 784 solvent recovery column, a water stream with a higher temperature than the
Table 6. Selectivity S12 of the mixture of the solvents for separating n-pentane(1) T6.1 and 1-pentene(2).
Mixture of the solvents Concentration
Volume fraction T6.2T6.3 No. ABof B,% S12 T6.4 1 Methyl cellosolve Nitromethane 0 1.69 T6.5 2 Methyl cellosolve Nitromethane 5 1.70 T6.6 3 Methyl cellosolve Nitromethane 100 2.49 T6.7 4 Pyridine Butyrolactone 0 1.60 T6.8 5 Pyridine Butyrolactone 32.1 1.79 T6.9 6 Pyridine Butyrolactone 100 2.17 T6.10 7 Ethyl methyl ketone Butyrolactone 0 1.62 T6.11 8 Ethyl methyl ketone Butyrolactone 50 1.79 T6.12 9 Ethyl methyl ketone Butyrolactone 100 2.17
Adapted from Cui et al. (2001). 中国科技论文在线 http://www.paper.edu.cn
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T7.1 Table 7. The examples of the mixture of the solvents for decreasing the boiling point.
T7.2 No. Systems Mixture of the solvents
T7.3 1 C4 hydrocarbons ACN + water T7.4 2 Aromatics and non-aromatics NMP + water T7.5 3 Benzene and non-aromatics NFM + an additive
785 boiling point of the solvent is needed to heat the solvent to reach its boiling 786 point at normal pressure. That means much energy with high level has to be 787 consumed. If the solvent recovery column is operated at below normal 788 pressure, the degree of vacuum pressure must not be too high due to the 789 restriction of the temperature of condensing water at the top. Therefore, the 790 best strategy is to find the suitable additive to make into a mixture to 791 decrease the boiling point. It is evident that the boiling point of the additive 792 is relatively lower than the basic solvent, but higher than the components to 793 be separated. Table 7 illustrates the examples of the mixture of the solvents, T7 794 in which the function of the additive is to decrease the boiling point (Tian 795 et al., 2001b; Zhu et al., 2002). 796 However, one question arises, that is, the separation ability of the 797 mixture of the solvents may decline. The fact is that, at some time, the 798 separation ability of the mixture of the solvents is improved as the ex- 799 ample of Table 4, where the mixture of ACN and water not only improves 800 the separation ability, but also decreases the boiling point. At other time, 801 the separation ability of the mixture of the solvents indeed declines, but 802 not obviously. The reason is that as the temperature decreases, there is a 803 positive effect on the separation ability of the mixture of the solvents. The 804 temperature effect on selectivity is given by
dðlg S0 Þ L0 À L0 12 ¼ 1 2 ð5Þ dð1=TÞ 2:303R
0 805806 where Li is partial molar heat of solution, component at infinite dilution in 807 the solvent, T is the absolute temperature(K), and R is gas constant 808 (8.314J/(mol K)). 0 809 It can be obtained that lgS12 is proportional to the reciprocal absolute 810 temperature. This means that although the separation ability of the additive 811 is weaker than that of the basic solvent, the relatively low boiling point of 812 the mixture of the solvents in some degrees offsets the negative influence 813 of the separation ability of the additive, which leads to no apparent de- 814 crease of the separation ability of the mixture of the solvents. That is one 815 reason why some researchers are willing to improve the solvent by de- 816 creasing the boiling point. 中国科技论文在线 http://www.paper.edu.cn
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817 In addition, the mixture of the solvents can raise the yield ratio of the 818 product. Zhu et al. studied an extractive distillation process for recovering 819 high purity of benzene from pyrolysis gasoline. The selected solvent is the 820 mixture of N-formylmorpholine(NFM) and an additive. When the weight 821 fraction of the additive is in the range of 5%–20%, the yield ratio of 822 benzene increases from 99.0% to 99.8% and the bottom temperature of the 823 solvent recovery column is less than 190°C.
824 3.2.4. The Advantages and Disadvantages of Extractive 825 Distillation with Liquid Solvent
826 In most cases, the solvent and feed mass ratio(i.e. solvent ratio) in 827 the extractive distillation with liquid solvents is very high, up to 5–8. For 828 example, for the separation of C4 hydrocarbon using N,N-dimethylfor- 829 mamide(DMF) or acetonitrile(ACN) as the solvents, the solvent ratio is 830 7–8 in industry, which leads to much consumption of energy. However, 831 in systems where solubility considerations permit their use, the separation 832 ability of the solid salts is much greater than that of the liquid solvents, 833 and thus the solvent ratio is generally low. The reason why extractive 834 distillation with liquid solvent is more widely used in industry rather than 835 extractive distillation with solid salt is that there is no problems of dis- 836 solution, reuse and transport for the liquid solvent because it is in the 837 liquid phase under the operation condition of extractive distillation process. 838 In a word, the advantages of extractive distillation with liquid solvent 839 predominate over its disadvantages. On the contrary, the disadvantages of 840 extractive distillation with solid salt predominate over its advantages.
841 3.3. Extractive Distillation with the Combination 842 of Liquid Solvent and Solid Salt
843 3.3.1. The Definition of Extractive Distillation with Liquid 844 Solvent and Solid Salt
845 Like the extractive distillation with solid salt or liquid solvent, in 846 certain systems where solubility permits, it is feasible to use a combina- 847 tion of liquid solvent and solid salt dissolved into the liquid phase, rather 848 than only salt or liquid solvent, as the separating agent for extractive 849 distillation. Therefore, the extractive distillation process in which the com- 850 bination of liquid solvent and solid salt is used as the separating agent is 851 called extractive distillation with the combination of liquid solvent and 852 solid salt. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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853 3.3.2. The Process of Extractive Distillation with the 854 Combination of Liquid Solvent and Solid Salt
855 See Figure 4 in the 1st chapter(the process of extractive distillation 856 with liquid solvent). That is to say, the processes of extractive distillation 857 with liquid solvent and with the combination of liquid solvent and solid 858 salt, are the same.
859 3.3.3. Case Studies
860 Extractive distillation witxh the combination of liquid solvent and solid 861 salt as the separating agent is a new process for producing high-purity pro- 862 ducts. This process integrates the advantages of liquid solvent(easy opera- 863 tion) and solid salt(high separation ability). In industrial operation, when only 864 salt is used, dissolution, reuse and transport of solid salt is quite a problem. 865 The concurrent jam and erosion limits the industrial value of extractive 866 distillation with solid salt only. However, the mixture of liquid solvent and 867 solid salt can avoid the defects and realize continual production in industry. 868 Extractive distillation with the combination of liquid solvent and solid salt 869 can be suitable either for the separation of polar systems or for the separation 870 of non-polar systems, which are mentioned afterwards, respectively. 871 Herein, we select aqueous alcohol solutions, i.e., ethanol/water and 872 isopropanol/water as the representatives of polar systems and C4 mixture as 873 the representatives of non-polar systems. It is known that both anhydrous 874 alcohol and 1,3-butadiene are basic chemical raw materials. Anhydrous al- 875 cohol is not only used as chemical reagent and organic solvent, but also used 876 as the raw material of many important chemical products and intermediates; 877 1,3-Butadiene mainly comes from C4 mixture and is utilized for the syn- 878 thesis of polymers on a large scale. It is reported (Lei et al., 2002a,c,d) that 879 the systems of aqueous alcohol and C4 mixture are able to be separated by 880 extractive distillation. Because these two materials are very important in 881 industry, the separation of them by extractive distillation is interesting.
882 Separation of the Systems of Ethanol/Water 883 and Isopropanol/Water 884 Firstly the equilibrium data of the ethanol(1)-water(2) system, which 885 corresponded well with the reference data (Gmehling and Onken, 1977), 886 were measured. It is verified that the experimental apparatus was reliable. 887 Then the measurements were respectively made for the system ethanol(1)- 888 water(2)-ethylene glycol(solvent/feed volume ratio is 1:1) and the system 889 ethanol(1)-water(2)-ethylene glycol-CaCl2(solvent/feed volume ratio is 1:1 890 and the concentration of salt is 0.1g/ml solvent) at normal pressure. 891 On the other hand, measurements were also made for the system of iso- 892 propanol(1)/water(2)/ ethylene glycol/ glycollic potassium at normal pressure 中国科技论文在线 http://www.paper.edu.cn
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893 (Lei et al., 2002a). Isopropanol(1)/ water(2) and the mixture of ethylene glycol 894 and glycollic potassium were blended with the feed/solvent volume ratio of 895 1:1. The mixture of ethylene glycol and glycollic potassium were prepared 896 from ethylene glycol and potassium hydroxide with the weight ratio 5:1 and 897 4:1 respectively. During preparation, the mixture of ethylene glycol and po- 898 tassium hydroxide were fed into a distillation column and the water produced 899 was removed. Afterwards, the experimental VLE data were measured. 900 The experimental results from both systems show that under the same 901 liquid composition the mole fraction of alcohol in the vapor phase with salt 902 is higher than that without salt. It means that adding salt to ethylene glycol 903 is advisable for improving the separation ability of the solvent with alcohol 904 separated as a light component and water as a heavy component.
905 Separation of C4 Mixture 906 When acetonitrile(ACN) is regarded as an basic solvent, organic sol- 907 vents including water and salts will be added to it (Lei et al., 2002d). The 1 908 aim is to find the effect of them on aaj in the solvent ACN. Among organic 909 solvents, water and ethylenediamine are better additives than other solvents. 910 However, it is found that a little of solid salt added to ACN can effectively 911 improve the relative volatility and the effect of solid salt is close to water and 912 stronger than ethylenediamine. But in the former time ACN was mixed with 913 water to separate C4 mixture with the defect that ACN is prone to hydrolyze, 914 which leads to equipment corrosion and operation difficulty. 915 On the other hand, N,N-dimethylformamide(DMF) is another solvent 916 commonly used to separate C4 mixture. Due to the same reason as ACN, 917 DMF is used as a single solvent. It is expected to modify it with solid salt. 918 Many substances are strongly soluble in DMF including many kinds of 919 solid salts. The influence of solid salts and organic additives on the sepa- 920 ration ability of DMF is tested (Lei et al., 2002c,d). The same phenomenon 921 exists as with ACN. Salts added to DMF also improve the relative vola- 922 tilities of C4 to some extent, and at the same concentration the effect of the 923 salts is more apparent than that of organic solvents. Moreover, if some 924 factors such as relative volatilities, price, erosion, source and so on are 925 considered, the salts NaSCN and KSCN are the best additives.
926 3.3.4. The Advantages and Disadvantages of Extractive 927 Distillation with the Combination of Liquid 928 Solvent and Solid Salt
929 As said above, extractive distillation with the combination of liquid 930 solvent and solid salt as the separating agent integrates the advantages of 931 liquid solvent(easy operation) and solid salt(high separation ability). From 932 the above discussion, we know that whether it is of separating polar sys- 933 tems or non-polar systems, extractive distillation with the combination of 中国科技论文在线 http://www.paper.edu.cn
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934 liquid solvent and solid salt may be feasible. So it is concluded that 935 extractive distillation with the combination of liquid solvent and solid salt 936 is a promising separation method. If we meet with the problems about 937 extractive distillation in the future, it is wise to try to solve it by adding 938 solid salt. 939 Unfortunately, many solid salts are corrosive to the equipment and 940 easies to decompose at high temperature. In some cases the kinds of solid 941 salts that we can select are small. An economic calculation must be made in 942 determining the final salts. The benefit from adding salts in the production 943 should exceed the price of salts and other charges. 944 On the other hand, since the amount of solid salts added to the liquid 945 solvents is often small, the role of solid salts in improving the separation 946 ability is doomed to be limited. Besides, liquid solvents are volatile, which 947 inevitably pollutes the top product of the extractive distillation column. 948 Accordingly, new separating agents should be explored to avoid these 949 problems brought out by liquid solvent and solid salt.
950 3.4. Extractive Distillation with Ionic Liquid
951 3.4.1. The Definition of Extractive Distillation with Ionic Liquid
952 Like the extractive distillation with solid salt or liquid solvent or the 953 combination, in certain systems where solubility permits, it is feasible to 954 use an ionic liquid dissolved into the liquid phase, rather than only salts or 955 liquid solvents, as the separating agent for extractive distillation. Therefore, 956 the extractive distillation process in which ionic liquid is used as the sep- 957 arating agent is called extractive distillation with ionic liquid. 958 Then, what are ionic liquids? Ionic liquids are salts consisting entirely of 959 ions, which exist in the liquid state at ambient temperature, i.e. they are salts 960 that do not normally need to be melted by means of an external heat source. 961 Ionic liquids are often used in the chemical reaction (Gordon, 2001; 962 Gregory and Mahdi, 2002; Hagiwara and Ito, 2000; Olivier, 1999; Welton, 963 1999; Zhao et al., 2002). The cases of applications on the separation 964 process are a few, especially rarely reported on the extractive dis- 965 tillation(only one patent found) (Arlt et al., 2001). 966 Ionic liquids are called ‘‘green’’ solvents. When they are used in the 967 extractive distillation, the ‘‘green’’ is brought out in the following aspects:
968 1. Negligible vapor pressure, which means that ionic liquids don’t 969 pollute the product at the top of the column. However, when the 970 liquid solvent or the combination of liquid solvent and solid salt 971 is used as the separating agent, the solvent may be entrained more 972 or less into the product. In the case of strict restriction for the 中国科技论文在线 http://www.paper.edu.cn
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973 impurity of the product, it is advisable to use ionic liquids as the 974 separating agents. 975 2. A wide liquid range of about 300°C with a melting point around 976 room temperature. This temperature range in most cases corre- 977 sponds with the operation condition of the extractive distillation. 978 3. A wide range of materials including inorganic, organic and even 979 polymeric materials are soluble in ionic liquids, which ensures that 980 the ionic liquids have an enough solubility for the components to 981 be separated, and can play a role in increasing the relative vola- 982 tility in the liquid phase. 983 4. Potential to be reused and recycled. Due to their non-volatility, 984 ionic liquids are easy to be recovered from the components to be 985 separated, and the simplest way is by evaporation in a tank. 986 5. Many ionic liquids are high thermal and chemical stability with 987 or without water under the operation temperature of extractive 988 distillation. However, some excellent liquid solvents, which are 989 commonly used until now, such as acetontrile(ACN), dimethyl- 990 formamide(DMF), N-methyl-pyrrolidone(NMP), etc., are not ther- 991 mal and chemical stability with or without water, and are easy to 992 be decomposed.
993 The most common ionic liquids in use are those with alkylammonium, 994 alkylphosophonium, N-alkylpyridinium, and N, N-dialkylimidazolium 995 cations. However, much more ionic liquids are synthesized based on 1,3- 996 dialkylimidazolium cations with 1-butyl-3-methylimidazolium [bmim]+- 997 being probably the most common cation(See Figure 17). The most common F17 À À À À À À 998 anions are [PF6] , [BF4] , [SbF6] , [CF3SO3] , [CuCl2] , [AlCl4] , À À À À À 2À 999 [AlBr4] , [AlI4] , [AlCl3Et] ,[NO3] , [NO2] and [SO4] . Ionic liquids 1000 of this type have displayed the useful combination of low melting point, 1001 along with high thermal and chemical stability necessary for extractive 1002 distillation process. 1003 The most widely used methodology in the synthesis of ionic liquids is 1004 metathesis of a halide salt of the organic cation with a group 1 or ammonium
Figure 17. Typical 1-alkyl-3-methylimidazolium cations and the abbreviations used to refer to them. R = Me, R1 = Et: [emim]+; R = Me, R1 = n-Bu: [bmim]+; R = Me, R1 = n-hexyl: [hmim]+; R = Me, R1 = n-octyl: [omim]+. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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Figure 18. One synthesis route for ionic liquids.
1005 salt containing the desired anion. Alternatively, the halide salt of the organic 1006 cation may be reacted with a Lewis acid. Figure 18 illustrates a synthesis F18 1007 route, in which synthesis of one ionic liquid may be carried out under an À 1008 inert atmosphere because such substances as AlCl3, [AlCl4] are sensitive to 1009 air and moisture.
1010 3.4.2. The Process of Extractive Distillation with Ionic Liquid
1011 The process of extractive distillation with ionic liquid may be same to 1012 the process of extractive distillation with solid salt (See Figures 14 and 15).
1013 3.4.3. Case Study
1014 Extractive distillation with ionic liquid as the separating agent is a very 1015 new process for producing high-purity products. This process integrates the 1016 advantages of liquid solvent(easy operation) and solid salt(high separation 1017 ability). But compared with extractive distillation with the combination of 1018 liquid solvent and solid salt, it has no problem of entrainment of the solvent 1019 into the top product of the column. 1020 Extractive distillation with ionic liquids can be suitable either for the 1021 separation of polar systems or for the separation of non-polar systems. The 1022 following polar and non-polar systems have been investigated (Arlt et al., 1023 2001): ethanol/water, acetone/methanol, water/acetic acid, tetrahydro- 1024 furan(THF)/water and cyclohexane/benzene. It is found that the separation 1025 effect is very apparent for these systems when the corresponding ionic 1026 liquids are used. However, it is cautious that the suitable ionic liquid should 1027 have enough solubility for the given components to be separated. The 中国科技论文在线 http://www.paper.edu.cn
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Table 8. Vapor-liquid equilibrium data of cyclohexane(1) and toluene(2) at normal pressure, temperature T/K, liquid phase x1, and vapor phase y1, mole fraction for all T8.1 cases, solvent/feed volume ratio is 1:1.
T8.2 No. T/°C x1 x2 y1 y2 T8.3 No separating agent T8.4 1 97.3 0.1267 0.8733 0.2468 0.7532 T8.5 2 99.3 0.2614 0.7386 0.4488 0.5512 T8.6 3 96.0 0.3534 0.6466 0.5852 0.4148 T8.7 4 89.0 0.5021 0.4979 0.7167 0.2833 T8.8 5 89.1 0.6516 0.3484 0.7604 0.2396 T8.9 6 88.9 0.7738 0.2262 0.8336 0.1664 T8.11T8.10 [AlCl4]À as the anion T8.12 1 105.5 0.1267 0.8733 0.2989 0.7011 T8.13 2 98.0 0.2614 0.7386 0.5592 0.4408 T8.14 3 94.0 0.3534 0.6466 0.6608 0.3392 T8.15 4 86.8 0.5021 0.4979 0.7644 0.2356 T8.16 5 86.0 0.6516 0.3484 0.8541 0.1459 T8.17 6 81.6 0.7738 0.2262 0.8819 0.1181 À T8.19T8.18 [BF4] as the anion T8.20 1 88.2 0.5021 0.4979 0.6876 0.3124 À T8.22T8.21 [PF6] as the anion but ionic liquid is in the aqueous solution T8.23 1 90.6 0.5021 0.4979 0.5756 0.4244
1028 reason why ionic liquid can greatly raise the relative volatility is the same 1029 as that of using solid salt, that is, due to the salt effect. 1030 Recently, we measured the equilibrium data of the cyclohexane(1)- 1031 toluene(2) system in which three kinds of ionic liquids are selected as the 1032 separating agents. Ionic liquids were prepared according to the synthesis 1033 route shown in Figure 18, and [bmim]+was the cation. Their molecular 1034 structures have been verified by HNMR technique. The experimental VLE 1035 data at normal pressure are listed in Table 8, in which the mole fractions T8 1036 are on solvent free basis. 1037 As shown in Table 8, among three kinds of ionic liquids, [bmim]+ À + À + À + 1038 [AlCl4] , [bmim] [BF4] and [bmim] [PF6] , the ionic liquid of [bmim] 1039 [AlCl4]À is best for separating cyclohexane and toluene because in the case 1040 of same liquid composition(x1 = 0.5021, x2 = 0.4979) the corresponding 1041 vapor composition of cyclohexane is the greatest. This means that in this 1042 case the relative volatility of cyclohexane to toluene is also the largest. 1043 Moreover, it can be seen that under the same liquid composition the relative 1044 volatility of cyclohexane to toluene, when the ionic liquid of 1045 [bmim]+[AlCl4]À is used as the separating agent, is apparently larger than 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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1046 when no separating agent is used. Since cyclohexane and toluene can be 1047 regarded as the representatives of aromatics and non-aromatics, the ionic 1048 liquid of [bmim]+[AlCl4]À should be a potential solvent for separating 1049 aromatics and non-aromatics.
1050 3.4.4. The Advantages and Disadvantages of Extractive 1051 Distillation with Ionic Liquid
1052 As said above, extractive distillation with ionic liquids as the separating 1053 agent, like the combination of liquid solvent and solid salt, integrates the 1054 advantages of liquid solvent(easy operation) and solid salt(high separation 1055 ability). Whether it is of separating polar systems or non-polar systems, 1056 extractive distillation with ionic liquids may be feasible. So it is concluded 1057 that extractive distillation with ionic liquid is another promising separation 1058 method besides extractive distillation with the combination of liquids sol- 1059 vent and solid salt. 1060 Unfortunately, extractive distillation with ionic liquid as the separating 1061 agents has some disadvantages, that is, it often takes much time to prepare 1062 ionic liquid and the prices of the materials used for synthesizing ionic liquid 1063 are somewhat expensive. These disadvantages may hold back the wide 1064 application of this technique in industry, even if the advantages of this 1065 technique are very attractive.
1066 4. EXPERIMENT TECHNIQUE OF 1067 EXTRACTIVE DISTILLATION
1068 The expressions of selectivity and relative volatility are given in Eqs. 1 1069 and 2, respectively. Since the activity coefficients depend on the phase 1070 compositions, and the role of the solvent tends to increase with an increase 1071 in its concentration, it is common practice to consider the situation of 1072 infinite dilution. Then the definition of selectivity at infinite dilution 1073 becomes, from Eq. 2: ! 1 1 gi Sij ¼ 1 ð6Þ gj
10741075 which also represents the maximum selectivity. 1076 A relation analogous to Eq. 6 holds for the relativity volatility, that is,
1 0 1 gi Pi aij ¼ 1 0 ð7Þ gj Pj
10771078 which also represents the maximum relativity volatility. 中国科技论文在线 http://www.paper.edu.cn
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1079 In order to evaluate the possible separating agents, the goal of the 1080 experimental technique is to obtain the value of selectivity, or relativity 1081 volatility, or activity coefficient from which selectivity and relativity vol- 1082 atility can be deduced, at infinite dilution. 1083 These values may be found in some certain data banks. But it is a 1084 common situation that the systems concerned are not included. In this case, 1085 we have to rely on the experiments. There are four experimental methods to 1086 obtain selectivity, or relativity volatility, or activity coefficient at infinite 1087 dilution, i.e. direct method, gas-liquid chromatography method, ebullio- 1088 metry method, and inert gas stripping and gas chromatography method.
1089 4.1. Direct Method
1090 This is an older method, involving extrapolation of classical vapor- 1091 liquid equilibrium(VLE) data (Ellis and Jonah, 1962; Hilmi et al., 1970). 1092 By plotting experimental data in the form of versus liquid composition(x) 1093 and extrapolating the curve to x = 0, the activity coefficient at infinite 1094 dilution g1 is deduced. Therefore, it is required that all of the VLE at total 1095 concentration should be measured, which leads to much time consumption 1096 and getting inaccurate results. 1097 There are many famous experimental instruments to measure the VLE 1098 at normal or vacuum pressure (Beatriz et al., 2000; Furter, 1992; Furter, 1099 1993; Jose et al., 1999; Jose et al., 2001). One kind of experimental 1100 instruments such as Rose still, Othmer still and their modifications can be 1101 bought in the market. The common characteristics of these instruments are 1102 that they belong to recycling vapor-liquid equilibrium still and the data 1103 under boiling, isobaric conditions are measured. Each experiment is done 1104 for each data. 1105 Another kind of experimental instruments to measure the VLE at 1106 normal or vacuum pressure (Wu et al., 1988), such as Stage-Muller dy- 1107 namic still and so on, are used to obtain the data under isothermal condi- 1108 tion. Each experiment is done for each data. 1109 In the recent year, one important experimental instrument, i.e. 1110 HSGC(Headspace-gas chromatography) method, to measure the VLE at 1111 normal or vacuum pressure has been developed to obtain the data under 1112 isothermal condition. Each experiment is done for many data. 1113 In the references (Fawzi et al., 2000; Seiler et al., 2002a,b), this 1114 method is introduced. It combines a headspace sampler and a GC(gas 1115 chromatograph) in order to determine the composition of a vapor phase. 1116 Samples of different liquid(10ml) are filled into vials(capacity 20ml) and 1117 sealed with air-tight septa. To ensure thermodynamic equilibrium, the vials 1118 are mixed at equilibrium temperature for 24h and then transferred into the 1119 headspace oven. Inside the oven of the headspace sampler, the system is 中国科技论文在线 http://www.paper.edu.cn
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1120 mixed by an agitation mode and equilibrated again for another 5h. A 1121 pneumatically-driven thermostated headspace sampler takes a sample of 1122 approximately 0.05g out of the vials’ vapor phase in equilibrium with the 1123 liquid phase. A specially designed sample interface delivers the sample to 1124 the GC column of the GC. 1125 The HSGC method is the most promising direct method, and can finish 1126 the VLE measurement in a relatively short time. But it is a pity that the 1127 drawback of extrapolating the experimental data can’t be overcome yet. 1128 However, if the VLE at high pressure is required for the studied system 1129 such as C3, C4 hydrocarbons, direct method may be very tedious and diffi- 1130 cult, and is better to be replaced by other methods such as gas-liquid chro- 1131 matography method, and inert gas stripping and gas chromatography method.
1132 4.2. Gas-Liquid Chromatography Method
1133 This method is commonly used for determining experiments values of 1134 g1. It involves measuring the retention behavior of a solute in an inert carrier 1135 gas stream passing through a column containing a solvent-coated solid 1136 support (Donohue et al., 1985; Thomas et al., 1982; Vega et al., 1997). 1137 The equation used in calculating g11 is:
1 1 RTf2Zm expðÀV2 P=RTÞn1 g2 ¼ s s ð8Þ f2P2ðVR À VmÞ
11381139 where Zm is compressibility of he mixture, f2 is vapor-phase fugacity co- 1140 efficient of solute, VR is retention volume of solute, Vm is volume of the 1 1141 mobile phase, V 2 is molar volume of solute at infinite dilution, and n1 is 1142 moles of solvent on the column. This relation assumes that equilibrium is 1143 achieved throughout the column, that the solute is sufficiently dilute to be 1144 within the Henry’s law region, and that the packing is inert relative to the 1145 solvent and the solute. 1146 There are several advantages in using the gas-liquid chromatography 1147 method. Firstly, it is the method that g1 may be found directly. Secondly, 1148 commercially available equipment can be used and the techniques are well 1149 established. Further, although solvent purity is crucial, solute purity is not 1150 critical as separation is achieved by the chromatograph. Finally, the method 1151 is quick on the condition that the solvent is determined beforehand. Up to 1152 30 data points may be measured in the course of a day’s run. Conventional 1153 static technique in the direct method may take hours or days to obtain one 1154 or two limiting activity coefficients, and even the ebulliometric method to 1155 be discussed afterwards, can measure only four values in a day’s run with 1156 existing equipment. 中国科技论文在线 http://www.paper.edu.cn
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Extractive Distillation 163 1157 However, the most apparent defect of this method is that, when one 1158 solute system has been given and different solvents are to be evaluated, the 1159 column packing should be made once for each solvent. We know that the 1160 preparation of the column packing is a very tedious work and the time 1161 consumed is not endurable. 1162 The minor defects of this method are encountered when solvents nearly 1163 as volatile as the solutes are used. The stripping of solvent from the column 1164 packing introduces impurities into the carrier gas and thus reduces the 1165 sensitivity of the detector. This means that many solutes can’t be detected 1166 in certain volatile solvents.
1167 4.3. Ebulliometric Method
1168 Ebulliometric method is a rapid and robust method of measuring pres- 1169 sure-temperature-liquid mole fraction(PTx)data (Olson, 1989; Qian et al., 1170 1981). PTx data are measured in an ebulliometer which is a one-stage total 1171 reflux boiler equipped with a vapor-lift pump to spray slugs of equilibrated 1172 liquid and vapor upon a thermometer well. The advantages of ebulliometric 1173 method are:
1174 1. Degassing is not required. 1175 2. Equipment is simple, inexpensive, and straightforward to use. 1176 3. Data can be measured rapidly. 1177 4. Either Px or Tx data can be measured. ÀÁ 1178 @T For a binary system, the procedure is to measure @x P in a twin-ebul- 1179 liometer(differential) arrangement where a pure component is charged to 1180 both ebulliometers and small amounts of the second component are added to 1 1181 one ebulliometer.ÀÁ Activity coefficient at infinite dilution g is deduced by 1182 @T extrapolating @x x¼0, which leads to getting inaccurate results. One ex- 1183 pression of g1 is written as, "# P DHV dT g1 ¼ 1 À ð9Þ 1 P0 RT2 dx 1 1 x1¼0
11841185 where dT ¼ T À T þ dx 1 2 DT 1 x1¼0 x x 1 2 x1¼0 0 11861187 DT = TÀx1T1Àx2T2, P is the total pressure, P1 is the saturated pressure of 1188 solute 1, T is the absolute temperature, R is gas constant, and DHv is the 1189 evaporation latent heat of solute 1. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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1190 When solute 1 is added into the solvent S, the concentration of solute 1, 1191 x1, is determined. Only if the boiling point different of the mixture is mea- 1192 sured, then the activity coefficient can be calculated from Eq. 9. Moreover, if 1193 solute 1 is in the very dilution, then it is reasonable to think that the activity 1 1194 coefficient may be g1 . 1195 For extractive distillation, a ternary system, consisting of solvent S, 1196 components A and B to be separated, are encountered while one solvent S is 1197 evaluated. In this case selectivity, or relative volatility at infinite dilution in 1 1 1 1198 solvent S, can be obtained by carrying out two g (gA , gB ) experiments. 1199 From this viewpoint, ebulliometric method is somewhat tedious.
1200 4.4. Inert Gas Stripping and Gas Chromatography Method
1201 Leroi et al. (Leroi et al., 1977) proposed a new method to measure g1, 1202 S1 and a1, i.e. the combination of inert gas stripping and gas chromatog- 1203 raphy method, which has the merits of a high level of accuracy and repro- 1204 ducibility (Chen et al., 1988; Duhem and Vidal, 1978). One schematic 1205 diagram of the experimental apparatus is depicted in the reference (Lei et al., 1206 2002d). The principle of the method is based on the variation of the vapor 1207 phase composition when the highly diluted components of the liquid mixture, 1208 controlled to be below 0.01mol/l, are stripped from the solution by a constant 1209 flow of inert nitrogen gas with a flow rate 20ml/min. In the stripping cell, the 1210 outlet gas flow is in equilibrium with the liquid phase, and gas is injected 1211 into the gas chromatograph by means of a six-way valve at periodic intervals. 1212 The peak areas of solutes are recorded by the integrating meter. 1213 The most important part in this method is the equilibrium cell. One 1214 modification to the structure proposed by Lei involves enlarging the gas- 1215 liquid interface and increasing the contact time between gas and liquid 1216 phases by means of a spiral path. Two similar cells, a presaturation cel and 1217 a stripping cell, are used. The configuration of the modified equilibrium cell 1218 is described in the reference (Lei et al., 2002d). The relative volatility can 1219 be obtained by this method in a shorter time than by other methods. 1220 According to the experimental principle we can derive the relation- 1 1221 ship of activity coefficient gi at infinite dilution. It is given by the 1222 following equation:
1 NRT A0 gi ¼ 0 ln ð10Þ DPi t A
12231224 where N is the total number of mole of solvent in the dilution cell at 1225 time t, and A0 and A are respectively the original peak areas of the solute 1226 and the peak area at intervals of the time t. 中国科技论文在线 http://www.paper.edu.cn
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1227 For extractive distillation, we usually take on the selectivity or relative 1228 volatility at infinite dilution as the standard of evaluating the solvent. The 1 1 1229 selectivity Sij and the relative volatility aij at infinite dilution are re- 1230 spectively given by the following equations:
1 A =A P0 1 gi lnð i0 iÞ j Sij ¼ 1 ¼ 0 ð11Þ gj lnðAj0=AjÞPi
12311232 1 0 1 gi Pi lnðAi0=AiÞ aij ¼ 1 0 ¼ ð12Þ gj Pj lnðAj0=AjÞ
1 1 12331234 It is evident that the selectivity Sij and the relative volatility aij are co- 1235 ordinate in evaluating the separation ability of the solvent. For the sake of 1 1 1 1236 simplicity, it is better to select aij . Comparing with gi , the data of aij is 1237 easy to be obtained accurately because it needn’t measure N and t which 1238 perhaps bring some extra errors in the experiment. Thus if the peak area A 1 1239 of the solute at different time is known, aij is able to be calculated 1240 according to Eq. 12 in which only the peak area A is required. It has been 1241 verified by Lei (Lei et al., 2002d) that the results obtained by this method 1242 are reliable, and very suitable for the evaluation of different solvents. 1243 The comparison of gas-liquid chromatography method(method 1) and 1244 inert gas stripping and gas chromatography method(method 2) for sepa- 1245 rating C4 mixture with ACN and DMF is given in Table 9 (Chemical T9 1246 Engineering Department of Zhejiang University, 1973; Lei et al., 2002d), 1247 where we use the subscript (1–5) for butane, 1-butene, 2-trans-butene, 2- 1248 cis-butene and 1,3-butadiene respectively. It can be seen that the difference 1249 between these two methods is below 15%. 1250 It is concluded by comparison of these four methods that inert gas 1251 stripping and gas chromatography method should be firstly recommended,
Table 9. The comparison of gas-liquid chromatography method(method 1) and inert T9.1 gas stripping gas chromatography method(method 2).
T9.2 ACN(20°C) DMF(50°C)
Method Method Deviation Method Method Deviation T9.3 1 2 % 1 2 %
1 T9.4 a15 3.48 3.82 9.44 3.82 3.32 13.09 1 T9.5 a25 2.13 2.11 0.01 2.40 2.12 11.67 1 T9.6 a35 – 1.81 – 1.94 1.79 7.73 1 T9.7 a45 1.47 1.63 10.88 1.65 1.61 2.42 中国科技论文在线 http://www.paper.edu.cn
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1252 secondly the HSGC technique in the direct method and finally gas-liquid 1253 chromatography method. However, it is hard to find the references about 1254 ebulliometric method in the recent year.
1255 5. CAMD OF EXTRACTIVE DISTILLATION PR
1256 It is tiresome to choose the best solvent from thousands of different 1257 substances for a given system through experiments only. There are some 1258 methods to pre-select the possible solvents by calculation, such as Pierotti- 1259 Deal-Derr method, Parachor method, Weimer-Prausnitz method, and CAMD, 1260 among which CAMD is both the recent development and the most impor- 1261 tant, and has completely replaced other calculation methods for screen- 1262 ing solvents.
1263 5.1. CAMD for Screening Solvents
1264 CAMD(computer-aided molecular design) is developed in the 1980’s 1265 and widely used in such unit operations as gas absorption, liquid–liquid 1266 extraction, extractive distillation and so on (Churi and Achenie, 1996; 1267 Harper et al., 1999; Hostrup et al., 1999; Joback and Stephanopoulos, 1995; 1268 Marcoulaki and Kokossis, 1998; Mavrovouniotis, 1998; Meniai et al., 1998; 1269 Ourique and Telles, 1998; Pistikopoulos and Stefanis, 1998; Raman and 1270 Maranas, 1998; Sinha et al., 1999; Venkatasubramanian et al., 1995). 1271 CAMD breaks new way in selecting the possible solvents by largely re- 1272 ducing experiments. The application of CAMD in chemical engineering 1273 is mostly based on the UNIFAC group contribution (Franklin, 1949; 1274 Gmehling et al., 1982; Jorgensen et al., 1979; Macedo et al., 1983). In- 1275 calculable feasible molecules will be formulated by UNIFAC groups in 1276 accordance with certain rules, but in terms of given target properties, the 1277 desired molecules are screened by computers. The groups of UNIFAC 1278 provide building blocks for assembling molecules. CAMD is essential the 1279 inverse of property prediction by group contribution. Given a set of de- 1280 sirable properties, it is proposed to find a combination of structural groups, 1281 satisfying the property specification. In most cases, more than one solution 1282 is produced. Thus, a screening is needed since only one of the alternatives 1283 must be chosen for the specified problem. At this point, factors such as 1284 corrosion, prices, source, etc. should be taken into consideration. Of course, 1285 it is a procedure after CAMD. 1286 The solvents are divided into three parts, liquid solvents, solid salt and 1287 ionic liquid which can be designed respectively by CAMD. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
Extractive Distillation 167
1288 5.1.1. CAMD of Liquid Solvents
1289 Procedure of CAMD for the Liquid Solvents 1290 The UNIFAC groups are used as building blocks for the synthesis of 1291 liquid solvents. CAMD for liquid solvents is conducted in the following 1292 four steps (Gani and Brignole, 1983; Gani and Fredenslund, 1993; Gani et 1293 al., 1991; Joback and Reid, 1987; Lei et al., 2002e; Pretel et al., 1994; Reid 1294 et al., 1987): group sorting and pre-selection, combination of groups, pre- 1295 diction of target properties, final selection of solvents. 1296 In the procedure, both explicit properties and implicit properties are 1297 considered. Explicit properties can be computed according to the thermo- 1298 dynamic method. But relative volatility among them is the key to extractive 1299 distillation. For this reason, compounds are ordered by the values of their 1300 relative volatility. For CAMD for extractive distillation, implicit properties 1301 such as toxicity, cost, stability and material source are important. The 1302 compounds that do not satisfy the implicit properties are crossed out from 1303 the order. The remaining compounds ranked in front of the order are the 1304 objects of the best possible solvents we seek. As a result, by CAMD, the 1305 work to search for the best solvents is reduced. The final solvents will be 1306 tested with only a minimum number of experiments.
1307 Program of CAMD for the Liquid Solvents 1308 Many CAMD algorithms have been proposed. These may be broadly 1309 classified as interactive, combinatorial, knowledge-based and mathematical 1310 programming methods. Recently, a new method, genetic algorithm, is put 1311 forward, which retains the efficiency of mathematical programming while 1312 still perform well in difficult search spaces. 1313 In our work, CAMD for extractive distillation has been programmed 1314 with Visual Basic for Windows. This program is a combinatorial method, 1315 and can provide much information necessary for the extractive distillation. 1316 CAMD is an easy-to-use software with user friendly interface. One with a 1317 little knowledge of extractive distillation can become familiar with it in a 1318 few hours.
1319 Application of CAMD for the Liquid Solvents
1320 Separation of C4 Mixture with ACN. Extractive distillation is 1321 widely used in purification of 1,3-butadiene from C4 mixture. ACN is a 1322 frequently used basic solvents and is inconvenient to be completely 1323 replaced by other solvents to enhance the relative volatility of C4 mixture. 1324 Therefore, water is usually added to ACN as a cosolvent. But the mixed 中国科技论文在线 http://www.paper.edu.cn
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1325 ACN has a disadvantage, that is, it tends to give rise to hydrolysis, which 1326 contributes to the loss of ACN and corrosion of equipment during pro- 1327 duction. So, a more efficient additive is required to substitute water. 1328 2-Butene and 1,3-butadiene are key components in C4 separation because 1329 their separation is difficult. So, it is reasonable to only consider 2-butene and 1330 1,3-butadiene as the representative of C4 mixture for solvent evaluation. 1331 CAMD for this separation has been performed on a PC (Pentium 2 1332 286MHz) (Lei et al., 2002e). Water that has ever been used as the additive 1333 (Bannister and Buck, 1969; Coogler, 1967) is sought, which indicates that the 1334 designed result are reliable to some degree. After considering such implicit 1335 properties as toxicity, boiling point, chemical stability and hydrolysis, the 1336 molecules, (H2O), (CH3 3CH2 CH3COO), (CH3 CH2 CH3 CO), (2CH2NH2) 1337 and (CH3 CH2 OH), are regarded as the possible additives. 1338 The additives generated by CAMD are tested by experiments, and it 1339 proved that water and ethylenediamine are good potential additives of ACN 1340 of all liquid solvents. Therefore, if we consider the hydrolysis influence of 1341 ACN, ethylenediamine is the best substance as liquid solvent that is added 1342 to ACN. 1343 Thus, by means of the tool, CAMD, we can find the best additive, 1344 ethylenediamine, from a great number of liquid solvents with only a few 1345 experiments. It is shown that CAMD can effectively decrease the amounts 1346 of experiments and help researchers to explore the useful substances in a 1347 short time.
1348 Separation of Aromatics and Non-Aromatics. In industry the 1349 methods for separating aromatics and non-aromatics are liquid–liquid ex- 1350 traction and extractive distillation (Liu et al., 1995; Rawat and Gulati, 1981; 1351 Wardencki and Tameesh, 1981; Zheng et al., 1997). Liquid–liquid extrac- 1352 tion as an old method is almost replaced by extractive distillation in most 1353 cases. It is assumed that cyclohexane and benzene are taken on as the rep- 1354 resentation of aromatics and non-aromatics respectively because they are the 1355 key components in the separation process. In order to find the potential 1356 solvents, CAMD for this separation is used (Yang et al., 2001). The re- 1357 striction for the solvents are listed in the following order.
1358 1. Maximum molecular weight: 240 1359 2. Minimum boiling point: 383.15K 1360 3. Maximum boiling point: 553.15K 1361 4. Maximum melting point: 278.15K 1362 5. Minimum selectivity at infinite dilution: 3.0 1363 6. Maximum activity coefficient of component i at infinite dilution in 1 1364 solvent (gi,s ): 12 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
Extractive Distillation 169
1365 7. Maximum activity coefficient of solvent at infinite dilution in 1 1366 component i (gs,j ):
1367 0 Pi 0 Ps 13681369 1370 By means of CAMD, No.1, NMP, and No.2, DMF, are regarded as the 1371 potential solvents. Fortunately, both solvents have ever been reported as the 1372 solvents (Liu et al., 1995; Rawat and Gulati, 1981; Wardencki and 1373 Tameesh, 1981; Zheng et al., 1997). So in means that the result of CAMD 1374 are reliable in this case. 1375 Braam (Braam and Izak, 2000) also studied this system by causing AQ2 1376 CAMD and found that aniline is the possible best solvent, resulting in a 1377 relative volatility of 2.65(UNIFAC) with cyclohexane in the distillate. The 1378 only solvent not to result in a higher relative volatility is acetonyl acetone 1379 which has a higher boiling point than aniline. However, it happened that 1380 NMP is common for both cases in separating aromatics and non-aromatics. 1381 Unfortunately, DMF as a commonly used solvent is neglected by Braam. 1382 The reason may be that DMF forms minimum boiling point azeotropes with 1383 non-aromatic hydrocarbons with 6–8 carbon atoms(i.e, cyclohexane and 1384 heptane) (Cesar et al., 1997), which causes solvent losses with the distillate. 1385 In order to decrease the solvent losses, the addition of steam to the dis- 1386 tillation column above the solvent feed has been recommended. Steam 1387 breaks the DMF-hydrocarbons azeotrope, and DMF entrained by the dis- 1388 tillate can be recovered by washing it with water or ion exchange. So the 1389 post-treatment after extractive distillation is somewhat complicated, which 1390 contributes to no economics of the whole process.
1391 Separation of Ethanol and Water. The mixture forms a mini- 1392 mum-boiling azeotrope, and extractive distillation may be used as an al- 1393 ternative to azeotropic and pressure-swing distillation. The solvent given in 1394 the reference for this system is ethylene glycol (Lei et al., 2002d). This 1395 solvent will allow the recovery of ethanol in the distillate with a predicted 1396 relative volatility of 2.54(UNIFAC). In contrast to this, the first solvent 1397 generated by CAMD for this system, hexachlorobutadiene, is predicted to 1398 cause a relative volatility more than 3 times this value. This solvent was 1399 tested and performed very well (Braam and Izak, 2000). 1400 Solvents were also generated to reverse the relative volatility of the 1401 system to facilitate the recovery of water in the distillate. In this case, the 1402 best solvent calculated by CAMD is dodecane. The reason may be that water 1403 and dodecane can form azeotrope. So this type of distillation should be the 中国科技论文在线 http://www.paper.edu.cn
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1404 combination of azeotropic distillation and extractive distillation. However, 1405 no solvents could be found in the reference for this separation.
1406 Other Systems. The systems of acetone/methanol, ethanol/ethyl 1407 acetate and methanol/methyl acetate, have been studied by Braam by using 1408 CAMD (Braam and Izak, 2000). For separating acetone and methanol, di- 1409 methyl sulfoxide(DMSO), generated by CAMD, is found to be the best 1410 alternative to water that is commonly used in industry; for separating 1411 ethanol and ethyl acetate, diethylene glycol and DMSO, generated by 1412 CAMD, were tested by experiments to be potential alternatives; for sepa- 1413 rating methanol and methyl acetate, tetrachloroethylene, generated by 1414 CAMD, was tested and found to perform very well. Therefore, it is sug- 1415 gested that tetrachloroethylene should replace 2-methoxy ethanol which is 1416 now used as the industrial solvent. However, it is known that tetra- 1417 chloroethylene is toxic, which doesn’t conform to the requirement of im- 1418 plicit properties. From this viewpoint, tetrachloroethylene is toxic, which 1419 doesn’t conform to the requirement of implicit properties. Form this 1420 viewpoint, tetrachloroethylene isn’t the best as the solvent.
1421 5.1.2. CAMD of Solid Salts
1422 Procedure of CAMD for the Solid Salts 1423 There is only one reference with regard to the work of CAMD for solid 1424 salts despite the frequent use of solid salts as separating agents in chemical 1425 engineering (Lei et al., 2002e), say nothing of ionic liquid. With the de- 1426 velopment of extractive distillation, the extractive distillation with the 1427 combination of liquid solvent and solid salts is becoming more and more 1428 applied in industry. Therefore, it is interesting to consider the salting effect in 1429 regard to CAMD. 1430 As a simplified presumption, a salt is thought to be composed of one 1431 positive ion and one negative ion which are regarded as the groups of salts. 1432 It is consequently easy to assemble the ions into molecules in the same way 1433 as liquid solvents. These ions are collected and listed Table 10. In the T10 1434 procedure of CAMD, the combination rule is simply that the chemical 1435 valence of a salt must be zero, which impossible to lead to combination 1436 explosion because most molecules are composed of just two groups, one 1437 positive ion and the other negative ion. Therefore, both liquid solvents and 1438 solid salts can be designed in one CAMD program, and this goes further 1439 step in the application of CAMD. 1440 The key of CAMD for solid salts is the selection of an appropriate 1441 thermodynamic method to obtain the target properties. However, the ther- 1442 modynamic method for solids salts is very scare and inaccurate. That is 1443 where the limitation of CAMD for solid salts lie. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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T10.1 Table 10. Ions of CAMD for solid salts.
T10.2 Ions Groups
T10.3 Cation Li+ Na+ K+ Rb+ + + + + T10.4 Cs Ag NH4 Cu T10.5 Mg2+ Ca2+ Sr2+ Ba2+ T10.6 Be2+ Fe2+ Zn2+ Cu2+ T10.7 Al3+ Fe3+ T10.8 Anion OHÀ FÀ ClÀ BrÀ À À À À T10.9 I SCN NO3 NO2 À À À T10.10 CH3COO ClO4 BF4
Adapted from Lei et al. (2002e).
1444 Application of CAMD for the Solid Salts 1445 There is only one example reported in the reference (Lei et al., 2002e), 1446 i.e. the system of ACN/C4. CAMD for this separation has been performed 1447 on a PC(Pentium 2 286 MHz). The constraints for the system of ACN/C4 1448 include pre-selected group type number, expected group number, temper- 1449 ature, minimum infinite dilute relative volatility, concentration of additive 1450 and key components. Moreover, the salts must meet the requirement of 1451 implicit properties, e.g. chemical stability, oxidation, material source, price 1452 factor and solubility in ACN, etc. These factors taken in, the solid salts 1453 NaSCN and KSCN are excellent additives. On the basis of CAMD, some 1454 experiments are performed to investigate the effect of solid salts. 1455 Simulation of extractive distillation process for ACN and ACN/NaSCN 1456 with a 10wt% salt concentration is carried out. The results show that the 1457 required solvent flowrate decreases 11.8% and much energy is saved in 1458 the extractive distillation column when solid salt is added. Therefore, the 1459 mixture of ACN and solid salt performs more satisfactorily than a single 1460 ACN solvent. Apart from this, the mixture has no problem about disso- 1461 lution, reuse and transport of salt, which is often brought on by using a 1462 single salt.
1463 5.1.3. CAMD of Ionic Liquids
1464 It is a pity that by now, there is no report of CAMD for ionic liquids. 1465 From the 3rd chapter, we know that ionic liquids may become potential 1466 excellent separating agents in extractive distillation. So CAMD for ionic 1467 liquids is a very interesting topic. 1468 Since ionic liquid is still a kind of salt, the program of CAMD for ionic 1469 liquids should be similar to that for solid salts. Likewise, an ionic liquid can 中国科技论文在线 http://www.paper.edu.cn
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T11.1 Table 11. Typical cation/anion combinations in ionic liquids.
Coordination T11.2 Cations Anions ability of anions
À À À T11.3 Alkylammonium, [BF4] , [PF6] , [SbF6] , Weak (neutral) À À T11.4 Alkylphosophonium, [CF3SO3] , [CUCl2] , À À À T11.5 N-alkylpyridinium, N, [AlCl4] , [AlBr4] , [AlI4] , À À À T11.6 N-dialkylimidazolium [AlCl3Et] , [NO3] , [NO2] , 2À T11.7 [SO4] + + + À À T11.8 PR4 ,SR4 ,NR4 [Cu2Cl3] , [Cu3Cl4] , None (acidic) À À T11.9 [Al2Cl7] , [Al3Cl10]
1470 be assumed to be composed of one positive ion and one negative ion, which 1471 are regarded as the groups of salts. It is consequently easy to assemble the 1472 ions into molecules. In the procedure of CAMD, the combination rule for 1473 ionic liquids is also same to that for solid salts. So it indicates that CAMD 1474 for ionic liquids, solid salts and liquid solvents, can be placed in a same 1475 program, which facilitates the exploitation and use of CAMD. 1476 These ions are collected and listed in Table 11, where R is alkyl (Zhao T11 1477 et al., 2002). 1478 The key of CAMD for ionic liquids is the selection of an appropriate 1479 thermodynamic method to obtain the target properties. For extractive dis- 1480 tillation, the most important of all the explicit properties is the activity 1481 coefficient, which can be used to calculate relative volatility or selectivity, 1482 and thus evaluate the possible ionic liquids. 1483 Today, there are two methods commonly used for predicting activity 1484 coefficient, i.e. UNIFAC(UNIQUAC functional group activity coefficient) 1485 model and MOSCED(modified separation of cohesive energy density) 1486 model. The UNIFAC model is the most widely used, and based on the 1487 contributions of the constituent groups in a molecule. Since the first version 1488 of UNIFAC, several revisions and extensions have been proposed to im- 1489 prove the prediction of activity coefficient. The MOSCED model is an 1490 extension of regular solution theory to mixtures that contain polar and 1491 hydrogen bonding components. The cohesive energy density is separated 1492 into dispersion forces, dipole forced, and hydrogen bonding with small 1493 corrections made for asymmetry. However, it is very possible that group 1494 parameters of the selected ionic liquid can’t be found in the present limited 1495 parameter table of UNIFAC and MOSCED models. In this case, COSMO- 1496 RS(Conductor-like Screening Model for Real Solvents) model is selected 1497 and used as an alternative to UNIFAC and MOSCED models (Gmehling, 1498 1998; Thomas and Eckert, 1984). 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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1499 COSMO-RS model is a novel and efficient method for the priori 1500 prediction of thermophysical data of liquids, and has been developed since 1501 1994 (Klamt, 1995; Klamt and Eckert, 2000; Klamt et al., 1998). It is 1502 based on unimolecular quantum chemical calculations that provide the 1503 necessary information for the evaluation of molecular interactions in 1504 liquids. It can be applied to nearly any system for which no group para- 1505 meters are available in the UNIFAC and MOSCED models. Moreover, 1506 COSMO-RS model is open for a large number of qualitative improvements 1507 and functional extensions. A correction for misfit charge interactions 1508 will improve the accuracy of the electrostatic part and enable calculations 1509 for ions. 1510 In COSMO-RS model, the activity coefficient of component i is related 1511 with chemical potential and given as follows: m À m0 g ¼ exp i i ð13Þ i RT
0 15121513 where mi is the chemical potential of component i in the mixture, mi the 1514 chemical potential in the pure liquid substance, T the absolute temperature 1515 and R gas constant. The chemical potential can be solved by using the exact 1516 equation resulting from statistical thermodynamics. 1517 Therefore, it is possible to select the potential ionic liquid by means of 1518 CAMD. However, it is known that the ionic liquids must meet the re- 1519 quirement of implicit properties that can’t be obtained by calculation. In 1520 this case it is better to set up a data bank of commonly used ionic liquids. 1521 Table 12 shows some implicit properties of commonly used ionic liquids T12 1522 (Zhao et al., 2002), which can be input into the CAMD program for 1523 screening ionic liquids. By combination of explicit and implicit properties, 1524 it is believed that the best ionic liquid can be found by CAMD.
1525 5.2. Other Methods for Screening Solvents
1526 5.2.1. Pierotti-Deal-Derr Method
1527 Apart from CAMD, the potential solvents may be evaluated by other 1528 prediction methods. Pierrotti-Deal-Derr method is one of those methods 1529 (Duan, 1978) that can be used to predict the activity coefficient at infinite 1530 dilution and thus deduce relative volatility at infinite dilution, the most 1531 important explicit property in extractive distillation. 1532 Activity coefficients at infinite dilution are correlated to the number of 1533 carbon atoms of the solute and solvent(n1 and n2). For the members of T12.1 Table 12. Some implicit properties of commonly Chen and Li, Lei, used ionic liquids. 174
Liquid temperature T12.2 (°C) Solubility in common solventsc Color (with Density Petroleum Acetic T12.3 Ionic liquida xb impurities) (g/ml) Lowest Highest Water Methanol Acetone Chloroform ether Hexane anhydride Toluene
T12.4 [bmim]BF4 Light yellow 1.320 À48.96 399.20 s s s s i i i i
T12.5 [bmim]PF6 Light yellow 1.510 13.50 388.34 i s s s i i s i 120026627_SPM_32_02_R1_092503 T12.6 [bmim]Cl/AlCl3 0.50 Light brown 1.421 À88.69 263.10 r r s s i i s s T12.7 0.55 Light brown 1.456 À94.44 286.59 r r s s i i s s T12.8 0.60 Light brown 1.481 À95.87 316.34 r r s s i i s s T12.9 [emim]Br/AlCl3 0.50 Purplish 1.575 13.61 272.51 r r s i i i s i black T12.10 0.55 Brownish 1.656 6.45 294.02 r r s i i i s i black T12.11 0.60 Brownish 1.995 À19.08 345.34 r r s i i i s i black T12.12 [emimi]PF6 Yellow 1.426 2.71 304.65 i i s s i i s i T12.13 N-butylpyridine/ 0.50 Yellow 1.412 18.80 240.00 r r s i i i s s AlCl3 T12.14 0.55 Brownish 1.430 33.73 245.39 r r s i i i s s yellow T12.15 0.60 Brownish 1.497 18.11 260.2 r r s i i i s s yellow T12.16 (CH3)3NHCl/ 0.66 Brownish 1.621 À67.90 80.25 r r s i i i s s 2AlCl3 yellow
a[bmim] = 1-butyl-3-methylimidazolium, [emim] = 1-ethyl-3-methylimidazolium. b Apparent mole fraction of AlCl3. cs: soluble; i: insoluble; r: may react with each other. Adapted from Zhao et al. (2002). 中国科技论文在线 http://www.paper.edu.cn
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1534 homologous series H(CH2)n1X1 (solute) in the members of the homologous 1535 series H(CH2)n2Y2:
1 F2 n1 C1 2 lg g1 ¼ A12 þ þ B2 þ þ D0ðn1 À n2Þ ð14Þ n2 n2 n1
15361537 where the constants are functions of temperature, B2 and F2 are functions of 1538 the solvent series, C1 is a function of the solute series, A12 is a function of 1539 both, and D0 is independent of both. 1540 For zero members of a series, e.g. water for alcohol, no infinite value 1541 for g1 is obtained. Instead, by convention, any terms containing an n for 1542 the zeromember are incorporated in the corresponding coefficient. So for n- 1543 alcohols in water: 1 lg g1 ¼ K þ B2n1 þ C1=n1 ð15Þ 2 15441545 Notice that the term D0(n1Àn2) is incorporated into the K constant because 1546 D0 is smaller than the other coefficients by a factor of 1000. Therefore, this 1547 term is insignificant. In Eq. 15 only K is a function of the solute and 1548 solvent. B2 is always the same when water is the solvent and C1 is the same 1549 for n-alcohol solutes. This is shown better from the following homologous 1550 series in water at 100°C.
1 0:230 n-Alcohols: lg g1 ¼À0:420 þ 0:517n1 þ ð16Þ n1 15511552
1 0:320 n-Aldehydes: lg g1 ¼À0:650 þ 0:517n1 þ ð17Þ n1 15531554 where the coefficient B is the same in both cases.
1555 5.2.2. Parachor Method
1556 Activity coefficients at infinite dilution are obtained from the follow- 1557 ing relationship: 1 lg g1 ¼ ½U1=2 À CU1=22 ð18Þ 1 2:303RT 1 2 15581559 v Ui ¼ DHi À RT ð19Þ V 15601561 where Ui is potential energy of component i, DHi is enthalpy of evapo- 1562 ration, C is a constant, a function of temperature, the parachor ratio of the 1563 two components, and the number of carbon atoms in the solute and solvent 1564 molecules, T is the absolute temperature, and R is gas constant. About the 1565 same variety of systems covered in the Pierrotti-Deal-Derr method, is 1566 covered in this approach. 中国科技论文在线 http://www.paper.edu.cn
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1567 5.2.3. Weimer-Prausnitz Method
1568 Starting with the Hildebrand-Schatchard model for non-polar mixtures, 1569 Weimer and Prausnitz developed an expression for evaluating values of 1570 hydrocarbons in polar solvents: 1 2 2 V2 V2 RT ln g2 ¼ V2½ðl1 À l2Þ þ t1 À 2c12þRT ln þ 1 À ð20Þ V1 V1
15711572 where Vi is the molar volume of pure component i, li is the non-polar 1573 solubility parameter, ti is the polar solubility parameter, T is the absolute 1574 temperature, and R is gas constant. The subscript 1 represents the polar 1575 solvent and subscript 2 is the hydrocarbon solute with
2 c12 ¼ kt1 ð21Þ 15761577 Later Helpinstill and Van Winkle suggested that Eq. 20 is improved by 1578 considering the small polar solubility parameter of the hydrocarbon (olefins 1579 and aromatics):
1 2 2 RT ln g ¼ V2½ðl1 À l2Þ þðt1 À t2Þ À 2c 2 12 V V þRT ln 2 þ 1 À 2 V1 V1 ð22Þ 15801581 2 c12 ¼ kðt1 À t2Þ ð23Þ 15821583 For saturated hydrocarbons,
2 c12 ¼ 0:399ðt1 À t2Þ ð24Þ 15841585 For unsaturated hydrocarbons,
2 c12 ¼ 0:388ðt1 À t2Þ ð25Þ 15861587 For aromatics,
2 c12 ¼ 0:447ðt1 À t2Þ ð26Þ
15881589 The term c12 corresponds to the induction energy between the polar and 1590 non-polar components. Since no chemical effects are included, the corre- 1591 lation should not be used for solvents showing strong hydrogen bonding. 1592 It is evident that the parameters of these three methods are incomplete, 1593 which leads to their very limited application in extractive distillation. So 1594 these methods are rarely reported in the recent references. 1595 However, CAMD based on group contribution methods is very desir- 1596 able. By means of CAMD, the experiment working is greatly decreased in a 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
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1597 search for the best solvents. CAMD as a useful tool plays an important role 1598 in finding the solvent and shortening the search time. It is believed that 1599 with the development of CAMD, it would be possible to be extended and 1600 applied in many more fields.
1601 6. THEORY OF EXTRACTIVE DISTILLATION
1602 Until now, there are two theories related to extractive distillation, i.e. 1603 Prausnitz and Anderson theory which gives semi-quantitative explanations, 1604 and Scaled Particle Theory which gives complete quantitative results. The 1605 two theories provide a molecular basis for explaining why some solvents 1606 can increase the relative volatility of the components to be separated, and 1607 more selective than others.
1608 6.1. Prausnitz and Anderson Theory
1609 Separation of hydrocarbon mixtures by extractive distillation has been 1610 practiced industrially for many years, even though there has been only 1611 limited understanding of the fundamental phase equilibria which forms the 1612 thermodynamic basis of this operation. In general, it is known that the 1613 addition of polar solvents to hydrocarbon mixture results in increased 1614 volatilities of paraffins relative to naphthenes, olefins, diolefins and alkynes, 1615 and in increased volatilities of naphthenes relative to aromatics. Therefore, 1616 the addition of a polar solvent enables facile separation by distillation of 1617 certain mixtures which otherwise can only be separated with difficulty. The 1618 Prausnitz and Anderson theory (Prausnitz and Anderson, 1961) tries to 1619 explain the solvent selectivity in extractive distillation of hydrocarbons from 1620 the viewpoint of molecular thermodynamics and intermolecular forces. The 1621 interaction forces between the solvent and the component are broadly di- 1622 vided into two types, i.e. physical force and chemical force. The true state 1623 in the solution is undoubtedly a hybrid of these two forces.
1624 6.1.1. Physical Force
1625 The selectivity is related to the various energy terms leading to the 1626 desired nonideality of solution which is the basis of extractive distillation, 1627 and can be expressed in such a manner.
2 2 2 RT ln S23 ¼½d1pðV2 À V3Þ þ ½V2ðd1n À d2Þ À V3ðd1n À d3Þ
þ½2V3x13 À 2V2x12 ð27Þ 中国科技论文在线 http://www.paper.edu.cn
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178 Lei, Li, and Chen
16281629 where subscripts 1–3 represent solvent, the light component and the heavy 1630 component to be separated by extractive distillation, respectively, and Vi is 1631 the molar volume of component i. 1632 The three bracketed terms in Eq. 27 show, respectively, the separate 1633 contributions of physical force to the selectivity, i.e. the polar effect, the 1634 dispersion effect and the inductive effect of the solvent. It is convenient to 1635 rewrite Eq. 1 as
RT ln S23 ¼ P þ D þ I ð28Þ 16361637 where
P ¼ d2 ðV À V Þ 16381639 1p 2 3
D V 2 V 2 16401641 ¼ 2ðd1n À d2Þ À 3ðd1n À d3Þ
I ¼ 2V3x13 À 2V2x12 16421643 It is found that the polar term P is considerably larger than the sum of D 1644 and I, and frequently very much larger. Thus, Eq. 28 becomes
2 RT ln S23 ¼ d1pðV2 À V3Þ ð29Þ 16451646 Of course in the special case where components 2 and 3 are identical in 1647 size, the polar term vanishes. This means that the physical force can’t play 1648 a role in separating hydrocarbon mixture. In this case, the chemical force is 1649 dominant, and can be used to explain the separation phenomena, as is 1650 discussed in the following text. 1651 Eq. 29 not only shows the effect of molecular size but also predicts that 1652 when one separates hydrocarbons of different molar volumes, the selectivity 1653 is sensitive to the polar solubility parameter. It indicates that the effec- 1654 tiveness of a solvent depends on its polarity, which should be large, and on 1655 its molar volume, which should be small. 1656 One example of separating C4 mixture is given to illustrate the 1657 physical force. It is known that the order of the molar volume of C4 1658 mixture is in the following Butane > butene > butadiene > butyne
16591660 According to Eq. 29, the order of volatilities of C4 mixture is in the 1661 same following > > > 16621663 Butane butene butadiene butyne 1664 However, in order to have a much higher selectivity, the polar solu- 1665 bility parameter of the solvents should be as great as possible. That is why 中国科技论文在线 http://www.paper.edu.cn
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T13.1 Table 13. The solvents for separating ethane and ethylene.
T13.2 Dipole moment Relative volatility Selectivity
1 1 T13.3 Solvents m(debyes) a12 S12 T13.4 Toluene 1.23 0.84 0.83 T13.5 Xylene o-1.47 0.90 0.89 T13.6 m-1.13 T13.7 p-0 T13.8 Tetrahydrofuran 5.70 0.97 0.96 T13.9 Butyl acetate 6.14 0.95 0.94 T13.10 Ethyl acetate 6.27 1.01 1.00 T13.11 Pyridine 7.44 1.02 1.01 T13.12 Acetone 8.97 1.08 1.07 T13.13 ACN 11.47 1.24 1.23 T13.14 DMF 12.88 1.20 1.19 T13.15 Dimethyl sulfoxide 13.34 1.19 1.18 T13.16 NMP 13.64 1.14 1.13
Adapted from Yi et al. (2001).
1666 such solvents as ACN, DMF and NMP with high polarity and small molar 1667 volume, are used for this separation. 1668 Another example is concerned with the separation of ethane(1) and 1669 ethylene(2) by extractive distillation (Yi et al., 2001). The experimental 1670 results are listed in Table 13, in which the solvent polarity is character- T13 1671 ized by dipole moment. It can be seen from Table 13 that with the 1672 increase of dipole moment, relative volatility and selectivity at infinite 1673 dilution also approximately rise, which is consistent with the Prausnitz 1674 and Anderson theory.
1675 6.1.2. Chemical Force
1676 For components of identical size, solvent polarity is not useful and 1677 selectivity on the basis of a physical effect is not promising. For such cases 1678 selectivity must be based on chemical force which will selectively increase 1679 the interaction between the solvent and the components. However, sepa- 1680 ration of components of identical size is very rare. 1681 The chemical viewpoint of solutions considers that nonideality in so- 1682 lution arises because of association and solvate. In accordance with this 1683 concept, the true species in solution are loosely bonded aggregates con- 1684 sisting of two (or more) molecules of the same species(association) or of 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
180 Lei, Li, and Chen
1685 different species(solvate). That is to say, the solvent and the component can 1686 form complexes. Such complexes are believed to be the result of acid-base 1687 interactions following the Lewis definitions that a base is an electron donor 1688 and that an acid is an electron accepter. 1689 For example, for the separation of C4 hydrocarbons, the fluidity of 1690 electron cloud is different for the group C–C(no double-bond), C C(only 1691 one double-bond), C C–C C(two double-bonds), C C(one trinary- 1692 bond), and in the following order
C À C < C ¼ C < C ¼ C À C ¼ C < C C
16931694 The greater the fluidity is, the easier the group is to be polarized. Ac- 1695 cordingly, the basicity of C4 hydrocarbons is in the same order
C À C < C ¼ C < C ¼ C À C ¼ C < C C
16961697 which means that the chemical force between solvent and butyne is the 1698 greatest, while it is the smallest between solvent and butane. 1699 So the volatilities of C4 hydrocarbons are in the following order Butane > butane > butadiene > butyne
17001701 which is consistent with the conclusion resulting from experiment. 1702 A schematic diagram of complex formation between DMF and cis-2- 1703 butene is given in Figure 19. F19 1704 In fact, in the solution physical and chemical forces exist at the same 1705 time. Just in some cases one is predominant, and the other is minor. 1706 The limitation of this theory is that it is only suitable for the separation 1707 of non-polar systems. However, in extractive distillation, polar-polar and 1708 non-polar –polar systems are often met, and at this time the molecular 1709 interaction may be more complicated. But the idea of physical and chemical
Figure 19. Schematic diagram of chemical interaction(Arrow shows donation of electrons.) 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
Extractive Distillation 181
1710 forces is valuable and may be adopted. For instance, for the separation of 1711 acetic acid and water with tributylamine as the separating agent by ex- 1712 tractive distillation, the chemical force between acetic acid and tributyla- 1713 mine is very strong and has been verified by IR and GC-MS techniques(See 1714 Chapter 3, 3.2.3 Case studies).
1715 6.2. Scaled Particle Theory
1716 6.2.1. The Relationship of Salt Effect and Relative Volatility at 1717 Infinite Dilution
1718 Due to the unique advantages, extractive distillation with the combi- 1719 nation of liquid solvent and solid salt as an advanced technique has been 1720 received more attention. However, selection of solid salt needs theoretical 1721 guidance. In this case, scaled particle theory is recommended to apply for 1722 extractive distillation, and tries to interpret the effect of solid salt on ex- 1723 tractive distillation. Moreover, it provides a theoretical support for opti- 1724 mizing the solvents when needed. 1725 Nowadays there are many theories (Li, 1988; Masterton and Lee, 1970; 1726 Pierotti, 1976; Shoor and Gubbins, 1969) about salt effect such as the 1727 electrostatic theory of Debye-McAulay in dilute electrolyte solutions, in- 1728 ternal pressure theory of McDevit-Long, salt effect nature of Huangziqing, 1729 electrolyte solution theory of Pitzer and scaled particle theory, in which the 1730 first four theories require the experimental data to correlate or make some 1731 simplification with a little accuracy or have no wide range to apply. But the 1732 scaled particle theory, which is deduced from thermodynamics and statis- 1733 tical physics, has defined physical meaning, and the required molecular 1734 parameters are readily available. Especially, in the recent years, the study of 1735 scaled particle theory has been farther developed. But, first of all, scaled 1736 particle theory can be applied to extractive distillation with the combination 1737 of liquid solvent and solid salt. Therefore, it is reasonable to deal with salt 1738 effect of extractive distillation with this theory. 1739 The relationship of salting coefficients and relative volatilities at infi- 1740 nite dilution can be deduced and expressed as follows
a1 s ðks1Àks2Þcs 1 ¼ 10 ð30Þ a0
1 1 17411742 where as and a0 represent relative volatilities at infinite dilution with salt 1743 and no salt. 1744 Eq. 30 constructs a bridge between microscale and macroscale. Even 1745 if the calculated salting coefficients are not accurate due to the current 中国科技论文在线 http://www.paper.edu.cn
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182 Lei, Li, and Chen
1746 limitation of scaled particle theory, it is not difficult to judge the magnitude 1747 of ks1 and ks2 by the conventional solution knowledge and decide whether it 1748 is advantageous to improve the relative volatilities with salt. From Eq. 30, it 1749 concludes that if ks1 > ks2 with low salt concentration the relative volatilities 1750 of components to be separated will be increased by adding salt, and the more 1751 great the difference between ks1 and ks2, the more apparent the effect of 1752 improving relative volatility. 1753 Four systems DMF/C4, ACN/C3, ethylene glycol/ethanol/water and 1754 ethylene glycol/acetone/methanol are investigated (Lei et al., 2002f; Liao 1755 et al., 2001). As to the first two non-polar solutes systems, the non-elec- 1756 trolytes bring out salting-out in the solutions. But for the final two polar 1757 solutes systems, the effect is salting-in. 1758 For separating non-polar system, e.g. DMF/C4 and ACN/C3, the rel- 1759 ative volatilities at infinite dilution calculated by scaled particle theory 1760 correspond well with experimental values. The reason may be that C4 and 1761 C3 are non-polar components and the sizes of them are not large, which 1762 lead to the accurate results. It is of interest to compare the relative con- 1763 tribution of the three terms ka, kb; and kg to the salting coefficient, ks.Itis 1764 noted that kg is small. Therefore, the salting coefficients ks mainly depends 1765 upon the relative magnitudes of ka and kb. 1766 However, for separating polar systems, salt coefficients are not easy to 1767 be accurately calculated in terms of scaled particle theory. But it does not 1768 influence our analysis on whether it is advantageous to add salt to a system 1769 because in most cases, it is not difficult for us to qualitatively judge the 1770 relative magnitude of salt coefficient of each component. But in the past the 1771 interpretation of the phenomena is very vague (Duan et al., 1980; Lei et al., 1772 1982; Zhang et al., 1984; Zhao et al., 1989, 1992). 1773 Although the application of scaled particle theory to the calculation of 1774 salt effect has the great advantages that the required molecular parameters 1775 are readily available, it is limited in some degrees. For polar solutes, scaled 1776 particle theory can only provide qualitative analysis according to Eq. 30. 1777 The reason may be that the hydrogen bonding between polar solutes and 1778 polar solvent is very complicated and greater than van der Waals bonding. 1779 By now, only van der Waals bonding is considered in scaled particle the- 1780 ory. So it is normal that quantitative calculation for polar solutes is very 1781 difficult and inaccurate. We know any theory has its own deficiency, but 1782 we believe that with the development of scaled particle theory, the problem 1783 will be solved one day. 1784 Anyway, scaled particle theory is extended to solve the problem of 1785 extractive distillation with the combination of solvent and salt, and will 1786 promote the development of extractive distillation that is always an im- 1787 portant separation method. 中国科技论文在线 http://www.paper.edu.cn
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Extractive Distillation 183
1788 7. MATHEMATIC MODEL OF 1789 EXTRACTIVE DISTILLATION
1790 For design of distillation process, two types of modeling approaches 1791 have been developed: the equilibrium(EQ) stage model and the non-equi- 1792 librium (NEQ) stage model. The most difference between these two models 1793 is that the mass and heat transfer rates should be considered in every tray in 1794 the NEQ stage model. For extractive distillation, the process can be sim- 1795 ulated either by EQ stage model or by NEQ stage model.
1796 7.1. EQ Stage Model
1797 7.1.1. Model Equations
1798 The schematic diagrams of a tray column for EQ stage model are 1799 shown in Figure 20. In general, the assumptions adopted are the following F20 1800 (Komatsu, 1977; Komatsu and Holland, 1977; Jelinek and Hlavacek, 1976):
1801 1. Operation reaches steady state; 1802 2. System reaches mechanical equilibrium in every tray; 1803 3. The vapor and liquid bulks are mixed perfectly and assumed to be 1804 at thermodynamic equilibrium; 1805 4. Heat of mixing can be neglected; 1806 5. Reactions take place in the liquid phase; if reactions take place in 1807 the vapor phase, the actual vapor compositions are replaced by 1808 transformed superficial compositions.
Figure 20. Schematic representation of an EQ stage. 中国科技论文在线 http://www.paper.edu.cn
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184 Lei, Li, and Chen
1809 6. The condenser and reboiler are considered as an equilibrium tray; 1810 7. If the catalyst is used, then the amount of catalyst in each tray of 1811 reaction section is equal; 1812 8. If the catalyst is used, then the influence of catalyst on the mass 1813 and energy balance is neglected because the catalyst concentration 1814 in the liquid phase is often low.
1815 The equations that model EQ stages are known as the MESHR equa- 1816 tions. MESHR is an acronym referring to the different types of equation. 1817 The M equations are the material balance equations. The total material 1818 balance takes the form
dM j ¼ V þ L þ F Àð1 þ rV ÞV Àð1 þ rLÞL ð31Þ dt jþ1 jÀ1 j j j j j Xr Xc þ vi;kRk; jej k¼1 i¼1
18191820 where F, V, L are feed, vapor and liquid flowrates, respectively, t is time, 1821 Rk,j is reaction rate, vi,k is the stoichiometric coefficient of component i in 1822 reaction k, e is reaction volume or amount of catalyst, and Mj is the hold-up 1823 on stage j. With very few exceptions, Mj is considered to be the hold-up 1824 only of the liquid phase. It is more important to include the hold-up of the 1825 vapor phase at higher pressures. The component material balance(neglecting 1826 the vapor hold-up) is
dM x j i; j ¼ V y þ L x þ F z dt jþ1 i; jþ1 jÀ1 i; jÀ1 j i; j Xr V L Àð1 þ rj ÞVjyi; j Àð1 þ rj ÞLjxi; j þ vi;kRk;jej ð32Þ k¼1
18271828 where x, y are mole fraction in the liquid and vapor phase, respective. 1829 In the material balance equations given above rj is the ratio of side- 1830 stream flow to interstage flow:
V V L L rj ¼ Sj =Vj; rj ¼ Sj =Lj ð33Þ
18311832 The E equations are the phase equilibrium relations
yi; j ¼ Ki; jxi; j ð34Þ
18331834 where ki, j is chemical equilibrium constant. The S equations are the sum- 1835 mation equations 中国科技论文在线 http://www.paper.edu.cn
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Extractive Distillation 185
Xc Xc xi; j ¼ 1; yi; j ¼ 1 ð35Þ i¼1 i¼1
18361837 The enthalpy balance is given by
dM H j j ¼ V HV þ L HL þ F HF dt jþ1 jþ1 jÀ1 jÀ1 j j V V L L Àð1 þ rj ÞVjHj Àð1 þ rj ÞLjHj À Qj ð36Þ
18381839 where H is molar enthalpy, and Q is heat duty. 1840 There is no need to take separate account in Eq. 36 of the heat gen- 1841 erated due to chemical reaction since the computed enthalpies include the 1842 heats of formation. 1843 The R equations are the reaction rate equations. For the reactive ex- 1844 tractive distillation it is known that chemical reaction is reversible, and the 1845 reaction rate is assumed to be zero. That is to say, the chemical equilibrium 1846 is reached in every tray. 1847 Under steady-state conditions all of the time derivatives in the MESH 1848 equations are equal to zero. Newton’s method(or a variant thereof) for 1849 solving all of the independent equations simultaneously is an approach 1850 nowadays widely used. But other methods also frequently appear, e.g. the 1851 relaxation method. In this method the MESH equations are written in un- 1852 steady-state form and are integrated numerically until the steady-state so- 1853 lution has been found, is used to solve the above equations.
1854 7.1.2. Case Studies
1855 EQ stage model is used for the process simulation of extractive 1856 distillation, and can obtain the necessary information for various purposes. 1857 In what follows we discuss the steady state of extractive distillation and 1858 extractive distillation with chemical reaction(i.e. reactive extractive dis- 1859 tillation) in the EQ stage model. It would be nearly impossible to cite 1860 every paper about the EQ stage model, but we try to be comprehensive in 1861 our coverage.
1862 Steady State Analysis 1863 Analysis of operation state is still a topic of considerable interest in the 1864 distillation community. Especially in the catalytic distillation process the 1865 non-linearity phenomena are very prominent, and multiple steady state is 1866 easy to appear. 1867 For the separation of C4 mixture by extractive distillation with DMF, the 1868 operation state is analyzed by changing the feeding location of solvent, which 中国科技论文在线 http://www.paper.edu.cn
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186 Lei, Li, and Chen
1869 is a sensitive parameter. The feeding location of solvent into the extractive 1870 distillation column for recovering butane is variable, but other operating 1871 conditions are kept constant. The relation of the top molar composition of 1872 butane with the feeding stage of solvent is shown in Figure 21 (the stage is F21 1873 numbered from the top to the bottom). When the solvent is fed in the vicinity 1874 of No.20 stage, the composition change is very abrupt and two operation 1875 states(OS) are found. However, only No.1 operation state(OS) is desirable 1876 because in this case the top molar composition of butane is much higher. 1877 Similarly, only the feeding location of solvent into the extractive dis- 1878 tillation column for recovering 1,3-butadiene is variable, but other operating 1879 conditions are kept constant. In this case the top molar composition of 1,3- 1880 butadiene is stable because the amount of vinylacetylene(VAC) is so small 1881 that the 1,3-butadiene composition is not affected. The function of this 1882 column is to remove VAC from 1,3-butadiene, so the change of the bottom 1883 flowrate of VAC should be obvious. The relation of the bottom flowrate of 1884 VAC (kmol/h) with the feeding stage of solvent S is shown in Figure 22 F22 1885 (the stage is numbered from the top to the bottom). It can be seen from 1886 Figure 22 that when solvent S is fed in the vicinity of No.30 stage, two 1887 operation states are also found. However, only No.1 operation state is
Figure 21. The relation of the top composition of butane with the feeding stage of solvent S in the extractive distillation column. 中国科技论文在线 http://www.paper.edu.cn
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Figure 22. The relation of the bottom flowrate of VAC with the feeding stage of solvent in the extractive distillation column.
1888 desirable because in this case the amount of VAC removed from 1,3-buta- 1889 diene is greater. 1890 Therefore, by using of EQ stage model, we can find which operation 1891 state is the best or the worst.
1892 Reactive Extractive Distillation 1893 In the EQ stage model, if there exists no chemical reaction, the inde- 1894 pendent equations are simplified, only solving MESH equations. However, 1895 for reactive extractive distillation, EQ stage model is somewhat compli- 1896 cated, especially when there exist more than one chemical reactions. Sep- 1897 aration of acetic acid and water with tributylamine as the separating agent 1898 is just the case. 1899 As mentioned in the front, the following reversible chemical reaction 1900 may take place:
þ À HAc þ R3N À! À R3NH Á OOCCH3
+ À 19011902 where HAc, R3N and R3NH Á OOCCH3 represent acetic acid, tributyla- 1903 mine and the salt formed by the reaction, respectively. 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
188 Lei, Li, and Chen
1904 The chemical equilibrium constant Kp at 25°C can be deduced, and 1905 thus the relation of chemical equilibrium constant with temperature is 1906 expressed by Eq. 4. 1907 On the other hand, it is known that aggregation of acetic acid mole- 1908 cules in the vapor phases occurs. If only aggregation of two molecules is 1909 considered, the following reversible chemical reaction may take place:
2A1 À!À A2
19101911 where A1 is the monomer of acetic acid, and A2 is the dimer of acetic acid. 1912 K In this case, the chemical equilibrium constant A2 is written, ZV A2 KA ¼ ð37Þ 2 P ZV 2 ð A1 Þ 19131914 where Z is the superficial molecular number in the vapor phase. 1915 K In addition, A2, the function of temperature, can also be calculated by 1916 the following empirical equation (Shi et al., 1999a,b),
lg KA2 ¼ eA2 þ oA2 =T ð38Þ 19171918 where eA2 = À10.4205, oA2 = 3166. 1919 The key in the simulation of extractive distillation process is the se- 1920 lection of an accurate VLE model to solve the EQ stage model of the col- 1921 umn. In general, the Wilson, NRTL and UNIQUAC equations, which are 1922 suitable for systems composed of many components and can deduce the 1923 multi-components system from binary systems, are used. 1924 Although the VLE data can be obtained by experiments and then cor- 1925 related by the iterative solution using the maximum likelihood regression, the 1926 interaction parameters of the VLE model may be various under different 1927 assumptions. Table 14 shows the interaction parameters of Wilson, NRTL T14
Table 14. Interaction parameters of Wilson, NRTL and UNIQUAC equations for the T14.1 system of water(1) and acetic acid(2) (unit: J/mol).
T14.2 Equation A12 A21 Dy1 T14.3 1. Not considering two-molecule aggregation T14.4 Wilson 3766.34 À3301.3 0.0204 T14.5 NRTL (a = 0.47) 87.86 363.04 0.0205 T14.6 UNIQUAC À2882.96 4149.57 0.0241 T14.7 2. Considering two-molecule aggregation T14.8 Wilson 3566.01 À818.99 0.0164 T14.9 NRTL (a = 0.47) 2861.58 À120.11 0.0164 T14.10 UNIQUAC À1553.11 2321.62 0.0185 中国科技论文在线 http://www.paper.edu.cn
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Table 15. Interaction parameters of Wilson, NRTL and UNIQUAC equations for the T15.1 system of water(1) and tributylamine (2) (unit: J/mol).
T15.2 Equation A12 A21 Dy1 T15.3 Wilson 16425.00 22378.10 0.0213 T15.4 NRTL (a = 0.3) 9245.76 2940.54 0.0202 T15.5 UNIQUAC 791.39 302.74 0.0191
1928 and UNIQUAC equations for the system of water(1) and acetic acid(2) under 1929 two cases: considering two-molecule aggregation and not, respectively; Ta- 1930 ble 15 shows the interaction parameters of Wilson, NRTL and UNIQUAC T15 1931 equations for the system of water(1) and tributylamine(2); Table 16 shows T16 1932 the interaction parameters of Wilson, NRTL and UNIQUAC equations for 1933 the system of acetic acid(1) and tributylamine(2) under three cases: not 1934 considering two-molecule aggregation and reversible chemical reaction, only 1935 considering reversible chemical reaction, and considering two-molecule ag- 1936 gregation and reversible chemical reaction at the same time, respectively. 1937 In these tables, an average deviation Dy1 is calculated as, PN jDyj Average deviation ¼ i¼t N 19381939 where N is the times of experiments.
Table 16. Interaction parameters of Wilson, NRTL and UNIQUAC equations for the T16.1 system of acetic acid(1) and tributylamine (2) (unit: J/mol).
T16.2 Equation A12 A21 Dy1 T16.3 1. Not considering two-molecule aggregation and reversible chemical reaction T16.4 Wilson 7373.29 À3558.84 0.1158 T16.5 NRTL (a = 0.2) À1553.97 5263.68 0.1160 T16.6 UNIQUAC À1707.19 4830.12 0.1156 T16.7T16.8 2. Only considering reversible chemical reaction T16.9 Wilson 7550.79 À3478.02 0.1182 T16.10 NRTL (a = 0.2) 346.69 3413.18 0.1181 T16.11 UNIQUAC À1551.01 4697.44 0.1200 T16.12 3. Considering two-molecule aggregation and reversible chemical T16.13 reaction simultaneously T16.14 Wilson 8731.21 À2265.00 0.0580 T16.15 NRTL (a = 0.2) 1496.1 4118.9 0.0596 T16.16 UNIQUAC À1505.47 5463.82 0.0552 中国科技论文在线 http://www.paper.edu.cn
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T17.1 Table 17. The comparison of the experimental and calculated values.
T17.2 Ttop/°C Water concentration at the top, wt% T17.3 No. Exp. EQ1 EQ2 EQ3 Exp. EQ1 EQ2 EQ3
T17.4 1 99.9 99.7 99.5 99.7 99.74 96.20 96.30 96.46 T17.5 2 100.2 99.8 99.9 99.6 99.81 94.09 92.10 97.51 T17.6 3 99.6 100.0 99.5 99.7 99.77 98.78 99.30 96.08 T17.7 4 99.9 100.5 99.8 99.5 99.86 94.73 94.77 98.55
1940 It can be seen from Table 14 that the difference of interaction para- 1941 meters between two cases is somewhat obvious, but the average deviation 1942 in both cases is small enough. The reason may be that the activity coef- 1943 ficients of water and acetic acid are not apparently changed at the total 1944 concentration, and the interaction parameters of the VLE models can be 1945 well correlated. Even so, it manifests that the influence of two-molecule 1946 aggregation on the interaction parameters can’t be ignored. 1947 In Table 16, the difference of interaction parameters among three cases 1948 is also somewhat obvious. Moreover, the average deviation in the third case 1949 is small enough. The reason may be that this case is approximate to the 1950 actual situation, and the VLE model can accurately describe the real ac- 1951 tivity coefficients. Besides, it manifests that the influence of both two- 1952 molecule aggregation and reversible chemical reaction on the interaction 1953 parameters can’t be ignored. 1954 By using of EQ stage model, the extractive distillation process of 1955 separating water and acetic acid is simulated. In the simulation, three cases 1956 are concerned, i.e. . not considering two-molecule aggregation and re- 1957 versible chemical reaction(EQ1 model); . only considering reversible 1958 chemical reaction(EQ2 model); . considering two-molecule aggregation 1959 and reversible chemical reaction simultaneously(EQ3 model). 1960 The experimental data, as well as the calculated results from the EQ 1961 stage models(EQ1, EQ2 and EQ3 models), are listed in Table 17. It is T17 1962 shown that the values of EQ3 model are closer to experimental results than 1963 those of EQ1 and EQ2 models. That means that EQ3 model reflects the real 1964 state of the system of water/acetic acid/ tributylamine more accurately. 1965 In order to further investigate the difference among EQ1, EQ2 and EQ3 1966 models, the composition and temperature distributions along the extractive 1967 distillation column under the same operation condition are given in 1968 Tables 18 and 19, where subscripts 1, 2 ,3 and 4 represent water, acetic T18/T19 1969 acid, tributylamine and salt produced by the reaction, respectively. The tray 中国科技论文在线 xrcieDistillation Extractive
T18.1 Table 18. The composition distribution along the extractive distillation column for the EQ1, EQ2 and EQ3 models.
T18.2 x1 X2 x3 x4 T18.3 No. EQ1 EQ2 EQ3 EQ1 EQ2 EQ3 EQ1 EQ2 EQ3 EQ1 EQ2 EQ3 120026627_SPM_32_02_R1_092503 T18.4 1 0.0004 0.0002 0.0 0.4970 0.4762 0.4906 0.5029 0.4904 0.4769 0.0 0.0331 0.0326 T18.5 2 0.0029 0.0015 0.0 0.8910 0.8736 0.7917 0.1058 0.1013 0.1775 0.0 0.0235 0.0308 T18.6 3 0.0068 0.0036 0.0 0.9350 0.9266 0.8419 0.0582 0.0552 0.1318 0.0 0.0145 0.0263 T18.7 5 0.0301 0.0164 0.0 0.9168 0.9142 0.8577 0.0531 0.0549 0.1177 0.0 0.0145 0.0245 T18.8 7 0.1200 0.0680 0.0 0.8270 0.8580 0.8584 0.0530 0.0590 0.1169 0.0 0.0149 0.0246 T18.9 9 0.3430 0.2268 0.0012 0.6015 0.7046 0.8569 0.0553 0.0556 0.1178 0.0 0.0130 0.0242 T18.10 11 0.6184 0.4690 0.0474 0.3255 0.4562 0.8133 0.0561 0.0638 0.1152 0.0 0.0108 0.0241 T18.11 13 0.4862 0.5194 0.1237 0.3006 0.3512 0.5910 0.2133 0.1164 0.2411 0.0 0.0129 0.0441 T18.12 15 0.4885 0.6162 0.1617 0.3061 0.2759 0.5971 0.2054 0.0981 0.2149 0.0 0.0098 0.0263 http://www.paper.edu.cn T18.13 17 0.6334 0.6576 0.1619 0.2318 0.2096 0.5430 0.1348 0.1224 0.2607 0.0 0.0104 0.0344 T18.14 19 0.6612 0.6618 0.1486 0.2029 0.2091 0.5256 0.1359 0.1205 0.3008 0.0 0.0086 0.0250 T18.15 21 0.7236 0.7523 0.2890 0.1386 0.1274 0.3986 0.1378 0.1145 0.2927 0.0 0.0058 0.0196 T18.16 23 0.7939 0.7414 0.8163 0.0722 0.0688 0.0925 0.1340 0.1870 0.0874 0.0 0.0028 0.0038 T18.17 24 0.8717 0.8570 0.8705 0.0328 0.0321 0.0348 0.0955 0.1093 0.0930 0.0 0.0016 0.0018 T18.18 25 0.9730 0.9721 0.9680 0.0115 0.0112 0.0106 0.0155 0.0166 0.0213 0.0 0.0001 0.0 191 中国科技论文在线 http://www.paper.edu.cn
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Table 19. The temperature distribution along the extractive distillation column for the EQ1, EQ2 and T19.1 EQ3 models.
T19.2 T/K
T19.3 No. EQ1 EQ2 EQ3
T19.4 1 404.83 404.90 424.45 T19.5 2 393.75 393.71 420.07 T19.6 3 392.51 392.37 419.40 T19.7 5 391.41 391.26 419.19 T19.8 7 388.34 388.14 419.18 T19.9 9 382.65 382.43 418.97 T19.10 11 377.76 377.65 411.03 T19.11 13 378.06 376.89 395.89 T19.12 15 378.73 376.62 394.63 T19.13 17 376.89 375.60 392.60 T19.14 19 376.44 375.62 393.41 T19.15 21 376.44 374.16 384.81 T19.16 23 374.12 373.22 374.53 T19.17 24 373.30 372.88 373.36 T19.18 25 372.84 372.69 372.83
1970 is numbered from the bottom to the top. It can be seen from Table 18 that 1971 the calculated results of EQ1 model correspond well with those of EQ2 1972 model on a large part of trays. However, in the vicinity of the tray feeding 1973 the mixture of acetic acid and water, i.e. from No. 9 to No.15, the dif- 1974 ference of liquid composition distribution is enlarged, which may be due to 1975 the relatively great influence of the feed mixture on the chemical reaction 1976 between acetic acid and tributylamine. But the difference of the composi- 1977 tion and temperature distributions between EQ3 model and EQ1(or EQ2) 1978 model is evident. This indicates that the influence of two-molecule aggre- 1979 gation is more obvious than that of reversible chemical reaction. Anyway, 1980 EQ3 model is more suitable for the design and optimization of extractive 1981 distillation process than EQ1 and EQ2 models. 1982 On the other hand, the effect of the solvent, tributylamine, on the 1983 separation of water and acetic acid, can be verified by simulation. It can be 1984 seen from Table 17 that water composition at the bottom of the extractive 1985 distillation column is nearly equal to zero. That is to say, the mixture of 1986 water and acetic acid can be effectively separated by extractive distillation 1987 with tributylamine as the separating agent. 中国科技论文在线 http://www.paper.edu.cn
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1988 7.2. NEQ Stage Model
1989 The NEQ stage model for extractive distillation should follow the 1990 philosophy of rate-based models for conventional distillation. Unfortu- 1991 nately, it has been rarely reported on the NEQ stage model for extractive 1992 distillation. The reason may be that extractive distillation is a special dis- 1993 tillation mode, and building an NEQ stage model for a distillation process 1994 is not as straightforward as it is for the EQ stage model. If the chemical 1995 reaction is involved, we need to simply add an equation to take account of 1996 the effect of chemical reaction on a tray or section of packing. As we know, 1997 the NEQ stage model is more complicated than the EQ stage model. In the 1998 NEQ stage model, the design information on the column configuration must 1999 be specified so that mass transfer coefficients, interfacial areas, liquid hold- 2000 ups, etc. can be calculated. Therefore, for any new invented configuration 2001 of the column, many experiments have to be done in advance to obtain the 2002 necessary model parameters. Evidently, it is too tedious, and much time 2003 will be spent on the design of extractive distillation process. Fortunately, as 2004 pointed out by Lee and Dudukovic (Lee and Dudukovic, 1998), a close 2005 agreement between the predictions of the EQ and NEQ stage models can be 2006 found if the tray efficiency or HETP(height equal to a theoretical plate) is 2007 known. So the EQ stage model is widely used in extractive distillation. 2008 The model equations of NEQ stage for extractive distillation are similar to 2009 those for reaction distillation (Baur et al., 2000, 2001; Higler et al., 1999a,b; 2010 Lei et al., 2003; Taylor and Krishna, 2000). Though sophisticated NEQ stage 2011 model is available readily, detailed information on the hydrodynamics and 2012 mass transfer parameters for the various hardware configurations is woefully 2013 lacking in the open reference. Moreover, such information may have vital 2014 consequences for the calculated results of the distillation column. There is a 2015 crying need for research in this area. It is perhaps worth noting here that 2016 modern tools of computational fluid dynamics could be invaluable in devel- 2017 oping better insights into hydrodynamics and mass transfer in the distillation 2018 column. But the computing time may be greatly prolonged. 2019 In the field of reaction distillation(RD), recent NEQ modeling works have 2020 exposed the limitations of EQ stage model for final design and for the de- 2021 velopment of control strategies. NEQ stage model has been used for com- 2022 mercial RD plant design and simulation. Thus, it is conjecturable that NEQ 2023 stage model would be widely applied in extractive distillation, and EQ stage 2024 model only has its place for preliminary design. The research on NEQ stage 2025 model for extractive distillation should be strengthened in the near future. 2026 But both in the EQ stage model and in the NEQ stage model, the 2027 thermodynamic and physical properties are required. Moreover in the NEQ 中国科技论文在线 http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503
194 Lei, Li, and Chen
2028 stage model the mass and energy transfer models are also necessary. Inter- 2029 ested readers can refer to the references (Kooijman and Taylor, 1991; 2030 Krishna and Wesselingh, 1997; Reid et al., 1987; Seader and Henley, 1998; 2031 Tong, 1996).
2032 8. COMPARISON OF EXTRACTIVE DISTILLATION 2033 AND ADSORPTION DISTILLATION
2034 Adsorption distillation as a new separation method seems attractive and 2035 in this work is compared with traditional extractive distillation. The system 2036 of ethanol and water is selected because ethanol is a basic chemical ma- 2037 terial and solvent used in the production of many chemicals and inter- 2038 mediates. Especially in the recent year, ethanol is paid more attention 2039 because it is an excellent alternative fuel and has a virtually limitless po- 2040 tential for growth.
2041 8.1. Definition of Adsorption Distillation
2042 There are two kinds called adsorption distillation. One has presently 2043 been proposed by Abu Al-Rub (Fahmi et al., 1999; Fawzi et al., 2000). It 2044 involves replacement of the inert packing material in packed-bed distilla- 2045 tion column by an active packing material. The active packing materials 2046 used by Abu Al-Rub for separating ethanol and water are 3A˚ or 4A˚ mo- 2047 lecular sieves, which are thought to be able to alter the VLE of ethanol and 2048 water considerably. 2049 On the other hand, we think that some ion-exchange resins can be 2050 assumed to be composed of one anion which has generally large molecular 2051 weight and one cation which is generally a single metal ion. Since salt 2052 effect may plays a role in raising the relative volatility of ethanol to water, 2053 ion-exchange resins can also be selected as the packing material in the 2054 adsorption distillation column. In this work we only discuss this kind of 2055 adsorption distillation. 2056 Another kind of adsorption distillation has been put forward by Cheng 2057 et al (1999a,b). It doesn’t involve replacement of internals of distillation. 2058 Tiny solid particles are used as the adsorption agent, and blended with 2059 liquid phase in the column. The enhancement of gas-liquid mass transport 2060 by the tiny solid particles has been proved, and the mechanism model is 2061 proposed. However, when adding solid particles to liquid phase, the packed 2062 column is prone to be jammed. From this viewpoint, the tray column is 2063 more suitable for this kind of adsorption distillation. Even so, it is difficult 中国科技论文在线 http://www.paper.edu.cn
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2064 to run the distillation column because the solid particles are accumulated 2065 after a long stay. With regard to this content, the interested readers can 2066 refer to the references. 2067 The comparison of adsorption distillation and extractive distillation for 2068 separating ethanol and water is very interesting because adsorption distil- 2069 lation is an attractive ‘‘new’’ technique and extractive distillation is an 2070 ‘‘old’’ technique. The most commonly used separating agents in the ex- 2071 tractive distillation are ethylene glycol, the mixture of ethylene glycol and 2072 one salt, CaCl2 and N, N-dimethylformamide(DMF) (Lei et al., 2002d, 2073 Shealy et al., 1987). 2074 The comparison of azeotropic distillation and extractive distillation has 2075 been made in the 1st chapter. Today, azeotropic distillation is almost 2076 replaced by extractive distillation for separating ethanol and water.
2077 8.2. Process Experiment
2078 The experimental flow sheet of extractive distillation process with two 2079 columns(extractive distillation column and solvent recovery column) has 2080 been established in the laboratory and is shown in Figure 23. The extractive F23 2081 distillation column was composed of three sections, (I)rectifying section of 2082 30mm(diameter) Â 800mm(height), (II)stripping section of 30mm(dia- 2083 meter) Â 400mm(height) and (III)scrubbing section of 30mm(diameter) 2084 Â 150mm(height). The solvent recovery column was composed of two 2085 sections, (I)rectifying section of 30mm(diameter) Â 650mm(height) and
Figure 23. The two column process for extractive distillation. 1. Extractive dis- tillation column; 2. Solvent recovery column. 中国科技论文在线 http://www.paper.edu.cn
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2086 (II)stripping section of 30mm(diameter) Â 300mm(height). The two col- 2087 umns were packed by a type of ring-shape packing with the size of 2088 4mm(width) Â 4mm(height). The theoretical plates are determined by use 2089 of the system of n-heptane and methylcyclohexane at infinite reflux, having 2090 18 theoretical plates(including reboiler and condenser) for extractive 2091 distillation column and 14 theoretical plates(including reboiler and con- 2092 denser) for solvent recovery column. 2093 In the extractive distillation process experiment, the mixture of ethyl- 2094 ene glycol and CaCl2 was used as the separating agent. The reagents, 2095 ethylene glycol and CaCl2, were of analytical purity, and purchased from 2096 the Beijing Chemical Reagents Shop, Beijing, PRC. Composition analyses 2097 of the samples withdrawn from the top of extractive distillation column 2098 were done by a gas chromatography (type Shimadzu GC-14B) equipped 2099 with a thermal conductivity detector. Porapark Q was used as the fixed 2100 agent of the column packing, and hydrogen as the carrier gas. The column 2101 packing was 160°C and the detector 160°C. The data were dealt with by a 2102 SC 1100 workstation. In terms of peak area of the components, the sample 2103 compositions could be deduced. 2104 The experimental results of extractive distillation column are given in 2105 Table 20, where the feed concentration is ethanol of 0.3576 mole fraction T20 2106 and water of 0.6424 mole fraction. It is shown that high-purity ethanol(a- 2107 bove 99.0%wt), up to 0.9835 mole fraction(almost 99.5%wt), is obtained 2108 under low solvent/feed volume ratio 2.0 and low reflux ratio 0.5, which 2109 means that the mixture of ethylene glycol and CaCl2 is effective for the 2110 separation of ethanol and water by extractive distillation. 2111 In order to test the effect of molecular sieves and ion-exchange resins on 2112 the separation of ethanol and water in the adsorption distillation column, a set 2113 of experimental apparatus was established and shown in Figure 24. The col- F24 2114 umn was composed of two sections, (I) 30mm(diameter) Â 600mm(height) 2115 and (II) 30mm(diameter) Â 600mm(height). The first section was firstly 2116 filled with stainless steel ring-shape packing with the size of 3mm width 2117 and 3mm height, then filled with 3 A˚ sphere-shape molecular sieves with
T20.1 Table 20. Experimental results of extractive distillation column.
T20.2 The top composition Solvent/feed T20.3 No. volume ratio Reflux ratio Ttop/K x1(ethanol) x2(water) T20.4 1 0.5 0.5 351.1 0.8506 0.1494 T20.5 2 1.0 0.5 351.1 0.9674 0.0326 T20.6 3 2.0 0.5 351.1 0.9835 0.0165 中国科技论文在线 http://www.paper.edu.cn
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Figure 24. The experimental apparatus used for separating ethanol and water by adsorption distillation.
2118 the diameter of 4–6mm, and finally filled with ion-exchange resins (the À 2119 type is D072 Styrene-DVB, and composed of the anion RSO3 and the 2120 cation Na+.) with the diameter of 1.20–1.30mm. The second section was 2121 always filled with stainless steel ring-shape packing with the size of 3mm 2122 width and 3mm height in both experiments. In the beginning the feed 2123 mixture entered into the bottom tin with 2500ml. Then the batch distillation 2124 column started to run. In the experiment it was assumed that the bottom 2125 and top concentration was keep constant because the operation time was 2126 not too long and concentration change in the bottom tin was negligible. 2127 The organic solvents, ethanol, was of analytical purity. Ethanol and 3 A˚ 2128 molecular sieves were purchased from the Beijing Chemical Reagents 2129 Shop, Beijing, PRC. Ion-exchange resins were purchased from the Chem- 2130 ical Plant of Nankai University. Water was purified with ion-exchange 2131 resins. Composition analyses of the samples withdrawn from the top and 2132 bottom were done by a gas chromatography (type GC4000A) equipped with 2133 a thermal conductivity detector. Porapark Q was used as the fixed agent of 2134 the column packing, and hydrogen as the carrier gas. The column packing 2135 was 160°C and the detector 160°C. The data were dealt with by a BF9202 中国科技论文在线 http://www.paper.edu.cn
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T21.1 Table 21. The experimental results of adsorption distillation.
T21.2 The top of the column The bottom of the column
T21.3 T/K x1 x2 T/K x1 x2 T21.4 No separating agent T21.5 354.15 0.8057 0.1943 355.55 0.5930 0.4070 T21.6 354.15 0.8063 0.1937 355.55 0.5940 0.4060 T21.7 Molecular sieves T21.8 354.55 0.8463 0.1537 356.05 0.5093 0.4907 T21.9 354.55 0.8443 0.1557 356.35 0.5163 0.4837 T21.10 Ion-exchange resins T21.11 354.25 0.8205 0.1795 356.05 0.5093 0.4907 T21.12 354.25 0.8183 0.1817 356.35 0.5163 0.4837
2136 workstation. In terms of peak area of the components, the sample compo- 2137 sitions could be deduced. 2138 Under the infinite reflux condition, the experimental results are given 2139 in Table 21, where x1 and x2 are molar fraction of ethanol and water in T21 2140 the liquid phase, respectively, and T is temperature(K). The fresh mo- 2141 lecular sieves and ion-exchange resins are used and not replaced during 2142 the operation. 2143 It can be seen from the above that the enhancement of molecular sieves 2144 and ion-exchange resins on the separation of ethanol and water is very lim- 2145 ited, and high-purity ethanol(above 99.0%wt) can not be obtained, at most up 2146 to 0.8463 mole fraction(94.8%wt) and 0.8205 mole fraction (93.8%wt) re- 2147 spectively in the case study. Therefore, although adsorption distillation, in 2148 which the solid separating agent is used and the operation process is simple, 2149 may be attractive, it seems to be unfeasible in the actual distillation process.
2150 8.3. VLE Experiment
2151 In order to further compare the separation effect of different separating 2152 agents used in adsorption distillation and extractive distillation, a typical 2153 recycling Vapor–liquid equilibrium cell having the volume 60ml, was 2154 utilized to measure the VLE of aqueous ethanol system. Salt and the sol- 2155 vent ethylene glycol were blended with the solvent/feed volume ratio 1:1 2156 and the concentration of salt was 0.1g(salt)/ml(solvent). In the cell the 2157 mixture with about 40ml was heated to boil at normal pressure. One hour 2158 later, the desired phase equilibrium was achieved. At that time, small 2159 samples in the vapor and liquid phases were removed by transfer pipet with 2160 1ml in order to make the equilibrium not to be destroyed. 中国科技论文在线 http://www.paper.edu.cn
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2161 Firstly we measured the equilibrium data of the ethanol(1)-water(2) 2162 system which corresponded well with the reference data. It is verified that 2163 the experimental apparatus was reliable. Then the measurements were re- 2164 spectively made for the system ethanol(1)-water(2)-ethylene glycol(solvent/ 2165 feed volume ratio is 1:1), and the system ethanol(1)-water(2)-ethylene gly- 2166 col-CaCl2(solvent/feed volume ratio is 1:1 and the concentration of salt 2167 is 0.1g/ml solvent) at 1 atm. The experimental VLE data are plotted in 2168 Figure 25 where the mole fractions are on solvent free basis, along with the F25 2169 results from the references with 4 A˚ molecular sieves as the separating agent. 2170 It can be seen from the above figure that the separation effect of 2171 molecular sieves on ethanol and water is much less than that of ethylene 2172 glycol and the mixture of ethylene glycol and CaCl2. Moreover, the sepa- 2173 ration ability of the mixture of ethylene glycol and CaCl2 is higher than that 2174 of single ethylene glycol, which shows that it is an effective way to im- 2175 prove ethylene glycol by adding a little of salt.
Figure 25. VLE of ethanol and water at 1 atm for different separating agent. ^— ethanol(1)-water(2)-ethylene glycol-CaCl2; .—ethanol(1)-water(2)-ethylene glycol; 6—ethanol(1)-water(2)—300g of 4 A˚ molecular sieves; —ethanol(1)-water(2). 中国科技论文在线 http://www.paper.edu.cn
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2176 On the other hand, it is well-known that the mixture of ethanol and water 2177 is very difficult to be separated when the mole fraction of ethanol in the 2178 liquid phase is over 0.895. However, the VLE change in the presence of 2179 molecular sieves is not apparent especially when the mole fraction of ethanol 2180 in the liquid phase is in the range of 0.895–1.0. An important experimental 2181 operation condition should also be mentioned when measuring VLE of the 2182 ethanol-water-molecular sieves system. That is, ‘‘fresh’’ molecular sieves 2183 were used for each run. It indicates that only dehydrated molecular sieves 2184 can raise the relative volatility of ethanol to water and the used ‘‘old’’ 2185 molecular sieves have little effect on this separation if not dehydrated in 2186 time. However, it is a pity that the actual operation condition in the ad- 2187 sorption distillation column is quite different from that in the VLE experi- 2188 ment. When molecular sieves are selected as active packing materials for 2189 separating ethanol and water in the column, high-purity ethanol is not an- 2190 ticipated because molecular sieves may be quick to be saturated by water. 2191 That is why we obtained the result in the process experiment of adsorption 2192 distillation. To obtain high-purity ethanol as in the extractive distillation 2193 process, the molecular sieves should be constantly regenerated. It is very 2194 tedious to often unload and load the molecular sieves from the column. 2195 Moreover, much energy and equipment cost will be consumed in the process 2196 of regenerating molecular sieves. Thus, we think that molecular sieves are 2197 not good separating agent for adsorption distillation. The same applies for 2198 ion-exchange resins. 2199 N, N-dimethylformamide(DMF) is also reported as the separating agent 2200 for separating ethanol and water (Shealy et al., 1987). In this work DMF is 2201 improved by adding a little of salt to make into a mixture. If some factors 2202 such as solubility, price, erosion, source and so on are considered, the salt 2203 NaSCN is the prospective additive. The separation ability of DMF, the 2204 mixture of DMF and NaSCN, ethylene glycol and ion-exchange resins are 2205 compared when the composition of feed mixture in the VLE cell is ethanol 2206 of 0.7861 mole fraction and water of 0.2139 mole fraction. The result is 2207 given in Table 22, where the solvent/feed volume ratio is 1.0. 2208 It is well-known that relative volatility provides a useful index for se- 2209 lection of suitable separating agent. Relative volatility is defined as follows:
y1x2 a12 ¼ ð39Þ y2x1
22102211 Obviously, a12 larger than unity is desired. Thus, the following conclusions 2212 can be drawn from Table 22: T22
2213 1. Adding a little of salt to DMF can improve the relative volatility of 2214 ethanol to water. 中国科技论文在线 http://www.paper.edu.cn
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Table 22. Relative volatility of different separating agent for separating ethanol T22.1 and water.
T22.2 The separating agent Relative volatility a12 T22.3 No separating agent 1.05 T22.4 DMF 1.31 T22.5 The mixture of DMF and NaSCN(0.10gNaSCD/ml DMF) 1.63 T22.6 The mixture of DMF and NaSCN(0.20gNaSCD/ml DMF) 2.08 T22.7 Ethylene glycol 2.23 T22.8 Ion-exchange resins 1.16
2215 2. The separation ability of ethylene glycol is greater than that of 2216 single DMF, and approximate to the mixture of DMF and 2217 NaSCN(0.20gNaSCD/ml DMF). 2218 3. The separation ability of ion-exchange resins is lest among the 2219 separating agents. It is not advisable that ion-exchange resins are 2220 used for separating agent for the separation of ethanol and water. The 2221 reason may be that the molecular weight of ion-exchange resins is so 2222 high that the amount of function group Na+is very little and can not 2223 apparently influence the relative volatility of ethanol to water.
2224 Therefore, by combining the results of Figure 25 and Table 22, it is 2225 evident that the best separating agent is the mixture of ethylene glycol and 2226 CaCl2, and the corresponding separation process is extractive distillation. 2227 So, the separation technique using active packing materials for sepa- 2228 rating ethanol and water seems very attractive, but can not replace common 2229 extractive distillation process by now because molecular sieves and ion- 2230 exchange resins as solid separating agent are much more difficult to be 2231 regenerated than liquid separating agent, and the separation effect is also 2232 less than that of the separating agents currently used in the extractive 2233 distillation process.
2234 9. CONCLUDING REMARKS
2235 Since the solvent is the core of extractive distillation, more attention 2236 should be paid on the selection of the potential solvent. The procedure of 2237 selecting solvents is composed of two steps: calculation screening and 2238 experimental screening. The main method of calculation screening is 2239 CAMD, by which the amount of experiment work is greatly reduced. 中国科技论文在线 http://www.paper.edu.cn
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2240 However, until now, CAMD has been largely used for the search of liquid 2241 solvents, and the application on the solid salt is very few. The main reason 2242 may be that the quantitative calculation of salt effect is difficult and the 2243 thermodynamic method is incomplete. But the solid salt as one part of the 2244 solvent(e.g. combination of the liquid solvent and solid salt), is becoming 2245 more and more applied in industry. Moreover, it is a pity that by now 2246 CAMD for ionic liquids is completely blank. Ionic liquids with many 2247 unique advantages are a kind of special solvent in extractive distillation. 2248 We believe that with the development of CAMD, more studies would be 2249 concerned with ionic liquids. However, CAMD is limited by the accuracy 2250 of the group contribution methods that are used. The availability of accurate 2251 parameters for these methods is of crucial importance. 2252 There are four methods for experimental screening, among which inert 2253 gas stripping and gas chromatography method is more promising. It is 2254 advisable that the potential solvents are first worked out by CAMD, and 2255 then tested by experiments. 2256 Frankly speaking, the theories of extractive distillation is very scarce, 2257 let alone their accuracy. The Prausnitz and Anderson theory is based on 2258 semi-quantitative explanations, and the quantitative calculation is very 2259 difficult. Even so, this theory is only suitable for the hydrocarbon systems. 2260 Scaled particle theory constructs a bridge between microscale and macro- 2261 scale. This theory can quantitatively calculate the salt effect and thus de- 2262 duce relative volatilities at infinite dilution with salt. However, it is limited 2263 for polar solutes. The reason may be that only van der Waals bonding is 2264 currently considered in the scaled particle theory, and the hydrogen bonding 2265 between polar solutes and polar solvent is very complicated and greater 2266 than van der Waals bonding. But the scaled particle theory can provide 2267 qualitative analysis for polar solutes. Even so, this theory is only suitable 2268 for extractive distillation with the combination of solvent and salt. Anyway, 2269 the scaled particle theory is relatively perfect in extractive distillation. 2270 Two types of modeling approaches, i.e. EQ and NEQ stage models, can 2271 be used for the design of extractive distillation process. The EQ stage model 2272 is mature and can be found in many references on extractive distillation or 2273 other special distillations. NEQ stage model is rarely reported in open 2274 references on extractive distillation, although it is widely used in other 2275 special distillation such as RD. The reason may be that extractive distillation 2276 is a special distillation mode, and building an NEQ model for a distillation 2277 process is not as straightforward as it is for the EQ stage model. However, in 2278 the field of reaction distillation(RD), recent NEQ modeling works have 2279 exposed the limitations of EQ stage model for final design and for the 2280 development of control strategies, which is never mentioned in extractive 2281 distillation. It may be due to the weaker non-linearity of modeling equations 中国科技论文在线 http://www.paper.edu.cn
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2282 in extractive distillation than in RD. Only in reactive extractive distillation is 2283 chemical reaction involved. Even in reactive extractive distillation, the 2284 chemical equilibrium constant is small in order to ensure that the solvent is 2285 easy to be recovered in the solvent recovery column. Therefore, the influence 2286 of chemical reaction on modeling equations is relatively not remarkable. 2287 Since adsorption distillation is an attractive new separation method, the 2288 comparison of it and extractive distillation is an interesting work. However, 2289 by process and VLE experiments, it is found that the separation technique 2290 using active packing materials for separating ethanol and water can’t re- 2291 place common extractive distillation. This shows that extractive distillation 2292 is still a promising and challenging separation method, and more advan- 2293 tageous than both adsorption distillation and azeotropic distillation in the 2294 field of special distillations.
2295 ACKNOWLEDGMENTS
2296 Partial support for our work comes from the help of Professor W. 2297 Arlt(Technical University of Berlin, Institut fuer Verfahrenstechnik, Fach- 2298 gebiet Thermodynamik und Thermische Verfahrenstechnik, Germany). The 2299 authors are grateful to China Petroleum and Chemical Corporation for fi- 2300 nancial support of this work.
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