http://www.paper.edu.cn 120026627_SPM_32_02_R1_092503 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]. 121 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 120026627_SPM_32_02_R1_092503 122 Lei, Li, and Chen 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 120026627_SPM_32_02_R1_092503 Extractive Distillation 123 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 120026627_SPM_32_02_R1_092503 124 Lei, Li, and Chen 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 120026627_SPM_32_02_R1_092503 Extractive Distillation 125 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