Application of Membrane Separation Method to Concentrate Products On

Resources Processing 51:148–157 (2004) RESOURCES PROCESSING Original Paper

Application of Membrane Separation Method to Concentrate Products on the Process Producing Environmentally Adaptable Deicer by means of Wet Oxidation of Organic Wastes

Yuichi SUGAI1, Jin FANGMING2, Heiji ENOMOTO2 and Takehiko MORIYA3

1Venture Business Laboratory, Akita University 2Graduate School of Environment Study, Tohoku University 3Tohoku Electric Power Co., Inc.

Abstract The research is proceeding on the process development to manufacture calcium acetate which is an environmentally adaptable deicer by means of wet oxidation of organic wastes. In the current research, separation and concentration of calcium acetate in an aqueous solution obtained by the process were investigated, using reverse osmosis membrane and ion-permeable membrane independently and in combination. Calcium acetate was highly concentrated with each methods. Moreover, separation of acetic acid and formic acid, which was a product produced by a wet oxidation reaction of organic matter as well as acetic acid, was possible by performing multi-stage separation and concentration using a reverse osmosis membrane.

Key words: Acetic acid, Formic acid, Separation, Concentration, Reverse osmosis membrane, Electrodialysis

1. Introduction moisture because the reaction takes place in water and that it has the benefit of being able to prevent Research with regard to effective use of various the secondary generation of toxic substances such organic wastes has been conducted in recent as dioxins. years. Conversions to thermal energy and to use- Wet oxidation is a hydrothermal reaction with ful substances using chemical reactions are meth- addition of oxidants for oxidation of target sub- ods of effective use of waste. Conversion to use- stances in water. This method could make possi- ful substances in particular is desired from the ble the conversion to useful substances through perspective of effective use of resources. chemical reactions as well as conversion to ther- Hydrothermal reaction is expected as a method mal energy. This is particularly effective for a of using organic waste through chemical conver- waste containing a large amount of moisture, such sion to useful substances. This is a method of al- as raw kitchen garbage. lowing organic waste to react with water in water Raw kitchen garbage consists of various sub- at a high temperature and pressure and to convert stances so that separation is difficult, and it in- it into useful substances; for instance, research cludes a large amount of moisture (over 70%). with regard to conversion of plastic into oil or raw Conversion through composting and/or ferment- materials has been reported [1–3]. In addition, a ing action of microorganisms has been primarily hydrothermal reaction is characterized by the fact performed as a method of using this garbage. that preprocessing such as drying is not needed However, the former has limits in the processing even with waste containing a large amount of quantity, while the latter involves processing of waste remained after fermentation, which may cause another problem; neither will lead to wide- Accepted 3 February 2004 spread use as general-purpose techniques.

148 RESOURCES PROCESSING Application of Membrane Separation Method to Concentrate Products of Wet Oxidation of Organic Wastes On the other hand, wet oxidation can be a stuffs, flavoring, as well as raw material for phar- method for effective use of raw kitchen garbage maceuticals), although it does not readily apply to with a highly efficient conversion to thermal ener- separation in instances of bulk handling at indus- gy even if target substances include moisture of trial purity. more than 70%, so use of this method with raw Thus, separation and/or concentration of lower kitchen garbage for power generation is being in- carboxylic acids (acetic acid and formic acid) in vestigated [4]. In addition, results obtained indi- an aqueous solution and their calcium salts (calci- cate that organic matter is decomposed via car- um acetate and calcium formate) were investigat- boxylic acids with a wet oxidation reaction and ed in the current study, using a non-cellulose ace- that acetic acid is the most refractory compound tate reverse osmosis membrane and ion- of the carboxylic acids [4]. Thus, investigation is permeable membrane independently and in com- preceding from the viewpoint of more actively us- bination. Formic acid is also a product produced ing this refractory acetic acid; one way, the con- by a wet oxidation reaction of organic matter and version to calcium acetate, is being investigated its calcium salt is expected to have the same ac- [5]. The spread of future use of calcium acetate as tion as with calcium acetate as a road deicer. an environmentally adaptable deicer is expected, However, the effect of calcium formate on the en- although cost will be a problem for this. Thus, re- vironment is still unclear, so separation of acetic search is proceeding on process development to acid and formic acid (calcium acetate and calcium manufacture calcium acetate at a lower cost using formate) was also investigated in the current garbage as well as agricultural waste, which as- study. Moreover impurities such as dicarboxylic sures a stable supply such as rice straw and rice acids are also included in solutions obtained by hulls, and oyster shells, which are fishery waste, wet oxidation of organic matter, so the possibility as an inexpensive calcium source. The possibility of methods for separating these impurities and of new techniques for use of organic waste has target substances was also investigated. been discovered through this research. As previously described, methods of separation 2. Experimental and/or concentration required when the useful product is water-soluble have been emphasized as 2.1 Testing materials part of the establishment of techniques for con- Aqueous solutions prepared by commercially version of organic waste into useful substances available acetic acid, formic acid, calcium acetate, using hydrothermal reactions. This is the same as and calcium formate of all reagent grade (Wako for a calcium acetate manufacturing process using Pure Chemical Industries, Ltd.) as well as an wet oxidation. Thus, methods for separation and/ aqueous solution (hereafter termed as a calcium or concentration of carboxylic acids and their cal- carboxylate (CC) solution) obtained by reacting cium salts were investigated in the current study oyster shells and an aqueous solution obtained by with a calcium acetate manufacturing process in a wet oxidation reaction [6] of rice hulls (hereaf- mind. ter termed as an oxidized solution) was used as With regard to methods of separation and/or the sample solution. concentration of aqueous organic matter, applica- 2.2 Experimental method with a reverse osmo- tion of methods using membranes was attempted. sis membrane Methods of separation and/or concentration using A batch membrane filtration device (Nitto Den- membranes are often used when desalinating sea- ko Corporation) as shown in Fig. 1 was used for water, although they are also partly used in sepa- the experiments and NTR-759HR and NTR- ration and/or concentration of aqueous organic 70SWC (both with an effective membrane area of matter such as alcohols and sugars. However, sep- 32 cm2) were used for flat membranes. These are aration and/or concentration of lower carboxylic aromatic polyamide membranes; their perfor- acids in particular would be difficult for a cellu- mance and applicable conditions are shown in lose acetate membrane as is generally used for Table 1. NTR-759HR is primarily used for dem- these uses; that research has also been left out. In ineralization of various types of water for indus- addition, a method that selectively extracts organ- trial use and provides excellent saline rejection ic matter in an aqueous solution using supercriti- performance in low-pressure operations. NTR- cal carbon dioxide has also been investigated. 70SWC is primarily used for desalination of sea- However, the extraction cost for this method is water; it has higher salt rejection than the NTR- high, so in the current stage it applies to a relative- 759HR and is assumed to also have the capability ly small amount of a high-purity substance (food- to reject permeation of organic matter such as ag-

Vol. 51, No. 3 (2004) 149 Yuichi SUGAI, Jin FANGMING, Heiji ENOMOTO and Takehiko MORIYA ricultural chemicals and organic halogen com- id, calcium acetate, and calcium formate. Gas pounds. The device is a mechanism whereby the chromatograph with a flame ionization detector sample solution in the container is pressurized (GC-FID, for acetic acid and formic acid) and 1H- with nitrogen gas while stirred with a stirrer and nuclear magnetic resonance (1H-NMR, for calci- water molecules in the sample solution selectively um acetate and calcium formate) were used for permeate the flat membrane and are discharged quantitative analysis and high performance liquid outside the container. Thus, the target substance is chromatography (HPLC) was also used for quali- concentrated in the container. tative analysis. These analysis conditions are First, the sample solution is placed in the con- shown in Table 2. These are the same as the meth- tainer and stirred at a speed (about 500 rpm) such ods used in previously reported research [5–7], so that air bubbles do not appear in the solution. details on the method of analysis are not de- Next, given the recommended pressures for use of scribed here. The experiment was finished when the flat membranes and withstanding pressure of the solute concentration in the permeating solu- the experiment device, the solution is pressurized tion became sufficiently stable and the solute re- with nitrogen gas so as to reach 1.5 MPa for the jection rate (= 1 − (solute concentration in perme- NTR-759HR and 4.0 MPa for the NTR-70SWC ating solution)/(solute concentration in and the experiment is performed. When handling concentrate)) at that point was used for evaluation pure water with these conditions, the solution that of separation performance using these separation permeates the flat membrane (permeating solu- membranes, that is, it was used as an indicator of tion) flows out at a flow rate of 1.5–2.0 ml/min for the applicability of this method. either flat membrane. During the experiment the 2.3 Experimental method using electrodialysis permeating solution was occasionally sampled; A desktop electrodialyser (Asahi Kasei Chemi- the permeating solution, as well as the solution re- cals Corporation, Micro-Acilyzer S-1) was used maining in the container after the experiment was for the experiment; Aciplex AC-220 (fractional finished, was analyzed for acetic acid, formic ac- molecular weight: 300, effective membrane area: 20 cm2) was used for the ion exchange cartridge. The principle of the experiment apparatus is shown in Fig. 2. The ion exchange cartridge used in this experiment alternately positioned two cation-exchange membranes and anion-exchange membranes; the sample solution ( in the figure), the electrode solution () as well as the recovery solution () were present between individual membranes. When voltage was applied to the sample solution, cations and anions in the sample solution tend to move to respectively to the cath- ode and anode, although only cations permeate the cation-exchange membrane and only anions permeate the anion-exchange membrane, so this is the mechanism where target cations and anions are concentrated in the region as indicated by in the figure. Electrodialysis allows selective con- Fig. 1 Schematic illustration of the experimental appara- centration of an ionic substance alone in water, so tus for reverse osmosis. separation of calcium acetate and calcium for-

Table 1 Material, operating limits and performance of the membranes NTR-759HR NTR-70SWC Material Aromatic polyamide Aromatic polyamide pH range for use 2 ~ 10 2 ~ 10 Maximum pressure [MPa] 3.0 6.9 Maximum Temperature [°C] 40 40 Turbidity (Fouling index) ≤ 4 ≤ 4 NaCl Rejection* [%] (Operating pressure [MPa]) 99.5 (1.5) 99.6 (5.6) *25°C, pH: 6.5

150 RESOURCES PROCESSING Application of Membrane Separation Method to Concentrate Products of Wet Oxidation of Organic Wastes

Table 2 Analysis conditions tion were circulated at a constant flow rate; fixed GC (HEWLETT PACKARD HP5890 SERIES II) voltage was applied and the experiment was start- Column : HP-INNOWax ed. During the experiment, the recovery solution Carrier : Helium was occasionally sampled and the experiment was Oven : 50°C (1.0 min) to 200°C (0 min) at ended at the point when electrical conductivity 10°C/min reached 250 µS/cm on the order of drinking water. Injection : Splitless, 1 µl; inlet at 250°C Calcium acetate and calcium formate in the re- Detector : FID 250°C covery solution sampled during the experiment and the recovery solution after the experiment HPLC 1 Column : Shodex Rspak KC-811 ended were quantitatively analyzed with the H- = Eluent : 0.1% H3PO4 NMR. The concentration rate ( (solute concen- Flow rate : 1.0 ml/min tration in the recovery solution)/(initial solute Column temperature: 40°C concentration)) was used as an indicator for eval- Injection : 10 µl uation of the applicability of this method. Detector : Waters 410 (RI) NMR (JEOL JNM-LA300) 3. Results Observed nucleus : 1H Observed frequency: 300 MHz 3.1 Concentration of acetic acid and formic acid The concentration experiment for acetic acid mate, which are ionic substances in a CC solution, and formic acid was performed only with the re- and non-ionic substances and concentration can verse osmosis membrane. Results are summarized be expected. in Table 3. A calcium acetate aqueous solution, a calcium First, in the concentration experiment for acetic formate aqueous solution, as well as a CC solu- acid, 300 ml of an acetic acid aqueous solution tion were used as the sample solutions. A 40 ml of with initial concentration of 4000 ppm was placed sample solution was placed in a sample tube and in the container and the experiment was per- the experiment was performed with 2 ml of recov- formed until 215 ml of permeating solution was ery solution (distilled water) and 20 ml of elec- obtained. trode solution (0.5 N sodium nitrate aqueous solu- Changes in the acetic acid concentration in tion). With the ion exchange cartridge used in this each permeating solution sampled with respect to experiment, half of the ions in the sample solution the cumulative volume of permeating solution are moved to the electrode solution, so the concentra- shown in Fig. 3. The acetic acid concentration in tion rate was up to 10 fold even at a maximum. the permeating solution increased with time First, the sample solution and electrode solu- elapsed as shown in the figure because the acetic acid in the container concentrated with time. For the NTR-759HR, about 2/5 of acetic acid in the solution in the container permeated; for the NTR- 70SWC, about 1/4 tended to permeate. The final solute rejection rate was 0.62 for the NTR- 759HR; in contrast, it was 0.77 for the NTR- 70SWC. That is, the NTR-70SWC has better rejection (concentration) performance for acetic acid. In addition, for the NTR-70SWC, the initial acetic acid concentration was changed to 500 and 10000 ppm, and the same experiment was performed. The results were a solute rejection rate of 0.85 for 500 ppm and of 0.79 for 10000 ppm; the same extent of concentration was obtained up to approximately 10000 ppm regardless of the initial acetic acid concentration. Based on the above results, the NTR-70SWC may be applicable to concentration of acetic acid, Fig. 2 Schematic illustration of the experimental appara- although about 1/5 of the acetic acid permeates tus for electrodialysis. this flat membrane, so a concentration process of

Vol. 51, No. 3 (2004) 151 Yuichi SUGAI, Jin FANGMING, Heiji ENOMOTO and Takehiko MORIYA

Table 3 Experimental results of concentrating formic acid / Acetic acid with reverse osmosis NTR-759HR NTR-70SWC Formic acid Acetic acid Formic acid Acetic acid Initial conditions Concentration [ppm] 4000 4000 4000 500 4000 10000 Volume [ml] 200 300 200 300 Concentrated solution Concentration [ppm] 4148 9656 4388 1386 11440 28334 Volume [ml] 127 75 126 91 74 91 Permeated solution Concentration [ppm] 3531 2117 3349 106 1478 3294 Volume [ml] 73 225 74 209 226 209 Performance of membrane Concentration coefficiency 1.04 2.41 1.10 2.77 2.86 2.83 Rejection coefficiency 0.1 0.62 0.13 0.85 0.77 0.79 Permeation flow [m3/(m2 · hr)] 0.027 0.030 0.035 0.049 0.030 0.030

Fig. 3 concentration of acetic acid in the permeated solution at the experiment with reverse osmosis. Fig. 4 concentration of formic acid in the permeated solution at the experiment with reverse osmosis. several stages may be needed with the reverse os- the molecular weight of formic acid is smaller mosis membrane. This concentration process is than that of acetic acid. Though the final solute re- described in detail in section 4.1. jection rate for the NTR-70SWC was higher, it Next, in the concentration experiment for for- was too low, which suggested that concentration mic acid, 200 ml of a formic acid aqueous solution of formic acid with a reverse osmosis membrane with initial concentration of 4000 ppm was placed was difficult. in the container and the experiment was per- In this manner, a substantial difference in the formed until 74 ml of permeating solution was ob- solute rejection rate for acetic acid and formic tained. acid was seen even when the same flat membrane Changes in the formic acid concentration in the was used. This conversely implies that it may be permeating solutions with time are shown in used in separation of acetic acid and formic acid. Fig. 4. Here again the tendency of the formic acid Together with the aforementioned concentration concentration to increase in the permeating solu- process for acetic acid, the separation process for tion with time is observed. The permeation rejec- acetic acid and formic acid is also described in de- tion performance for individual flat membranes tail in section 4.2. was low with respect to formic acid; 7/8 of formic 3.2 Concentration of calcium acetate and acid permeated with the NTR-759HR and 3/4 per- calcium formate meated with the NTR-70SWC. This is because The concentration experiment for calcium ace-

152 RESOURCES PROCESSING Application of Membrane Separation Method to Concentrate Products of Wet Oxidation of Organic Wastes tate and calcium formate was performed using a Table 4 Experimental results of concentrating calcium reverse osmosis membrane and electrodialysis. formate / calcium acetate with reverse osmosis 3.2.1 Concentration experiment with a reverse Calcium Calcium osmosis membrane formate acetate First, in the concentration experiment for calci- Sample um acetate, 300 ml of an aqueous solution with Concentration [ppm] 10212 9894 initial calcium acetate concentration prepared at Volume [ml] 300 300 9894 ppm was placed in the container and the experiment was performed until 200 ml of permeat- Concentrated solution ing solution was obtained. Concentration [ppm] 30330 29588 Volume [ml] 100 100 Results are summarized in Table 4. Calcium acetate in the permeating solution was not detected; Permeated solution the calcium acetate concentration in the concen- Concentration [ppm] 0 0 trate was 29600 ppm. Thus, the fact that the NTR- Volume [ml] 200 200 70SWC is highly superior in rejection (concentra- Performance of membrane tion) performance of calcium acetate became ap- Concentration coefficiency 2.97 2.99 parent. In addition, the same results were obtained Rejection coefficiency 1.00 1.00 even with calcium formate. Permeation flow [m3/(m2 · hr)] 0.033 0.038 The fact that the molecular weights of these substances are greater than those of acetic acid and formic acid may be a reason why a high con- to a calcium acetate aqueous solution with an ini- centration rate was obtained for both calcium ace- tial concentration prepared at 10600 ppm and the tate and calcium formate. In addition, the fact that experiment was performed. NTR-70SWC is particularly superior in rejection Results are summarized in Table 5. The calci- performance for ionic substances can also be cited um acetate concentration in the recovery solution as a reason. obtained was 50863 ppm; a concentration rate of Thus, a reverse osmosis membrane has ex- 4.8 fold was obtained. In addition, the quantity of tremely high rejection (concentration) perfor- electricity required for the experiment was 311 C. mance with respect to calcium acetate and calci- The reason the concentration rate stopped at about um formate. That is, concentration with a reverse half of the maximum concentration rate is be- osmosis membrane is highly likely to be an effec- cause part of the water molecules for movement tive method, through a process facilitating con- of both acetic acid ions and calcium ions moved centration after reacting a calcium agent in the ox- to the recovery solution and the recovery solution idized solution containing acetic acid and formic increased. Considering that the recovery solution acid, for concentration of both at the same time. after the experiment ended was 2.1 times the 3.2.2 Concentration experiment using amount compared to the initial stage of the exper- electrodialysis iment, concentration equivalent to the maximum First, concentration experiment for calcium ac- concentration rate occurred in this experiment. etate, an average of 4.3 V of voltage was applied In addition, as a result of changing the initial

Table 5 Experimental results of concentrating calcium formate / calcium acetate with electrodialysis Calcium formate Calcium acetate Sample Concentration [ppm] 5000 50000 4489 10600 45000 Impressed voltage [V] 4.3 7.0 10.0 4.3 4.3 7.0 10.0 4.3 4.3 Volume [ml] 40 40 40 40 40 Recovery solution Concentration of acetic acid [ppm] 21600 26700 27700 104100 21500 24100 26900 50863 92300 Volume [ml] 3.3 2.9 2.3 3.8 3.2 2.9 2.2 4.2 3.9 Performance of membrane Concentration coefficiency 4.32 5.34 5.54 2.08 4.80 5.37 5.99 4.80 2.05 permeation flow [m3/(m2 · hr)] 0.031 0.057 0.100 0.311 0.034 0.063 0.109 0.015 0.350 quantity of electricity [C] 164 143 137 1655 148 128 123 311 1510

Vol. 51, No. 3 (2004) 153 Yuichi SUGAI, Jin FANGMING, Heiji ENOMOTO and Takehiko MORIYA concentration and performing the same experiment, an almost constant concentration rate was obtained with a calcium acetate concentration up to 10000 ppm. Moreover, it concentrated to about 10% of about 2.1 fold even with a calcium acetate aqueous solution of a high concentration of 4.5%. With regard to calcium formate as well, a concentration rate of 4.3 fold was obtained for 5000 ppm and a concentration rate of 2.1 fold was obtained for 5.0%. Next, an experiment was performed in which the applied voltage was changed. The experiment was performed with the initial calcium acetate concentration of 4500 ppm and the average applied voltage at 4.3, 7.0, and 10.0 V. Changes in the calcium acetate concentration in the recovery Fig. 5 Effect of impressed voltage at the experiment solution with time are shown in Fig. 5. First, a re- with electrodialysis on concentration of acetic acid in the recovery solution. duction in the experiment time and an increase in the concentration rate was facilitated with a higher applied voltage. The concentration rate in- membrane method and electrodialysis. The initial creased from 4.4 fold to 6.0 fold with increasing calcium acetate concentration in the CC solution the applied voltage. The amount of the increase in used in the experiment was 619 ppm; the calcium the recovery solution was lower with a higher ap- formate concentration was 5500 ppm. Results ob- plied voltage in this experiment; this contributed tained from the experiments using reverse osmo- to an increase in the concentration rate. In addi- sis membrane method and electrodialysis are tion, the increase in the applied voltage leads to a summarized in Table 6. Respective results are de- decrease in the quantity of electricity along with a scribed below. reduction in the running time. The quantity of 3.3.1 Concentration experiment using a electricity required in individual experiments was reverse osmosis membrane 148 C (at the average applied voltage of 4.3 V), 300 ml of CC solution was placed in the con- 128 C (7.0 V), and 123 C (10.0 V). tainer and the experiment was performed until As summaries, electrodialysis is also extremely 150 ml of permeating solution was obtained. effective in concentration of calcium acetate and Results indicated that calcium acetate and cal- calcium formate, and raising the applied voltage cium formate were both not detected in the per- is effective in improving the concentration rate meating solution; concentration rates of 1.9 fold and reducing the quantity of electricity. and 1.8 fold that were nearly equivalent to respec- 3.3 Concentration experiment for the CC tive maximum concentration rates were obtained. solution In addition, throughput for the CC solution was The concentration experiment for the CC solu- about 0.03, which decreased compared to han- tion was also performed using a reverse osmosis dling of calcium acetate or calcium formate alone.

Table 6 Experimental results of concentrating calcium formate / calcium acetate in CMA solution Reverse osmosis Electrodialysis Sample Calcium formate Calcium acetate Calcium formate Calcium acetate Concentration [ppm] 5500 619 5500 619 Volume [ml] 300 40 Concentrated solution / Recovery solution Concentration [ppm] 9735 1156 25960 2863 Volume [ml] 150 3.5 Performance of membrane Concentration coefficiency 1.77 1.87 4.72 4.63 Permeation flow [m3/(m2 · hr)] 0.031 0.031 quantity of electricity [C] — 222

154 RESOURCES PROCESSING Application of Membrane Separation Method to Concentrate Products of Wet Oxidation of Organic Wastes Low molecular weight organic matter in the CC mosis membrane for the CC solution, and the re- solution (such as water-insoluble calcium oxalate) sidual solution in the sample tube after electrodi- may cause clogging when permeating the flat alysis of the CC solution, and organic matters in membrane. these solutions were examined. Chromatograms Thus, throughput decreased somewhat, al- for each solution are shown in Fig. 6. As shown in though solute rejection performance is similar to Fig. 6(a), numerous peaks were detected for sub- that in the experiment using respfective substanc- stances thought to be several types of dicarboxyl- es independently as previously described. Impuri- ic acids and unidentified organic matter besides ties (organic matter such as dicarboxylic acids) acetic acid and formic acid in the CC solution. In are also present in a CC solution together with contrast, most peaks detected with analysis of the calcium acetate and calcium formate, although the CC solution were not noted in the permeating so- influence of impurities on the concentration of lution after the experiment using a reverse osmo- calcium acetate and calcium formate is slight. sis membrane, which indicated that impurities However, as described later in 4.1, impurities in a were concentrated as well. However, a marked CC solution are also concentrated with this meth- decrease on the order of that with the permeating od. Thus, it is effective in concentration of calci- solution mentioned above was not noted for the um acetate and calcium formate aqueous solution residual solution in the sample tube after elec- with impurities separated with other methods. trodialysis and organic matter detected at the 3.3.2 Concentration experiment using elec- same extent as the CC solution was also found to trodialysis be present. An average of 4.3 V of voltage was applied and These results may imply that electrodialysis is the experiment was performed. Results indicated applicable with respect to separation of calcium that a concentration rate of 4.6 fold was obtained acetate and calcium formate, and impurities. for calcium acetate and 4.7 fold was obtained for Moreover, more impurities may be separated with calcium formate. In addition, the relationship between concentration of calcium salt of organic acids and the electric power consumed was also the same as in the experiment using respective substances independently. The above results may imply that data obtained in the experiment using calcium acetate and calcium formate independently can be used in design of a concentration process for a mixture of calcium acetate and calcium formate using electrodialysis.

4. Investigation of the concentration process

Based on what has previously been described, the high likelihood of being able to use membrane separation in separation and/or concentration of acetic acid or calcium acetate and formic acid or calcium formate became apparent. In this section, based on results previously described, three substances in the CC solution (calcium acetate, calcium formate, and other substances) were differen- tiated and respective separation and/or concentration processes were investigated. 4.1 Process to separate the concentrate of calcium acetate and calcium formate from other substances As mentioned previously, there are impurities in a CC solution besides calcium acetate and cal- Fig. 6 HPLC chromatogram of original CC solution (a), cium formate. Thus, HPLC analysis was per- permeated solution after reverse osmosis experi- formed regarding the CC solution, the permeating ment (b), and sample solution after electrodialysis solution after the experiment using a reverse os- (c).

Vol. 51, No. 3 (2004) 155 Yuichi SUGAI, Jin FANGMING, Heiji ENOMOTO and Takehiko MORIYA electrodialysis through use of an ion-exchange stage concentration is again concentrated (third- membrane of appropriate fractional molecular stage concentration); concentrate obtained () weight. That is, more selective concentration of and the permeating solution obtained in second- calcium acetate and calcium formate is possible stage concentration () are mixed and concen- through use of an ion-exchange membrane with a trated (fourth-stage concentration). The concen- fractional molecular weight of about 100. trate obtained through this () undergo mixture 4.2 Selective separation and concentration reflux with the stock solution () and the same process for calcium acetate concentration process was repeated. Acetic acid As mentioned in 3.1, acetic acid must be selec- in the recovery solution obtained by the above tively separated in stages of the oxidized solution separation and concentration process () was to selectively separate and concentrate calcium concentrated about 7 times with respect to the acetate. For this purpose, the NTR-70SWC is ef- stock solution. In addition, the acetic acid concen- fective because it has a substantial difference in tration was concentrated 4 times with respect to the rejection rate for acetic acid and formic acid, the formic acid concentration, suggesting the pos- although a separation and concentration process sibility of selective separation and concentration in several stages is needed. of acetic acid. The recovery rate for acetic acid in Thus, as mentioned in 3.1 as well, a separation this process (acetic acid in and in the figure) and concentration process for acetic acid as is 95%; concentration with better efficiency can shown in Fig. 7 was investigated. This process is be facilitated. The mixed solution of the stock so- explained below in accordance with figures. lution and the reflux solution in the figure First, 300 ml of stock solution respectively in- would have an acetic acid concentration of 4213 cluding acetic acid and formic acid at 4000 ppm ppm; formic acid concentration would be 4247 was used for feed solution. The amount of perme- ppm. Acetic acid and formic acid are almost the ated solution is 2/3 of the amount of aqueous so- same concentration, so selective separation and lution to concentrate. The acetic acid rejection concentration of acetic acid of almost the same rate is 0.80 and the formic acid rejection rate is extent is facilitated even if this process is repeat- 0.15. The concentrate obtained in first-stage con- ed. centration ( in the figure) is again concentrated In this process, impurities are also concentrated (second-stage concentration) and the concentrate in the recovery solution, so electrodialysis is suit- obtained () serves as the recovery solution. In ed to removal of impurities. That is, impurities are addition, the permeating solution obtained in first- removed by adding calcium agents to the recov-

Fig. 7 Flow sheet of the process for concentrating acetic acid.

156 RESOURCES PROCESSING Application of Membrane Separation Method to Concentrate Products of Wet Oxidation of Organic Wastes ery solution obtained and performing electrodial- duced by wet oxidation, additional organic matter ysis as a calcium acetate solution and pure calci- (impurities) is present after the reaction. Howev- um acetate solution at a higher concentration can er, the influence of impurities on the concentra- be obtained. tion of calcium acetate and calcium formate is Here, a separation and concentration process slight. was investigated with a small batch experimental (5) Electrodialysis is suited to separation of cal- device, although a simpler separation and concen- cium acetate and calcium formate in a CC solu- tration process with good efficiency can be ex- tion and impurities such as dicarboxylic acids. pected for a continuous concentration device (6) Separation of acetic acid and formic acid is equipped with a hollow fiber reverse osmosis possible by performing multi-stage separation and membrane module as used at the practical level. concentration using a reverse osmosis membrane.

5. Conclusion Acknowledgment

In the current research, the applicability of The authors would like to express thanks to membrane separation for separation and/or con- Asahi Kasei Chemicals Corporation (Astom Cor- centration of acetic acid, formic acid, calcium ac- poration at present), which kindly rented us a etate and calcium formate in an oxidized solution bench-top electrodialyzer. The authors are grate- and CC solution was investigated and the possi- ful to Mr. Zhou Zhouyu, graduate student for bility of separation of impurities and separation of Ph.D., for his support in analyses. calcium acetate and calcium formate were also discussed. The results are summarized as follows. References (1) With concentration using acetic acid and formic acid aqueous solutions and a reverse osmosis 1. T. Moriya, H. Enomoto: Shigen-to-Sozai, membrane, acetic acid had a solute rejection rate 115, 4, pp. 245–251 (1999) of about 0.80; in contrast, formic acid remained at 2. T. Moriya, H. Enomoto: Shigen-to-Sozai, a solute rejection rate of about 0.13. Concentra- 115, 8, pp. 591–596 (1999) tion of formic acid is difficult with a reverse os- 3. T. Moriya, H. Enomoto: Kagaku-Kogaku- mosis membrane. Ronbun-Syu, 25, 6, pp. 940–946 (1999) (2) Regarding calcium acetate and calcium for- 4. J. Fangming, A. Kishita, H. Enomoto: mate, there was an extremely high concentration Haikibutsu-Gakkai-Ronbun-Shi, 10, 5, pp. with either reverse osmosis membrane or elec- 257–266 (1999) trodialysis. That is, a process that facilitates con- 5. J. Fangming, A. Kishita, T. Moriya, H. centration after reacting a calcium agent in the ox- Enomoto, N. Sato: Shigen-to-Sozai, 116, 4, idized solution containing acetic acid and formic pp. (2000) acid is effective. 6. J. Fangming, A. Kishita, T. Moriya, H. (3) With electrodialysis, an increase in the con- Enomoto, N. Sato: Shigen-to-Sozai, 116, 4, centration rate and throughput speed and a reduc- pp. (2000) tion in the required quantity of electricity are fa- 7. J. Fangming, Y. Yoshiyama, T. Moriya, H. cilitated by raising the applied voltage. Enomoto, N. Sato: Shigen-to-Sozai, 117, 8, (4) When acetic acid and formic acid are pro- pp. (2001)

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