Ind. Eng. Chem. Res. 1988, 27, 1330-1332
1330
COMMUNICATIONS Solvent Extraction and Recovery of Ethanol from Aqueous Solutionst The distribution coefficients of ethanol and water from aqueous solutions to various organic solvents were measured, and separation factors were calculated. Several solvents had ethanol distribution coefficients greater than 0.1 and separation factors for ethanol/water greater than 10. The ethanol was stripped from selected solvents into a carrier gas and recovered by condensation. A conceptual processing scheme is proposed for separating and recovering ethanol from aqueous solutions by solvent extraction followed by gas stripping.
I. Introduction There is a need for new, energy-efficient separation methods for recovering ethanol from fermentation liquors because of the high energy requirements for traditional distillation methods. Conventional distillation techniques often require 30-60% of the product combustion energy to separate the ethanol from the water. When this amount of energy is added to the other processing energy requirements, 90-130% of the heating value of the ethanol is consumed in its recovery. Several alternatives to distillation have been described or proposed for separating ethanol from aqueous solutions (Hartline, 1979). These alternatives include absorption processes using cellulosic materials, inorganic desiccants, and molecular sieves; membrane separations using materials that are selectively permeable to either ethanol or water; and solvent extraction using organic solvents or critical fluids such as carbon dioxide. The separation of ethanol from specific reagents and well-defined aqueous solutions by solvent extraction has been reported by several investigators. Roddy measured the distribution of ethanol and water to several organic liquids a t different temperatures by a tracer-counting technique (Roddy, 1981; Roddy and Coleman, 1983). Equilibrium distribution coefficients for ethanol, butanol, and acetone in various solvents have been measured (Dadgar and Foutch, 1986), and separation factors for removal of these produds from water have been calculated. Other schemes have utilized fluorocarbon extractants (Levy, 1981,1984,1986) and combined a distillation process with a gasoline-extraction process (Leeper and Wankat, 1982). Tedder et al. (1986) have described a process to recover ethanol from low-grade fermentates by combining solvent extraction and extractive distillation. Mixed solvent systems have also been considered for use as ethanol extractants (Mitchell et al., 1987; Munson and King, 1984). Several factors influencing the selection of an appropriate ethanol extractant have been discussed in terms of Lewis acidity and steric effects (Munson and King, 1984). In this paper, we present experimental data on the distribution coefficients of ethanol and water for several solvents as well as data on the recovery of the extracted ethanol. The ethanol was stripped from the solvent into a carrier gas and then condensed. A conceptual processing scheme is proposed for separating and recovering ethanol from aqueous solutions by solvent extraction followed by gas stripping. 0888-5885/88/2627-1330$01.50/0
11. Experimental Methods and Procedures A. Solvent Extraction. Solvents were of the highest grade commercially available and were used without further purification. The extraction measurements were conducted using a 10% (by volume) solution of ethanol in water with an equal volume (5 mL) of solvent a t room temperature. During the course of this study, room temperature varied from 21 to 24 "C. However, the temperature remained constant during each extraction experiment. The mixtures were contained in glass separatory funnels with Teflon closures and were shaken vigorously for 20 min on a wrist-type shaker. The mixtures were then centrifuged to remove any entrained phases. The solvent phases were analyzed for ethanol and water by gas chromatography (0.003- x 1.80-m Chromosorb 101/Poropak Q, He carrier, thermal conductivity detector). In general, the enzymatic analyses gave slightly higher values for distribution coefficients. Some of the alcohols used as extractants could also react in the enzymatic assay. Also, some solvents could interfere with the gas chromatographic analysis of ethanol, particularly in dilute aqueous solutions. The enzymatic assay was used to verify the gas chromatographic analyses, and the combination of the two analytical methods served to identify any such problems. In addition, the aqueous phases were analyzed for ethanol, using an enzymatic assay (Calbiochem-Behring Stat-Pack) as well as gas chromatography. B. Ethanol Stripping. Selected solvents were sparged with argon for 60 min and then mixed with an equal volume of 50 vol % aqueous ethanol for 30 min to fully load the solvent. The aqueous and organic phases were separated and centrifuged to remove any entrained phases. The concentration of ethanol in the aqueous phase was determined by enzymatic analysis before and after extraction. Volume changes as a result of the ethanol extraction were also measured. About 40 mL of the resulting pregnant organic was then charged to the sparger of the stripping system, which was maintained at a constant temperature. The organic was then sparged with argon, which was chosen for experiments because it is inert. Another gas such as nitrogen or air would be chosen for the actual process, depending on availability, cost, and reactivity. The exit gas was passed through a trap, which was cooled to -70 "C to condense the ethanol from the gas stream. The volumes and compositions of the liquids in the stripper and ethanol trap were then determined. Tests were conducted at stripper temperatures of 50, 65, and 75 "C, using sparge gas flows of 2.5, 5.0, 7 . 5 , and 10 vol of 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 1331 Table I. Distribution Coefficients a n d Separation Factors for Ethanol a n d Water Using Various Solvents" distribution coeff, D," separation for for factor, S!:?: solvent water ethanol 0.007 6 0.40 53 2,4-dimethyl-3-heptanol 0.025 0.73 29 2-ethyl-1-butanol 0.43 23 2-ethyl-1-hexanol 0.0189 0.0118 0.51 43 3-ethyl-3-hep tan01 0.041 1 0.46 11 1-octanol 0.25 19 1-tridecanol 0.0131 0.00385 0.13 34 di-n-butyl phthalate 0.26 28 n-butyl acetate 0.0093 0.21 23 isobutyl acetate 0.0092 0.00178 0.22 124 chloroform 0.23 161 1 M BAMBP in Freon 214b 0.001 43
" [Phase ratio = 1:1]. BAMBP is [4-sec-butyl-2-(a-methylbenzy1)lphenol; Freon 214 is tetrachlorotetrafluoropropane. gas/vol of liquid/min (vvm). 111. Results and Discussion
The candidates for solvents were screened by determining their distribution coefficients for ethanol and water and their separation factors. Solvents which produced emulsions that remained after centrifugation were discarded without further study. The distribution coefficients and separation factors are defined as distribution coefficient = D," = concentration in solvent phase (1) concentration in aqueous phase separation factor = SEkyz = distribution coefficient of ethanol (2) distribution coefficient of water Solvents with experimental distribution coefficients for ethanol greater than 0.15 and a separation factor greater than 10 were considered to be potential candidates for extractants in the proposed process. A distribution coefficient of 0.15 was estimated to be that which required as much energy in the processing scheme as did the distillation methods. A separation factor of 10 was selected as the minimum value acceptable to meet product specifications and to minimize the energy consumption and processing steps necessary to remove water from the ethanol after solvent extraction. Other factors considered included the solubility of the solvent in water, solvent boiling point (vapor pressure), phase separation characteristics, solvent toxicity, and solvent cost. The calculated separation factors for several of the solvents tested, based on gas chromatographic analyses, and the distribution coefficients for ethanol and water are shown in Table I. The solvents listed in Table I are representative of those which had ethanol distribution coefficients greater than 0.1 and separation factors greater than 10. The aliphatic alcohols appeared to be the most promising solvents for extraction process application, based upon several criteria mentioned earlier. Other solvents evaluated included several Freons and chlorinated hydrocarbons. The distribution coefficients compared favorably with previously reported values for the solvents (Roddy, 1981; Dadgar and Foutch, 1986; Munson and King, 1984; Mitchell et al., 1987). Direct comparison of such data can be misleading since each study used different temperatures, different feed ethanol concentrations, different analytical methods, and different solvent purities. Nevertheless, as shown in Table 11, there is reasonable
Table 11. Comparison of Ethanol Distribution Coefficients a n d Ethanol/Water Separation Factors for t h e Solvents Used i n t h e S t r i m i n g Studies ethanol distribution separation extractant coeff factor reference 2-ethyl-l0.43 23 this study hexanol 0.47 21 Dadgar and Foutch, 1986 0.66 24 Munson and King, 1984 this study 2-ethyl-l0.73 29 14.6 Mitchell et al., 1987 butanol 0.94 Munson and King, 1984 0.97 19.6 Roddy, 1981 0.69 30 11 1-octanol 0.46 this study 11 0.53 Dadgar and Foutch, 1986 11 0.64 Munson and King, 1984 12 0.50 Roddy, 1981 Table 111. Recovery of Ethanol by Gas Stripping with Argon solvent 2-ethyl-1- 2-ethyl-lbutanol hexanol 1-octanol 1. initial ethanol concn in 195 177 188 pregnant org, kg/m3 2. ethanol concn in product, 590 710 638 kg/m3 3. ethanol recovery, % 21.2 17.1 14.4 Table IV. Effect of Stripping Gas Flow Rate a n d Temperature on Ethanol Recovery from 2-Ethyl-1-hexanol run number 1 2 3 4 50 50 65 75 1. temp 7.5 10 2. strip gas flow rate, w m 2.5 5 3. initial ethanol concn in stripper, kg/m3 177 182 173 187 4. ethanol concn in product, kg/m3 710 603 660 600 5. ethanol recovery, % 17 27 60 77
agreement in the extraction data for the three solvents, 2-ethyl-1-hexanol,2-ethyl-1-butanol, and 1-octanol, which were selected for gas-stripping studies. These solvents had good phase separation characteristics, ethanol distribution coefficients of >0.4, and boiling points of 149-195 " C and were relatively inexpensive (e.g., $0.84/kg for 2-ethyl-1-hexanol). Initial tests were performed using organics heated to 50 " C . The argon flow rate was 2.5 w m (1.0 X m3/min). The stripping time was 2 h for 2-ethyl-1-hexanol and 1-octanol and 4 h for 2-ethylbutanol. The results are summarized in Table 111. The effects of stripping temperature and gas flow rate on the ethanol recovery were measured using 2-ethyl-lhexanol solvent and the previously described procedure. The results are summarized in Table IV. Tests were conducted at sparger temperatures of 50,65, and 75 "C, using stripping gas flows of 2.5, 5.0, 7.5, and 10.0 vvm. Ethanol recovery was measured after 2 h of gas sparging. The ethanol stripping rate was estimated by measuring the amount of product recovered at 15-20-min intervals. The stripping rates were 0.17,0.27, and 0.48 g of ethanol/h/wm for 50, 65, and 75 "C, respectively. The best batch test showed a 77% recovery of ethanol from the solvent phase in 2 h at 10.0 w m and 75 "C. The starting solvent phase contained 187 kg of ethanol/m3 of solvent mixture. This test also resulted in the carryover of about 2.2% of the solvent into the cold trap. Since the ethanol recovery increased with gas flow rate, the cold trap appeared to be efficient at removing the ethanol. The unrecovered ethanol was therefore considered to be left in the pregnant organic and recoverable by further stripping.
1332 Ind. Eng. Chem. Res., Vol. 27, No. 7, 1988 SOLVENT + ETHANOL
COLUMN FEED
-1
FILTER
ETHANOL-RICH FEED
SOLIDS
FEED RECYCLE AOUEOUS RAFFINATE
to evaluate possible heat and energy recovery steps, solvent recovery from the aqueous stream, and waste treatment. The effect of ethanol concentration in the aqueous feed stream must also be evaluated because it has been pointed out that selectivity of solvents tends to decrease with increasing aqueous ethanol concentrations (Tedder et al., 1986). However, from an energy requirement standpoint, solvent extraction appears to be a reasonable alternative to distillation for recovery of ethanol from aqueous solutions.
Acknowledgment
GAS-LIQUID SEPARATOR CHILLER
J. E. Attrill, of the ORNL Analytical Chemistry Division, gas chromatographic analyses. J. W. Roddy provided helpful suggestions and discussions. We thank J. W. Roddy and S. M. Robinson for technical review, Martha Stewart for editorial assistance, and D. J. Weaver and A. J. Miller for manuscript preparation. This research was sponsored by the US.Department of Energy under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
I . ~_ performed the HEATEXCHANGER
ETHANOL-RICH PRODUCT
q5-p@
~
GAS COMPRESSOR
Figure 1. Conceptual process flow sheet for ethanol extraction and recovery.
A conceptual process flow sheet for recovery of ethanol from aqueous solution by solvent extraction and gas stripping has been developed based on the above data (Figure 1). Clarification of the aqueous ethanol-rich feed solution may be necessary to ensure a feed solution that is compatible with the operation of the solvent extraction column, although columns are available which treat unclarified fermentation liquids. The ethanol will be extracted into the organic solvent (e.g., 2-ethyl-1-hexanol) in the countercurrent extraction column. The raffinate stream, that is, the exhausted aqueous effluent, is discarded or recycled, depending on the ethanol concentration. The extract, containing solvent and ethanol, is heated in a heat exchanger and is then transferred into a stripping column where a heated inert gas contacts the solution by countercurrent flow to strip the ethanol from the solvent. The ethanol-rich gas is cooled to condense the ethanol, which is removed in a gas-liquid separator. The separated solvent is recycled to the extraction column, and the stripping gas is recompressed and recycled to the stripping column. For applications requiring pure ethanol, it would be necessary to remove the small amount of entrained solvent in the product. Heat-exchange optimization between various process streams would make the process more energy efficient. Energy use comparisons were made based on a heating value for ethanol of 23.55 X lo9 J/m3 (84480 BTU/gal) and a plant designed to produce 295 m3/day (78000 gal/day) of 95 % ethanol. Traditional distillation would require 7 X lo9 to 14 X lo9 J/m3 (25000-50000 BTU/gal) for the distillation step. Solvent extraction followed by gas stripping was estimated to require about 2.8 X lo9 J/m3. Removal of entrained solvent, if needed, would require some additional energy. Material and equipment costs for the solvent extraction step were estimated to be comparable to the distillation step, except for the additional compressor costs. Additional studies are required
Registry No. Freon 214, 2268-46-4; BAMBP, 2622-83-5; ethanol, 64-17-5; water, 7732-18-5; 2,4-dimethyl-3-heptanol, 19549-72-5; 2-ethyl-1-butanol, 97-95-0; 2-ethyl-1-hexanol, 104-76-7; 3-ethyl-3-heptano1, 19780-41-7; 1-octanol, 111-87-5; 1-tridecanol, 112-70-9; di-n-butyl phthalate, 84-74-2; n-butyl acetate, 123-86-4; isobutyl acetate, 110-19-0; chloroform, 67-66-3.
Literature Cited Dadgar, A. M.; Foutch, G. L. Biotechnol. Bioeng. Symp. 1986, 15, 611-620. Hartline, F. F. Science (Washington,D.C.) 1979, 206, 41-42. Leeper, S. A.; Wankat, P. C. Ind. Eng. Chem. Process Des. Dev. 1982, 21, 331-334. Levy, S. U.S. Patent 4260836, 1981. Levy, S. U.S. Patent 4 424 275, 1984. Levy, S. U S . Patent 4568643, 1986. Mitchell, E. J.; Arrowsmith, A.; Ashton, N. Biotech. Bioeng. 1987, 30, 348-351. Munson, C. L.; King, C. J. Ind. Eng. Chem. Process Des. Deu. 1984, 23, 109-115. Roddy, J. W. Ind. Eng. Chem. Process Des. Deu. 1981,20, 104-108. Roddy, J. W.; Coleman, C. F. Ind. Eng. Chem. Fundam. 1983,22, 51-54. Tedder, D. W.; Tawfik, W. Y.; Poehlein, S. R. “Applications” In Chemical Separations; King, C. Judson; Navratil, James D., Eds.; Litervan Literature: Denver, CO, 1986; Vol. 11, pp 247-254.
* To whom correspondence should be addressed. ‘Research sponsored by the U S . Department of Energy under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.
B. Z a n e Egan,* Douglas D. Lee, David A. McWhirter Chemical Technology Division Oak Ridge National Laboratory Oak Ridge, Tennessee 37831 Received for review October 7 , 1987 Revised manuscript received March 14, 1988 Accepted March 29, 1988
