I n d . Eng. Chem. Res. 1989,28, 1112-1113
1112
Narasimhan, C. S.; Deshpande, V. M.; Ramnarayan, K. Selective Hydrogenation of a$-unsaturated Aldehydes to Unsaturated Alchols Over Mixed Ruthenium-Tin Boride catalysts. J . Chem. Soc., Chem. Commun. 1988, 2, 99. Schreifels, J. A,; Maybury, P. C.; Swartz, W. E., Jr. X-Ray Photoelectron Spectroscopy of Nickel Boride Catalysts: Correlation of Surface States with Reaction Products in the Hydrogenation of Acetonitrile. J . Catal. 1980, 65, 195. Snappe, R.; Bourneville, J. P. Alcohol Production by Catalytic Hydrogenation of Organic Acid Esters. Ger. Offen DE 3217429,1982. Traverse, H.; Bourneville, J. P.; Martino, G. New Route to Rhodium Tin Bimetallic Catalysts Selective for the Hydrogenation of Esters into Alcohols. 8th International Conference on Catalysis; Pachema, T., Ed.; Verlag Chemie Weinheim Deerfield Beach: Bond, 1984; Vol. 111, p 89.
* To whom all correspondence should be addressed. Chakravarthula S. Narasimhan* Vinayak M. Deshpande, Krishnan Ramnarayan Alchemie Research Centre Belapur Road Thane 400601, India Received for review September 22, 1988 Accepted April 18, 1989
CORRESPONDENCE Comments on "Solvent Extraction and Recovery of Ethanol from Aqueous Solutions" Sir: Egan et al. (1988) believe they have found a new low-cost method for recovering and concentrating ethanol from aqueous solutions: They say that the method allows them to obtain 95 vol 70 aqueous ethanol using 20-40% less energy than that required by traditional distillation. However, we-want to point out that it is not feasible to obtain ethanol concentrations of 95 vol % (or even greater than 85 vol %) using this method. The difficulty which is not considered by the authors lies in the modification of the volatilities of ethanol and water when a solvent like 2-ethyl-1-hexanol is present in the mixture. In an ethanol-water binary mixture at atmospheric pressure, the volatility of ethanol is much greater than that of water when the concentration of ethanol is less than 85 vol '3%. In the 85-95 vol 70range, the volatility of ethanol is still greater but both volatilities are closer when the concentration of ethanol is increased, and both of them are equal for 97 vol '70: the azeotropic point. However, the presence of a solvent like 2-ethyl-1-hexanol causes a reduction of the relative volatility of ethanol to water (let us consider, for example, that the main characteristic of the extractive distillation is this change of relative volatility when a solvent is added). The result is a water volatility similar to or even greater than the ethanol volatility in many ternary mixtures which contain a high concentration of 2-ethyl-1-hexanol. In this way, the water is stripped from the solvent as easily as ethanol, and it is not possible to obtain concentrated solutions of ethanol (greater than 85 vol % ) since the water/ethanol ratio in the extract which feeds the stripping column is not small enough because the selectivity of the solvent in the extraction is low. This effect can be seen in Table IV of their communication. For example, let us consider the product obtained with the best batch test (no. 4) containing 600 kg/m3 of ethanol, which shows only 77% recovery. Since the volatility of 2-ethyl-1-hexanol is extremely low, the product obtained can be considered to contain basically water and ethanol with a concentration of 76 vol '70. This concentration is much less than the 95 vol % sought by the authors. On the other hand, in a paper previously published in this journal (Ruiz et al., 1987), a complete study of the
Table I. Distribution Coefficients ( K J Calculated by UNIQUAC for Different Liquid-Phase Mole Fractions (xi) and Temperatures ( T ) in Water (W)-Ethanol (E)-2-Ethsl-l-hexanol (EH)Mixtures P, kPa T,"C XW XE XEH Kw K E K E H 100 100 100 100 100
50 75 100 75 75
0.22 0.22 0.22 0.17 0.30
0.28 0.28 0.28 0.09 0.35
0.50 0.50 0.50 0.74 0.35
0.6 1.4 3.6 1.7 1.0
0.4 1.0 2.4 1.0 0.9
0.003 0.003 0.048 0.011 0.014
liquid-liquid equilibrium of the ternary system waterethanol-2-ethyl-1-hexanolwas presented. In this work, a set of UNIQUAC parameters to describe the liquidliquid and vapor-liquid equilibria of the system was calculated. With these parameters, it is possible to estimate the distribution factor Ki= yi/xi (xi and yi represent the mole fractions of component i in the liquid and in the vapor in equilibrium) when the composition of the liquid, the temperature, and the pressure are set. The greater Ki is, the greater the concentration of i is in the vapor, and therefore there is more volatility of that component. For example, in Table I, the K:s obtained using different conditions are shown. The liquid compositions are some of those in extracts with 2-ethyl-1-hexanol that were published previously (Ruiz et al., 1987). For example, in Table I, it can be seen that the K's of 2-ethyl-1-hexanol are very small and the ICs of water and ethanol are similar. By use of the Ki's obtained with the UNIQUAC parameters, the conceptual process flow sheet presented by the authors and particularly the stripping column were analyzed. Several calculations were performed for different compositions of feed (organic phases of the equilibrium data (Ruiz et al., 1987)), feed temperatures (between 25 and 100 "C), flow rates, temperatures of the argon gas (50-100 "C), and pressures in the top and bottom plates (between 0.1 and 10 bar). The problem was solved on the design condition that the ethanol mole fraction in the stream of exhausted solvent be less than 0.2 mol % in order to allow the solvent to be recycled to the extraction column. The Edminster group method approximation (Henley and Seader, 1981) was applied. The K's were calculated by the UNIQUAC equation. For example, the
0888-5885/89/ 2628- 1112$01.50/0 0 1989 American Chemical Society
Ind. Eng. Chem. Res. 1989, 28, 1113 Table 11. Initial Conditions and Calculated Results in One of the Best Tests on the Stripping Column design conditions calcd results no. of equilib stages = 3 feed: flow rate = 1 mol/s exhausted solvent: flow rate = 0.5 mol/s temp = 75 “C temp = 20.8 O C composition: X W = 0.22, XE = 0.28, composition: XEH = 0.5 %W = 0.0001, XE = 0.0002, stripping gas: XEH = 0.998 min flow rate = 5 mol/h ethanol-rich gas: flow rate = 25 mol/h flow rate = 25.5 mol/s temp = 50 “C temp = 75 O C composition: Ar composition: exhausted solvent: ethanol xw = 0.0088, X E = 0.0109, concn less than 0.02 mol Ti XEH = 0.98 pressures in the bottom and composition of top plates: 110 and 100 kPa liq after condensation of gas: 82 vol % ethanol
conditions and results of one of the best tests are shown in Table 11. By use of this method, all the calculated ethanol concentrations were less than 85 vol %. Therefore, the energy cost of this process is not comparable to that of the traditional distillation to 95 vol % (bearing in mind that the
1113
concentration from 85 to 95 vol % is the most important factor contributing to the energy cost of the distillation because of the similarity of the volatilities of water and ethanol in this range of concentrations). For these reasons, the method is not as useful and cheap as the authors claim in the communication, and we believe that it cannot compete with other methods such as traditional distillation. Registry No. E, 64-17-5; EH, 104-76-7.
Literature Cited Egan, B. Z.; Lee, D. D.; McWhirter, D. A. Znd. Eng. Chem. Res. 1988,
27, 1330. Henley, E. J.; Seader, J. D. Equilibrium Stage Separation Operations in Chemical Engineering; Wiley: New York, 1981. Ruiz, F.; Gomis, V.; Botella, R. F. Znd. Eng. Chem. Res. 1987,26,699.
Francisco Ruiz,* Vicente Gomis, Pilar Blasco Divisidn de Ingenieria Quimica Universidad de Alicante Aptdo, 99 Alicante, Spain
Response to Comments on “Solvent Extraction and Recovery of Ethanol from Aqueous Solutions” Sir: We appreciate the information supplied by Ruiz et al. (1989), pertaining to the limitations of 2-ethyl-lhexanol as one of the ethanol extractants discussed in our previous communication (Egan et al., 1988). Of the several solvents that we studied a t the time, 2-ethyl-1-hexanol hexanol appeared to be the best based on experimentally determined distribution coefficients, stripping data, and other criteria discussed in our paper. In light of the information generated by the ternary-phase diagram construction techniques, perhaps another solvent might be more suitable for producing the higher ethanol concentrations that we projected in our suggested process. The search for the ideal solvent for ethanol extraction continues, and the phase diagram construction techniques of Rub et al. should be helpful in screening future solvent choices. Registry No. 2-Ethyl-1-hexanol, 104-76-7; ethanol, 64-17-5.
Literature Cited Egan, B. Z.; Lee, D. D.; McWhirter, D. A. Znd. Eng. Chem. Res. 1988,
27, 1330. Ruiz, F.; Gomis, V.; Blasco, P. Znd. Eng. Chem. Res. 1989, preceding paper in this issue.
B. Zane Egan,* Douglas D. Lee, David A. McWhirter Oak Ridge National Laboratory‘ Chemical Technology Division Oak Ridge, Tennessee 37831
‘Operated by Martin Marietta Energy Systems, Inc., under Contract DE-AC05-840R21400 with the US Department of Energy.
0888-5885/89/2628-1113$01.50/0 0 1989 American Chemical Society
