Ind. Eng. Chem. Res. 1998, 37, 599-603
599
Salt Effects in Extraction of Ethanol from Aqueous Solution: 2-Ethylhexanol + Sodium Chloride as the Solvent V. Gomis,* F. Ruiz, N. Boluda, and M. D. Saquete Departamento de Ingenierı´a Quı´mica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain
Liquid-liquid equilibrium data for the quaternary system water-ethanol-2-ethylhexanolsodium chloride have been determined experimentally at 25 °C. The data obtained make it possible to carry out the design calculation of a proposed suitable extraction process for separating ethanol and water using 2-ethylhexanol as the solvent when sodium chloride is added. The analysis of the process shows that the advantages that the salt addition offers to the extraction and recovery of the ethanol are presently limited. Introduction The process of recovery by liquid-liquid extraction of ethanol manufactured in a fermentation process could be improved by the effect of strong electrolyte addition. The salt addition could considerably improve the characteristics as extractant of the solvents. In these lines, Malinowski and Daugulis (1994) studied the effect of sodium chloride and potassium chloride on the extraction using different solvents. They studied the effect of the salts on the distribution coefficient for ethanol and the separation factor with respect to water. But all the determinations were done on an aqueous feed containing 6 wt % ethanol. The design calculations obviously cannot be done using only these data because the two parameters depend on the ethanol concentration. On the other hand, in two previous works (Ruiz et al., 1987, 1988) different processes for the extraction of ethanol were studied depending upon the volatility of the solvent. The design calculations of each process were carried out, and the properties that the solvent should offer to decrease the energetic requirements of the extraction process were studied. The solvent chosen in one of the works (Ruiz et al., 1987) was 2-ethylhexanol. This solvent has a large distribution coefficient for ethanol but a small selectivity with respect to water. The addition of a salt such as sodium chloride could improve the distribution coefficient and selectivity. In this paper, liquid-liquid equilibrium (LLE) data for the quaternary system water (W)-ethanol (E)-2ethylhexanol (EH)-sodium chloride (S) at 25 °C are presented. The results obtained allow the analysis of the improvements on the characteristics of 2-ethylhexanol as an extractant of ethanol when sodium chloride is added. Experimental Section All chemicals (Merck) were used as supplied. Equilibrium data were obtained by preparing mixtures of known overall mass composition, stirring vigorously, and settling at a constant temperature of 25 °C. The process of mixing and settling was repeated several * Author to whom correspondence should be addressed. Telephone: (34)-65903400, ext. 3362. Fax: (34)-65903826. E-mail:
[email protected].
times over for at least 48 h to ensure equilibrium was reached. After that, samples were taken from both phases and analyzed. The 2-ethylhexanol and ethanol in both phases were determined by gas chromatography using a 2 m × 3 mm column packed with Porapack Q 80/100. The column temperature was 240 °C, and detection was carried out by flame ionization. To obtain quantitative results for the organic phase, an internal standard method was used, where 1-hexanol was the standard compound used. Since the concentration of 2-ethylhexanol in the aqueous phase to be determined is very low, these samples could not be diluted with water. As indicated by Gomis et al. (1994), this dilution would prevent phase separation effects when an organic internal standard was added. Therefore, it was not possible to use an internal standard method and an external method was used by preparing standards containing the four components of the samples. The concentration of salt in the aqueous phase was determined from the mass of the solid residue obtained by evaporation at 120 °C of a known mass of the sample. As the chloride concentration in the organic phase was low, it was measured by titration with silver nitrate after adding water to an aliquot of the sample. An equivalent amount of sodium was assumed to be present. The amount of water in the organic phase was determined by the Karl Fischer method using a Mettler DL18 Karl Fischer titration and was checked by chromatographic analysis under the same conditions as used previously and by thermal conductivity. Water in the aqueous phase was determined from the material balance in that phase. Three sets of initial heterogeneous mixtures were selected, each one characterized by the value of [XS/(XS + XW)]0 ) 0.1, 0.2, and 0.25. Xi is the weight percentage of component i. The different mixtures were prepared so that XS + XW ) XEH: the ethanol levels (L) in the initial global mixture were increased stepwise for each set until the solid-liquid-liquid region was reached. Duplicates of each heterogeneous mixture were prepared and analyzed. The relative accuracy of the weight fraction measurements ranged between approximately 1% for the majority compounds and 5% for the minority compounds. The concentrations obtained of the four components in both phases with the value of the accuracy and the known overall composition of the
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600 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 1. Tie Line Data (wt %) for Water (W)-Ethanol (E)-2-Ethylhexanol (EH)-Sodium Chloride (S) at 25 °Ca aqueous phase L
XW
XE
XEH
organic phase XS
XW
XE
XEH
XS
0.00 5.48 11.4 16.3 22.5 28.6 34.3 39.0 43.1 49.1
97.3 91.7 84.8 79.1 71.5 63.8 55.7 48.5 41.4 33.2
ND ND ND 0.01 0.03 0.09 0.17 0.38 0.76 0.92
0 5 10 15 20 25 30 35 40 45
90.0 85.3 81.2 77.5 74.0 70.8 67.4 64.7 62.2 58.5
0.00 4.85 9.27 13.3 16.9 20.0 23.2 25.5 27.0 29.0
[XS/(XS + XW)]0 ) 0.1 0.02 10.0 2.66 0.03 9.78 2.84 0.03 9.46 3.77 0.04 9.14 4.60 0.07 9.02 5.92 0.12 9.05 7.59 0.27 9.14 9.75 0.49 9.28 12.1 1.04 9.77 14.8 1.75 10.7 16.8
0 5 10 15 20 25
79.5 76.9 74.4 72.6 70.3 67.8
0.00 3.31 6.30 8.16 10.6 12.1
[XS/(XS + XW)]0 ) 0.2 ND 20.5 1.94 0.01 19.7 2.63 0.01 19.3 3.22 0.02 19.2 4.12 0.03 19.1 5.32 0.01 20.1 6.65
0.00 6.82 13.6 20.1 26.8 33.1
98.1 90.5 83.2 75.8 67.8 60.0
ND 0.01 0.02 0.03 0.08 0.17
0 5
74.7 72.7
0.00 2.44
[XS/(XS + XW)]0 ) 0.25 ND 25.3 1.69 ND 24.9 2.49
0.00 7.77
98.3 89.7
ND 0.05
a
ND, no detectable composition.
heterogeneous mixture were used to check the mass balance and fit the results following a method of data reconciliation (Gomis et al., 1997). The results reported correspond to the values obtained using this method.
Phase-Equilibrium Data Table 1 shows the experimental equilibrium data (weight percent) for the system water (W)-ethanol (E)2-ethylhexanol (EH)-sodium chloride (S) at 25 °C for each one of the sets of tie lines determined, each one characterized by the global initial ratio [XS/(XS + XW)]0. The values of L for the initial mixtures are also included. Figure 1 shows the ternary representations for each one of the sets of initial mixtures selected on a free salt basis. For comparison, the system without salt (Ruiz et al., 1987) has been included. A change in the slope of the tie lines is observed when the salt ratio is increased. In order to study the effect of the salt in the distribution coefficient of ethanol and the separation factor with respect to water, Figures 2 and 3 have been represented. As a consequence of the bivariance of the two-phase state in a quaternary system (degrees of freedom ) 2 for constant pressure and temperature), it is necessary to represent the variations of the distribution coefficients and separation factor against two parameters characteristic of the conjugated phases. The two parameters chosen for the representations are the ratio XS/(XS + XW) and the concentration of ethanol in one of the phases, for example in the organic phase, which we consider to be the most influential. Since the solubility of the salt and water in the organic phase is low, the XS/(XS + XW) ratio in the aqueous phase remains almost the same value as that in the initial global mixture.
Figure 1. Ternary representations on a free salt basis of the several sets of tie lines. W ) water, EH ) 2-ethylhexanol, and E ) ethanol.
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 601
Figure 2. Distribution coefficient of ethanol ((XE)org/(XE)aq) versus ethanol concentration in the organic phase (wt %) for different values of the XS/(XS + XW) ratio.
Figure 3. Separation factor ((XE)org/(XE)aq)/((XW)org/(XW)aq) of ethanol with respect to water versus ethanol concentration in the organic phase (wt %) for different values of the XS/(XS + XW) ratio.
As is observed in these figures, the distribution coefficient increases greatly with the XS/(XS + XW) ratio
in the aqueous phase but only slightly with the ethanol concentration. The separation factor also increases with
602 Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 Table 2. Flow Rate (mol/s) and Temperature (°C) of the Streams of Figure 3 with salt stream
flow rate
W
E
1 2 3 4 5 6 7 8 9 10 11 12
100 125.139 86.111 77 1050 1146.859 28.25 19.999 1126.86 0.14 0.03 6
80 20.681 79.999
20 25.168 0.002
0.001 20.68 0.001
19.998 5.17 19.998
without salt EH 79.26 0.14 77 1050 1126.86 2.40
S 0.03 5.97
1126.86 0.14
flow rate
W
E
100 169.819 80.141 104 1170 1293.859 45.96 19.999 1273.86 0.14
80 33.561 79.999
20 28.388 0.002
EH
0.001 33.56 0.001
19.998 8.39 19.998
temp
107.87 0.14 104 1170 1273.86 4.01 1273.86 0.14
0.03 6
25 93 25 25 100 170 117 78 184 25 170 25
Table 3. Energetic Requirements (MJ/s) of the Equipment of Figure 4 reboiler duty of column Q reboiler duty of column R condenser duty of column R duty of heat exchanger A duty of heat exchanger B duty of heat exchanger C
Figure 4. Flow sheet of the separation process: P, extraction column; Q, dehydration column; R, solvent recovery distillation column; F, filter; M, mixer; A, B, C, and D: heat exchangers.
the XS/(XS + XW) ratio. However, it decreases considerably with the ethanol concentration. In this way the importance of obtaining data with different concentrations of ethanol is demonstrated. Separation Process Example Figure 4 shows a conceptual flow sheet of a suitable extraction process to produce anhydrous alcohol when a solvent such as 2-ethylhexanol is used in combination with the addition of sodium chloride. The flow sheet is quite similar to that with 2-ethylhexanol without salt (Ruiz et al., 1987). Stream 1 containing 20 mol % ethanol and coming from the beer still of a typical fermentation plant is fed to the extraction column P. An extractive distillation column Q dehydrates the extract, producing a bottom stream without water. After removing a small amount of precipitated salt in the distillation column with the filter F, this stream only contains ethanol and 2-ethylhexanol and is transferred to a second distillation column R, where it is separated into an overhead stream containing only ethanol and a bottom stream containing the solvent which is recycled. In order to study the improvement in the process when salt is added, the design calculations with and without salt have been carried out under the same conditions. The chosen conditions were as follows: ∑feed: ethanol, 20 mol/s; water, 80 mol/s. In the process with salt: sodium chloride, 6 mol/s. ∑extraction column: 5 equilibrium stages, raffinate containing less than 0.002 mol/s of ethanol. Temperature, 25 °C. ∑extractive distillation column: 24 equilibrium stages, a bottom stream containing less than 0.001 mol/s of
with salt
without salt
28.26 63.31 -57.63 -3.96 -1.44 -28.35
33.16 74.94 -68.64 -5.36 -2.31 -31.16
water, an overhead stream containing water/ethanol 80/ 20 (the same as the feed). ∑solvent recovery distillation column: 10 equilibrium stages, 4 overhead streams containing less than 0.0001 mol/s of 2-ethylhexanol and a bottom stream containing less than 0.0001 mol/s of ethanol. Process calculations were carried out using CHEMCAD III (1993). The equilibriums were modeled using the UNIQUAC model (Abrams and Prausnitz, 1975). Parameters for the liquid-liquid equilibrium with salt were obtained by correlation of the experimental data of Table 1. Parameters for the liquid-liquid-vapor equilibrium were obtained from the literature (Ruiz et al., 1987). The results of the flow rate, compositions of the streams, and energetic requirements are shown in Tables 2 and 3. As can be observed in the tables, the increases in the distribution coefficient and separation factor when salt is used produce important improvements in the extraction process: the flow rate of 2-ethylhexanol necessary for the extraction (stream 4) and the flow rate of water in the extract (stream 2) decrease notably. However, the economic comparison of both processes shows that the improvement is not very significant: With respect to the cost of the equipment, the most important differences are a consequence of the different sizes of columns Q and R. The diameters of tower Q and R of the process without salt should be 4% and 10%, respectively, greater than those of the process with salt. In any case, the consequent increase of cost (compensated in part by the mixer M and the filter F) would be very slight compared to the total cost of the installation. With respect to the operation cost, as is shown by Black (1980), who analyzed other similar processes to separate ethanol, the costs for supplying steam for the reboilers and cooling water for the condensers and heat exchangers represent most of the operating cost. On the other hand, the use of the salt would increase the cost of the process since the cost of the salt recycling would be impracticable. The calculation of the operation costs of both processes, with values of 0.005 $/MJ for steam used in reboilers, 0.0001 $/MJ for cooling water, and 0.03 $/kg of salt, would result in the cost of the
Ind. Eng. Chem. Res., Vol. 37, No. 2, 1998 603
process with salt being 13% cheaper. However, the salt would complicate the operation of the process, especially of column Q, due to the presence of small amounts of salt. Therefore, we consider that the advantages that the salt addition offers to the extraction and recovery of ethanol are limited at present. Acknowledgment The authors thank DGICYT (Spain) for the financial aid of Project PB93-0946. Literature Cited Abrams, D. S.; Prausnitz, J. M. Statistical thermodynamics of liquid mixtures: a new expression for the excess Gibbs energy of partly and completely miscible systems. AIChE J. 1975, 21, 116. Black, C. Distillation modeling of ethanol recovery and dehydration process for ethanol and gasohol. Chem. Eng. Prog. 1980, 76 (9), 78. CHEMCAD III. Process Flowsheet Simulator, Version 3.2; Chemstations Inc.: Houston, TX, 1993.
Gomis, V.; Ruiz, F.; De Vera, G.; Lo´pez, E.; Saquete, M. D. Liquidliquid-solid equilibria for the ternary systems water-sodium chloride-1-propanol or 2-propanol. Fluid Phase Equilib. 1994, 98, 141. Gomis, V.; Ruiz, F.; Asensi, J. C.; Saquete, M. D. Procedure for checking and fitting experimental liquid-liquid equilibrium data. Fluid Phase Equilib. 1997, 129, 15. Malinowski, J. J.; Daugulis, A. J. Salt effects in extraction of ethanol, 1-butanol and acetone from aqueous solutions. AIChE J. 1994, 40 (9), 1459. Ruiz, F.; Gomis, V.; Botella, R. F. Extraction of ethanol from aqueous solution 1. Solvent less volatile than ethanol: 2-ethylhexanol. Ind. Eng. Chem. Res. 1987, 26, 696. Ruiz, F.; Gomis, V.; Botella, R. F. Extraction of ethanol from aqueous solution 2. A solvent more volatile than ethanol: dichloromethane. Ind. Eng. Chem. Res. 1988, 27, 648.
Received for review May 27, 1997 Revised manuscript received November 13, 1997 Accepted November 14, 1997 IE970372P
