648
Ind. Eng. Chem. Res. 1988,27, 648-650
Extraction of Ethanol from Aqueous Solution. 2. A Solvent More Volatile Than Ethanol: Dichloromethane F r a n c i s c o Ruiz,* Vicente Gomis, and Rogelio F. Botella Divisidn de Ingenieria Qdmica, Universidad de Alicante, Aptdo 99, Alicante, Spain
Liquid-liquid equilibrium data for the ternary system water-ethanol-dichloromethane have been determined experimentally a t 25 "C and correlated simultaneously together with vapor-liquid equilibrium data by using the UNIQUAC model. A suitable extraction process for separating ethanol and water using dichloromethane as the solvent has been chosen, and the design calculations have been carried out to determine the energetic requirements. The properties that another solvent should offer t o decrease these energetic requirements have been studied. Liquid-liquid extraction is one possible means of acomplishing the recovery of anhydrous ethanol from a fermentation broth. In part 1of this work (Ruiz et al., 1987), the processes for the extraction of the ethanol were divided into two types, depending upon the volatility of the solvent: those using a solvent less volatile than ethanol (part 1)and those using a solvent more volatile than ethanol (part 2). Dichloromethane is the solvent chosen in this part to show the advantages and disadvantages of the second type of process. Ternary liquid-liquid equilibrium (LLE) data for the water (W)-ethanol (E)-dichloromethane (D) system have been measured a t 25 OC and correlated simultaneously together with vapor-liquid equilibrium (VLE) data for the systems water-ethanol and ethanol-dichloromethane. The results obtained permit the calculation of the solvent circulation rates and energetic requirements of a suitable extraction process and enable the properties that another solvent should offer to decrease these requirements to be studied. Experimental Section All chemicals (analytical reagent grade) were supplied by Merck. The contents of volatile impurities were determined by gas chromatographic analysis: all compounds contained negligible amounts of impurities (less than 0.2 wt 5%). The analytical methods and experimental conditions to determine the binodal c w e and the equilibrium data were the same as in the previous work (Ruiz et al., 1987). Basically they were the cloud point method and the chromatographic analysis of each one of the phases in which a synthetic heterogeneous initial mixture splits.
Phase Equilibrium D a t a Table I shows the mutual solubility data (weight percent) for the system water (W)-ethanol (Ebdichloromethane (D)a t 25 "C.The tie lines for this ternary system appear in Table 11. Figure 1 shows the phase diagram (weight percent) for this ternary system. Correlation of LLE-VLE D a t a As in the previous paper, the simultaneous representation of both vapor-liquid and liquid-liquid equilibrium data is necessary for the design of the separation operation of the components of the extract since, for a t least part of the operation, it is possible that there exist two liquid phases in addition to a vapor phase. The UNIQUAC equation (Abrams and Prausnitz, 1975) was used to carry out this representation. The LLE data obtained were correlated together with VLE data located in compendiums such as that of Gmehling and Onken (1977): Data were obtained by Stabnikov et al. (1972) for 0888-5885/88/2627-0648$01.50/0
Table I. Mutual Solubility Data (wt % ) for Water (W)-Ethanol (E)-Dichloromethane (D) at 25 OC xW
XE
xD
XW
xE
61.8 51.1 43.9 37.3 31.7
31.5 35.8 37.1 37.5 36.8
6.7 13.1 19.0 25.2 31.5
25.3 19.8 13.9 7.6 4.1
36.4 34.8 31.9 25.1 18.5
xD
38.3 45.4 54.2 67.3 77.4
Table 11. Tie Line Data (wt %) for Water (W)-Ethanol (E)-Dichloromethane (D) at 25 "C initial aqueous phase organic phase mixture xw Xn x w xw, x L"
x,
0.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 25.0 30.0
0.21 0.25 0.34 0.40 0.47 0.59 0.88 1.31 1.99 4.11 8.35
0.0 0.45 1.03 1.95 2.76 4.15 6.00 8.71 11.8 18.4 26.9
99.8 99.3 98.6 97.6 96.8 95.3 93.1 90.0 86.1 77.5 64.8
98.7 94.7 90.3 86.6 82.7 78.9 75.4 72.2 69.1 64.8 58.2
0.0 3.95 8.31 11.9 15.7 19.2 22.2 25.0 27.5 30.4 33.8
n
1.29 1.33 1.38 1.48 1.62 1.95 2.36 2.84 3.43 4.83 7.96
"Ethanol level (defined as L = XE in the global initial mixture).
Table 111. Constants of the Antoine B / ( C T )" component A water 8.0713 ethanol 8.1122 dichloromethane 7.0808
+
Equation: log P , = A
B
C
1730.63 1592.86 1138.81
233.43 226.18 231.45
-
"P,= vapor pressure (mmHg); T = temperature ("C). Table IV. UNIQUAC Interaction Parameters for the Ternary LLE and VLE Data: Water (W)-Ethanol (EkDichloromethane (D) W E D W
0.0
E D
52.2 753.5
69.8 0.0 487.0
329.6 -135.2 0.0
the system water-ethanol a t 1.013 bar and by Aleksandrova et al. (1971) for the system ethanol-dichloromethane at the same pressure. The parameter estimation procedure and the objective function used in this work are the same as those used in the previous part (Ruiz et al., 1987). The pure-component molecular structure constants for the UNIQUAC equation are those given by Prausnitz et al. (1980). Vapor pressures of the three components were calculated by the Antoine equation. The Antoine constants are listed in Table 111. 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 4, 1988 649 E
A
0
GO.OBAL I N I T I AMLi x r u n r EXlRfME
O
OF
SOLUBILITV
Tlf
I
T
INE
POINT
1 Figure 1. Phase diagram (wt %) for the ternary system water (W)-ethanol (E)-dichloromethane (D) at 25 "C.
Figure 3. Flow sheet of the separation process. P = extraction column. Q = azeotropic distillation column. R = solvent recovery distillation column. Table V. Flow Rate (mol/s) and Composition (mol %) of All the Streams of Figure 3 flow rate, stream mol/s W, mol ?& E, mol ?& D, mol % 1 2 3 4 5 6 7 8 9 10
Figure 2. Comparison between the experimental and calculated LLE and VLE data (mol %) for the system water (W)-ethanol (E)-dichloromethane (D). Points represent the experimental data and lines the calculated ones.
The estimated UNIQUAC binary interaction parameters obtained for this system are presented in Table IV. The experimental and calculated LLE and VLE data (mole percent) are shown in Figure 2. These UNIQUAC parameters allow the calculation of VLE, LLE, and vapor-liquid-liquid equilibrium (VLLE) data for this system following the conventional procedures described in the previous paper.
Separation Process Example Figure 3 shows a conceptual flow sheet of a suitable extraction process to produce anhydrous alcohol when a solvent like dichloromethane, more volatile than ethanol, is used. The process is based on the extraction of ethanol with dichloromethane and separation of the dissolved water in the extract by azeotropic distillation: the boiling point of dichloromethaneis 40 "C, but there is a minimum boiling azeotrope formed by the heterogeneous mixture of water and dichloromethane. The boiling point of this azeotrope is 38 "C and the composition of the vapor phase in equilibrium is 6.7 mol % water and 99.3 mol % dichloromethane. As in the previous paper, stream 1containing 20 mol % ethanol and coming from the beer still of a typical fermentation plant is fed to extraction column P which produces a raffinate (stream 3) containing less than 0.02 mol % ethanol. The output extract (stream 2) goes to azeotropic distillation column Q. A decanter is used to separate the two liquid phases formed by condensing stream 5 and to provide a dichloromethanephase as reflux to the column. The bottom stream from column Q (stream 8) and a portion of the organic phase from the decanter
5 10.04 4 8.78 8.07 0.27 2.60 7.17 21.91 1
80 3.00 99.7 0.50 4.40 97.2 1.20 0.20 0.50
20 10.0 0.02
1.90 2.47 1.90 13.0
87.0 0.28 99.5 93.7 0.33 96.9 86.8 99.5
100
(stream 7) are transferred to distillation column R, where they are separated into an overhead stream containing the solvent (stream 4) and a bottom stream containing only ethanol (stream 10). Process calculations were carried out by using the Torres-Marchal (1981) graphical design procedure for column Q. Because of the small amount of water going into column R, this was calculated as a binary column using the McCabe-Thiele procedure and assuming that water leaves the column with the dichloromethane. Bubble point feeds were assumed. As the molar heats of vaporization of the compounds involved are widely different (about 40 kJ/mol for both ethanol and water and 30 kJ/mol for dichloromethane), pseudo molecular weights were taken so that the latent heats thus calculated were equal for all pseudocomponents. The equilibrium curves were obtained by using the UNIQUAC model and the parameters obtained in the correlation. VLE and VLLE data were used. For the calculation of extraction column P with a operating temperature of 25 "C, the Hunter and Nash graphical method (1932) was used. The flow rates and compositions of all the streams of the process are summarized in Table V. The calculation number of equilibrium stages and the feed plate location are shown in Figure 3 on each column. The greatest energetic requirements of the process are the latent heat for the vaporization of streams 5 and 9 in columns Q and R (about 30(8.07 + 21.9) = 899 kJ/mol of ethanol). As was postulated by Munson and King (1984) and Zacchi et al. (1983), the energetic requirements of the process are very great and represent an important fraction of the energy that would be liberated if the alcohol is burnt (1360 kJ/mol). Therefore, the use of this solvent is economically unattractive. The energetic requirements are even greater than those calculated for 2-ethylhexanol in
650
Ind. Eng. Chem. Res. 1988,27, 650-657
the first part of this work. However, both distillation columns operate at low temperatures; therefore, the boiler energy can be supplied either by waste heat or simply by solar energy. A low boiling solvent, with properties for the ethanol extraction substantially superior to dichloromethane, has not been studied until now. However it may be possible to find a solvent with properties more favorable than dichloromethane. These properties should be those which reduce the energy of streams 5 and 9. (1) A greater distribution coefficient for ethanol is needed to increase the concentration of ethanol in stream 2 and thus to reduce the amounts of solvent to be evaporated. (2) A greater separation factor is needed to reduce the concentration of water in stream 2 and thus reduce the reflux ratio in column Q and the flow rate of stream 5. (3) The minimum-boiling heterogeneous azeotrope formed with water-ethanol-solvent should have a vaporphase composition in equilibrium with a greater water/ solvent ratio. In this way, the reflux ratio in column Q and the flow rate of stream 5 would decrease. (4) In the entire region of compositions, the relative volatilities of solvent to ethanol should be much greater than 1. Relative volatilities close to 1 should be avoided. In this way, the reflux ratio in column R and the flow rate of stream 9 would decrease. (5) The latent heat of the solvent should be less to reduce the energetic requirements of the process which needs to evaporate streams 5 and 9.
(6) If the boiling point of the solvent were lower (even slightly lower than the room temperature), extraction column P would need to be operated at pressures slightly above atmospheric. The cost of the energetic requirements of columns Q and R would however be very economical. Registry No. HOCH2CH3,64-17-5; CH2C12,75-09-2; HzO, 7732-18-5.
Literature Cited Abrams, D. S.; Prausnitz, J. M. AZChE J. 1975, 21, 116. Alekaandrova, M. V.; Boldina, L. A,; Komarova, V. F.; Kistereva, N. V.; Fedorova, T. S. Sb. Nauch. Tr. Vladimir. Politekh. Znst. 1971, 12, 140.
Gmehling, J.; Onken, U. Vapor-Liquid Equilibrium Data Collection; Chemistry Data Series Vol. 1, Parts 1 and 2; DECHEMA: Frankfurt, 1977. Hunter, T. J.; Nash, A. W. J. SOC.Chem. Znd. 1932, 51, 2851'. Munson, C. L.; King, C. J. Ind. Eng. Chem. Process Des. Dev. 1984, 23, 109. Prausnitz, J.; Anderson, T.; Grens, E.; Eckert, C.; Hsieh, R.; O'Connell, J. Computer Calculations for Multicomponent Vapor-Liquid and Liquid-Liquid Equilibria; Prentice Hall: Englewood Cliffs, NJ, 1980. Ruiz, F.; Gomis, V.; Botella, R. F. Znd. Eng. Chem. Res. 1987,26(4), 696. Stabnikov, V. N.; Matyushev, B. Z.; Protsyuk, T. B.; Yushchenko, N. M. Pishch. Prom. 1972, 15, 49. Torres-Marchal, C. Chem. Eng. 1981, 19, 134. Zacchi, G.; Aly, G.; Wennersten, R. ISEC'83, Denver, CO, 1983.
Received for review June 16, 1987 Revised manuscript received November 17, 1987 Accepted December 2, 1987
Continuous, Regenerative, Two-Dimensional Extraction. 1. Experimentation and Computer Simulation Shankar Nataraj,?William L. Wehrum, and Phillip C. Wankat* School of Chemical Engineering, Chemical and Metallurgical Engineering Building, Purdue University, West Lafayette, Indiana 18105
Experimental and computer simulation results for extraction using several variations of regenerated, two-dimensional cascades are presented. Shifts in temperature were used t o regenerate the solvent for the systems diethylamine, toluene, water and citric acid, 50/50toluene-triisoctylamine, water. The modest separations achieved were in qualitative agreement with theory. Much larger separation is predicted when the equilibrium shift is larger. The basic features of continuous, regenerative, two-dimensional extraction can be demonstrated with the simplest such cascade, depicted in Figure 1. The chemical system involved is that where a single solute, say, A, is distributing between two solvents species: the solvent, S, and the diluent, D. The thermodynamic equilibrium relationship in the two-phase system, in its simplst form, is characterized by the complete immiscibility of S and D, and a linear partition coefficient, K , defined as the ratio of the concentration of A in D to that in S. To regenerate the solvent, the equilibrium distribution of A between D and S must be sensitive to some parameter such as temperature, pH, or the concentration of another ionic species. Without loss of generality, this parameter is referred to as temperature. Let the partition ratio be Kh a t the hot temperature Th and K , a t the cold temperature T, and assume, for specificity, that Kh >> K,. 'Current address: Air Products and Chemicals, Allentown, PA 18105. 0888-5885/88/2627-0650$01.50/0
The cascade in Figure 1 is a pair of equilibrium stages (e.g., mixer-settlers). One of these stages is held a t temperature Th,the other at T,. The solvent, S, flows between the two stages with a flow rate S and is recycled. The diluent stream, containing both D and solute A dissolved in it at a concentration yo, is fed to both stages at flow rates D and nD as shown. The diluent streams exiting the two stages constitute the products of the process. The concentrations indicated for the diluent product streams in m and K , Figure 1 are for the extreme case where Kh 0. Even though these concentrations can be calculated from material balances and equilibrium relationships, physical reasoning is invoked here. In the cold stage, K , 0, and the diluent yields all of its solute to solvent S, which stores it. The diluent exits solute-free. The solute-laden solvent circulates to the hotter stage, where Kh a,and S gives up all of ita solute to D. Thus, the diluent exits this stage with its concentration increased n-fold. The temperature, in effect, acts as the signal which tells the solvent where to take up or yield the solute.
-
-
-
0 1988 American Chemical Society
-
