I n d . Eng. C h e m . Res. 1990,29, 1383-1395
1383
Multicomponent Three-phase Azeotropic Distillation. 3. Modern Thermodynamic Models and Multiple Solutions Brett P. Cairns and Ian A. Furzer* Department of Chemical Engineering, University of Sydney, Sydney, New South Wales, Australia 2006
A new simulation method using an extended phase-stability and phase-splitting algorithm has been programmed, and new three-phase regions are identified in published azeotropic distillation examples. If phase splitting is ignored, then an erroneous composition profile can be produced. The modern thermodynamic models UNIFAC-VLE, UNIFAC-LLE, modified UNIFAC, UNIQUAC, NRTL, ASOG, and modified ASOG have been investigated in the simulation model. The sensitivity of the selection of thermodynamic model on temperature and composition profiles has been studied for a wide range of systems. The changes in the number of theoretical stages required for the separation is dependent on the choice of thermodynamic model. A worked example of ethanol dehydration with 2,2,4-trimethylpentane shows the importance of reflux ratio, feed plate location, and number of stages on the separation. Multiple solutions for azeotropic distillation columns reported in the literature can be rationalized by the inclusion of a phase-splitting algorithm in the simulation model. A new set of multiple solutions is reported for the system ethanol-water-2,2,4-trimethylpentane. Part 2 of this publication by Cairns and Furzer (1990b) was concerned with reviewing the previous simulation models and worked examples for three-phase distillation and the development using a modified phase-stability analysis of a new simulation method. This new method has been programmed and named NSHET and used to predict the separation that had been previously estimated by those worked examples. The examples simulated are given in part 2, Table I, numbers 3 and 5-10 covering a wide range of system components. Figure 1 shows the composition profile along the column for the system 1propanol (1)-water (2)-l-butanol (3), Figure 2 for the system ethanol (1)-water (2)-l-butanol (3),and Figure 3 for the system propylene (1)-benzene (2)-n-hexane (3)water (4) using UNIFAC-VLE thermodynamics. The full graphical results are given in Figures 1-9, and where phase splitting was detected, the overall liquid compositions have been shown. All solutions were obtained by starting the Newton-Raphson iterations from the overall feed composition on all stages. The initial temperature map was set by linear interpolation between the top plate and reboiler estimates shown in Table I. Antoine coefficients were taken from Sinnott et al. (1983). The number of iterations required to reach the solution for a tolerance of 1.0 X lo4 is also given in Table I, along with the maximum allowable variable corrections. The number of iterations required for all problems was generally less than 10. Boston and Shah (1979) solved the acetone-chloroform-water example of Figure 4 for initial temperature estimates of up to 150 K outside the solution. Table I shows a similar analysis where the bottoms temperature estimate is some 100 K away from the final solution. For a ATMAxequal to 10 K and AXMM,equal to 0.10 mole fraction, NSHET converged in 11 iterations. When the maximum temperature correction was doubled to give ATMAX equal to 20 K, seven iterations were required. The algorithm also converged in only eight iterations for the poor starting point where the reboiler and top plate estimates were reversed, Table I. The examples of Figures 7 and 8 are taken from the work of Pratt (1947), who gives results for hand calculations for both problems. The UNIQUAC parameters have been taken from Gmehling and Onken (1977) and are directly fitted to the experimental data measured by Pratt (1947). The example of Figure 7 shows that NSHET predicts that the three-phase region extends down to the
Table I. Initial Temperature Estimates, Maximum Variable Corrections, and Number of Iterations top plate reboiler no. of fig no. temp, “C temp, O C AT” AXmaX iterations 1 2 3 4 4 4 4 5 6 7 8 9
88.0 88.0 -55.0 46.0 46.0 46.0 160.0 83.0 70.0 70.0 70.0 63.0
93.0 100.0 0.0 50.0 160.0 160.0 46.0 85.0 77.0 82.0 82.0 78.0
10 10 10 10 10 20 20 10 10 10 10 10
0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10 0.10
4 7 7 4 11 7 8 6 5 7 9 10
sixth plate. This compares with Pratt’s original hand calculation which shows two liquid phases on the top two stages. The second case, Figure 8 however, matches Pratt’s hand calculations with two liquid phases on the top two plates. Figure 1 shows the solution of the l-propanol-waterbutanol example proposed by Block and Hegner (1976) using UNIFAC-VLE thermodynamics. The butanol concentrates sufficiently on the lower plates to initiate a liquid-phase split. Figure 2 shows the solution for a similar problem, given by Ross (1979), where the propanol of the Block and Hegner (1976) example is replaced by ethanol. Again the three-phase region is found in the bottom section of the column. The results shown in Figure 3 are for the four-component hydrocarbon and water systems treated by Boston and Shah (1979). Using UNIFAC-VLE, NSHET predicts that the bottom two stages are within the three-phase region as water concentrates toward the reboiler, leaving propylene to be collected overhead. This compares with Boston and Shah (1979) who used the Margules equation to calculate the liquid-phase activity coefficients and found that the bottom four stages had two liquid phases. Figure 5 shows the results for the final problem presented by Boston and Shah (19791, which involves separating l-butanol, water, and butyl acetate in a five-stage column. NSHET, using UNIFAC-VLE thermodynamics, predicts that the top 80% of the column has two liquid phases due to the high concentration of water. Boston and Shah (1979) used NRTL and found that only the top two stages
Osss-5a85/90/2629-1383~02.50/0 0 1990 American Chemical Society
1384 Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 z o 9:
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Figure 8. Acetonitrile (1)-trichloroethylene (2)-water (3) UNIQUAC thermodynamics.
were within the three-phase region. Figure 6 shows the results of the system considered by Buzzi Ferraris and Morbidelli (1982) to illustrate their approximate model. Using NRTL, Buzzi Ferraris and Morbidelli (1982) calculated that only the top 4 stages would experience a liquid-phase split, whereas, UNIFACVLE and NSHET predict that the three-phase region extends to the 15th stage. Finally, Figure 9 shows the results for an ethanolwater-benzene azeotropic column which has approximately 70% of the column in the three-phase region. These
profiles, generated with UNIFAC-VLE, match those calculated by Baden (1984). If the possibility of liquid-phase splitting is ignored, a two-phase solution which always exists for a distillation problem, may in fact be satisfied by a three-phase solution. In many cases, depending on the thermodynamic model, the pseudo-two-phase solution is not very different from the actual three-phase solution. Indeed, many authors suggest using the two-phase solution as the initial conditions for the full three-phase calculations. However, in other cases, the three-phase solution may differ markedly,
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1385 L
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found on a single plate around the feed. The effect of ignoring this phase split is minimal as shown in Figure 10. The example shown in Figure 11 was first proposed by Kinoshita et al. (1983) and later by Vickery and Taylor (1986) for a single liquid phase only. The problem involves removing water from acetone in the presence of furfural. Vickery and Taylor (1986) have noted that the furfural tends to concentrate almost exclusively within the bottom section of the column, and this has been seen to adversely affect convergence. In fact, when phase splitting is considered, the reboiler is within the three-phase region. The effect of ignoring this phase split results in some component profile deviations around the lower four stages (Figure 11). The differences in the predictions become more important as the number of stages within the three-phase region increase. Figure 12 shows the two- and three-phase solution for the acetone-chloroform-water problem in part 2 (Table 11,number 7) in which the entire column is within the three-phase region. Although the two-phase solution generally follows the three-phase profiles, large differences are apparent toward the top of the column. The last example, Figure 13, which has also been investigated by Baden (1984), shows marked differences that can result when liquid-phase splitting is ignored in azeotropic distillation. If the two-phase solution is analyzed, it is found that only the top stage is unstable and should be split to form two liquid phases. This compares with the three-phase solution where the top 17 stages have two liquid phases. Furthermore, as shown in Figure 14, the temperature in the top section of the column for the
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and care should be taken before accepting a two-phase solution. To illustrate these differences, Figures 10-14 show the results obtained by NSHET for various separations when phase splitting was ignored (two phase) and when it was considered (three phase). Figure 10 presents the component profiles for the column proposed by Kistenmacher (1982) to desulfurize methanol in the presence of trace hydrocarbons. The solution shown is for a column without side streams and with a total condenser, whereas Baden (1984) solves the problem with two side streams and a partial condenser. In both cases, two liquid phases are only
1386 Ind. Eng. Chem. Res., Vol. 29, No. 7 , 1990
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Figure 15. 1-Propanol (1)-water (2)-l-butanol(3) 1-propanol profile.
Figure 16. 1-Propanol (1)-water (2)-l-butanol (3) water profile.
two-phase solution is actually infeasible, as it is below the predicted azeotropic minimum temperature. This highlights the importance of performing the full three-phase calculations, rather than relying on the profiles that can be generated by ignoring the liquid-phase split.
Effect of Thermodynamic Model The variety of different activity coefficient models available raises concern over which model will provide accurate equilibrium predictions for use in distillation calculations. To quantify expected errors, the usual procedure is to compare the predictions of a particular model and parameter set to experimentally measured equilibrium data and arrive at a mean deviation. This, however, can be misleading, especially for highly nonideal systems, because it is possible that the overall mean error may appear acceptable and yet some specific regions of the concentration space may be relatively poorly predicted. Furzer (1988) has made a critical examination of the predicted VLLE data in the system ethanol (1)-water (2)-benzene (3) using modified UNIFAC and the published experimental data for this system. Cairns and Furzer (1988) showed that for the methanol-acetone-chloroform system different thermodynamic models could place the same overall feed mixture in different distillation regions. The result was that very different product compositions were predicted for the same feed composition. This sensitivity was due to relatively poor correlation of experimental data by some of the models for dilute methanol concentrations, even though the overall mean deviations were of the order of 0.02 mole fraction. The problem for three-phase distillation is further complicated because of the liquid-phase split. The scarcity of VLLE and experimental three-phase distillation data make comparisons difficult. Table I in part 2 shows that the most often attempted three-phase distillation problem in the literature has been the 1-butanol-water-1-propanol example given by Block and Hegner (1976), which has commonly been solved by using the NRTL model. To examine the predictions of other models, NSHET was used to solve the problem using UNIFAC-VLE (Gmehling et al., 1982),UNIFAC-LLE (Magnussen et al., 1981), ASOG (Kojima and Tochigi, 1979), modified UNIFAC (Larsen et al., 19871, modified ASOG (Tochigi et al., 19811, UNIQUAC (Abrams and Prausnitz, 1975),and NRTL (Renon and Prausnitz, 1968). UNIQUAC and NRTL parameters were taken from Gmehling and Onken (1977) and Antoine coefficients were taken from Sinnott et al. (1983). The results for the system propanol (1)-water (21-1butanol (3) for all models are shown in Figures 15-19. The composition predictions of UNIFAC-VLE and modified UNIFAC were virtually the same, although the temperature profiles show that modified UNIFAC predicted con-
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Figure 19. Comparison of predicted number of stages with three phases.
sistently higher temperatures than UNIFAC-VLE. UNIFAC-LLE, however, provided very poor composition profiles and also predicted much higher temperatures than the other models. This is probably to be expected, as the
Ind. Eng. Chem. Res., Vol. 29, No. 7, 1990 1387 Table 11. Predicted Absolute Mean Vapor Composition and Temperature Errors for I-Propanol-l-Butanol-Water
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mean comp error, mole fraction 0.0054 0.0390 0.0033 0.0156 0.0077 0.0118 0.0126
model UNIFAC-VLE UNIFAC-LLE modified UNIFAC ASOG modified ASOG NRTL UNIQUAC
mean temp error, K 0.342 2.753 0.463 0.287 0.680 0.628 0.499
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