Extractive Distillation of Hydrocarbons with Dimethylformamide

Sep 1, 1997 - and toluene mixture using N,N-dimethylformamide was studied both experimentaly and by simulation, using a commercial process simulator, ...
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Ind. Eng. Chem. Res. 1997, 36, 4934-4939

Extractive Distillation of Hydrocarbons with Dimethylformamide: Experimental and Simulation Data Ce´ sar Ruiz, Jose´ Coca,* Aurelio Vega, and Fernando V. Dı´ez Department of Chemical and Environmental Engineering, University of Oviedo, 33071 Oviedo, Spain

The influence of the extractive solvent/hydrocarbons feed ratio, reflux ratio, and column configuration on the separation of aromatics from a hexane, heptane, cyclohexane, benzene, and toluene mixture using N,N-dimethylformamide was studied both experimentaly and by simulation, using a commercial process simulator, phase equilibria predictions by the models NRTL, Wilson, and UNIQUAC, and model parameters available in the literature. The results show that the efficiency for the separation of aromatic/non-aromatic hydrocarbon mixtures increases when the solvent/hydrocarbon feed ratio and the number of column plates are increased but is unaffected by the reflux ratio within the experimental range. Simulations predict qualitatively the influence of the operation variables on the column performance but are not accurate enough for design purposes. Introduction Benzene is a key chemical in the chemical industry, as raw material for the production of polymer precursors, solvents, dyes, drugs, herbicides, etc. The world production of benzene is more than 20 × 106 tons/yr (Vervalin, 1991). Before World War II, benzene was basically produced from coal, but nowdays, more than 90% is obtained from petroleum, mainly by recovery from catalytic reformates and pyrolisis gasoline and by dehydroalkylation of toluene. These processes yield mixtures of aromatics and paraffinic hydrocarbons, olefins, and naphthenes, the aromatic concentration ranging from 20% to 90%. The separation of benzene and other aromatic compounds from these mixtures is carried out by liquid-liquid extraction, azeotropic distillation, and extractive distillation. Extractive distillation is a suitable separation process, especially for streams with an aromatic content between 65 and 90 wt %. The extractive solvent (one single component or a mixture) modifies the vapor-liquid equilibria of the mixture, and a benzene-rich stream is obtained. The extractive solvent has a lower volatility than the components in the mixture and is added near the top of the distillation column. It shows usually stronger interactions with the aromatic compounds, lowering their volatility, and hence, they are recovered in the bottoms stream. Non-aromatic compounds are recovered as distillate. Aromatic hydrocarbons are separated from the extractive solvent in an additional distillation column, and the extractive solvent is recirculated to the extractive distillation column. The extractive solvent must show, besides high selectivity, high miscibility with the feed to be treated, stability, low volatility, specific heat, reactivity, corrosivity, toxicity, and availability at a reasonable price. Several compounds have been proposed as extractive solvents for the separation of aromatics from hydrocarbon mixtures, such as sulfones (Lee, 1986; Lee and Combs, 1987, 1988), morpholines (Preusser et al., 1970; Lackner and Emmrich, 1971), mixtures of phthalic anhydride, maleic anhydride, and adiponitrile (Berg, 1983), phenol (Black, 1960; Esso R&E, 1964), dialkyl* Telephone: +34.8.5103508. Fax: +34.8.5103434. Email: [email protected]. S0888-5885(97)00203-0 CCC: $14.00

amides (Mikitenko et al., 1972), and pyrrolidones (Mu¨ller et al., 1973), but only a few of them have been used industrially. One of the most widely used solvents is N,N-dimethylformamide (DMF), a solvent used in the process of the Institut Francais du Petrole (IFP). DMF is a highly selective solvent and has a relatively low boiling point but forms minimum boiling point azeotropes with non-aromatic hydrocarbons with 6-8 carbon atoms (i.e., cyclohexane and heptane), which causes solvent losses with the distillate. In order to decrease the solvent losses, the addition of steam to the distillation column above the solvent feed has been recommended. Steam breaks the DMF-hydrocarbons azeotrope (Mikitenko et al., 1972; Mikitenko and Asselineau, 1977) and the DMF entrained by the distillate can be recovered by washing it with water (Lehmann et al., 1978) or ion exchange (Cohen et al., 1975). Although widely used industrially, little information is available in the open literature about the influence of operation conditions on the performance of the extractive distillation processes. Laboratory tests in extractive distillation are timeconsuming and expensive because of the large number of parameters involved. It would be desirable to predict the experimental data with the help of available simulation programs, but unfortunately, their applicability to highly nonideal systems, such as those involved in extractive distillation, is very limited, partially because of the low quality of the interaction parameters available in the literature, generally obtained for binary systems. The aim of this work is to study the influence of the operation variables and column configuration on the performance of the separation of aromatics and aliphatic hydrocarbons by extractive distillation with DMF and to compare the experimental results with those predicted with a commercial simulation program, using phase-equilibrium parameters available in the literature. Experimental Section Distillation Experiments. Extractive distillation experiments were carried out in a 50-mm-internal diameter sieve plate glass column (Normschliff Gera¨tebau, Wertheim, Germany), consisting of up to three sections of five plates each. The column is silver-plated © 1997 American Chemical Society

Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 4935

Figure 1. Scheme of the experimental setup.

and vacuum-jacketed and is equipped with ports at different heights for introduction of the feed, sampling, and temperature measurement. The reflux ratio and level of liquid in the boiler are controlled automatically. The hydrocarbon mixture and extractive solvent are pumped by alternative metering pumps and preheated in temperature-controlled tanks. The column was operated with either two or three of the available sections. The hydrocarbons mixture was introduced between the two upper sections (plate 5 or 10, counting from the boiler up) at 50 °C with a flow rate of 2.1 g/min, and the extractive solvent was fed in the upper part of the column, at 85 °C, and its flow rate adjusted according to the desired solvent/feed ratio. A scheme of the experimental setup is shown in Figure 1. The feed to the column consisted of a synthetic mixture of hydrocarbons containing 11.5 wt % hexane, 3.5 wt % heptane, 11.5 wt % cyclohexane, 79.5 wt % benzene, and 2.0 wt % toluene. All chemicals, including DMF, were Panreac pure grade with a minimum purity of 98% and were used with no previous treatment. The mixture composition is similar to that of a pyrolisis gasoline, stabilized by catalytic hydrogenation and preconcentrated in aromatics by rectification. Experiments were started by filling the boiler with the hydrocarbon mixture and DMF in the same ratios, as if they were fed to the column in the continuous process, the heating was started, and the column was operated at total reflux. Once the plates were filled with liquid, the pumps, reflux regulator, and boiler level controller were connected, and the column was allowed

to reach steady state, indicated by a constant-temperature profile along the column. The samples of the bottoms and distillate products were withdrawn after 2 h of steady-state operation. Feed, distillate, and bottoms products were analyzed by GC (Varian 3400 with automatic injector, Model 8200), with a 50-m × 0.2-mm-i.d. (0.33-µm film) methylsilicone (OV-1) fused silica capillary column, helium as carrier gas, and a FID detector. The hydrocarbons in the feed and distillate were analyzed at a constant temperature of 35 °C, while for the bottoms samples, a programmed temperature analysis was needed with heating ramp of 30 °C/min, starting at 35 °C and ending at 245 °C. Cyclohexene was used as the internal standard, and injections of 0.5-µL samples were performed by the sandwhich technique, using dichloromethane as the eluent and a split ratio of 430:1. Distillation Simulation. Simulation of the extractive distillation process was carried out with a commercial process simulator which can simulate the operation of distillation columns, among other process units. Rigorous distillation calculations were carried out assuming ideal equilibrium stages by solving the mass and enthalpy balances, coupled with the equilibrium relations, using the inside-out algorithm (Boston and Sullivan, 1974), modified by Saeger and Bishnoi, for highly nonideal systems (PRO/II Manual, 1989). The program can calculate vapor-liquid equilibria by estimating the liquid activity coefficients using the Wilson (1964), NRTL (Renon and Prausnitz, 1968; Scott, 1956), and UNIQUAC models (Abrams and Prausnitz, 1975). The vapor pressures are calculated by the Antoine equation, and the enthalpies are calculated by the equation of Peng and Robinson (1976). The simulation program can use the compound properties available in its own library or data input for other compounds. These data, besides the parameters for vapor-liquid equilibrium estimation, are the molecular weight, critical properties, density, normal boiling point, etc. The operation variables which must be specified to carry out the simulation are the number of equilibrium stages, temperature, pressure, composition and inlet plate of the feed, column pressure profile, and energy specifications for the boiler and condenser (in this case, boiler heat duty, and condition of total bubblepoint condenser). The simulation output provides information on the temperature, flow rate and composition of overhead and bottoms streams, and condenser refrigeration duty. In this work, simulations were performed using the Wilson, NRTL, and UNIQUAC models and binary interaction parameters available in the literature (Gmehling et al., 1990). Two different sets of parameters were used: (i) parameters obtained from isobaric equilibrium data at 101 kPa and (ii) parameters recommended by DECHEMA, obtained from different equilibrium data and optimized to fit several consistency tests. The parameters used are listed in Tables 1-3. Results and Discussion The column efficiency was determined by operating the column at total reflux with a mixture of toluene and methylcyclohexane, as recommended by Onken and Artl (1989). Theoretical stages of the column were calculated by the McCabe and Thiele method, using the equilibrium data given by Tyminski and Klepanska (1977). The overall efficiencies obtained were 86% for the column with 10 plates and 81% for the column with 15 plates.

4936 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997 Table 1. Wilson Binary Interaction Parameters (Gmehling et al., 1990) DECHEMA

isobaric

comp 1

comp 2

λ12-λ11

λ21-λ22

λ12-λ11

λ21-λ22

hexane hexane hexane hexane hexane benzene benzene benzene benzene cyclohexane cyclohexane cyclohexane heptane heptane toluene

benzene cyclohexane heptane toluene DMF cyclohexane heptane toluene DMF heptane toluene DMF toluene DMF DMF

38.2946 609.7706 315.3933 83.6461 1153.2528 143.2586 -87.5741 23.9459 424.3076 8.7998 -279.2808 545.1342 5.6428 438.8451 -513.5916

267.4496 -293.3773 -337.4396 165.512 1804.9766 119.695 608.8005 -0.2429 -92.5815 70.0848 623.0145 1919.6611 226.1746 1899.2489 1131.1118

171.3205 381.825 315.6461 83.3933 1153.2528 184.37 207.385 377.976 -209.703 -146.0728 -82.4345 545.1342 64.4582 438.8451 983.7811

220.1709 -195.7709 -337.4396 165.512 1804.9766 99.7937 189.2954 -354.9859 686.8662 291.7352 274.8449 1919.6611 187.3255 1899.2489 27.1682

Table 2. NRTL Parameters (Gmehling et al., 1990) DECHEMA

isobaric

comp 1

comp 2

λ12-λ11

λ21-λ22

R12

λ12-λ11

λ21-λ22

R12

hexane hexane hexane hexane hexane benzene benzene benzene benzene cyclohexane cyclohexane cyclohexane heptane heptane toluene

benzene cyclohexane heptane toluene DMF cyclohexane heptane toluene DMF heptane toluene DMF toluene DMF DMF

-74.0742 -369.3735 -285.9412 18.8142 1291.8625 324.2508 1489.461 -45.1484 -318.9368 407.6494 797.4637 1387.1391 -89.9142 1602.9124 1381.916

380.7098 513.5146 252.9977 224.0904 1338.1616 -61.5585 -913.704 61.3096 651.9001 -316.0632 -429.4265 743.054 324.126 1232.6425 -577.9433

0.3022 0.3035 0.3018 0.3015 0.403 0.3 0.2035 0.3035 0.3007 0.3068 0.3036 0.4227 0.2931 0.5592 0.3128

-106.6131 -369.3735 -285.9412 18.8142 1291.8625 228.1244 597.6609 111.1157 736.7867 524.2103 355.9066 1387.1391 -30.576 1602.9124 -2260.2463

488.4492 513.5146 252.9977 224.0904 1338.1616 47.8677 -213.0923 -121.2437 -251.4046 -412.5457 -156.6677 743.054 268.3335 1232.6425 3666.1775

0.2994 0.3035 0.3018 0.3015 0.403 0.3014 0.3017 0.3033 0.3074 0.3053 0.3065 0.4227 0.2986 0.5592 0.0711

Table 3. UNIQUAC Binary Interaction Parameters (Gmehling et al., 1990) DECHEMA

isobaric

comp 1

comp 2

λ12-λ11

λ21-λ22

λ12-λ11

λ21-λ22

hexane hexane hexane hexane hexane benzene benzene benzene benzene cyclohexane cyclohexane cyclohexane heptane heptane toluene

benzene cyclohexane heptane toluene DMF cyclohexane heptane toluene DMF heptane toluene DMF toluene DMF DMF

274.7865 -317.696 452.111 62.3601 575.0424 -46.4152 428.1659 -1.8972 -282.4537 39.9574 506.0203 695.7144 242.9335 816.8464 677.5064

-164.3716 433.0089 -352.4246 -19.2546 88.5955 140.7182 -284.1689 13.139 400.0562 -30.1693 -327.06 -75.7081 -172.3662 -164.7572 -402.7637

125.3113 -253.788 452.111 62.3601 575.0424 -31.6945 -19.2976 -59.9728 325.7296 252.5871 270.5354 695.7144 69.1066 816.8464 -362.4183

-26.1184 323.8685 -352.4246 -19.2546 88.5955 133.9432 116.1749 62.5854 -182.3086 -209.603 -185.7878 -75.7081 -22.3781 -164.7572 722.7436

A constant efficiency along the column was considered, as the efficiency variations along the column were found to be relatively small ((10%). The effects of boiler duty, reflux and solvent/feed ratio, and column configuration on the process performance were determined both experimentaly and by simulation. The results are shown by plotting the aromatic hydrocarbons content of the bottoms product vs the fraction of the aromatics fed to the column which are recovered in the bottoms stream, both on a solvent-free basis, Figure 2. The optimal column operation corresponds to points near the upper right corner (high purity and recovery of aromatics in the column bottoms). The experimental boiler duty is not easy to determine, because the effective heat transferred to the boiler is not given just from the electric power consumption, as

there are heat losses which depend on variables such as ambient temperature and humidity, and could not be estimated accurately. Qualitatively, increasing the boiler duty corresponds to points toward the left sides in Figures 2-6, i.e., a low heating rate yields a high recovery but low purity of aromatics in the bottoms stream, and the opposite is true for high heating rates. It has been found that, under the experimental conditions used, the aromatics losses with the distillate correspond to benzene, while the non-aromatics withdrawn with the bottoms stream are cyclohexane and heptane. In conventional distillation, the separation efficiency is higher for higher reflux ratios. In extractive distillation, the effect of increasing the reflux ratio is 2-fold: as the reflux ratio increases, so does the difference in

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Figure 2. Percentage of aromatics in the column bottoms (DMFfree basis), as a function of the percentage of aromatics recovered in this stream. Influence of reflux ratio. Experimental and simulation (NRTL-DECHEMA parameters) results. (0) Experimental, reflux ratio 3, solvent/feed ratio 3. (O) Experimental, reflux ratio 6, solvent/feed ratio 3. (9) Simulation (NRTL), reflux ratio 3, solvent/feed ratio 3. (b) Simulation (NRTL), reflux ratio 6, solvent/feed ratio 3.

Figure 3. Percentage of aromatics in the column bottoms (DMFfree basis), as a function of the percentage of aromatics recovered in this stream. Influence of extractive solvent/hydrocarbons feed ratio, column with 10 plates. Experimental and simulation (NRTLDECHEMA parameters) results. (0) Experimental, reflux ratio 3, solvent/feed ratio 3. (O) Experimental, reflux ratio 3, solvent/feed ratio 4. (9) Simulation (NRTL), reflux ratio 3, solvent/feed ratio 3. (b) Simulation (NRTL), reflux ratio 3, solvent/feed ratio 4.

concentration between liquid and vapor along the column, but simultaneously, it causes dilution of the extractive solvent along the column, and hence, its selectivity decreases (Eisenlohr et al., 1975). The effect of the reflux ratio on the 10-plate column performance for a solvent/feed ratio of 3 is shown in Figure 2, where experimental and simulation results are plotted for external reflux ratios of 3 and 6. No substantial difference is observed between the results for both reflux ratios. The simulation results, unless otherwise stated, correspond to the NRTL model using the interaction parameters recommended by DECHEMA. As boiler energy consumption increases for higher reflux ratios, it is more economically efficient to operate the column at the lower reflux ratio. It would be expected that the extractive solvent/ hydrocarbons feed ratio would influence the aromatics separation. As this ratio increases, the concentration of extractive solvent in the column increases and the concentration of the other components decreases, and therefore, their activity coefficients are higher. This

Figure 4. Percentage of aromatics in the column bottoms (DMFfree basis), as a function of the percentage of aromatics recovered in this stream. Influence of extractive solvent/hydrocarbons feed ratio, column with 15 plates. Experimental and simulation (NRTLDECHEMA parameters) results. (0) Experimental, reflux ratio 3, solvent/feed ratio 3. (O) Experimental, reflux ratio 3, solvent/feed ratio 4. (9) Simulation (NRTL), reflux ratio 3, solvent/feed ratio 3. (b) Simulation (NRTL), reflux ratio 3, solvent/feed ratio 4.

Figure 5. Percentage of aromatics in the column bottoms (DMFfree basis), as a function of the percentage of aromatics recovered in this stream. Influence of number of column plates. Experimental and simulation (NRTL-DECHEMA parameters) results. (0) Experimental, column with 10 plates. (O) Experimental, column with 15 plates, configuration A. (4) Experimental, column with 15 plates, configuration B. (9) Simulation (NRTL), column with 10 plates. (b) Simulation (NRTL), column with 15 plates, configuration A. (2) Simulation (NRTL), column with 15 plates, configuration B.

increase is larger for the more volatile compounds, resulting in an increase of the relative volatility of the non-aromatic compounds and a better separation (Buell and Boatright, 1947; Lee, 1986). Furthermore, by increasing the solvent/feed ratio, a larger column diameter, solvent inventory, and energy consumption would be needed. Figures 3 and 4 show the influence of solvent/feed ratio on the column performance, for the operation with 10 and 15 plates, respectively, and a reflux ratio of 3. The experimental and simulation results show the expected trend of improved separation for higher solvent/feed ratios, in both cases. The influence of the number of plates and column configuration is apparent from the data shown in Figure 5, corresponding to operation with a reflux ratio of 3 and a solvent/feed ratio of 3. The column performance improves when the number of plates is increased from 10 to 15, the best results being obtained when the 5 plates are added above the hydrocarbons feed plate

4938 Ind. Eng. Chem. Res., Vol. 36, No. 11, 1997

Figure 6. Percentage of aromatics in the column bottoms (DMF free basis), as a function of the percentage of aromatics recovered in this stream. Comparison of experimental and simulation results. Column with 15 plates, reflux ratio 3, solvent/feed ratio 3. (O) Experimental. ([) Simulation, NRTL, DECHEMA parameters. (]) Simulation, NRTL, isobaric parameters. (2) Simulation, UNIQUAC, DECHEMA parameters. (4) Simulation, UNIQUAC, isobaric parameters. (9) Simulation, Wilson, DECHEMA parameters. (0) Simulation, Wilson, isobaric parameters.

(configuration A). On the contrary, simulation results predict that better column performance is obtained if plates are added below the feed plate. A comparison of the simulation and experimental overall results can be made from the data plotted in Figure 6. It may be observed that, except for UNIQUAC simulations with isobaric parameters, the shape of the simulation curves is similar to that of the experimental results. However, simulations predict a lower purity in the aromatics in the column bottoms than the experimental values in the zone of high purity and not very high recovery and a higher purity in the zone of very high aromatics recovery. The best agreement with experimental results, in the zone of high purity in aromatics, is obtained with the NRTL and Wilson models using the DECHEMA interaction parameters, while the same models with isobaric interaction parameters give better results for the very high aromatics recovery region. The simulation results do not agree with the experimental ones in the whole range of aromatics recovery. The discrepancies between the experimental and simulation results are caused by the unaccuracy of the thermodynamic model with the available interaction parameters to predict multicomponent vapor-liquid equilibria. The comparison of the experimental boiler and condenser temperatures with model predictions, for the multicomponent mixture, shows that the model overpredicts the bottoms temperature (mixture rich in DMF) an average of 4.3 °C, while it underpredicts the condenser temperature (mixture poor in DMF) an average of 2.4 °C, while comparing the experimental results for the system DMF-benzene-heptane presented by Blanco (1995) with the simulation results for this system shows an average absolute error in predicting the boiling point of 4.3 °C and average relative errors in predicting the vapor compositions of 6.6% (benzene), 16.2% (heptane), and 25.6% (DMF). An important cause of these discrepancies might be the low quality of some binary interaction parameters, especially those corresponding to the systems DMF-saturated hydrocarbons, as they were obtained from poor equilibrium data (very inmiscible systems). Column bottoms consist of all the DMF and toluene

Figure 7. Percentage of cyclohexane fed to the column recovered in the column bottoms, as a function of the percentage of aromatics recovered in this stream. Column with 15 plates, reflux ratio 3, solvent/feed ratio 3. Experimental and simulation (DECHEMA parameters) results. (0) Experimental. (9) Simulation, NRTL. (b) Simulation, UNIQUAC. (O) Simulation, Wilson.

Figure 8. Percentage of heptane fed to the column recovered in the column bottoms, as a function of the percentage of aromatics recovered in this stream. Column with 15 plates, reflux ratio 3, solvent/feed ratio 3. Experimental and simulation (DECHEMA parameters) results. (0) Experimental. (9) Simulation, NRTL. (b) Simulation, UNIQUAC. (O) Simulation, Wilson

fed to the column, benzene, and cyclohexane and heptane as non-aromatic impurities. Neither DMF nor toluene was detected in the overhead product, and no hexane was detected in the column bottoms. Simulations predict that over 99.4% DMF and 99.98% toluene and below 0.16% hexane fed to the column are withdrawn with the column bottoms, which agrees well with the experimental results. The experimental (column with 15 plates, reflux ratio 3, solvent/feed ratio 3) and simulation (DECHEMA interaction parameters) results for the cyclohexane withdrawn with the column bottoms, as a function of the aromatics recovered in the column bottoms, are shown in Figure 7. Cyclohexane is entrained by the solvent in higher ratios than hexane because of its higher polarity. The amount of cyclohexane withdrawn with the bottoms product is higher than 1%, only when aromatics recovery is higher than 95%. The non-aromatic compound present in the column bottoms in larger amounts is heptane, because, in spite of its nonpolar nature, its boiling point is almost 20 °C higher than that of benzene. About 5% of the heptane fed to the column is retained in the column bottoms for aromatics recovery of 90%; this amount increases sharply for higher aromatics recoveries, Figure 8. Simulations predict cyclohexane and heptane bottom contents higher than the experimental values, for

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aromatics recoveries lower than 90%, and lower for higher aromatics recoveries. Conclusions The efficiency for the separation of aromatics/nonaromatics hydrocarbon mixtures by extractive distillation with DMF increases when the solvent/hydrocarbon feed ratio and the number of column plates are increased but is unaffected by the reflux ratio in the range studied. Toluene in the column bottoms and hexane in the column overhead are separated completely, while the separation of cyclohexane and mainly heptane is not so efficient, especially for high recoveries of aromatics in the column bottoms. Several conclusions can be drawn for the industrial extractive distillation process. If toluene-free benzene is to be produced, toluene must be separated either before or after the extractive distillation, as it will go with benzene in the extractive distillation column bottoms. Hexane is easily separated, so its content in the feed to the extractive distillation column is not critical. Cyclohexane and mainly heptane are partially withdrawn with the column bottoms as non-aromatic impurities, so increasing their concentrations in the column feed increases the difficulty of the separation. The operation variable more adequate to be manipulated by the column operator in order to control the column performance is the extractive solvent/hydrocarbons feed ratio. Computer simulation using the commercial process simulator and vapor-liquid interaction parameters available in the literature is a useful tool for the study of the influence of feed composition and operation variables on the performance of the extractive distillation column, but the results are not accurate enough for design purposes if aromatics are to be produced according to very strict specifications, i.e., to be used as petrochemical raw material. Acknowledgment This work has been carried out with the financial support of Repsol Petro´leo S. A. and the Spanish Interministerial Commission for Science and Technology under Grant PTR92-0039. Literature Cited Abrams, D. S.; Prausnitz, J. M. Statistical thermodynamics of liquid mixtures: A new expression for the excess Gibbs energy of partly or completely miscible systems. AIChE J. 1975, 21, 116. Berg, L. Separation of benzene and toluene from close boiling nonaromatics by extractive distillation. AIChE J. 1983, 29, 961. Black, C. Extractive Distillation. U.S. Patent 2,961,383, 1960. Blanco, B. Estudio del equilibrio entre fases de hidrocarburos presentes en una corriente de refinerı´a aplicado a su separacio´n mediante destilacio´n extractiva. Ph.D. Dissertation, Universidad de Burgos, Burgos, Spain, 1995. Boston, J. F.; Sullivan, S. L. A new class of solutions methods for multicomponent, multistage separation processes. Can. J. Chem. Eng. 1974, 52, 52.

Buell, C. K.; Boatright, R. G. Furfural extractive distillation for separation and purification of C4 hydrocarbons. Ind. Eng. Chem. 1947, 39, 695. Cohen, G.; Gracco, F.; Mikitenko, P. Process for separating aromatic hydrocarbons by extractive distillation. U.S. Patent 3,919,078, 1975. Eisenlohr, K. H.; Mu¨ller, E.; John, K. P. A method of recovering pure aromatics from mixtures of hydrocarbons containing aromatics by extractive distillation. British Patent 1,398,219, 1975. Esso Research & Engineering. Pure benzene separation from mixtures of saturated and unsaturated benzene-containing hydrocarbons. German Patent 1,161,546, 1964. Gmehling, J.; Onken, U.; Arlt, W. Vapor-liquid equilibrium data collection. Chemistry Data Series; DECHEMA: Frankfurt, 1990; Vol. 1. Lackner, K.; Emmrich, G. Higher octanes, less benzene. Hydrocarbon Process. 1971, 67, 67. Lee, F. M. Use of organic sulfones as the extractive distillation solvent for aromatics recovery. Ind. Eng. Chem., Process Des. Dev. 1986, 25, 949. Lee, F. M.; Combs, D. M. Two-liquid phase extractive distillation for aromatics recovery. Ind. Eng. Chem. Res. 1987, 26, 564. Lee, F. M.; Combs, D. M. Two-liquid phase extractive distillation for upgrading the octane number of catalytically cracked gasoline. Ind. Eng. Chem. Res. 1988, 27, 118. Lehmann, P.; Oehler, R.; Seyfarth, U.; Ulbrecht, H.; Wachowiack, E. Process for the recovery of non-aromatic solvent from a raffinate hydrocarbon component. British Patent 1,522,933, 1978. Mikitenko, P.; Asselineau, L. Proce´de´ de separation d’hydrocarbures aromatiques par distillation extractive. French Patent 2,395,974, 1977. Mikitenko, P.; Cohen, G.; Asselineau, L. Proce´de´ de purification de benze`ne et de tolue`ne par distillation aze`otropique-extractive. French Patent 2,176,488, 1972. Mu¨ller, E.; Eisenlohr, K. H.; Hohfeld, G.; John, P. A process for producing pure benzene by extractive distillation. British Patent 1,302,325, 1973. Onken, U.; Arlt, W. Recommended test mixtures for distillation columns; IChEn: Rugby, 1989. Peng, D. Y.; Robinson, D. B. A two constant equation of state. Ind. Eng. Chem. Fundam. 1976, 15, 59. Preusser, G.; Richter, K.; Schulze, M. Extractive recovery of highly pure aromatics from hydrocarbon mixtures containing high amounts of nonaromatics. German Patent 2,013,298, 1970. PRO/II Manual, Simulation Sciences Inc., May 1989. Renon, H.; Prausnitz, J. M. Local compositions in thermodynamic excess functions for liquid mixtures. AIChE J. 1968, 14, 135. Scott, R. L. Corresponding states treatment of nonelectrolyte solutions. J. Chem. Phys. 1956, 25, 193. Tyminski, B.; Klepanska, A. Vapor-liquid equilibriums in the methylcyclohexane-toluene system. Inz. Chem. 1977, 7, 193. Vervalin, C. H. Benzene demand growth to diminish according to DeWitt analysis. Hydrocarbon Process. 1991, 70, 29. Wilson, G. M. Vapor-liquid equilibrium. XI. A new expression for the excess free energy of mixing. J. Am. Chem. Soc. 1964, 86, 127.

Received for review March 11, 1997 Revised manuscript received July 10, 1997 Accepted July 11, 1997X IE9702035

Abstract published in Advance ACS Abstracts, September 1, 1997. X