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Ind. Eng. Chem. Res. 2004, 43, 1274-1278
SEPARATIONS Simulation of Salt-Containing Extractive Distillation for the System of Ethanol/Water/Ethanediol/KAc. 1. Calculation of the Vapor-Liquid Equilibrium for the Salt-Containing System Jiquan Fu† Center of Chemical Engineering, Beijing Key Laboratory, Beijing Institute of Clothing Technology, Beijing 100029, P.R. China
Quaternary vapor-liquid equilibrium (VLE) data were measured for the system of ethanol/ water/ethanediol/potassium acetate (KAc). Binary salt-containing VLE data were correlated with the salt-containing local composition model (SCLCM). For a multicomponent system, ternary and quaternary VLE data predicted by SCLCM are in agreement with the literature data and data in this work. The influence of KAc, ethanediol, and the mixture of KAc and ethanediol on the system was investigated by the SCLCM method, and the results showed that the additives have different influences on the volatility of ethanol and water. The calculation method of the salt-containing VLE is simple and effective for the system. Introduction Extractive distillation is an important process for separating the mixture of compounds with azeotropic behavior, but it has the disadvantage of using large amounts of solvent. A large solvent ratio (usually >5) causes not only great energy consumption but also a heavier liquid load in the column, leading to higher operating cost, shorter dwell time, lower plate efficiency, and a higher actual plate number. To extend the application field of extractive distillation, it is essential to improve the solvent efficiency and reduce the solvent consumption. Salt-containing distillation is a distillation process using salt as the separating agent. For mixtures with a binary azeotrope, a certain amount of salt can eliminate the azeotrope entirely. Usually the relative volatility can increase several times by adding even small amounts of salt into the mixtures. Disadvantages of the process are related to the restoration of salt, the conveyance of salt, and so on; thus, the disadvantages limit the application of salt-containing distillation. Duan et al.1 have developed a new process called saltcontaining extractive distillation by combining the advantages of extraction and those of salt-containing distillation. Using mixtures of solvent with salt as the extractive agents, the process overcomes the disadvantages of conventional extractive distillation because salt has a pronounced effect on the relative volatility of solvent species and, at the same time, maintains the advantages of the conventional process, such as easy cycle and conveyance of the extractive agent. On the basis of the results of their studies on the system of ethanol/ water/ethanediol/KAc, an industrial plant (2300T/Y) was established and top-quality products were pro† Tel.: 0086-010-64288287. E-mail:
[email protected].
Fax:
0086-010-64288287.
duced. In comparison with the conventional ethanol extractive distillation process, the solvent ratio is 4-5 times smaller and the tower is 3-4 times shorter in the salt-containing extractive distillation process. The satisfactory effects of this process were demonstrated by running the plant for more than 2 years. After 2 years, Lei et al.2 reported another study on the saltcontaining extractive distillation process for the system of tert-butanol/water/ethanediol/KAc in a similar device (3500T/Y). Fu3 reported a simulation method of salt-containing extractive distillation, and the method has been used successfully in computer simulation for industrial columns of salt-containing distillation of the above system of tert-butanol/water/ethanediol/KAc. Because Duan et al.1 did not publish the theory stage numbers and plate effect in the columns and other research information about this system cannot be found in the literature, it is difficult to simulate the process of the salt-containing extractive distillation. Despite the difficulties described previously, the study on the simulation method of saltcontaining extractive distillation is still worth doing because it is a very typical example of investigating the similar systems. In this paper, the height equivalent to a theoretical plate of the packing tower was calculated using the information provided by the Chemical Engineering Handbook, which enabled the determination of the stage numbers. The simulation of the column then was able to be carried out. The purpose of this work is to investigate whether the simulation method3 is suitable for the system of ethanol/water/ethanediol/KAc. Experimental Section Although the salt-containing multicomponent vaporliquid equilibrium (VLE) data for the system of ethanol/ water/ethanediol/KAc is important to examine the predicted results, no such data have been published. Therefore, the VLE data were measured in this paper.
10.1021/ie0302600 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/29/2004
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Chemicals. The reagents used were ethanol (>99.5 wt %), ethanediol (>99.8 wt %), and KAc (>99.0 wt %) supplied by Beijing Chemical Reagent Corp. (Beijing, China). Distilled water was used, and KAc was dried at 120 °C for 10 h. Experimental Equipment and Measurement. The improved Dvorak-Boublik still was used because of its good reproducibility. The temperature was measured with a mercury-in-glass thermometer (1/10 division). A device consisting of a 220C type pressure gauge and a PDRC-1C/2C type display supplied by MKS Corp. (Burlington, MA), whose accuracy is 0.013 kPa, was used to measure the equilibrium total pressure. The equilibrium data were determined at 101.210 kPa ((0.040 kPa). The experimental technique and procedure were the same as those described in earlier papers.4-7 To keep the concentrations of ethanediol (x3 ) 0.320) and the salt (x4 ) 0.022) constant, it is necessary to analyze the liquid sample and to prepare the new liquidphase samples to replace the previous samples in order to approach the above selected concentrations of x3 ) 0.320 and x4 ) 0.022 in the measurement process. When x3 ) 0.320 ( 0.003 and x4 ) 0.022 ( 0.003, the composition-adjusting process was stopped. Analysis Method. Vapor-phase compositions were measured by chromatography, and the compositions of the solvent and salt in the liquid phase were measured by the method of combining titration and gas chromatographic analyses (the CTGT method).8 The procedure is as follows: (1) The solvent component Xi (salt free) in the liquid phase can be obtained by gas chromatographic analysis, and it can be translated into mass percent (Wi). (2) Analyze the salt equivalent concentration (NS) in the liquid phase with titration. (3) Calculate the salt mass percent WS in the liquid equation WS ) NSMS/(NSMS + 1000Fmix solvent), where Fmix solvent ) ∑WiFi. (4) Calculate the salt mass (SW) based on 1 mol of mixed solvent by solving the equation WS ) SW/(∑XiMi + SW), with nS ) SW/MS. (5) Determine the mole fraction xi of the solvent component with xi ) Xi/(1 + nS) and the mole fraction xS of the salt with xS ) nS/(1 + nS), where nS ) SW/MS. VLE data for the quaternary system in whole mole fraction range of ethanol/water at x3 ) 0.320 and x4 ) 0.022 were determined and presented in Figure 2. Calculation of the Salt-Containing VLE To simulate of salt-containing extractive distillation, models of VLE are needed; they are reported widely. Sander et al.9 gave a good review of the earlier models. Using the salt-containing local composition model (SCLCM), Fu10 correlated 18 binary systems (using Wilson, NRTL, and UNIQUAC) and predicted the results of 5 ternary systems, which agreed well with the results predicted by a pseudobinary approach. Fu8,11 studied the ternary and quaternary systems methanol/ ethanol/water and NH4NO3/NH4Cl/NH4Br. Total absolute deviations were ∆T h ) 0.18 °C and ∆yj ) 0.015. The model was successfully used in the simulation of saltcontaining extractive distillation.3 It is also used in this work. The advantage of SCLCM is that salt-containing systems are dealt with by means of the local composition model of a conventional miscible system, in which the
Figure 1. Predicted vs experimental values for the system of ethanol (1)/water (2)/KAc (4).
Figure 2. Predicted vs experimental values for the system of ethanol (1)/water (2)/ethanediol (3)/KAc (4).
salt is assumed as a “solvent”. The calculation method used in the conventional miscible system is well-known. For example, to calculate the quaternary system of ethanol(1)/water(2)/ethanediol(3)/KAc(4), six binary data are needed: data of saturated vapor pressure containing salt, ethanol/KAc, water/KAc, and ethanediol/KAc and data of binary VLE in conventional miscible systems ethanol/water, water/ethanediol, and ethanol/ethanediol. It is well-known that binary data are only required to predict multicomponent data by the local composition model, so the quaternary VLE can be predicted when salt is considered as a “solvent”. The correlation of the binary data and estimation of parameters using SCLCM are as follows. When the vapor phase is assumed to have ideal gas behavior, the VLE relation is
yiP ) P0i γixi
(1)
For salt-containing binary data, ysolvent ) 1 and ysalt ) 0, so
PSi ) P0i γixi γi was calculated by the NRTL model.
(2)
1276 Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 Table 1. Correlation Results for the Binary System of Ethanol (1)/Water (2)/Ethanediol (3)/KAc (4) (NRTL Model, J/mol) ∆gi,j/Ri,j component
1
2
3
4
1 2 3 4
0.0/0.0 1488.48/0.27 -203.77/0.3704 -747.88/1.0
-216.18/0.27 0.0/0.0 916.02/0.2916 -1225.98/1.0
1644.05/0.3704 -712.40/0.2916 0.0/0.0 -1094.65/0.20
1733.07/1.0 828.88/1.0 13146.80/0.20 0.0/0.0
The definition of the salt concentration in SCLCM is the mole fraction defined by the mole number of salt and solvent:
∑ni
xsolvent,salt ) ni/
(3)
ni ) mole number of solvent and salt. The definition of the mole fraction of salt and solvent was chosen in order to calculate the enthalpy of salt conveniently in the simulation of the columns. The binary data were correlated by a maximumlikelihood method,12 and the correlated binary parameters can give the “best” overall representation of both the experimental results and an optimal prediction result.13 The object function is m
F)
∑j
[
(Pcj - Pej )2 σPj2
+
(Tcj - Tej )2 σTj2
+
c e 2 - x1j ) (x1j
+
σx1j2
]
c e 2 - y1j ) (y1j
σr1j2
Figure 3. X-Y diagram for the system of ethanol (1)/water (2) at different concentrations of KAc (4) (salt free).
(4)
To the salt-containing binary system (y1 ) 1 and y2 ) 0), the salt was regarded as a nonvolatile component, so the last item on the right-hand side of eq 4 is zero, and σPj2, σTj2, σx1j2, and σy1j2 are the error variances of P, T, x1, and y1. In this work, the standard deviation values are the following: σP ) 0.133 kPa, σT ) 0.05 K, σx1 ) 0.001, and σy1 ) 0.003. The parameters correlated using the NRTL model for the systems of ethanol/water, ethanol/ethanediol, water/ ethanediol,14,15 ethanol/KAc,16 ethanediol/KAc,3 and water (2)/KAc (4)17 are shown in Table 1. Of course, other methods such as the least-squares method or the optimize method can be used to estimate the binary model parameters. Estimation of the binary parameters between solvents is very well-known. This kind of software can be used directly to estimate the model parameters between the solvent and salt. The form of experimental data is Pj, Tj, x1j and y1j (y1j ) 1.0); here, basic data are the Antoine constant of the solvent and salt. We can input the Antoine constant (in this work, A ) -20.0, B ) 0.0, and C ) 0.0), which can make saturated vapor pressures of the salt approach zero. The software does not need any changes in the process of parameter estimation. Prediction of the Multicomponent System VLE and an X-Y Diagram of the Ethanol (1)/Water (2) Binary System Ternary System VLE Prediction. Ternary experimental data for the system of ethanol (1)/water (2)/KAc (4) at different salt concentrations were reported by Schmitt and Vogelpohl.18 We can investigate the effect of the SCLCM method based on deviations between the
predicted value and the experimental data. The curves of calculated and experimental data at x4 ) 0.052 and x4 ) 0.102 are shown in Figure 1. It is shown that the predicted values agreed with the experimental data at x1 > 0.25, while predicted values are smaller than the experimental data at x1 < 0.25. In general, the results of the ternary system prediction are satisfactory. This shows that the SCLCM method and the parameters of the NRTL model are suitable for the ternary systems. Quaternary System VLE Prediction. It is possible to compare the each datum predicted by SCLCM with the experimental one. In this work, values of x3 and x4 (x3 ) 0.320 and x4 ) 0.022) are used as constants for the prediction. The boiling points at isobaric (101.210 kPa) have been calculated by SCLCM for the quaternary system; thus, the X-Y equilibrium relationship for the system of ethanol (1)/water (2) (ethanediol + KAc free) can be calculated after processing of the X-Y data computed by boiling point calculation. A comparison of the predicted values with the experimental ones is showed in Figure 2. The results of the comparison are satisfactory. This shows that the SCLCM method and the parameters of the NRTL model are suitable for the quaternary system. Thus, the SCLCM method can be used to investigate the VLE relationship of the system in this work. Investigation of the VLE Relationship in This Work. To investigate the influence of different KAc concentrations, ternary VLE data (x3 ) 0) for the system of ethanol (1)/water (2)/KAc (4) were predicted at four constant x4 values (x4 ) 0, 0.01, 0.03, and 0.05). The X-Y equilibrium relationships (KAc free) are illustrated in Figure 3. It is shown that the relative volatility between components 1 and 2 increased with the increase in the KAc concentration and that the azeotropic composition is a function of the salt concentration. It was observed that the azeotropic point can be eliminated only by adding KAc.
Ind. Eng. Chem. Res., Vol. 43, No. 5, 2004 1277
be required compared to that of the pure agent extractive distillation under the same stage numbers. Conclusion The comparison results of the predicted values and the experimental data showed that the SCLCM method is suitable for the calculation of VLE for the ternary system of ethanol/water/KAc and the quaternary system of ethanol/water/ethanediol/KAc. The SCLCM method can be used to simulate the salt-containing extractive distillation process with the system presented in this paper. Notation
Figure 4. X-Y diagram for the system of ethanol (1)/water (2) at different concentrations of ethanediol (3) (solvent free).
∆g ) NRTL model parameter m ) number of experimental points P ) pressure T ) temperature x ) liquid-phase mole fraction y ) vapor-phase mole fraction X ) liquid-phase mole fraction at KAc, or ethanediol, and/or KAc + ethanediol free Y ) vapor-phase mole fraction at KAc, or ethanediol, and/or KAc + ethanediol free Superscripts and Subscripts c ) calculated value e ) experimental value
Literature Cited
Figure 5. X-Y diagram for the system of ethanol (1)/water (2) at different agents (agents free).
To study the influence of different ethanediol concentrations, similar work has been done for the system of ethanol (1)/water (2)/ethanediol (3), at x3 ) 0.0, 0.1, 0.3, and 0.5. The X-Y equilibrium relationships are reported in Figure 4. It can be seen that by adding only the ethanediol one can also eliminate the azeotropic point, but the concentration of ethanediol is higher. It is a classical VLE calculation of the ternary soluble system; the NRTL model can give good predicted results. From Figure 4, we may find an interesting phenomenon: when x1 < 0.25, the expected “solvent” effect of ethanediol was not observed. Figure 5 shows the X-Y relationship of the predicted VLE for the binary system of ethanol (1)/water (2) at x3 ) 0.4500, x4 ) 0.0500 (average mole fraction of a mixed agent within column I), at x3 ) 0.0, x4 ) 0.05, and at x3 ) 0.50, x4 ) 0.0. It can be clearly seen from the curves that the relative volatility of ethanol (1)/water (2) with the mixed agent of ethanediol and KAc is obviously higher than that with pure ethanediol, when the concentration of the agent is the same. It is implied that fewer theoretical stage numbers were required for saltcontaining extractive distillation when the separation demand is the same. In other words, less solvent will
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Received for review March 21, 2003 Revised manuscript received December 3, 2003 Accepted December 5, 2003 IE0302600
