Ind. Eng. Chem. Res. 2000, 39, 3901-3906
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Investigating Polymeric Entrainers for Azeotropic Distillation of the Ethanol/Water and MTBE/Methanol Systems Adnan M. Al-Amer*,† Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
In this work, selected polymeric entrainers have been investigated to assess their capability of breaking the azeotrope of ethanol/water and MTBE/methanol systems. Solubility testing and group contribution model calculations were used to guide in the initial selection of potential polymers. Experimental VLE measurements were performed to determine whether the selected polymers are capable of breaking the azeotrope. We have found polymeric entrainers capable of breaking the azeotrope for the ethanol/water system. Poly(ethylene glycol) at 10 wt % and poly(acrylic acid) at 0.45 wt % did break the azeotrope for the ethanol/water system. This conclusion is based on composition and temperature data. Other polymers used with the ethanol/water system might be capable of breaking the azeotrope, but we could not conclusively determine this from the collected data. From the results obtained for the MTBE/methanol system, we were not able to definitively identify such entrainers. This is because of the difficulty in finding a polymer that will substantially dissolve in both MTBE and methanol and, at the same time, will provide the required specific interaction with each component. The experimental VLE data for selected systems were fit to the UNIQUAC model. A satisfactory fit was obtained, and the parameters are reported. Introduction Complete separation of azeotropic mixtures requires either the coupling the distillation columns with other separation methods such as adsorption, membranes, and extraction or the use of more complex distillation schemes based on a modification of the equilibrium to effect the complete separation. Although many new separation techniques are being developed, distillation will remain the method of choice for large-scale separation of nonideal mixtures including azeotropic mixtures.1 Separation of such mixtures is achieved by use of one of the enhanced distillation methods. These include extractive distillation, salt distillation, pressureswing distillation, reactive distillation, and azeotropic distillation. The latter method involves the use of entrainers to alter the relative volatility of the components and break the azeotrope. The choice of separation method depends on the specific system and economics.2 Much research on entainer selection and distillation system configuration has been performed. In 1991, Laroche et al.3 presented practical solutions for choosing the best entrainers required for separating binary azeotropes into pure components. They also suggested the feasible flow sheet of separation sequences for each entrainer. Their analysis showed that a good entrainer is a component that breaks the azeotrope and yields high relative volatilities between the two azeotropic constituents. They used equivolatility curve diagrams to compare entrainers. In 1996, Widago and Seider4 reviewed recent work on the separation of azeotropic mixtures. They examined the important considerations in the selection of entrain* Telephone: 8602353.Fax: 8602334.E-mail: alamer@kfupm. edu.sa. † Previous publications by the author appeared under his subfamily name of Al-Jarallah.
ers as success in azeotropic distillation is largely determined by the choice of entrainer. In the past, entrainers were selected using a trial-and-error procedure with much reliance on experimental data. This results in a waste of time and resources. More recently, entrainers have been selected on the basis of their potential for producing feasible designs. Widago and Seider presented simple rules using maps of distillation lines in preference to residue curves for screening the many possible entrainers. These maps are useful in early design stages, providing specifications for the desired separations. Bekiaris and Morari (1996)5 extended the multiplicity analysis from ternary systems (two components plus and entrainer) to quaternary systems for the case of infinite flux and infinite number of trays. They showed how their work could be useful for the selection of entrainer. Guttinger and Morari (1996)6 studied multiple steady states in two or more columns typically used in ternary azeotropic distillation. They found that, for the intermediate entrainer scheme, multiplicity could occur for all feed compositions. Safrit and Westerberg (1997)7 examined the case of a continuously flowing extractive agent to facilitate the separation of azeotropic mixtures. They showed the sensitivity of the separation’s profile to the entrainer flow rate. Group contribution calculations reported in the literature revealed a number of suitable low-molecularweight entrainers.8 One possible guide for polymeric entrainers is to choose the polymeric counterparts of those low-molecular-weight entrainers. For example, the following entrainers were found suitable for breaking the azeotrope in the ethanol/water system: acetic acid, 2-aminoethanol, N,N-dimethylformamide (DMF), ethylene glycol, meso-2,3-butanediol, and morpholine. Ace-
10.1021/ie0000515 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/08/2000
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tic acid and ethylene glycol are already used in the industry. Therefore, in this research, we tested their polymeric counterparts: poly(acrylic acid) and poly(vinyl alcohol). However, one should keep in mind that terminal functional groups are sometimes lost in the polymerization, such as in ethylene glycol, or packed very closely along the main chain, which might reduce the effectiveness of these polymers. The advantage of using such polymers is that they remain in the liquid phase. They can be separated by ultrafiltration. Hence, the separation can be completed in a smaller number of columns. Thus, in this work, selected polymeric entrainers have been investigated to assess their capability of breaking the azeotrope of the ethanol/water and MTBE/methanol systems. Solubility testing and a group contribution model calculations were used to guide in the initial selection of potential polymers. Experimental VLE measurements were performed to determine whether the selected polymers are capable of breaking the azeotrope. The VLE data were fit using the UNIQUAC model. Criteria for Entrainers Selection Because there were a very large number of polymers that might have been applicable, the following criteria were used to guide the selection of the polymers: (1) Polymer availability and cost. In general, the polymer should be available commercially at reasonable cost. (2) Polymer solubility. The polymer should be soluble in the system. The solubilities of the selected polymers must be large enough to allow for sufficient specific interactions to attract one of the components and thus break the azeotrope. For the ethanol/water system, the polymers must be polar in order to dissolve in water. (3) Group contribution calculations. To obtain a relative ranking of the ability of the potential polymers to break the azeotrope, one can use a group contribution model. Group contribution models such as UNIFAC do not require a set of fitted parameters. It is sufficient to know the structure of the components to obtain the VLE behavior (assuming that the group parameters are available). These methods are approximate and are expected to only give a qualitative indication as to which polymer is the best, which is the second best, and so on. Group contribution models allow for the calculation of activity coefficients based on groups rather than whole molecules. Calculations for a polymer can be done by selecting the proper groups for that polymer in addition to the groups for the two volatile components. The criterion for a useful polymer is the relative volatility, R, at the azeotropic temperature at a total pressure of 1 atm. The MTBE/methanol system forms an azeotrope at a temperature of 55 °C and atmospheric pressure with xi ) yi ≈ 0.7.9 The relative volatility is equal to unity for the azeotrope. Using a group contribution model, the activity coefficient γ1 is calculated for different added amounts of the selected polymers. For this system, the objective is to have a polymer with strong interactions with MTBE and weak interactions with methanol. This will lead to a value of R12 lower than unity. This is the desired behavior for this system because the feed to the distillation column, which comes from the reactor, has an MTBE mole fraction close to unity. For the ethanol/ water system, the objective is to increase R12 above unity.
Table 1. Systems with Entrainers system ethanol/water
entrainer
wt %
poly(ethylene glycol) (MW ) 1000)
3 5 10 poly(ethylenimine) (BDH cat#15047) 3 5 15 poly(acrylic acid) (MW ) 2000) 0.45 poly(2-acrylamido-2-methyl-1-propane 5 sulfonic acid) (Aldrich cat#19197-3) MTBE/methanol poly(ethylene glycol) (MW ) 1000) 3.3 tetraethylene glycol 3.3 triethylene glycol 3.3
The details of the initial list of polymers and the results of the solubility tests and the group contribution calculations are given in ref 10. On the basis of the solubility tests and the relative volatility calculations mentioned above, we selected 11 systems with entrainers to run the VLE experiments. These are given in Table 1. The selected polymeric entrainers for ethanol/water include nonionic and ionic polymers. For the methanol/ MTBE system, it was difficult to find a polymer soluble in both components. This fact limited our study to one polymer [poly(ethylene glycol)]. However, we have included the olegomeric counterparts of this polymer, tetraethylene glycol and triethylene glycol. These compounds have very low vapor pressures at the temperature of the VLE and, therefore, were assumed to be nonvolatile in our calculations. Experimental Section Analar-grade MTBE and methanol were used for the MTBE/methanol system. Refractometer model RFM 340, Bellingham & Stanley Ltd. (BS), was used for the measurement of refractive index. This instrument was calibrated and used for the analysis of methanol/MTBE mixtures. This analysis technique gave better results than gas chromatography. For the MTBE/methanol systems with polymers, we also used the refractive index. We prepared a calibration curve for each polymer weight fraction. Analar-grade ethanol and distilled water were used for the ethanol/water system. Karl Fischer titration techniques using Mettler KF titrator model DL30 were used for the analysis of ethanol/water mixtures as we found that this method gives the most satisfactory results compared with gas chromotographic and refractive index methods. The VLE measurements were made using the Fischer VLE model 0602 constant-pressure still operated at atmospheric pressure. More details on the experimental procedure are provided in ref 10 Results and Discussion Experimental VLE Data. First, only mixtures of the volatile components (ethanol/water and MTBE/methanol) without the entrainers were used. The purpose of this step was to test the VLE equipment and verify its proper functioning by comparison to literature data from Perry11 and to verify the reproducibility of the measurements. A sample result from this test is shown in Figure 1 for the ethanol/water system without entrainer. From this figure, good agreement with published data is verified. The agreement ensures us about the good
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3903
Figure 1. Ethanol/water xy diagram from the literature and from this work.
Figure 2. xy diagram for methanol/MTBE with 3.3 wt % poly(ethylene glycol). The curve is the UNIQUAC fit.
functioning of the still and the reliability of the experimental procedure and analysis technique. The presence of an azeotrope is judged from the composition data crossing the x ) y line and from the presence of a minimum value in temperature. Thus, the criteria for the success of a polymer entrainer in breaking the azeotrope is the absence of this behavior, from both curves to be conservative. For all the systems with entrainers in Table 1, the VLE data were plottted as xy and Txy curves. Selected results of VLE data with the entrainers for both systems are plotted in Figures 2-10. The criteria for a successful entrainer were not met for all of the MTBE/methanol systems investigated. Figure 2 shows a sample of a typical result for this system with 3.3 wt % poly(ethylene glycol) in which the VLE data crossed the x ) y line. Figure 3 confirms this
Figure 3. Txy diagram for methanol/MTBE with and without 3.3 wt % poly(ethylene glycol). The curve is the UNIQUAC fit for the ternary mixture.
Figure 4. xy diagram for ethanol/water with 10 wt % poly(ethylene glycol). The curve is the UNIQUAC fit.
result as a minimum in temperature is clearly shown. Thus, none of the polymers tested for this system broke the azeotrope. The failure to break the azeotrope in the methanol/MTBE system is due to the difficulty in finding a polymer that will substantially dissolve in both MTBE and methanol and, at the same time, will provide the required specific interaction with each component. The search for such a polymer requires an extensive study. For the ethanol/water system, poly(ethylene glycol) at 10 wt % and poly(acrylic acid) at 0.45 wt % did break the azeotrope. This is concluded from the composition and from the temperature data. Even if there is some error in composition date, the absence of a minimum in temperature confirms the absence of an azeotrope. These results are shown in Figures 4-7 for these systems. The rest of the polymer systems tested for the
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Figure 5. Txy diagram for ethanol/water with and without 10 wt % poly(ethylene glycol). The curve is the UNIQUAC fit for the ternary system.
Figure 7. Txy diagram for ethanol/water with and without 0.45 wt % poly(acrylic acid). The curve is the UNIQUAC fit for the ternary mixture.
Figure 6. xy diagram for ethanol/water with 0.45 wt % poly(acrylic acid). The curve is the UNIQUAC fit.
Figure 8. xy diagram for ethanol/water with 5% polyethyleneimine.
ethanol/water system came very close to the x ) y line, but we are not sure if they crossed this line. In other words, it is not certain that they broke the azeotrope. For example, the results for 5% and 15% polyethyleneimine are shown in Figures 8 and 9. The Txy plot for the latter system is shown in Figure 10; as one can see it is not clear that a minimum in temperature is not present. Therefore, a conservative conclusion can be made that these polymers did not break the azeotrope. Although we found two polymers that were capable of breaking the azeotrope for the ethanol/water system, the gap between the equilibrium curve and the x ) y line is small. This means that a large number of trays will be needed in distillation columns. More work is needed to increase the difference between x and y in the ethanol-rich region.
Fitting VLE Data. A number of models were developed recently to specifically represent the activity coefficients of polymers in solutions. In this work, we choose not to use these models because of the low molecular weights of our polymers and the fact that we are more concerned with the behavior of the volatile components. The collected VLE data were fit to the UNIQUAC model. This model was chosen because of its wide acceptance in the literature and its accuracy in representing VLE data for a wide range of systems. Equilibrium was represented by the equation
yiP ) xiγiPisat
(1)
For binary systems, there are two parameters, but for ternary systems, there are six parameters. The parameters obtained in this work are listed in Table 2. We fit
Ind. Eng. Chem. Res., Vol. 39, No. 10, 2000 3905 Table 2. UNIQAC Fit of Experimental Data SSEb
|err ymax|c
0.0042
0.042
-567.662
0.0067
0.013
-606.714
0.0006
0.015
419.911
0.0014 0.0036
0.020 0.037
543.105
0.0048
0.042
-174.236
-153.013
0.0052
0.038
-8.43557 -868.854 -581.793
-481.231 -801.525 -633.953
0.0018 0.0394 0.0033
0.019 0.184 0.043
exptl systema
θ1
MeOH-MTBE MeOH-MTBE-T3G 3.3 wt % MeOH-MTBE-T4G 3.3 wt % MeOH-MTBE-PEG 3.3 wt % EtOH-water EtOH-water-PAA 0.45 wt % EtOH-water-PEI 15 wt % EtOH-water-PEG 10 wt % PEG 5 wt % PEG 3 wt % PEG
-57.062
412.822
-66.223
-335.967
532.446
146.72
-231.677
-74.578
-341.708
580.129
172.605
-233.72
-68.843 123.993
-176.808 -31.511
461.326
-28.7472
297.008
3197.53
-113.489
114.868
-136.413
-10.264
-209.725
-47.7105 -120.099 -11.1501
-1123.99 269.347 -2278.72
171.571 423.963 110.854
1270.69 -199.267 5461.57
θ2
θ3
θ4
-3024.71
θ5
29.3171 6221.23
θ6
a T3G ) triethylene glycol; T4G ) tetraethylene glycol; PEG ) poly(ethylene glycol); PAA ) poly(acrylic acid); and PEI ) poly(ethylene imine). b SSE ) sum of square error in fitted yi. c |err ymax| ) maximum absolute fit residual.
Figure 9. xy diagram for ethanol/water with 15 wt % polyethyleneimine.
Figure 10. Txy diagram for ethanol/water with and without 15 wt % polyethyleneimine.
the vapor composition to the experimental data via
The VLE data are plotted in Figures 1-10, along with the UNIQUAC prediction in most cases.
yi ) xiγiPisat/P
(2)
where γi is the activity coefficient of component i calculated from the UNIQUAC equation, with τij ) exp(-Rij/T) and τij ) 1 for all i ) j. For the two-component systems, i.e., methanol/MTBE and ethanol/water, a12 ) θ1 and a21 ) θ2, whereas for the three-component systems (methanol/MTBE/polymer or ethanol/water/polymer), a12 ) θ1, a13 ) θ2, a21 ) θ3, a23 ) θ4, a31 ) θ5, and a32 ) θ6. The experimental data for the various systems were fit to the UNIQUAC equation and the resulting values of θ are summarized in the Table 2. The table also shows the sum of square error in the fitted yi value and the maximum absolute error. The fit is satisfactory, except for ethanol/water with 5% PEG. The large maximum error for this weight fraction is probably due to experimental error. The fits to the other two weight fractions, 3% and 10%, are satisfactory. We perform the fit for PEI in ethanol/water for only the highest weight fraction (15%).
Conclusions No polymer among the polymers studied was found to break the azeotrope for the MTBE/methanol system. For the ethanol/water system, at least two polymers, 10 wt % poly(ethylene glycol) and 0.45 wt % poly(acrylic acid), have been found to break the azeotrope for this system. The UNIQUAC fit for the VLE data has been found to be satisfactory. The UNIFAC group contribution prediction was not satisfactory in predicting the relative volatility. More accurate group contribution methods would make the polymer-design part more reliable. Acknowledgment The support of the King Fahd University of Petroleum and Minerals and SABIC for Project ChE/SABIC/96-4 is duly acknowledged. The help of my colleague and coinvestigator, Professor E. Z. Hamad, is greatly appreciated.
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Nomenclature Psat ) Saturation vapor pressure T ) Temperature xi ) Mole fraction of component i in liquid phase yi ) Mole fraction of component i in vapor phase aij ) binary interaction parameter γi ) Activity coefficient of component i
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(6) Guttinger, T. E.; Morari, M. Multiple Steady States in Homogeneous Separation Sequences. Ind Eng. Chem. Res. 1996, 35, 4597. (7) Safrit, B. T.; Westerberg, A. W. Improved Operational Policies for Batch Extractive Distillation Columns. Ind. Eng. Chem. Res. 1997, 36, 436. (8) Foucher, E.; Doherty, M.; Malone, M. Automatic Screening of Entrainers for Homogeneous Azeotropic Distillation. Ind. Eng. Chem. Res. 1991, 30, 760. (9) Alm, K.; Ciprian, M. Vapor Pressures, Refractive Index at 20.0 °C, and Vapor-Liquid Equilibrium at 101.325 kPa in Methyl tert-Butyl Ether-Methanol System. J. Chem. Eng. Data 1980, 25, 100. (10) Al-Amer, A. M.; Hamad, E. Z. Investigating Polymeric Entrainers for Azeotropic Distillation of MTBE/Methanol and Ethanol/Water Systems. Final Report, Project CHE/SABIC/96-4, Deanship of Research and Graduate Studies, King Fahd University of Petroleum & Minerals, Saudi Arabia, 1999. (11) Seader, J. D.; Zdzislaw, M. K. Distillation. In Chemical Engineer’s Handbook; Perry, R. B., Green, D., Eds.; McGrawHill: New York, 1984.
Received for review January 11, 2000 Accepted July 22, 2000 IE0000515
