Heat-Integrated Pressure-Swing-Distillation Process for Separation of

Article pubs.acs.org/IECR

Heat-Integrated Pressure-Swing-Distillation Process for Separation of Tetrahydrofuran/Methanol with Different Feed Compositions Yinglong Wang,* Peizhe Cui, and Zhen Zhang College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, China S Supporting Information *

ABSTRACT: A process optimization is carried out to separate binary azeotropic mixtures of tetrahydrofuran and methanol by pressure-swing distillation according to the pressure-sensitive property of the binary system. Rigorous steady-state simulation that is based on the minimization of the total annual cost for partially and fully heat-integrated pressure-swing-distillation processes is implemented on Aspen Plus following the sequential iterative optimization procedure. The feasible sequence of high pressure and low pressure of two columns in the pressure-swing-distillation process and the suitable heat integration scheme are both determined by the feed composition.

1. INTRODUCTION Tetrahydrofuran (THF) is one of the most commonly used solvents in the chemical and the pharmaceutical industries due to its excellent dissolution ability. The production of steroid drugs faces the problem of separating solvent which contains THF and methanol. The separation and recycle of THF and methanol are of high economic significance and environmental importance. However, difficulties occur in the separation process because a minimum-boiling azeotrope is formed in the binary system.1 This means simple distillation, which is based on the difference in the volatility of each component of the mixture, is unable to accomplish the efficient separation of the two components. An effective method for separating azeotropes is special distillation such as extractive distillation and pressure-swing distillation.2−4 In the published literature, most are focused on extractive distillation for THF and methanol separation.5−10 These studies contribute to the separation of THF and methanol to obtain high-purity THF. However, a third component is introduced to the product inevitably by extractive distillation. The pressure-swing-distillation (PSD) process, commonly used to separate azeotropic mixtures based on the shift of the relative volatilities and azeotropic compositions by changing the system’s pressure, is another suitable separation method for the separation of azeotropes. Efficient separation is achieved by two columns operating at two different pressures, which are determined by the composition variation of the azeotrope with the pressure. For the minimum-boiling azeotrope, highpurity product streams are produced from the bottom of the columns. The distillate stream from the first column enters the second column, from the top of which the distillate stream recycles back to the first column.4 PSD is an attractive and valuable topic worthy of study in the case where the composition of the azeotrope changes significantly with pressure. PSD has gained widespread attention from scholars in recent years.11−21 Studies on separation of homogeneous azeotrope mixtures by PSD were carried out by Repke et al.11,12 Cho et al. focused on the use of PSD to separate the mixture of THF and water and the mixture of acetonitrile and water.14,18 © 2014 American Chemical Society

The THF and water system was also studied by Hamad using global energy optimization strategies in steady-state systems.20 All these achievements provide an efficient method to separate azeotrope mixtures. To make the distillation process more energy-efficient, good efforts have been made in the past few decades, especially in the study of heat integration.22−25 One of the important inherent features of pressure-swing distillation is the opportunity for heat integration because the temperatures of the two columns are different as a result of different pressures. There are two heat integration schemes for a PSD process. One case is that the condenser in the high-pressure column can be integrated with the reboiler in the low-pressure column (condenser/reboiler type); the other integration is between the stripping section in the low-pressure column and the rectifying section in the highpressure column (rectifying/stripping type).26 Both schemes have obvious advantages in saving energy for the separation of azeotropes with PSD. The two schemes are effective energysaving methods, and many scholars are interested in this topic.27−36 Flores-Tlacuahuac et al. studied the internally heatintegrated PSD process for bioethanol separation.29 The fully heat-integrated PSD process was used by Xu et al. to separate a phenol and cyclohexanone mixture.34 Thermally integrated and dividing-wall columns were used for comparison in the work of Modla.35 PSD for the separation of THF and methanol was studied by Xu.10 His work demonstrated the effectiveness of the PSD process for the separation of THF and methanol although heat integration was not taken into consideration. Therefore, in this article, PSD processes for THF and methanol with partial and full heat integration are optimized based on economic considerations, and the optimization considering the different feed compositions that are rarely noticed in the PSD research is also presented. Received: Revised: Accepted: Published: 7186

January 17, 2014 March 7, 2014 April 2, 2014 April 2, 2014 dx.doi.org/10.1021/ie500235f | Ind. Eng. Chem. Res. 2014, 53, 7186−7194

Industrial & Engineering Chemistry Research

Article

2. DESIGN Modeling and optimization of PSD process design were performed for the separation of THF and methanol from an

Figure 2. xy diagram of THF/methanol azeotrope at 1 and 10 atm.

activity model with the built-in binary interaction parameters in Aspen Plus was chosen as the property package in the calculation. The purity of the products was set as 99.9 mol % for both components in all cases studied. The impact of pressure on azeotropic composition and azeotropic point for the THF and methanol binary system is shown in Figure 1. It is clear that the azeotropic composition changes significantly with pressure, which means pressureswing distillation is likely economically feasible. 2.1. Basis of Economic Calculation. The progress of PSD is carefully optimized according to the economic optimization which is based on the minimization of the total annual cost (TAC). TAC consists of the operating cost (annual energy cost) and the capital investment that is equal to the total capital investment divided by a 3-year payback period.37,38 The major investment for a distillation system is the column vessel and the two heat exchangers (i.e., reboiler and condenser). The formula to calculate the length of the vessel (L) is L = 0.61(NT − 2) + 3, where 0.61 refers to the typical distance between the trays (tray spacing with a unit of meters), NT − 2 is the number of trays (in Aspen Plus, the first stage is the reflux drum and the last stage is the reboiler), and 3 is an empirical value which means the height of the head and bottom of a column. The diameter of a column is determined by the “Tray Sizing” function in Aspen Plus, and “sieve tray” was used. Small items such as reflux drums, pumps, valves, and pipes are usually not considered because their costs are much lower compared with the costs of the column vessels and heat exchangers. The basis of the economics and the sizing relationships and parameters are listed in Table 1.39−42 Fc refers to the impact of pressure on investment. 39,40 Furthermore, the operating time of the distillation system is set at 8000 h/year. 2.2. Selection of Pressure. The composition of the azeotrope changes significantly with pressure. The larger the shift is, the smaller the recycle flow rate and the energy consumption in the two reboilers will be, which means that it is appropriate to select a higher pressure in the high-pressure column.17 However, the pressure should not be higher than is compatible with the choice of heating steam. The minimum temperature difference required in the reboiler is 20 K; therefore, 10 atm is selected for the high-pressure column so that medium-pressure steam can be used in the high-pressure column, which can significantly reduce the energy costs,

Figure 1. Effect of pressure on azeotrope composition and temperature.

Table 1. Basis of Economics and Equipment Sizing column diameter: Aspen tray sizing

length (L) = 0.61(NT − 2) + 3 vessel (diameter and length in meters)

capital cost = 17640Fc(ID)1.066 (L)0.802 condensers (area in m2) heat-transfer coefficient (KC) = 0.852 kW/(K·m2) differential temperature (ΔTC) = reflux-drum temperature − 310 K QC AC = K CΔTC

capital cost = 7296Fc(A C)0.65 reboilers (area in m2) heat-transfer coefficient (KR) = 0.568 kW/(K·m2) differential temperature (ΔTR) = steam temperature − base temperature QR AR = K CΔTC

capital cost = 7296Fc(AR )0.65 energy costs LP steam (433 K) = $7.72/GJ MP steam (457 K) = $8.22/GJ HP steam (537 K) = $9.88/GJ total capital cost TAC = + annual energy cost payback period payback period = 3 years

azeotropic mixture using a commercial process simulator (Aspen Plus). The accuracy of simulated results strongly depends on the quality of physical property model parameters.16 The boiling points of THF and methanol are 339.12 and 337.66 K, respectively. The mutual exclusion between THF and methanol molecules leads to the formation of a minimumboiling azeotrope in the binary system with a composition of 50.79 mol % THF at 332.94 K under atmospheric pressure. The predicted azeotropic compositions at different pressures by the NRTL model have good agreement with the experimental data published in Azeotropic Data.1 Therefore, the NRTL 7187

dx.doi.org/10.1021/ie500235f | Ind. Eng. Chem. Res. 2014, 53, 7186−7194


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Figure 3. Sequential iterative optimization procedure.

methanol at different pressures are shown in Figure 2. The azeotropic composition changes from 50.79 mol % THF at 1 atm to 15.37 mol % THF at 10 atm.

compared with the utilization of high-pressure steam. Furthermore, the pressure of low-pressure column is set at 1 atm so that cooling water with a temperature of 310 K can be used in the condenser. In conclusion, low-pressure steam was used in the low-pressure column operated at 1 atm, and medium-pressure steam was used in the high-pressure column operated at 10 atm. The xy diagrams for the system of THF and

3. OPTIMIZATION Take the case of 25 mol % THF feed composition as an example. The feed temperature is set at 320 K. For an 7188



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Figure 4. Effect of feed locations on TAC.

Figure 5. Effect of reflux ratios and number of stages on TAC.

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Figure 6. Flow sheet of optimized PSD process for feed composition of 25 mol % THF.

Figure 7. Flow sheet of partially heat-integrated PSD process for feed composition of 25 mol % THF.

locations (NF1, NF2 and NFR) need to be optimized. The

optimization, process design variables and optimization variables must be specified. In this work, the purities of two bottom products are selected as design variables. In order to achieve the required product purities and minimize TAC, the number of stages in LP and HP columns (N1 and N2), the reflux ratios of the two columns (RR1 and RR2), and the feed

optimization procedures shown in Figure 3 follow the sequential iterative optimization procedure,34 and appropriate changes are made to consider the optimization of column sequence and the feed composition. 7190



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on the LP column (NFR) are optimized to achieve the minimum TAC. The results are shown in Figure 4. The optimization of feed locations is the innermost loop. The next step is to optimize the reflux ratios of the two columns. Increasing the reflux ratios contributes to improving product purity. However, the increasing of reflux ratios also makes the heat duties in the bottoms increase. Therefore, under the precondition of ensuring the purities of the bottoms, there is an optimal reflux ratio that minimizes the TAC, as shown in Figure 5. During the optimization of the reflux ratio, feed locations at the inner loop are also optimized. The last step is to optimize the number of stages (NT) of the two columns. The increase of NT leads to an increase of the separation effect, but the equipment investment increases at the same time. The effect of NT1 and NT2 on TAC is shown in parts c and d, respectively, of Figure 5, in which each point corresponds to an optimal design (i.e., minimum TAC). Stream information, heat duties, equipment sizes, and operating conditions of the optimized process flow sheet can be found in Figure 6. The TAC calculated for the optimized PSD system is $1,043,467.0. 3.2. Partial Heat Integration. The columns of the above PSD process both have independent reboilers and condensers. The large temperature difference between the condenser of the HP column (410.0 K) and the reboiler of the LP column (337.7 K) makes heat integration possible in PSD.22,23 The heat duties are not equal (991.04 kW for the condenser of the high-pressure column and 1992.54 kW for the reboiler of the low-pressure column), so an auxiliary reboiler is required in the LP column. After partial heat integration is carried out for the above PSD, the TAC is further reduced to $821,989.5. The optimized process flow sheet for partial heat integration is shown in Figure 7. 3.3. Full Heat Integration. The design of the fully heatintegrated PSD process is carried out on the basis of partial heat

Figure 8. Effect of reflux ratios on fully heat-integrated process.

3.1. Procedures. First of all, the mixture flow rate is assigned as 100 kmol/h. There are initially 35 stages in both columns. The mixture is fed into the LP column on the middle stage. Initial estimate values for the reflux ratios of the columns are both 2. The high-purity methanol product leaves the column from the bottom of LP column, the distillate stream enters the HP column for which the bottom stream is highpurity THF, and the distillate stream recycles back to the LP column. The “Design Spec” and “Vary” features of the LP column block in Aspen Plus is used to adjust the distillate flow rate to obtain 99.9 mol % methanol, and the bottom flow rate of the HP column is manipulated to obtain 99.9 mol % THF using the “Design spec/Vary” function of the HP column block. After fixing the number of stages and reflux ratios of the two columns, feed locations such as LP column feed stage (NF1), HP column feed stage (NF2), and recycle stream feed location

Figure 9. Flow sheet of fully heat-integrated PSD process for feed composition of 25 mol % THF. 7191



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Figure 10. Flow sheet of partially heat-integrated PSD process for feed composition of 75 mol % THF.

Figure 11. Flow sheet of fully heat-integrated PSD process for feed composition of 75 mol % THF.

9. The fully heat-integrated process with a TAC of $773,384.8 shows a better performance in economy compared with partial integration.

integration. In the fully heat-integrated process, no auxiliary reboiler is used. The only heat input is the HP column reboiler duty QR2. To achieve this “neat” configuration, the “flowsheet design spec” function in Aspen Plus is used. The RR2 is manipulated to make the sum of the LP column reboiler duty QR1 and the HP column condenser duty QC1 equal to 0, while the reflux ratio of the LP column (RR1) is varied to minimize the TAC, which is shown in Figure 8. The flow sheet of the optimized fully heat-integrated PSD process is shown in Figure

4. PRESSURE-SWING-DISTILLATION SCHEMES FOR VARIOUS FEED COMPOSITIONS An interesting phenomenon that occurred during our investigation is that the composition of the mixture affects the sequence of the high and low pressures of the two columns 7192



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scheme are best for the mixture with 75 mol % THF. The conclusion that different pressure-swing-distillation schemes should be selected when separating mixtures with various compositions will remind the engineer of detailed study during PSD design. It will save a lot of energy for industrial applications. In our opinion, various azeotropic systems should be studied to discover the relationship between the Txy diagram type and the optimum PSD scheme which will facilitate the PSD design.

in PSD if the minimization of the TAC is considered during the process design. The process studied above is PSD with a column sequence of LP−HP for the THF−methanol system when the feed composition is 25 mol % THF. For a PSD process with a column sequence of HP−LP, the separation can also be achieved but the TAC is much larger. Therefore, the LP−HP column sequence is suitable for a feed composition of 25 mol % THF. However, for a feed composition of 75 mol % THF, the column sequence should be HP−LP, which can be explained by the xy diagram (Figure 2). If the mixture with 75 mol % THF is first fed to the LP column, the feed composition lies on the right of the azeotrope composition of 1 atm. Then the bottom product is high-purity THF and the distillate with a composition a little larger than the azeotrope composition (50.79 mol % THF) is fed to the HP column. The feed composition of the HP column still lies on the right side of the azeotrope composition, and the bottom product is still THF. Therefore, the column sequence should not be LP−HP. The optimization of the PSD for separation of THF and methanol with a composition of 75 mol % THF also follows the procedures mentioned above. The heat duties are not equal (979.37 kW for the condenser of the high-pressure column and 586.09 kW for the reboiler of the low-pressure column), so an auxiliary condenser is required in the HP column for partial integration. When an auxiliary condenser is introduced in the PSD process, the TAC is reduced from $714,624.5 to $568,696.5. After full heat integration is carried out for the PSD process, the TAC changes to $577,743.6. The optimized flow sheets for partial and full heat integration are shown in Figures 10 and 11, respectively. Another interesting finding is that different feed compositions lead to different heat integration schemes, which is worth further discusson. First, compared with the case of 25 mol % THF, the heat input in the LP column (i.e., −QC1 ≥ QR2) is 0 for both partial and full integration, so no additional heating steam is needed for the LP column which can reduce a lot of costs. Second, it is shown in the optimized flow sheets that the magnitudes of the HP column reboiler duty QR1 and the HP column condenser duty QC1 in the fully heat-integrated process are slightly smaller than those of the partial heat integration process, which is beneficial to reduce TAC. However, the LP column diameter and the LP column condenser duty QC2 in the fully heatintegrated process are much larger than those of the partially heat-integrated process, which means that a larger column shell and a larger condenser result in a larger equipment investment. Therefore, the TAC of the fully heat-integrated process is a little larger than that of the partially heat-integrated process. Thus, during the optimization of the PSD processes, not only the column sequence (depending on feed composition) but also the different heat integration schemes should be considered and compared according to the detailed calculation results.

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ASSOCIATED CONTENT

S Supporting Information *

Tables S1 and S2 listing design parameters and economics for 25 and 75 mol % THF mixtures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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NOTATION A = heat transfer area of heat exchanger [m2] B = bottom stream flow rate [kmol/h] D = distillate flow rate [kmol/h] ID = diameter of the column [m] K = heat transfer coefficient L = length of the vessel [m] NT = number of stages Q = heat duty [kW] RR = reflux ratio TAC = total annual cost [$/y] xD = composition of the distillate xB = composition of the bottom stream

Indices

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1, 2 = column index co = cooler C = condenser R = reboiler

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5. CONCLUSIONS The separation of THF and methanol by the pressure-swingdistillation process with a focus on heat integration for different feed compositions is investigated in this paper. The optimization for the process is carried out in detail to find the process conditions under which the minimum TAC can be obtained. For the mixture with 25 mol % THF, the LH−HP column sequence and fully heat-integrated scheme are suitable, while the HP−LP column sequence and partial heat integration 7193



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