Employing TopâBottom Recycled Reactive Distillation to the

Article pubs.acs.org/IECR

Employing Top−Bottom Recycled Reactive Distillation to the Separations of Adipic Acid and Glutaric Acid Esterifications Xinhui Yao, Kejin Huang,* Haisheng Chen, and Shen Li College of Information Science and Technology, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ABSTRACT: For the separations of ternary/quaternary reacting mixtures with the most unfavorable relative volatilities (i.e., the reactants are the lightest and the heaviest components with the product/products as the intermediate one/ones), the reactive distillation column with a top-bottom external recycle (RDC-TBER) was already demonstrated to be considerably advantageous over the currently available process schemes [Chen, H.; Huang, K.; Liu, W.; Zhang, L.; Wang, S.; Wang, S. J. Enhancing Mass and Energy Integration by External Recycle in Reactive Distillation Columns. AIChE J. 2013, 59 (6), 2015]. In the present study, the RDC-TBER is further evaluated in terms of the separations of two more complicated reacting systems: the adipic acid and glutaric acid esterifications, which feature a two-stage cascade reaction kinetics besides the most unfavorable ranking of relative volatilities (i.e., the relative volatilities of the intermediate and final products are within those of the lightest and heaviest reactants). In comparison with the flowsheet by conventional reactive distillation column, the flowsheet by the RDC-TBER leads to sharply reduced capital investment, operating cost, and total annual cost (TAC). Moreover, the reduction of the TAC tends to increase in case the two-stage cascade reaction involved becomes difficult to proceed. These outcomes indicate that the arrangement of a top−bottom external recycle helps to enhance internal mass integration and/or internal energy integration between the reaction operation and the separation operation involved and facilitates the RDC-TBER to be a favorable option for the separations of adipic acid and glutaric acid esterifications.

1. INTRODUCTION As a hybrid of reaction operation and separation operation, reactive distillation columns can render great advantages in capital investment and operating cost over the conventional reactor/separator/recycle systems for some reaction and separation operations.1−4 This is, however, not always true because the ranking of the relative volatilities of reacting components can present a great influence to the performance of reactive distillation columns.5−7 Recently, Tung and Yu classified reactive distillation systems into six broad categories according to the ranking of the relative volatilities of reacting components and provided corresponding process configurations for all of the categories they made.8 For the separation of quaternary reacting mixtures with two reactants as the lightest and the heaviest components and two products as the intermediate ones, the conventional reactive distillation column (CRDC) was found to lose totally its competition with the conventional reactor/separator/recycle systems, and the reason lay on the fact that the lightest and the heaviest reactants unconverted tended to accumulate in the top and bottom and could not be forced in close contact in the reactive section to favor the desired forward reaction. The case represented actually the severest circumstance of the application of reactive distillation columns and was termed the system separating reacting mixtures with the most unfavorable ranking of relative volatilities. To tackle this problem, Tung and Yu once advocated a reactive distillation column with two reactive sections located in the top and bottom, respectively. Although a certain extent of improvement could be expected in system performance, it still involved a serious drawback. In the case of the reaction operation involving thermal effect, whether exothermic or endothermic, adverse internal energy integration © 2013 American Chemical Society

could occur in one of the two reactive sections, leading inevitably to the degradation in system performance. As shown in Figure 1, we recently proposed a reactive distillation column with a top−bottom external recycle (RDC-TBER), which featured transferring one of the unconverted reactants from one end where it accumulated to the other end where the reactive section was located.9,10 In terms of the thermodynamic characteristics of the reaction operation involved (i.e., exothermic or endothermic), the reactive section should be arranged either at the bottom (Figure 1a) or at the top (Figure 1b), with the external recycle directed from the top to bottom or inversely. The external linkage between the top and bottom helps to boost reaction conversion rate and enhances internal mass integration and/or internal energy integration between the reaction operation and the separation operation involved.11−13 Therefore a significant improvement in system performance can be expected. So far, the RDC-TBER has already been evaluated intensively in terms of the separations of various ternary or quaternary reacting mixtures with the most unfavorable ranking of relative volatilities. Although the RDCTBER was exclusively found to be favorable in comparison with the CRDC and reactive distillation column with two reactive sections in the top and bottom, respectively, its feasibility and applicability still needs to be carefully examined in terms of the separations of more complicated reacting mixtures with the most unfavorable ranking of relative volatilities. Received: Revised: Accepted: Published: 16870

August 10, 2013 September 30, 2013 November 7, 2013 November 7, 2013 dx.doi.org/10.1021/ie402622f | Ind. Eng. Chem. Res. 2013, 52, 16870−16879

Industrial & Engineering Chemistry Research

Article

dimethyl adipate (DMA) and water (H2O) with monomethyl adipate (MMA) as an intermediate product. k1

AA + MeOH HooI MMA + H 2O k −1

(1.1)

k2

MMA + MeOH HooI DMA + H 2O k −2

(1.2)

The reaction rates can be expressed by a quasihomogeneous model. −dCAA /dt = k1CAAC MeOH − k −1C MMAC H2O

(2.1)

−dCMeOH/dt = k1CAACMeOH − k −1CMMAC H2O + k 2CMMACMeOH − k −2CDMAC H2O

(2.2)

dCMMA /dt = k1CAACMeOH − k −1C MMAC H2O − k 2CMMA CMeOH + k −2C DMAC H2O

(2.3)

dC H2O/dt = k1CAACMeOH − k −1CMMAC H2O + k 2CMMA CMeOH − k −2C DMAC H2O

dC DMA /dt = k 2CMMACMeOH − k −2C DMAC H2O

(2.4) (2.5)

where Ci represents the molar compositions (mol/cm3) of component i, and the forward and backward reaction rate constants (cm6/(gcat·min·mol)) can be expressed as

Figure 1. Scheme of the RDC-TBER for A + B ⇌ C + D with αA > αC > αD > αB: (a) for exothermic reactions and (b) for endothermic reactions.

In the present study, the RDC-TBER is to be employed to the separations of adipic acid and glutaric acid esterifications, which are characterized by two-stage cascade reactions in addition to the most unfavorable ranking of relative volatilities. Hung et al. once conducted the synthesis and design of these complicated systems with the application of the CRDC and derived a flowsheet comprising a CRDC, a conventional distillation column recycling the lightest reactant unconverted to the CRDC, and a product distillation column with an overhead decanter.14,15 The process is termed the CRDC flowsheet throughout the current work. We attempt to replace the CRDC with the RDC-TBER and then redesign the flowsheet, giving rise to the RDC-TBER flowsheet. Through strict comparison between the RDC-TBER and CRDC flowsheets in the aspects of capital investment (CI), operating cost (OC), and total annual cost (TAC, which includes utility cost and discounted capital investment by a specified payback time period), the advantages of the RDC-TBER are examined. Moreover, the effect of the external recycle on the performance of the RDC-TBER flowsheet is also analyzed, and some concluding remarks are finally summarized in the last section of the article.

k1 = (5.857 × 106)e−4097.8/ T

(3.1)

k −1 = (5.875 × 106)e−4097.8/ T

(3.2)

k 2 = (2.024 × 106)e−4201.1/ T

(3.3)

k −2 = (7.906 × 105)e−4201.1/ T

(3.4)

The equilibrium constants are independent of temperatures since the thermal heats of the reactions can be ignored. K1, eq = k1/k −1 = 0.997

(4.1)

K 2, eq = k 2/k −2 = 2.56

(4.2)

The normal boiling points of the reacting components, AA, MMA, DMA, H2O, and MeOH are 337.47 °C, 261.76 °C, 235.68 °C, 100.0 °C, and 64.53 °C, respectively. It is noted that the reactants MeOH and AA are the lightest and the heaviest components, respectively, with the intermediate and final products MMA, DMA, and H2O as the intermediate ones. Thus the reactive distillation columns to be designed belong to the system separating reacting mixtures with the most unfavorable ranking of relative volatilities. In particular, from the perspective of each reaction involved, the reactive distillation columns to be designed can still be seen as a system separating reacting mixtures with the most unfavorable ranking of relative volatilities. The processes to be developed here are simulated using the commercial software Aspen Plus. The NRTL-HOC property method is employed to describe vapor−liquid equilibrium and the Aspen Plus built-in parameters are adopted to compute the fugacity coefficients of the reacting components involved. The relevant physicochemical parameters are taken from refs 14 and 15, where their accuracy was ascertained.

2. EXAMPLE I: EMPLOYING THE RDC-TBER TO THE SEPARATION OF ADIPIC ACID ESTERIFICATION 2.1. Problem Description. Adipic acid (AA) and methanol (MeOH) undergo two reversible cascade reactions to form 16871

dx.doi.org/10.1021/ie402622f | Ind. Eng. Chem. Res. 2013, 52, 16870−16879


Article

Figure 2. Flowsheet for the separation of adipic acid esterification: (a) CRDC flowsheet and (b) RDC-TBER flowsheet.

Table 1. Comparison between the CRDC Flowsheet and RDC-TBER Flowsheet of Example I CRDC flowsheet

RDC-TBER flowsheet

parameter

CRDC

stripper

PDC

RDC-TBER

diameter (m) CI (103 $/yr) OC (103 $/yr) TAC (103 $/yr) overall CI (103 $/yr) overall OC (103 $/yr) overall TAC (103 $/yr) comparison of CI (%) comparison of OC (%) comparison of TAC (%)

1.443 128.242 291.037 419.279

1.056 69.899 162.178 232.077 202.273 472.712 674.985 100 100 100

0.247 4.132 19.497 23.629

1.434 128.415 289.545 417.960

2.2. CRDC Flowsheet. The flowsheet includes a CRDC, a stripper recycling the lightest reactant unconverted to the CRDC, and a product distillation column (PDC) with an overhead decanter. Four constraints need to be considered during process design: (1) temperature of the reactive section should be below 190 °C to avoid the deactivation of catalyst, (2) MMA composition in the outlet stream of the CRDC

stripper 0.604 36.348 54.998 91.346 168.895 364.039 532.934 83.50 77.01 78.95

PDC 0.247 4.132 19.496 23.628

should be less than 0.07 mol % to ensure the DMA composition in the bottom withdrawal of the PDC over 99.04 mol %, (3) MeOH composition in the bottom withdrawal of the stripper should be below 1 mol %, and (4) DMA composition should be over 99.04 mol % in the bottom withdrawal of the PDC. In the light of a grid-search method, Hung et al. generated an optimum design of the CRDC 16872



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flowsheet.14 While the CRDC includes totally 22 stages with the bottom 13 stages working as the reactive section, the stripper and PDC contain, respectively, 14 and 3 stages. The reactants AA and MeOH are both fed to the bottom reboiler of the CRDC in which the cascade reaction is allowed to occur. On the basis of the same steady-state operating conditions, we simulated the CRDC flowsheet with our developed model, and the obtained outcomes showed essentially a good agreement with those of Hung et al. In order to make a consistent comparison with the RDC-TBER flowsheet, hereinafter we will use our developed model to estimate the performance of the CRDC flowsheet, and the relevant results are indicated in Figure 2 and Table 1. 2.3. RDC-TBER Flowsheet. For the fair comparison between the CRDC flowsheet and RDC-TBER flowsheet, it is stipulated here that the number of stages in the RDC-TBER, stripper, and PDC of the latter flowsheet is kept the same as in the CRDC, stripper, and PDC of the former flowsheet. In particular, the same amount of catalyst is assumed in the RDCTBER as in the CRDC. The decision variables for process design include the location and flow rate of the outlet stream of the RDC-TBER, the number of reactive stages, and the flow rate of the external recycle from the top to the bottom of the RDC-TBER. Figure 3 depicts the relationship between the TAC and these decision variables. Here, the formulas by Douglas are employed to estimate the CI and OC,16 and a payback time of 3 years is adopted to calculate the TAC. The outlet stream of the RDC-TBER is found to locate at stage 4 (Figure 3a) with a flow rate of 79 kmol/h (Figure 3b). The reactive section consists of 9 stages, which is located from stage 14 to the bottom of the RDC-TBER (Figure 3c), and the flow rate of the external recycle is 95 kmol/h (Figure 3d). Below this value, it becomes impossible to find a feasible process design that meets all the constraints and product specifications. This is why the TAC takes actually a monotonous relationship with the flow rate of the external recycle in this situation instead of a parabolic one as usually expected. The resultant optimum design of the RDC-TBER flowsheet is sketched in Figure 2b. 2.4. CRDC Flowsheet versus RDC-TBER Flowsheet. Since the flow rate and composition of the bottom withdrawal of the stripper are kept the same in the RDC-TBER flowsheet as in the CRDC flowsheet, the design of the PDC shows no changes at all by the replacement of the CRDC with the RDCTBER. Figure 4 plots the relationship between the flow rate of the external recycle and the flow rate of the recycle from the stripper to the RDC-TBER under the condition of satisfying all the constraints and product specifications. It can readily be noted that they take approximately an inversely proportional relationship, implying that the external recycle of the RDCTBER has a similar effect on the reaction conversion as the recycle from the stripper to the RDC-TBER. However, their impacts on the overall system performance are quite different, and this can be inferred from the profiles of liquid compositions and temperatures of the CRDC and RDC-TBER as shown in Figure 5. The MeOH/DMA composition in the outlet stream of the RDC-TBER appears to be much lower/higher than that of the CRDC, and this demonstrates the advantages of using an external recycle from the top to the bottom of the RDC-TBER to replace a certain amount of the recycle from the stripper to the RDC-TBER in the RDC-TBER flowsheet. The comparison between the CRDC flowsheet and RDCTBER flowsheet is conducted in Table 1. The RDC-TBER flowsheet is found to be dramatically superior to the CRDC

Figure 3. TAC versus the key decision variables in the RDC-TBER flowsheet of Example I: (a) location of the outlet stream of the RDCTBER, (b) flow rate of the outlet stream of the RDC-TBER, (c) number of reactive stages of the RDC-TBER, and (d) flow rate of the external recycle. 16873



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Figure 4. Flow rate of the external recycle versus the flow rate of the recycle from the stripper to the RDC-TBER under the condition of satisfying all the constraints and product specifications.

flowsheet, securing a reduction of the CI, OC, and TAC by 16.50%, 22.99%, and 21.05%, respectively.

3. EXAMPLE II: EMPLOYING THE RDC-TBER TO THE SEPARATION OF GLUTARIC ACID ESTERIFICATION 3.1. Problem Description. Extremely analogous to the kinetics of adipic acid esterification with MEOH, glutaric acid (GA) and MeOH react through two reversible cascade reactions to produce dimethyl glutarate (DMG) and water (H2O) with monomethyl glutarate (MMG) as an intermediate product. k1

GA + MeOH HooI MMG + H 2O k −1

(5.1)

k2

MMG + MeOH HooI DMG + H 2O k −2

(5.2)

The reaction rates take the same expression as in eq 2 and the rate constants whose units are cm6(gcat·min·mol) are given as follows. k1 = (3.346 × 107)e−4417.4/ T

(6.1)

k −1 = (6.416 × 103)e−84.5/ T

(6.2)

k 2 = (3.487 × 105)e−2042.2/ T

(6.3)

k −2 = (2.024 × 106)e−4201.1/ T

(6.4)

The equilibrium constants depend heavily on temperatures because the two reactions are highly endothermic and exothermic, respectively. K1,eq = k1/k −1 = (5.215 × 103)e−4332.9/ T = 0.295 (at T = 443 K)

(7.1)

K 2,eq = k 2/k −2 = (1.723 × 10−1)e 2158.9/ T = 22.53 (at T = 443 K)

(7.2)

Figure 5. Profiles of liquid compositions and temperatures for Example I: (a) liquid compositions of the CRDC, (b) temperature of the CRDC, (c) liquid compositions of the RDC-TBER, and (d) temperature of the RDC-TBER.

With reference to the database of the commercial software Aspen Plus, the normal boiling points of the reacting components, GA, DMG, MMG, H2O, and MeOH, are found to be 322.13 °C, 197.12 °C, 172.28 °C, 100.0 °C, and 64.53 °C, respectively. While the reactants MeOH and GA are the lightest and the heaviest components, the intermediate and final products MMG, DMG, and H2O are the components in

between. Thus the reactive distillation columns to be designed should again be regarded as a kind of systems separating reactive mixtures with the most unfavorable ranking of relative 16874



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Figure 6. Flowsheet for the separation of glutaric acid esterification: (a) CRDC flowsheet and (b) RDC-TBER flowsheet.

Table 2. Comparison between the CRDC Flowsheet and RDC-TBER Flowsheet of Example II CRDC flowsheet

RDC-TBER flowsheet

parameter

CRDC

MRDC

PDC

RDC-TBER

diameter (m) CI (103 $/yr) OC (103 $/yr) TAC (103 $/yr) overall CI (103 $/yr) overall OC (103 $/yr) overall TAC (103 $/yr) Comparison of CI (%) comparison of OC (%) comparison of TAC (%)

0.925 75.150 83.900 159.050

0.738 62.957 77.896 140.853 143.460 182.979 326.439 100 100 100

0.356 5.353 21.183 26.536

0.921 76.540 84.920 161.460

volatilities. The flowsheets to be developed are still simulated using the commercial software Aspen Plus with the same methods as in Example I to estimate the vapor−liquid equilibrium relationship and the fugacity coefficients of the reacting components involved. The relevant physicochemical parameters are taken again from refs 14 and 15.

MRDC 0.645 53.918 59.633 113.551 135.813 165.746 301.559 94.67 90.58 92.38

PDC 0.356 5.355 21.193 26.548

3.2. CRDC Flowsheet. According to Hung et al.,14 the flowsheet includes a CRDC, a methanol recovery distillation column (MRDC), and a PDC with an overhead decanter. Their optimum design of the CRDC flowsheet is shown in Figure 6a and the detailed outcomes are listed in Table 2. The CRDC includes totally 22 stages and the bottom 12 stages are designated as the reactive section. The MRDC and PDC 16875



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Figure 7. TAC versus the key decision variables in the RDC-TBER flowsheet of Example II: (a) location of the outlet stream of the RDC-TBER, (b) flow rate of the outlet stream of the RDC-TBER, (c) number of reactive stages of the RDC-TBER, (d) flow rate of the external recycle, and (e) feed location of the MRDC.

decision variables. The outlet stream of the RDC-TBER is found to locate at stage 4 (Figure 7a) with a flow rate of 66 kmol/h (Figure 7b). There are 12 stages in the reactive section arranged in the bottom part of the RDC-TBER (Figure 7c), and the flow rate of the external recycle is 6 kmol/h (Figure 7d). The feed to the MRDC is introduced onto stage 7, one stage above the location in the corresponding unit of the CRDC flowsheet (Figure 7e). The resultant optimum design of the RDC-TBER flowsheet is sketched in Figure 6b. 3.4. CRDC Flowsheet versus RDC-TBER Flowsheet. The design of the PDC shows no changes, either, in this situation. Figure 8 plots the relationship between the flow rate of the external recycle and the flow rate of the recycle from the MRDC to the RDC-TBER under the condition of satisfying all the constraints and product specifications. Again, an approximate inversely proportional relationship is noticed between

contain 23 and 3 stages, respectively, and the reactants GA and MeOH are fed to the bottom reboiler of the CRDC where the cascade reaction is still allowed to occur. 3.3. RDC-TBER Flowsheet. Similar to Example I, the RDCTBER includes an external recycle from the top to the bottom because the esterification of glutaric acid with methanol is essentially highly endothermic. In addition to the assumption of the same amount of catalyst in the CRDC and RDC-TBER, the number of stages in the RDC-TBER, MRDC, and PDC of the RDC-TBER flowsheet is still kept the same as in the CRDC, MRDC, and PDC of the CRDC flowsheet. The decision variables for process design include the location and flow rate of the outlet stream of the RDC-TBER, the number of reactive stages, the flow rate of the external recycle from the top to the bottom of the RDC-TBER, and the feed stage of the MRDC. Figure 7 depicts the relationship between the TAC and these 16876



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Figure 8. Flow rate of the external recycle versus the flow rate of the recycle from the MRDC to the RDC-TBER under the condition of satisfying all the constraints and product specifications.

them. Although the external recycle of the RDC-TBER exhibits a similar effect on the reaction conversion as the recycle from the MRDC to the RDC-TBER, their impacts on the overall system performance are different, and this can be detected from the profiles of liquid compositions and temperatures of the CRDC and RDC-TBER as shown in Figure 9. The MeOH/ DMG composition in the outlet stream of the RDC-TBER is much lower/higher than that of the CRDC, and this demonstrates the advantages of using an external recycle from the top to the bottom of the RDC-TBER to replace a certain amount of the recycle from the MRDC to the RDCTBER in the RDC-TBER flowsheet. The comparison between the CRDC flowsheet and RDCTBER flowsheet is conducted in Table 2. The RDC-TBER flowsheet is found again to be superior to the CRDC flowsheet, securing a reduction of the CI, OC, and TAC by 5.33%, 9.42% and 7.62%, respectively.

4. DISCUSSION It is worthwhile to analyze here the detailed mechanism that leads to the reduction of CI, OC, and TAC in Examples I and II with the addition of an external recycle to the RDC-TBER flowsheet. Although the external recycle of the RDC-TBER exhibits a similar effect on the reaction conversion as the recycle from the second distillation column (i.e., the stripper or MRDC) to the RDC-TBER, the allowance of the purification of the lightest reactant unconverted in the top section above the outlet stream of the RDC-TBER helps to reinforce internal mass integration and/or internal energy integration between the reaction operation and the separation operation involved in addition to the reduction of the flow rate of the mixtures to be processed by the second distillation column (i.e., the stripper or MRDC). Despite the fact that the heat duty of reboiler appears to decrease slightly in Example I and even increase somehow in Example II (note the fact that this also accompanies with the recycle of more water to the reactive section in the RDC-TBER flowsheet than in the CRDC flowsheet), the modification still leads to not only the performance improvement in the RDCTBER but also the dramatic alleviation of the separation work in the second distillation column (i.e., the stripper or MRDC). This is why the RDC-TBER flowsheet can cut the CI, OC, and TAC considerably in comparison with the CRDC flowsheet. The use of sidestreams in the RDC-TBER flowsheet does have a favorable effect toward the performance of the RDCTBER flowsheet; however, it should not be considered here as the primary reason for such great degrees of improvement in

Figure 9. Profiles of liquid compositions and temperatures for Example II: (a) liquid compositions of the CRDC, (b) temperature of the CRDC, (c) liquid compositions of the RDC-TBER, and (d) temperature of the RDC-TBER.

system performance over the CRDC flowsheet. Since the locations for withdrawing the sidestreams are only three stages below the top stage in the two examples studied, their impacts 16877



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21376018 and The Doctoral Programs Foundation of Ministry of Education of China under Grant 20100010110008.

should be rather limited. On the other hand, the addition of an external recycle between the top and the bottom of the RDCTBER serves actually to be the main impetus for the great enhancement of system performance from the CRDC flowsheet to the RDC-TBER flowsheet. It is interesting to compare here the effect of the RDC-TBER on the improvement of system performance between the two RDC-TBER flowsheets for the adipic acid and glutaric acid esterifications, respectively. While for the separation of adipic acid esterification the TAC has been cut by 21.05% in comparison with the corresponding CRDC flowsheet, for the separation of glutaric acid esterification, the number has been lowered to 7.62%. The great difference is mainly due to the fact that glutaric acid esterification proceeds much faster than adipic acid esterification, and as a result, the addition of an external recycle between the top and bottom of the RDC-TBER presents a much weaker impact to the separation of glutaric acid esterification than to the adipic acid esterification. This interpretation can be confirmed by the fact that the enhancement in the composition of DMA is much greater from the outlet stream of the CRDC to the outlet stream of the RDC-TBER of Example I than the one in the composition of DMG from the outlet stream of the CRDC to the outlet stream of the RDC-TBER of Example II. Moreover, the ratio between the flow rate of the external recycle and the recycle flow rate from the second distillation column (i.e., the stripper or the MRDC) to the RDC-TBER is also a good indication to this interpretation. While for the separation of adipic acid esterification the ratio is 95 kmol/h:49 kmol/h = 1.9, for the separation of glutaric acid esterification, the ratio becomes only 6 kmol/h:36 kmol/h = 0.17.

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REFERENCES

(1) Taylor, R.; Krishna, R. Modelling Reactive Distillation. Chem. Eng. Sci. 2000, 55, 5183. (2) Malone, M. F.; Doherty, M. F. Reactive Distillation. Ind. Eng. Chem. Res. 2000, 39, 3953. (3) Kaymak, D. B.; Luyben, W. L. Quantitative Comparison of Reactive Distillation with Conventional Multiunit Reactor/ Column/ Recycle Systems for Different Chemical Equilibrium Constants. Ind. Eng. Chem. Res. 2004, 43, 2493. (4) Tang, Y. T.; Chen, Y. W.; Huang, H. P.; Yu, C. C.; Hung, S. B.; Lee, M. J. Design of Reactive Distillations for Acetic Acid Esterification. AIChE J. 2005, 51, 1683. (5) Kaymak, D. B.; Luyben, W. L. Effect of Relative Volatility on the Quantitative Comparison of Reactive Distillation and Conventional Multi-unit Systems. Ind. Eng. Chem. Res. 2004, 43, 3151. (6) Chen, C. S.; Yu, C. C. Effects of Relative Volatility Ranking on Design and Control of Reactive Distillation Systems with Ternary Decomposition Reactions. Ind. Eng. Chem. Res. 2008, 47, 4830. (7) Thotla, S.; Mahajani, S. Reactive Distillation with Side Draw. Chem. Eng. Process. 2009, 48, 927. (8) Tung, S. T.; Yu, C. C. Effects of Relative Volatility Ranking to the Design of Reactive Distillation. AIChE J. 2007, 53, 1278. (9) Chen, H.; Huang, K.; Zhang, L.; Wang, S. Reactive Distillation Column with a Top-Bottom External Recycle. Ind. Eng. Chem. Res. 2012, 51, 14473. (10) Chen, H.; Huang, K.; Liu, W.; Zhang, L.; Wang, S.; Wang, S. J. Enhancing Mass and Energy Integration by External Recycle in Reactive Distillation Columns. AIChE J. 2013, 59, 2015. (11) Huang, K.; Nakaiwa, M.; Wang, S. J.; Tsutsumi, A. Reactive Distillation Design with Considerations of Heats of Reaction. AIChE J. 2006, 52, 2518. (12) Sun, J.; Huang, K.; Wang, S. Deepening Internal Mass Integration in Design of Reactive Distillation Columns, 1: Principle and Procedure. Ind. Eng. Chem. Res. 2009, 48, 2034. (13) Huang, K.; Lin, Q.; Shao, H.; Wang, C.; Wang, S. A Fundamental Principle and Systematic Procedures for Process

5. CONCLUSIONS In the current study, the RDC-TBER has been applied to the separations of two two-stage cascade reacting mixtures (i.e., the adipic acid and glutaric acid esterifications with MEOH) with the most unfavorable ranking of relative volatilities (i.e., the reactants are the lightest and the heaviest components with the intermediate and final products in between). The RDC-TBER flowsheet appears to be dramatically superior to the CRDC flowsheet because of the addition of an external recycle between the top and the bottom of the RDC-TBER. It serves to strengthen internal mass integration and/or internal energy integration between the reaction operation and the separation operation involved in the RDC-TBER, leading to a considerable reduction in the capital investment and operating cost of the stripper and MRDC. These results have demonstrated again that the RDC-TBER is a more favorable option than the CRDC in the separations of complicated reacting mixtures with the most unfavorable ranking of relative volatilities.

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NOTATION AA = adipic acid C = molar composition, mol/cm3 CI = capital investment, $/yr CRDC = conventional reactive distillation column DMA = dimethyl adipate DMG = dimethyl glutarate F = feed flow rate, kmol/h GA = glutaric acid H2O = water k = reaction rate costant, cm6/(gcat·min·mol) Keq = equilibrium constant MeOH = methanol MMA = monomethyl adipate MMG = monomethyl glutarate MRDC = methanol recovery distillation colum OC = operating cost, $/yr PDC = product distillation colum Q = reboiler heat duty, kW RDC-TBER = reactive distillation column with a top-bottom external recycle R = reflux flow rate, kmol/h T = temperature, K TAC = total annual cost, $/yr α = relative volatility

AUTHOR INFORMATION

Corresponding Author

*Phone: +86 10 64434801. Fax: +86 10 64437805. E-mail: [email protected] (K. Huang). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The current work is financially supported by The National Science Foundation of China under Grants 21076015 and 16878



Article

Intensification in Reactive Distillation Columns. Chem. Eng. Process. 2010, 49, 294. (14) Hung, S. B.; Lai, I. K.; Huang, H. P.; Lee, M. J.; Yu, C. C. Reactive Distillation for Two-Stage Reaction Systems: Adipic Acid and Glutaric Acid Esterifications. Ind. Eng. Chem. Res. 2008, 47, 3076. (15) Lee, M. J.; Lai, C. H.; Wang, T. B.; Lin, H. M. Vapor-Liquid Equilibrium of Mixtures Containing Adipic Acid, Glutaric Acid, Dimethyl Adipate, Dimethyl GLutarate, Methanol, and Water. J. Chem. Eng. Data 2007, 52, 1291. (16) Douglas, J. M. Conceptual Design of Chemical Processes; McGrawHill: New York, 1988.

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