A Systematic Method for the Development of Operating Policies for

Article pubs.acs.org/IECR

A Systematic Method for the Development of Operating Policies for Two-Step Processes with Semibatch Reactive Distillation Po-Hsien Lee, Yu-Lung Kao, and Jeffrey D. Ward* Department of Chemical Engineering National Taiwan University Taipei 10617, Taiwan S Supporting Information *

ABSTRACT: Semibatch reactive distillation (SBRD) is a variation on batch reactive distillation (BRD) in which one reactant or an entrainer is fed to the process continuously during the batch. Qi and Malone (Qi, W; Malone, M. F. Semibatch Reactive Distillation for Isopropyl Acetate Synthesis. Ind. Eng. Chem. Res. 2011, 50, 1272−1277) suggested SBRD for the synthesis of isopropyl acetate with acetic acid fed continuously at the top of the column to reduce the composition of the alcohol in the distillate product. The distillate product from the SBRD is a mixture of water and ester which can subsequently be separated by inverted batch distillation (IBD). The optimization of such a two-step batch process involves trade-offs between the two steps (SBRD and IBD). In this work, an algorithm for systematic design of such processes is developed and applied to processes for the synthesis of isopropyl acetate and ethyl acetate. The key variable connecting the two batch steps is the composition of alcohol in the SBRD distillate product. Allowing a higher alcohol composition makes SBRD operation easier but IBD operation more difficult. The algorithm identifies the alcohol concentration which minimizes total process time, which is closely related to energy consumption, as well as side feed flow rate and reflux ratio profiles during the batches.

1. INTRODUCTION Distillation is the most widely used method of separation in the chemical industry, and possible modes of operation include continuous, batch, and semibatch.1 Batch or semibatch operation may be more suitable than continuous operation for small-volume products and offers advantages including shorter process development time, easier startup and shutdown, and flexible operation. When reaction occurs in the pot or on the stages of a batch distillation column, the unit is called batch reactive distillation (BREAD), and when a side feed is incorporated in a BREAD process, the unit is called semibatch reactive distillation (SBRD). Figure S1 in the Supporting Information shows the configuration of a semibatch distillation column for which operation is similar to that of conventional batch distillation (CBD). The difference is that instead of adding all material in the pot initially, in an SBRD process one or more species are fed continuously during the batch via a side feed. A number of researchers have studied BREAD and SBRD process designs. Guo et al.2,3 first studied the feasibility of BREAD by considering residue curve maps and reaction equilibrium. Chin et al.4 provided a new feasibility evaluation procedure for complex BREAD system including rectifierdecanter and homogeneous and heterogeneous BREAD columns. Chin and Lee5 developed a method for evaluating the liquid still composition trajectory of BREAD system. Steger et al.6 provided a graphical feasibility method to investigate BREAD in middle-vessel columns (MVC) with the fully reactive, nonreactive, and hybrid configurations. © 2016 American Chemical Society

Esterification reactions are an important class of reactions which are usually reversible and therefore well-suited to reactive distillation. Huss et al.,7 Tang et al.,8,9 Lei et al,.10,11 and Hu et al.12 studied reactive distillation for processes with esterification reactions in continuous systems with different design configurations. Esterification of acetic acid with various alcohols has been studied as a model system in BREAD and SBRD design.13−15 Synthesis of ethyl acetate and isopropyl acetate by BREAD is challenging because the lightest stationary point is a ternary azeotrope of alcohol, acetate, and water. Mujtaba and Macchietto16 developed optimized strategies for BREAD processes for the ethyl acetate system without considering the azeotropes in the system. Steger et al.6 provided five configurations for an MVC column to produce ethyl acetate by semibatch reactive distillation with acetic acid as side feed and infinite reflux ratio. Modla17 described a reactive pressure swing two-column process. Chin et al.5 used chloroform as a light entrainer in the isopropyl acetate system. Qi and Malone13 suggested an alternative using SBRD for the isopropyl acetate system with acetic acid as a side feed. They showed that in this way the concentration of alcohol in the distillate can be reduced efficiently. A second nonreactive inverted batch distillation (IBD) operation can be employed to separate the product IPAC from water. Therefore, SBRD has Received: Revised: Accepted: Published: 8602

January 20, 2016 July 3, 2016 July 11, 2016 July 11, 2016 DOI: 10.1021/acs.iecr.6b00275 Ind. Eng. Chem. Res. 2016, 55, 8602−8615

Article

Industrial & Engineering Chemistry Research Table 1. Kinetic Model for the IPAC System kinetic modela (Amberlyst 15)

k1 (T = 363 K)

Keq (T = 363 K)

2.26 × 10−4

8.7

Langmuir−Hinshelwood/Hougen-Watson Model

r = mcat

k1(aHOACaIPOH − aIPACaH2O/K eq) (1 + KHOACaHOAC + KIPOHaIPOH + KIPACaIPAC + KH2OaH2O)2

⎛ 68620.43 ⎞⎟ k1 = 7.667 × 10−5 exp⎜23.81 − ⎝ RT ⎠ KHOAC = 0.1976, KIPOH = 0.2396, KIPAC = 0.147, KH2O = 0.5079 a

Units: r, (kmol/s); mcat, (kgcat); k1, (kmol/kgcat·s); R = 8.314 (J/mol·K); T (K).

Table 2. Kinetic Model for the ETAC System kinetic modela (Amberlyst 35)

k1 (T = 363 K)

Keq (T = 363 K)

61.47

2.96

Pseudohomogeneous Model

r = mcat (k1C HOACCETOH − k −1C ETACC H2O) ⎛ − 6105.6 ⎞ ⎟ k1 = 1.24 × 109 exp⎜ ⎝ T ⎠ ⎛ − 5692.1 ⎞ ⎟ k −1 = 1.34 × 108 exp⎜ ⎝ T ⎠ a

Units: r, (mol/min); mcat (gcat); k1 (cm6/gcat·mol·s); R = 8.314 (J/mol·K); T (K), Ci (mol concentration).

the catalyst, 770 (kg/m3), is assumed to be the same as for the isopropyl acetate system. 2.2. Thermodynamic Model. Certain physical properties of each species in the IPAC system are given in Table S1 in the Supporting Information. The thermodynamic model used for the IPAC system is the universal quasichemical (UNIQUAC) activity coefficient model. Dimerization of acetic acid in the vapor phase is described by the Hayden−O’Connell model. Thermodynamic calculations are performed using Aspen Properties. A comparison between the predicted azeotropes from the Aspen UNIQUAC model and experimental azeotropic result from Horsley19 is shown in Table 3. The IPAC system has one ternary heterogeneous azeotrope and three binary homogeneous azeotropes.

the potential to improve the production efficiency. However, they considered only the SBRD step, not the IBD step, and therefore do not determine the optimal alcohol concentration in the BREAD product. In this work we study the optimization of two-step batch processes for the production of ethyl acetate and isopropyl acetate. Acetic acid and the alcohol are both impurities in the SBRD product but the second column (IBD) is only effective in removing the alcohol, in the SBRD a constraint is set on the acetic acid concentration in the distillate product. The amount of alcohol impurity in the SBRD product is a design variable and the most important variable affecting both columns. When the constraint on the alcohol impurity is relaxed, the energy required for the SBRD decreases and the energy required for the IBD increases. The algorithm identifies the value of the alcohol impurity in the SBRD product that minimizes total energy consumption.

Table 3. Azeotropes in the IPAC System UNIQUAC simulation

2. MODELS AND METHODS 2.1. Reaction Kinetics. The reaction of isopropyl alcohol and acetic acid to form water and isopropyl acetate is a reversible reaction. A solid acidic ion-exchange resin catalyst (Amberlyst 15) is assumed.

component IPOH−IPAC−H2Oa IPAC−H2Oa IPOH−H2O

isopropyl alcohol + acetic acid

IPOH−IPAC

↔ water + isopropyl acetate a

Table 1 shows the reaction kinetic model taken from Gadewar et al.18 The density of the catalyst is assumed to be 770 (kg/ m3) which is used to convert the rate of reaction from weight basis into volume basis in the modeling code. Ethanol and acetic acid also react reversibly to form ethyl acetate and water. A different solid acidic ion-exchange resin catalyst (Amberlyst 35) is assumed in this case.

mole fraction (0.2152, 0.4065, 0.3783) (0.5924, 0.4076) (0.6451, 0.3549) (0.6641, 0.3359)

temp (°C) 75.39 76.83 79.37 80.43

experimental data mole fraction (0.1377, 0.4938, 0.3885) (0.5982, 0.4018) (0.6875, 0.3125) (0.6508, 0.3492)

temp (°C) 75.5 76.6 82.5 80.1

Heterogeneous azeotropes are denoted by boldface.

Physical properties of species in the ethyl acetate system are shown in Table S2 in the Supporting Information. The thermodynamic model used for this system is the nonrandom two-liquid (NRTL) activity coefficient model, and again the dimerization of acetic acid in the vapor phase is modeled with the Hayden−O’Connell model. The binary interaction parameters are taken from Tang et al.8 Table 4 shows a comparison between the simulated azeotropes from the NRTL model and experimental result from Horsley.19 The ETAC system has two binary homogeneous azeotropes and one

ethanol + acetic acid ↔ water + ethyl acetate

Table 2 shows the reaction kinetic model which is taken from Lai et al.11 Ci is the concentration of species i. The density of 8603

DOI: 10.1021/acs.iecr.6b00275 Ind. Eng. Chem. Res. 2016, 55, 8602−8615

Article

Industrial & Engineering Chemistry Research

the total liquid feed to the decanter, that is, R = L/(L+D) where L is the liquid reflux rate and D is the distillate rate. For a decanter with two liquid phases the reflux ratio for each phase is defined with respect to the flow rates of that phase only. Also, a constant vapor rate is assumed which is the same for both processing steps (i.e., SBRD and IBD). This corresponds to making the best use of available capital equipment, since the vapor rate is constrained by the column diameter (to prevent liquid entrainment). Although it would be difficult to measure and control the vapor rate during column operation, the model can be used to infer a reboiler duty or reboiler temperature profile which can achieve a nearly constant vapor rate. It is also assumed in this work that because the vapor rate is constant in both processing steps, the total energy consumption is closely related to the total combined batch time of the two processing steps. Like other modeling assumptions this is only an approximation of the exact behavior of the process since the molar enthalpy of vaporization of the components is different and therefore the rate of energy consumption will be somewhat different depending on the composition of the pot if the vapor rate is constant. However, the pot composition profile is likely to be similar for different operating policies and therefore the total combined batch time is closely related to the total amount of energy consumed.

Table 4. Azeotropes in the ETAC System NRTL simulation component

mole fraction

ETOH− ETAC−H2O ETAC−H2Oa ETOH−H2O ETOH−ETAC

(0.1070, 0.6074, 0.2856) (0.6869, 03131) (0.4571, 0.5429) (0.9016, 0.0984)

a

temp (°C) 70.09 70.37 71.81 78.18

experimental data mole fraction (0.1126, 0.5789, 0.3085) (0.6885, 0.3115) (0.462, 0.538) (0.9037, 0.0963)

temp (°C) 70.23 70.38 71.81 78.17


ternary homogeneous azeotrope. Compared with the IPAC system, most things are similar but the boiling points of all stationary points are somewhat lower and the ternary azeotrope is outside the liquid−liquid region. For each system a single set of binary interaction parameters was found to accurately describe both vapor−liquid and liquid− liquid equilibria. Flash calculations were performed in Aspen Properties, and the results were transferred to MATLAB by a custom FORTRAN routine. 2.3. Column Configurations. In this study, both systems have two processing steps. The first step is semibatch reactive distillation (SBRD), and the second step is inverted batch distillation (IBD). This section describes the configuration of each unit. Figure 1 shows a schematic diagram of the SBRD process and the nonreactive IBD. In this work it is assumed that reaction occurs only in the pot. The column section above the side feed is called the rectifying section and the section below the side feed is called the extractive section. The condenser condenses all overhead vapor into liquid. Then the condensed liquid is stored in the reflux drum, which is also a decanter. Therefore, two receivers are used to collect the organic phase and the aqueous phase separately. Organic reflux ratio and aqueous reflux ratio can be manipulated independently. A process model is built in MATLAB. The following assumptions are used to develop the model: (1) constant molar liquid holdup on trays (2) negligible vapor holdup (3) reaction takes place only in the pot, not on the trays (4) trays have constant molar overflow The equations that constitute the model are provided in the Appendix. In this work, reflux ratios are defined with respect to

3. ISOPROPYL ACETATE SYSTEM 3.1. Design Concept. Table 5 shows the boiling points of all stationary points (pure components and azeotropes) of the Table 5. Boiling Points of Stationary Points in the IPAC System at 1 atm

a

component

boiling point (°C)

mole fraction

IPOH−IPAC−H2Oa IPAC−H2Oa IPOH−H2O IPAC−IPOH IPOH IPAC H2O HOAC

75.39 76.83 79.37 80.43 82.05 88.52 100.02 118.01

(0.2152, 0.4065, 0.3783) (0.5924, 0.4076) (0.6451, 0.3549) (0.6641, 0.3359) 1 1 1 1


Figure 1. Schematic diagrams of an SBRD with a decanter and a nonreactive IBD with a decanter. 8604


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purity IPAC product can be collected in the bottom receiver and high purity water is left in the decanter. 3.2. Development of Operating Policies. In the IPAC system, there are many variables that can be adjusted which are highly interdependent, including initial charge ratio, vapor flow rate, total number of stages, reflux ratio, and others. To decrease the numbers of variables, in this work it is assumed that the column properties are fixed. Therefore, three manipulated variables remain for the SBRD operation: (1) organic reflux ratio, (2) aqueous reflux ratio, and (3) side feed flow rate. In the IBD, the only manipulated variable is the boilup ratio. Among the three manipulated variables, the influence of the aqueous reflux ratio is investigated first. Three cases are compared to see whether the result can be improved by changing the aqueous reflux ratio. The aqueous reflux ratio conditions of the three cases are shown in Table S3 in the Supporting Information. In these cases, the optimal design is determined for each aqueous reflux ratio using the procedure described later. The others parameters in all cases are the same at the beginning of the process. The results indicate that the organic reflux ratio profile and side feed flow rate profile are similar for all three values of aqueous reflux ratio. The results are listed in Table S3 in the Supporting Information and Table 6. The results given in Table S3 show

IPAC system at 1 atm pressure. Figure 2 shows a ternary diagram for IPOH, IPAC and water including the liquid−liquid

Figure 2. Ternary diagram for IPOH, IPAC, and WATER system. The liquid−liquid envelope is calculated at 40 °C.

envelope at 40 °C. Venimadhavan et al.14 propose a method using conventional batch reactive distillation for esterification reactions that is feasible for the IPAC system. The lightest stationary point which is a ternary azeotrope (IPOH−IPAC− H2O) is located in the two-liquid region and will split into two phases in the decanter. However, when a liquid mixture with the composition of the ternary azeotrope splits into two liquid phases, the aqueous phase contains only 95 mol % water, and 3 mol % of the reactant IPOH is lost in the organic phase. According to Qi and Malone,13 SBRD can improve the production efficiency and produce high purity water in the aqueous phase. In this study, a process with two steps is designed. The first unit is an SBRD which uses HOAC as an inherent entrainer so that a mixture of water and IPAC with only a small amount of IPOH is collected at the top. The second unit is an IBD column to further purify the main product IPAC. In the SBRD, operation consists of two periods. The first period is total reflux. The second period is the product collection period. IPOH is charged in the pot at the beginning. When the process starts, HOAC is fed into the column. In the extractive section, the HOAC extracts IPOH down to decrease the IPOH composition in the distillate. In the rectifying section, the concentration of HOAC can be reduced to a very small amount. The distillate is dragged close to the H2O−IPAC edge of the ternary diagram and splits into two phases in the decanter. High purity water can be collected in the aqueous phase receiver and a binary mixture of two products (IPAC and H2O) can be collected in the organic phase receiver. An excess of HOAC accumulates in the pot at the end of the process. The organic product is used in the second step (IBD) as an initial charge. In the IBD, the organic phase is initially charged in the reflux drum before the process starts. After the process starts, the lightest component which is binary azeotrope of IPAC and water can be obtained as the distillate and the heaviest component which is the main product IPAC can be collected at the bottom of the column by refluxing all of the organic phase back to the IBD column and allowing the aqueous phase to accumulate in the decanter. At the end of the process, high

Table 6. Pot Composition (comp) and Holdup at the End of the Process aqueous reflux ratio

IPOH

HOAC

water

IPAC

0.95

0.0316 0.91 0.0357 0.84 0.0317 0.69

0.7466 21.39 0.7539 17.84 0.7967 17.28

0.2018 5.78 0.1850 4.38 0.1368 2.97

0.0200 0.57 0.0255 0.60 0.0348 0.75

0.90 0.85

comp hold up comp hold up comp hold up

that the aqueous reflux ratio has only a small effect on the process operation. A lower aqueous reflux ratio leads to a slightly better result both for the process time and the product purity. Table 6 also shows a small improvement in the pot composition which has a higher purity of HOAC at the end of the process. However, when the aqueous reflux ratio is lower than 0.80, IPOH will come up from the bottom of the column as shown in Figure 3. The first 8 h is the total reflux period. Then in the early part of the product collection period (8−12 h), compared with a reflux ratio of 0.85, when the aqueous reflux ratio is equal to 0.8 the composition of alcohol in the distillate is much higher than the constraint. Therefore, the aqueous reflux ratio is fixed at 0.85 in the subsequent optimization. Table 7 shows the remaining variables which require pairing between manipulated variable and constrained variable. Because both variables greatly affect both impurities simultaneously, it is difficult to manipulate both variables at the same time during the optimization. When the side feed flow rate is increased, the IPOH composition in the reflux drum will decrease and HOAC composition will increase. However, when the organic reflux ratio is increased, both constrained variables decrease simultaneously. Also, a small change in the organic reflux ratio will have a big influence on the constrained variables. It is difficult to maintain the values of both controlled variables 8605


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Figure 3. Reflux drum composition profiles for two different aqueous reflux ratios.

to the process time. Therefore, for optimization, at the beginning of the product collection period the organic reflux ratio is set to the minimum value which can meet both constraints. Thus, for the IPAC system, the design method leads to the flowsheet shown in Figure 4, which can be applied for the design of the IPAC SBRD process. The flowchart can be applied to design processes for different values of the constraint on the alcohol composition, and the value of the alcohol constraint that minimizes the combined batch time can be determined. In the IBD, the final goal is to collect 90% of the IPAC in the organic product from the SBRD. For example, if the total organic product is 11 kmol including 9 kmol of IPAC, the process ends when the 8.1 kmol IPAC has been collected. The product constraint is that the IPAC product must be 99% pure. The boilup ratio is manipulated to control the purity of IPAC. In this work, processes are designed with constraints on the product yield; that is, the yield of each step is required to be 90% for an overall yield of 81%. The yield is a design variable, and so in theory it could be determined automatically by the algorithm, for example the value of the yield that maximized the profit of the process could be determined. However, since unconsumed material is recycled to the next batch, this would require consideration of batch-to-batch operation which is beyond the scope of this work (although it is demonstrated later that batch-to-batch recycle of unconsumed acetic acid is feasible). Alternatively, the proposed method could be evaluated for different values of the yield specification and the engineer could then make a choice among the options based on engineering judgment. When the constraint on the IPOH composition in the organic SBRD product is increased (more IPOH is allowed), the SBRD process time decreases and the IBD process time increases. Therefore, the simulation is conducted for several values of the IPOH constraint. The time required for the IBD and the SBRD processes are added together to determine the total process time. Finally, the total process time for different IPOH constraints is determined and the IPOH constraint

Table 7. Variables for SBRD Operation manipulated variables

constrained variables

organic reflux ratio side feed flow rate

IPOH composition in reflux drum HOAC composition in reflux drum

precisely at the constraints. Furthermore, the concentration of HOAC in the reflux drum changes only from 10−3 to 10−5 during the entire process, so it is difficult to control HOAC impurity at a precise value. Therefore, a spec is set for the IPOH impurity in the reflux drum but only an upper limit is set for the HOAC impurity in the reflux drum. The side feed flow rate is manipulated to achieve the IPOH concentration objective in the reflux drum. The organic reflux ratio is manipulated if necessary to keep HOAC concentration less than 10−3 in the reflux drum. When the process starts, the side feed flow rate of HOAC is fixed at 1 kmol/h under total reflux until the IPOH impurity in the distillate is lower than the constraint. During the product collection period, the goal is to collect 9 kmol IPAC which is 90% of the initial charge of IPOH. The side feed flow rate is changed every 2 h to meet the constraint on the IPOH composition. The organic reflux ratio is changed only when the HOAC concentration in the distillate is higher than the constraint (0.1%) which is checked every 2 h. For example, for the first 2 h period in the product collection period (8−10 h), the initial value for the side feed flow rate is 1 kmol/h which comes from the previous period. However, at the end of this time interval (10 h), the IPOH impurity is higher than constraint. Therefore, the side feed flow rate is increased slightly, and the same 2 h interval is simulated again. The process is repeated until the IPOH impurity meets the constraint. Then, the process can proceed to next 2 h time interval. Also, among the two manipulated variables, the organic reflux ratio is changed before the side feed flow rate. That is, if neither impurity meets the constraint, the organic reflux ratio is changed first and the time interval is simulated again. The step size of the side feed flow rate is 0.01 kmol/h and the step size of the organic reflux ratio is 0.1. The reflux ratio is closely related 8606


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kmol; (4) holdup of reboiler, 0.5 kmol; (5) decanter temperature, 40 °C; (6) vapor rate, 5kmol/h. Table 8 shows the minimum process time for different IPOH composition constraints. As the constraint on the IPOH composition in the SBRD product is increased (more IPOH is allowed), the organic reflux ratio and SBRD process time decrease in the SBRD. Conversely, the IBD process time increases. The results show that the required time for the SBRD changes only slightly compared to that for the IBD process as the constraint on the IPOH composition is increased. The minimum total process time corresponds to a constraint of 1% IPOH in the SBRD product. Details about the combined process (SBRD and IBD) with the miniumum process time are as follows. The constraint on the IPOH composition is 1%. Simulation results for the SBRD operation are shown from Figure 5 to Figure 9. The side feed flow rate profile and the organic reflux ratio profile are shown in Figure 5. The first 8 h is a total reflux period which lasts until the IPOH composition in the reflux drum is less than 1%. In this period, the side feed flow rate is fixed at 1 kmol/h. Then the organic reflux ratio is reduced to 0.83 which can satisfy the constraints during the product collection period. As the operation continues, the side feed flow rate becomes smaller. At the later part of the process (24−27.4 h), the IPAC composition becomes lower which causes more IPOH to go up from the pot, so the side feed flow rate is increased to extract the IPOH from the vapor. At the same time, due to the increase in the side feed flow rate, the HOAC impurity in the reflux drum increases. Therefore, the organic reflux ratio also increases as the side feed flow rate increases. The total amount of HOAC which is added as the side feed is 27.9 kmol. Figure 6 shows the pot composition profile and the pot component holdup profile. At the beginning, the pot is filled with pure IPOH. After the process starts, HOAC is added into the column and reacts with IPOH to form water and IPAC. Hence, the amount of IPOH decreases with time. An excess of HOAC is added into the column resulting in the accumulation of HOAC in the pot because HOAC is the heaviest component. Also, IPAC and water are generated in the pot and both concentrations gradually increase during the early part of the process (0−7 h). After that, IPAC and water concentration decreases because of the accumulation of HOAC. After 8 h of total reflux, both IPAC and water are collected in the receiver during the product collection period, so the concentration of IPAC and water decreases in the pot. Figure S2 in the Supporting Information shows the reflux drum composition profile. Figure 7 shows the organic receiver composition profile and holdup. Figure 8 show the aqueous receiver composition profile and holdup. During the first 8 h of the total reflux period, the HOAC side feed extracts IPOH back to the pot, so the IPOH concentration decreases gradually in the distillate. The total reflux period ends when the IPOH impurity in the reflux drum is less than 1%. During the product

Figure 4. Flowchart for IPAC system operating policy development.

which corresponds to the minimum total process time is identified. Since both processes have the same vapor rate, the time required is closely related to the amount of energy consumed. 3.3. Results. In the first step of the process, an SBRD model is built with following setup: (1) total number of stages, 30; (2) pot diameter, 1.5 m; (3) pot height, 2 m; (4) total condenser temperature, 40 °C; (5) initial charge, 10 kmol of IPOH; (6) constant pressure of 1 atm; (7) holdup on each tray, 0.02 kmol; (8) vapor rate, 5 kmol/h; (9) tray of side feed, 5. In the second step of the process, an IBD model is built with the following setup: (1) total number of trays, 10; (2) constant pressure of 1 atm on each tray; (3) holdup on each tray, 0.06

Table 8. Total Process Time for Different Values of the IPOH Composition Constraint IPOH constraints on reflux drum in SBRD

0.5%

1%

2%

3%

4%

5%

SBRD

0.84 28.30 0.51 1.71 30.00

0.83 27.46 0.55 1.93 29.31

0.82 27.16 0.59 2.40 29.55

0.82 26.96 0.66 3.29 30.25

0.81 26.88 0.75 4.97 31.85

0.81 26.70 0.78 5.68 32.38

initial organic reflux ratio process time (h) IBD boilup ratio process time (h) total process time (h)

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Figure 5. Manipulated variable profiles for the IPAC system.

Figure 6. Pot composition profile and component hold up profile for the IPAC system.

Figure 7. Organic receiver composition and accumulation profiles for the IPAC system.

collection period, the IPOH concentration in distillate continues to decrease. Therefore, the reflux drum composition approaches the H2O-IPAC edge. Only 1% IPOH and a trace amount of HOAC are found in the distillate. A mixture which is primarily IPAC and water can be collected in the organic receiver. Table 9 shows the receiver composition and holdup at the end of the process. An excess of HOAC with a purity of 78% remains in pot. A mixture of IPAC and water with 1% IPOH

and a trace amount HOAC is collected in the organic receiver. The amount of the IPAC in the organic receiver is 9.00 kmol, consistent with the yield specification. The second unit in this process is an IBD column for the purification of the IPAC product. The initial charge is the contents of the organic receiver from the SBRD unit, which contains a total of 11.6 kmol including 9.00 kmol of IPAC. Figure S3 in the Supporting Information shows the decanter 8608


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Figure 8. Aqueous receiver composition and accumulation profiles for the IPAC system.

Table 9. Receiver Composition (comp) and Holdup at the End of the Process organic receiver IPOH HOAC H2O IPAC Total

Table 10. Product Receiver Composition and Holdup at the End of the Process

aqueous receiver

comp

holdup (kmol)

comp

holdup (kmol)

0.0139 0.0006 0.1327 0.8528

0.15 0.01 1.40 9.00 10.6

0.0020 0.0003 0.9930 0.0047

0.01 1 × 10−03 4.60 0.02 4.6

IPOH HOAC H2O IPAC Total

composition

holdup (kmol)

0.0047 0.0008 0.0030 0.9915

0.04 0.01 0.02 8.10 8.17

SBRD process requires much more time. When the constraint on the isopropyl alcohol composition in the SBRD product, which is the most important variable affecting both columns, is increased, the SBRD process time decreases and the IBD process time increases. However, the required time for the SBRD process changes only slightly compared with to IBD process as the amount of IPOH in the distillate is increased. Therefore, the minimum process time is at a constraint of only 1% IPOH. Also, because of the constant vapor rate, the process time is closely related to the energy consumption. The value of the IPOH constraint that minimizes total energy consumption can be determined.

component holdup profile. Figure 9 shows the product receiver composition and component holdup profile. In Figure S3, because in the IBD column only the organic phase is refluxed while the aqueous phase is retained in the decanter, the IPAC concentration decreases with time in the column. Figure 9 shows that the product purity of IPAC increases during most of the process time. At the end of the process, a high purity IPAC product can be collected in the receiver and high purity water is left in the decanter. Table 10 lists the receiver composition and holdup at the end of the process. High purity IPAC (0.9915) is collected. The yield of IPAC in the second unit IBD is 0.900 and total yield of IPAC is 0.810. In this section, the optimization of the IPAC system using SBRD and IBD is studied. Compared to the IBD process, the

4. ETHYL ACETATE SYSTEM 4.1. Design Concept. Table 11 shows the boiling points of all stationary points (pure components and azeotropes) of the

Figure 9. Product receiver composition and accumulation profile for the IPAC system. 8609


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process. As the process proceeds, the manipulated variables which are the organic reflux ratio and side feed flow rate change only slightly for different values of the aqueous reflux ratio. The results are listed in Table S4 in the Supporting Information and Table 12. The results in Table S4 show that

Table 11. Boiling Points of Stationary Points in the ETAC System at 1 atm

a

component

boiling point (°C)

mole fraction

ETOH−ETAC−H2O ETAC−H2Oa ETOH−ETAC ETAC ETOH-H2O ETOH H2O HOAC

70.09 70.37 71.81 77.20 78.18 78.31 100.02 118.01

(0.1070, 0.6074, 0.2856) (0.6869, 0.3131) (0.4571, 0.5429) 1 (0.9016, 0.0984) 1 1 1

Table 12. Pot Composition (comp) and Hold up at the End of Process aqueous reflux ratio 0 0.1

Heterogeneous azeotrope is denoted by boldface.

0.2

IPAC system at 1 atm pressure. The method using conventional batch reactive distillation for esterification reaction suggested by Venimadhavan et al.14 for the IPAC system is not feasible for the ETAC system because the lightest stationary point which is a ternary azeotrope (ETOH−ETAC−H2O) is not located in the liquid−liquid region. Therefore, the distillate in the decanter will not separate into two phases to produce a high purity product. For the SBRD system, HOAC can be used as a side feed entrainer to extract ETOH from the distillate and drag the top vapor composition from the ternary azeotrope toward the ETAC−H2O edge as shown in Figure 10. Then the distillate becomes nearly a binary mixture.

0.3 0.4

hold up comp hold up comp hold up comp hold up comp hold up comp

ETOH

HOAC

water

ETAC

0.03 0.0008 0.03 0.0007 0.03 0.0008 0.03 0.0008 0.03 0.0007

36.83 0.8909 36.83 0.8918 36.86 0.8905 36.85 0.8919 36.84 0.8928

3.40 0.0823 3.37 0.0816 3.40 0.0821 3.34 0.0808 3.30 0.0800

1.07 0.0260 1.07 0.0259 1.10 0.0266 1.10 0.0265 1.09 0.0264

the aqueous reflux ratio has only a small effect on the process. A lower aqueous reflux ratio leads to a slightly better result for both the process time and the product purity. Table 12 shows that lower aqueous reflux ratio also causes only a slight difference in the pot composition. Therefore, zero aqueous reflux ratio was determined to be optimal and was applied for all further work for this process. The operating policy for the ETAC system is similar to that of the IPAC system. However, two significant differences in the vapor−liquid equilibrium properties were observed. First, compared with the IPAC system, in the ETAC system HOAC is not as inclined to go up to the top of the column because the top temperature is lower. Second, HOAC is less effective as an entrainer in extracting the alcohol in the ETAC system than in the IPAC system. Therefore, some changes were made to the operating policy at the beginning of the ETAC process. In the IPAC system, the HOAC side feed flow rate was fixed at 1 kmol/h in the total reflux period until the IPOH composition in the distillate was lower than 1%. Compared with the IPAC system, the ETAC system can tolerate a higher HOAC flow rate without HOAC accumulating in the distillate, but it is more difficult to reduce the ETOH composition in distillate to a low level (1%). Therefore, in ETAC system, the side feed flow rate during the total reflux period was made a manipulated variable. Also, the specification of the ETOH composition at the end of the total reflux period is changed according to the specification in the product collection period. Table S5 in the Supporting Information shows the pairing between manipulated variables and constrained variables for the ETAC system. Other aspects of the operating policy during the product collection period were the same as for the IPAC system. Therefore, for the ETAC system the design method leads to the flowsheet shown in Figure 11, which can be applied to design the ETAC SBRD process. As for the IPAC system, calculations are performed for different values of the constraint on the alcohol composition in the distillate. Table 13 shows an example of the results with a 2% ETOH constraint when the side feed flow rate is changed during the total reflux period. The minimum SBRD process time corresponds to an initial side feed flow rate of 3.3 kmol/h.

Figure 10. Ternary diagram for the ETOH, ETAC, and WATER system. The liquid−liquid envelope is calculated at 40 °C.

In the ETAC system, the design concept is similar to the IPAC system. There are two steps in the process. The first step is SBRD with the side feed entrainer HOAC to collect a binary mixture of ETAC and water. The second step is an IBD column to further purify the main product from the organic receiver and recover high purity ETAC. Development of Operating Policies. The optimization and operating policy of the ETAC system are similar to those of the IPAC system. Among the three manipulated variables, the effect of the aqueous reflux ratio is checked first. Five cases are compared to see whether the result can be improved by changing the aqueous reflux ratio. The values of the aqueous reflux ratio in the five cases considered were between 0 and 0.5. In these cases, the optimal design is determined using the procedure described previously for the IPAC system. The other parameters in all cases are the same at the beginning of the 8610


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As for the IPAC system, when the constraint on the alcohol composition is increased (more alcohol is allowed), the SBRD process time decreases and the IBD process time increases. Therefore, again as for the IPAC system, an optimal value of the alcohol composition can be identified which minimizes the total process time. 4.2. Simulation. For the first step of the process, a SBRD model is built with following setup: (1) total number of stages, 30; (2) pot diameter, 1 m; (3) pot height, 2 m; (4) total condenser temperature, 40 °C; (5) initial charge, 10 kmol of ETOH; (6) constant pressure of 1 atm; (7) holdup on each tray, 0.02 kmol; (8) vapor rate, 5 kmol/h; (9) tray of side feed, 5. For the second step of the process, an IBD model is built with the same specifications as that used in the IPAC system. Figure 12 shows SBRD process times for different side feed flow rates and different ETOH composition constraints. As

Figure 12. Process time for different side feed flow rates and ETOH composition constraints.

before, for each constraint, the minimum batch time required for each of the two steps is determined as shown in Table 14. Table 14. Total Process Time for Different Values of the ETOH Composition Constraint ETOH constraints on reflux drum in SBRD SBRD

initial organic reflux ratio initial side feed flow rate (kmol/h) process time (h) IBD boilup ratio process time (h) total process time (h)

Figure 11. Flowchart for ETAC system operating policy development.

Table 13. Result of Changing Initial Side Feed Flow Rate side feed flow rate in total reflux period (kmol/h)

total reflux time (h)

initial organic reflux ratio in product collection period

total process time

3.7 3.5 3.3 3.1 2.9 2.7 2.5 2.3 2.1 1.9

5 5 5 6 6 7 8 10 13 16

0.91 0.90 0.88 0.86 0.86 0.86 0.85 0.85 0.83 0.80

18.0 16.7 14.7 15.8 15.6 16.9 17.1 19.2 21.9 24.6

1%

1.5%

2%

2.5%

0.91

0.89

0.88

0.86

0.84

3.7

3.3

3.3

3.1

2.9

47.6

36.2

29.8

26.1

24.1

0.87 10.8

>0.99 >50

0.68 3.6 51.2

0.72 4.3 40.5

0.78 6.0 35.8

3%

36.9

The minimum total process time is found when the constraint on the composition of the ETOH impurity is 2%. Furthermore, when the ETOH composition is higher than 3%, the distillate composition leaves the two-liquid region at the end of the IBD process, making the process infeasible. The following are the details of the optimized process with the minimum process time. In the first step of the process, the simulation results are shown in Figure 13 to Figure 17. The side feed flow rate profile and the organic reflux ratio profile are shown in Figure 13. The first 10 h is a total reflux period until the ETOH composition in the reflux drum becomes lower than 8611


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Figure 13. Manipulated variable profiles for the ETAC system.

Figure 14. Pot composition profile and holdup profile for the ETAC system.

Figure 15. Organic receiver composition and accumulation profiles for the ETAC system.

2%. In this period the side feed flow rate is fixed at 3.3 kmol/h. Then the organic reflux ratio is reduced to 0.88 which can satisfy the constraints during the product collection period. As the operation continues, the side feed flow rate becomes smaller and finally becomes zero. At the end of the process (28−30.62 h), the ETAC composition in the pot decreases which causes more HOAC to travel up from the pot, so the organic reflux ratio is increased to reduce the HOAC content in the vapor. The total amount of HOAC which is added as the side feed is 58.6 kmol.

Figure 14 shows the pot composition profile and the pot component holdup profile. HOAC is added into the column and reacts with ETOH to form water and ETAC. Hence, the amount of ETOH decreases with time. An excess of HOAC added into the column results in the accumulation of HOAC in the pot. After 10 h of total reflux period, both ETAC and water are collected in the receiver during the product collection period. Figure S4 in the Supporting Information shows the reflux drum composition profile. Figure 15 shows the organic receiver composition profile and holdup. Figure 16 shows the 8612


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Figure 16. Aqueous receiver composition and accumulation profiles for the ETAC system.

holdup profile. Figure 17 shows the product receiver composition and component holdup profile. From Figure S5, because the IBD column only recycles the organic phase and retains the aqueous phase, the ETAC concentration decreases with time in column. Figure 17 shows that the product purity of ETAC increases during most of the process time and then gradually decreases at the end of the process. At the end of the process, high purity ETAC product can be collected in the product receiver and high purity H2O is left in the decanter. Table 16 lists the receiver composition and holdup at the end of the process. High purity ETAC (0.9902) is collected. The

aqueous receiver composition profile and holdup. Only 2% ETOH and a trace amount of HOAC are in the distillate. A mixture which is primarily ETAC and water can be collected in the organic receiver. Table 15 shows the receiver composition and holdup at the end of the process. An excess of HOAC with a purity of 89% Table 15. Receiver Composition (comp) and Holdup at the End of the Process organic receiver ETOH HOAC H2O ETAC total

aqueous receiver

comp

holdup (kmol)

comp

holdup (kmol)

0.0190 0.0001 0.1713 0.8096

0.21 1.27 × 10−03 1.91 9.01 11.1

0.0063 0.0000 0.9819 0.0117

0.03 5.26 × 10−05 3.96 0.05 4.04

Table 16. Product Receiver Composition and Holdup at the End of the Process ETOH HOAC H2O ETAC total

remains in the pot. A mixture of ETAC and H2O with 2% impurity ETOH and trace amount HOAC is collected in the organic receiver. The amount of the ETAC in the organic receiver is 9.01 kmol which matches the specification. The second unit of this process is the IBD column for purification of the product ester. An initial charge of 11.1 kmol from the organic receiver containing 9.01 kmol of ETAC is placed in the decanter in the IBD. Figure S5 in the Supporting Information shows the decanter composition and component

composition

holdup (kmol)

0.0044 0.0002 0.0053 0.9902

0.04 1.39 × 10−03 0.04 8.14 8.23

yield of ETAC in the second unit IBD is 0.903, and the total yield of ETAC is 0.814. The holdup remaining in the SBRD pot at the end of the first step is 53.4 kmol which is mostly HOAC. Since this large

Figure 17. Product receiver composition and accumulation profile for the ETAC system. 8613


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vapor rate is constant and equal for both processing steps, the process time is closely related to the energy consumption and minimizing the total process time is essentially equivalent to minimizing the total energy consumption.

amount of reactant would otherwise be wasted, it would be desirable to recycle it as part of the side feed for the next batch. This case was also simulated, and after several batches, the pot composition and holdup reaches a steady state which is shown in Table 17. Therefore, it is concluded that the holdup remaining in the SBRD pot can be recycled to the next batch.

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S Supporting Information *

Table 17. Pot Composition and Hold up after Several Batches ETOH HOAC H2O ETAC total

composition

holdup (kmol)

0.0011 0.8375 0.1414 0.0200

0.08 65.79 11.10 1.57 78.55

ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b00275. Physical properties of pure components in the IPAC and ETAC systems; optimization results; dynamic model; notation; supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +886-2-2366-3037. Fax: +886-2-2369-1314. E-mail: jeff[email protected].

In this section, optimization of the ETAC system is discussed. Compared with the IPAC system, the operating policy is changed during the total reflux period, because the ETAC system can tolerate a higher HOAC flow rate and it is more difficult to reduce the ETOH composition in the distillate. Compared to the IBD process time, the SBRD process time is much longer. When the constraint on the isopropyl alcohol impurity is increased, the SBRD process time decreases, and the IBD process time increases. The minimum total process time was found to occur for an ETOH composition constraint of 2%. Also, due to the constant vapor rate in both units, the total process time is closely related to the total energy consumption.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Taiwan Ministry of Science and Technology is gratefully acknowledged.

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REFERENCES

(1) Mujtaba, I. M. Batch Distillation Design and Operation; Imperial College Press: London, 2004. (2) Guo, Z.; Ghufran, M.; Lee, J. W. Feasible Products in Batch Reactive Distillation. AIChE J. 2003, 49, 3161. (3) Guo, Z.; Lee, J. W. Feasible Products in Batch Reactive Extractive Distillation. AIChE J. 2004, 50, 1484−1492. (4) Chin, J.; Lee, J. W.; Choe, J. Feasible Products in Complex Batch Reactive Distillation. AIChE J. 2006, 52, 1790−1805. (5) Chin, J.; Lee, J. W. Estimation of still trajectory for batch reactive distillation systems. Ind. Eng. Chem. Res. 2008, 47, 3930−3936. (6) Stéger, C.; Lukács, T.; Rév, E.; Meyer, M.; Lelkes, Z. A Generic Feasibility Study of Batch Reactive Distillation in Hybrid Configurations. AIChE J. 2009, 55, 1185−1199. (7) Huss, R. S.; Chen, F.; Malone, M. F.; Doherty, M. F. Reactive Distillation for Methyl Acetate Production. Comput. Chem. Eng. 2003, 27, 1855−1866. (8) Tang, Y. T.; Huang, H. P.; Chien, I. L. Design of a Complete Ethyl Acetate Reactive Distillation Column system. J. Chem. Eng. Jpn. 2003, 36, 1352−1363. (9) Tang, Y. T.; Chen, Y. W.; Huang, H. P.; Yu, C. C.; Hung, S.-B.; Lee, M. J. Design of Reactive Distillations for Acetate Acid Esterification. AIChE J. 2005, 51, 1683−1699. (10) Lai, I. K.; Hung, S. B.; Hung, W. J.; Yu, C. C.; Lee, M. J.; Huang, H. P. Design and Control of Reactive Distillation for Ethyl and Isopropyl Acetates Production with Azeotropic Feeds. Chem. Eng. Sci. 2007, 62, 878−898. (11) Lai, I. K.; Liu, Y. C.; Yu, C. C.; Lee, M. J.; Huang, H. P. Production of High-Purity Ethyl Acetate Using Reactive distillation: Experimental and Start-Up Procedure. Chem. Eng. Process. 2008, 47, 1831−1843. (12) Hu, S.; Zhang, B. J.; Hou, X. Q.; Li, D. L.; Chen, Q. L. Design and Simulation of an Entrainer-enhanced Ethyl Acetate Reactive Distillation Process. Chem. Eng. Process. 2011, 50, 1252−1265. (13) Qi, W.; Malone, M. F. Semi-batch Reactive Distillation for Isopropyl Acetate Synthesis. Ind. Eng. Chem. Res. 2011, 50, 1272− 1277. (14) Venimadhavan, G.; Malone, M. F.; Doherty, M. F. A Novel Distillate Policy for Batch Reactive Distillation with Application to the Production of Butyl Acetate. Ind. Eng. Chem. Res. 1999, 38, 714−722.

5. CONCLUSIONS In this work, two systems are studied to find the best overall operation of processes involving SBRD and IBD in sequence. Both systems use acetic acid as a side feed entrainer to substantially reduce the amount of alcohol in the distillate. Owing to the similar design concepts and operating policies, they can readily be compared. In the SBRD, a mixture of two products, water and acetate, can be collected in an organic receiver. One major difference between two systems is the aqueous reflux ratio. In the IPAC system, the aqueous reflux ratio should not be less than 0.85. However, the ETAC system can operate without aqueous reflux. As the aqueous reflux ratio decreases, water will be removed from the column and the distillate composition will approach the minimum boiling stationary point which is a ternary azeotrope. Therefore, the ETAC system requires a larger side feed to maintain the specification of the alcohol impurity in the product. In the IPAC system, too much HOAC cannot be tolerated in the column. If the aqueous reflux ratio in the IPAC system is reduced, the result is that either the IPAC composition or HOAC composition will exceed the constraint. However, in the ETAC system, the side feed flow rate can be much higher, even 3 times greater than that of the IPAC system, so zero aqueous reflux ratio was found to be best in the ETAC system. In both systems, the SBRD process time accounts for most of the total process time. When the constraint on the alcohol impurity is increased, the SBRD process time decreases and the IBD process time increases. Therefore, there is an impurity composition corresponding to the minimum combined process time that can be determined. This composition is 1% for the IPAC system and 2% for the ETAC system. Also, because the 8614


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Industrial & Engineering Chemistry Research (15) Hsu, L. W. Semi-batch Reactive Extractive Distillation for Production of Acetic Acid Esters with Different Alcohols. MS dissertation. National Taiwan University, Department of Chemical Engineering, Taipei. Taiwan, 2013. (16) Mujtaba, I. M.; Macchietto, S. Efficient Optimization of Batch Distillation with Chemical Reaction Using Polynomial Curve Fitting Techniques. Ind. Eng. Chem. Res. 1997, 36, 2287−2295. (17) Modla, G. Reactive Pressure Swing Batch Distillation by a New Double Column System. Comput. Chem. Eng. 2011, 35, 2401−2410. (18) Gadewar, S. B.; Malone, M. F.; Doherty, M. F. Feasible Region for a Countercurrent Cascade of Vapor-Liquid CSTRs. AIChE J. 2002, 48, 800−814. (19) Horsley, L. H. Azeotropic Data III; Advances in Chemistry Series 116; American Chemical Society: Washington, DC, 1973.

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A Systematic Method for the Development of Operating Policies for

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