Closed Operation of Multivessel Batch Reactive Distillation Processes

Article pubs.acs.org/IECR

Closed Operation of Multivessel Batch Reactive Distillation Processes Yu-Lung Kao,†,‡ Georg Fieg,‡ and Jeffrey D. Ward*,† †

Department of Chemical Engineering National Taiwan University, 10617 Taipei, Taiwan Institute of Process and Plant Engineering, Hamburg University of Technology, 21073 Hamburg, Germany

‡

ABSTRACT: Closed operation of batch reactive distillation process with three vessels (reflux drum, reboiler drum, and middle vessel) is studied using Aspen Plus and Aspen Dynamics. The results show that the rate of accumulation of product in the middle vessel is an important optimization variable. Batch capacity increases and energy consumption decreases as the rate of accumulation increases; however, beyond a certain value the process is found to be infeasible because the reactive vessel dries out before the desired product purity can be achieved. In all cases the batch capacity is found to be higher and the energy consumption lower for closed operation compared to both conventional operation and operation with off-cut.

1. INTRODUCTION Reactive distillation, which combines reaction and distillation in one unit, has the potential to reduce both operating and capital costs compared to conventional processes. Distillation can remove one or more product species from the reaction zone, which increases the reaction conversion. Reaction can change the composition profile in the column and, therefore, overcome distillation boundaries caused by azeotropes. Continuous reactive distillation processes have been investigated by many researchers and widely employed in industry.1 However, as the demand for specialty chemicals, fine chemicals, pharmaceuticals, and biochemical products increases, batch-wise operation of reactive distillation (BREAD) has received more attention. Compared with continuous reactive distillation, batch reactive distillation is more suitable for small scale production and products with seasonal demand, and the investment cost is relatively low. It is also more flexible in operation. That is, an existing distillation column can be used for different processes by changing the operating recipe.2 A multivessel batch distillation column is similar to a conventional batch distillation column (CBD), but with one or more intermediate product vessels connected to the column, as shown in Figure 1a. With additional product vessels, instead of collecting different products sequentially at the top of the column, different product species can be collected simultaneously during the batch. A special operation mode called closed operation is often applied for multivessel batch distillation column. Instead of collecting products in separate product accumulators, products are collected in one or more intermediate product vessels as well as the reflux drum and reboiler, as shown in Figure 1b. This configuration has the following advantages: (1) The operation is simpler because no product is withdrawn from the column. (2) The final product purity is more secure because the product vessels are connected © 2017 American Chemical Society

to the distillation column all the time. The product in the product vessel can be further purified even if the average product purity has dropped below the purity specification (3) The energy consumption may be reduced.3 Multivessel batch distillation with closed operation has been studied by many researchers for nonreactive batch distillation processes. The control of closed operation of a multivessel batch distillation was first studied by Hasebe et al.3 They proposed to calculate the final holdup in each vessel in advance and then to control the level in each vessel to keep the holdup constant during the batch. However, a disadvantage of this control scheme is that it requires a rather complicated adjustment when the feed composition is not known exactly. Therefore, Skogestad and co-workers proposed a temperature feedback control structure.4,5 The idea is to control the temperature at certain locations along the column by adjusting the reflux flow out of each vessel. In this case, the product concentration is indirectly controlled. Later on, the feasibility of a temperature control was also investigated experimentally.6 A wide variety of optimal operation problems (reflux ratio, initial feed distribution, product withdrawal rate, and reboiler duty) of multivessel batch distillation were investigated by Furlonge et al.7 A rigorous model validated by experiments was used to study the behavior of the column during startup for closed operation by Grützmann and co-workers.8,9 They provided process insights and general guidelines to improve industrial application. On the basis of the understanding of the startup period, the optimization of an open-loop control structure was also investigated.10 Compared with a feed-back temperature control Received: Revised: Accepted: Published: 3655

December 8, 2016 February 14, 2017 March 14, 2017 March 14, 2017 DOI: 10.1021/acs.iecr.6b04756 Ind. Eng. Chem. Res. 2017, 56, 3655−3670

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Industrial & Engineering Chemistry Research

Figure 1. Multivessel batch distillation column diagram.

As a result, a middle-vessel column (MVC) configuration with off-cut collection was investigated to improve the performance of these two systems by Kao and Ward.12 Reactants are charged in the middle vessel, and one of the products is withdrawn from the column at the top or at the bottom while the other product accumulates in the middle vessel. Off-cut is withdrawn at the opposite end of the column after the collection of the withdrawn product. Compared with conventional operation (inverted distillation column), the result showed an improvement of about 60% in the CAP for MVC for the case studied. However, the disadvantage of this operation is that only half of the column stages are used effectively at any time during the operation. For example, for Type IIp processes, during the product collection period, only the upper part of the column is used to purify the lighter product. Then, during the off-cut period, only the lower part of the column is used to purify the heavier product. By contrast, as will be shown in this work, for closed operation, the entire column is used effectively during the entire batch. The simplest multivessel batch distillation column, a middlevessel column (MVC) shown in Figure 1c, is used for the process. There are three vessels (reflux drum, middle vessel, and reboiler) in this configuration and the general strategy is to collect one product each in two of the vessels and drive the reactants to the remaining vessel where they are consumed by reaction. For example, for type IIp system, the reaction takes place in the reboiler, the lighter product is collected in the reflux drum, and the heavier product is collected in the middlevessel. The remainder of this article is organized as follows. In section 2, the reaction system and the process description are introduced. In section 3, the simulation tool for the process model is presented. In section 4, simulation results are shown and discussions are made. Finally, conclusions are drawn in section 5.

structure, the optimal open-loop control structure requires a shorter batch time. However, to our knowledge, closed operation has never been studied for batch reactive distillation processes. Therefore, in this work dynamic operation of a multivessel batch reactive distillation process with closed operation is attempted. A quaternary reactive system A + B ⇔ C + D is considered, the same as that considered previously by Kao and Ward11 for BREAD process design with off-cut collection. This reactive system can be classified into six distinct types depending on the boiling points of the reactants and products. In the classification, LLK (lighter-than-light key), LK (light key), HK (heavy key), and HHK (heavier-than-heavy key) are used to denote the four components in the system in an increasing order of boiling point. In one case (LLK + HHK ⇔ LK + HK) operation is infeasible without an entrainer because it is impossible to collect either product at the top or bottom of the column. For the remaining five systems, optimal operation recipes with offcut collection were proposed. The results showed that when both products are the lighest or heaviest species, that is HK + HHK ⇔ LLK + LK (Type IIp) or LLK + LK ⇔ HK + HHK (Type IIr), the process performance based on batch capacity was worse than for other cases. Batch capacity (CAP) is defined as the total moles of on-spec products collected divided by the total time of the batch: NP

CAP =

∑i = 1 Pi tb

(1)

where NP is the number of products and Pi is the amount of product i collected during the batch. It is equivalent to the production rate during the batch. Two reasons explain the relatively poor performance of Type IIp and Type IIr systems. First, the boiling point of the two products are close which makes the separation difficult. Second, the off-cut collection is limited by the batch configuration (conventional batch distillation and inverted batch distillation column) studied.

2. REACTION SYSTEM AND PROCESS DESCRIPTION 2.1. Reaction System. First, an ideal reactive system is used in this work to reduce the difficulty of converging the 3656

DOI: 10.1021/acs.iecr.6b04756 Ind. Eng. Chem. Res. 2017, 56, 3655−3670

Article

Industrial & Engineering Chemistry Research flowsheet in Aspen Plus. In other words, ideal vapor−liquid equilibrium and constant relative volatilities are assumed. The constant relative volatilities are αLLK/αLK/αHK/αHHK = 8/4/2/ 1. In addition, a simple second-order liquid phase reversible reaction based on mole fraction is assumed. The reaction rate Ri (mol/s) for component i can be expressed as

For Type IIp, the HK + HHK ⇔ LLK + LK system, the two heavier reactants are charged in the reboiler at the bottom. The reaction is assumed to take place only in the charge drum in this work. Initially, the column is operated at total reflux. That is, the input and output flow rates of each vessel are equal. As the reaction takes place in the reboiler, two lighter products are produced and tend to go to the upper part of the column. When the LLK purity in the reflux stream achieves the product purity specification, accumulation of LLK in the reflux drum begins. The reflux flow rate is decreased so that product LLK with the specified purity gradually accumulates in the reflux drum. Because LLK accumulates at the top, the concentration of the other product LK increases in the lower part of the column. Then, the liquid output from the middle vessel is decreased so that liquid containing both products begins to accumulate in the middle vessel. The LK concentration in the middle vessel will gradually increase during the process. Because both of the products are removed from the reboiler, a high reaction conversion can be achieved. At the end of the batch, product LLK and product LK are collected in the reflux drum and middle vessel, respectively, and the unconsumed reactants (off-cut) remain in the reboiler. On the contrary, Type IIr, the LLK + LK ⇔ HK + HHK system, is the mirror image of the Type IIp process. The two lighter reactants are loaded in the reflux drum where the reaction takes place. As the process proceeds, high purity product HHK accumulates in the reboiler, and the product HK accumulates in the middle vessel where the purity gradually increases. At the end of the batch, the unconsumed reactants will remain in the reflux drum. In addition to MVC with closed operation, MVC with off-cut collection operation proposed by Kao and Ward is also simulated to make a comparison with closed operation.12 Both Type IIp (using ideal system) and Type IIr (using DMA system) processes are illustrated and compared. For the Type IIp process, reactants HK and HHK are charged in the middle vessel where the reaction takes place. High purity product LLK is withdrawn and collected at the top of the column. Product LK accumulates in the middle vessel and can be purified in a shorter time by off-cut withdrawal from the bottom of the column in the later part of the batch. For the DMA process reactants are also charged in the middle vessel. Water (HHK) is withdrawn from the bottom, and DMA (HK) accumulates in the middle vessel and is purified by off-cut removal from the top of the column.

R i = νiV (kFXA XB − kBXCXD)

where νi is the stoichiometric coefficient which is negative for reactant and positive for product, V is the reactive holdup (mol), kF and kB are the forward and backward specific reaction rates (1/s), respectively, and Xi is the liquid mole fraction for component i. This reaction expression is generic and is applied for all systems studied in this work, only the components A, B, C, and D are different for different cases. The thermodynamic and kinetic parameters used in the ideal quaternary system are listed in Table 1. Other physical properties of the four components required for simulation in Aspen Plus are based on those of propane. Table 1. Thermodynamic and Kinetic Parameters in the Ideal System activation energy (cal/mol) specific reaction rate at 366 K (kmol s−1 kmol−1) heat of reaction (cal/mol) heat of vaporization (cal/mol) relative volatilities component vapor pressure constantsa ln PSi = AVP,i − BVP,i/T a

forward (EF) backward (EB) forward (kF)

12000 17000 0.008

backward (kB) λ ΔHV (LLK/LK/HK/HHK) LLK LK AVP 13.04 12.34 BVP 3862 3862

0.004 −5000 6944 8/4/2/1 HK HHK 11.65 10.96 3862 3862

Pressure is in bar, and temperature is in Kelvin.

In addition to the ficticious system, a real reactive system, the production of 1,1-dimethoxyethane (or dimethylacetal, DMA) is also studied to complement the ideal system study. The reaction is acetaldehyde (294.21 K) + 2 methanol (337.68 K) ⇔ DMA (337.45 K) + water (373.17 K). DMA can be used as a raw material for the manufacture of fragrances and pharmaceutical products. It is also an important intermediate for synthesis of various industrial chemicals. This reactive system was also used by Qi to show the advantages of batch reactive distillation using an inverted batch distillation column over conventional design.13 The boiling point ranking and the MeOH−DMA−acetaldehyde ternary map are shown in Figure 2. Water does not form an azeotrope with any other species in the system. A distillation boundary is formed due to the MeOH−DMA azeotrope, and therefore, the next species to be distilled from the bottom of the column after water is DMA rather than methanol. Therefore, the process behaves as a Type IIr system. The details of thermodynamic and kinetic models can be found in a paper by Kao and Ward.12 2.2. Process Description. In this work, closed operation is employed for MVC. That is, no product or off-cut is withdrawn from the column. Instead, they will accumulate in the reboiler, middle vessel, and reflux drum, which are connected to the column during the whole batch process. Therefore, the accumulated products always have a chance to be purified if the product purity drops below the purity specification during the batch.

3. MODEL DEVELOPMENT A number of simulators have been developed over the years to handle batch distillation processes: BATCHFRAC,14 Aspen Batch Modeler, Aspen BatchSep,15 and others. The column configuration (only conventional batch distillation column) or the implementation of reaction in the column is restricted for these developed simulators which limits the investigation of multivessel batch reactive distillation. Some researchers have built models in a programming language, such as MATLAB, 11,12 FORTRAN,16 Aspen Custom Moldeler,8,9 or gPROMS.7,17 However, building a process model in a programming language can be very time-consuming, especially when a rigorous model is desired. Therefore, different levels of simplifying assumptions are usually made for the model. For example, the vapor holdups and column hydraulics are ignored, and ideal vapor liquid equilibrium is assumed. 3657


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Figure 2. MeOH−DMA−acetaldehyde ternary map and the boiling point ranking of DMA system.

Figure 3. Process flowsheet of closed operation in Aspen Plus.

In this work, Aspen Plus and Aspen Dynamics are used for process simulation because they permit the rapid development of rigorous process models. Aspen includes extensive physical property libraries, built-in models of different operation units, rigorous numerical integration algorithms and models of control elements. Luyben studied the application of Aspen Dynamics to simulate a nonreactive batch distillation process with middle-vessel column configuration.18 His paper provides a step-by-step procedure for developing such a process model. Compared with a previous application by Chien et al. in which

the process is simulated from an empty column (start-up period is also considered),19 the approach used in this work is simpler and more straightforward. The procedure for establishing the model can be summarized as follows. First, the components and their physical properties and the flowsheet required for the process are built in Aspen Plus. Also, the size of each piece of equipment which is related to the vapor and liquid load and the holdup capacity is carefully specified because the equipment sizes determine the batch operation condition. Then, the file is exported to Aspen 3658


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Industrial & Engineering Chemistry Research Table 2. Equipment Sizes for Ideal System Processes unit column

reflux drum of upper column sump of upper column middle vessel reboiler

column


column

reflux drum of upper column sump of upper column reboiler

size

remark

Type IIp Closed Operation Process tray diameter: 1.2 m simple tray weir height: 0.05 m tray spacing: 0.6m length: 6 m initial liquid volume diameter: 3 m height: 2 m initial liquid volume diameter: 1 m length: 6 m initial liquid volume diameter: 3 m height: 8 m initial liquid volume diameter: 4 m Type IIr Closed Operation Process tray diameter: 1.2 m simple tray weir height: 0.05 m tray spacing: 0.6m length: 8 m initial liquid volume diameter: 4 m height: 2 m initial liquid volume diameter: 1 m length: 6 m initial liquid volume diameter: 3 m height: 8 m initial liquid volume diameter: 4 m Type IIp Off-Cut Collection Process tray diameter: 1.2 m simple tray weir height: 0.05 m tray spacing: 0.6m length: 6 m initial liquid volume diameter: 3 m height: 8 m initial liquid volume diameter: 4 m height: 6 m initial liquid volume diameter: 3 m

initial liquid molar holdup about 1.0 kmol (depend on the composition) fraction: 0.05

33.7 kmol

fraction: 0.1

2.4 kmol

fraction: 0.05

18.9 kmol

fraction: 0.8

1013.5 kmol

about 1.0 kmol (depend on the composition) fraction: 0.6

1614.0 kmol

fraction: 0.1

2.5. kmol

fraction: 0.05

29.4 kmol

fraction: 0.05

36.7 kmol


33.7 kmol

fraction: 0.8

1011.7 kmol

fraction: 0.05

18.9 kmol

3.1. Steady-State Design in Aspen Plus. Figure 3 shows the flowsheet developed in Aspen Plus to represent a middlevessel column. The flowsheet mainly consists of three parts: upper column (RadFrac model: 10 stages including a condenser), middle vessel (Flash2 model), and lower column (RadFrac model: 10 stages including a reboiler). There are also fictitious feed, distillate, vent, and bottoms streams that are required for the steady-state process to converge in Aspen Plus. For the type IIp system, 500 kmol/h of feed consisting of equal composition of HK and HHK is charged into the reboiler of the lower column. The reboiler duty of the lower column is specified as 2.1 MW, and the distillate rate of the upper column is specified as 5 kmol/h. Conversely, 500 kmol/h of feed consisting of equal composition of LLK and LK is fed to the reflux drum of the upper column for the Type IIr system. Similarly, the reboiler duty of the lower column is specified as 2.1 MW, while the bottom rate of the lower column is specified as 5 kmol/h. The stage pressure drop is set at 0.01 bar and the pressure in the reflux drum is set at 0.92 bar for Type IIp process and 1.0 bar for Type IIr process. The size of each vessel and column section and the initial liquid holdups are summarized in Table 2. The steady-state design of type IIp off-cut collection process is shown in Figure 4. Feed is supposed to be charged in the middle vessel where reaction takes place. However, the Flash2

Dynamics. Next, while in dynamic mode, changes such as closing inlet and outlet valves are made to convert the process model from steady-state operation to batch operation. In addition, the operating recipe is specified by a “task” in Aspen Dynamics, and controllers used to realize the operating recipe are added. Then, the process model is ready for the simulation. Details can be found in a paper by Luyben.18 The modeling of the start-up period from a dry column is not considered by using this method. Instead, the initial condition of the process simulations in Aspen Dynamics is the steady-state result in Aspen Plus, which can be considered to be the end of the startup period. It is assumed that the end of the start-up period is achieved when the column is operated under total reflux/boilup without reaction until a steady state is achieved. One additional step is required to model the reaction in a batch reactive distillation process that is not required for a nonreactive process. The reaction and kinetic rate expressions are included in the Aspen Plus simulation, but the reactive holdup is set to be a very small value (10−8 kmol in this work) in Aspen Plus. In this way very little reaction occurs in the initial steady-state flowsheet. Later, during dynamic simulation, the reactive holdup is increased to a desired value in a short period of time at the beginning of the batch process in Aspen Dynamics. 3659


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The steady-state flowsheet for closed operation and off-cut collection operation for the DMA process were set up in a similar manner. The details are not described here, but the equipment sizes used in this work are shown in Table 3. 3.2. Export to Aspen Dynamics and Build the Control Structures. After the steady-state result is successfully converged, the file is exported to Aspen Dynamics using a pressure driven simulation. The initial steady-state operating condition is modified to prepare for the modeling of the batch process by closing all the input and output streams while at the same time maintaining the holdup in each vessel at the desired steady-state value. A detailed procedure for accomplishing this can be found in Luyben’s work.18 3.2.1. Type IIp Closed Operation Process. Once the desired initial condition is achieved, a control structure is implemented to realize the operating recipe. A “task” as shown in Figure 5a is used to automatically give commands during the batch simulation to implement the recipe specified for Type IIp closed operation. Figure 6 shows the control structure and the values of the tuning parameters of the controllers. In this work, feedback control is employed and the controllers are tuned manually starting with the default values given by Aspen Dynamics. A pressure controller (Upper_CondPC) to control the condenser pressure is automatically added by Aspen Dynamics after the file is exported. A concentration controller (CCtop) is employed to maintain the mole fraction of product LLK in the reflux drum by manipulating the reflux mass flow rate. Initially, the controller is set manual mode. A level controller (LCsump) is employed to maintain the holdup level in the sump of the upper column by manipulating the position of valve VUP (liquid stream between the upper part of the column and the middle vessel). The middle-vessel flow ratio (MVFR) is defined as the output molar flow rate from the middle vessel divided by the input molar flow rate to the middle vessel. This ratio is used to determine the product accumulation rate in the middle vessel and is controlled by a flow rate

Figure 4. Process flowsheet of off-cut collection operation in Aspen Plus.

unit in Aspen Plus cannot include a reaction. Therefore, the sump of the upper column functions as the middle vessel here, and the feed is charged to it. The RadFrac model is also used for the upper column (10 stages including a condenser) and lower column (10 stages including a reboiler). The flow rate of feed consisting of equal composition of HK and HHK is 168 kmol/h. The reboiler duty of the lower column is also specified as 2.1 MW, and the distillate rate of the upper column is specified as 83 kmol/h. The equipment sizes are also summarized in Table 2. Table 3. Equipment sizes for DMA system unit column


column

reflux drum of upper column sump of upper column reboiler

size

remark

DMA Closed Operation Process tray diameter: 0.97 m simple tray weir height: 0.05 m tray spacing: 0.6m length: 6 m initial liquid volume diameter: 3 m height: 2 m initial liquid volume diameter: 1 m length: 6 m initial liquid volume diameter: 2 m height: 6 m initial liquid volume diameter: 2 m DMA Off-Cut Operation Process tray diameter: 0.97 m simple tray weir height: 0.05 m tray spacing: 0.6m length: 2 m initial liquid volume diameter: 1 m height: 6 m initial liquid volume diameter: 3 m height: 4 m initial liquid volume diameter: 2 m 3660

initial liquid molar holdup about 1.0 kmol (depend on the composition) fraction: 0.7

752.1 kmol

fraction: 0.5

21.3 kmol

fraction: 0.05

24.3 kmol

fraction: 0.05

24.1 kmol


1.6 kmol

fraction: 0.7

752.8 kmol

fraction: 0.05

16.9 kmol


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Figure 5. Tasks for different processes used in Aspen Dynamics: (a) Type IIp closed operation; (b) Type IIr closed operation; (c) Type IIp off-cut collection operation; (d) DMA off-cut collection operation.

controller (FC). The manipulated variable of this controller is the position of valve VM (liquid stream between the middle vessel and the lower part of the column), and the initial set point of the controller is 1. The task shown in Figure 5a implements the following procedure: At time 0.5 h, the reactive holdup in the reboiler is increased to 100 kmol in 0.5 h, so that the reaction starts to take place. When the LLK concentration in the condenser is equal to the purity specification (95 mol %), the controller CCtop is changed from manual mode to automatic mode, and the set point is specified as 0.95. Then, 1 h later, the set point (MVFR) of the flow rate controller is decreased from 1 to a certain value (in the figure it is 0.92), which must be less than 1 so that the holdup in the middle vessel will increase. This value is assumed to be a constant in this work. The batch process ends when the concentration of LK in the middle vessel

achieves the purity specification (95 mol %). If the LK concentration cannot be achieved because the MVFR is too low, as will be discussed in section 4.1, the batch process also ends when the holdup remaining in the reboiler is less than 10 kmol to prevent the reboiler from drying out. However, in this case the process is infeasible. For this problem formulation, the only variable for optimization is the MVFR value. 3.2.2. Type IIr Closed Operation Process. The task and control structure for Type IIr closed operation are shown in Figure 5b and Figure 7. The operating recipe described by the task can be understood in a similar way, and the control structure is similar to the Type IIp process. A level controller is used to maintain the holdup in the sump of the upper column. A flow rate controller is used to control the MVFR (initially, it is set 1) which determines the rate of product accumulation in the middle vessel. In this case, the concentration of product 3661


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Figure 6. Control structure of Type IIp closed operation process.

Figure 7. Control structure of Type IIr closed operation process.

achieves the specified product purity (0.95 mol %), distillate is withdrawn from the column and collected in a product accumulator. A constant concentration policy to collect distillate is used. Therefore, a concentration controller is employed to achieve this goal by manipulating the position of valve VD (distillate stream), which is totally closed before the start of collection of the LLK. The liquid level in the reflux drum of the upper column and the liquid level in the reboiler of the lower column are also controlled by manipulating the mass reflux flow and the position of valve VUP during the batch. After a certain amount of time (10 h in the figure), valve VB (bottom stream) is opened and off-cut collection begins from the bottom of the column. The valve position is assumed to be a constant during the off-cut collection period. The criterion for the end of the batch is that the concentration of LK in the

HHK in the reboiler is controlled by manipulating the mass reflux flow at the top. The control structure of the DMA process with closed operation is similar. However, an alternative method to control the concentration of water (HHK) in the reboiler is used. Instead of manipulating the mass reflux flow, the reboiler duty of the lower column is manipulated to control the water concentration for the DMA system. The detailed task and process flowsheet can be easily modified from the ideal system case. 3.2.3. Off-Cut Collection Operation Process. For the Type IIp off-cut collection process, the task and the control structure is shown in Figure 5c and Figure 8. At time 0.5 h, the reactive holdup in the sump of upper column increases to 100 kmol in 0.5 h. Then after the concentration of LLK in the condenser 3662


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Figure 8. Control structure of Type IIp off-cut collection operation process.

Figure 9. Control structure of DMA off-cut collection operation process.

to control the water concentration in the reboiler by manipulating the position of valve VB. Two level controllers are added to maintain the holdup level in the reflux drum and reboiler. The manipulated variables of these two controllers are the mass reflux flow and the reboiler duty, respectively. The task can be understood in the same way as the Type IIp system, and the two optimization variables are the time to withdraw offcut and the valve position of VD which determines the off-cut flow rate at the top.

middle vessel achieves the purity specification (95 mol %). For this problem formulation, the optimization variables of this process are the time at which the withdrawal of off-cut begins and the valve position of valve VB which determines the off-cut withdrawal flow rate. The task and control structure for the DMA off-cut collection process are shown in Figure 5d and Figure 9. The bottom flow is collected with a constant water concentration (95 mol %). Therefore, a concentration controller is employed 3663


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Industrial & Engineering Chemistry Research Table 4. Batch Simulation Results under Different MVFRs for the Type IIp Closed Operation Process

a

MVFR

time (h)

product LLK (kmol)

LLK purity (mol frac)

product LK (kmol)

LK purity (mol frac)

off-cut (kmol)

CAP (kmol/h)

1a 0.98 0.96 0.94 0.92 0.91

92.3 23.5 24.1 24.8 25.6 25.7

552.8 506.7 515.5 524.6 533.8 537.7

0.95 0.95 0.95 0.95 0.95 0.95

505.2 110.3 217.0 330.7 452.0 510.5

0.95 0.95 0.95 0.95 0.95 0.949b

0 441.3 352.8 202.9 72.3 9.8

11.46 26.21 30.33 34.43 38.54

No accumulation in the middle vessel. bLK product purity is not achieved.

Figure 10. Composition time profiles and holdup time profiles for the Type IIp system without product collection in the middle vessel (MVFR = 1).

reboiler duty Qreb = 2.1 MW is applied. As mentioned in section 3.2.1, the optimization variable is the MVFR. Table 4 shows the batch simulation results for different values of the MVFR. The table includes the total batch time, total product amount and its corresponding purity, off-cut amount (cut that does not meet any product specifications), and the objective function CAP. An MVFR equal to 1 means that the holdup in the middle vessel remains small. In this case, the MVC column functions as a conventional batch distillation column in which product LLK and product LK accumulate in the reflux drum and reboiler, respectively. The advantage of this operation is that all reactants are converted to valuable products and no off-cut is produced at the end of the batch. However, the LK product purity can only be achieved by a high reaction conversion. That is, the total amount of unconsumed reactants in the reboiler must be less than 5 mol % of the total product amount in the reboiler. In addition, the accumulation of LK in the reboiler will suppress the reaction and reduce the reaction rate. Therefore, a long batch time is required for achieving the high reaction

Note that a temperature controller can be used to replace the concentration controller employed to control the product concentration in the reflux drum or reboiler for different processes in this work as proposed by Wittgens et al.4 Temperature measurement may be easier to implement in practice than concentration measurement.

4. SIMULATION RESULTS In this work, the batch capacity (CAP) is used as the objective function to determine the optimal operation. The constraint is that the purity of both products at the end of the batch must be at least 95 mol %. In section 4.1, the results of closed operation and off-cut collection operation for Type IIp system are presented together for comparison. Section 4.2 presents the results of Type IIr system with closed operation including results for DMA process. 4.1. Type IIp System. 4.1.1. Closed Operation Process. Initially, an equal-molar reactant feed is charged into the reboiler so that it is 80% full (about 1013 kmol). A constant 3664


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Figure 11. Composition time profiles and holdup time profiles for optimal Type IIp closed operation process (MVFR = 0.92).

conversion (92.3 h). Figure 10 shows the composition time profiles in the reflux drum, middle vessel, and reboiler and their corresponding holdups. The concentration specification for the LLK at the top can be reached quickly, and after that, the accumulation of LLK begins. Most LLK is collected in 40 h. The remainder of the batch time is used for purifying the LK to the specification in the reboiler. As a result, compared with other cases in which product LK accumulates in the middle vessel (MVFR is less than 1), the CAP is much smaller. As the MVFR decreases from 0.98 to 0.92, the accumulation rate of LK in the middle vessel increases. This is expected because as the MVFR decreases the removal rate of product from the reboiler increases. Intuitively, the faster the products are removed from the reboiler and collected, the higher the CAP is. It can be observed from Table 4 that the amount of product LK collected increases significantly because the total batch times required for achieving the purity specification of product LK are similar (the total batch times increase slightly as the MVFR decreases). Therefore, the CAP increases as the MVFR decreases. However, when the MVFR is smaller than

Figure 12. CAP vs valve position with different withdrawal times for the Type IIp off-cut collection operation process. Valve position of 5%, 7%, and 9% correspond to average off-cut withdrawal molar flow rate of 9.7, 13.9, and 18.6 kmol/h, respectively.

Table 5. Batch Simulation Results under Different Withdrawal Times for the Type IIp Off-Cut Collection Operation Process withdrawal time (h)

opt valve position (%)

time (h)

product LLK (kmol)

LLK purity (mol frac)

product LK (kmol)

LK purity (mol frac)

off-cut (kmol)

CAP (kmol/h)

27.1 22.1 17.1 12.1 11.1 10.1

7 7 7 8 8 8

40.1 37.5 34.5 31.0 30.6 30.2

410.9 400.0 385.1 362.5 357.6 352.3

0.95 0.95 0.95 0.95 0.95 0.95

367.0 357.8 346.0 309.8 306.2 302.7

0.95 0.95 0.95 0.95 0.95 0.95

197.9 218.1 244.6 303.0 311.6 320.3

19.40 20.23 21.17 21.69 21.70 21.69

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Figure 13. Composition time profiles and holdup time profiles for the optimal Type IIp off-cut collection operation process (withdrawal time = 11.1 h and valve position = 8%, which equals to an average off-cut withdrawal flow rate = 16.0 kmol/h).

Table 6. Batch Simulation Results under Different MVFRs for the Type IIr Closed Operation Process

a

OR

time (h)

product HHK (kmol)

HHK purity (mol frac)

product HK (kmol)

HK purity (mol frac)

off-cut (kmol)

CAP (kmol/h)

1a 0.98 0.95 0.96 0.94 0.93

148.9 57.2 57.4 57.1 56.4 51.0

887.6 818.3 843.3 854.5 863.2 846.9

0.95 0.95 0.95 0.95 0.95 0.95

771.3 285.3 547.7 679.2 804.6 837.4

0.95 0.95 0.95 0.95 0.95 0.933b

0 590.2 303.1 160.5 26.6 9.9

11.14 19.29 24.24 26.85 29.58

No accumulation in the middle vessel. bHK product purity is not achieved.

0.92, the LK purity in the middle vessel cannot be achieved until the reboiler is almost empty (less than 10 kmol). This happens because when the MVFR is too small, the separation between LLK and LK will not be good enough. There will be more than 5 mol % of LLK in the middle vessel at the end of the batch. This situation is considered to be an infeasible design in this work. Although it is still possible to further purify the LK purity under closed operation (for example, by adjusting the MVFR to a value greater than one), which is one of the advantages of closed operation, this option will not be considered in this work. Therefore, when the MVFR equals to 0.92, the process has maximum CAP equal to 38.5 kmol/h. The composition time profiles and the holdups of the optimal case are shown in Figure 11. Similarly, the concentration of LLK reaches the purity specification soon after the batch begins, after which accumulation in the reflux drum begins. After 1 h, the accumulation of LK in the middle vessel also begins according to the operating recipe. At first, the concentration is about 70 mol %, and it gradually increases to achieve the purity specification. The accumulation rate of LLK

is controlled by the concentration controller and decreases during the batch because less and less LLK is left in the reboiler. On the contrary, the accumulation rate of LK is a constant, and controlled by the MVFR. Because both of the products are removed from the reboiler (almost all LLK and part of the LK), the concentration of reactants can be maintained relatively high even during the latter part of the batch (about 20 mol % each), the forward reaction rate is higher compared to the process where only LLK is removed from the reaction zone. 4.1.2. Off-Cut Collection Process. For the off-cut collection process, the optimization variables are the time at which off-cut is withdrawn from the bottom begins and the valve position controlling the withdrawal flow rate. To make a fair comparison with closed operation, the same amount and composition of feed is charged in the middle vessel, and the same constant reboiler duty is applied. Figure 12 shows the CAP versus valve position for different values of the off-cut withdrawal time. For different values of the withdrawal time, the optimal valve position does not vary too much. Table 5 shows the batch 3666


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Figure 14. Composition time profiles and holdup time profiles for optimal type IIr closed operation process (MVFR = 0.94).

Table 7. Simulation Results of Optimal Closed Operation and Off-Cut Collection Operation for DMA System time (h)

product water (kmol)

25.4

281.0

19.9

239.8

water purity (mol frac)

product DMA (kmol)

DMA purity (mol frac)

off-cut (kmol)

CAP (kmol/h)

total energy (MW)

Optimal Closed Opration: MVFR = 0.9 0.95 248.7 0.95 15.1 20.89 28.6 Optimal off-Cut Collection Operation: Withdrawal Time = 12.3 h and Valve Position =50% 0.95 134.3 0.95 173.5 18.80 25.7

product/energy (kmol/MW) 18.52 14.56

withdrawal removes the unconsumed reactants in the middle vessel, which helps the LK purity in the middle vessel to achieve the specification. Compared with the operation without LK accumulation in the middle vessel (MVFR = 1, conventional batch distillation process), the CAP is improved significantly by 89.3%. This result is consistent with the result in our previous work. The improvement is due to the fact that high reaction conversion is not needed because off-cut removal can help to meet the product purity specification. Moreover, optimal closed operation is compared with the optimal off-cut collection operation. The closed operation significantly improves the process by 77.6% of CAP. Closed operation is a better operating strategy for the following two reasons. First, both products are removed from the reaction zone during the batch which improves the forward reaction. Second, the separation stages are used more efficiently. For offcut collection operation, only the upper column is used for separating LLK product from the middle vessel whereas the lower column is only used during the off-cut collection. On the contrary, for closed operation, both upper and lower columns are used for separating products from reactants during the whole batch time. The lower column separates both LLK and LK products from the reactants, and the upper column

simulation results for different withdrawal times with optimized valve positions. The table includes the optimal valve position, total batch time, product amount and its purity, off-cut amount, and CAP. When the off-cut is withdrawn later in the process, more product and less off-cut are produced. However, a longer batch time is required. There is a trade-off between product yield and the total batch time. When the withdrawal time equals to 11.1 h and valve position is equal to 8%, the process has the maximum CAP of 21.70 kmol/h. Figure 13 shows the composition time profiles in the condenser, middle vessel, reboiler for the optimal case. For off-cut collection operation, LLK product and off-cut are withdrawn from the column and collected in separate accumulators. Instead of showing the composition time profiles in these accumulators, Figure 13 shows the instantaneous composition of the distillate and bottom flow (the same composition as in the condenser and reboiler) which are collected in these accumulators. Figure 13 also shows the holdups of the LLK accumulator, middle vessel, and off-cut accumulator. It can be seen that the collection of LLK at the bottom begins at 2.1 h and the collection of off-cut at the top begins at 11.1 h. From the composition time profile in the reboiler and middle vessel, it can be seen that the off-cut 3667


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Figure 15. Composition time profiles and holdup time profiles for optimal DMA closed operation process (MVFR = 0.90).

Figure 16. Composition time profiles and holdup time profiles for optimal DMA off-cut collection operation process (withdrawal time = 12.3 h and valve position = 50%, which equals to an average off-cut withdrawal flow rate = 22.7 kmol/h). 3668


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efficient use of the separation stages. The composition time profiles and holdups time profiles of both optimal designs are shown in Figure 15 and Figure 16. For closed operation, high purity water accumulates in the reboiler soon after the batch starts. DMA with an increasing purity accumulates in the middle vessel. Note that a high concentration of DMA in the middle vessel can only be achieved when the liquid composition in the reflux drum is located in the lower distillation region on the ternary map (Figure 2) where DMA is the stable node rather than MeOH. This explains the late concentration rise of DMA (HK product) compared to the ideal Type IIr system. On the contrary, for off-cut collection operation it can be seen from the composition time profile of the middle vessel that DMA can be purified in a shorter period of time by off-cut removal. However, more DMA (product accumulating in the middle vessel) is lost along with the off-cut removal than in Type IIp off-cut collection operation process due to the MeOH-DMA azeotrope. This explains why the yield of DMA product is less than that of water.

separates LLK from the middle vessel. Therefore, both reaction and separation are more efficient for the closed operation process. 4.2. Type IIr System. 4.2.1. Closed Operation Process. In this case, 1614 kmol of equal molar reactant feed is charged in the reflux drum (reaction zone), and a constant reboiler duty = 2.1 MW is applied. For the ideal system, the HHK concentration in the reboiler is controlled by manipulating the mass reflux flow from the reflux drum in the upper column. The MVFR is the only optimization variable. Similarly, Table 6 shows the batch simulation results for different MVFRs. Because the process is the mirror image of Type IIp closed operation process, the CAP trend is similar. When no product is collected in the middle vessel (MVFR = 1), the process has a very low CAP because the reaction conversion required to satisfy the HK product purity specification in the reflux drum takes a long time. As the MVFR decreases, the CAP increases because more HK is collected in the middle vessel. However, when MVFR is smaller than 0.94, the HK purity specification cannot be achieved until less than 10 kmol of the liquid is left in the reflux drum. As a result, the process has a maximal CAP of 29.95 kmol/h when MVFR equals 0.94. The composition time profiles and holdup time profiles of the optimal design are shown in Figure 14. High purity product HHK can be collected in the reboiler starting from early in the batch. On the contrary, the purity of product HK in the middle vessel gradually increases until it reaches the purity specification. 4.2.2. DMA Process. Both closed operation and off-cut collection operation are presented here. For the closed operation process, a stoichiometric feed is also used. That is, a feed consisting of 1/3 of acetaldehyde and 2/3 of methanol with 752 kmol is charged in the reflux drum where the reaction takes place. The initial reboiler duty is 2.1 MW, and the reactive holdup used is 0.02 kmol. In this case, the water concentration in the reboiler is controlled by manipulating the reboiler duty. Controlling the water concentration by manipulating the mass reflux flow as in the ideal system was also attempted; however, it was observed that control performance was significantly worse in this case. The effect of the reboiler duty on the water concentration is more direct. By contrast, the mass reflux flow needs to travel all the way down the column through two vessels where level control is implemented. These three controllers interact which makes the control structure more unstable. This phenomenon is not observed in the ideal system because most of the physical properties are assumed to be the same for different components. Again, the same feed condition and initial reboiler duty are used for off-cut collection operation process. The optimal designs with optimized variables for both operations are shown in Table 7. Similar to the result of the ideal system, the performance of closed operation is better than operation with off-cut. Compared with off-cut collection operation, the optimal CAP is improved by 11.1%. However, for both processes the reboiler duty is a manipulated variable that changes with the process time. The total batch time therefore does not provide a good estimate of the energy consumption. Therefore, the total heat consumption of the reboilers is calculated for the optimal designs, and the amount of product produced per unit energy consumed is also shown in Table 7. In this case, the amount of product per unit energy consumed for closed operation is 27.2% higher than for off-cut collection operation. Similar to previous results, closed operation proves to be a better strategy due to the more favorable reaction conditions and more

5. CONCLUSIONS In this work, closed operation of Type II batch reactive distillation processes is studied by modeling using Aspen Plus and Aspen Dynamics. The simplest form of a multivessel distillation column, a middle-vessel column (MVC), is used under closed operation. An ideal reactive system and a real chemistry system, the production of 1,1-dimethoxyethane (DMA), are investigated. For Type IIp systems, the reaction takes place in the reboiler, the lighter product is collected in the reflux drum, and the heavier product is collected in the middle vessel. On the contrary, for the Type IIr system, the reaction takes place in the reflux drum, the heavier product is collected in the reboiler, and the lighter product is collected in the middle vessel. For all cases considered, the accumulation rate of product in the middle vessel was an important optimization variable. As the accumulation rate was increased, the batch capacity increased but beyond a certain value the process operation became infeasible because the reactive vessel dried out before the product purity specifications were achieved. Results further show that compared with operation with off-cut collection, the CAP is significantly improved by 77.6% for the ideal system and product recovery per unit energy consumption is improved by 27.2% for the DMA system. Therefore, it is concluded that closed operation is an attractive alternative for batch reactive distillation for Type II systems.

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AUTHOR INFORMATION

Corresponding Author

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

Jeffrey D. Ward: 0000-0003-0727-7689 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank our colleagues in the Institute of Process and Plant Engineering for the help with computer simulations. This project was funded by the Sandwich Program of Ministry of Science and Technology, R.O.C. and German Academic Exchange Service (DAAD). 3669


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REFERENCES

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