Simulated Moving Bed Reactor for the Synthesis of 2-Ethylhexyl

Article pubs.acs.org/IECR

Simulated Moving Bed Reactor for the Synthesis of 2‑Ethylhexyl Acetate. Part I: Experiments and Simulations Vivek Chandra Gyani, Bhoja Reddy, Rahul Bhat, and Sanjay Mahajani* Department of Chemical Engineering, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India ABSTRACT: Reactive chromatography (RC) is a promising technology, wherein equilibrium limitations are overcome by simultaneous reaction and adsorptive separation in a single unit. This operation, on a large scale, can be performed conveniently in a simulated moving bed reactor (SMBR) in continuous mode. In this work, we demonstrate the use of an SMBR for the synthesis of 2-ethylhexyl acetate through experiments and simulations. The kinetic model and Langmuir adsorption isotherm, which serve as inputs to the simulator, were taken from an earlier study (Gyani and Mahajani Sep. Sci. Technol. 2008, 43, 2245). A parametric study was performed using the experimentally validated simulator by varying one of the parameters at a time, to yield desired performance in terms of conversion, purity, productivity, and desorbent consumption. The trends in the results are explained with the help of variations in the solid and liquid concentrations. The simulation results indicate that an SMBR unit containing eight columns is able to offer almost quantitative conversion and purity with a typical desorbent requirement of 46.35 mol of acetic acid per mole of product and a productivity of 32.01 mol of product per kilogram of adsorbent per day. Further optimization of the performance and application of a systematic design algorithm are developed and presented in part II of this work.

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INTRODUCTION In the chemical and pharmaceutical industries, the performance of conventional processes can be significantly improved by integration of individual processes into a single unit. The objective is to reduce equipment size to save capital, energy, and operating costs for the desired performance. Multifunctional reactors, which integrate reaction and separation, have received significant attention in the past two decades.1,2 The working principles of various types of multifunctional reactors and their applications have been reviewed in detail elsewhere.3,4 Currently, the industrial realization of such multifunctional units is mainly limited to reactive distillation, which might not be a suitable option for molecules that are difficult to evaporate, such as those typically of interest in the fine-chemical and pharmaceutical industries. Close-boiling or azeotropic mixtures can also sometimes impose limitations on the use of reactive distillation. Furthermore, in some cases, it is also possible that the catalyst used cannot withstand the high temperatures employed in distillation columns. Multifunctional reactors other than reactive distillation have not seen commercial success because of some technical barriers. Significant efforts are necessary on a laboratory level to assess their technical feasibility and potential for commercialization. Reactive chromatography (RC) is one such promising use of multifunctional reactors that has found several applications in recent years. These include esterification,5−12 etherification,13 acetalization,14−17 isomerization,18 inversion,19 and hydrolysis.20 An exhaustive review of all types of chromatographic reactors and simulated-moving-bed-reactor (SMBR) applications can be found elsewhere.21 RC involves a chromatographic reactor in which the reaction and adsorptive separation of products takes place simultaneously. Chromatographic reactors mainly involve one fluid phase, either liquid or gas, and a solid phase. The catalyst can be either a solid or a fluid. If the catalyst is a solid, then it can be the same material as the adsorbent, or it © 2014 American Chemical Society

can be mixed with the adsorbent. RC is mostly suitable for reactions limited by equilibrium or selectivity and becomes more competitive for nonvolatile and temperature-sensitive components. However, the reaction conditions should be compatible with those required for adsorptive separation. Adsorption is an inherently unsteady-state process unless one manages to move solids continuously during the course of the operation. A true moving bed reactor (TMBR) is one such reactor in which solid movement is continuous. However, the movement of solids is extremely difficult because it causes mixing and attrition. The practical implementation of TMBRs is achieved in simulated moving bed reactors (SMBRs), wherein the solid movement is simulated by switching the inlet and outlet ports appropriately. Although a reasonable number of experimental and simulation studies on SMBRs have been reported in the literature, to the best of our knowledge, to date, there is still no major commercial application of an SMBR in the chemical industry. This means that, in addition to detailed theoretical studies, an attempt should also be made to explore several potential applications of this technology to widen its scope. The objective of the present work was to evaluate the applicability of SMBRs for one such commercially important esterification reaction, namely, the synthesis of 2-ethylhexyl acetate from 2-ethylhexanol and acetic acid using Amberlyst-15 as the catalyst and adsorbent. Esterification reactions are very common reactions, especially in the pharmaceutical, perfumery, and polymer industries, in which both heterogeneous and homogeneous catalysts have been widely used. 2-Ethylhexyl Received: Revised: Accepted: Published: 15811

May 21, 2014 July 26, 2014 September 2, 2014 September 2, 2014 dx.doi.org/10.1021/ie502090z | Ind. Eng. Chem. Res. 2014, 53, 15811−15823

Industrial & Engineering Chemistry Research

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Figure 1. Schematics of (a) an SMBR and (b) the equivalent TMBR for the equilibrium reaction AcH + 2-EH = 2-EHAc + water, where AcH is the desorbent, 2-EH is the limiting reactant, 2-EHAc is the least-retained product, and water is the most-retained product.

Figure 2. Schematic of the SMBR experimental setup (red, feed; blue, desorbent; green, raffinate; and pink, extract).

because SMBRs offer almost the same performance as TMBRs without actual movement of the solid phase. This is achieved by connecting several beds in an arrangement such that the locations of the inlet and outlet ports are switched periodically. The time period between the successive switches of the ports is called the switch time. Figure 1a shows a schematic diagram of an SMBR unit and its operating principle for the esterification reaction acetic acid (AcH) + 2-ethylhexanol (2-EH) = 2ethylhexyl acetate (2-EHAc) + water. The affinities of these components toward the adsorbent follow the order water > AcH > 2-EH > 2-EHAc.5 The SMBR unit consists of a set of interconnected columns of uniform geometry having a packing (e.g., ion-exchange resin) that functions both as an adsorbent and as a catalyst. As shown in Figure 1a, there are two inlet fluid streams (feed and desorbent) and two outlet fluid streams (raffinate and extract) that divide the unit into four sections (labeled I−IV). A mixture of reactants AcH and 2-EH is used as the feed; component AcH, having an affinity next to that of water, is used as the desorbent. At steady state, for the complete separation of the products, the less-retained component 2EHAc and the more-retained component water elute at the raffinate and extract ports, respectively, along with excess and unreacted AcH. The inlet and outlet ports are switched simultaneously in the direction of fluid flow by one column length to realize the virtual countercurrent motion of the solid.

acetate is widely used as a solvent for cellulose nitrate and many natural and synthetic resins. Experimental studies to obtain reaction kinetics and adsorption isotherms have been performed in our laboratory, and the feasibility of reactive chromatography for this reaction was demonstrated on a laboratory-scale fixed-bed chromatographic reactor (FBCR) in our previous work.5 In this work, an SMBR simulator, developed based on an appropriate model, was experimentally validated by performing laboratory experiments. Furthermore, parametric studies were performed by one-parameter continuation, and the results were interpreted to arrive at a feasible design giving the desired performance. This work provides insight into the operation and design of SMBRs and forms a basis for further studies on the simulation-based conceptual design algorithm presented in part II of this work. There has been considerable research on the design of SMBRs based on optimization using mathematical techniques such as sequential quadratic programming (SQP) algorithm.10,22 In this work, we followed a different approach that involving understanding the effects of parameters and then derived an algorithm for conceptual design.

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SIMULATED MOVING BED REACTOR Working Principle of SMBRs. Interest in simulated moving bed reactors has been growing in recent years, mainly 15812

dx.doi.org/10.1021/ie502090z | Ind. Eng. Chem. Res. 2014, 53, 15811−15823


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The sectional flow rates are the key parameters that determine the feasibility of SMBR operation.23 They are maintained in such a way that the regenerations of the mostretained component (water) from the solid phase and of the least-retained component (2-EHAc) from the liquid phase occur in sections I and IV, respectively. In sections II and III, the sectional flow rates are such that the desorption of 2-EHAc and adsorption of water occur after the conversion of reactant 2-EH. The corresponding TMBR configuration is shown in Figure 1b. Because these two configurations are conceptually similar, it is always more convenient and easier to interpret the simulation results of an SMBR assuming it to be conceptually similar to the corresponding TMBR. Experimental Setup and Procedure. The SMBR experimental setup used in the present work consisted of four columns (shown in Figure 2), that is, a single column in each section, in contrast to the two columns in each section shown in Figure 1a. Each column (40 cm in length and 2.54 cm in inner diameter) was equipped with four on/off solenoid valves, two at the inlet and two at the outlet. The feed line and desorbent line were connected to the inlet, whereas the raffinate and extract lines were connected to the outlets of all of the columns. The outlet of each column was connected to another column either directly (columns II and III and columns IV and I, as shown in Figure 2) or through internal pumps (columns I and II and III and IV, as shown in Figure 2). In an SMBR, one can vary switch time and four flow rates, of which at least one flow rate should be the internal flow rate. In our experiments, we varied two internal flow rates, namely, one in section II (Q2) and the other in section IV (Q4), and two external flow rates, namely, those of the feed (QF) and desorbent (QD), using four peristaltic pumps. Solenoid valves and pumps were operated through customized LabVIEW software. Instructions for LabVIEW were provided through a MATLAB code to achieve required functioning of valves and pumps. For any column, when the solenoid valve corresponding to the feed was open, the raffinate valve was also opened at the outlet of the same column. Similarly, when the solenoid valve corresponding to desorbent was opened, the extract valve was also opened at the outlet of the same column. At any point in time, feed was introduced onto one of the columns, and the desorbent was introduced onto the other column. After the switch time, the feed, desorbent, raffinate, and extract ports were switched simultaneously, thereby simulating movement of the solid. The temperatures inside the columns were maintained by providing electrical heaters on the outer surface of each column. All connecting lines (feed line, desorbent line, and internal lines), excluding product lines (raffinate and extract lines), were properly insulated to minimize heat losses. Desorbent, in a desorbent reservoir, was preheated to a desired temperature to reduce the heat load on the electric tape. The feed was preheated in the connecting lines, before entering the column. The raffinate and extract streams were collected over the switch time. Also, raffinate and extract samples were withdrawn every 5 min, and the samples were analyzed using gas chromatography. SMBR Model. The mathematical model of the SMBR used in this work is based on a model of an FBCR, which happens to be the basic building block. Switching is incorporated to realize a virtual movement of the solid through multiple such columns. SMBR model equations, with proper initial and boundary

conditions, are reported in the literature.13 The mass balance of the ith component in the jth section during the Nth switch period is given by ∂Cij(N )

(N )

(1 − ε) ∂Γ ij ρp ∂t ∂t ε (N ) vj ∂Cij ∂ 2Cij(N ) (1 − ε) + Ej + =− ρp R ij(N ) ε ∂z ε ∂z 2 +

(1)

(N) where C(N) are the concentrations of component i in ij and Γij the bulk and adsorbed phases, respectively, for section j during the Nth switch time; Ej and vj are the axial dispersion coefficient and the superficial fluid velocity, respectively, in section j; ε is the interparticle column void fraction; ρp is the adsorbent density; and R(N) ij is the net production rate of component i in section j during the Nth switch period. Note that the pressure drop and swelling of the adsorbent are ignored, which implies constant velocity and concentration along the length of the reactor. As reported by Gyani and Mahajani,5 the reaction rate during the Nth switch time for the synthesis of 2-ethylhexyl acetate is given by

R ij(N ) =

(N ) (N ) (N ) (N ) υi[k f CAcH, jC 2‐EH, j − k bC 2‐EHAc, jCwater, j] (N ) (N ) 2 [1 + kAcHCAcH, j + k waterCwater, j]

(2)

where υi is the stoichiometric coefficient of species i; kAcH and kwater are the adsorption constants for acetic acid and water, respectively; and kf and kb are the forward and backward reaction rate constants, respectively. The temperature dependencies of the forward and backward reaction rate constants are expressed as k f = k f0 exp( −Ef /R gT )

(3a)

k b = k b0 exp( −E b /R gT )

(3b)

k0f

k0b

where and are the pre-exponential factors for the forward and backward reactions, respectively; Ef and Eb represent activation energies of the forward and backward reactions, respectively; Rg represents the ideal gas constant; and T is the temperature in kelvin. It is observed that an activity-based rate model makes the computation even more intensive compared to a concentrationbased rate model. The two kinetic models agree reasonably well in terms of the prediction of FBCR performance.5 Hence, the concentration-based rate model was used for the SMBR simulations in this work for computational simplicity. The solid-phase and liquid-phase concentrations are in equilibrium and are related by the adsorption isotherm. The Langmuir adsorption isotherm during the Nth switch time is given by Γ(ijN ) =

K i Γi∞Cij(N ) NC

1 + ∑i = 1 K iCij(N )

(4)

where Ki and Γ∞ i are the adsorption equilibrium constant and the capacity of the adsorbent for component i, respectively, and NC is the total number of components in the liquid mixture. The initial conditions are given as follows: when N = 0, 15813

Cij(N ) = Cijinitial = 0

(5)



when N ≥ 1,

Cij(N )

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⎧C (N − 1) for j = 1, 2, ..., N − 1 ⎪ i,j+1 col =⎨ ⎪C (N − 1) for j = N ⎩ i1 col

Table 1. Kinetic Parameters of the Concentration-Based LHHW Rate Model5

(6)

The boundary conditions are given as follows: Desorbent node v v Ci(,Np +) q + 1 = 4 Ci(,Np +) q + D CiD z=0 z=L v1 v1

(7)

z=0

= Ci(,Np +) q + r

z=L

(8)

Feed node Ci(,1N )

value 75783.2 40104.7 8.63 × 1013 1.88 × 107 0.11 1.29

Table 2. Adsorption Isotherm Parameters5

Extract node Ci(,Np +) q + r + 1

parameter Ef (J/mol) Eb (J/mol) k0f (mol kg−1 h−1) k0b (mol kg−1 h−1) kAcH kwater

= z=0

v2 (N ) v Ci , p + q + r + s + F CiF z=L v3 v3

(9)

component

Ki (cm3/mol)

Γ∞ i (mmol/g)

acetic acid 2-ethylhexanol 2-ethylhexyl acetate water

413.20 284.52 196.12 1232.02

22.35 7.4 2.55 45.3

Raffinate node Ci(,Np +) 1

z=0

= Ci(,Np )

z=L

noted that the computational time decreased to as low as 4−5 h when appropriate initial conditions were assigned, typically those obtained as a steady-state solution in the previous run under almost identical operating conditions. The desired performances are normally sought in terms of high conversion, high purity, high productivity, and lowest desorbent consumption. These performance parameters are calculated as described in the next section. SMBR Performance Parameters. The SMBR process performance parameters chosen were conversion, purity, productivity, and desorbent consumption. Purity is defined as the mole fraction of the desired product in the raffinate and extract ports, excluding the solvent concentration because it is in excess

(10)

where v1 = v4 + vD

(11)

v2 = v1 − vX

(12)

v3 = v2 + vF

(13)

v4 = v3 − vR

(14)

Here, v1, v2, v3, and v4 are the velocities in sections I, II, III, and IV, respectively, whereas vD, vX, vF, and vR are the fluid velocities of the desorbent, extract, feed, and raffinate, respectively. p, q, r, and s are the numbers of columns in sections III, IV, I, and II, respectively. Determining the optimum values of all SMBR parameters based on numerical simulations is computationally intensive.8 An alternate approach is based on performing a parametric study by directly observing the computed steady-state concentration profiles within the unit at the end of the switch time. Parametric studies of SMBRs have been reported by several investigators for various applications.8,13,24,25 The mass balance equation (eq 1), the kinetic equation (eqs 2 and 3), and the adsorption isotherm equation (eq 4), along with the proper initial and boundary conditions (eqs 5−10), describe the SMBR system completely. The method of lines was used to solve these equations. The concentration-based kinetic model and the Langmuir adsorption isotherm model proposed in our earlier study5 served as inputs to the simulator. The kinetic parameters (of eqs 2 and 3) and adsorption isotherm parameters (of eq 4) for the 2-EHAc system are summarized in Tables 1 and 2, respectively. Space discretization was performed using the finite backward difference method to convert the set of partial differential equations (PDEs) into several coupled ordinary differential equation (ODE) initial value problems (IVPs). A sufficient number of grid points was used in all simulations of the SMBR to minimize the numerical error. The discretized equations were integrated using the ode15s solver of MATLAB. Simulation of an SMBR is a computationally expensive task, as it required approximately 15−16 h to complete 60 switches on an Intel Core 2 Quad processor (Q6600 at 2.4 GHz) with 4 GB of RAM when the columns initially were devoid of any reactants. It should also be

Raffinate purity PuR =

t+t ∫t * C2R‐EHAc dt t+t R ) dt ∫t * (C2R‐EH + C2R‐EHAc + Cwater

(15)

Extact purity PuX =

t+t X dt ∫t * Cwater t+t X ) dt ∫t * (C2X‐EH + C2X‐EHAc + Cwater

(16)

Conversion (X) is the ratio of the amount of limiting reactant (2-EH) reacted to the amount of limiting reactant (2-EH) introduced into the SMBR Conversion X=1−

QR ∫

t+t*

t

C2R‐EH dt + Q X ∫

t

Q FC2F‐EHt *

t+t*

C2X‐EH dt (17)

Productivity (PR) is defined as the number of moles of desired product (i.e., 2-ethylhexyl acetate) realized in raffinate stream per kilogram of adsorbent per unit time Productivity PR = 15814

QR ∫

t

t+t*

C2R‐EHAc dt

(1 − ε)VcolNcolt *ρp

(18)



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interpret the results and discuss the effects of different parameters in detail. Parametric Study. The SMBR unit has 13 parameters, including SMBR configuration, column dimensions, switch time, feed concentration, desorbent concentration, four sectional flow rates, feed flow rate, desorbent flow rate, raffinate flow rate, and extract flow rate. For a given SMBR configuration, the column dimensions, feed concentration, and desorbent concentration, the number of degrees of freedom for the SMBR reduces to five (switch time and four flow rates). It can be noted that only the inlet and outlet flow rates do not explicitly define the sets of four parameters. It can be seen from the node balance equations (eqs 11−14) that at least one internal flow rate needs to be defined, along with three other flow rates. Therefore, theoretically, any four flowrate combinations consisting of either three external and one internal, or two external and two internal, or one external and three internal, or four internal flow rates can be chosen. In our simulations, unless mentioned otherwise, we have chosen the flow-rate combination as three external and one internal flow rates, namely, QF, QR, QD, and Q3. Parametric studies using the experimentally validated simulator were performed by varying one of the SMBR parameters at a time while keeping all other parameters constant. Several simulations were performed, and a reasonably good configuration that gives close-to-quantitative conversion and the desired purity with respect to the desired product was obtained. We call this the base-case simulation, the results of which are explained first. The results of the parametric study are then compared with those of the base-case simulation, and an explanation is provided for the differences in performance. This helps elucidate the working principles of the SMBR and determine a set of operating parameters giving the desired performance. However, as explained later, it does not guarantee an optimum design. The effects of changes in the SMBR configuration that mainly affect productivity are not considered in this work. These effects are presented in detail in part II of this work. The configuration of the SMBR used for our simulations involved two columns (column length = 30 cm, diameter = 1.5 cm, ε = 0.42) in each section, packed with Amberlyst-15 resin (ρp = 800 kg/m3). The temperature of each column was maintained at 353 K. The feed concentration was selected as equimolar in all simulations to ensure that the desorbent stream acted only as a

Finally, desorbent consumption (DC) is defined as the number of moles of acetic acid consumed as the desorbent per unit mole of 2-ethylhexyl acetate realized in the raffinate stream13 Desorbent consumption DC =

D F [Q DCAcH + Q F(CAcH − XC2F‐EH)]t * t+t Q R ∫ * C2R‐EHAc dt

(19)

t

In eqs 15−19, QF, QD, QR and QX are the volumetric flow rates of feed, desorbent, raffinate and extract, respectively; t represents the beginning of the switching time; and Ncol and Vcol are the total number of columns in the SMBR unit and the volume of a single column, respectively.

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RESULTS AND DISCUSSION Comparison of Experimental and Simulated Results. The operating conditions used for the SMBR experiment are summarized in Table 3. In this case, the cyclic steady state was Table 3. SMBR Experimental Operating Conditions parameter

value

temperature (°C) switch time (min) feed flow rate (mL/min) desorbent flow rate (mL/min) flow rate in column II (mL/min) flow rate in column IV (mL/min)

80 30 1.65 13 7.15 4.25

realized after the eighth cycle (32 switches). The performance parameters selected were conversion, raffinate purity, and extract purity, which were averaged over the switch time. These parameters as determined through experiments (X = 98.44, PuR = 78.4, and PuX = 95.91) were compared with the simulator predictions (X = 97.5, PuR = 76.4 and PuX = 99.04). Similarly, the dynamic profiles of the raffinate and extract compositions at cyclic steady state from experiment and simulation are compared in Figure 3. The experimental results agrees well with the simulation results. Any mismatch might be due to the nonideality of the reactor and temperature fluctuations (±6 °C) inside the reactor. The average temperature over the switch time inside the reactor was maintained at the desired level (80 °C). In the subsequent sections, we

Figure 3. Comparison of experimental and simulated profiles after cyclic steady state. Lines represent predictions from simulations, and points represent experimental results. 15815



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compositions reported in Figure 4 are at the end of the switch. The x axis depicts the position along the eight columns. It is important to note that the origin of the length axis is always the feed node. Hence, when we refer to length, it is the length relative to the feed node, which is physically moving after each switch. It is known that the system is never in true steady state. The cyclic or pseudo-steady state is said to be achieved when the concentration profiles along the length in consecutive switches are identical. It should be noted that feed and desorbent always enter at the inlet of column 1 and the end of column 4, respectively, whereas the raffinate and extract are withdrawn at the ends of columns 2 and 6, respectively. The outlet of column 8 is recycled back to the inlet of column 1, namely, the feed port. The raffinate and extract composition profiles are plotted in panels a and b, respectively, of Figure 5 for three successive switches, namely, the 46th, 47th, and 48th switches, to show the behavior at cyclic steady state. Here, one can observe that, at cyclic steady state, the concentration profiles do not change in the consecutive switch periods. Also, it is evident that only 2ethylhexyl acetate and water, along with the excess acetic acid, are present at the raffinate and extract ports, respectively. This dynamic behavior continues to repeat in each switch cycle, giving rise to a cyclic steady state. Here, one can observe that, in section III, the reaction occurs around the feed port, where 2ethylhexanol becomes completely converted within the second column of section III, and as a result, 2-ethylhexyl acetate along with acetic acid are obtained at the raffinate port. In section IV, 2-ethylhexyl acetate is adsorbed to give pure acetic acid within the first column. As a result, in the second column of this section, only pure desorbent (acetic acid) is present, which is recycled back to section I. In section I, the adsorbent column is completely regenerated, and hence, only acetic acid is present throughout the section. In section II, only water and acetic acid are present, as water is adsorbed here, which is desired for the complete conversion later in section III. The performance realized for the base-case simulation at cyclic steady state is summarized in Table 4. The separation of 2-ethylhexyl acetate and water in the reactive zone is responsible for the enhancement of the conversion, purity, and productivity, which can be maximized by the proper choice of the section flow rates and switch time. In this case, we obtain a conversion of 99.38%, a raffinate purity of 99.38%, a productivity of 26.98 mol of 2-EHAc (kg of adsorbent)−1 day−1,

solvent because the stoichiometric ratio of acetic acid to alcohol in the esterification reaction is 1:1. The desorbent was taken as acetic acid (99.9% purity) for all simulations. For a given set of conditions, one can also vary feed concentration to arrive at an optimum feed concentration giving the maximum productivity and minimum desorbent consumption.26 Base-Case Simulation. The inputs used for simulations and the performance parameters for the base-case simulation are reported in Table 4 for the production of 36 g/h of 2ethylhexyl acetate. Table 4. Operating and Performance Parameters of the SMBR at Cyclic Steady State for the Base-Case Simulationa parameter

value

switch time, t* (s) conversion of 2-EH (%) raffinate purity (%) extract purity (%) productivity [mol of 2-EHAc (kg of adsorbent)−1 day−1] desorbent consumption [mol of AcH/(mol of 2-EHAc)]

1000 99.38 99.38 99.99 26.98 65.45

a

Operating parameters: QF = 0.74 mL/min, QR = 2.76 mL/min, QD = 13.79 mL/min, Q3 = 3.17 mL/min, AcH/2-EH feed ratio = 1:1 (mol/ mol), temperature = 353 K, column length = 30 cm, reactor diameter = 1.5 cm, SMBR unit configuration = 2−2−2−2.

Figure 4 shows the concentration profiles of all of the components at the end of 48th switch (cyclic-steady-state). The

Figure 4. Concentration profile at the end of the 48th switch (cyclic steady state).

Figure 5. Concentration profiles at the (a) raffinate and (b) extract ports for successive switch periods during cyclic steady state (switch time = 1000 s). 15816



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Figure 6. Effects of switch time on the performance of the SMBR: (a) conversion and purity, (b) desorbent consumption and productivity.

water. Thus, the resin, because of its lower solid pseudovelocity, has a limited ability to remove water. Therefore, the excess water that is not adsorbed in section III finds outlet through the liquid stream leaving section III. The same stream goes into section IV, where it gets adsorbed on the relatively fresh resin that enters this section. This results in a large amount of water in the resin leaving section IV and entering the reaction zone (i.e., section III). Hence, the reaction rate is adversely affected, thereby reducing the performance of the SMBR at longer switch times. A further increase in switch time (>3000 s) affects the performance of section IV, and hence, 2ethylhexyl acetate is not adsorbed completely. It moves with the liquid stream from section IV to section I and adversely affects the extract purity. Desorbent consumption decreases significantly with increasing switch time until 800 s and then becomes constant until 1000 s. With a further increase in switch time (up to 2000 s), water starts breaking through from the outlet of section III; the desorbent consumption starts increasing (until 3000 s) and becomes constant afterward. We found that the productivity decreases due to the lower conversion and poor separation of 2ethylhexyl acetate from water in these regions. We observed here that a switch time in the range of 800−1000 s is optimal in terms of maximum conversion, lowest desorbent consumption, maximum productivity, and maximum purities of 2-ethylhexyl acetate in the raffinate stream and of water in the extract stream. Effects of Column Length. The length of the reactor was varied over the range of 15−60 cm, and its effects on the SMBR performance parameters are shown in Figure 8. An increase in the length of the reactor increases the catalyst/adsorbent loading and also the volume of the reactor. Moreover, the solid pseudo-velocity is also a strong function of the length of the column for a given switch time and increases with increasing column length. The length of the column is thus an important parameter that influences both the reaction and separation. The performance parameters are therefore highly sensitive to a change in column length. Figure 8a shows that there is an increase in the conversion of 2-ethylhexanol and raffinate purity with increasing length up to 30 cm, whereas the extract purity remains constant. Further, the conversion decreases slightly, whereas the purity is affected significantly after a length of 37 cm. This can be explained as follows: With increasing column length until 30 cm, the residence time of the components increases, leading to a rise in conversion. For shorter columns, the solid pseudo-velocity becomes smaller, as a result of which the relative fluid velocity

and a desorbent consumption of 65.45 mol of AcH/(mol of 2EHAc) for the chosen operating conditions. As explained earlier, an SMBR is a discrete version of a TMBR. In a TMBR, there is actual movement of the solid at a certain flow rate. On the other hand, an SMBR operates on the basis of the virtual movement of solid due to the simultaneous switching of the feed, raffinate, desorbent, and extract ports. Hence, conceptually, the two configurations are similar, and thus, as mentioned before, the effects of all of the parameters in an SMBR can be very well explained by viewing the SMBR as a TMBR, that is, by considering the solid flow rate as the bed volume divided by the switch time. Effects of Switch Time. The effect of switch time (t*) on SMBR performance was studied by keeping all other conditions the same as in the base case. As mentioned before, the performance of the SMBR was evaluated in terms of conversion, extract and raffinate purities, productivity, and desorbent (acetic acid) consumption. The effects of switch time on the performance of the SMBR are shown in Figure 6. Switch time mainly influences the solid-phase pseudovelocity and, hence, the residence time of resin in each section. Here, one can observe that the conversion, productivity, and purities at the raffinate and extract ports increased initially with increasing switch time (from 200 to 800 s). At higher switch times (800−1000 s), they become almost insensitive, and after a certain limit (∼1000 s), there is a sudden fall in all of the performance parameters except the extract purity. Extract purity is affected only after a very large switch time (3000 s). Beyond a switch time of 3000 s, the conversion, productivity, and desorbent consumption become insensitive to further change. This effect can be explained with the help of the bulk- and solid-phase concentrations of the individual species at the inlets of each section. The variations in these concentrations at the end of every switch with respect to changing switch time are shown in Figure 7. At lower switch times (200−800 s), the solid-phase pseudo-velocity is very high, which results in less efficient desorption or regeneration of the resin phase in section I. Less desorption results in water slippage through the resin that leaves section I and enters section IV. It is for this reason that the resin entering section III, in which the major part of the reaction takes place, carries a reasonably large amount of water, thereby reducing the conversion and, hence, the productivity and raffinate purity. Because of the presence of water in section IV, ester is not adsorbed completely onto the adsorbent. This ester exits through the extract, making it less pure in terms of water concentration. On the other hand, for higher switch times (1000−2000 s), the resin leaving section III is saturated with 15817



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Figure 7. Effects of switch time: Variation in inlet concentrations of the liquid and solid phases at the end of a switch time during cyclic steady state.

in section III increases. Because of this, the concentration fronts shift toward the raffinate port and eventually contaminate the raffinate port with water. It also indicates that, because of the low solid pseudo-velocity, water is not completely adsorbed in section III, which results in its slippage at the raffinate port.

This leads to a decrease in raffinate purity. For very long columns (beyond 37 cm), both purities decrease dramatically. In this case, a higher solid pseudo-velocity leads to a reduction in solid−liquid contact, resulting in a lower efficiency of regeneration in section I. Water slips through the resin that 15818



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Figure 8. Effects of column length on the performance of the SMBR unit: (a) conversion and purity, (b) productivity and desorbent consumption.

Figure 9. Effects of desorbent flow rate on the performance of the SMBR unit: (a) conversion and purity, (b) productivity and desorbent consumption.

Figure 10. Concentration profiles at the end of a switch during cyclic steady state along the length of the SMBR unit for different desorbent flow rates QD: (a) 6.9 and (b) 13.79 mL/min.

leaves section I and enters sections IV and III, affecting the performances of sections IV and III. Thus, the raffinate and extract purities are hampered at higher column length. Productivity decreases monotonically with increasing length, as can be understood from eq 18. The desorbent consumption is defined as the number of moles of acetic acid used per mole of 2-ethylhexyl acetate produced. Hence, at greater lengths, the desorbent consumption increases to a very high value because 2-EHAc production at the raffinate port is hampered with a decrease in extract purity. Hence, we can conclude that, under the given conditions, the best SMBR performance can be obtained for a column length close to 33 cm. The results can

also be interpreted well by plotting the variation in the inlet concentration to each column, as shown earlier in Figure 7. Effects of Desorbent Flow Rate. Figure 9 shows that the conversion, raffinate purity, and productivity increase with increasing value of QD and then become constant, whereas the extract purity remains unaffected. In addition, the desorbent consumption increases linearly, as anticipated from eq 19. At very low QD, the flow rate in section I (i.e., Q1) becomes low, so that the solid in section I is not regenerated completely, which decreases the raffinate purity. It is evident from Figure 10, which shows the axial concentration profile, that as the desorbent flow rate increases, the regeneration of resin in 15819



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Figure 11. Effects of raffinate flow rate on the performance of the SMBR unit: (a) conversion and purity, (b) productivity and desorbent consumption.

Figure 12. Concentration profiles at the end of a switch during cyclic steady state along the length of the SMBR unit for different raffinate flow rates QR: (a) 1.38 and (b) 2.75 mL/min.

Figure 13. Effects of the flow rate in section III on the performance of the SMBR unit: (a) conversion and purity, (b) productivity and desorbent consumption.

Effects of Raffinate Flow Rate. Because Q3 is constant, there is a maximum limit on the value of QR (i.e., Q3). Figure 11 shows that the extract purity and productivity increase with increasing QR and finally become insensitive to further changes in QR. The desorbent consumption decreases with increasing QR and finally attains a constant value. The conversion and raffinate purity remain unaffected with increasing raffinate flow rate. These results can be explained from the axial profile. Figure 12 shows that an increase in QR improves the separation of 2-ethylhexyl acetate and water as a result of the reduction in the fluid velocity in sections IV and section I. This leads to an

section I is improved. This results in a reduction in the concentration of water in the outgoing resin from section I to a vanishing level, thereby improving the conversion taking place in section III. To summarize, increasing the desorbent flow rate improves conversion and purity, and beyond a certain value, the performance remains unaffected. It is known that, at very high desorbent flow rates, although all of the performance targets are met, the cost of downstream processing to recover fresh solvent is bound to increase. Hence, the desorbent flow rate of 15 mL/ min, which is just sufficient to regenerate the solid in section I, is considered optimal under the conditions of interest. 15820



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Figure 14. Concentration profile at the end of a switch during cyclic steady state along the length of the SMBR unit for different section III flow rates Q3: (a) 3.17 and (b) 4.53 mL/min.

Figure 15. Effects of feed flow rate on the performance of the SMBR unit: (a) conversion and purity, (b) productivity and desorbent consumption. Operating parameters: QX = 11.77 mL/min, QD = 13.79 mL/min, Q2 = 2.43 mL/min, AcH/2-EH feed ratio = 1:1 (mol/mol), temperature = 353 K, switching time (t*) = 1000 s, reactor length = 30 cm; reactor diameter = 1.5 cm, SMBR unit configuration = 2−2−2−2.

Figure 16. Concentration profile at the end of a switch during cyclic steady state along the length of the SMBR unit for different feed flow rates QF: (a) 0.74 and (b) 2.21 mL/min.

parameter, and we selected the section III flow rate to examine its effect. There is a minimum limit on Q3 (i.e., QR) such that Q4 is positive. Figure 13 shows that the conversion, raffinate purity, extract purity, and productivity all decrease with increasing Q3, whereas the desorbent consumption increases with increasing Q3. An increase in Q3 results in an increase in the internal flow rates in all sections. This increase in internal flow rates decreases the residence times of the liquid in the corresponding sections. The decrease in residence time in section III causes the composition fronts to move toward the raffinate port, and

increase in the adsorption of 2-ethylhexyl acetate in section IV, thereby improving the extract purity increasing QR. Even though the raffinate flow rate reaches its maximum limit (i.e., Q3), the conversion and purities are within acceptable limits, which indicates that one can eliminate section IV under these conditions without compromising the desired performance. Effects of Flow Rate in Section III. By fixing three external flow rates, the fourth external flow rate becomes fixed by the mass balance equation. As mentioned earlier, even after fixing all of the external flow rates, one can vary the internal flow rate independently. One of the section flow rates can be chosen as a 15821



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mathematical model was solved using the experimentally determined adsorption and kinetic parameters reported in our previous work.5 The SMBR simulator was validated using four-column SMBR experiments. The targets for performance parameters such as conversion, purity, productivity, and desorbent consumption were achieved by choosing the right flow rates in the sections and the right switch time. Both purities and conversions close to 99% were realized in an SMBR unit of eight columns (2−2−2−2) with a productivity of 32.01 mol of 2-EHAc (kg of adsorbent)−1 day−1 and a desorbent consumption of 46.35 mol of AcH/(mol of 2-EHAc) for an equimolar feed composition. Because the parametric study was based on one-parameter continuation, there is further scope to reduce desorption consumption and increase productivity, and part II of this work further elucidates this idea by proposing a systematic design procedure.

as a result, the conversion and raffinate purity decrease, as shown in Figure 14b. Similarly, the decrease in residence time in section IV results in the presence of ester at the extract port. For a fixed feed flow rate, because of the reduced conversion, the productivity decreases with increasing Q3. Thus, by definition, the desorbent consumption increases with a decrease in the production rate of ester at the raffinate port. As the conversion, purities, and productivities decrease with increasing Q3, the optimum value of Q3 is the minimum value, which is just greater than raffinate flow rate, that is, 2.8 mL/min. Effects of Feed Flow Rate. The effects of the feed flow rate (QF) on the SMBR performance were studied for the given values of QX, QD, Q2 and switch time (1000 s). Figure 15 shows that the conversion, raffinate purity, and desorbent consumption decrease with increasing QF, whereas the productivity at the raffinate port increases with increasing QF. The extract purity remains unaffected with increasing QF. With an increase in the feed flow rate, the molar flow rates of acetic acid and 2ethylhexanol in the feed stream increase, and as a result, more 2-ethylhexyl acetate and water are produced in section III, which increases their productivity in the raffinate stream. An increase in QF also increases the fluid velocity in section III, which results in a shorter residence time in section III. The composition fronts move toward the raffinate port, as shown in Figure 16b, and as a result, the conversion and raffinate purity decrease. The parametric studies revealed that there is a complex interplay among the parameters considered and that obtaining the optimal values of the performance parameters is a challenging task. The design corresponding to the best performance of the SMBR unit configuration containing eight columns (2−2−2−2) for a given feed composition is given in Table 5.

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Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Department of Science and Technology, New Delhi, India, is gratefully acknowledged. We also thank Prof. Sharad Bhartiya for his technical input.

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Table 5. Optimal Performance Parameters at Cyclic Steady State for an Eight-Column SMBR Unita parameter

value

switch time, t* (s) conversion of 2-EH (%) raffinate purity (%) extract purity (%) productivity [mol of 2-EHAc (kg of adsorbent)−1 day−1] desorbent consumption [mol of AcH/(mol of 2-EHAc)]

1000 99.47 99.85 99.21 32.01 46.35

AUTHOR INFORMATION

a

Operating parameters: QF = 1 mL/min, QR = 2.85 mL/min, QD = 12.74 mL/min, Q3 = 3.22 mL/min, length of reactor = 33 cm, reactor diameter = 1.5 cm, AcH/2-EH feed ratio = 1:1 (mol/mol), temperature = 353 K, SMBR unit configuration: 2−2−2−2

The optimum values of operating conditions that we obtained from the parametric study might not represent the global optimum. These are the best operating points around the initial values of operating conditions that we chose. To determine the overall optimum values, we should scan the complete operating region and perform parametric analysis around each point. Hence, there is a need to propose a systematic design strategy for an SMBR giving optimum performance, which is elaborated in part II of this work.

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CONCLUSIONS A simulation-based parametric study was performed for the synthesis of 2-ethylhexyl acetate in an SMBR using bifunctional Amberlyst-15 as the selective adsorbent and catalyst. A 15822

NOMENCLATURE 2-EH = 2-ethylhexanol 2-EHAc = 2-ethylhexyl acetate AcH = acetic acid Ci,j = liquid- (bulk-) phase concentration of species i in section j, mol/L DC = desorbent consumption, mol of AcH/mol of 2-EHAc E = axial dispersion coefficient, cm2/s Eb = activation energy of the backward reaction, J/mol Ef = activation energy of the forward reaction, J/mol FBCR = fixed-bed chromatographic reactor kb = backward rate constant, mol g−1 h−1 k0b = Arrhenius pre-exponential factor for the backward rate constant, mol g−1 h−1 kf = forward reaction rate constant, mol g−1 h−1 k0f = Arrhenius pre-exponential factor for the forward rate constant, mol g−1 h−1 ki = adsorption constant of species i in the LHHW model, L/ mol Ki = adsorption constant in the Langmuir isotherm for species i, L/mol N = number of switches Ncol = total number of columns in the SMBR unit NC = number of components p = number of columns in section III PR = productivity, mol of 2-EHAc (kg of adsorbent)−1 day−1 PuR = raffinate purity PuX = extract purity q = number of columns in section IV QD = desorbent flow rate, mL/min QF = feed flow rate, mL/min Qj = volumetric flow rate in section j, mL/min QR = raffinate flow rate, mL/min QX = extract flow rate, mL/min dx.doi.org/10.1021/ie502090z | Ind. Eng. Chem. Res. 2014, 53, 15811−15823


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(12) Reddy, B.; Mahajani, S. Feasibility of Reactive Chromatography for the Synthesis of n-Propyl Acetate. Ind. Eng. Chem. Res. 2014, 53, 1395. (13) Zhang, Z.; Hidajat, K.; Ray, A. K. Application of Simulated Countercurrent Moving-Bed Chromatographic Reactor for MTBE Synthesis. Ind. Eng. Chem. Res. 2001, 40, 5305. (14) Silva, V. M. T. M.; Rodrigues, A. E. Dynamics of a Fixed-Bed Adsorptive Reactor for Synthesis of Diethylacetal. AIChE J. 2002, 48, 625. (15) Gandi, G. K.; Silva, V. M. T. M.; Rodrigues, A. E. Synthesis of 1,1-Dimethoxyethane in a Fixed Bed Adsorptive Reactor. Ind. Eng. Chem. Res. 2006, 45, 2032. (16) Graça, N. S.; Pais, L. S.; Silva, V. M. T. M.; Rodrigues, A. E. Dynamic Study of the Synthesis of 1,1-Dibutoxyethane in a Fixed-Bed Adsorptive Reactor. Sep. Sci. Technol. 2011, 46, 631. (17) Rodrigues, A. E.; Silva, V. M. T. M. Industrial Process for Acetals Production in a Simulated Moving Bed Reactor. U.S. Patent 2008/0287714, 2008. (18) Minceva, M.; Gomes, P. S.; Meshko, V.; Rodrigues, A. E. Design Methodology and Operation of a Simulated Moving Bed Reactor for the Inversion of Sucrose and Glucose−Fructose Separation. Chem. Eng. J. 2008, 140, 305. (19) Azevedo, D. C. S.; Rodrigues, A. E. Design Methodology and Operation of a Simulated Moving Bed Reactor for the Inversion of Sucrose and Glucose−Fructose Separation. Chem. Eng. J. 2001, 82, 95. (20) Yu, W.; Hidajat, K.; Ray, A. K. Determination of Adsorption and Kinetic Parameters for Methyl Acetate Esterification and Hydrolysis Reaction Catalyzed by Amberlyst 15. Appl. Catal. A Gen. 2004, 260, 191. (21) Silveston, P. L.; Hashimoto, K.; Kawase, M. Chromatographic Reactors. In Periodic Operation of Chemical Reactors; Silveston, P. L., Hudgins, R. R., Eds.; Butterworth-Heinemann: Oxford, U.K., 2013; Chapter 20, pp 569−595. Hashimoto, K.; Kawase, M.; Silveston, P. L. Simulated Moving Bed Chromatographic Reactors. In Periodic Operation of Chemical Reactors; Silveston, P. L., Hudgins, R. R., Eds.; Butterworth-Heinemann: Oxford, U.K., 2013; Chapter 21, pp 597− 635. (22) Ströhlein, G.; Lode, F.; Mazzotti, M.; Morbidelli, M. Design of Stationary Phase Properties for Optimal Performance of Reactive Simulated-Moving-Bed Chromatography. Chem. Eng. Sci. 2004, 59, 4951. (23) Lode, F.; Francesconi, G.; Mazzotti, M.; Morbidelli, M. Comparing True Countercurrent and Simulated Moving-Bed Chromatographic Reactors. AIChE J. 2003, 49, 977. (24) Silva, V. M. T. M.; Rodrigues, E. Novel Process for Diethylacetal Synthesis. AIChE J. 2005, 51, 2752. (25) Fricke, J.; Meurer, M.; Dreisorner, J.; Schmidt-Traub, H. Effect of Process Parameters on the Performance of a Simulated Moving Bed Chromatographic Reactor. Chem. Eng. Sci. 1999, 54, 1487. (26) Gyani, V. C. Reactive Chromatography. Ph.D. Thesis, IIT Bombay, Mumbai, India, 2010; pp 84−89.

r = number of columns in section I Rg = ideal gas constant, J mol−1 K−1 Rij = rate of the reaction of species i in section j, mol g−1 s−1 RC = reactive chromatography s = number of columns in section II SMBR = simulated moving bed reactor t = time, s T = temperature, K t* = switch time, s TMBR = true moving column reactor Vcol = volume of single column, cm3 vD = fluid velocity of through desorbent node, cm/s vF = fluid velocity of through feed node, cm/s vj = superficial fluid velocity of section j, cm/s vR = fluid velocity of through raffinate node, cm/s vX = fluid velocity of through extract node, cm/s X = conversion Greek Letters

Γ∞ i = adsorption capacity of component i, mol/g Γ(N) ij = concentration of component i in the adsorbed phase in section j during the Nth switch time ε = column void fraction υi = stoichiometric coefficient of component i ρp = density of adsorbent particles, g/L

Subscripts and Superscripts

col = column D = desorbent F = feed i = component i j = section j N = number of switches R = raffinate X = extract

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REFERENCES

(1) Westerterp, K. R. Multifunctional Reactors. Chem. Eng. Sci. 1992, 47, 2195. (2) Agar, D. W. Multifunctional Reactors: Old Preconceptions and New Dimensions. Chem. Eng. Sci. 1999, 54, 1299. (3) Kulprathipanja, S. Reactive Separation Process; Taylor & Francis: New York, 2002. (4) Sundmacher, K.; Kienle, A.; Seidel-Morgenstern, A. Integrated Chemical Processes; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2005. (5) Gyani, V. C.; Mahajani, S. Reactive Chromatography for the Synthesis of 2-Ethylhexyl Acetate. Sep. Sci. Technol. 2008, 43, 2245. (6) Mazzotti, M.; Kruglov, A.; Neri, B.; Gelosa, D.; Morbidelli, M. A Continuous Chromatographic Reactor: SMBR. Chem. Eng. Sci. 1996, 51, 1827. (7) Kawase, M.; Suzuki, T. B. E. N.; Inoue, K.; Yoshimoto, K.; Hashimoto, K. Increased Esterification Conversion by Application of the Simulated Moving-Bed Reactor. Chem. Eng. Sci. 1996, 51, 2971. (8) Lode, F.; Houmard, M.; Migliorini, C.; Mazzotti, M.; Morbidelli, M. Continuous Reactive Chromatography. Chem. Eng. Sci. 2001, 56, 269. (9) Gelosa, D.; Ramaioli, M.; Valente, G.; Morbidelli, M. Chromatographic Reactors: Esterification of Glycerol with Acetic Acid Using Acidic Polymeric Resins. Ind. Eng. Chem. Res. 2003, 42, 6536. (10) Ströhlein, G.; Assunçaõ , Y.; Dube, N.; Bardow, A.; Mazzotti, M.; Morbidelli, M. Esterification of Acrylic Acid with Methanol by Reactive Chromatography: Experiments and Simulations. Chem. Eng. Sci. 2006, 61, 5296. (11) Deshmukh, K. S.; Gyani, V. C.; Mahajani, S. M. Esterification of Butyl Cellosolve with Acetic Acid Using Ion Exchange Resin in Fixed Bed Chromatographic Reactors. Int. J. Chem. React. Eng. 2009, 7, 1. 15823


Simulated Moving Bed Reactor for the Synthesis of 2-Ethylhexyl

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