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Journal of Process Control 18 (2008) 215231Overall control strategy of a coupled reactor/columnsprocess for the production of ethyl acrylate

I-Lung Chien *, Kay Chen, Chien-Lin Kuo

Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 106, Taiwan

Received 1 December 2006; received in revised form 10 February 2007; accepted 20 February 2007Abstract

Ethyl acrylate (EA) is widely used in industry as a precursor for varnishes, adhesive, and finishes of papers and textiles. This impor-tant ester can be produced directly from ethanol (EtOH) and acrylic acid (AA) via esterification reaction with the presence of sulphuricacid as homogeneous catalyst. The proposed design flowsheet of this process includes a CSTR reactor coupled with a rectifier and anoverhead decanter. In order to further purify the final EA product, another stripper is needed with its top vapor recycled back to decan-ter. The simplest and industrial easily applicable overall control strategy will be investigated with only one tray temperature control loopin each of the two columns. The final proposed overall control strategy of this process is found to be different than another similar cou-pled reactor/columns process published earlier [I-L. Chien, Y.P. Teng, H.P. Huang, Y.T. Tang, Design and control of an ethyl acetateprocess: coupled reactor/column configuration, J. Proc. Cont. 15 (2005) 435449]. Both EtOH and AA feed flow rates are used as manip-ulated variables in the overall control strategy with CSTR heat duty left as throughput manipulator for the overall process. The final EAproduct with stringent specifications of 0.1 wt% EtOH and 0.005 wt% AA impurities can be achieved with this proposed overall controlstrategy despite feed flow rate and feed composition disturbances. 2007 Elsevier Ltd. All rights reserved.

Keywords: Ethyl acrylate; Esterification reaction; Reactive distillation; Coupled reactor/columns; Optimum design; Overall control strategy1. Introduction

Ethyl acrylate (EA) is widely used in industry as a pre-cursor for varnishes, adhesive, and finishes of papers andtextiles. This important ester can be produced directly fromethanol (EtOH) and acrylic acid (AA) via esterificationreaction with the presence of sulphuric acid as homoge-neous catalyst. Only in a very recent paper [2], the kineticsof this esterification reaction has been given. There is nopaper in the literature on the subject of the production ofethyl acrylate, thus the results of this paper should be use-ful to other researchers.

In this study, overall control strategy of this processwith coupled reactor/columns configuration will be stud-0959-1524/$ - see front matter 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.jprocont.2007.02.006

* Corresponding author. Tel.: +886 2 27376652; fax: +886 2 27376644.E-mail address: chien@ch.ntust.edu.tw (I-Lung Chien).ied. The principal behind the coupled reactor/columns con-figuration, similar to reactive distillation, is that thecontinuous removal of products from the esterificationreaction mixture by distillation reduces the backward reac-tion rate. The advantage of the coupled reactor/columnsconfiguration over reactive distillation according to Yiand Luyben [3] include: the existing reactor/columns inthe plant can be retrofitted for this usage; easy maintenanceof the overall system; larger reactor holdup and differentreaction temperature can easily be designed; etc.

In a three-paper series by Yi and Luyben [35], theystudied the design and control of various coupled reac-tor/column systems. The studied systems include: a binaryreactor/rectifier, a binary reactor/stripper, a multicompo-nent reactor/rectifier, a multicomponent reactor/rectifier/stripper, and a more complex process that consists of acoupled reactor/stripper, two distillation columns andone recycle stream. Their studied systems are very simple,

mailto:chien@ch.ntust.edu.tw

216 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231ideal chemical systems and also no liquidliquid equilib-rium is considered. Chiang et al. [6] studied a coupled reac-tor/column system for the production of amyl acetate.Their process is much simpler than the studied ethyl acry-late process because amyl acetate has the highest boilingpoint in the system. Their system with the configurationof reactor with rectifier on top and stripper on the bottomproduces amyl acetate from the bottom of the stripper andalso produces water through aqueous phase of a decanter.Chien et al. [1] proposed a coupled reactor/column config-uration for the production of ethyl acetate. Their processflowsheet configuration is very similar to the ones in thispaper. However, the overall control strategy is differentthan the one will be developed for this system.

The organization of this paper is as follows. The ther-modynamic properties of this four-component system andthe kinetics of this esterification reaction will be given inSection 2. The design flowsheet of a complete coupled reac-tor/column system will be proposed in Section 3. A finalEA product purity of over 99.5 wt% will be obtained inthe proposed design with stringent specifications of0.1 wt% EtOH and 0.005 wt% AA impurities. In Section4, the overall control strategy of this process will be inves-tigated. Only one tray temperature control loop in each ofthe two columns (rectifier and stripper) will be used. Feedflow rate and feed composition disturbances will be usedto test the overall control strategy. Some concludingremarks will be drawn in the final section.2. Thermodynamic and kinetic model used in the simulation

There are total of four azeotropes in this system includ-ing two homogeneous azeotropes of EtOH + H2O andEtOH + EA and two heterogeneous azeotropes ofEA + H2O and EtOH + EA + H2O. In order to accuratelyrepresent the overall system, liquid activity coefficientmodel was used for the vaporliquidliquid equilibrium.A suitable NRTL (nonrandom two-liquid) model parame-ter set has been established with excellent prediction of thecompositions and temperatures for the four azeotropes inthis system. In this NRTL parameter set, the Aspen Plus

built-in binary-pair parameters of AAEA and AAH2Owere used. For the EtOHH2O, EtOHEA, EAH2O pairs,binary parameters were obtained to fit well the LLEboundary of these three components. For the one pair ofAAEtOH that does not have the Aspen Plus built-inNRTL binary parameters, the Dortmund modified UNI-FAC group contribution estimation method [7,8] was usedto obtain the remaining thermodynamic model parameters.Vapor association of Acrylic acid due to dimerization hasalso been included by using the second virial coefficientof the HaydenOConnell [9] model in the vapor phase.The Aspen Plus built-in association parameters wereemployed to compute fugacity coefficient.

The kinetic model of this esterification reaction is fromthe paper by Witczak et al. [2] with diluted sulphuric acidas homogeneous catalyst. The reaction can be seen asbelow:

C2H5OHEtOH

CH2CHCOOHAA

$ CH2CHCOOC2H5EA

H2O 1

The kinetic equation is

r k1C cat C2AAC2EtOH C2EAC

2H2O

K2

!mol=dm3 min

where

k1 3:26 106 exp15900RT

dm12=mol4 min

K 2:71 104 exp 6490RT

and R is gas constant (1.987 cal/mol/K) and Ccat is as-sumed to have value of 0.15 mol/dm3. In the kinetic equa-tion, all concentrations are with unit of (mol/dm3) andtemperature in K.

3. Design flowsheet of the complete process

The RCM for the EtOHEAH2O three-componentsystem and AAEAH2O three-component system can beseen in Figs. 1 and 2. From these two figures, the highestboiling point temperature of the whole system includingthe pure components and azeotropes is the acrylic acid(AA) at 141.19 C and the lowest temperature of the wholesystem is the EtOH + EA + H2O three-component azeo-trope at 75.29 C. The two products (ethyl acrylate andwater) of this esterification reaction are neither the lightestnor the heaviest component in the system, thus the com-plete designed process will need to be more complex incomparison with the other reactive distillation papers inthe literature.

The proposed design of this process including a CSTRreactor coupled with a rectifier (without heat source). Theheat input in the CSTR totally vaporizes the reactor outletstream to vapor phase and continuously enter the rectifierfrom the bottoms to promote the forward reaction further.The bottom liquid stream from the rectifier containingmostly heavy boiler AA is recycled back to the CSTR.The composition of the top vapor stream from the rectifieris close to the lightest boiler of ternary azeotrope of EtO-H + EA + H2O. This stream after sub-cooling to 40 Ccan be naturally separated inside a decanter to formorganic and aqueous phases. Extra water is added in thedecanter to maintain suitable composition inside of theliquidliquid boundary. The water purity of the aqueousphase is quite high, thus it can go to a waste water treat-ment plant for discharge. The organic phase compositionby natural liquidliquid separation has the benefit of cross-ing the distillation boundary into a desirable region toobtain pure ethyl acylate product (see Fig. 1). This organic

EA WATER

AA

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

(141.19 C)

(99.40 C)(81.04 C)

(100.02 C)

Fig. 2. RCM of AAEAH2O three-component system.

EA WATER

EtOH

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9 (78.20 C)

(77.87 C)

(99.40 C)

(78.31 C)

(100.02 C)(81.04 C)

(75.29 C )

Fig. 1. RCM of EtOHEAH2O three-component system.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 217phase stream is partly refluxed and is partly designed tofeed into another stripper with reboiler for further purifica-tion into the final EA product. The top vapor of this strip-per with composition near the top vapor of the rectifier is

Organic Reflux

AA

Steam

Aqueous

EtOH

Decanter

Steam

EA

Reboiler

Water

CSTR

Fig. 3. Conceptual design of the overall process flowsheet.

218 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231also condensed and then fed into the decanter. The bottomstream of the stripper is the final EA product with stringentspecifications of 0.1 wt% EtOH and 0.005 wt% AA impuri-ties in this product stream. This conceptual design of theoverall process flowsheet can be seen in Fig. 3.

The design flowsheet is selected based on the maximiza-tion of Total Annual Profit (TAP) for the overall system.This TAP includes: the product value minus the costs oftwo feed streams, minus annualized capital costs, minustotal utility costs, and minus the waste water treatmentColumn 1=24 stagesColumn 2=7 stagesCSTR volume=126 cum

Water flow rate (mol/min)0 20 40 60 80 100 120

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5.30e+5

5.35e+5

5.40e+5

5.45e+5

5.50e+5

5.55e+5

5.60e+5

Water flow rate=50 mole/minCSTR volume=126 cumColumn 2=7 stages

Tray of Column 122 23 24 25 26

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5.562e+5

5.563e+5

5.564e+5

5.565e+5

5.566e+5

5.567e+5

5.568e+5

Fig. 4. Optimization result with EtOcost. The reason to add product value and the costs oftwo feed streams in the TAP calculation is because thetwo feeds are not necessarily fixed at equal molar ratio.The annualized capital costs follow directly from the calcu-lation procedure in Douglas [10] with the annual capitalcharge factor of 1/3 was used. The annualized equipmentcost includes CSTR, column shells, column trays, reboilers,and condensers. The utility cost including the steam andcooling water costs are calculated the same way as inChiang et al. [6]. The waste water treatment cost is calcu-Water flow rate=50 mole/minColumn 1=24 stagesColumn 2=7 stages

CSTR Volume (cum)125 126 127 128 129 130 131 132 133

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5.46e+5

5.48e+5

5.50e+5

5.52e+5

5.54e+5

5.56e+5

5.58e+5

Water flow rate=50 mole/minCSTR volume=126 cumColumn 1=24 stages

Tray of Column 25 9

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5.561e+5

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5.568e+5

6 7 8

H feed flow rate of 96 mol/min.

Table 1Optimum process design for this system

CSTR holdup (m3) 64

Rectifier total stages(where 20th stage is the CSTR)

20

Stripper total stages(including the reboiler)

7

Fresh EtOH feed flow rate(g mol/min)

102

EtOH feed composition 82.2 mol% EtOH17.8 mol% H2O

Fresh AA feed flow rate(g mol/min)

66

AA feed composition 100 mol% AAVapor flow rate from CSTR

to rectifier (g mol/min)258

Bottom liquid flow rate from rectifier toCSTR (g mol/min)

90

CSTR heat duty (KW) 151.8Water injection rate into the

decanter (g mol/min)100.0

Organic reflux flow rate (g mol/min) 84.05Organic outlet flow rate into

stripper (g mol/min)145.55

Stripper reboiler duty (KW) 70.10Aqueous outlet flow rate (g mol/min) 204.1EA product flow rate (g mol/min) 63.9

EA product composition 99.78 mol% (99.89 wt%) EA0.0069 mol% (0.005 wt%) AA0.213 mol% (0.10 wt%) EtOH1 104 mol% H2O

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 219lated with the estimation of $0.053/m3 (given in Table 3.4of Turton et al. [11] textbook).

The design and operating variables that need to bedetermined include AA/EtOH feed ratio, CSTR holdup,total stages of the rectifier, total stages of the stripper,and the water addition rate into the decanter. An iterativesequential optimization procedure is proposed to find theoptimal flowsheet of the overall system. In the design ofthe process flowsheet, pure AA feed composition isassumed while the EtOH feed stream is practically assumedto contain 82.2 mol% EtOH and 17.8 mol% H2O.

The acrylic acid flow rate is fixed at 66 mol/min to setthe throughput of the overall process. The organic refluxflow rate from the decanter is manipulated to hold theAA impurity in the final EA product to be at the specifica-tion of 0.005 wt%. The reboiler duty of the stripper ismanipulated to hold the EtOH impurity in the final EAproduct to be at the specification of 0.1 wt%. The iterativesequential optimization procedure is to find the optimiza-tion result at a particular EtOH feed flow rate. In eachoptimization search, four design and operating variablescan be changed including CSTR holdup volume, rectifyingcolumn total stage, stripping column total stage, and wateraddition rate. Iterative procedure is needed to find the opti-mization result. Fig. 4 shows an example of the optimiza-tion result with EtOH feed flow rate at 96 mol/min. Inthis case, the water addition rate to achieve the highestTAP is at 50 mol/min, the CSTR holdup volume is126 m3, rectifying column total stage is 24, and strippingcolumn total stage is 7. Notice that in this case, CSTRholdup volume below 126 m3 cannot satisfy the AA impu-rity specification in final EA product stream.

Summarizing the optimization results at various EtOHfeed flow rates, Fig. 5 shows that the optimal EtOH feedflow rate is at 102 mol/min. At this flow rate, the TAP ismaximized at $5.95 105. The optimum pure AA and pureEtOH feed ratio is calculated to be 1:1.27, not at exactlyequal molar ratio. With this optimum feed ratio, otheroptimum design and operating variables are: water addi-EtOH flow rate (mol/min)94 96 98 100 102 104 106 108 110

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5.6e+5

5.7e+5

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6.0e+5

Fig. 5. Summary of optimization results at various EtOH feed flow rates.tion rate at 100 mol/min, CSTR holdup volume at 64 m3,rectifying column total stage at 20, and stripping columntotal stage at 7. The final overall process flowsheet can beseen in Table 1. Notice from the final flowsheet, the speci-fications of the two impurities are met and the final EAproduct purity is at 99.78 mol% (or 99.89 wt%).

4. Overall control strategy of this process

Some straightforward inventory and other loops aredetermined first. These include the following: the organicphase level is controlled by the organic outlet flow tostripper; the aqueous phase level is controlled by the aque-ous outlet flow; the bottom level of the stripper is con-trolled by the final product flow; the top pressures ofthe rectifier and stripper are controlled at 1.1 atm by thetop vapor flow; and the temperature at decanter are con-trolled at 40 C by the condenser duty. The organic refluxratio is fixed by a ratio scheme where the value of theratio can be set by a tray temperature control loop if nec-essary. The extra water flow rate into the decanter is ratioto a throughput manipulator yet to be determined. Sum-marizing of the basic loops in the process can be seen inFig. 6.

For the candidate overall control strategies studied inthis paper, some other prerequisite assumptions are out-lined below. Firstly, all control loops are in conventionalPID form for easier industrial applications. Secondly,

Organic Reflux

AA

Steam

Aqueous

EtOH

Decanter

Steam

EA

Reboiler

Water

CSTR

PC PCLC LC

LC

FC

FC

FC

FC

1 1

196

207

TCTC

X

Ratio to athroughputmanipulator

Fig. 6. Basic regulatory control strategy of the overall process.

220 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231only one tray temperature control loop is investigated ineach of the rectifier and stripper column to avoid strongloop interactions. Thirdly, the manipulated variable ofthe tray temperature control loop for the stripper arestraightforwardly determined as the stripper reboilerduty.

The manipulated variables of the remaining two mostimportant control loops are yet to be determined. Thesetwo control loops are the CSTR level control loop andthe rectifier tray temperature control loop. The CSTR levelcontrol loop unlike the other level control loops needs toinclude integral mode to make sure the reaction volumeis maintained. The candidate manipulated variables forthese two important loops are: AA feed flow, EtOH feedflow, CSTR heat duty, and organic reflux ratio. Thus, theplanning of the overall control strategy is to select twoout of the above four manipulated variables to controlCSTR level and one tray temperature at rectifier. For theremaining two manipulated variables, one can be used asthe throughput manipulator to set the production rate ofthe overall process, and the other is fixed throughout vari-ous disturbance changes.

4.1. Determine of tray temperature control point and the

control structure

Closed-loop sensitivity analysis similar to the one usedin Lee et al. [12] will be used here to determine the traytemperature control point at rectifier and also at stripper.The purpose for the closed-loop sensitivity analysis is dif-ferent in this paper. In Lee et al. [12], the control structurewas already set and the closed-loop analysis is solely usedto determine the temperature control point. However, inthis paper, the closed-loop sensitivity analysis is not onlyused to determine the tray temperature control point atrectifier and at stripper, but it is also used to screen outthe possible worse control structures in the overall controlstrategy.

Because there are four free manipulated variableswhich could be used for the CSTR level control loop andrectifier tray temperature control loop, five possible controlstructures considered are listed below:

CS1: EtOH feed flow and CSTR heat duty are used asmanipulated variables for the CSTR level and recti-fier tray temperature loops, AA feed flow is used asthroughput manipulator, and organic reflux ratio isfixed throughout various disturbance changes.

CS2: AA feed flow and CSTR heat duty are used asmanipulated variables for the CSTR level and recti-fier tray temperature loops, EtOH feed flow is usedas throughput manipulator, and organic reflux ratiois fixed throughout various disturbance changes.

CS3: EtOH feed flow and AA feed flow are used as manip-ulated variables for the CSTR level and rectifier traytemperature loops, CSTR heat duty is used asthroughput manipulator, and organic reflux ratio isfixed throughout various disturbance changes.

CS4: EtOH feed flow and organic reflux ratio are used asmanipulated variables for the CSTR level and recti-fier tray temperature loops, AA feed flow is used asthroughput manipulator, and CSTR heat duty isfixed throughout various disturbance changes (onlyratio to measurable AA feed flow).

CS5: AA feed flow and organic reflux ratio are used asmanipulated variables for the CSTR level and recti-fier tray temperature loops, EtOH feed flow is usedas throughput manipulator, and CSTR heat duty isfixed throughout various disturbance changes (onlyratio to measurable EtOH feed flow).

Trays of 2nd Column1 2 3 4 5 6 7

Del

ta te

mpe

ratu

re (

C)

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4AA-5%EtOH(H2O)+20%EtOH(H2O)-20%throughput+10%throughput-10%

Organic reflux ratio fixed & AA fixed

Trays of 1ST Column1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Del

ta te

mpe

ratu

re (

C)

-1.5

-1.0

-0.5

0.0

0.5

1.0

Fig. 7. Closed-loop sensitivity plot with organic reflux ratio and AA feedfixed.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 221There is another control structure which uses CSTRheat duty and organic reflux ratio as manipulated variablesfor the CSTR level and rectifier tray temperature loops.However, in this case, both EtOH feed flow and AA feedflow will be fixed, thus will be infeasible to cope with feedcomposition changes.

The screening of the above five possible control struc-tures can easily be made by the closed-loop sensitivityanalysis described below using process simulation tool.In the following, two ideal composition control loops areassumed to be present which holds the specifications of0.1 wt% EtOH and 0.005 wt% AA impurities in the prod-uct stream by varying two chosen manipulated variables.One of the manipulated variables has been pre-determinedto be the stripper reboiler duty which controls the EtOHimpurity at 0.1 wt% in the product stream and the othermanipulated variable to hold AA impurity at 0.005 wt%in the product stream can be varied due to different con-trol structure. Three disturbance changes are made inthe closed-loop simulations, they are 10% changes inthe throughput manipulator; 20% changes in the EtOHfeed water concentration; and the changes of water con-centration in the AA feed from 0 mol% to 5 mol%. Forexample, the closed-loop simulation for control structureCS1 can be achieved with: fixing AA impurity in the prod-uct stream at 0.005 wt% by varying EtOH feed flow rate;fixing CSTR level at original setpoint by varying CSTRheat duty; and fixing EtOH impurity in the product streamat 0.1 wt% by varying stripper reboiler duty. Thus, forcontrol structure CS1 in the simulation runs, organicreflux ratio and AA feed rate are fixed with AA feed rateonly moves when throughput manipulator changes arerequired.

For each disturbance case, the temperature profiles ofrectifier and stripper under perfect composition controlcan be obtained for the above mentioned three disturbancechanges. By comparing the temperature profiles for thedisturbance changes versus the base case, the deviationsof the temperature profiles of rectifier and stripper canbe plotted. Because this is an ideal disturbance rejectioncondition with both product specifications hold at theiroriginal values, one could choose a tray temperature con-trol point for rectifier and another one for stripper andusing two tray temperature control loops to replace theideal dual-composition control loops. The chosen temper-ature control point for rectifier or stripper should be theone with the least deviation of the tray temperature.Fig. 7 shows such plot for control structure CS1 underabove mentioned three kinds of disturbance changes.The focus of observing the least deviations of the temper-atures should be made for the feed composition changes.The reason is because the disturbance change for thethroughput manipulator is considered as known distur-bance, thus, some calculating scheme to adjust the setpointvalue of the temperature control loop as proposed inHuang et al. [13] can easily be applied to hold the productspecifications.From Fig. 7, the suitable choice of the tray temperaturecontrol point for rectifier is at tray #5 and for stripper isalso at tray #5. Notice that for rectifier, the tray #5 gavevery small deviations under 20% changes in the EtOHfeed water concentration, however, the temperature devia-tion under 5% change in the AA feed concentration gaveconsiderably larger temperature deviation. From theseclosed-loop simulations, one could predicts that by usingcontrol structure CS1 with two tray temperature controlpoints at tray #5 of rectifier and tray #5 at stripper, thecontrol performance should perform well under 20%changes in the EtOH feed water concentration. However,for 5% changes in the AA feed concentration there willbe some deviations in final product impurity especiallyfor AA impurity. Of course the chosen tray temperaturecontrol points should be evaluated so that enough open-loop sensitivity is there between the manipulated variable(in this case EtOH feed rate) and rectifier tray #5 temper-ature and also between stripper reboiler duty and stripper

Organic reflux ratio fixed & Q1 fixed

Trays of 1ST Column1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Del

ta te

mpe

ratu

re (

C)

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.15

0.20

AA-5%EtOH(H2O)+20%EtOH(H2O)-20%throughput+10%throughput-10%

222 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231tray #5 temperature. This open-loop sensitivity evaluationwill be shown later in this section.

Similar closed-loop sensitivity simulations can be runfor other control structures CS2, CS3, CS4, and CS5. Figs.811 show the temperature deviations for control struc-tures CS2CS5, respectively. By observing these plots,the tray temperatures for various control structures withthe least temperature deviation for rectifier are at tray#5 for CS2 and CS3 and at tray #4 for CS4 and CS5.For the stripper, the one with the least temperature devia-tion are all at tray #5. The control at stripper should beeasy and should give acceptable control performancebecause the temperature deviations at tray #5 under vari-ous disturbances are all not very large. On the other hand,the control at rectifier is more difficult and should drawmore attention. The control structure CS3 gave the leasttemperature deviations at tray #5 of rectifier under vari-ous disturbance changes while CS4 and CS5 gave quitelarge temperature deviations at the least temperature devi-ation of tray #4 under 5% change in the AA feedconcentration.Organic reflux ratio fixed & EtOH fixed

Trays of 1ST Column1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Del

ta te

mpe

ratu

re (

C)

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Trays of 2nd Column

1 2 3 4 5 6 7

Del

ta te

mpe

ratu

re (

C)

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4AA-5% EtOH(H2O)+20%EtOH(H2O)-20%throughput+10%throughput-10%

Fig. 8. Closed-loop sensitivity plot with organic reflux ratio and EtOHfeed fixed.

Trays of 2nd Column1 2 3 4 5 6 7

Del

ta te

mpe

ratu

re (

C)

-0.20

-0.15

-0.10

-0.05

0.00

0.05

0.10

Fig. 9. Closed-loop sensitivity plot with organic reflux ratio and CSTRduty fixed.From the above simply closed-loop sensitivity analysis,we can abandon further dynamic investigation of controlstructures CS4 and CS5 and focus on CS1 to CS3 first.Only if dynamic behaviors of CS1 to CS3 are not accept-able, we will re-investigate CS4 and CS5 then.

4.2. Loop pairing and dynamic considerations

From the above analysis, it is better to fix organic refluxratio, thus, the manipulated variables used for the rectifiertray temperature at tray #5 and CSTR level loops shouldbe picked from the below three manipulated variables:EtOH feed; AA feed; or CSTR heat duty. Fig. 12 showsthe open-loop dynamic response with organic reflux ratiofixed and step changes of either EtOH feed, AA feed, orCSTR duty. When one of the three manipulated variablesstep changes, the remaining two manipulated variablesare kept at their original base case values. The first obser-vation is that none of the dynamic responses can be mod-

Trays of 2nd Column1 2 3 4 5 6 7

Del

ta te

mpe

ratu

re (

C)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6AA-5% EtOH(H2O)+20%EtOH(H2O)-20%throughput+10%throughput-10%

Q1 fixed & AA fixed

Trays of 1ST Column1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Del

ta te

mpe

ratu

re (

C)

-1.5

-1.0

-0.5

0.0

0.5

1.0

Fig. 10. Closed-loop sensitivity plot with CSTR duty and AA feed fixed.

Trays of 2nd Column1 2 3 4 5 6 7

Del

ta te

mpe

ratu

re (

C)

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6AA-5%EtOH(H2O)+20%EtOH(H2O)-20%throughput+10%throughput-10%

Q1 fixed & EtOH fixed

Trays of 1ST Column1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Del

ta te

mpe

ratu

re (

C)

-1.5

-1.0

-0.5

0.0

0.5

1.0

Fig. 11. Closed-loop sensitivity plot with CSTR duty and EtOH feedfixed.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 223eled as first-order plus deadtime process. A suitable modelto use should be the integrating plus deadtime model form.Also from this figure, one observed that the dynamicresponse of CSTR heat duty versus tray #5 temperatureof rectifier is very problematic, thus this pairing shouldbe avoid. The inverse response behavior can be explainedby the combinatory effects of this tray temperature dueto CSTR heat duty and also from the changing of theCSTR level (reactor volume). It is also notice that theopen-loop effect of AA feed to tray #5 temperature of rec-tifier is considerably smaller than that from EtOH feed,thus, it is better to use EtOH feed to control tray #5temperature.

Since CSTR heat duty is not suitable to use as themanipulated variable for the tray #5 temperature controlloop, one of the candidate overall control strategy is touse CSTR heat duty to control CSTR level. This pairingwas also used in Chien et al. [1]. Fig. 13 shows anotherreason why AA feed is not suitable to control tray #5temperature. With CSTR heat duty manipulating CSTRlevel in automatic mode, we can do open-loop stepresponse for changes in AA or EtOH feed flow rate.From Fig. 13, one observe that the open-loop responsewith AA feed as the manipulated variable exhibits largeinverse response. Let us take AA feed +1% changes asan example. Because AA is a high-boiler with normalboiling point at 118 C, thus increasing AA feed intothe system should eventually make tray #5 temperatureto increase. However, increasing AA feed flow rate willalso result in increasing the CSTR level, thus makingthe CSTR heat duty to drop which temporarily decreas-ing tray #5 temperature causing the inverse response.Increasing EtOH feed flow rate will not have the inverseresponse because EtOH is a low-boiler thus eventuallywill make tray #5 temperature to drop which coincidewith the effect of decreasing CSTR heat duty due to levelcontrol action. Thus, the first overall control strategy tofurther dynamically evaluate the control performance isto use CSTR heat duty to control CSTR level and to

T 5 o

f 1st c

olum

n (

C)

94.4

94.6

94.8

95.0

95.2

95.4

95.6

Q1 +1%Q1 -1%

Time (hr)0 10 15 20 25 30

T 5 o

f 1st c

olum

n (

C)

92

94

96

98

100

ETOH feed +1%ETOH feed -1%

CST

R le

vel (

m)

4.356

4.358

4.360

4.362

4.364

4.366

4.368

4.370

4.372

ETOH feed +1%ETOH feed -1%

T 5 o

f 1st c

olum

n (

C)

94.2

94.4

94.6

94.8

95.0

95.2

95.4

95.6

95.8

AA feed +1%AA feed -1%

CST

R le

vel (

m)

4.358

4.360

4.362

4.364

4.366

4.368

4.370

AA feed +1% AA feed -1%

CST

R le

vel (

m)

4.355

4.360

4.365

4.370

Q1 +1%Q1 -1%

5Time (hr)

0 10 15 20 25 305Time (hr)

0 10 15 20 25 305

Time (hr)0 10 15 20 25 305

Time (hr)0 10 15 20 25 305

Time (hr)0 10 15 20 25 305

Fig. 12. Open-loop responses with step changes in either of EtOH feed, AA feed, or CSTR duty.

224 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231use EtOH feed flow rate to control tray #5 temperatureof rectifier. AA feed will be used as the throughputmanipulator and other control loops are explained previ-ously. This overall control strategy is denoted as ControlStrategy (I). This overall control strategy is similar towhat Chien et al. [1] used in their ethyl acetate system.However, in their control strategy the tray temperatureat rectifier was controlled by manipulating the acid feedflow rate rather than the EtOH feed flow rate as is usedin our system.

Another overall control strategy from previous CS3 is touse CSTR heat duty as throughput manipulator and lettingorganic reflux ratio to be fixed under various disturbances.Note again from previous Fig. 9, this CS3 results in theleast temperature deviations under perfect compositioncontrol. In this CS3, AA and EtOH feed flow rates willbe used as the manipulated variables for the CSTR levelloop and tray #5 temperature control loop at rectifier.The loop pairing selection is to use AA feed to controlCSTR level and to use EtOH feed to control tray #5 tem-perature at rectifier since the effect of AA feed to this traytemperature is considerably smaller as can be seen in previ-ous Fig. 12. This second overall control strategy using twofeed flow rates to control CSTR level and one tray temper-ature at rectifier is denoted as Control Strategy (II). Kay-mak and Luyben [14] in their reactive distillation columncontrol study proposed to use two feed flow rates to con-trol two tray temperatures at the reactive distillation col-umn. Although their control strategy is different than ourstudy, the concept of using heat duty as throughput manip-ulator is the same.4.3. Open-loop sensitivity analysis to verify the suitability

of temperature control points

Before the dynamic evaluation of closed-loop perfor-mances for the two overall control strategies, the open-loopsensitivity analysis will be performed to verify that the tem-perature control point determined by closed-loop sensitiv-ity analysis in Section 4.1 is really workable or not. Inorder to have the tray temperature control loop to workwell, the chosen manipulated variable needs to haveenough open-loop sensitivity to the controlled tray temper-ature. For the open-loop sensitivity plots of Control Strat-egy (I), CSTR level loop is in automatic mode manipulatedby the CSTR heat duty. Similarly for the open-loop sensi-tivity plots of Control Strategy (II), the CSTR level loop isin automatic mode manipulated by AA feed flow rate. Sim-ilar open-loop sensitivities are observed for either of Con-trol Strategies (I) or (II). Fig. 14 shows the exampleresult for Control Strategy (II). From this figure, it isshown that enough open-loop sensitivity is observed forthe selected temperature control points. The same conclu-sion can be made for Control Strategy (I).

4.4. Closed-loop dynamic simulation results

Rigorous dynamic simulator, Aspen DynamicsTM, wasused for all the closed-loop dynamic simulations. The moredifficult to handle unmeasured feed composition changeswill be tested first on Control Strategies (I) and (II).Fig. 15 shows the closed-loop dynamic simulation withAA feed concentration changes from totally AA to include

CSTR Level Q1 in auto mode

Time (hr)0 10 15 20 25 30

T 5of

1st C

olum

n (

C)

93.5

94.0

94.5

95.0

95.5

96.0

96.5

AA Feed +1%AA Feed -1%

CSTR Level Q1 in auto mode

T 5of

1st C

olum

n (

C)

92

93

94

95

96

97

98

99

EtOH Feed +1%EtOH Feed -1%

5

Time (hr)0 10 15 20 25 305

Fig. 13. Open-loop responses for tray #5 temperature of rectifyingcolumn with step changes in either AA feed or EtOH feed while CSTRlevel manipulating its duty is in auto mode.

Trays of 2nd Column1 2 3 4 5 6 7

Tem

pera

ture

(oC

)

75

80

85

90

95

100

105

Base caseQ2 +1%Q2 -1%

Trays of 1ST Column

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Tem

pera

ture

(oC

)

80

90

100

110

120

Base caseEtOH feed +0.01%EtOH feed -0.01%

Fig. 14. Open-loop sensitivity plot for the rectifying and strippingcolumns while CSTR level is controlled by AA feed.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 2255% water at t = 2 h. The three important control loops(tray #5 temperature at rectifier, tray #5 temperature atstripper, and CSTR level) are all controlled back to theirsetpoint values. The rather slowness of the dynamicresponse is mainly due to the needed large volume of theCSTR reactor in the optimized flowsheet. With a smallerCSTR reactor, the stringent AA impurity specification inthe final product can not be satisfied. Since interactionsbetween the CSTR level loop and rectifier T5 loop are quitesevere as can be seen in previous Fig. 12, together with theunusual inverse response characteristics in the off-diagonalelement (rectifier T5 vs. Q1), the pattern of the closed-loopdynamic response is rather unusual.

By looking at the final product composition, althoughthe EtOH impurity in the final product stream holds nicelyusing this control strategy, however, there is some deviationof the AA concentration with this impurity changing from0.005 wt% to 0.0052 wt%. This agrees with the closed-loopsensitivity plot in previous Fig. 7 where ideally the temper-ature setpoint at rectifier should be adjusted a little in theface of AA feed composition disturbance.Fig. 16 shows the simulation results for 20% changesin the EtOH feed composition (water contents in this feedstream from 17.8 mol% to 21.35 mol% or from 17.8 mol%to 14.24 mol%) at t = 2 h. The control strategy performswell for this type of disturbance with both the AA andEtOH impurities in the product stream holding very closeto their specifications.

The closed-loop responses of these two types of feedcomposition changes for Control Strategy (II) are shownin Figs. 17 and 18. It is noticed that for Control Strategy(II) both AA and EtOH impurities are very close to theirspecifications despite various feed composition changes.This agrees with the earlier findings via steady-state simu-lation (see Fig. 9) where no adjustment of the temperaturesetpoints are needed for AA or EtOH feed compositionchanges.

It is also noticed that the transient response also favorControl Strategy (II) in that the ranges of variations inthe two product impurities during the transient periodare much narrower for Control Strategy (II). For example,

Time (hr)0 50 100 150 200 250 300

Fin

al p

rodu

ct A

A C

once

ntra

tion

2.5e-5

3.0e-5

3.5e-5

4.0e-5

4.5e-5

5.0e-5

5.5e-5

6.0e-5

Time (hr)0 50 100 150 200 250 300

Fin

al p

rodu

ct E

tOH

Con

cent

ratio

n

9.900e-4

9.950e-4

1.000e-3

1.005e-3

1.010e-3

1.015e-3

Time (hr)0 50 100 150 200 250 300

T5

of 1

st C

olum

n (o

C)

92

93

94

95

96

Time (hr)0 50 100 150 200 250 300

EtO

H F

eed

Rat

e (m

ol/m

in)

94

96

98

100

102

104

Time (hr)0 50 100 150 200 250 300

T5

of 2

nd C

olum

n (o

C)

99.615

99.620

99.625

99.630

99.635

99.640

99.645

Time (hr)0 50 100 150 200 250 300

Reb

oile

r D

uty

of 2

nd C

olum

n (K

W)

60

62

64

66

68

70

72

Time (hr)0 50 100 150 200 250 300

CS

TR

Lev

el (

m)

4.360

4.361

4.362

4.363

4.364

4.365

Time (hr)0 50 100 150 200 250 300

Reb

oile

r D

uty

of 1

st C

olum

n (K

W)

140

142

144

146

148

150

152

154

AA Concentration-5%

5.20e-51.007e-3

Fig. 15. Closed-loop performance for Control Strategy (I) with AA feed concentration changes from 100 mol%AA to 95 mol%AA at t = 2 h.

226 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231with 20% changes in the EtOH feed composition, therange of variations in AA impurity is from 0.0040 wt% to0.0063 wt% for Control Strategy (II). However, much lar-ger range of variations is observed for Control Strategy(I) with range from 0.0028 wt% to 0.0086 wt%.

The controller tuning for either control strategy followsthe same tuning rules. The CSTR level control loop istuned first with IMC-PID tuning method of Chien andFruehauf [15] using integrating plus deadtime model. Theclosed-loop time constant is set to be twice of the modelapparent deadtime. For the Control Strategy (II), sincethe CSTR level is controlled by the AA feed rate whichhave direct influence on the level, the tuning is tighter thanthe result of Control Strategy (I). However, by lookingat the dynamic response of AA feed rate in Fig. 17,although the tuning is tight, the manipulated variable isquite acceptable with small overshoot. After this importantlevel control loop is put into automatic mode, the temper-ature loop at stripper is tuned next followed by the tuningof the temperature loop at rectifier. The dynamics of the

Time (hr)0 50 100 150 200 250 300

Fina

l pro

duct

AA

Con

cent

ratio

n

2e-5

3e-5

4e-5

5e-5

6e-5

7e-5

8e-5

9e-5

Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

0 50 100 150 200 250 300

Fina

l pro

duct

EtO

H C

once

ntra

tion

9.850e-4

9.900e-4

9.950e-4

1.000e-3

1.005e-3

1.010e-3

1.015e-3

0 50 100 150 200 250 300

T 5 o

f 1st

Col

umn

(oC

)

91

92

93

94

95

96

97

98

99

0 50 100 150 200 250 300Et

OH

Fee

d R

ate

(mol

/min

)

94

96

98

100

102

104

106

108

110

0 50 100 150 200 250 300

T 5 o

f 2nd

Col

umn

(oC

)

99.59

99.60

99.61

99.62

99.63

99.64

99.65

0 50 100 150 200 250 300

Reb

oile

r Dut

y of

2nd

Col

umn

(KW

)

67

68

69

70

71

72

73

74

0 50 100 150 200 250 300

CST

R L

evel

(m)

4.360

4.362

4.364

4.366

4.368

4.370

0 50 100 150 200 250 300

Reb

oile

r Dut

y of

1st C

olum

n (K

W)

146

148

150

152

154

156

158

EtOH Feed H2O Concentration +20%EtOH Feed H2O Concentration -20%

4.90e-5

1.002e-3

5.03e-5

9.974e-4

Fig. 16. Closed-loop performance for Control Strategy (I) with EtOH feed concentration 20% changes at t = 2 h.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 227temperature loop at stripper is much faster than the one atrectifier, thus it is tuned first and put into automatic modebefore the tuning of the final temperature loop at rectifier.Integrating plus deadtime model was also used with theclosed-loop time constant to be twice of the model appar-ent deadtime. The tuning of this final temperature controlloop is crucial for the overall control performance. Thesuitable combination of the controller gain and reset timeis important to achieve faster closed-loop response withoutmuch of the oscillation.

Since Control Strategy (II) works better to handle theunmeasured feed composition disturbances, this controlstrategy is further tested with simulation study for the casewith throughput changes. For this control strategy,increasing or decreasing the production rate of the finalproduct needs to go through the changes of the CSTR heat

0 50 100 150 200 250 300

Fina

l pro

duct

AA

Con

cent

ratio

n

4.84e-54.86e-54.88e-54.90e-54.92e-54.94e-54.96e-54.98e-55.00e-55.02e-55.04e-5

0 50 100 150 200 250 300

Fina

l pro

duct

EtO

H C

once

ntra

tion

9.980e-4

9.990e-4

1.000e-3

1.001e-3

1.002e-3

1.003e-3

1.004e-3

0 50 100 150 200 250 300

T 5 o

f 1st

Col

umn

(oC

)

94.85

94.90

94.95

95.00

95.05

95.10

0 50 100 150 200 250 300Et

OH

Fee

d R

ate

(mol

/min

)100.4

100.6

100.8

101.0

101.2

101.4

101.6

101.8

102.0

0 50 100 150 200 250 300

T 5 o

f 2nd

Col

umn

(oC

)

99.59

99.60

99.61

99.62

99.63

99.64

99.65

0 50 100 150 200 250 300

Reb

oile

r Dut

y of

2nd

Col

umn

(KW

)

67.0

67.5

68.0

68.5

69.0

69.5

70.0

70.5

0 50 100 150 200 250 300

CST

R L

evel

(m)

4.36405

4.36410

4.36415

4.36420

4.36425

4.36430

4.36435

0 50 100 150 200 250 300

AA fe

ed R

ate

(mol

/min

)

65.5

66.0

66.5

67.0

67.5

68.0

68.5

69.0

69.5

AA Concentration-5%

5.02e-5 1.003e-3

Time (hr) Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

Fig. 17. Closed-loop performance for Control Strategy (II) with AA feed concentration changes from 100 mol%AA to 95 mol%AA at t = 2 h.

228 I-L. Chien et al. / Journal of Process Control 18 (2008) 215231duty. Fig. 19 shows the simulation results with 10%changes of the CSTR heat duty. With these changes ofthe CSTR heat duty, the two feed flow rates are changedaccordingly with only small deviations in the AA productimpurity. Because this disturbance is considered as aknown load change, thus similar to the paper by Huanget al. [13], a planning of the temperature setpoint adjust-ment versus each CSTR heat duty can easily be made fromprocess simulation to circumvent the impurity deviationproblem. For example, with 10% changes in the through-put manipulator (CSTR heat duty), the setpoint of tray #5temperature at rectifier should decrease around 0.3 Caccording to Fig. 9 so that AA impurity in the final productcan return back to 0.005 wt%.

With above 10% changes of the CSTR heat duty, theproduct rate is changed from 63.8824 mol/min to69.66846 mol/min (a +9.1% increase in production) orfrom 63.8824 mol/min to 58.0364 mol/min (a 9.1%

Time (hr)0 50 100 150 200 250 300

Fin

al p

rodu

ct A

A C

once

ntra

tion

3.5e-5

4.0e-5

4.5e-5

5.0e-5

5.5e-5

6.0e-5

6.5e-5

Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

0 50 100 150 200 250 300

Fin

al p

rodu

ct E

tOH

Con

cent

ratio

n

9.850e-4

9.900e-4

9.950e-4

1.000e-3

1.005e-3

1.010e-3

1.015e-3

0 50 100 150 200 250 300

T5

of 1

st C

olum

n (o

C)

93.5

94.0

94.5

95.0

95.5

96.0

96.5

97.0

0 50 100 150 200 250 300

EtO

H F

eed

Rat

e (m

ol/m

in)

92

94

96

98

100

102

104

106

108

110

112

0 50 100 150 200 250 300

T5

of 2

nd C

olum

n (o

C)

99.600

99.605

99.610

99.615

99.620

99.625

99.630

99.635

99.640

0 50 100 150 200 250 300

Reb

oile

r D

uty

of 2

nd C

olum

n (K

W)

67

68

69

70

71

72

73

0 50 100 150 200 250 300

CS

TR

Lev

el (

m)

4.3640

4.3641

4.3642

4.3643

4.3644

4.3645

0 50 100 150 200 250 300

AA

Fee

d R

ate

(mol

/min

)

63

64

65

66

67

68

69

EtOH Feed H2O Concentration +20%

EtOH Feed H2O Concentration -20%

4.98e-5

4.95e-5

1.00e-3

9.96e-4

Fig. 18. Closed-loop performance for Control Strategy (II) with EtOH feed concentration 20% changes at t = 2 h.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 229decrease in production). This demonstrates that CSTRheat duty can be used as a throughput manipulator tosmoothly increase or decrease the production rate of thefinal product. The final proposed overall control strategyfor this process can be seen in Fig. 20.

5. Conclusions

In this paper, design and control of a complete ethylacrylate process with coupled reactor/columns configura-tion has been investigated. Unlike other paper in the liter-ature with only one reactive distillation column toproduce products like methyl acetate or butyl acetate, thisoverall process flowsheet is more complex including aCSTR, a rectifying column, a decanter, and another strip-ping column. The design procedure is based on the max-imization of total annual profit (TAP) for the overallsystem. The final EA product is having very high purityof 99.78 mol% (or 99.89 wt%) with two impurities ofAA and EtOH meeting stringent specifications of

Time (hr)0 50 100 150 200 250 300

Fina

l pro

duct

AA

Con

cent

ratio

n

3.5e-5

4.0e-5

4.5e-5

5.0e-5

5.5e-5

6.0e-5

6.5e-5

7.0e-5

Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

Time (hr) Time (hr)

0 50 100 150 200 250 300

Fina

l pro

duct

EtO

H C

once

ntra

tion

9.20e-4

9.40e-4

9.60e-4

9.80e-4

1.00e-3

1.02e-3

1.04e-3

1.06e-3

1.08e-3

0 50 100 150 200 250 300

T 5 o

f 1st

Col

umn

(oC

)

93

94

95

96

97

98

0 50 100 150 200 250 300

EtO

H F

eed

Rat

e (m

ol/m

in)

85

90

95

100

105

110

115

120

0 50 100 150 200 250 300

T 5 o

f 2nd

Col

umn

(oC

)

99.50

99.55

99.60

99.65

99.70

99.75

0 50 100 150 200 250 300

Reb

oile

r Dut

y of

2nd

Col

umn

(KW

)

6062646668707274767880

0 50 100 150 200 250 300

CST

R L

evel

(m)

4.362

4.364

4.366

4.368

0 50 100 150 200 250 300

AA F

eed

Rat

e (m

ol/m

in)

5658606264666870727476

Throughput +10%Throughput-10%

4.77e-5

1.01e-35.24e-5

9.89e-4

Fig. 19. Closed-loop performance for Control Strategy (II) with 10% changes in CSTR duty at t = 2 h.

230 I-L. Chien et al. / Journal of Process Control 18 (2008) 2152310.005 wt% AA and 0.1 wt% EtOH in the final productstream.

For the overall control strategy of this process, astraightforward procedure has been followed which usesclosed-loop sensitivity analysis to screen out the worsecandidates of overall control strategy and to pick thesuitable temperature control points. The final recom-mended overall control strategy is found to be differentthan another similar coupled reactor/columns process pub-lished earlier [1]. Both AA and EtOH feed flow rates areused as manipulated variables in the overall control strat-egy to control CSTR level and one tray temperature at rec-tifier with CSTR heat duty left as the throughputmanipulator for the overall process. The final EA productwith stringent specifications of EtOH and AA impuritiescan be achieved with this proposed overall control strategydespite various feed composition disturbances andthroughput changes.

Organic Reflux

AA

Steam

Aqueous

EtOH

Decanter

Steam

EA

Reboiler

Water

CSTR

PC PCLC LC

LC

LC

FC

FC

FC

FC

1 1

196

207

TCTC

X

X

TC

TC

(throughput manipulator)

Fig. 20. Proposed overall control strategy of this process.

I-L. Chien et al. / Journal of Process Control 18 (2008) 215231 231Acknowledgements

The first author, I-Lung Chien, take this opportunity tothank Prof. Dale Seborg for his guidance and valuable ad-vices over the years. This paper was prepared while I-Lungwas back visiting Department of Chemical EngineeringUCSB after completing his Ph.D. degree as Dales studentmore than 20 years ago. There is an old Chinese sayingwhich conveys I-Lungs appreciation to Prof. Seborg:Once becoming a teacher of yours, always respect himas your father. At this occasion of Dales 65th birthday,I-Lung wish him happiness always and longevity.

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[4] C.K. Yi, W.L. Luyben, Design and control of coupled reactor/column systems Part 2. More complex coupled reactor/columnsystem, Comput. Chem. Eng. 21 (1997) 4767.[5] C.K. Yi, W.L. Luyben, Design and control of coupled reactor/column systems Part 3. A reactor/stripper with two columns andrecycle, Comput. Chem. Eng. 21 (1997) 6986.

[6] S.F. Chiang, C.L. Kuo, C.C. Yu, D.S.H. Wong, Design alternativesfor the amyl acetate process: coupled reactor/column and reactivedistillation, Ind. Eng. Chem. Res 41 (2002) 32333246.

[7] U. Weidlich, J. Gmehling, A modified UNIFAC model. 1. Predictionof VLE, HE and c1, Ind. Eng. Chem. Res. 26 (1987) 13721381.

[8] J. Gmehling, J. Li, M. Schiller, A modified UNIFAC model. 2.Present parameter matrix and results for different thermodynamicproperties, Ind. Eng. Chem. Res. 32 (1993) 178193.

[9] J.G. Hayden, J.P. OConnell, A generalized method for predictingsecond virial coefficients, Ind. Eng. Chem. Proc. Des. Dev. 14 (1975)209216.

[10] J.M. Douglas, Conceptual Process Design, McGraw-Hill, New York,1988.

[11] R. Turton, R.C. Bailie, W.B. Whiting, J.A. Shaeiwitz, Analysis,Synthesis, and Design of Chemical Processes, Prentice-Hall, Engle-wood Cliffs, NJ, 1998.

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Overall control strategy of a coupled reactor/columns process for the production of ethyl acrylateIntroductionThermodynamic and kinetic model used in the simulationDesign flowsheet of the complete processOverall control strategy of this processDetermine of tray temperature control point and the control structureLoop pairing and dynamic considerationsOpen-loop sensitivity analysis to verify the suitabilityof temperature control pointsClosed-loop dynamic simulation results

ConclusionsAcknowledgementsReferences