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Chemical Engineering and Processing 82 (2014) 88–100

Contents lists available at ScienceDirect

Chemical Engineering and Processing:Process Intensification

j ourna l ho me page: www.elsev ier .com/ locate /cep

ontrol analysis of an extractive dividing-wall column used forthanol dehydration

alvador Tututi-Avilaa,b, Arturo Jiménez-Gutiérrezb, Juergen Hahna,∗

Department of Biomedical Engineering, Department of Chemical & Biological Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USADepartamento de Ingeniería Química, Instituto Tecnológico de Celaya, Ave. Tecnológico y García Cubas S/N, 38010 Celaya, Gto., México

r t i c l e i n f o

rticle history:eceived 22 March 2014eceived in revised form 27 May 2014ccepted 28 May 2014

eywords:

a b s t r a c t

This paper deals with design and control of an extractive dividing-wall distillation column (EDWC) forethanol dehydration using ethylene glycol as entrainer. An initial design, based on a section analogyprocedure for a conventional extractive distillation sequence, was obtained and then used in an opti-mization process to minimize the total annual cost. It was shown that the EDWC can result in significantsavings over the conventional process. As these savings sometimes go along with a decrease in the control

ioethanolthanolhermally coupled columnividing-wall columnxtractive distillation

properties, an investigation of two control structures for the EDWC and one for the conventional columnconfiguration was performed next.

It was observed in closed-loop simulations that the EDWC with an appropriate structure exhibited goodcontrol properties and that its closed-loop responses were similar to those obtained for the operation ofa conventional extractive distillation system.

© 2014 Elsevier B.V. All rights reserved.

. Introduction

Price increases of fossil fuels, along with tighter environmen-al policies, have motivated the search for alternative processesith lower energy consumption and carbon emissions. Bioethanol

s one of the most promising alternatives for producing sustain-ble biofuels in the short term. Bioethanol can be easily integratednto existing fuel systems at 5–85% mixtures with gasoline with-ut the need for any modification of current engines [1]. The mostopular blend for light-duty vehicles is known as E85, and con-ains 85% bioethanol and 15% gasoline. This mixture leads to aetter oxidation of hydrocarbons because of its high oxygen con-ent, thus reducing the amount of aromatic compounds and carbon

onoxide emissions [2]. Bioethanol can be produced using dif-erent raw materials such as corn, sugar cane, and lignocellulosiciomass at an industrial scale. Ethanol is produced via fermentationhere the output of the fermenter is generally a diluted aque-

us solution containing bioethanol in the range of 5–12 wt.% thateeds to be further concentrated for its use in combustion engines

3,4]. The current ASTM specification for water in fuel ethanoltates a maximum of 1.0 vol.% (1.3 wt.%). Other countries and gov-rnments have established lower allowed water concentrations.

∗ Corresponding author. Tel.: +1 518 276 2138; fax: +1 518 276 3035.E-mail address: hahnj@rpi.edu (J. Hahn).

ttp://dx.doi.org/10.1016/j.cep.2014.05.005255-2701/© 2014 Elsevier B.V. All rights reserved.

According to international standards, bioethanol must have a purityof 99.0–99.8 wt.%, or even higher, for its use in fuel applications.

Two major steps are usually used to reach the requiredethanol purity levels due to a binary ethanol–water azeotrope(95.6 wt.% ethanol). Ethanol is first pre-concentrated to approxi-mately 92–94 wt.% using ordinary distillation, a step that usuallyaccounts for a large fraction of the total energy requirement of abiorefinery [4]. In the second stage, ethanol is further dehydrated toyield anhydrous ethanol. Several processes can be used for the sec-ond stage, such as pervaporation, azeotropic distillation, extractivedistillation, pressure-swing distillation, liquid–liquid extraction,adsorption, or a combination of techniques. Extractive distillation(ED) has been the preferred option for large-scale production ofbioethanol fuel [5]. ED requires the use of, e.g., a liquid solvent,ionic liquid, dissolved salt, a mixture of volatile liquid solvent anddissolved salt, or a hyperbranched polymer. ED is performed usu-ally in a sequence of two columns, where the first one purifies theethanol and the other one splits water from the solvent, which isrecycled to the first column (Fig. 1a). Hernandez [6] and Kiss andSuszwalak [7] have proposed the use of thermally coupled distil-lation columns for extractive distillation. Fig. 1b and c shows thestructure of such systems and their implementation as dividing-

wall columns [8–10].

Dividing-wall columns (DWCs) represent a typical example ofprocess intensification since they can result in reductions in bothcapital investment and energy costs. The DWC technology has

S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100 89

F tillatiow

pDn[beud[acBToEcoanstOpat[D

tadfdtBpRsepLotpaftomi

ig. 1. Alternatives for the separation of ethanol–water mixtures via extractive disith a side rectifier; (c) EDWC system.

roved a feasible option for several separation problems [7,11–13].WC for ternary separations can be considered as a proven tech-ology, with over 100 columns reported in operation worldwide14]. The extension of DWC systems for extractive distillation haseen recently analyzed, showing that its application to dehydratethanol could potentially reduce both energy and capital costs byp to 17% [6,7,15–17]. Some industrial applications of extractiveivided wall columns (EDWCs) have been reported in the literature18]. Uhde applied the DWC technology for the Morphylane processt the Arsol Aromatics GmbH (formerly Aral Aromatics), with a feedapacity of 28,000 mt/year. Another EDWC was implemented byASF SE for the production of butadiene from a C-cut [14,18–21].he implementation of EDWCs requires a proper understandingf their dynamic behavior and control properties. Although theDWC design offers the convenience that two reboilers from theonventional design are combined into one reboiler, one degreef freedom is lost for control purposes [11]. Due to this loss of

degree of freedom, thermally coupled structures were origi-ally assumed to be difficult to control, however, they have beenhown to provide suitable control properties, sometimes even bet-er than those of conventional distillation arrangements [22,23].ne conclusion that can be drawn from these studies is that controlroperties of thermally coupled systems should be investigated on

case by case basis as it is not possible to determine a priori whathe control properties of a particular system are. Kiss and Bildea24] have presented an overview of control studies for standardWC systems, which does not include EDWCs.

A few works have addressed the control of conventional extrac-ive bioethanol processes. Gil et al. [25] reported the controlnalysis of a bioethanol process using glycerol as entrainer. Theyeveloped a control structure that exhibited good control per-ormance in response to feed disturbances. Control of extractiveistillation processes to dehydrate alcohols has been reported inhe works by Arifin and Chien [26] and Luyben [27,28]. Macien andrito [29] studied the dynamic behavior of an extractive distillationrocess to dehydrate ethanol using ethylene glycol as entrainer.amirez-Marquez et al. [30] and Segovia-Hernandez [31] havetudied the dynamic properties of thermally coupled columns forthanol dehydration. They used composition controllers and set-oint tracking tests to analyze closed-loop performance. Ling anduyben [32] studied temperature control of DWC avoiding the usef expensive and high-maintenance composition analyzers. Twoypes of temperature control structures were studied using a four-oint temperature control. The control structures consisted of andjustable liquid split. Effective control was achieved with a dif-erential control structure. Recently, Dwivedi et al. [33] showed

hat vapor split (see Fig. 1c) can be effectively used as a degreef freedom during practical operation of DWCs, allowing energyinimization along with the use of an adjustable liquid split. Even

n the light of these developments on the control of dividing-wall

n: (a) conventional extractive distillation sequence; (b) thermally coupled system

columns, no studies on the dynamics and control of EDWCs forethanol dehydration using temperature controllers under com-monly occurring disturbances can be found in the literature, despitethe potential for significant energy savings of such thermally cou-pled systems.

In this work, we investigate the design and control of a novelEDWC structure for the dehydration of bioethanol using ethyleneglycol as entrainer. Temperature control structures are developedon the basis of sensitivity analysis [34]. Two control structures forthe EDWC are explored to maintain product quality. A compari-son between an EDWC structure and a conventional sequence isalso presented to further examine operational aspects of the EDWCimplementation.

2. Design of extractive distillation systems

The feed stream is assumed to contain 84% mol (93 wt.%)ethanol, as is commonly the case for a feed from a pre-concentrationstage. The feed flowrate is taken as 45.36 kmol/h (equivalent to abioethanol production rate of 17 KTPY), at 351 K and 1 atm. Prod-uct specifications of 99.5 wt.% of ethanol and water in the overheadsare assumed, with 99.5 wt.% of ethylene glycol in the bottoms of therecovery column (main column in the EDWC). Condenser pressureis set at 1 atm in both columns of each configuration. A tray pres-sure drop of 0.0068 atm is assumed. The NRTL property method isused as this non-ideal mixture contains polar components. AspenPlus simulations were performed using the RADFRAC unit model.

2.1. Steady state design

The base design of the separation sequences was obtained byminimizing the total annual cost (TAC). The EDWC was designedfrom the structure of a thermally coupled system (Fig. 1b). Its initialdesign is based on a section analogy procedure for a conventionalextractive distillation sequence [35] (Fig. 1a), which is then used inan optimization process. The design procedure determines the traystructure of each column, the tray position for the interconnect-ing streams, and the operating conditions (such as pressure andreflux ratios or values of the interconnecting streams) that mini-mize the total annual cost, which includes annual capital costs andoperating costs. The minimization is subject to the design purityrequirements:

min(TAC) = f (Nk, Rk, Dk, NFk, S, NFS)

subject to

ym ≥ xm, ∀m

where Nk is the number of stages in column k, Rk and Dk arereflux and distillate flowrates, NFk is the feed stage of each of thecolumns, S is the solvent flow, NFS is the solvent feed stage in the

90 S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100

Table 1Design of the conventional and the thermally coupled arrangement.

E-conventional EDWC alternative

C1 C2 C1 C2

Number of stages 17 19 21 17Feed stage 11 11 11 17Feed stage of extractive agent 4 – 3 –Interconnection stage – – 17 17Feed flowrate of ethanol (kmol/h) 38.1024 0 38.1024 0.0114Feed flowrate of water (kmol/h) 7.2576 7.5940 7.2576 7.3291Feed flowrate of EG (kmol/h) 69.6045 69.5999 67.51 0.4594Purity of bioethanol (wt.%) 99.50 99.50Purity of water by-product (wt.%) 99.50 99.50Purity of ethylene glycol recycle (wt.%) 99.66 99.95Reflux ratio 0.64 0.81 0.39 1.1Operating pressure (atm) 1 1 1 1Column diameters (m) 0.60 0.31 0.65Tray space 0.4 0.3 0.4QHX (kW) cooler 309.4 318.1QC (kW) 685.04 140.14 580.10 89.89Total QC (kW) 825.85 669.99QR (kW) 907.57 230.06 992.48Total QR (kW) 1137.63 992.48Economic performance

Total investment cost (TIC) (106 $/year) 0.2382 0.22150.3543 0.30900.3782 0.3312

eiEt

ulbCtftBss

atftttmt

vcTe

sai

eRhpwi

Total operating cost (TOC) (106 $/year)

Total annual cost (TAC) (106 $/year)

xtractive column, and ym and xm are vectors of obtained and spec-fied purities for the m product streams. The design variables of theDWC include the interconnecting vapor flow rate (vapor split) andhe location of the interconnecting stage.

The optimization of the separation sequences was performedsing genetic algorithms coupled with the Aspen Plus process simu-

ator. The parameters of the genetic algorithm were set as suggestedy Bravo-Bravo et al. [36] with 2500 individuals and 40 generations.ross over and mutation factors were set to 0.8 and 0.05, respec-ively. The sizing relationships and economic factors were takenrom Luyben [37]. A price of $9.88 per GJ of steam was assumed. Forhe TAC calculations a plant lifetime of ten years was considered.ecause the two columns have small diameters, we specified a traypacing of 0.4 m for the main column and 0.3 m for the rectifyingection.

Table 1 gives the designs obtained for the two arrangementsfter the optimization procedure. The energy savings provided byhe EDWC scheme amount to 13% for the heating duties and 19%or the cooling requirements. In economic terms, savings in capi-al investment are also observed, which together with energy costsranslated into 12.4% savings with respect to the optimal design ofhe conventional extractive distillation sequence. Thus, the opti-

um arrangement of the EDWC provides a significant reduction ofhe energy consumption at steady state.

The liquid composition and temperature profiles of the con-entional sequence are shown in Fig. 2. A remixing effect in theomposition of water can be observed in the extractive column.he energy savings provided by the EDWC reflect the reduction orlimination of such remixing effect.

The liquid, vapor and temperature profiles of the EDWC arehown in Fig. 3. High-purity water and ethanol streams are obtaineds overhead products on each side of the wall, while ethylene glycols recovered as the bottoms product.

For the control implementation, the Aspen Plus design wasxported to a pressure-driven simulation in Aspen Dynamics.eflux drum and base volumes were specified to provide 5 min of

oldup for 50% liquid level. Pumps and valves were sized to giveroper pressure drops to handle changes in flow rates. The mostidely used technique for selecting a temperature control location

s the slope criterion [34], which consist of selecting a tray whereFig. 2. Liquid composition and temperature profiles for the conventional distillationsequence.

S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100 91

Fig. 3. Temperature and composition profiles for the EDWC.

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

−2

0

2

4

6

Stage

Gai

n

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19−0.02

−0.01

0

0.01

0.02

Stage

Gai

n

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

−0.5

0

0.5

1

Stage

Gai

n

+0.1% QR −0.1% QR

Extractive Column

Recovery Column

Recovery Column

+0.1% QR2 −0.1% QR2

+0.1% R2 −0.1% R2

Fig. 5. Open-loop sensitivity analysis of the conventional process.

Fig. 4. Control strategy for the conventional process.

92 S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100

Table 2Temperature controller parameters for the conventional system.

Controller TC14 TC18 TC3 TCRECYCLE

Controlled variable (◦C) T14 = 107.45 T18 = 187.76 T3 = 115.17 TRecycle = 114.76Manipulated variable QR1 QR2 /F2 R2 Qcooler

Transmitter range (◦C) 57.45–157.45 137.76–237.76 65.17–165.17 64.76–164.76Controller output range 0–1815.14 kW 0–0.0216 0–1.647 −618.8 to 0 kWOpen loop gain (%/%) 15.58 1.83 2.96 4.12Time constant (min) 11.43 5.15 8.93 3.64Dead time (min) 1.02 0.90 0.99 0.63IMC lambda (min) 2.27 1.80 1.79 4.0Controller gain (%/%) 0.3352 1.6978 1.7799 0.2399Controller integral time (min) 11.85 5.6 9.43 3.99

0 1 2 3 4 530

405060

F / [

kmol

/hr]

0 1 2 3 4 5

60

80

S / [

kmol

/hr]

0 1 2 3 4 530

40

50

D1 /

[km

ol/h

r]

0 1 2 3 4 5

6

8D

2 / [k

mol

/hr]

0 1 2 3 4 5114

115116117

T 3 / [°

C]

0 1 2 3 4 54

567

R2 /

[km

ol/h

r]

0 1 2 3 4 50.9930.9940.9950.9960.997

x D1

(E) /

[−]

0 1 2 3 4 50.985

0.99

0.995

1

x D2

(W) /

[−]

0 1 2 3 4 5

100

120

Time / [hr]

T 14 /

[°C

]

0 1 2 3 4 5186

188

190

Time / [hr]

T 18 /

[°C

]

+20% F −20% F

Fig. 6. Dynamic responses for feed flow rate disturbances of the conventional system.

tAotwcttEf

strategy is to maintain the two product purities as close as pos-sible to their nominal values in the face of commonly occurringdisturbances.

he largest change in temperature from tray to tray is observed.nother commonly used approach is the sensitivity criterion, basedn the tray with the largest change in temperature for a change inhe manipulated variable. In this study, temperature control loopsere implemented following the slope criterion and the sensitivity

riterion, which provide a simple and effective method for selectingemperature control tray locations in extractive distillation sys-ems [11,12,25,27,28]. The comparison of control structures for theDWC presented here are based on PID loops within a multiloopramework.

3. Control of the conventional extractive distillationprocess

Control of conventional extractive distillation systems has beenstudied by several authors [12,25–28,37]. On the basis of this works,a basic control structure for the conventional extractive distilla-tion system is developed in this section. The goal of the control

S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100 93

0 1 2 3 4 544.5

4545.5

46

F / [

kmol

/hr]

0 1 2 3 4 569.5

7070.5

71

S / [

kmol

/hr]

0 1 2 3 4 536

384042

D1 /

[km

ol/h

r]

0 1 2 3 4 54

6

8

D2 /

[km

ol/h

r]

0 1 2 3 4 5110

115

120

T 3 / [°

C]

0 1 2 3 4 55.2

5.45.65.8

R2 /

[km

ol/h

r]0 1 2 3 4 5

0.994

0.995

0.996

x D1

(E) /

[−]

0 1 2 3 4 50.9930.9940.9950.9960.997

x D2

(W) /

[−]

0 1 2 3 4 5100

105110115

Time / [hr]

T 14 /

[°C

]

0 1 2 3 4 5186

188

190

Time / [hr]

T 18 /

[°C

]

88 mol% Ethanol 80 mol% Ethanol

sition

pwfRl(cufltflsr

ivrt

rotasiT

Fig. 7. Dynamic responses for feed compo

Fig. 4 reports the basic control scheme proposed for flow, level,ressure and temperature control. The top pressure of each columnas controlled by manipulating the condenser duties. The total

eed was flow-controlled and used as a throughput manipulator.eflux drum levels for both columns were controlled by manipu-

ating the distillate flow rates (D1 and D2). The entrainer flow rateS) to feed flow rate (F) ratio was fixed. An important inventoryontrol loop was related to the bottom level of the recovery col-mn, which was controlled by manipulating the entrainer makeupow as shown in Grassi [38] and Luyben [28]. The base level forhe extractive column was controlled by manipulating the bottomsow rate. The reflux ratio in the extractive column was held con-tant at its nominal value during disturbances. A feed-forward QR/Fatio was implemented in the recovery column.

The entrainer feed temperature was controlled by manipulat-ng the heat removal rate in the cooler. The remaining manipulatedariables can be used to configure tray temperature loops. Thus, theeboiler duties of both columns were used to control the tempera-ure of a particular stage for each column.

An open loop sensitivity analysis with ±0.1% changes in theeboiler duty of the extractive and recovery columns was carriedut, and the results are shown in Fig. 5. For the extractive distilla-ion column, the temperature of the 14th stage (T14) was selected

s the control point. In the case of the recovery column, the 18thtage (T18) shows the largest slope of the temperature profile, ands also the most sensitive to changes in the reboiler duty. However,18 was controlled by manipulating the feed-forward QR2 /F2 ratio,

disturbances of the conventional system.

which has been shown to reduce the transient deviation in the bot-toms purity [27,32,39]. The effect of the reflux ratio on the recoverycolumn has a higher gain (see Fig. 5) than the one from the reboiler.Therefore, another controller was added so that the temperature ofan additional stage could be controlled by manipulating the refluxratio (R2) in the recovery column. The temperature at the 3rd stage(T3) had the largest slope (see Fig. 5) and the highest sensitivity forchanges in utility duties, so it was selected as a controlled point.The overall control scheme is depicted in Fig. 4.

As for controller tuning, pressure control loops in both columnswere tightly tuned with Kc = 20 and �1 = 12 min. Level control wasachieved via Proportional-only controllers with Kc = 2. Flow con-trollers were set as Proportional-Integral controllers with Kc = 0.5and �1 = 0.3 min. PI temperature controller settings were deter-mined using Internal Model Control (IMC) tuning rules [40–43].Step changes of 1.0% of the steady-state value in the input vari-ables were implemented and the open loop dynamic responseswere recorded. The dynamic responses were fitted to first orderplus time delay transfer functions. Each temperature controller wasassumed to perform with a 1-min deadtime in the control loop.The resulting controller parameters are given in Table 2. It shouldbe noted that the transmitter ranges of the key temperature con-trollers is determined by their nominal values [28]. Ranges for the

controlled outputs are also provided.

Changes in the operating conditions of the pre-concentrationcolumn could give rise to disturbances in the feed compositionto the extractive distillation system. Therefore, we considered two

94 S. Tututi-Avila et al. / Chemical Engineerin

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21−4

−2

0

2

4

Stage

Gai

n

+0.1% QR −0.1% QR

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21−0.5

−0.25

0

0.25

0.5

Stage

Gai

n

+0.1% R1 −0.1% R1

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

−0.5

0

0.5

1

Stage

Gai

n

+0.1% R2 −0.1% R2

Main Column

Main Column

Side Rectifier

Fig. 8. Open-loop sensitivity analysis of the EDWC.

Fig. 9. Overall control strategy of the EDW

g and Processing 82 (2014) 88–100

ethanol composition disturbances, from 84 to 88 mol% ethanol andfrom 84 to 80 mol% ethanol, at time = 0.2 h. Fresh feed flow distur-bances of ±20% at 0.2 h were also considered. Figs. 6 and 7 showthe closed-loop results.

Fig. 6 shows the results for a 20% feed flow disturbancesapplied to the system at time = 0.2 h. The temperature controllersresponded relatively fast and the product compositions were main-tained quite close to their required specifications. The time requiredto reach the steady state was approximately 1 h.

In the case of feed composition disturbances, the two bottomplots of Fig. 7 show the temperature control point for each col-umn. Temperature controllers handled the disturbance reasonablywell by bringing the temperatures back to their setpoints. It canbe observed that the ethanol concentration in the extractive col-umn and the water concentration in the recovery column were heldquite close to their nominal values, ensuring the quality of the topproducts of both columns. It should be noted that the ethanol com-position was higher than 99 wt.% ethanol at all times, an essentialpoint for fuel applications. Overall, the proposed control strategyshowed good dynamic behavior in the face of feed composition andfeed flow rate disturbances.

4. Control of the EDWC separation system

Although a considerable amount of existing literature focuseson control of conventional DWCs, there are only a limited num-ber of studies on the control of EDWC [11,13,44,45] and noneof these studies focused on ethanol dehydration where temper-ature controllers have been employed. In this section two controlstructures are proposed to stabilize the EDWC for the ethanol dehy-dration. The two structures are based on previous studies involvingother systems, i.e., either involving a different mixture, a differentarrangement, different measured properties, or some combinationof these factors. For the dynamic analysis, the tray location for tem-perature control was selected using the same criteria as above. An

open loop sensitivity test with ±0.1% changes in the reboiler dutyof the main column and ±0.1% changes in the reflux ratios of bothcolumns were carried out. The results of the sensitivity analysis areshown in Fig. 8.

C without adjustable vapor split.

S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100 95

Table 3Temperature controller parameters for the EDWC without adjustable vapor split.

Controller TC14 TC18 TC4 TCRECYCLE

Controlled variable (◦C) T14 = 113.16 T18 = 161.40 T4 = 108.82 TRecycle = 108.0Manipulated variable R1 QR R2 Qcooler

Transmitter range (◦C) 63.16–163.16 111.40–211.40 58.82–158.82 58.0–158.0Controller output range 0–0.7760 0–1984.96 kW 0–0.3122 −636.2 to 0 kWOpen loop gain (%/%) 2.54 27.23 1.07 2.21Time constant (min) 23.27 7.92 11.32 5.26Dead time (min) 1.75 1.06 1.02 0.63IMC lambda (min) 6.0 2.0 2.2 4.0Controller gain (%/%) 1.5866 0.1551 5.0254 0.6306Controller integral time (min) 24.14 8.45 11.83 5.57

0 1 2 3 4 530

405060

F / [

kmol

/hr]

0 1 2 3 4 55060708090

S / [

kmol

/hr]

0 1 2 3 4 530

40

50

D1 /

[km

ol/h

r]

0 1 2 3 4 556789

D2 /

[km

ol/h

r]

0 1 2 3 4 5100110120130140150

T 4 / [°

C]

0 1 2 3 4 50

123

R2 /

[km

ol/h

r]

0 1 2 3 4 50.98

0.9850.99

0.9951

x D1

(E) /

[−]

0 1 2 3 4 50.97

0.98

0.990.995

1

x D2

(W) /

[−]

0 1 2 3 4 590

100110120130

Time / [hr]

T 14 /

[°C

]

0 1 2 3 4 5120140160180200

Time / [hr]

T 18 /

[°C

]

+20% F −20% F

Fig. 10. Dynamic responses for feed flow rate disturbances of the EDWC without adjustable vapor split.

Table 4Temperature controller parameters for the EDWC with adjustable vapor split.

Controller TC14 TC18 TC4 TCRECYCLE

Controlled variable (◦C) T14 = 113.16 T18 = 161.40 T4 = 108.82 TRecycle = 108.0Manipulated variable VR/QR QR R2 Qcooler

Transmitter range (◦C) 63.16–163.16 111.40–211.40 58.82–158.82 58.0–158.0Controller output range 0–4.3661 0–1984.96 kW 0–0.3122 −636.2 to 0 kWOpen loop gain (%/%) 3.41 27.15 1.07 2.21Time constant (min) 9.84 7.78 11.32 5.26Dead time (min) 1.04 1.09 1.02 0.63IMC lambda (min) 2.0 2.0 2.2 4.0Controller gain (%/%) 1.519 0.1649 5.0254 0.6306Controller integral time (min) 10.36 8.33 11.83 5.57

96 S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100

0 1 2 3 4 544.5

4545.5

46F

/ [km

ol/h

r]

0 1 2 3 4 567

67.5

68

S / [

kmol

/hr]

0 1 2 3 4 535

40

45

D1 /

[km

ol/h

r]

0 1 2 3 4 56

789

D2 /

[km

ol/h

r]

0 1 2 3 4 5106

108110112

T 4 / [°

C]

0 1 2 3 4 50.60.8

11.2

R2 /

[km

ol/h

r]

0 1 2 3 4 50.9

0.95

1

x D1

(E) /

[−]

0 1 2 3 4 5

0.6

0.8

1

x D2

(W) /

[−]

0 1 2 3 4 56080

100120140

Time / [hr]

T 14 /

[°C]

0 1 2 3 4 5150

160170180

Time / [hr]

T 18 /

[°C]

88 mol% Ethanol 80 mol% Ethanol

Fig. 11. Dynamic responses for feed composition disturbances of the EDWC without adjustable vapor split.

Fig. 12. Overall control strategy of the EDWC with adjustable vapor split.

S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100 97

0 1 2 3 4 530

405060

F / [

kmol

/hr]

0 1 2 3 4 55060708090

S / [

kmol

/hr]

0 1 2 3 4 530

40

50

D1 /

[km

ol/h

r]

0 1 2 3 4 556789

D2 /

[km

ol/h

r]

0 1 2 3 4 5106

108110112

T 4 / [°

C]

0 1 2 3 4 50.5

11.5

2

R2 /

[km

ol/h

r]

0 1 2 3 4 50.9930.9940.9950.9960.997

x D1

(E) /

[−]

0 1 2 3 4 50.9940.9940.9950.9960.997

x D2

(W) /

[−]

0 1 2 3 4 5110

112114116

Time / [hr]

T 14 /

[°C

]

0 1 2 3 4 5140150160170180

Time / [hr]

T 18 /

[°C

]

+20% F −20% F

isturb

4a

rateloosrrm

sm(m(toFca

Fig. 13. Dynamic responses for feed flow rate d

.1. Temperature control structure for the EDWC without andjustable vapor split

The first control structure explored was based on the oneeported in Wu et al. [11]. They reported a control structure withoutdjustable vapor split. Fig. 4 shows the basic control scheme. Essen-ial changes were made to this system, taking the conventionalxtractive distillation as a basis. An important inventory controloop was added which manipulates the entrainer makeup flow inrder to affect the bottom level of the main column. The base levelf the side rectifier was controlled by manipulating the recycletream flow rate (LR). A fixed ratio control was used to hold theatio between the vapor sidedraw flow (VR) to the heat input to theeboiler (QR) [46]. It has been demonstrated that this ratio can beanipulated in industrial operations [33].The temperature at the 18th stage (T18) of the main column was

elected, based upon a sensitivity analysis, as the control point foranipulating the reboiler duty. The temperature at the 14th stage

T14) of the main column was used to control the reflux ratio of theain column. In the case of the side rectifier column, the 4th stage

T4) showed the largest slope in the temperature profile and washe most noticeably affected tray by changes in the reflux ratio. The

verall control scheme tested in the dynamic simulation is shown inig. 9. IMC tuning was used to determine the PID parameters. Theontroller tuning parameters for the key temperature controllersre given in Table 3.

ances of the EDWC with adjustable vapor split.

Fig. 10 gives the responses of the EDWC column for a 20%change in the feed flow rate of the feed stream. The compositionsof the two product streams (xD1 and xD2 ) were controlled for largefeed flowrate disturbances. However, Fig. 11 shows that this con-trol strategy cannot retain the desired product quality when feedcomposition disturbances are tested. This problem arises becausethe low reflux ratio (0.39) in the main column cannot return thetemperature at the 14th stage back to its nominal value; as a con-sequence, large settling times and poor control performance wereobserved. Therefore, an alternative control structure is explorednext to handle such disturbances.

4.2. Temperature control structure for the EDWC with anadjustable vapor split

Luyben [46] has proposed a temperature control structure thatsuccessfully stabilizes conventional thermally coupled columnswith side rectifiers, in which an adjustable vapor split is used tocontrol a particular temperature stage in the main column. It shouldbe noted that although other works have analyzed several controlstructures for DWC systems, including the use of the vapor split asa degree of freedom [33,47], the use of the adjustable vapor split for

EDWCs has been studied in few works [13,44], for other chemicalsystems. This configuration will be used for the EDWC for ethanoldehydration investigated in this work. The control configurationshown below follows the one described in Section 4.1, but assumes

98 S. Tututi-Avila et al. / Chemical Engineering and Processing 82 (2014) 88–100

0 1 2 3 4 544.5

4545.5

46

F / [

kmol

/hr]

0 1 2 3 4 567

67.5

68

S / [

kmol

/hr]

0 1 2 3 4 535

373941

D1 /

[km

ol/h

r]

0 1 2 3 4 556789

10

D2 /

[km

ol/h

r]

0 1 2 3 4 5107

108109110

T 4 / [°

C]

0 1 2 3 4 50.5

11.5

2

R2 /

[km

ol/h

r]0 1 2 3 4 5

0.9930.9940.9950.9960.997

x D1

(E) /

[−]

0 1 2 3 4 50.99

0.995

1

x D2

(W) /

[−]

0 1 2 3 4 5105

110115120

Time / [hr]

T 14 /

[°C]

0 1 2 3 4 5158

160162164

Time / [hr]

T 18 /

[°C]

88 mol% Ethanol 80 mol% Ethanol

Fig. 14. Dynamic responses for feed composition disturbances of the EDWC with adjustable vapor split.

0 1 2 3 4 50.993

0.994

0.995

0.996

x D1

(Eth

anol

) / [−

]

0 1 2 3 4 50.985

0.987

0.989

0.991

0.993

0.995

0.997

x D2

(Wat

er) /

[−]

0 1 2 3 4 50.9945

0.995

0.9955

Time / [hr]

x D1

(Eth

anol

) / [−

]

0 1 2 3 4 50.992

0.994

0.996

Time / [hr]

x D2

(Wat

er) /

[−]

0.997

0.995

0.993

Conventional EDWC

-20%F

-20%F

+20%F

+20%F

+20%F

+20%F

-20%F-20%F

88 mol% E

88 mol% E

80 mol% E

80 mol% E

80 mol% E

80 mol% E

88 mol% E

88 mol% E

Fig. 15. Comparison of conventional arrangement and EDWC for feed disturbances.

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S. Tututi-Avila et al. / Chemical Engi

hat an adjustable vapor split is available to control a given trayemperature in the main column. In this case, the temperature athe 14th stage (T14) was controlled by manipulating the ratio of theapor sidedraw flow (VR) to the reboiler heat input (QR), with theeflux ratio in the main column (R1) being held constant. The pro-osed control scheme is shown in Fig. 12. Table 4 gives the resultsrom the tuning procedure for the four temperature control loops.

The control strategy with adjustable vapor split provided atable regulatory control against both feed flow rate and feed com-osition disturbances. Fig. 13 shows the responses of the EDWColumn for a 20% change in the feed flow rate of both feed streams at.2 h. Stable regulatory control was achieved with product puritieseturning close to their specifications in approximately 1.5 h. Allray temperatures returned quickly to their set points.

Fig. 14 displays the results for changes in the ethanol feed com-osition. One can observe that the proposed control structure forhe EDWC handled such disturbances quite effectively, and theemperature control points returned quickly to their set points.roduct purity deviations were small. The new control structure,herefore, provided a suitable option for the control of the EDWCystem.

A comparison of the responses obtained for the extractiveividing-wall column and those of the conventional sequence isade, using the best control structures for each case. It should

e noted that in both cases four key temperature controllers aresed. Fig. 15 summarizes the responses under the considered dis-urbances in feed flowrate and feed composition of the ethanoltream. It can be observed that the responses are quite similar, withhort stabilization times of the controlled variables. These resultsllustrate that the control properties of complex EDWC arrange-

ents can be comparable to those of a regular sequence, however,he EDWC arrangement has the potential to offer significant energyavings.

. Conclusions

The design and control of an ethanol dehydration processia extractive distillation using ethylene glycol as entrainer haveeen presented. An optimal steady state design of a conventionalequence was first obtained, from which the design of an extractiveividing-wall column was implemented. The EDWC structure pro-ided savings in both capital and energy consumption. Additionally,he EDWC requires fewer equipment units and has a reduced plantootprint.

For the conventional process, a temperature control structureas proposed, which showed good control for both columns whileaintaining the required high purity of the products. Two different

emperature control structures where implemented for the EDWCystem based on the availability of the vapor split inside of the col-mn as a degree of freedom. An initial control structure withoutdjustable vapor split did not provide a proper behavior. However,he system yielded a suitable response under feed composition andeed flowrate disturbances after the vapor split had been includeds a variable in the control scheme. For the tests conducted here,he closed-loop performance of the thermally coupled system wasimilar to the one of the conventional arrangement. These resultshow that the EDWC provides a promising alternative for the sepa-ation of ethanol/water via extractive distillation, since the energyavings provided by such complex arrangements are significant, yetts dynamic performance under feedback control is comparable toonventional extractive distillation systems.

cknowledgements

S. Tututi appreciates the financial support for visiting schol-rs from Conacyt, Mexico, through the program “Estancias

[

[

g and Processing 82 (2014) 88–100 99

Posdoctorales y Sabáticas al Extranjero para la Consolidación deGrupos de Investigación 2011–2012 (Register No. 184961)”. Also,financial support from Conacyt from project CB-43898 is gratefullyacknowledged.

References

[1] M. Balat, H. Balat, C. Öz, Progress in bioethanol processing, Prog. Energy Com-bust. Sci. 34 (2008) 551–573.

[2] J.A. Quintero, M.I. Montoya, O.J. Sánchez, O.H. Giraldo, C.A. Cardona, Fuelethanol production from sugarcane and corn: comparative analysis for aColombian case, Energy 33 (2008) 385–399.

[3] L.M. Vane, Separation technologies for the recovery and dehydration of alcoholsfrom fermentation broths, Biofuels Bioprod. Bioref. 2 (2008) 553–588.

[4] H.-J. Huang, S. Ramaswamy, U.W. Tschirner, B.V. Ramarao, A review of sepa-ration technologies in current and future biorefineries, Sep. Purif. Technol. 62(2008) 1–21.

[5] A.K. Frolkova, V.M. Raeva, Bioethanol dehydration: state of the art, Theor.Found. Chem. Eng. 44 (2010) 545–556.

[6] S. Hernández, Analysis of energy-efficient complex distillation options to purifybioethanol, Chem. Eng. Technol. 31 (2008) 597–603.

[7] A.A. Kiss, D.J.P.C. Suszwalak, Enhanced bioethanol dehydration by extractiveand azeotropic distillation in dividing-wall columns, Sep. Purif. Technol. 86(2012) 70–78.

[8] G. Kaibel, Distillation columns with vertical partitions, Chem. Eng. Technol. 10(1987) 92–98.

[9] A.C. Christiansen, S. Skogestad, K. Lien, Complex distillation arrangements:extending the petlyuk ideas, Comput. Chem. Eng. 21 (Supplement) (1997)S237–S242.

10] B. Kolbe, S. Wenzel, Novel distillation concepts using one-shell columns, Chem.Eng. Proc.: Proc. Int. 43 (2004) 339–346.

11] Y.C. Wu, P.H.-C. Hsu, I.L. Chien, Critical assessment of the energy-saving poten-tial of an extractive dividing-wall column, Ind. Eng. Chem. Res. 52 (2013)5384–5399.

12] Z. Fan, X. Zhang, W. Cai, F. Wang, Design and control of extraction distillationfor dehydration of tetrahydrofuran, Chem. Eng. Technol. 36 (2013) 829–839.

13] M. Xia, B. Yu, Q. Wang, H. Jiao, C. Xu, Design and control of extractive dividing-wall column for separating methylal–methanol mixture, Ind. Eng. Chem. Res.51 (2012) 16016–16033.

14] I. Dejanovic, L. Matijasevic, Z. Olujic, Dividing wall column—a breakthroughtowards sustainable distilling, Chem. Eng. Proc.: Proc. Int. 49 (2010) 559–580.

15] M. Errico, B.-G. Rong, G. Tola, M. Spano, Optimal synthesis of distillation systemsfor bioethanol separation. Part 2. Extractive distillation with complex columns,Ind. Eng. Chem. Res. 52 (2013) 1620–1626.

16] M. Errico, B.-G. Rong, Synthesis of new separation processes for bioethanolproduction by extractive distillation, Sep. Purif. Technol. 96 (2012) 58–67.

17] A.A. Kiss, R.M. Ignat, Innovative single step bioethanol dehydration in an extrac-tive dividing-wall column, Sep. Purif. Technol. 98 (2012) 290–297.

18] Ö. Yildirim, A.A. Kiss, E.Y. Kenig, Dividing wall columns in chemical processindustry: a review on current activities, Sep. Purif. Technol. 80 (2011) 403–417.

19] N. Asprion, G. Kaibel, Dividing wall columns: fundamentals and recentadvances, Chem. Eng. Proc.: Proc. Int. 49 (2010) 139–146.

20] B. Heida, G. Bohner, K. Kindler, Consider divided-wall technology for butadieneextraction, Hydr. Proc. 81 (2002), 50B–50H.

21] M. Jobson, Dividing wall distillation comes of age, Chem. Eng. 766 (2005) 30–31.22] S. Hernández, A. Jiménez, Controllability analysis of thermally coupled distil-

lation systems, Ind. Eng. Chem. Res. 38 (1999) 3957–3963.23] R. Gutiérrez-Guerra, J.G. Segovia-Hernández, S. Hernández, Reducing energy

consumption and CO2 emissions in extractive distillation, Chem. Eng. Res. Des.87 (2009) 145–152.

24] A.A. Kiss, C.S. Bildea, A control perspective on process intensification individing-wall columns, Chem. Eng. Proc.: Proc. Int. 50 (2011) 281–292.

25] I.D. Gil, J.M. Gómez, G. Rodríguez, Control of an extractive distillation process todehydrate ethanol using glycerol as entrainer, Comput. Chem. Eng. 39 (2012)129–142.

26] S. Arifin, I.L. Chien, Design and control of an isopropyl alcohol dehydrationprocess via extractive distillation using dimethyl sulfoxide as an entrainer, Ind.Eng. Chem. Res. 47 (2008) 790–803.

27] W.L. Luyben, Plantwide control of an isopropyl alcohol dehydration process,AIChE J. 52 (2006) 2290–2296.

28] W.L. Luyben, Comparison of extractive distillation and pressure-swing dis-tillation for acetone–methanol separation, Ind. Eng. Chem. Res. 47 (2008)2696–2707.

29] M.R.W. Maciel, R.P. Brito, Evaluation of the dynamic behavior of an extrac-tive distillation column for dehydration of aqueous ethanol mixtures, Comput.Chem. Eng. 19 (Supplement 1) (1995) 405–408.

30] C. Ramírez-Márquez, J.G. Segovia-Hernández, S. Hernández, M. Errico, B.-G.Rong, Dynamic behavior of alternative separation processes for ethanol dehy-dration by extractive distillation, Ind. Eng. Chem. Res. 52 (2013) 17554–17561.

31] J.G. Segovia-Hernandez, M. Vázquez-Ojeda, F.I. Gómez-Castro, C. Ramírez-Márquez, M. Errico, S. Tronci, B.-G. Rong, Process control analysis for intensifiedbioethanol separation systems, Chem. Eng. Proc.: Proc. Int. 75 (2014) 119–125.

32] H. Ling, W.L. Luyben, Temperature control of the BTX divided-wall column, Ind.Eng. Chem. Res. 49 (2009) 189–203.

1 neerin

[

[

[

[

[

[

[

[

[

[

[

[

[

[

00 S. Tututi-Avila et al. / Chemical Engi

33] D. Dwivedi, J.P. Strandberg, I.J. Halvorsen, H.A. Preisig, S. Skogestad, Activevapor split control for dividing-wall columns, Ind. Eng. Chem. Res. 51 (2012)15176–15183.

34] W.L. Luyben, Evaluation of criteria for selecting temperature control trays indistillation columns, J. Process Contr. 16 (2006) 115–134.

35] S. Hernández, A. Jiménez, Design of optimal thermally-coupled distillation sys-tems using a dynamic model, Chem. Eng. Res. Des. 74 (1996) 357–362.

36] C. Bravo-Bravo, J.G. Segovia-Hernández, C. Gutiérrez-Antonio, A.L. Durán, A.n.Bonilla-Petriciolet, A. Briones-Ramírez, Extractive dividing wall column: designand optimization, Ind. Eng. Chem. Res. 49 (2010) 3672–3688.

37] W.L. Luyben, Comparison of extractive distillation and pressure-swing distil-lation for acetone/chloroform separation, Comput. Chem. Eng. 50 (2013) 1–7.

38] V.I.I. Grassi, Process design and control of extractive distillation, in: W. Luyben

(Ed.), Practical Distillation Control, Springer, USA, 1993, pp. 370–404.

39] H. Ling, W.L. Luyben, New control structure for divided-wall columns, Ind. Eng.Chem. Res. 48 (2009) 6034–6049.

40] M. Morari, E. Zafiriou, Robust Process Control, Prentice Hall, Englewood Cliffs,NJ, 1989.

[

g and Processing 82 (2014) 88–100

41] D.E. Rivera, M. Morari, S. Skogestad, Internal model control: PID controllerdesign, Ind. Eng. Chem. Process Des. Dev. 25 (1986) 252–265.

42] D.E. Seborg, Process Dynamics and Control, 3rd ed., John Wiley & Sons, Hoboken,NJ, 2011.

43] S. Skogestad, Simple analytic rules for model reduction and PID controllertuning, J. Process Contr. 13 (2003) 291–309.

44] M. Xia, Y. Xin, J. Luo, W. Li, L. Shi, Y. Min, C. Xu, Temperature control for extrac-tive dividing-wall column with an adjustable vapor split: methylal/methanolazeotrope separation, Ind. Eng. Chem. Res. 52 (2013) 17996–18013.

45] H. Zhang, Q. Ye, J. Qin, H. Xu, N. Li, Design and control of extractive dividing-wall column for separating ethyl acetate–isopropyl alcohol mixture, Ind. Eng.Chem. Res. 53 (2013) 1189–1205.

46] W.L. Luyben, American Institute of Chemical Engineers. Distillation Design

and Control Using Aspen Simulation, 1st ed., John Wiley & Sons, Hoboken, NJ,2006.

47] D. Dwivedi, I.J. Halvorsen, S. Skogestad, Control structure selection for three-product Petlyuk (dividing-wall) column, Chem. Eng. Proc.: Proc. Int. 64 (2013)57–67.