control extract distil usin glycerol

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Computers and Chemical Engineering 39 (2012) 129–142

Contents lists available at SciVerse ScienceDirect

Computers and Chemical Engineering

journal homepage: www.elsevier .com/ locate /compchemeng

Control of an extractive distillation process to dehydrate ethanol using glycerolas entrainer

Iván D. Gil a,∗, Jorge M. Gómezb, Gerardo Rodríguez a

a Grupo de Procesos Químicos y Bioquímicos, Departamento de IngenieríaQuímica y Ambiental, UniversidadNacional de Colombia – Sede Bogotá, CiudadUniversitaria – Carrera 30

45-03, Bogotá, Colombiab Grupo de Diseño de Productosy Procesos,Departamento de IngenieríaQuímica, Universidad de los Andes, Carrera 1 Este 19A-40, Bogotá, Colombia

a r t i c l e i n f o

Article history:Received 23 April 2011

Received in revised form

27 December 2011

Accepted 12 January 2012

Available online 21 January 2012

Keywords:

Ethanol dehydration

Extractive distillation

Glycerol

Entrainer

Distillation control

a b s t r a c t

In this paper, an investigation of the design and control of an extractive distillation process to produce

anhydrous ethanol using glycerol as entrainer is reported. The extractive distillation process receives the

azeotropic mixture ethanol–water that is fed into a dehydration column in one of the intermediate stages

while at the same time glycerol is fed into one of the top stages. As overhead product high purity ethanol

is withdrawn and in the bottom stream a mixture of water/glycerol is sent to a recovery column. The

effects of the entrainer to feed molar ratio, reflux ratio, feed stage, feed entrainer stage and entrainer feed

temperature were studied to obtain the best design with minimal energy requirements. A control scheme

is developed in order to maintain stable operation for large feed disturbances. Dynamic simulations show

that is possible to use only one temperature control to hold the purity specifications.

© 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Distillation processes represent a high percentage of the sep-

aration operations used in the refining and chemical industries.

Distillation, in addition to being a very common operation, has

a strong impact on energy consumption of the processes and it

is used in the purification steps, where the products will have a

higher added value and specifications are more rigorous, which is

the case of dehydration step in the anhydrous ethanol production.

Anhydrous ethanol is widely used in the chemical industry as a

raw material in chemical synthesis of esters and ethers, and as sol-

ventin production of paint, cosmetics, sprays, perfumery, medicine

and food, among others. Furthermore, mixtures of anhydrous

ethanol and gasoline may be used as fuels, reducing environmen-

tal contamination and improving gasoline octane index, mainly

due to the addition of ethanol (Barba, Brandani, & Di Giacomo,

1985; Black, 1980; Chianese & Zinnamosca, 1990; Meirelles, Weiss,

& Herfurth, 1992). Among the most popular processes used in

ethanol dehydration, heterogeneous azeotropic distillation uses

solvents such as benzene, pentane, iso-octane and cyclohexane;

extractive distillation withsolvents and saltsas entrainers; adsorp-

tion with molecular sieves; and, processes that use pervaporation

∗ Corresponding author. Tel.: +57 1 3165672; fax: +57 1 3165617.

E-mail address: [email protected] (I.D. Gil).

membranes (Black, 1980; Gomis, Pedrasa, Francés, Font, & Asensi,

2007; Hanson, Lynn, & Scott, 1988; Pinto, Wolf-Maciel, & Lintomen,

2000; Ulrich & Pavel, 1988).

Heterogeneous azeotropic distillation has been widely stud-

ied in many papers and textbooks and widely applied in alcohol

industry to dehydrate ethanol (e.g. 60% of dehydration plants in

Brazil are azeotropic distillation based). However, heterogeneous

azeotropic distillation reports some disadvantages associated with

the high degree of nonlinearity, multiple steady states, distilla-

tion boundaries, long transients, and heterogeneous liquid–liquid

equilibrium, limiting the operating range of the system under dif-

ferent feed disturbances (Chien, Wang, & Wong, 1999; Widagdo &

Seider, 1996). Extractive distillation is a partial vaporization pro-

cess in the presence of a non-volatile separating agent with a

high boiling point, which is generally called solvent or entrainer,

and which is added to the azeotropic mixture to alter the rela-

tive volatility of the key component with no additional formation

of azeotropes (Black & Distler, 1972; Perry, 1992). The princi-

ple driving extractive distillation is based on the introduction

of a selective solvent that interacts differently with each of the

components of the original mixture and which generally shows

a strong affinity with one of the key components (Doherty &

Malone, 2001; Lee & Gendry, 1997). In the case of extractive

distillation many of the disadvantages found in azeotropic ones

are not present, because there is no heterogeneous liquid–liquid

equilibrium, no additional azeotropes are formed with the

0098-1354/$ – seefrontmatter © 2012 Elsevier Ltd. All rights reserved.

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130 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142

0,0

0,2

0,4

0,6

0,8

1,0

1,00,80,60,40,20,0

y E t h a n o l

x Ethanol

x-y Exp.

x-y Est. NRTL

50

90

130

170

210

250

290

1,00,80,60,40,20,0

T e m p e r a t u r e ( ° C )

x Ethanol

EtOH-Gly T-x Exp.

EtOH-Gly T-x Est. NRTL

EtOH-Gly T-y

Est. NRTL

0,0

0,2

0,4

0,6

0,8

1,0

1,00,80,60,40,20,0

y E t h a n o l

x Ethanol

x-y

Exp.

x-y Est. NRTL

75

80

85

90

95

100

105

1,00,80,60,40,20,0

T e m p e

r a t u r e ( ° C )

x-y Ethanol

EtOH-Water T-x Exp.

EtOH-Water T-y Exp.

EtOH-Water T-x Est. NRTL

EtOH-Water T-y Est. NRTL

0,0

0,2

0,4

0,6

0,8

1,0

1,00,80,60,40,20,0

y W a t e r

x Water

x-y Exp.

x-y Est. NRTL

50

90

130

170

210

250

290

1,00,80,60,40,20,0

T e m p e r a t u r e ( ° C

)

x-y Water

Water-Gly T-x Exp.

Water-Gly T-y Exp.

Water-Gly T-x Est. NRTL

Water-Gly T-y Est. NRTL

Fig. 1. T-xy and x– y experimental and predicted diagrams at 1 atm for the ethanol–water, water–glycerol and ethanol–glycerol systems (Carey & Lewis, 1932; Chen &

Thompson,1970; Coelho, dosSantos, Mafra, Cardozo-Filho, & Coraza, 2011).

addition of the entrainer and therefore there are no distillation

boundaries.

Distillation control is a very important topic and it has been

subject of study duringseveral decades by control engineersin aca-

demic andindustrial contexts(Hurowitz,Anderson, Duvall, & Riggs,

2003; Ross, Perkins, Pistikopoulos, Koot, & van Schijndel, 2001;

Shinskey,1996; Wolf-Maciel & Brito, 1995). The selectionof control

configuration appropriated for the distillation involves an initial

step where regulatory controls areimplementedin a good wayand

then the control problem is reduced to identify the best pairing

of the controlled and manipulated variables that allow obtaining

composition control in the column (Hurowitz et al., 2003; Luyben,

2006a; Skogestad, 1992). The available methodologies in the task

of select the configuration of composition control are multiple

and they use criteria based on steady state and dynamic mod-

els (Fruehauf & Mahoney, 1993; Luyben, 2006a, 2006b; Shinskey,

1977). Control of azeotropic and extractive distillation has been

subject of different studies. Luyben (2006b) studied a control struc-

ture for a multiunit heterogeneous azeotropic distillation process

that uses benzene as entrainerto dehydrate ethanol. Also the dehy-

drationof isopropylalcoholby extractive distillationusing ethylene

glycol (Luyben, 2006c) and dimethyl sulfoxide (Arifin & Chien,


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I.D. Gil et al. / Computers andChemical Engineering 39 (2012) 129–142 131

Fig. 2. T – xy and x– y experimental and predicted diagrams at 1 atm for the ethanol–water and water–glycerol systems (Carey & Lewis, 1932; Chen & Thompson, 1970). (a)

Residue curve map, (b) pseudo-binary vapor–liquid equilibrium for ethanol–water–glycerol system (Lee & Pahl, 1985).

2008) as entrainers has been reported. However, there are no stud-

ies that report ethanol dehydration using glycerol as entrainer and

considering at the same time the control strategy required to pro-

duce high purity ethanol and recover totally the glycerol stream.

The purpose of this work is to design and control an extractive

distillation process to produce anhydrous ethanol using glycerol

as entrainer. This is a new alternative for dehydrate ethanol taking

intoaccount thatglycerol is available at low costs as consequence of

the highproductionof thissubstance in the biodieselprocess. Addi-

tionally, it has been demonstrated thepotential effectof glycerol in

modifying the vapor–liquid equilibrium of the ethanol–water mix-

tureeliminating the azeotrope. The steady statedesign involves the

selection of the appropriate thermodynamic model and the study

of the effect of the main designvariables. The control strategy con-

siders thecontrol of only onetemperature on each columnin orderto be used for wider industrial applications and to provide good

product quality control.

2. Thermodynamic model

Ethanol–water mixture at atmospheric pressure has a

minimum-boiling homogeneous azeotrope at 78.1◦C of com-

position 89mol% ethanol. Thus, this mixture cannot be separated

in a single distillation column and if it is fed to a column operating

at atmospheric pressure, the ethanol purity in the distillate cannot

exceed 89mol% while high purity water can be produced out from

the bottom. The NRTL physical property model (Renon & Prausnitz,

1968) is used to describe the nonideality of the liquid phase and

the vapor phase is assumed to be ideal. The complete NRTL model

binary parameters are taken from Aspen Plus database.

The thermodynamic model prediction is validated with experi-

mental data from Carey andLewis(1932), and Chen and Thompson

(1970) f or ethanol–water and water–glycerol mixtures, respec-tively. The y–x and T–xy vapor–liquid equilibrium plots are shown

in Fig. 1, where it can be seen that the model fits the experimental

Fig. 3. Flowsheet for extractive distillation system.


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Fig. 4. Effectof RR and EFSon ethanol purity and reboiler energyconsumption of extractive distillation column.

data very well forthe case of ethanol–water mixture andwithsome

difficulties for the case of water–glycerol mixture in the zone of

lower water composition.

Ternary-phase diagrams with residue curves, distillation

boundaries and tie lines, provide very useful tool into the con-

straints encountered in the highly nonideal systems and they can

be used as simple method for identifying and designing feasible

extractive distillation sequences. Fig. 2a gives the residue curve

map for the ethanol–water–glycerol system calculated usingAspen

Split at 1atm and Fig. 2b shows the pseudo-binary vapor–liquid

equilibrium diagram for the same system with the experimen-

tal data reported by Lee and Pahl (1985). In Fig. 2b it is evident

that glycerol modifies the vapor–liquid equilibrium curve, elimi-nating the azeotrope and allowing obtaining high purity ethanol.

Lee and Pahl (1985) report that the glycols used as solvents elim-

inate the ethanol–water azeotrope and change the VLE curve.

Ethylene glycol is well known as solvent in extractive distilla-

tion of ethanol–water mixtures with a good performance results.

However, glycerol shows a better performance in modifying the

VLE curve favorably for distillation as consequence of its longer car-

bon chain length and the existence of oxygen in the carbon chain,

according to the results reported by Lee and Pahl (1985).

3. Steady state design

Extractive distillation includes an entrainer to increase the rel-

ative volatility of the key components of the feed. This process

is used to separate low relative volatility systems, or those that

have an azeotrope (Treybal, 1955). The process flowsheet of the

extractive distillation process is presented in Fig. 3. The process

hastwo columns, one forextractive separation and another for sol-vent recuperation. The azeotropic mixture (F1) and the entrainer

(S1) streams are fed to the extractive distillation column, where

the dehydration of the desired compound (ethanol) takes place.

The bottom product of the extractive distillation column feeds the

entrainer recovery column, where the entrainer (leaving from the

Fig. 5. Effect of RR and E/Fon ethanol purity and reboiler energy consumption of extractive distillation column.


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Fig. 6. Effect of EFT and RR on ethanol purity and reboiler energy consumption of extractive distillation column.

reboiler) is separated from water and is recycled to the extractive

distillation column.

Glycerol alters the liquid activity coefficients and in conse-

quence the relative volatility of ethanol–water mixture causing

water to move toward the bottoms and pure ethanol is withdrawn

at the top. The mixture water–glycerol (B1) from the bottoms of

extractive column is fed into the entrainer recovery column to

produce almost pure water in the distillate (D2) and high purity

glycerol in the column bottoms (B2). Glycerol will be recirculated

back to the extractive column. Notice that a small makeup glycerol

stream is required to balance small entrainer losses in both D1 and

D2 streams.

The minimal product specifications in the two columns are set

to be thefollowing: 99.5 mol% min. of ethanol in D1 and99.95 mol%min. of glycerol in B2 in order to be recycled to the extractive

distillation column. To establish the operating conditions for the

extractive distillation process, a sensitivity analysis was done to

determinethe main designvariables such as numberof stages(NS),

reflux molar ratio (RR), binaryfeed stage (BFS),entrainer feed stage

(EFS), entrainerfeed temperature (EFT)and entrainerto feed molar

ratio (E/F). The binary mixture was fed in the extractive distillation

column at azeotropic composition. The operating pressure in the

extractive distillation column is fixed a 1atm and for the case of

entrainer recovery column is fixed at 0.02 atm in order to avoid the

thermal degradation of glycerol because of high temperatures.

Theeffect of changingthe reflux molar ratio (RR), entrainer feed

stage (EFS) and entrainer to feed molar ratio (E/F) on the distil-

late composition and reboiler energy consumption of extractive

distillation column are shown in Figs. 4 and 5. It can be seen that

for a given entrainer feed stage (EFS) and entrainer to feed molar

ratio (E/F) there is an optimum reflux molar ratio (RR) that gives

maximum ethanol purity. However, the value of RR ratio must be

low to avoid energy wastes during operation. Reflux molar ratios

in a range of 0.3–0.5 reach composition requirements with lower

energy consumption in the reboiler as can be noted in Figs. 5 and

6. Entrainer feed stage should be located close to the condenser toimprove ethanol purity and its effect on energy consumption is no

significant (see Fig. 4). Entrainer to feed molar ratio (E/F) causes

a direct effect on the distillate purity. Sensitivity analyses, shown

in Fig. 5, show that increasing E/F ratio it is possible to have an

important improvement in the ethanol quality, without consider-

ably affecting energy consumption. At a constant reflux ratio, for

different values of E/F ratio within the interval 0.3–0.4, the energy

consumption increased in 4%. In the same way, maintaining the

60

80

100

120

140

160

180

200

20151050

T e m p e r a t u r e ( ° C )

Stage

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

20151050

L i q u i d C o m p o s i o n P r o fi l e

Stage

Ethanol

Water

Glycerol

Fig. 7. Temperature and composition profiles of the extractive distillation column.


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10

60

110

160

210

6420

T e m

p e r a t u r e ( ° C )

Stage

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

6420

L i q u i d C o

m p o s i o n P r o fi l e

Stage

Ethanol

Water

Glycerol

Fig. 8. Temperature and composition profiles of the entrainer recovery column.

E/F ratio at 0.3, and increasing values of the reflux ratio until a

distillate composition equivalent to the one obtained in the previ-

ous variation is reached; the increase in energy consumption was

30%. Consequently, the reflux ratio must be operated in the lowest

possible value, so the ratio E/F ratio can be manipulated to reachthe distillate composition without high energy consumption, and

reminding that high E/F ratios increase the energy consumption in

reboiler of the recovery column. Also, Figs. 4 and 5 show that the

change in reflux molar ratio (RR) hasa greater effecton the reboiler

energyconsumption than that of the entrainerfeed stage (EFS) and

entrainerto feed molar ratio (E/F).To achieve thedesired 99.5 mol%

of ethanol in D1, the entrainer to feed molar ratio must be about

0.45at a low reflux molar ratio of about 0.35.

Entrainer feed temperature (EFT) has an important effect on

the distillate composition and the reboiler energy consumption.

Several authors recommend considering EFT as design variable

and operating 5–15◦C below the top temperature of the extractive

distillation column (Doherty & Malone, 2001; Knight & Doherty,

1989). As it can be observed in Fig. 6, using high entrainer feed

temperatures demands high reflux molar ratios to reach a spec-

ified separation. This occurs because, as EFT is increased, part of the water found in the stage vaporizes, increasing the content of

water in the distillate and decreasing its purity. Then increasing RR

is necessary to compensate this effect. In conclusion, low reflux

operations need entrainer fed at temperatures between 70 and

80 ◦C to keep the distillate purity which is in accordance with the

value recommended by other authors taking into account that the

overhead temperature in the extractive column is 78◦C approxi-

mately. The leastenergy demand corresponds to low entrainer feed

temperatures and low reflux molar ratios.

Temperature and composition profiles in the two columns are

given in Figs. 7 and 8. It is noticed that the goal of extractive

Fig. 9. Final flowsheet design for the ethanol dehydration system.


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-4

-3

-2

-1

0

1

2

3

181716151413121110987654321

T e m p e r a t u r e D i ff e r e n c e ( ° C )

Stage

+5%

-5%

-15

-10

-5

0

5

10

15

654321

T e m p e r a t u r

e D i ff e r e n c e ( ° C )

Stage

+5%

-5%

Fig. 10. Sensitivity analysis for ±5% changes in extractive and recovery reboiler duties.

distillation is fulfilled in eliminating water going into the rectify-

ing section. Entrainer is fed on stage 3 and the azeotropic feed is

introduced on stage 10. Fig.9 shows the final flowsheet forthis sys-

tem. Itis important toobservethatthe overheadtemperature inthe

entrainer recovery column corresponds to 16 ◦C. Therefore, cool-

ingwatercannot be used as cooling mediumand a more expensive

cooling medium must be used instead.

4. Control strategy design

The development of the control strategy requires the conversion

of the steady state model in a dynamic one in order to evaluate the

effect of the maindisturbancesto the extractive distillation system.

Themodel developedin AspenPlus is exported toa pressure-driven

simulation in Aspen Dynamics. Before converting the Aspen Plus

model to Aspen Dynamics, the sizing of equipment is necessary.

The Pack-Sizing utility of the RadFrac distillation column block in

Aspen Plus is used to calculate the column diameters to be 0.85m

and 0.57m for the extractive and recovery column, respectively.

Reflux drums and base levels are calculated to provide 5 min of

holdup when at the 50% liquid level. Pumps and valves are sized

to provide adequate pressure drops over valves to handle changes

in flow rates appropriately (good rangeability). The Aspen Plus file

is pressure checked and exported into Aspen Dynamics. The top

pressures of the extractive and recovery column are set at 1 atm

and 0.02atm, respectively.

Fig. 11. Configuration of control strategy 1.


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104,50

105,00

105,50

106,00

106,50

107,00

107,50

108,00

1086420

T i n 1 7

t h s t a g e o f C - 1 ( ° C )

Time (h)

87 mol

%

Ethanol

85 mol % Ethanol

188,00

189,00

190,00

191,00

192,00

193,00

194,00

195,00

1086420

T i n 5 t h s t a g e o f C - 2 ( ° C )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

0,99540

0,99560

0,99580

0,99600

0,99620

0,99640

0,99660

0,99680

0,99700

1086420

X D 1 (

e t h a n o l )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

0,98700

0,98800

0,98900

0,99000

0,99100

0,99200

0,99300

1086420

X D 2

( w a t e r )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

82,00

83,00

84,00

85,00

86,00

87,00

88,00

89,00

90,00

1086420

D 1 ( k m

o l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

52,00

53,00

54,00

55,00

56,00

57,00

58,00

59,00

60,00

61,00

1086420

B 1 ( k m o l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

9,00

10,00

11,00

12,00

13,00

14,00

15,00

1086420

D 2 ( k m o l / h

)

Time (h)

87 mol % Ethanol

85 mol % Ethanol

44,90

44,95

45,00

45,05

45,10

45,15

45,20

45,25

45,30

1086420

B 2 ( k m o l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

Fig. 12. Dynamic responsesfor feed composition disturbances in thecontrol strategy 1.


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98,00

100,00

102,00

104,00

106,00

108,00

110,00

112,00

114,00

1086420

T i n 1 7 t h s t a g e o f C - 1 ( ° C )

Time (h)

Azeotropic Feed = +20%

Azeotropic Feed = -20%

186,00

188,00

190,00

192,00

194,00

196,00

198,00

200,00

1086420

T i n 5 t

h s t a g e o f C - 2

( ° C )

Time (h)



0,99620

0,99630

0,99640

0,99650

0,99660

0,99670

0,99680

0,99690

0,99700

0,99710

1086420

X D 1

( e t h a n o l )

Time (h)



0,97600

0,97800

0,98000

0,98200

0,98400

0,98600

0,98800

0,99000

1086420

X D 2

( w a t e r )

Time (h)



60,00

65,00

70,00

75,00

80,00

85,00

90,00

95,00

100,00

105,00

110,00

1086420

D 1 ( k m o l / h )

Time

(h)



30,00

35,00

40,00

45,00

50,00

55,00

60,00

65,00

70,00

1086420

B 1 ( k m

o l / h )

Time (h)



6,00

7,00

8,00

9,00

10,00

11,00

12,00

13,00

14,00

1086420

D 2 ( k m o l / h )

Time

(h)


Azeotropic Feed = -20%6,00

16,00

26,00

36,00

46,00

56,00

66,00

1086420

B 2 ( k m o l / h )

Time (h)



Fig. 13. Dynamic responses for feed flow disturbances in thecontrol strategy 1.


10/14


Fig. 14. Configuration of control strategy 2.

Initially, a basic regulatory control scheme is determined

through the various control loops as follows:

(1) Reflux drum levels for both columns are controlled by manip-

ulating the distillate valves located in the streams D1 and D2.

(2) The fresh feed to the extractive column is flow control in orderto guarantee the constant flowrate.

(3) The top pressures of both columns are controlled by manipu-

lating the corresponding condenser duties.

(4) The base level for extractive column is controlled by manipu-

lating the bottoms flow rate.

(5) The base level for recovery column is controlled by manipulat-

ing the makeup flow rate, according to the suggested by Grassi

(1992) and Luyben (2008) f or other extractive distillation sys-

tems.

(6) The entrainerfeed temperature is controlled at 80◦C bymanip-

ulating cooler duty.

(7) The entrainer flow rate is ratioted to the azeotropic feed and

the ratio is controlled by manipulatingthe bottoms flowrate of

the recovery column.(8) Reflux ratios are held constant in each column at their nominal

values during disturbances.It wasalso worked in other previous

works (Arifin & Chien, 2008; Luyben, 2008).

(9) The reboiler duties of both columns are used to control the

temperature in a particular stage of each column.

Temperature control stage location is selected applying two

criteria: (a) stage with a high slope in the temperature profile, and

(b) stage with high sensitivity to changes in reboiler duty. Temper-

ature profiles of both columns are shown in Figs.7 and 8. Also, Fig.

10 shows the results of an open loop sensitivity analysis with±5%

changes in reboiler duty of the extractive distillation column and

recovery column. For the extractive distillation column, the tem-

perature at the 17th stage has the higher slope in temperature and

additionally,as canbe seenin Fig. 10, it is a sensitivepoint; forthese

reasons the 17th stage is chosen as the control point. In the case of

recovery columnthe 2nd stage hasthe highest slope in the temper-

ature profile and the most noticeable stage to changes in reboiler

duty is the5th stage.Taking into account the importance of consid-

ering the dynamic response, the temperature control points in therectifying sections of the columns (e.g. the 8th stage in extractive

column and the 2nd stage in the recovery column) are not chosen

because of the largerdeadtime and topreventa poor control perfor-

mance. The 5th stage is chosen as the control point in the recovery

column.

The most control loops described above correspond to a typi-

cal distillation control configuration. Only two particular loops are

defined in a special manner: (1) the entrainer flow rate is ratioted

to the feed flow rate through the “ratio” multiplier which sends

the remote setpoint to the entrainer flow control which operates

on cascade and, (2) the base level in the recovery column is con-

trolled by manipulating the makeup entrainer flow rate. Because

the makeup entrainer flow rate is much smaller than the total

entrainer feed to the extractive column, the 5-min holdup time inthebase of the recovery column is notable to handle changes in the

entrainerflow rateand thebottom leveloscillates continuously.For

+20% changes of the feed flowrate the bottom level is continuously

dropping until it is empty and the valve is fully opened. To over-

come this situation a 10-min holdup time is fixed in the base of the

two columns. The overall control configuration initially proposed

is summarized in Fig. 11 as control strategy 1.

All level loops are Proportional only controllers with Kc = 2

for reflux drum levels according to the recommended by Luyben

(2002) and Kc = 10 for both base bottoms levels for faster

dynamics in the internal flow of the process. The pressure con-

trollers are Proportional-Integral with Kc =20 and I = 12min, the

default values used by Aspen Dynamics. All flow controllers

are Proportional-Integral with the settings recommended by


11/14


104,50

105,00

105,50

106,00

106,50

107,00

107,50

1086420

T i n 1 7

t h s t a g e o f C - 1

( ° C )

Time (h)

87 mol %

Ethanol

85 mol % Ethanol

187,00

188,00

189,00

190,00

191,00

192,00

193,00

194,00

195,00

196,00

1086420

T i n 5 t h s t a g e o f C - 2

( ° C )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

0,99590

0,99600

0,99610

0,99620

0,99630

0,99640

0,99650

0,99660

0,99670

0,99680

0,99690

1086420

X D 1 ( e t h a n o l )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

0,98600

0,98700

0,98800

0,98900

0,99000

0,99100

0,99200

0,99300

1086420

X D 2

( w a t e r )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

82,00

83,00

84,00

85,00

86,00

87,00

88,00

89,00

90,00

1086420

D 1 ( k m o

l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

52,00

54,00

56,00

58,00

60,00

62,00

64,00

1086420

B 1 ( k m o

l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

9,00

10,00

11,00

12,00

13,00

14,00

15,00

1086420

D 2 ( k m o l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

44,50

45,00

45,50

46,00

46,50

47,00

47,50

48,00

1086420

B 2 ( k m o l / h )

Time (h)

87 mol % Ethanol

85 mol % Ethanol

Fig. 15. Dynamic responsesfor feed composition disturbancesin thecontrol strategy 2.


12/14


98,00

100,00

102,00

104,00

106,00

108,00

110,00

112,00

114,00

1086420

T i n 1 7

t h s t a g e o f C - 1

( ° C )

Time (h)



190,00

191,00

192,00

193,00

194,00

195,00

196,00

197,00

198,00

1086420

T i n 5 t h s t a g e o f C - 2

( ° C )

Time (h)



0,99550

0,99600

0,99650

0,99700

0,99750

0,99800

0,99850

0,99900

1086420

X D 1 (

e t h a n o l )

Time (h)



0,93000

0,94000

0,95000

0,96000

0,97000

0,98000

0,99000

1086420

X D 2

( w a t e r )

Time (h)



60,00

65,00

70,00

75,00

80,00

85,00

90,00

95,00

100,00

105,00

110,00

1086420

D 1 ( k m o l / h )

Time (h)



30,00

35,00

40,00

45,00

50,00

55,00

60,00

65,00

70,00

1086420

B 1 ( k m o l / h )

Time (h)



6,00

7,00

8,00

9,00

10,00

11,00

12,00

13,00

14,00

1086420

D 2 ( k m o l / h )

Time (h)



6,00

16,00

26,00

36,00

46,00

56,00

66,00

1086420

B 2 ( k m o l / h )

Time (h)



Fig. 16. Dynamic responses for feed flow disturbancesin thecontrol strategy 2.


13/14


Table 1

Temperature controllers tuning parameters.

Parameter

TC – Column C-1

Ultimate gain 1.568

Ultimate period 4.2 min

Kc 0.4902

I 9.24min

TC – Column C-2

Ultimate gain 2.727Ultimate period 4.8 min

Kc 0.8523

I 10.56min

TC – Cooler

Open loop gain 6.77

Time constant 0.59 min

Dead time 0.6 min

Kc 0.13

I 0.899min

Luyben (2002) Kc =0.5 and I =0.3min and a filter time constant

F = 0.1min.The twotemperature control loops forthe columns are

closed loop tested for determining the ultimate gains and periods,

andTyreus–Luybentuningrule (Tyreus & Luyben, 1992) is used. For

the cooler temperature control loop, open loop tests are performedfor determining the PI tuning constants following the IMC-PI tun-

ing rule (Chien & Fruehauf, 1990). The results of those calculations

and the final controller tuning parameters are shown in Table 1.

Extractive distillation process is the finalstep in the ethanol pro-

duction. The dehydration column is fed with azeotropic ethanol

coming from a rectification column. Changes in the operating

conditions of this column could originate disturbances in the

feed composition to the extractive distillation system, particularly

diminishing the concentration of stream to lower values of ethanol

mole fraction. Here have been considered two ethanol composi-

tion disturbances to test the control strategy from 89 to 87mol%

ethanol and from 89 to 85mol% ethanol at time= 2 h. Additionally,

feed flow disturbances of ±20% also have been considered. Figs. 12

and 13 showthe closed-loop results for these disturbances appliedto the control strategy 1.

Analyzing the results for the feed composition disturbances pre-

sented in Fig. 12, in thetwo topplotsthe temperature control point

on each column works well in rejecting disturbances and the tem-

peratures back to their setpoints. The variation in temperature is

not higher than 3 ◦C in extractive column and 5 ◦C in the recov-

ery column, but the system rapidly achieves the steady state. In

parallel, the results for the feed flow rate disturbances presented

in Fig. 13 show that temperature control points vary 10 and 12◦C

for extractive and recovery column, respectively. This is due to the

most important effect that has the feed flow rate on the energy

required for the separation. Product purities are held quite close

to their specifications and they are mainly affected by changes in

the feed composition, however the quality of ethanol stream neveris negatively affected and the control strategy ensures the quality

product.Inventorycontrolloops are stabilized rapidly.In particular,

when the feed flow rate increases, the cascade controller increases

theentrainer flowrate fed to theextractive column making thebase

level of the recovery column begins to drop. Because the flow rates

of entrainer and feed to the extractive column have increased the

materialbalance is adjusted increasing the feed to therecovery col-

umn, which brings the base level back up. However, good results

only are obtained augmenting the holdup time in the base level

which implies increase the size of the sump level of the recovery

column to achieve good controllability and compensate the loop

poor dynamics.

In order to improve the dynamic performance an alterna-

tive control strategy 2 is proposed modifying slightly the control

structure. Now the base levels of both columns are controlled by

manipulating the bottom flow rates and the entrainer flow rate is

ratioted to theazeotropic feed andthe ratio is controlled by manip-

ulating the makeup flowrate. Again the entrainer flow rate control

is on cascade with the feed flow rate. The base levels are calculated

to provide 5 min of holdup when at the 50% liquid level. The rest

of the control loops are maintained as specified above. The overall

control configuration proposed is summarized in Fig. 14 as control

strategy 2.

Inthe case offeed compositiondisturbances,the twotopplotsof

Fig. 15 show the temperature control point on each column. As can

be seen the good tuning of the loop makes that the change in tem-

perature to bevery small andtherefore thesystemis notaffected by

the disturbances bringingthe temperatures back to their setpoints.

The variation in temperature for both columns is not higher than

3 ◦C whichis very desirable because the energyconsumption of the

system is not strongly affected in the transient periods and there-

fore the operationis more energyefficient. In the next two plots for

the composition of ethanol in the extractive column and water in

the recovery column, it can be observed the corresponding perfor-

mance verifyingthatthe composition or quality of the topproducts

of bothcolumnsis affected onlypositively, i.e. ensuring the product

purities are held quite close or higher to their specifications. Spe-

cially can be noticed that the ethanol composition always is higherthan 99.5 mol% ethanol.

Finally, in the case of the inventory loops, the mole flows of

distillate and bottoms are stabilized rapidly and in general all the

system takes about 2h approximately to come to a new steady

state. This favorable performance could be explained taking into

account the more direct effect of the bottoms flow rate valve of the

recovery column onthe base level andat thesametime the makeup

flow rate effect over entrainer feed rate to the extractive column.

Fig. 16 shows theresults forthe feed flowdisturbancesapplied over

the control strategy at time = 2 h. Once again, the temperature loop

responses relatively fast and the main products composition are

maintained at the specifications required. The time used to come

back to the steady state is about 3h. In this way, with the control

strategy 2 is easier to compensate the effects of feed compositionand feed flow rate disturbances because there is more direct effect

over the variables of interest, mainly in the case of base level of

the recovery columnand in the control of entrainerflow rate. From

the practice point of view is possible that the significant increase

in feed flow rate have influence on the transient response because

of small control valve installed in the make-up stream. In practice

it could be possible to select a good characteristic valve that allows

working with a good response when sporadically changes as high

as +20% are presented. However, this should be clearly verified for

the particular application.

5. Conclusions

This article presents the design and control of an ethanol

dehydration process via extractive distillation using glycerol as

entrainer. The design flowsheet is simulated using the NRTL ther-

modynamic model which describesappropriatelythe experimental

vapor–liquid equilibrium data and supports on a solid thermody-

namic basis the simulation results. From the sensitivity analysis

is possible to establish the main operating conditions of the sys-

tem and to determine the effect of the design variables. Reflux

ratio on the extractive distillation column has the greatest effect

on the energy consumption and it must be operated at low values.

Entrainer to feed molar ratio is useful in compensating changes

in some operating conditions without affecting the energy con-

sumption in an important manner compared with the effect of

the reflux ratio. Glycerol is an interesting candidate entrainer in


14/14


extractive distillation taking advantage of its low cost, high avail-

ability and great effect on azeotropic mixtures to improve the

relative volatility. The only possible drawback for ethanol dehydra-

tion via extractive distillation is that the two columns need to be

operated at higher temperatures demanding high pressure steam

to be used in the two reboilers.

Two control structures are developed and tested, providing

effective quality and production rate control. Control strategy 1

whichuses theentrainer makeupflow rate to control thebase level

in the recovery column has good control performance rejecting for

the disturbances in feed flow rate and feed composition, however,

it is necessary to adjust the size of sump level in order to obtain

good controllability and additionally the perturbations in temper-

ature of the control point of each column are important. On the

contrary, control strategy 2 which uses the entrainer makeup flow

rate to control the entrainer feed flow rate to the extractive col-

umn allows to achieve a soft-regulating control with a minimal

changes in the temperature profile for both columns and main-

taining the high purity of the products. Therefore, control strategy

2 is recommended for the extractive distillation of ethanol.

Acknowledgment

This work is supported by the Departamento Administrativo de

Ciencia,Tecnología e Innovación – Colciencias under grant research

project code 1101-452-21113.

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