control extract distil usin glycerol
TRANSCRIPT
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Computers and Chemical Engineering 39 (2012) 129–142
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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is withdrawn and in the bottom stream a mixture of water/glycerol is sent to a recovery column.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
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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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132 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142
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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I.D. Gil et al. / Computers andChemical Engineering 39 (2012) 129–142 133
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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134 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142
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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136 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142
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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I.D. Gil et al. / Computers andChemical Engineering 39 (2012) 129–142 137
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
Fig. 13. Dynamic responses for feed flow disturbances in thecontrol strategy 1.
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138 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142
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
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I.D. Gil et al. / Computers andChemical Engineering 39 (2012) 129–142 139
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.
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140 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142
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%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
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%
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)
Azeotropic Feed = +20%
Azeotropic Feed = -20%
Fig. 16. Dynamic responses for feed flow disturbancesin thecontrol strategy 2.
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I.D. Gil et al. / Computers andChemical Engineering 39 (2012) 129–142 141
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
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142 I.D. Gil et al. / Computers and Chemical Engineering 39 (2012) 129–142
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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