Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 67–73
Contents lists available at ScienceDirect
Process Safety and Environmental Protection
journal homepage: www.elsevier.com/locate/psep
Anhydrous ethanol production by extractive distillation:
A solvent case study
M.A.S.S. Ravagnani a,∗ , M.H.M. Reis a , R. Maciel Filho b , M.R. Wolf-Maciel b
a
b
State University of Maringá, Technology Center, Chemical Engineering Department, Maringá, PR, Brazil
State University of Campinas, Chemical Engineering School, Department of Chemical Process, Campinas, SP, Brazil
a b s t r a c t
Production of anhydrous ethanol in large scale has been made by extractive distillation using conventional solvents
like ethylene-glycol. In the present paper, extractive distillation process is studied to obtain pure ethanol using
ethylene-glycol and tetraethylene-glycol as solvents. Residue curve maps are used to analyse the proposed distillation processes in interpreting mixture behaviours and feasibility of distillation columns. The industrial process is
simulated at steady state from residue curve map analysis. Simulation results for the ethanol/water mixture using
ethylene-glycol, the conventional solvent, and tetraethylene-glycol, an alternative solvent, are presented. These
results showed that the process using tetraethylene-glycol is reliable, although it requires more energy than the
process with ethylene-glycol. However, ethylene-glycol has a considerable toxicity level while tetraethylene-glycol
is non-toxic.
© 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Keywords: Extractive distillation; Anhydrous ethanol; Tetraethylene-glycol; Residue curve maps; Clean technology;
Process simulation
1.
Introduction
Distillation is one of the most important separation processes,
mainly because it allows separating ideal and nonideal mixtures in large scale units. The separation of homogeneous
and heterogeneous azeotropic mixtures is of great industrial
interest, and a large number of such distillation columns are
in operation. Furthermore, the necessity in developing new
feasible sequences of distillation columns has increased, in
order to attend demands of clean and economical processes,
as pointed in Pinto et al. (2000).
The first important task before the synthesis of a separation process is to understand the mixture behaviour in terms
of its appropriate trajectory (i.e., vapour–liquid, liquid–liquid
or vapour–liquid–liquid equilibrium). Residue curve maps are
important graphical tools, related to vapour–liquid separations, according to Fien and Liu (1994), Widagdo and Seider
(1996), and Kiva et al. (2003). These authors presented very useful reviews about concepts and applications of residue curves
maps.
∗
According to Doherty and Malone (2001), for extractive
distillation processes, the analysis of the residue curve map
enables to decide if a solvent is thermodynamically appropriated to promote the mixture separation. In the residue curve
map, the pure solvent composition will be a stable node and
the minimum-boiling azeotropic point will be an unstable
node. Thus, the diagram will not have distillation boundaries,
and it will be possible to obtain all the components as pure
products, as shown in Doherty and Caldarola (1985).
Nowadays, some researchs are focus on systemize residue
curve analysis together with some additional tools, as unidistribution and univolatility curves, to find suitable processes
and eventually suitable entrainers for the separation of
zeotropic or azeotropic binary mixtures. Gerbaud et al. (2006)
implemented a procedure based on residue curve map analysis in a wizard computer tool to search for a suitable process
enabling the separation of binary mixtures by batch distillation. Brüggemann and Marquardt (2004) presented a shortcut
design method for the simultaneous determination of minimum entrainer feed flow rate and minimum reflux ratio based
Corresponding author. Tel.: +55 44 32614321; fax: +55 44 32614321.
E-mail addresses: ravag@deq.uem.br, mauro.ravagnani@hotmail.com (M.A.S.S. Ravagnani).
Received 18 December 2008; Received in revised form 24 November 2009; Accepted 26 November 2009
0957-5820/$ – see front matter © 2009 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
doi:10.1016/j.psep.2009.11.005
68
Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 67–73
Nomenclature
Ai , Bi , Ci , Di , Ei , Fi antoine equation parameters for the
component i
aij
NRTL non-temperature-dependent energy
parameter between components i and j
(cal/mol)
ALP
parameter of the NRTL equation
ALT
temperature-dependent parameter of the NRTL
equation (◦ C−1 )
NRTL temperature-dependent energy paramebij
ter between components i and j (cal/mol K)
c
component number
F
feed flow rate (mol/h)
Ki
thermodynamic constant of the component i
i
component
P
pressure (kPa)
Psat
saturation pressure of the component i (kPa)
i
R
ideal gas constant (cal/mol K)
S
solvent flow rate (mol/h)
T
absolute temperature (K)
t
temperature (◦ C)
molar liquid fraction of the component i
xi
yi
molar vapour fraction of the component i
Greek symbols
˛ij
NRTL non-randomness constant for binary
interaction (˛ij = ˛ji )
activity coefficient of the component i
i
warped time
˚i
fugacity coefficient of the component i
on the called nonlinear analysis. This nonlinear analysis is
a coupled of thermodynamic equilibrium equations with the
material and heat balances around each column tray. Frits
et al. (2006) applied an optimisation procedure to find all the
singular points of the profile maps and, then, to analyse the
feasibility of batch extractive distillation. Recently, RodriguezDonis et al. (2009a) showed that a priori knowledge of the
residue curve shape and the location of the univolatility curve
intersection with a diagram edge allows one to predict the
distillate product obtained by extractive distillation as a first
cut.
Extractive distillation processes are widely used in chemical industries for separating nonideal mixtures. In Langston
et al. (2005) a solvent, the heaviest component, is added causing an increase in the relative volatility of the key components
and new azeotropes are not formed. Moreover, the solvent is
completely miscible in the mixture, as can be seen in Seader
and Henley (1998).
A typical extractive distillation is the dehydration of
ethanol using ethylene-glycol as solvent. The separation of
the ethanol/water mixture is of great industrial interest, due
to the potential of ethanol as a renewable source of energy,
being used as addictive or complete substituting the gasoline,
besides to be the raw material for alcohol chemistry. Ethanol
is a relative clean-burning fuel. So, for this reason, the use
of ethanol can also reduce the pollution emitted to the air
(Vorayos et al., 2006).
Lee and Pahl (1985) reported that the extractive distillation
of the ethanol and water mixture consumes 50–80% of the
energy used in a typical fermentation ethanol manufacturing
process, showing the high necessity to improve this process
in order to minimize this energy consumption.
Nowadays, there is also an increasing preoccupation with
the environmental issues. Jin et al. (2004) reported the green
chemistry, the green engineering, and the industrial ecology
principles as methodologies specifically developed for ecologically considerate chemical engineering. The authors pointed
out the necessity in innovation to increase atomic utilization of reactants, efficiency in energy use, dematerialization,
non-toxicity, recyclability and creative systemic cycling of
materials for waste management.
There are new alternative processes to separate ethanol
from water. Solar distillation (Vorayos et al., 2006) can be used
to concentrate ethanol until 80 (%, v/v). Fatty acids are used as
liquid solvent. They are obtained from vegetable oils and animal (natural organic solvents) (Boudreau and Hill, 2006). Perez
et al. (2006) carried out a parametric study to demonstrate
the effects associated with changes in variables such as feed
flow rate, feed pressure, module feed side, ideal separation
factor and tube diameter. Some recent studies proposed the
utilization of saline extractive distillation process for ethanol
purification as pointed out in Pinto et al. (2000) and Ligero and
Ravagnani (2003).
Luyben (2009) presented a study considering the dynamic
control of a pervaporation system to produce anhydrous
ethanol. Guerreri (1992) stand out the pervaporation performance related to the distillation process to produce pure
ethanol.
Considering schemes of thermally integrated extractive
distillation process, Kim (2006) presented a new system of a
fully thermally coupled distillation column for the improvement of distillation column efficiency. The performance of the
system that has an extra column called postfractionator and
attached to the main column was examined with two industrial processes. Segovia-Hernández et al. (2006) presented
a comparative study of the energy-efficiency performance
between conventional distillation sequences and thermally
coupled distillation arrangements (TCDS).
However, most of the ethanol process industries are still
using the extractive distillation to obtain anhydrous ethanol.
Ethanol forms a minimum-boiling azeotrope with water
at about 90 mol% at 1 atm. This azeotrope must be broken
to achieve anhydrous ethanol. The usual solvent applied in
the industries to promote ethanol and water separation is
the ethylene-glycol. However, this compound has been related
with serious environmental problems. For instance, ethyleneglycol poisoning leads to fatal intoxication (Schladt et al.,
1998). In fact, ethylene-glycol has a low toxicity but it is in
vivo broken down by the liver enzyme alcohol dehydrogenase
to four organic acids: glycoaldehyde, glycolic acid, glyoxylic
acid and oxalic acid. According to Letha and Gregersen (2005),
the metabolites are cell toxins that can cause central nervous
system depression and cardio-pulmonary and renal failure.
Due to the ethylene-glycol toxicity, it could be forbidden in the next years, and, so, a new solvent must be
proposed. Tetraethylene-glycol can be a potential solvent
to substitute ethylene-glycol in the ethanol process industries. Tetraethylene-glycol does not form new azeotropes with
ethanol and/or with water, is completely miscible in the mixture, and is the heaviest component. Tetraethylene-glycol
with water/ethanol mixture forms a 1.0-1a class mixture (Kiva
et al., 2003) and thus meets the thermodynamic feasibility
criterion of extractive distillation (Knapp and Doherty, 1994;
Rodriguez-Donis et al., 2009a). It is necessary to know if
69
Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 67–73
tetraethylene-glycol is a suitable solvent to separate ethanol
and water.
In order to analyse both proposed processes with the
different solvents, the residue curve maps for the ethanol/
water/ethylene-glycol and the ethanol/water/tetraethyleneglycol systems were build up. A computational program,
developed in Fortran language, was used to calculate the
residue curves. After the system characterization, both
processes, with ethylene-glycol and tetraethylene-glycol as
solvents, were simulated by using the HYSYS simulator.
Langston et al. (2005) pointed out the high necessity to
have publications on computer simulations of extractive
distillation columns. Those authors used the commercial
simulator HYSYS to obtain results about the separation
of the systems acetone–methanol, methylacetate–methanol,
and methanol–chloroform.
2.
data
Residue curve maps and equilibrium
The concept of residue curve maps is related to a simple distillation process, where the liquid mixture is vaporized in a
still and the vapour formed at any instant is immediately
removed (Doherty and Perkins, 1978). This process is governed
by a set of nonlinear differential equations (Eq. (1)). Residue
curve maps can also be applied to continuous distillation. Van
Dongen and Doherty (1985) showed that the steady-state composition profile in a packed column at total reflux is identical
to a residue curve in a simple distillation process (where the
height of packing is equivalent to the warped time). Moreover, residue curves show the general behaviour of continuous
columns operating at finite reflux ratios (Doherty and Malone,
2001):
dxi
= (xi − yi )
d
i = 1, 2, . . . , c − 1
(1)
where xi and yi are the liquid and the vapour molar fractions
for component i, is the warped time, and c is the number of
components.
Before Eq. (1) integration, it is necessary to find a relationship between the liquid and the vapour phases. For an ideal
vapour ( = 1) in equilibrium with a nonideal liquid ( =
/ 1), the
equilibrium equations using the – approached can be used.
In this work, saturation pressures (Psat
) were calculated
i
applying the Antoine equation and the coefficients for this
equation are presented in Table 1, for each component (Reid
et al., 1988).
The liquid activity coefficients were calculated using NRTL
model (Renon and Praunitz, 1968) and are presented in
Table 2. Meirelles et al. (1992) obtained the NRTL parameters
for the ethanol/water/ethylene-glycol system by regression
of different experimental isobaric and isothermal data. The
interaction parameters for the tetraethylene-glycol/water and
Table 1 – Antoine vapour-pressure-equation
coefficientsa .
Components
Ethanol
Water
Ethylene-glycol
Tetraethylene-glycol
a
Ai
Bi
18.9119
18.3036
20.2501
16.0828
3803.98
3816.44
6022.18
2511.29
Ci
−41.68
−46.13
−28.25
−41.95
These parameters are used along with ln(Psat ) = A − B/(T + C), with
pressure in mmHg and temperature in K.
the tetraethylene-glycol/ethanol binary pairs were obtained
with regression of data generated using a Modified UNIFAC
model (Gmehling et al., 1993).
2.1.
Characterization and simulation results
Equilibrium data (Tables 1 and 2) were used for residue
curve calculations and in simulator fluid package for the
systems ethanol/water/ethylene-glycol and ethanol/water/
tetraethylene-glycol.
In order to analyse the process viability, extractive distillation processes with both proposed solvents were simulated
and parametrically optimised to minimize energy consumptions. Simulations were carried out in steady state using the
commercial simulator HYSYSTM (Hyprotech Ltd.). The flowsheet for the system ethanol/water/solvent is shown in Fig. 1.
The composition of the feed stream is 85% of ethanol and
15% of water (in molar basis), corresponding approximately to
the composition at the azeotropic point. Moreover, a pure solvent stream is fed some stages below the column top. Feed
and solvent stream positions are parametrically optimised.
The temperature of the feed stream corresponds to the saturated one. In this way, the vapour fraction in this stream is
equal to 0.
Fig. 1 – Flowsheet for the ethanol/water/ethylene-glycol
system.
Table 2 – NRTL parameters for the system ethanol (1)/water (2)/ethylene-glycol (3)/tetraethylene-glycol (4)a .
i
j
1
1
2
1
2
2
3
3
4
4
a
aij (cal/mol)
−105.5
3233.1
330.6
310.5
−90.05
aji (cal/mol)
787.1
−1040.1
−345.2
−442.1
3.856
bij (cal/mol K)
4.4
−22.2
1.9
0.0
0.0
bji (cal/mol K)
4.1
12.8
−2.2
0.0
0.0
ALP
0.171
0.370
0.186
0.300
−1.296
These parameters are used along with ˛ = ALP + ALT × t, Gij = exp(−˛ij ij ), and ij = (aij + bij T)/RT, for R = 1.987 cal/mol K.
ALT (◦ C−1 )
0.005228
−
–
0.0
0.0
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Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 67–73
Table 3 – Choice of the best feed stream position for the
extractive column for the ethanol/water/ethylene-glycol
system.
Feed stream position
20
25
28
30
32
35
37
Fig. 2 – Residue curve map for the system
ethanol/water/ethylene-glycol at 101.3 kPa.
Reboiler duty (107 kJ/h)
1.59
1.43
1.39
1.39
1.39
1.43
1.49
ratio solvent to feed flow rates (S/F), analysing the reboiler duty
requirement.
The practical number of stages refers to the number of
stages that the column almost did not show variations in the
reboiler duty, i.e., increasing the number of stages the decrease
in the reboiler duty is not considerable. The word “practical”
is used here to mean “feasible number of stages”, instead of
use the name “infinite number of stages”, through Underwood
shortcut method. The same is valid for “practical reflux ratio”
to differentiate it from the one calculated from the Underwood
method.
2.1.1.
Ethanol/water/ethylene-glycol
Fig. 3 – Reboiler duty versus number of stages of the
extractive column for the system
ethanol/water/ethyelene-glycol.
The calculated residue curve map (Fig. 2) shows that pure
ethylene-glycol is obtained as bottom product in a distillation process, from any initial liquid composition. Moreover,
there are no distillation boundaries in this diagram; so, it is
possible to obtain the three pure components in an extractive distillation process. According to this characterization
result, ethylene-glycol seems to be a reliable solvent to separate ethanol and water through out extractive distillation
process
At this point, it is quite important to mention that only thermodynamically it can be affirmed that the separation of the
ethanol and water mixture using ethylene-glycol as solvent
is possible. Moreover, Rodriguez-Donis et al. (2009b) showed
that the product sequence determined from the sole analysis of thermodynamic properties of residue curve maps and
even the occurrence of unidistribution lines and univolatility
A heat exchanger is used to cool the solvent stream that
comes from the conventional distillation column (solvent
recovery) and goes to the extractive distillation column. The
temperature that the solvent is fed in the extractive column
is an important parameter. If the temperature of the solvent
is close to its boiling point water can be vaporized to the top
of column and, so, the vaporized water can contaminate the
pure ethanol stream. Solvent stream temperature was ranged
from 25 ◦ C to solvent boiling point analysing the energy consumption and the convergence problem.
Pure ethanol is obtained as top product from the extractive column. The recovery column separates the water/solvent
mixture, in order to obtain almost pure water and pure solvent.
All the recovered solvent is returned to the extractive column.
The process was simulated in order to obtain almost pure
ethanol (99.0%, in molar basis). Initial specifications for the
simulations were total recovery from the fed ethanol and
reflux ratio was minimized looking for the ethanol purity.
The proposed processes were parametrically optimised in
terms of the number of stages, the positions of the feed and
the solvent streams, the solvent stream temperature, and the
Fig. 4 – Residue curve map for the system
ethanol/water/tetraehylene-glycol at 101.3 kPa.
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Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 67–73
Table 4 – Input and optimum conditions for the extractive distillation process with ethylene-glycol as solvent.
Material streams
Vapour fraction
Temperature (K)
Pressure (kPa)
Molar flow (mol/h)
Feed
Solvent
Pure ethanol
Bottom
0
351.29
101.3
100.0
0
298.15
101.3
300.0
0
351.31
101.3
85.00
0
428.15
101.3
315.0
Component mole fraction
Compositions
Ethanol
Water
Solvent
Feed
Solvent
Pure ethanol
Bottom
0.8500
0.1500
0.0000
0.0000
0.0000
1.0000
0.9908
0.0092
0.0000
0.0025
0.0451
0.9524
Energy streams
7
Heat flow (10 kJ/h)
Qc1
Qr1
94.96
1.393
Unit operations
Number of stages
Feed stream position
Solvent stream position
40
30
3
lines can be unambiguously. In terms of process viability and
optimisation, it is necessary that the operational variables be
well integrated in the process, such as: number of stages of
all distillation columns involved, feed and solvent locations,
recycled solvent temperature, ratio solvent to feed flow rates
(S/F), reflux ratios, reboiler duties and productivity.
Fig. 3 shows the reboiler duty versus the number of stages
of the extractive column using ethylene-glycol as solvent.
Analysing the presented results, it is observed that the number of practical stages is 40 (the stages are counted from the
top to the bottom, and the condenser and the reboiler are not
computed as stages).
The best ratio solvent to feed flow (S/F) is 3, since for smaller
values the simulation did not converge for the given specifications and for greater values the duty requirements increases.
Mainly for nonideal mixture separations, the position of
the feed stream influences on the optimised results. Depending on the feed and the solvent stream positions, the energy
consumption can be lesser or greater, or even it is not possible to get the simulation convergence. Table 3 shows the
values of the reboiler duty as function of the feed stream
position for the ethanol/water/ethylene-glycol system. It was
considered a column with 40 stages and the solvent stream
in the optimum position (stage number 3). The reboiler
Table 5 – Input and optimum conditions for the extractive distillation process with tetraethylene-glycol as solvent.
Material streams
Vapour fraction
Temperature (K)
Pressure (kPa)
Molar flow (mol/h)
Feed
Solvent
Pure ethanol
Bottom
0
351.29
101.3
100.0
0
298.15
101.3
200.0
0
351.31
101.3
85.00
0
470.45
101.3
215.00
Component mole fraction
Ethanol
Water
Solvent
Compositions
Feed
Solvent
Pure ethanol
Bottom
0.8500
0.1500
0.0000
0.0000
0.0000
1.0000
0.9914
0.0086
0.0000
0.0034
0.0664
0.9302
Energy streams
7
Heat flow (10 kJ/h)
Qc1
Qr1
34.38
1.798
Unit operations
Number of stages
Feed stream position
Solvent stream position
50
45
3
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Process Safety and Environmental Protection 8 8 ( 2 0 1 0 ) 67–73
duty is increased when the solvent stream position is above
stage 6. Feed stream position that gets the smallest energy
consumption is at stage number 30. For the recovery column, the best feed stream position is at the middle of the
column.
The obtained results showed that the best temperature
for the solvent stream is 25 ◦ C. Table 4 presents the input
and optimum results for anhydrous ethanol production with
ethylene-glycol as solvent.
2.1.2.
Ethanol/water/tetraethylene-glycol
Although simulation and practical results showed that
ethylene-glycol is a reliable solvent to promote ethanol/water
separation, some studies show that this solvent has a considerable toxicity level. In the present paper, tetraethylene-glycol
is presented as an alternative solvent to produce anhydrous
ethanol looking for new demands of clean and safe processes.
Fig. 4 shows that tetraethylene-glycol is also a reliable solvent to promote the separation of ethanol/water mixture by
extractive distillation process, since the residue curve map
does not present any distillation boundary.
However, all design and operation conditions must be
observed to guarantee the viability of the process and
its optimisation. This system was simulated using the
same input parameters and specifications used for the
ethanol/water/ethylene-glycol system.
The parametric optimisation was carried out and the
results obtained are summarised in Table 5.
Analysing these results, it was concluded that, using
tetraethylene-glycol as solvent to separate ethanol and
water, the number of practical stages is 50, corresponding to a reboiler duty equal to 1.8 × 107 kJ/h. It represents
a value 1.3 times greater than the energy spent in the
process with ethylene-glycol as solvent. Ethylene-glycol
has a enthalpy of vaporization equal to 61.9 ± 6.3 kJ/mol,
while the enthalpy of vaporization of tetraethylene-glycol is
99 ± 10 kJ/mol (Gallaugher and Hibbert, 1937). It explains the
higher heat duty required in the process with tetraethyleneglycol.
3.
Concluding remarks
Residue curve maps are important tools to understand
the behaviour of a mixture in its whole range of molar
composition. In the case of extractive distillation, one
of the main applications of these diagrams is in the
choice of appropriate solvents. The residue curve map of
the ethanol/water/tetraethylene-glycol system shows that
tetraethylene-glycol can be used to separate this azeotropic
mixture, after suitable choices of design and operating conditions of the process.
The system with tetraethylene-glycol as solvent needs
more energy than the system with ethylene-glycol. Moreover,
it is necessary more tetraethylene-glycol than ethylene-glycol.
Furthermore, when the solvent is the tetraethylene-glycol, the
extractive column must be larger.
However, an important consideration must be done in
this paper. Based on the environmental issues, the use of
tetraethylene-glycol as solvent is justified because it is nontoxic, while the ethylene-glycol has a considerable toxicity
level. Taking into account the considerable efforts that have
been done to substitute toxic solvents from industrial plants
the use of tetraethylene-glycol must be considered as a viable
alternative in substituting the ethylene-glycol in the anhydrous ethanol production in the ethanol distilleries.
As future works, the optimisation of energy consumption
in this process must be considered, as well as the application of other characterization tools, as the univolatility
curves.
Acknowledgements
The authors are grateful to CNPq (571683/19975 + 141893/2002-8), CAPES and Fapesp for the financial
support.
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