China Petroleum Processing and Petrochemical Technology
Scientific Research
2012,Vol. 14, No. 4, pp 89-96
December 30, 2012
Salt-Containing Extractive Distillation of 1-Propanol/Water System
1. Prediction of Salt Effect on Vapor Liquid Equilibrium
Fu Jiquan1; Fu Die2
(1. Center of Chemical Engineering, Beijing Key Laboratory, Beijing Institute of Fashion Technology,
Beijing 100029; 2. Department of Civil Engineering, Polytechnic Institute of NYU, NY 11201, U.S.A)
Abstract: This paper has studied the vapor-liquid equilibrium (VLE) for the salt-containing extractive distillation of the
1-propanol/water system. Binary VLE data were measured for the systems of 1-propanol (1)/KAc (4), ethanediol (3)/KAc
(4), and 1-propanol (1)/ethanediol (3), with the VLE data correlated with the NRTL model in order to obtain the model
parameters of these binary systems. The binary VLE data cited in technical literature were correlated to obtain the model
parameters for other binary systems. VLE data of ternary and quaternary systems predicted by the NRTL model agreed well
with the literature data. The influence of KAc, ethanediol, and the KAc/ethanediol mixture on volatility between 1-propanol and
water was investigated respectively. Test results showed that the above-mentioned materials (KAc, ethanediol) and their mixture
(KAc and ethanediol) have different influence on the volatility. When x3=0.5, and x4=0.05, the azeotropic point can be eliminated. The NRTL model method of salt-containing VLE is simple and effective for the prediction of the system’s VLE data.
Key words: salt effect, vapor liquid equilibrium, 1-propanol, NRTL model
1　Introduction
1-Propanol is widely used as an important chemical solvent. It is difficult to separate 1-prorpanol from water in
one tower because a binary azeotrope can be formed in
the 1-propanol/water system. Traditional separation methods for the system include extractive distillation and azeotropic distillation. They consume a large amount of energy
because a large amount of extractive solvent or azeotropic
solvent is used in order to remove the azeotropic point by
increasing the volatility between 1-propanol and water.
Study of new separation process is important for the purpose of energy economy. The salt-containing extractive
distillation is an effective method, which is initially introduced by Duan, et al.[1] and Lei, et al.[2], to separate the
ethanol/water system and the tert-butanol/water system.
And the industrial test has succeeded. It is important to
mention that the question of conveyance of solid salt can
be solved skillfully. Lei, et al.[3] investigated the process
for separation of 2-propanol from water by using a mixture of ethanediol+KAc(potassium acetate)+KOH as the
extractive agent. The laboratory experiments have shown
that the technology is feasible. Lately, Wang, et al.[4] studied the salt-containing extractive distillation for the system
of 1-propanol/water using both the experimental method
and the simulation method. However, there is not any publication with detailed studies on salt-containing VLE calculation method and simulation of salt-containing extractive
distillation of the 1-propanol/water system using mixed
extractive agents. Furthermore, some VLE studies have already been done on the 1-propanol/water/salt system.
In this work, a simple and effective VLE calculation
method, the NRTL model method[5], is used for the system of 1-propanol (1)/water (2)/ethanediol (3)/KAc (4).
(In this study KAc is assumed as the “solvent”.)
Hence six pairs of parameters of the NRTL model are needed.
To determine the parameters, we can either obtain the parameters from literature VLE data, or to measure the required
binary VLE data, which cannot be found in the literature.
The purposes of this work are: firstly, to provide a simple
and effective method for prediction of salt effects on VLE
for the system of 1-propanol (1)/water (2)/ethanediol (3)/
KAc (4); secondly, by investigating the influence of the
relative volatility between 1-propanol and water on KAc,
ethylene glycol and the two mixed solvents, to find the
Corresponding Author: Professor Fu Jiquan, Telephone: +8610-64288291; E-mail: [email protected].
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China Petroleum Processing and Petrochemical Technology
conditions for eliminating the azeotropic point. Therefore,
implementation of these tasks can confirm the thermodynamic feasibility of salt-containing extractive distillation process.
2　VLE Data Measurement
2.1　Selection of extractive agent system
Many kinds of salts, such as CaCl2, K2CO3 and KAc, have
the salt effect on the system of 1-propanol/water as referred
to in the literature[6-8]. KAc and ethanediol have been chosen as the mixed agents based on the following reasons:
KAc has the ability to increase the volatility between 1-propanol and water effectively; the mixture of ethanediol and
KAc has been successfully utilized in industrial devices for
the systems of ethanol/water and tert-butanol/water[1-2]. It is
important to investigate the validity of the ethanediol+KAc
mixture used in the system of 1-propanol/water.
2.2　Previous salt-containing VLE studies
In comparison with the study on isopropyl alcohol, the
study on the system of 1-propanol and water with salt is
less reported. According to the literature information[6],
Wu reported the results of the study on the ternary system
of 1-propanol/water/KAc, and measured the VLE data
when the salt molar fraction was 0.02, 0.04, 0.08, and
0.12, respectively. Quaternary experimental data for the
system of 1-propanol (1)/water (2)/ethanediol (3)/KAc
(4) at different concentrations of ethanediol and KAc
were reported by Li[7]. Zhang[8] reported the results of the
study on the ternary system of 1-propanol/water/CaCl2.
The VLE data were measured at a molar fraction of salt
equating to 0.018, 0.04, 0.065, and 0.846, respectively.
They used Furter’s model to correlate the data, and received a satisfactory results. Xu[9] reported the results of
study on the ternary system of 1-propanol/water/KF and
the ternary system of 1-propanol/water/K2CO3. Phasesplitting was identified by adding salt, and the LLE data
were measured at 25 ℃. The Piter and NRTL model were
used to correlate with the data obtained thereby. Fu [5]
reported the pertinent binary VLE data for the system of
tert-butanol/water/ethanediol/KAc, and since the VLE
data for the binary system of 1-propanol/KAc and 1-propanol/ethanediol have not been found in literature, the
related data were measured in this work.
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2012,14(4):89-96
2.3　Selection of calculation model
The model of vapor-liquid equilibrium is required in order
to calculate the salt-containing VLE data and to simulate
the salt-containing extractive distillation process. Prediction and correlation models of the salt effect on VLE are
extensively reported. Sander, et al.[10] presented a comprehensive review of the earlier models. Modernistic models
usually combine the long-range term with local composition model. The local composition model has been used to
predict the VLE data with salt successfully[11]. Fu[11] correlated eighteen binary systems (using the Wilson, NRTL
and UNIQUAC models) and predicted the results of study
on five ternary systems. The study coincided well with the
results predicted by the pseudo binary approach.
The advantage of the local composition model is that the
VLE calculation of salt-containing systems can be dealt
with by a conventional method used in miscible solvent
system which is a well-known and commonly used calculation method in chemical engineering. Only individual
model (i. e.: the Wilson, or NRTL, or UNIQAIC model)
is used in the calculation, in which the salt is assumed as
a “solvent”. The NRTL model was chosen in this work
and was named the NRTL model method. For the quaternary system of 1-propanol (1)/water (2)/ethandiol (3)/
KAc (4), six pairs of binary parameters of NRTL model
were required, namely: (1)1-propanol/water; (2) water/
ethanediol; (3)1-propanol/ethanediol; (4)1-propanol/KAc;
(5)water/KAc; and (6) ethanediol/KAc.
3　Experimental
Although a lot of VLE data for non-electrolyte solution or
electrolyte solution have been measured, the VLE data of
1-propanol/ethanediol system are not found in literature,
and the VLE data of the 1-propanol/KAc system and the
ethanediol/KAc system are needed to obtain parameters
of the model. Therefore the VLE data of three systems
were measured in this work.
3.1　Chemicals
The reagents used in experiments included 1-propanol (>99.7
m% pure), ethanediol (>99.8 m% pure), and KAc (>99.0 m%
pure). They were provided by the Beijing Chemical Reagent
Corporation. Distillated water was used in the experiments.
KAc reagent was dried at 120 ℃ for 6 hr.
Diao Rui, et al. Salt-Containing Extractive Distillation of 1-Propanol/Water System 1. Prediction of Salt Effect on Vapor Liquid Equilibrium
3.2　Experimental equipment
Table 2　Experimental data and correlation results for the
The salt-containing binary VLE (salt-containing saturated vapor pressure) for the system of 1-propanol/KAc
and the system of ethanediol/KAc were measured using
an ebulliometer[12-13]. Temperature was measured with a
mercury-in-glass thermometer (with a 1/10 ℃ graduation),
the exposed part was revised for compensating the error.
A device consisting of a 220C type pressure gauge and a
PDRC-1C/2C type display supplied by the MKS Corporation (U.S.A.) with an accuracy of 0.013 kPa, was used to
measure the total equilibrium pressure. The composition
of liquid phase was determined by the weight method.
The residual volume of the vapor phase was corrected.
The two groups of binary data with different pressures
and salt concentrations were determined with the results
shown in Table 1 and Table 2.
binary system of 1-ethanediol(3)/KAc(4)
No.
Pe, mmHg
ΔP, mmHg
t, ℃
Δt, ℃
x3e
Δx3
1
102.199
0.085
197.36
-0.15
1.0
-0.0038
2
103.505
0.051
198.36
-0.09
0.9858
-0.0036
3
93.376
-0.006
195.61
0.09
0.9858
0.0040
4
86.872
-0.069
193.34
0.10
0.9858
0.0045
5
80.075
-0.056
190.65
0.07
0.9858
0.0030
6
73.144
-0.032
187.76
0.05
0.9858
0.0022
7
103.492
0.019
199.31
-0.07
0.9679
-0.0038
8
93.403
-0.021
196.47
0.04
0.9679
0.0021
9
86.659
-0.107
194.76
0.17
0.9679
0.0102
10
79.995
-0.016
191.49
0.03
0.9679
0.0015
Table 1　Experimental data and correlation results for the
11
73.304
-0.008
188.60
-0.01
0.9679
-0.0004
binary system of 1-propanol(1)/KAc(4)
12
102.026
0.004
200.38
0.02
0.9509
0.0010
13
93.336
-0.044
197.87
0.08
0.9509
0.0051
14
86.885
-0.015
195.30
0.03
0.9509
0.0021
15
80.448
-0.023
192.92
0.04
0.9509
0.0028
16
73.571
0.084
189.31
-0.10
0.9509
-0.0061
17
101.879
0.031
201.20
-0.03
0.9357
-0.0018
18
94.202
0.016
198.75
-0.01
0.9357
-0.0009
19
86.645
0.059
195.63
-0.08
0.9357
-0.0055
20
79.968
0.004
193.56
-0.01
0.9357
-0.0005
21
73.437
0.044
190.46
-0.08
0.9357
-0.0050
No. Pe, mmHg ΔP, mmHg
1
100.120
-0.005
2
95.962
0.000
3
91.963
0.000
4
87.965
-0.001
5
83.966
-0.001
6
79.968
-0.001
7
75.970
-0.003
8
71.971
-0.001
9
100.386
0.021
10
94.882
0.024
11
91.990
-0.027
12
87.712
-0.061
13
83.766
0.009
14
79.328
-0.033
15
73.611
-0.107
16
70.918
-0.033
17
68.972
0.321
18
101.266
0.081
19
96.122
-0.035
20
92.043
-0.001
21
87.832
-0.015
22
83.873
-0.005
23
79.781
-0.025
24
74.610
-0.056
25
70.545
-0.060
26
101.319
0.024
27
97.548
0.017
28
92.043
0.025
29
88.031
0.009
30
83.833
0.027
31
79.861
0.043
32
75.743
0.040
33
71.638
-0.120
Mean
0.069
t, ℃
96.92
95.78
94.66
93.51
92.31
91.06
89.76
88.40
97.35
95.82
95.10
93.94
92.65
91.28
89.55
88.49
87.08
97.74
96.61
95.46
94.27
93.02
91.75
90.10
88.76
98.45
97.51
95.98
94.79
93.50
92.25
90.91
90.02
Δt, ℃
0.01
0.0
0.0
0.0
0.0
0.0
0.0
0.0
-0.01
-0.04
0.03
0.10
0.03
0.04
0.18
0.09
-0.47
-0.14
0.04
0.04
0.06
0.02
0.03
0.07
0.12
-0.07
-0.03
-0.03
-0.05
-0.06
-0.06
-0.05
0.17
0.11
x3e
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.9700
0.9700
0.9700
0.9700
0.9700
0.9700
0.9700
0.9700
0.9700
0.9420
0.9420
0.9420
0.9420
0.9420
0.9420
0.9420
0.9420
0.9160
0.9160
0.9160
0.9160
0.9160
0.9160
0.9160
0.9160
Notes: Δg14=2512.82 cal/mol, Δg41=-1165.93 cal/mol, α=0.30.
Δx3
0.0003
0.0
0.0
0.0001
0.0001
0.0001
0.0001
0.0001
-0.0001
-0.0003
0.0002
0.0007
0.0002
0.0003
0.0013
0.0006
-0.0031
-0.0021
0.0006
0.0007
0.0011
0.0004
0.0005
0.0011
0.0020
-0.0019
-0.0009
-0.0009
-0.0013
-0.0016
-0.0016
-0.0015
0.0054
0.0014
Mean
0.049
0.08
0.0040
Notes: Δg34=469.70 cal/mol, Δg43=-641.02 cal/mol, α=1.0.
For the system of 1-propanol/ethanediol, it is difficult
to determine the VLE data due to the large difference
between their boiling points (Dt=100 ℃). The improved
Dvorak-Boublik still was used to measure the data because of its good reproducibility. Both liquid and vapor
phase compositions were analyzed by gas chromatography. The above-mentioned method was used to measure
the temperature and pressure. The experimental data are
shown in Table 3. The thermodynamic test had been performed by the point test method, and the average value of
DP and Dy was 2.3211 kPa and 0.0096, respectively.
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China Petroleum Processing and Petrochemical Technology
Table 3　Experimental data and correlation results for the
binary system of 1-propanol(1)/ethanediol (3)
No.
t, ℃
Δt, ℃
x1e
Δx1
y1e
Δy1
1
196.93
0.14
0
0
0
0
2
162.78
0.08
0.0512
0
0.6964
0.0031
3
154.09
0.10
0.0734
0
0.7874
0.0048
4
143.75
0.08
0.1102
0
0.8610
0.0010
5
136.74
0.06
0.1455
0
0.8989
-0.0004
6
128.49
0.04
0.2054
0
0.9326
-0.0002
7
124.89
0.03
0.2411
0
0.9444
0.0003
8
120.66
0.01
0.2943
0
0.9562
0.0005
9
118.65
-0.01
0.3250
0
0.9612
-0.0005
10
115.62
-0.03
0.3800
0
0.9681
-0.0005
11
113.31
-0.06
0.4300
0.0001
0.9729
0.0004
12
111.24
-0.09
0.4820
0.0001
0.9769
0.0004
13
109.49
-0.12
0.5315
0.0001
0.9800
0.0004
14
107.50
-0.15
0.5945
0.0001
0.9835
0.0003
15
106.50
-0.17
0.6282
0
0.9851
0.0003
16
104.81
-0.20
0.6891
0
0.9878
0.0003
17
103.18
-0.24
0.7503
0
0.9904
-0.0001
18
102.05
-0.25
0.7945
0
0.9921
-0.0001
19
100.73
-0.27
0.8467
0
0.9942
0
20
99.41
-0.30
0.8991
0
0.9962
0.0001
21
98.78
-0.34
0.9244
0
0.9971
0
22
97.20
-0.30
0.9865
0
0.9995
0
23
96.86
-0.31
1.0
0
1.0
0
Mean
0.14
0
0.0006
Notes: Δg13=121.85 cal/mol, Δg31=157.92 cal/mol, α=0.30.
4　Calculation of Salt-Containing VLE
4.1　Relationship of VLE
The correlation of the binary data and estimation of parameters using the NRTL model are shown below.
The vapor phase was assumed as an ideal gas. The vaporliquid equilibrium relation is:
yi P = Pi 0γ i xi (1)
For the data of a salt-containing binary system, ysolvent =1,
ysalt =0, therefore,
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2012,14(4):89-96
S
0
Pi = Pi γ i xi (2)
and γi was calculated by the NRTL model[14]. The form of
the NRTL model is expressed by equation (3).
c
c


χ lτ lj Glj 
∑

j =1
j ij

τ ij − l =1
(3)
Gkj χ k 
j =1
∑
ki k
kj k 




The salt concentration is defined in terms of its molar
∑τ G χ
χG
ln γ i =
+∑
G χ
∑G χ
ji
ji
j
c
fraction. It can be expressed using the mole number of the
salt and the solvent:
n
xsolvent , salt = i (4)
∑ ni
where ni represents the mole number of solvent and salt.
The definition for use of salt and solvent concentration
was adequate for calculating the enthalpy of salt in the
simulation calculation of the columns.
4.2　Estimation of the model parameters
The binary data were correlated by the maximum likelihood method[15]. The correlated binary parameters can
provide an optimized overall representation of both the
experimental results and the optimal prediction results.
The object function is
2
2
2
2
m  Pc − Pe
( j j ) + (T jc − T je ) + ( x1cj − x1ej ) + ( y1cj − y1ej ) 
F = ∑

σ P2j
σ T2j
σ x21 j
σ y21 j
j 


(5)
For the salt-containing binary system, y1=1, and y2=0, in
which the salt was regarded as a nonvolatile component.
And the last item on the right-hand side of Eq. (5) is zero.
σ p2 j , σ T2j , σ x21 j and σ y21 j are the error variances of P, T,
x1 and y1, respectively. In this work, the standard devia-
tion values are shown as the following numbers: σP=1.0
mmHg, σT=0.05 K, σx1=0.001 and σv1=0.003.
The parameters correlated using the NRTL model for the
systems of 1-propanol/KAc, ethanediol/KAc, and 1-propanol/ethanediol are shown in Table 1, Table 2, and Table
3, respectively.
Actually, other methods such as the least square method
or the optimization method can also be used to estimate
the binary model parameters. Estimation of binary parameters between one solvent and the other one are known to
be very well. This kind of software can be used directly
for estimating model parameters between the solvent and
the salt. No modification is required in the course of pa-
Diao Rui, et al. Salt-Containing Extractive Distillation of 1-Propanol/Water System 1. Prediction of Salt Effect on Vapor Liquid Equilibrium
rameter estimation when the software is used.
In this work, the NRTL model was used to calculate the
salt effect on VLE using the specific model parameters
since different salts have differing salt effect on mixed
electrolyte solution.
For quaternary system consisting of 1-propanol (1)/water (2)/ethanediol (3)/KAc (4), the source of six binary
parameters of the NRTL model can be generalized as
follows: 1-propanol/water and water/ethanediol systems
are emanated from Gmehling’s data collection[16]; 1-propanol/ethanediol, 1-propanol/KAc, and ethanediol/KAc
systems are emanated from binary system data obtained
in this study; and the water/KAc system is emanated from
the literature[17].
5　Prediction of Multi-component System VLE
5.1　Prediction of ternary system VLE
5.1.1　Comparison between predicted value and
experimental data
The ternary experimental data for the system of 1-propanol (1)/water (2)/KAc (4) at different salt concentrations
were reported by Wu[6]. It is supposed that the NRTL
model’s binary parameters described in the literature and
data obtained in this study will be used to predict the ternary experimental data of other researchers. According
to this supposition, we can investigate the adaptability of
the NRTL model method based on deviations between the
predicted values and the experimental data. The curves
of calculated and experimental data at x4= 0.04 and 0.08,
respectively, are shown in Figure 1. It is obvious that the
trend of the predicted values agreed well with Wu’s experimental data at the whole x1 range. But deviations are
bigger between x1=0.1—0.3 when the KAc molar fraction
is 0.08 (x4=0.08). As a whole, the predicted results of ternary system are satisfactory. It shows that the NRTL model parameters estimated by binary data are satisfactorily
suited to the ternary system.
It is very important to verify whether the parameters
obtained from the binary system’s VLE data are appropriate for the prediction of quaternary system. The best
evaluation method is one the experimental data of which
can be used to check up the predicted values. It is possible
Figure 1　Predicted values using binary parameters vs.
experimental data for the system of 1-propanol (1)/
water(2)/KAc(4)
◆—CAL.:X4=0.04; ■—CAL.:X4=0.08;
▲—LIT.:X4=0.04; ●—LIT.:X4=0.08
to compare each value predicted by the NRTL model with
the experimental data. Fortunately, the quaternary experimental data for the system of 1-propanol (1)/water (2)/
ethanediol (3)/KAc (4) at different concentrations of ethanediol and KAc were reported by Li[7]. The data related
with the highest concentration of KAc were predicted
and compared, with the results shown in Figure 2. It is
verified that the predicted values tallied satisfactorily with
the experimental data at the whole concentration range. It
shows that the NRTL model parameters estimated by the
binary system data are suitable for the prediction of the
quaternary system data.
Figure 2　X-Y diagram for the system 1-propanol (1)/water
(2) with the mixed agent (agent free)
◆—LIT.; ■—CAL.
5.1.2　Influence of different KAc concentrations on
the system
Calculation of bubble point temperature is very useful for
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China Petroleum Processing and Petrochemical Technology
analyzing the influence of the salt effect on VLE. In order
to investigate the influence of different KAc concentrations, ternary VLE data (when x3=0) for the system of
1-propanol(1)/water(2)/KAc (4) were predicted at three
constant x4 values (x4=0.03, 0.05 and 0.12, respectively).
The X-Y equilibrium relationship (KAc free) is illustrated
in Figure 3. It is shown that the relative volatility between
1-propanol and water was increased with increase in the
KAc concentration, and that the azeotropic composition
was a function of salt concentration. It was observed that
the azeotropic point could not be eliminated by adding
KAc at x 4≤0.05. However, the azeotropic point could
be eliminated at x4=0.12. It can be seen in Figure 3 that
the predicted curves with different contents of salt were
hoisted proportionally, the azeotropic point moved with
an increasing KAc concentration, and the volatility between 1-propanol and water could be increased in the
whole range of liquid phase composition. This phenomenon indicates that adding KAc indeed could increase the
volatility between 1-propanol and water.
2012,14(4):89-96
ethanediol molar fraction reached 0.8 (x3=0.8). Hereby
the solvent/raw material ratio in extractive distillation has
reached 4 [x3/(x1+x2)]. It shows that the individual ethanediol solvent has a certain separation effect for the system
of 1-propanol/water. It is a classical VLE calculation of
ternary soluble system, and the NRTL model can lead to
a good predicted results. It can be seen from Figure 4 that
we can find other features: when x1<0.20, the expected
“solvent” effect of ethandiol was not obtained, and when
x1>0.20, the expected “solvent” effect of ethanediol became quite obvious.
Figure 4　X-Y diagram for the system of 1-propanol (1)/
water (2) at different concentrations of ethanediol (3)
(solvent free)
◆—X3=0, X4=0.0;
—X3=0.3, X4=0;
▲—X3=0.5,X4=0.0; ■—X3=0.8, X4=0.0
5.2　Prediction of quaternary system VLE
Figure 3　X-Y diagram for the system 1-propanol (1)/water
(2) at different concentration of KAc (4) (salt free)
◆—X3=0, X4=0.0;
—X3=0, X4=0.03;
▲—X3=0, X4=0.05; ■—X3=0, X4=0.12
5.1.3　Influence of different ethanediol concentrations on the system
In order to study the influence of different ethanediol
concentrations, similar research has been carried out for
the system of 1-propanol(1)/water (2)/ethanediol (3), at
x3=0.3, 0.5 and 0.8, respectively. The X-Y equilibrium relationships are illustrated in Figure 4. It shows that adding
only ethanediol could not eliminate the azeotropic point
at an ethanediol molar fraction of less than 0.5 (x3≤0.5),
while the azeotropic point could be eliminated when the
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The VLE behavior is investigated for the quaternary
system of 1-propanol (1)/water (2)/ethanediol (3)/KAc
(4) only by suing VLE calculation. The bubble point at
101.33 kPa is identified by the NRTL model for the quaternary system, and then the X-Y equilibrium relationship
for the system of 1-propanol (1)/water (2) (ethanediol
+KAc free) can be obtained after transforming the x-y
data. The influence of the agent and the mixed agent
(ethanediol+KAc) on volatility between 1-propanol and
water was investigated. Figure 5 shows the X-Y relationship of predicted VLE for the system of 1-propanol (1)/
water (2)/ethanediol (3)/KAc (4) at three cases including:
x 3=0.50 and x 4=0; x 3=0 and x 4=0.05; and x 3=0.50 and
x4=0.05, respectively.
At x3=0.0 and x4=0.05 and at x3=0.50 and x4=0.0, we can
verify that the X-Y relationship between 1-propanol and
Diao Rui, et al. Salt-Containing Extractive Distillation of 1-Propanol/Water System 1. Prediction of Salt Effect on Vapor Liquid Equilibrium
water is influenced by the salt or solvent individually.
It can be clearly seen from the curves (for the case of
x3=0.50 and x4=0.05) in Figure 5 that the relative volatility between 1-propanol and water using the mixed agent
is significantly higher than that using respectively pure
ethanediol or pure KAc, and the azeotropic point can be
eliminated under this condition. In addition, we must
specify these phenomena: in the range of x1 being less
than 0.20 (see Figure 5), the volatility between 1-propanol
and water cannot be increased even a mixed agent was
used, because the ethanediol effect is negative within this
x range despite the positive effect of KAc. This phenomenon has been verified by the results referred to in section
5.1.2.
should be equal to 0.05 or 0.12 at a fixed value of x3=
0.50 for the quaternary system of 1-propanol (1)/water
(2)/ethanediol (3)/KAc (4). The increase in salt concentration can evidently increase the separation efficiency
between 1-propanol and water.
Figure 6　X-Y diagram for the system 1-propanol (1)/water
(2) at different mixed agents (agent free)
◆—X3=0.0, X4=0; ■—X3=0.5, X4=0.05; ▲—X3=0.5, X4=0.12
Based on the above facts, it is implied that in comparison
with common extractive distillation process, fewer theoretic stage numbers are required for the salt-containing
extractive distillation process when the separation requirements are the same.
Figure 5　X-Y diagram for the system 1-propanol (1)/water
(2) at different mixed agents (agent free)
◆—X3=0, X4=0.0;
—X3=0, X4=0.05;
▲—X3=0.5, X4=0.0; ■—X3=0.5, X4=0.05
Considering the consumption of extractive distillation energy, a smaller solvent/raw material ratio should
be chosen. For the system of 1-propanol (1)/water (2)/
ethylene glycol (3), when x3= 0.5 (i. e.: the solvent/raw
material ratio is 1:1), the azeotropic point of 1-propanol/
water system has not been eliminated. When x3= 0.8 (i. e.:
the solvent/raw material ratio is 4:1), the azeotropic point
of 1-propanol/water system has been eliminated. Upon
taking into consideration the energy conservation issue,
the energy consumption of the second scenario would
be four times more than the first one. For conducting a
benign extractive distillation process, we should consider
to combine ethanediol with KAC at x 3=0.5. Figure 6
shows two kinds of combination situations with good industrial application prospects, which would favor x4 that
6　Conclusions
VLE values predicted by the NRTL model agreed well
with the literature data for the ternary and quaternary systems. The results on comparison of the predicted values
and the experimental data indicated that the NRTL model
method is suitable for the VLE calculation of the system
of 1-propanol(1)/water(2)/ethanediol (3)/KAc (4). When
ethanediol is used as the solvent at x3=0.8 (in which the
solvent/raw material ratio is 4:1), the azeotropic point of
1-propanol/water can be eliminated. When KAc is used
as the solvent, the azeotropic point can be eliminated at
x4=0.12. When ethanediol and KAc are used as a solvent
mixture at x3=0.5 and x4=0.05, the azeotropic point can be
eliminated. It is effective to separate the 1-propanol and
water using ethanediol and KAc as the mixed agent.
Notation
Δg—NRTL model parameter
m—number of experimental points
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China Petroleum Processing and Petrochemical Technology
P—pressure
T, t —temperature
x—liquid phase molar fraction
y—vapor phase molar fraction
X—liquid phase molar fraction at KAc, or ethanediol, and
or KAc+ethanediol free
Y—vapor phase molar fraction at KAc, or ethanediol, and
or KAc+ethanediol free
α—non-randomness parameter
Δ—experimental value subtracts calculating value
Superscripts and subscripts:
c—calculated value
e—experimental value
1, 2, 3, 4—1-propanol, water, ethanediol, and KAc
2012,14(4):89-96
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