Advanced Materials Research
ISSN: 1662-8985, Vols. 560-561, pp 79-85
doi:10.4028/www.scientific.net/AMR.560-561.79
© 2012 Trans Tech Publications, Switzerland
Online: 2012-08-24
Isobaric Vapor-Liquid Equilibria for the System Containing Acetic acid at
101.3kPa
Hua Xina, Xufeng Wangb, Joshua Qingsong Lic
The State Key Lab of Heavy Oil Processing, College of Chemical Engineering, China University of
Petroleum(East China) Qingdao, China
a
[email protected], [email protected], [email protected]
Keywords: vapor-liquid equilibria; acetic acid;n-pentyl acetate
Abstract. Isobaric vapor-liquid equilibria (VLE) data for water + acetic acid, acetic acid + n-pentyl
acetate, water + acetic acid + n-pentyl acetate systems have been measured at 101.33 k Pa using a
recirculating still. The nonideality of the vapor phase caused by the association of the acetic acid has
been corrected by the Hayden-O’Connellmethod. The three experimental binary data have been
correlated by the NRTL and UNIQUAC models. The obtained NRTL model parameters from binary
data have been used to predict ternary VLE data. The ternary predicted values obtained in this way
agree well with the experimental values.
Introduction
The separation of organic acids from aqueous solutions is industrially important, and azeotropic
distillation is an attractive process for such separation. Several acetates, such as isopropyl acetate[1],
butyl acetate[2], isobutyl acetate[3], and so on have been used as entrainers for the separation of acetic
acid. It is known that vapor-liquid equilibria data is vital to the simulation and design of the azeotropic
distillation process. Unfortunately, little work has been done on the vapor-liquid equilibria of the
water + acetic acid + entrainer system.
Little experimental study of the vapor-liquid phase equilibria of acetic acid + n-pentyl acetate
system at 101.33 kPa has been found. The aim of this article is mainly to investigate the vapor-liquid
phase equilibria of acetic acid + n-pentyl acetate, water + acetic acid + n-pentyl acetate systems at
101.33 kPa and supply basic data for the simulation and design of the azeotropic distillation process.
The water + acetic acid + ester systems are reactive systems because of the hydrolysis of the ester.
However, acetic acid can hold down the hydrolytic reaction, and the reaction rate is very slow.
Therefore, the hydrolysis of the ester is neglected.
It is known that the acetic acid molecules associate with each other to form stable dimers in both
the liquid and vapor phases. In the present study, the deviation from ideal gas behavior, caused by the
dimerization of acetic acid molecules and interaction between two molecules in the vapor phase, is
described with the Hayden-O’Connell(HOC) equation[4]. This theory has been commonly used to
calculate the vapor-liquid equilibria of systems with associating components. The nonidealities
caused by water + acetic acid interaction and the dimerization effect of acetic acid molecules in the
liquid phase are considered by the nonrandom two-liquids model (NRTL)[5] and the universal
quasi-chemical theory (UNIQUAC)[6]. In this work, both the NRTL and UNIQUAC models were
used in combination with the HOC method for correlating the vapor-liquid equilibria of binary
systems and predicting the vapor-liquid equilibria of the ternary systems containing the associating
component acetic acid.
Experimental Section
Materials. The chemicals used were acetic acid (glacial)(PA grade) with a stated minimum purity of
99.5 mass%, and n-pentyl acetate (≥98.5 mass %) all supplied by Sinopharm Chemical Reagent Co.,
Ltd, deionized water (PA grade) supplied by The State Key Lab of Heavy Oil Processing of China
University of Petroleum(East China). All of the solvents except water were distilled in a glass column
and further purified before use.
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80
Material Sciences and Technology
Apparatus and Procedure. Refractive indices were measured by a refractometer, model WYA-2W
with a precision of ±0.0001. The measured refractive indices of the pure liquids were compared with
published values in Table 1.
Table 1 Refractive Index of the Experimental Materials at 293.15 K
water
1.3330
1.3330
nDa
nDb
acetic acid
1.3716
1.3714
a
n-pentyl acetate
1.4023
1.4025
Taken from Robert et al.7 b This work.
Different types of VLE apparatuses have been designed to obtain VLE data. In this work, the
apparatus used was a recirculating still of the modified Rose type[8]. During the measurements,
temperature was measured by using a mercury thermometer. The uncertainty of the temperature
measurements was ±0.01K. The uncertainty of the measured vapor-phase mole fraction was 0.001.
The pressure was obtained at (101.33±0.04) kPa. The experimental procedures are almost the same as
those described previously[9].
Sample Analysis. Compositions of the water + aceticacid + n-pentyl acetate system were analyzed
using an Agilent6820 gas chromatograph with a thermal conductivity detector. A 2-m
chromatographic column was packed with Chromosorb 105. The carrier gas was hydrogen flowing at
60mL/min, and the column temperature was 473K. The injector and detector temperatures were 453K
and 473K, respectively. The detector current was 150 mA.
he separation of organic acids from aqueous solutions is industrially important, and azeotropic
distillation is an attractive process for such separation. Several acetates, such as isopropyl acetate[1],
butyl acetate[2], isobutyl acetate[3], and so on have been used as entrainers for the separation of acetic
acid. It is known that vapor-liquid equilibria data is vital to the simulation and design of the azeotropic
distillation process. Unfortunately, little work has been done on the vapor-liquid equilibria of the
water + acetic acid + entrainer system.
Results and Discussion
Experimental Data. Vapor-liquid equilibria for the binary systems water(1)+ acetic acid (2), and
acetic acid (2) + n-pentyl acetate (3) have been obtained at 101.33 kPa. The results are reported in
Table 2 and Table 3. Also, the vapor-liquid equilibria for the water (1) + acetic acid (2) + n-pentyl
acetate (3) ternary systems were obtained at 101.33 kPa, and the results are reported in Table.4.
To test the equilibria apparatus and operation method, VLE data of water-acetic acid system were
compared with the published data shown in Fig.1[10]. It can be found that the experimental data of
this work agreed well with the literature data. The test results show that the equilibrium apparatus and
operation method are applicable.
Vapor-Liquid Equilibria Model. The vapor-liquid equilibria equation can be expressed by the
following equation

L
Vi
s s
φi yi p = xi γ iφi pi exp 

( p − pis ) 
RT

(1)
fi is the fugacity coefficient of component i in the mixture, fiS is the fugacity coefficient at the
saturated vapor pressure, yi is the apparent mole fraction in vapor phase, xi is the mole fraction in
the liquid phase, gi is the activity coefficient in the liquid phase, p is the system pressure, and piS is the
saturate vapor pressure.
Advanced Materials Research Vols. 560-561
81
In the present work, the vapor-phase fugacity coefficients are computed by virial equation. The
second virial coefficient B is calculated with Hayden-O’Connell equation. The liquid-phase activity
coefficient is calculated by the solution models for the excess Gibbs free energy such as modified
UNIQUAC and NRTL, and the relationship expressed by the following equation[11].
ln γ
i
  nG E  
∂ 

RT 
=  

 T
∂ni
,P ,n
(2)
j≠i
Table 2 Experimental VLE Data for the Binary System water(1)+acetic acid(2)at 101.33 kPa
No.
1
2
3
4
5
6
7
8
9
10
11
T[K]
388.55
386.35
382.95
380.75
378.75
377.65
376.55
375.45
374.35
373.95
373.55
water(1)+acetic acid(2)
x1
0.0299
0.1149
0.1816
0.2814
0.3833
0.4575
0.7011
0.7667
0.8368
0.9279
0.9597
y1
0.0833
0.1932
0.2737
0.3967
0.5088
0.5901
0.7947
0.8439
0.8877
0.9473
0.9718
Table 3 Experimental VLE Data for the Binary System acetic acid(2) + n-pentyl acetate (3) at
101.33 kPa
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
T[K]
423.25
418.15
414.65
412.25
410.15
407.45
404.15
403.15
401.45
400.35
399.25
398.25
397.25
396.15
395.35
394.95
394.25
393.65
393.15
391.90
acetic acid(1)+n-pentyl acetate(2)
x1
0.0000
0.0513
0.1093
0.1267
0.1647
0.3010
0.3204
0.3564
0.4007
0.4783
0.5038
0.5575
0.6565
0.6888
0.7154
0.7694
0.8246
0.8820
0.9335
1.0000
y1
0.0000
0.1390
0.1780
0.2662
0.3501
0.4158
0.5297
0.5924
0.6235
0.6726
0.7145
0.7527
0.7916
0.8214
0.8500
0.8789
0.9098
0.9382
0.9642
1.0000
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Material Sciences and Technology
Table 4 Experimental VLE Data for the Ternary System water (1) + acetic acid (2) + n-pentyl
acetate (3) at 101.33 kPa
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
T[K]
384.32
381.53
379.50
377.81
376.68
384.40
381.09
378.81
377.39
376.02
383.90
380.57
378.25
376.38
383.87
380.37
377.56
383.82
379.88
377.13
383.68
water(1)+acetic acid(2)+n-pentyl acetate(3)
x1
x2
y1
0.0834
0.8744
0.1342
0.1268
0.8252
0.1630
0.1706
0.7799
0.2338
0.2150
0.7251
0.3235
0.3449
0.5950
0.4267
0.0863
0.8180
0.1216
0.1912
0.7293
0.2562
0.2311
0.6876
0.2849
0.2317
0.6748
0.3034
0.2699
0.6293
0.3840
0.0730
0.7833
0.1572
0.1403
0.7231
0.2231
0.1666
0.7033
0.2564
0.2247
0.6450
0.3367
0.0860
0.7418
0.1422
0.1245
0.6820
0.2158
0.1922
0.6202
0.2613
0.1100
0.6870
0.1962
0.1337
0.6413
0.2049
0.1718
0.6124
0.3013
0.0878
0.6490
0.2039
y2
0.8290
0.7893
0.7109
0.5952
0.4917
0.8067
0.6775
0.6228
0.5885
0.4723
0.7544
0.6721
0.6122
0.5201
0.7399
0.6463
0.5617
0.6708
0.6313
0.5001
0.6313
Fig.1 Isobaric VLE diagram of water (1)- acetic acid (2) binary system at 101.32 kPa
Calculation of Binary Vapor-Liquid Equilibria. To predict the VLE of the water + acetic acid +
entrainer system, it is very necessary to determine the binary adjustable parameters for each two
components in the system. For each binary system, the three corresponding parameters were
estimated with the maximum likelihood method by the following objective function[12]
2
2
2
2
n  p
Ti ,cal − Ti ,exp ) ( x1i ,cal − x1i ,exp ) ( y1i ,cal − y1i ,exp ) 
(
(
i ,cal − pi ,exp )

SSQ= ∑ 
+
+
+
(3)

σ p2i
σ T2i
σ x21i
σ y21i
i =1 


Advanced Materials Research Vols. 560-561
83
where n represents the number of experimental data points and the indices exp and cal represent the
experimental and calculated values. represent the estimated variance of each measured variable(p, T,
xi, yi). The summations are extended over all data points.
Fig.2 and Fig.3 show the correlation deviations of the vapor-phase composition and deviations of
temperature between the experimental and calculated values. The estimated binary parameters and the
average absolute deviation (AAD) between the calculated values and experimental values are
reported in Tab.5. From the average absolute deviation between calculated and experimental values, it
can be seen that there is good agreement between the experimental and the calculated data by both the
NRTL and UNIQUAC models. The NRTL model is better than the UNIQUAC model on the basis of
the average absolute deviations.
Fig.2 Experimental and calculated T-y1 diagram for acetic acid (1) + n-pentyl acetate (2)
at 101.33 kPa
Fig.3 Experimental and calculated T-y1 diagram for water(1) + acetic acid (2) at 101.33 kPa
Calculation of Ternary Vapor-Liquid Equilibria. In this work, no research was done on the phase
equilibria ofthe water + acetate system. The parameters of the water + n-pentyl acetate systems are
from published documents15, shown in Tab.6. The NRTL interaction parameters obtained from the
binary systems were used to predict the vapor-liquid equilibria of the water (1) + acetic acid (2) +
n-pentyl acetate (3) ternary system.
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Material Sciences and Technology
For the water + acetic acid + n-pentyl acetate system, the average absolute deviations (AAD) of the
vapor-phase mole fractions of water and acetic acid were 0.0357 and 0.0330, and the average absolute
deviation of the equilibria temperature was 0.65 K. The values of AAD show that the NRTL model
gives a good representation of the experimental data.
Table 5 Correlation Parameters and the Average Absolute Deviations for water(1)-acetic
acid(2)-n-pentyl acetate(3)
(1)+(2)
Model
UNIQUAC
NRTL
(2)+(3)
A12a
A21
α12
∆yb
264.59
-76.53
-
0.0081
125.06
672.60
1.33
0.0052
(1)+(3)d
A12
A21
α12
∆y
∆T[K]
A12
A21
α12
0.07
-59.88
187.69
-
0.0227
0.55
-
-
-
0.09
-73.29
486.83 0.47
0.0095
0.45
∆Tc[K]
3134.65 203.76
0.2
a. The binary adjustable parameters for various models are as follows:
NRTL, Aij=(gij - gjj); UNIQUAC, Aij=(uij - ujj).
b. ∆y = (1/N)∣ycal - yexp∣.
c. ∆T = (1/N)∣Tcal - Texp∣.
d. Taken from Lee et al.13
Table 6 Predictive Deviations for the water (1) + acetic acid(2) + n-pentyl acetate (3) Ternary System
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
∆T[T]
-0.87
-0.47
-0.17
-0.39
-0.18
0.40
-0.49
-1.86
-1.38
0.83
-1.27
0.93
-0.82
0.30
1.03
0.10
0.25
0.49
0.62
-0.49
0.35
∆y1
-0.0807
-0.0535
-0.0422
-0.0594
-0.0245
-0.0522
-0.0529
-0.0451
0.0247
-0.0474
0.0163
-0.0366
-0.0085
-0.0095
0.0214
0.0451
0.0156
0.0287
-0.0197
0.0569
0.0098
∆y2
-0.0254
0.0095
0.0147
0.0286
0.0179
-0.0177
-0.0198
-0.0459
-0.0164
0.0028
0.0078
0.0189
0.0574
-0.0498
-0.0154
-0.0567
-0.0987
-0.0478
-0.0647
-0.0478
-0.0298
Conclusions
The NRTL and UNIQUAC model can both satisfactorily correlate the vapor-liquid equilibria data
of binary systems. But the NRTL model is more accurate than the UNIQUAC model in correlating the
equilibria compositions of binary systems. The obtained NRTL interaction parameters were used to
predict the vapor-liquid equilibria of the water + acetic acid+ n-pentyl acetate systems. Ternary
Advanced Materials Research Vols. 560-561
85
predicted values agree well with the experimental data in the experimental range of compositions. It
can be said that NRTL-HOC model is suitable for correlating the vapor-liquid equilibria of binary
systemthe NRTL interaction parameters are good enough to represent the ternary system.
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10.4028/www.scientific.net/AMR.560-561
Isobaric Vapor-Liquid Equilibria for the System Containing Acetic Acid at 101.3kPa
10.4028/www.scientific.net/AMR.560-561.79
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