Ind. Eng. Chem. Res. 1987, 26, 1686-1691
1686
Y = mole fraction in the gas phase
mole fraction
z =
Greek Symbols
= parameter describing nonideality in the adsorbed phase
induced by interaction with surface (vacancy)
y = activity coefficient
7 = lateral interaction parameter
0 = fractional amount adsorbed based on a monolayer coverage
T = spreading pressure
4 = fugacity coefficient
a!
Subscripts
i = species or component
m = mixture
t = total
v = vacancy
e = experimental quantity
p = predicted quantity
Superscripts
s = surface phase
m = limiting or maximum value
Registry No. H2, 1333-74-0; CO, 630-08-0; CH,, 74-82-8; COP,
124-38-9; HzS, 7783-06-4; C, 7440-44-0.
Literature Cited
Cochran, T. W.; Kabel, R. L.; Danner, R. P. AZCHE J . 1985,31,268.
Costa, E.; Sotela, J. L.; Calleja, G.; Marron, C. AZCHE J . 1981,27,
5.
Friederich, R. 0.;Mullins, J. C. Znd. Eng. Chem. Fundam. 1972, 11,
439.
Grant, R. J.; Manes, M. Ind. Eng. Chem. Fundam. 1966, 5, 490.
Hyun, S. H.; Danner, R. P. J . Chem. Eng. Data. 1982,27, 196.
Lee, A. K. K. Can. J . Chem. Eng. 1973, 51, 688.
Lee, C. S.; O’Connell, J. P. AIChE J . 1986, 32, 96.
Lewis, W. K.; Gilliland, E. R.; Chertow, B.; Cadogan, W. P. Znd. Eng.
Chem. 1950, 42, 1319.
Markham, E. D.; Benton, A. F. J. Am. Chem. Soc. 1931, 53, 497.
Miller, G. W.; Knaebel, K. S.; Ikels, K. G. AIChE J . 1987, 33, 194.
Myers, A. L. Ind. Eng. Chem. 1968, 60, 45.
Myers, A. L.; Prausnitz, J. M. AIChE J . 1965, 11, 121.
Peng, D.-Y.; Robinson, D. B. Ind. Eng. Chem. Fundam. 1976,15,59.
Reich, R.; Zeigler, W. T.; Rogers, K. A. Ind. Eng. Chem. Process Des.
Deu. 1980, 19, 336.
Ritter, J. A. M. S. Thesis, State University of New York, Buffalo,
1986.
Ruthven, D. M.; Loughlin, K. F.; Holborow, K. A. Chem. Eng. Sci.
1973, 28, 701.
Saunders, J. T. M. S. Thesis, State University of New York, Buffalo,
1983.
Schay, G. J. Chem. Phys. Hungary, 1956,53, 691.
Talu, 0.; Zwiebel, I. AZChE J. 1986, 32, 1263.
Van Ness, H. C. Ind. Eng. Chem. Fundam. 1969,8, 464.
Wilson, R. J.; Danner, R. P. J . Chem. Eng. Data 1983, 28, 14.
Yang, R. T. Gas Separation by Adsorption Processes; Butterworth
Boston, 1987; Chapter 3.
Yon, C. M.; Turnock, P. H. AZChE Symp Ser. 1971, 67(117), 3.
Received for review October 28, 1986
Revised manuscript received April 24, 1987
Accepted May 1, 1987
Evaluation of an Equation of State Method for
Calculating the Critical Properties of Mixtures
J. Richard Elliott, Jr.
Department of Chemical Engineering, The University of Akron, Akron, Ohio 44325
Thomas E. D a u b e r t *
Department of Chemical Engineering, The Pennsylvania State University,
University Park, Pennsylvania 16802
The accuracy of an equation of state method for predicting critical properties of mixtures is evaluated
and compared to several empirical methods. The Soave equation of state is used with binary
interaction coefficients predicted from vapor-liquid equilibrium data. An extensive data base of
about 1500 points each for critical temperature and critical pressure is used in the evaluation. These
data include hydrocarbon mixtures and hydrocarbon-non-hydrocarbon mixtures with H2, N2,CO,
C 0 2 ,and H2S. The equation of state method is determined to be more accurate than the empirical
methods for critical temperature and critical pressure and slightly less accurate for critical volume.
Furthermore, the equation of state method predicts anomalous trends in critical loci which were
previously noncalculable by empirical methods.
When phase equilibria at high temperatures and pressures are considered, knowledge of the critical properties
of mixtures is essential. Many processes involving high
pressure are designed specifically to take advantage of the
unique phase behavior in the critical region. Enhanced
oil recovery with carbon dioxide and supercritical extraction provide two examples of such processes. Accurate
knowledge of the critical properties of the mixtures is
especially important for these types of processes.
Many correlations have been proposed for predicting the
critical properties of mixtures. Most of these correlations
have been empirical, and they have been limited in the
types of systems which they could represent. Empirical
correlations were evaluated by Spencer et al. (1973), and
the most accurate methods were recommended. One
0888-5885/87/2626-1686$01.50 f 0
modification has been made to the Chueh and Prausnitz
(1967) method in the API book (1986) as described later.
Since that time, the calculation of the critical properties
via an equation of state by applying the rigorous thermodynamic criteria at the critical point has become more
practical for common application. Peng and Robinson
(1977) evaluated their equation of state for 30 mixture
critical points by this method. Heidemann and Khalil
(1980) published an improved algorithm which was considerably more rapid and more robust than its predecessors. Michelsen and Heidemann (1981) have improved the
computation speed of this latter algorithm. Considering
these developments, a thorough evaluation of the merits
of a rigorous method relative to the empirical methods was
undertaken.
0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1687
Methods Compared
The empirical methods used to compare to the rigorous
method were those determined to be most accurate by
Spencer et al. (1973). These were the Li (1971) method
for critical temperature, the Kreglewski and Kay (1969)
method for critical pressure, and the Chueh and Prausnitz
(1967) method for all three critical properties.
Li (1971) Method for Critical Temperature:
n
Tcm = C ~ L T C ,
(1)
i=l
CxiVci
i=l
Kreglewski and Kay (1969) Method for Critical
Pressure:
r
pcm=
1
n
1
+ [5.808 + 4.93(C~i0i)]
X
i=l
[
]2XiPCi
Tc?;]
i=1
ponent mixtures. To evaluate the binary parameters,
binary critical data must be available for each component
pair in the mixture. Because of these limitations, the
method was not evaluated in this investigation.
As representative of the equation of state approach, the
Soave (1972) equation of state was investigated. The Soave
(1972) equation was implemented as described in Chapter
8 of the API book (1986). The binary interaction coefficients and correlations of binary interaction coefficients
recommended by that source were used directly with no
optimization of the parameters to fit the critical data. An
adaptation of the procedure of Moysan et al. (1983) for
binary interaction coefficients of hydrogen systems was
also implemented as described in the API book (1986).
Thus, the implementation of the equation of state method
for critical properties could fairly be termed predictive.
The implementation of the equation of state approach
requires trial and error solution for the two variables V,,
and T,,. The two equations which must be satisfied are
det [&I = 0
(15)
C=O
(16)
a2A
nTQij= nT anidnj
(3)
Chueh and Prausnitz (1967) Method for All Critical
Properties:
It
T,, = i=l
CBiT,,
n
V,, = C8iVC,+
i=l
P,, =
~
n
n
+ i=l jCBiBj~ij
=1
(4)
C0i0j~ij
i=lj=1
U
V,,
+ b)
+ Raj)RTcij1.5(Vci
+ Vcj)
0.291 - 0.04(~i+ ~ j )
(5)
(6)
1/4(aai
aij
=
+
(11)
= 0.0867 - 0.0125~i 0.011W:
(14)
Correlations for the binary parameters, T ~ and
,
vi,, are
available from Chapter 4 of the API book (1986). These
correlations were based on the data base used at the time
of the evaluation by Spencer et al. (1973).
One empirical method which has been proposed for
calculating critical properties of defined mixtures since the
previous investigations is that of Teja et al. (1983). The
method is limited, in that binary critical data cannot be
predicted. The method requires evaluation of binary interaction parameters to predict properties of multicomRbi
[
6 i # j # k , i # k
n n
RT
Vcm - b T,,'/2V,,(
1 i=j=k
hi,,= 3 i = j # k , i = k # j , j = k # i
where A = total Helmholtz free energy of the mixture.
Formulas for the derivatives of the Helmholtz energy with
respect to mole number are given by Heidemann and
Khalil (1980). Once V,, and T,, have been determined,
these values are substituted into the Soave (1972) equation
of state to obtain a value of P,,. The strategy of the
Heidemann and Khalil algorithm is to iterate on Tcmand
V,, in a nested manner instead of performing iterations
on both variables simultaneously as in a typical solution
of multiple nonlinear equations. Based on an initial guess
of V,,, T,, is iterated until eq 15 is satisfied. Closure of
eq 16 is checked, a new estimate for V,, is generated, and
the iteration on Tcmis carried out again. The advantage
of this strategy is that it leads to convergence on the desired root much more often than by use of simultaneous
iteration. Iteration on T , and V,, by the secant method
was found to be satisfactory for all calculations performed
in this investigation.
Data Base
Attention was restricted for this evaluation to hydrocarbon mixtures and mixtures containing hydrogen, nitrogen, carbon monoxide, carbon dioxide, and hydrogen
sulfide. An extensive compilation of critical properties
which included these types of mixtures was published by
Hicks and Young (1975). A second source which provided
data for a few systems not listed in Hicks and Young (1975)
was due to Kay (1972). Data from references after 1975
were also included. Points obviously inconsistent with
other similar data were eliminated so as not to bias the
evaluation.
The complete data set was recorded on magnetic tape.
A copy of the data set as well as the Fortran subroutine
for implementation of the Heidemann and Khalil (1980)
algorithm is available from the authors. Table I summa-
1688 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
Table I. Summary of Systems Studied for Critical Points
of Defined Mixtures
no, of
no. of points
binary systems
methane-hydrocarbon
hydrocarboh-hydrocarbon
systems T, P, V ,
15
139 137 85
138
1108 878 182
33
199 186 57
hydrocarbon-non-hydrocarbon
non-hydrocarbon-non-hydrocarbon
5
21 17
9
30
194 169 10
multicomponent
rizes the contents of the data set.
Results
The results of the investigation are summarized in Tables 11-IV for binary mixtures and in Table V for multicomponent mixtures.
For critical temperatures of binary mixtures, Table I1
shows that the accuracy of all the methods was roughly
equivalent. The Soave equation is slightly more accurate
for the non-hydrocarbon mixtures. The Chueh-Prausnitz
correlation failed for the n-hexane-acetylene system, as
negative absolute temperatures were calculated. The
reason for the failure is that the value of the correlation
parameter was outside the range of the polynomial correlation used to adapt the graphs of Chueh and Prausnitz,
showing that extrapolation of the method may be unreliable.
Table I11 shows that the critical pressure calculations
for binary mixtures are much more accurate when the
equation of state approach is used. This observation is
especially true for methane systems and for hydrocarbon-non-hydrocarbon systems.
Calculations of critical volumes of binary mixtures are
summarized in Table N.The Soave equation is inherently
wrong for the prediction of critical volumes because the
critical compressibility factor for pure compounds is fixed
at 2, = l j 3 . Two corrections of this shortcoming were
tested. Peneloux et al. (1982) have suggested a correction
scheme based on the Rackett equation being applied to
the liquid at a reduced temperature of 0.7. Unfortunately,
it was determined that this correction is not accurate for
the critical region. The second scheme involved calculating
correction factors for the pure compounds by using the
critical volumes of pure compounds from the data base and
then applying the molar average correction factor to the
mixture critical volume calculated from the Soave equa-
Table XI. Results of Evaluations for Critical Temperatures of Defined Binary Mixtures
NPTSO
NCANT*
% AAD'
% BIASd
AAD,' K
BIAS! K
A. Hydrocarbon-Hydrocarbon Systems
1. methane system
Soave
Li
Chueh-Prausnitz
Chueh-Prausnitz (revised)
2. non-methane systems
Soave
Li
Chueh-Prausnitzg
Chueh-Prausnitz (revised)
124
139
139
139
15
0
0
0
4.94
5.73
5.59
5.72
3.80
2.73
3.49
4.27
15.90
17.24
16.53
16.73
13.21
6.58
9.19
11.78
1108
1108
1104
1108
0
0
4
0
0.81
0.61
1.01
1.37
0.45
0.21
-0.88
-0.64
3.86
2.86
4.72
6.80
1.78
0.67
-4.10
-3.80
Soave
Li
Chueh-Prausnitz
Chueh-Prausnitz (revised)
B. Hydrocarbon-Non-Hydrocarbon Systems
188
11
1.81
1.01
0
5.02
-1.18
199
0
2.55
-0.09
199
0
2.24
-0.83
199
6.92
19.92
10.24
8.70
3.88
-5.93
-0.56
-3.84
Soave
Li
Chueh-Prausnitz
Chueh-Prausnitz (revised)
C. Non-Hydrocarbon-Non-HydrocarbonSystems
16
5
1.33
0.90
21
0
3.20
0.53
21
0
8.92
6.35
21
0
6.74
5.37
2.30
6.55
9.51
6.53
1.74
0.71
2.48
4.02
NPTS = total number of points. NCANT = number of points for which the method failed to provide a reasonable answer. For the
Soave equation, these failures could be overcome by a different initial guess which will vary according to the mixture being considered. %
AAD = ( l / N P T S ) C F [ l c a l c d a t l l / e x p t 1 ] 1 0 0 . % BIAS = ( l / N I " S ) C F [ ( c a l c d - exptl)/expt1]100.
= (l/NPTS)zFlcalcd
- exptll. BIAS = (l/NPTS)C, (calcd - exptl). #Failed for n-hexane-acetylene system.
Table 111. Results of Evaluations for Critical Pressures of Defined Binary Mixtures
NPTS
NCANT
% AAD
% BIAS
h,
bar
BIAS, bar
A. Hydrocarbon-Hydrocarbon Systems
1. methane systems
Soave
Kreglewski-Kay
Chueh-Prausnitz
2. non-methane systems
Soave
Kreglewski-Kay
Chueh-Prausnitz
122
137
137
15
0
0
6.57
23.79
7.79
-3.41
-23.15
-1.73
9.22
48.91
14.83
-5.33
-48.47
-2.83
878
878
878
0
0
0
2.35
3.82
7.67
-0.86
1.58
1.79
1.58
2.06
3.55
0.26
0.03
1.38
1.77
-5.73
-0.17
10.46
52.53
36.89
2.28
-37.21
-21.42
C. Non-Hydrocarbon-Non-HydrocarbonSystems
12
5
4.40
3.45
0
37.62
-36.59
17
0
30.45
-25.81
17
4.35
82.78
69.18
3.17
-82.00
-65.47
B. Hydrocarbon-Non-Hydrocarbon Systems
Soave
Kreglewski-Kay
Chueh-Prausnitz
Soave
Kreglewski-Kay
Chueh-Prausnitz
176
186
186
10
0
0
9.41
21.91
17.36
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1689
Table IV. Results of Evaluations for Critical Volumes of Defined Binary Mixtures
NPTS
NCANT
%AAD
% BIAS
A. Hydrocarbon-Hydrocarbon Systems
1. methane systems
Soave
85
0
19.56
19.02
Soave (corrected)
Chueh-Prausnitz
Chueh-Prausnitz (revised)
Kay’s rule
2. non-methane systems
Soave
S6ave (corrected)
Chueh-Prausnitz
Chueh-Prausnitz (revised)
Kay’s rule
85
85
85
85
0
0
0
0
13.96
9.02
11.24
44.92
-12.54
2.60
7.78
44.60
24.11
16.23
9.82
11.12
44.78
23.38
-14.78
0.93
5.97
44.22
182
182
182
182
182
0
0
0
0
0
21.80
7.58
8.02
8.06
12.70
18.60
-4.96
-4.61
-3.74
9.65
52.86
16.37
18.88
18.10
25.01
44.11
-9.86
-11.62
-10.45
17.75
Soave
Soave (corrected)
Chueh-Prausnitza
Chueh-Prausnitz (revised)
Kay’s rble
55
55
47
57
57
B. Hydrocarbon-Non-Hydrocarbon Systems
2
22.34
22.34
2
8.12
-1.55
10
5.51
1.92
0
9.02
3.36
0
13.60
13.37
32.43
9.93
7.74
10.79
16.03
32.43
-2.26
2.58
2.83
15.67
C. Non-Hydrocarbon-Non-Hydrocarbon Systems
9
0
15.01
15.01
9
0
3.39
-3.35
9
0
7.64
-6.34
8.84
8.84
9
0
9
0
4.31
4.31
13.93
2.90
7.03
7.81
3.68
13.93
-2.86
-6.02
7.81
3.68
Soave
Soave (corrected)
Chueh-Prausnitz
Chueh-Prausnitz (revised)
Kay’s rule
(I
AAD, cm3/amol BIAS,cm3/mol
Failed for hydrogen-n-decane and carbon dioxide-n-decane.
Table V. Results of Evaluations for Critical Properties of Defined Multicomponent Mixtures
critical temDerature
Soave
Li
Chueh-Prausnitz
critical pressure
Soave
Kreglewski-Kay
critical volume
Soave (corrected)
Chueh-Prausnitz
Kay’s rule
NPTS
193
194
194
NPTS
168
169
NPTS
10
10
10
NCANT
1
0
0
NCANT
1
0
NCANT
0
0
0
tion. This last approach is denoted as the Soave (corrected) method and proves to be reasonably accurate. Of
the empirical methods, the Chueh-Prausnitz is accurate
but failed for two mixtures, n-decane-hydrogen and ndecane-carbon dioxide, for the same reason as for failure
with critical temperatures.
Table V summarizes the results for multicomponent
systems. The Soave method yields reasonable results for
multicomponent systems and compares favorably with the
best empirical method ascertained in previous work.
% AAD
AAD, K
BIAS, K
6.70
5.69
4.62
-2.64
5.97
-4.43
AAD, bar
BIAS, bar
3.06
0.21
5.10
-4.63
AAD, cm3/gmol BIAS, cm3/mol
23.93
-23.93
18.21
-18.21
58.27
58.27
% BIAS
1.70
1.17
1.60
% AAD
3.33
4.62
% AAD
15.94
11.82
38.79
1.44
-0.61
-1.15
% BIAS
0.01
-3.86
% BIAS
-15.94
-11.82
38.79
PURE ACETYLENE
890
880
870
880
850
840
850
820
.-
e10
% 800
700
700
Discussion
The use of the Soave equation of state to predict the
critical points of defined mixtures is considerably more
difficult than the empirical methods in that the method
is only practical on a computer. This method offers two
important rewards as discussed below.
First, the equation of state method offers versatility in
representing anomalous critical behavior. Figure 1is an
example illustrating that the critical behavior of the etheneethyne system can be accurately represented by the
Soave equation if a nonzero interaction coefficient is used.
Figures 2 and 3 show excellent reproduction of the qualitative features of the experimental data smoothed and
plotted in Figures 4 and 5, respectively. Figure 6 shows
that qualitative agreement is also reasonable for a more
complicated system. Furthermore, the previous methods
cannot predict opposite signs of excess critical temperature
770
780
750
PURE ETHYLENE
720
710
..
I
J
I
35
40
45
I
50
I
55
I
I
I
I
BO
85
70
75
I
80
I
85
I
,
80
95
Tc, F
Figure 1. Critical locus for the ethene-ethyne system.
and excess critical pressure (e.g., the benzene-n-decane
mixture) nor can they predict the changing sign of the
excess critical pressure (e.g., the benzene-n-tridecane
mixture). Thus, the equation of state method appears to
be more reliable than the empirical method.
Second, the use of the equation of state method for
critical points and vapor-liquid equilibrium permits ex-
1690 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
U
W
U
I3
U
U
a
W
5
_1
<
2
k
0
LD
(0
0.0
0.1
0.3
0.2
0.4
0.5
0.6
0.7
0.8
0.9
W
1.0
0
X
M O L E FRACTION BENZENE
W
Figure 2. Excess critical temperature predicted by the Soave
equation for benzene-n-paraffin mixtures of hexane through tridecane.
00
0 1
0 2
0 3
0 4
05
06
08
07
09
10
M O L E FRACTION BENZENE
Figure 4. Experimental excess critical temperatures for benzenen-paraffin mixtures of hexane through tridecane.
60
-55
-60
0 0
0 1
0 2
0 3
0 4
0 6
0 5
0 7
0 8
0 9
1 0
MOLE FRACTION BENZENE
Figure 3. Excess critical pressure predicted by the Soave equation
for benzene-n-paraffin mixtures of hexane through tridecane.
Lo
ploration of phase equilibria over a wide range of conditions. Direct calculation of the critical locus before performing phase equilibrium calculations may save considerable effort if calculations are being carried out near the
critical region.
The failure of the previous adaptation of the ChuehPrausnitz method is due to the use of too many constants
in the polynomial equations used to represent the graphs
in the Chueh-Prausnitz article. To remedy this failure,
the curves were again regressed with careful consideration
of the statistical significance of each constant. The resulting model equations are much simpler and should be
more reliable for extrapolation. The revised equations
dealing with the parameters T,, and V , are given below:
--2712
TC, +
Tc2
[
- CT + 1.3918
~
CT
cV
for paraffin systems
= -0.0347 0
if component 1 or 2 is aromatic
= -0.0073 0
if component 1 or 2 is a non-hydrocarbon = -0.0621 0.1559
Results of using these correlations are given in Tables
( 0 0
W
0
x
-10
-20
-30
-40
0 0
0 1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0 9
0
M O L E FRACTION BENZENE
Figure 5. Experimental excess critical pressures for benzene-nparaffin mixtures of hexane through tridecane.
11-IV as Chueh-Prausnitz (revised). The new correlations
are slightly less accurate but gave reasonable results in all
cases.
Conclusions
It has been determined that an equation of state approach is significantly superior to empirical equations when
predicting the critical properties of mixtures. Not only are
the quantitative results more accurate but the method
predicts anomalous trends in the critical loci that most of
the empirical methods cannot even correlate. The calculational complexity of the equation of state approach is
substantially greater than that of the empirical methods,
but an efficient algorithm is available such that calculation
Ind. Eng. Chem. Res. 1987,26,1691-1695
0
0
0
PAK AND K A Y ( 1 0 7 2 )
-S O A V E EQUATION
!
w ol
h
P
1691
T,, = critical pressure of the mixture
V,, = critical volume of the ith component
V,, = critical volume of the mixture
x , = mole fraction of the ith component
Greek Symbols
vl1 = correlated binary parameter of the ij interaction for
critical volume
0, = volume fraction of the ith component
w , = acentric factor of the ith component
rll = correlated binary parameters of the i j interaction for
critical temperature
Literature Cited
-3
0 0
0 1
0 2
0 3
0 4
0 5
O B
0 7
0 8
o s
1 0
M O L E FRACTION TRIDECANE
Figure 6. Excess critical pressure calculated by the Soave equation
for the benzene-n-tridecane mixture.
times should not be prohibitive even on a personal computer.
Acknowledgment
This work was supported by the Refining Department
of the American Petroleum Institute.
Nomenclature
n = number of components
AN = composition change from test point
P,i = critical pressure of ith component
P,, = critical pressure of the mixture
TCi= critical temperature of the ith component
American Petroleum Institute Technical Data Book-Petroleum
Refining, 4th ed.; Daubert, T. E., Danner, R. P., Eds.; API:
Washington, D.C., extant 1986.
Chueh, P. L.; Prausnitz, J. M. AIChE J . 1967, J 3 , 1107.
Hicks, C. P.; Young, C. L. Chem. Reu. 1975, 75, 119.
Heidemann, R. A.; Khalil, A. M. AIChE J. 1980, 26, 769.
Kay, W. B. Final Report, API Project No. PPC 15.8, 1972; The Ohio
State University Research Foundation, Columbus, OH.
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Received for review June 10, 1986
Accepted May 26, 1987
A Kinetic Study of the Disproportionation of Potassium
Benzoate?
V. V. S. R e v a n k a r and L. K. Doraiswamy*
National Chemical Laboratory, Pune 411 008, India
A kinetic study of the disproportionation of potassium benzoate catalyzed by cadmium halides to
potassium terephthalate has been carried out in the temperature range 390-430 "C. The reaction
scheme has been represented by a two-step consecutive reaction followed by a parallel reaction
catalyzed by the reaction product of the consecutive scheme. The rate constants for these individual
reactions have been obtained under various reaction conditions. The apparent rate constants at
410 "C have been correlated with catalyst concentration and carbon dioxide gas pressure. Arrhenius
parameters for the individual steps have also been evaluated.
Terephthalic acid (TPA) finds extensive application in
the polymer industry. By far the most important use is
in the manufacture of synthetic fibers of polyester type
(notably Dacron and Terylene); second in importance is
its use as an intermediate for polyester film (Mylar and
Videne). Limited quantities are also used in the manufacture of TPA-based plasticizers.
There are several routes for the manufacture of TPA.
But commercially the liquid-phase oxidation of p-xylene
is followed. Generally, p-xylene is recovered from the
C8-aromatic fraction of naphtha reformate. However,
separation of p-xylene from other hydrocarbons, especially
*To whom correspondence should be addressed.
NCL Communication 4034.
from m-xylene and ethylbenzene, is very difficult and
costly. Hence, alternative raw materials have been sought.
Among them p-isopropyltoluene and benzenecarboxylic
acids are the most important ones. Considerable work in
this direction was carried out in Japan, USSR, and USA
in the late 1960s. The thermal disproportionation of the
potassium salt of benzenecarboxylic acid to terephthalic
acid seems to be an attractive route but has not yet been
commercialized due to mechanical and engineering problems.
Various workers have studied this disproportionation
reaction, and their results seem to suggest three different
mechanisms as discussed in our earlier paper (Revankar
et al., 1987). These are (i) bimolecular mechanism, (ii)
carboxylation-decarboxylation mechanism, and (iii) active
0888-5885/87/2626-1691$01.50/0 0 1987 American Chemical Society