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. Kreglewski, A.; Kay, W. B. J . Phys. Chem. 1969, 73, 3359. Li, C. C. Can. J . Chem. Eng. 1971, 19, 709. Michelsen, M. L.; Heidemann, R. A. AIChE J . 1981, 27, 521. Moysan, J. M.; Huron, M. J.; Paradowski, H.; Vidal, J. J. Chem. Eng. Sci. 1983, 38, 1085. Pak, S. C.; Kay, W. B. Ind. Eng. Chem. Fundam. 1972, 11, 255. Peneloux, A.; Rauzy, E.; Freze, R. Fluid Phase Equilibr. 1982,8, 7. Peng, D. Y.; Robinson, D. B. AIChE J . 1977,23, 137. Soave, G. Chem. Eng. Sci. 1972,27, 1197. Spencer, C. F.; Daubert, T. E.; Danner, R. P. AIChE J . 1973,19,522. Teja, A. S.; Gurg, K. B.; Smith, R. L. Ind. Eng. Chem. Process Des. Deu. 1983, 22, 672.
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
1692 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
species, intermediate phenyl anion type mechanism. Of these, the last one appears to be the most acceptable and accounts satisfactorily for intermediate products also (mono-, di-, and tribenzene carboxylates) (Furuyama, 1967). The mechanism is COOK
I
8
+
Cadmium complex1
Cd2+
,K+
v COOK
I
K+
WOK
+
Cd2+
+
C02
COOK
I
COOK
COOK
The kinetics of this important reaction has not been studied in detail. Ogata and Sakamoto (1964) have studied the kinetics of potassium benzoate disproportionation with Cd2+as catalyst using potassium cyanate as solvent. They have expressed their rate equations as
(3) where B = potassium benzoate, I = potassium isophthalate, T = potassium terephthalate, and Cd = cadmium catalyst. They obtained values of rate constants in the range 3.72-37.8 X s-l at temperatures in the range 365-410 "C. Their value of activation energy for isophthalate formation is 38.1 kcal/mol and for terephthalate formation is 38.0 kcal/mol. Ogata and Nakajima (1965) fitted their experimental data to the rate equation r = k [ CdIz][benzoate]
(4)
Khlestkin et al. (1977) suggested that the liberation of carbon dioxide is the rate-determining step in potassium benzenecarboxylate conversion and have expressed the reactivity in terms of initial decarboxylation rate
WdOpe= 78.3Ck - 0.1
(5)
where WdOpBis the decarboxylation rate and Ck is the catalyst concentration used. Morikawa et al. (1960) found that the activity of the carboxylic acid depends on both the concentration and nature of the catalyst, and they have suggested that the catalyst greatly influences the final stage of the reaction. None of the authors have explained the kinetics of the disproportionation reaction satisfactorily. Much work needs to be carried out to explain some of the mechanistic and engineering problems. The object of the present work is to study the kinetics of disproportionation of potassium
benzoate to potassium terephthalate using cadmium chloride catalyst. Experimental Section Feed Preparation. Aqueous solutions (10%) of potassium benzoate and catalyst were prepared in hot water (60 "C) and cold water, respectively. The proportions of the two in the reaction mixture were varied by weight. The catalyst solution was slowly added to the hot potassium benzoate solution with stirring, and the resultant solution was evaporated to dryness. The last traces of moisture were removed in a vacuum oven at 100 "C. The resultant powder was amorphous in nature. Experimental Procedure. The experimental unit consisted of a stainless steel bomb reactor of 7-cm i.d. and 12-cm height with a suitable arrangement for cooling. It was connected to a surge tank to minimize pressure fluctuations. The pressure was recorded by means of a pressure gauge. The other parts of the experimental setup were the same as those described in our earlier paper (Revankar et al., 1987). A very fine powder of potassium benzoate charge was spread on the reactor plate. Initially, the reactor was flushed with carbon dioxide gas and pressurized to the required pressure by adjusting the needle valves. The reactor was then lowered into a furnace which was kept at the desired temperature. The required reaction temperature was obtained within 1 min of insertion. The reaction was stopped after the desired time interval by lifting the reactor from the furnace and quenching it in chilled water. The carbon dioxide gas in the reactor was blown out by opening the needle valve. The cooled solid product mass was removed from the reactor for analysis. Analysis. The chemical analysis of the sample was carried out by following the methods described by Ogata and Nakajima (1965) as well as by Khlestkin et al. (1977). The results were confirmed by NMR, with the concentration of the benzoate and terephthalate calculated from integrated NMR spectra. Discussion It was assumed that terephthalic acid is formed directly from the benzoate salt. In the course of work on the kinetics of potassium benzoate rearrangement, we have found that the overall reaction is more complicated than this and that the terephthalate is formed, at least in part, via discrete intermediates as explained by Kraus et al. (1961). During the series of experimental runs, we found that in all cases the rate of formation of terephthalate product follows an S-shaped curve, indicating the probable existence of a parallel autocatalytic reaction involving the conversion of intermediates (catalyzed by the product potassium terephthalate). The intermediates were analyzed and found to be mainly potassium phthalates, potassium isophthalates, and potassium benzenecarboxylates. The concentrations of these intermediates invariably pass through a maximum. Lumping the intermediates together, one can represent the overall reaction as a two-step consecutive reaction followed by an autocatalytic reaction
-+ -
A
R
k,
S
R
k3
kz
2s
S
(catalyst) (autocatalytic)
(6)
(7) where R represents the intermediate products lump. The possibility of potassium benzenecarboxylate itself catalyzing the reaction has been predicted earlier (Khlestkin et al., 1976, 1977; Ratusky, 1967), and it is a well-known
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1693 1
o p
Cb-4L"S1 T E MPE R bTUR E PRESSLRE
5%
TEMPER I T U R E
4 I OC '
410 C '
1"
1 C4TbLIST TEMPER A T i R E
5 % 4 IC'O
CATALYST TEMPERATURE
1 Lo/&
PRESSURE
@
CbTbLYST TEMPERATURE
5% 410'C
PRESSURE
6 Jkglsm'
5% 395'C it k~ /cm'
i@ CbTbLYST TEMPERATURE PRESSURE
5% 41Ot It ka/Cm'
,-K-benloo?e
b,,., 1 r
0
TIME,min
20
40 TIME,min
60
80
Figure 1. Product distribution curves at different temperatures.
fact that all benzene carboxylates rearrange to more stable terephthalate (Khlestkin et al., 1977; Sorm and Ratusky, 1966). Figure 1 shows the variation of reactant and product concentrations with reaction time. The intermediate product passes through a maximum as the reaction progresses and then decreases slowly as it is converted to terephthalate. The product formation is observed to be the highest at the point of maximum intermediate concentration. At higher temperatures, there is a sharp decrease in the rate of intermediate formation. Presumably, this is due to the faster rate of conversion of intermediates to terephthalate. Figure 2 shows the effect of carbon dioxide pressure on terephthalate formation. As the pressure is increased, more of the benzoate is converted to potassium terephthalate, but the total increase in conversion is not very pronounced. In all the above runs, the catalyst (CdC12)concentration was kept constant at 5% by weight. A series of runs were carried out in which the catalyst concentration was varied in order to investigate its effect on overall conversion and the rate of terephthalate formation. The results are plotted in Figure 3. At low catalyst concentrations (1%by weight), the maximum concentration of intermediate as well as the final conversion to product terephthalate are low. As the catalyst loading increases, the overall rate of terephthalate formation increases. As expected, the reaction rates increase with increased catalyst loading. Figure 4 shows the variation of reactant and product concentrations with time for different catalyst loadings. It is observed that the rate of reaction is higher with Cd12 than with CdC03 or CdCb The average catalytic activity is in the following order: nitrates, sulfates
