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Kinetics of the synthesis of bisphenol A
Applied Catalysis, 37 (1988) 129-138 Elsevier Science Publishers B.V., Amsterdam -
129
Printed in The Netherlands
Kinetics of the Synthesis of Bisphenol A K. JERABEK*, J. ODNOHA* and K. SETINEK Institute of Chemical Process Fundamentals, CzechoslovakAcademy of Sciences, 165 02 Prague 6 (Czechoslovakia) (Received 15 April 1987, accepted 24 August 1987)
ABSTRACT The kinetics of the synthesis of bisphenol A from acetone and phenol on an ion-exchange catalyst promoted by partial neutralization of acid groups with 2-mercaptoethyl amine was investigated in the temperature range 50-85°C. A Langmuir-Hishelwood type rate equation was used for treatment of the experimental data. The rate constant and the adsorption coefficient of water as the reaction product are temperature dependent, while the adsorption coefficient of phenol and acetone can be regarded as constants within the given range of temperatures. The effect of adsorption of bisphenol A need not be included in the kinetic equation. The effect of the reverse reaction is apparent at temperatures higher than 7O’C. The equation thus obtained may be used in a description of integral data up to high conversions.
INTRODUCTION
2,2-Bis (4’ -hydroxyphenyl)propane (commercial names dian, bisphenol A) is one of the starting materials in the production of epoxy resins and polycarbonates. On an industrial scale it is produced by the acid-catalyzed condensation of acetone and phenol. Among others strongly acidic ion exchangers are used as the catalysts in this synthesis. Their advantages are low corrosion of the production equipment and the possibility of continuous production. The rate and selectivity of this reaction are greatly improved by the presence of compounds containing a mercapto group in the reaction mixture. There is very little information in the literature on the kinetics of this condensation; the only study of it was published by Reinicker and Gates [ 11.They used a stirred batch reactor and investigated the kinetics at one temperature only (91 “C ) , in the absence of a promoter containing a mercapto group. They tried to investigate a modified catalyst containing sulphhydryl grouns, but the chosen procedure, namely partial esterification of the acid groups with 2-mercaptoethanol, was unsuitable for this purpose. This procedure has been described in the patent literature [ 21, but it does not result in the assumedbonding *Present address: Research institute for Chemical Equipments, Prague, Czechoslovakia.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
130
of the promoter to the ion exchanger. We have found, however, that instead of this, dehydration of 2-mercaptoethanol takes place and the product of this dehydration contaminates the polymer. Their results made Reinicker and Gates [l] come to the conclusion, which was in fact wrong, that compounds with sulphhydryl groups bound to the ion exchanger did not possess a promoting effect. It is known, however, that by employing other methods of modification, e.g. the partial reduction of sulpho groups [ 31 or the partial neutralization of the acidic groups with cysteamine (2-mercaptoethyl amine) [ 41 it is possible to significantly raise both the reaction rate (ca. five to ten times) and the selectivity. In the present work we have investigated the kinetics of the synthesis of bisphenol A using a catalyst modified with cysteamine. We took measurements within a sufficiently broad range of temperatures, so that the eventual description could also be employed for an industrial reactor operating in a non-isothermal regime. EXPERIMENTAL
Chemicals
Acetone, reagent grade (Lachema, Brno) was dried over a molecular sieve. Phenol, analytical grade (Reactivul, Rumania) was repurified by distillation. Gel-type ion exchanger Ostion KSC-1 (Spolchemie, astf n/L), declared divinylbenzene content 4%, was recycled between the Na+ and H+ form four times (the last recycling to the H+ form), washed with distilled water and eluted with a 10% aqueous solution of cysteamine hydrochloride (Cillag Chemie, Switzerland), in an amount corresponding to 15% of the capacity of the acid ion exchanger. The ion exchanger was left to stand overnight in contact with this solution. On the following day it was washed with distilled water and dried at 110°C for 12 h. Apparatus
and procedure
of measurement
Kinetic measurements were carried out in a glass flow microreactor, magnetically stirred, with a volume of about 5 cm3. The amount of catalyst varied between 0.05 and 1.5 g, according to the conditions of measurement. To accelerate the establishment of the stationary state the catalyst was pre-swollen in acetone for at least 1 h. Excess acetone was then removed, the reactor rinsed with the reaction mixture, and the reaction mixture fed at the required rate (15-45 cm3/h). The first sample for analysis was taken after 40 cm3 of the reaction mixture had passed through the reactor. After that, the rate of feeding was changed if necessary and further samples were taken after each pasage of 20 cm3 of the mixture. It was verified that such conditions were sufficient for
131 TABLE 1 Dependence of the reaction rate of bisphenol A synthesis on the particle size of the catalyst Molar ratio of acetone : phenol 1: 8,70 o C Particle size of the catalyst (mm)
r (mmol/g* h)
>0.8 0.63-0.80 0.35-0.63 0.22-0.35
19.2 30.3 28.2 31.8
the establishment of a stationary state of the reactor’s operation. Conversion was ascertained by measuring the ratio of the concentration of phenol and bisphenol A formed by liquid chromatography. A cartridge glass column ( CGC ) p?istroje, Praha) was used packed with Separon Six C&, 5 pm (Laboratorni in the measurements. The carrier phase was a water-methanol mixture (1: 2 ) . A variable-wavelength UV analyzer UVM-4 (V$vojove dfiny CSAV, Praha) , adjusted to the wavelength 290 nm, was used as the detector. In repeated measurements, using the same conditions, the statistical scatter of reaction rate values varied about 10%. RESULTS AND DISCUSSION
Control tests (Table 1) verified that with a catalyst of particle size 0.35-0.63 mm, the effect of ion-exchanger particle size on the results could be neglected. The first series of kinetic measurements were carried out at 70’ C. The effect of the acetone-to-phenol molar ratio on the reaction rate in the range between 1: 2 and 1: 32 was investigated. The conditions were chosen so that the conversion of acetone as the key component of the reaction mixture did not exceed 10%. For the initial stage of the data treatment the measured reaction rates (ratios of conversion to the space time W/F) were regarded as the initial reaction rates (r”) . The experimental data could then be correlated using simplified kinetic equations, derived with the supposition that the products had a negligible effect (Table 2 ) . In addition to the kinetic equations, in which various forms of interactions between the reactant and a single type of active’site were assumed (eqns. 1-11), equations corresponding to the views of Reinicker and Gates [l] were also included in the set (eqns. 12-14). According to these equations, the synthesis of bisphenol A on an ion-exchange catalyst proceeds with the participation of two interpenetrating phases. One of these phases, the polar one, contains acid sites and sorbs acetone only, while the other contains
132 TABLE 2 Set of kinetic equations Equation no.
Q(K)
Rate equation
(mmol’/g”- h’)
1240
(1)
r”=kcAcp/(l+KAcA+Kpcp)
(2)
r”=kcAc~/(l+KAcA+Kpcp)
2
(3)
r”=kcAcp/(l+KAcA+KPcp)
3
(4)
r”=kcAcp/(l+KAcA+KPcp)
(5)
r,,=kcAc~/(l+KAcA+Kpcp)
(6)
r”=kc,&/(1+K,\~~+K~~~)~
(7)
r”=k~~c~/(l+K~c~+K~c~)~
(8)
r”=kc,c~/(l+KAcA+Kpcp)
(9)
r”=kc,c2,/(1+K~c~+Kpcp)*
226
(10)
r”=kc~~~/(l+K~c~+K~c~)~
(11)
r~~=k~~c~/(l+I(ZAc~+K~c~)*
279 1056
4
544 1293 5297 885 417
4
296 246
(12)
~~‘=~~,c~/(~+K~c~)-(~+K~c~+K~c~)~
(13)
r~‘=kc,c~/(1+K~c,)~(1+K,~~+K~~~)~
296 246
(14)
r”=kcAc~/(l+K~c,)~(1+KAc,+Kp~p)4
219
r, reaction rate (mmol g- ’h-’ ) c, concentration (mol dm-“) k, rate constant (variable dimension) K, adsorption coefficient (dm” mol- ’)
su@cripts: A, acetone P, phenol
hydrocarbon chains on the polymer backbone and sorbs both acetone and phenol. The experimental data were processed using a non-linear regression method which utilizes Marquardt’s algorithm [ 5 1. The minimum sum of the squared deviations between the calculated and experimental reaction rate values Q(K) .zhieved with the individual models (the last column in Table 2 ) showed that the best description of the experimental data was obtained by using one of the equations of Reinicker and Gates (eqn. 14). However, a slightly higher Q(K) value corresponded to a considerably simpler equation (eqn. 9). Such a small difference between the adequacy of the description is probably due to the higher number of constants in eqn. (14) compared with eqn. (9) and does not justify a somewhat exotic view regarding the existence of two separate phases in an ion-exchange catalyst. Therefore, further development of the kinetic description was made using eqn. (9). The preferred high value of the exponent in the denominator (both eqn. 9 and 14 ) is in agreement with the observed dependence of ion-exchange catalyst activity on the change of concentration of active centres [ 61.
133 TABLE 3 Kinetic equation Equation
no.
including
the effect of water
Rate equation
Q(K).,,
(151 (16) subscript: W. water
The efects of the reaction products, water and bisphenol A on the reaction rate were examined separately by investigating changes in the reaction rate after the addition of one of the products to the reaction mixture of acetone and phenol in the molar ratio 1: 8. This ratio corresponded to the maximum in the dependence of the rate of bisphenol A synthesis on the ratio of the two reactants. The effect of wat.er was investigated for additions of 0.2-2.0 wt.% to the reaction mixture. The upper limit corresponds to almost 100% conversion of acetone in the reaction mixture used. Experimental data measured in the presence of water were correlated by eqn. (9) which was supplemented by two forms of the water term in the denominator (Table 3). With respect to large differences in the absolute values of the reaction rates in this assembly of data the optimal correlation was sought on the basis of the adjusted criterion Q(K) rel, Q
(K)
re,
=
‘=P
r
rtheor =P
The results showed (last column, Table 3) that eqn. (15) was much more adequate than eqn. (16). In Fig. 1, the dependence of the reaction rate on the concentration of water calculated using eqn. (15) is compared with the experimental data. The statistical scatter of the measured data from the original value is indicated at the particular experimental points. The fact that the starting compounds already contained a small quantity of water should however also be considered. According to the results of gas chromatographic analysis, the water content in the starting compounds after the purification procedures used was constant at about 0.11 wt.%. The reaction rate measured with a mixture without addition of water was therefore plotted twice. The solid line corresponds to zero water concentration in the reaction mixture, i.e. as estimated while determining the kinetic equation; the broken line indicates the reaction rate for real water concentration. Fig. 1 shows that in view of the high sensitivity of the reaction towards water concentration, the kinetic equation must contain the adsorption term reflecting the effect of water, and the actual concentration of water must be considered, even at low conversions. The results obtained in similar experiments with additions of bisphenol A
134
Fig. 1. Dependence of the reaction rate on water concentration in the reaction mixture calculated using eqn. (15); cw, molar water concentration in the mixture ( mol/dm3); r, reaction rate (mmol/g* h) ; o, experimental data.
are summarized in Table 4. They showed that the kinetic equation need not include the adsorption for b&phenol A. This reaction product is obviously not adsorbed by the catalyst. Table 5 shows the results of experiments performed at high values of the space time ( W/F), when relatively high conversions could be reached. A comparison between the reaction rates measured and calculated using eqn. (15) revealed that under these conditions a satisfactory description of the kinetics of synthesis of bisphenol A did not require a rate equation with the term for the reverse reaction. In a further series of measurements the data assembly was supplemented by data measured at 50, 60 and 85°C. The effect of both a change in the acetone-phenol ratio and the addition of water was evaluated. The final data set contained 101 experimental points. These data were treated by non-linear regression, using minimization of the relative sum of deviations squared, Q(K) rel.For all data in the set, the concentration of water was calculated as TABLE 4 Comparison of reaction rates measured after the addition of bisphenol A to the reaction mixture with values calculated from eqn. (15) Content of bisphenol A ( mol/dm” ) 0.207 0.443
P (mmol/g*h) Experimental
Calculated
23.3 19.3
21.8 19.3
135
TABLE 5 Comparison of reaction from eqn. (15)
rates calculated
at higher conversions
of acetone with values calculated
Molar ratio acetone: phenol 1: 8,7O”C Conversion acetone (%)
of
17.9 25.0 34.2 44.8
r (mmol/g.
h)
Experimental
Calculated
11.1 7.8 5.3 3.4
11.7 8.0 4.9 2.8
the sum of the initial water content (0.11wt.% ) , of the increase in the water content due to the reaction (on the basis of acetone conversion), and of the addition of water to the reaction mixture (if any). The acetone and phenol concentrations were also corrected for the effect of conversion. The whole set was treated at once; for all constants of eqn. (15) a temperature dependence was assumed according to the Arrhenius or Van ‘t Hoff equation, respectively. The results thus obtained are given in Table 6. A pronounced temperature dependence was found only for the rate constant, k, and for the adsorption coefficient of water, K,. This corresponds to the assumed physical meaning of constants of the kinetic equation. The constant k (a multiple of the rate constant of the surface reaction and of the adsorption coefficients of acetone and phenol) increased with temperature, while the adsorption coefficient of water, as the equilibrium constant of an exothermal reaction between water and -S03H groups, decreased with temperature. The energy of interaction of acetone and phenol with the catalyst was considerably weaker. The observed temTABLE 6 Two variants of constants determined for eqn. (15) in the treatment of experimental data for all temperatures together assuming that the temperature dependences of all constants obey the Arrhenius equation (see text) k
Q(K),,,
KV
KP
K.4
A’
E *t
A
E
A
E
A
E
11100 24600
17400 20500
0.102 0.479
- 4450 0
0.144 0.110
400 0
0.00244 0.00214
-21100 -21200
*A, pre-exponential factor in the Arrhenius **E, activation energy or heat of adsorption
or Van ‘t Hoff equation, (J mol-‘).
variable dimension.
4.48 4.77
136
perature dependence of KA and KF was so small, that within the measured temperature range a satisfactory description of the kinetics may be obtained also using values of the adsorption coefficients of acetone and phenol which are independent of temperature (second line in Table 6). For comparison, the complete data set was also treated by using an equation based on views forwarded by Reinicker and Gates [ 11 and derived from eqn. (14) by adding a term for water adsorption. The minimum Q(K) rel= 10.7 thus obtained, however, was much larger than in the case of eqn. (15) (Table 5). The isothermal data were best described by the original eqn. (14) (Table 2); however, the temperature dependence of constants of this equation obviously does not satisfy exponential relations, and the equation was therefore found to be unsatisfactory for a description of kinetic data measured at several temperature levels simultaneously. An important step in the verification of the validity of the kinetic equation was its comparison with integral data. The first series of measurements was performed with a reactor provided with a fixed catalyst bed consisting of a glass tube of inner diameter 8 mm. The catalyst packing (3 g in the dry state) occupied a bed height (in the working condition) of 17 cm. This is much less than in industrial reactors of this type, where the bed height is usually greater than 1.5 m. Due to this, the linear flow-rate of the reaction mixture through the reactor was very low, and the functioning of the catalyst was considerably affected by external diffusion. To obtain a fit between experiment and theory, the theoretical values had to be multiplied by an effectiveness factor fr0.4. Under laboratory conditions it was not possible to increase the linear flow-rate of the reaction mixture in the stationary catalyst bed high enough to guarantee elimination of the effect of external diffusion. Therefore, to obtain integral data, a stirred batch reactor had to be used. The transformation of an ion-exchange catalyst from the dry state in which it is stored into its working swollen state requires a considerable time [6,7]. The experiments were therefore arranged so that a glass microreactor was first used in the regime of a stirred flow reactor, similar to the preceeding experiments, until a stationary state was reached. After that, the feeding of the reaction mixture was stopped and the reactor then passed to the regime of a stirred batch reactor. Samples (about 10 mm3) were taken from the reactor at suitable time intervals. After the experiment had been completed, the amount of the reaction mixture in the reactor was determined by weighing. It was verified t.hat sampling during the experiment could cause a negligible error of 1 wt.% of the reactor charge at most. The value of W/J’was calculated from the mass of the reactor charge and the time of the reaction, starting from the cessation of flow of the reaction mixture. For the graphic presentation in Fig. 2 a correction was added for the W/F theoretically needed in an integral reactor for a conversion reached at the moment of transition from the regime of stirred flow reactor to that of batch reactor. The curves in Fig. 2 are the result of
137
Fig. 2. Comparison of experimental and theoretical dependences of conversion n ( % ) on the space time W/F (g cat* h/mol acetone) for various temperatures. Starting molar ratio phenol : acetone, F/A = 8. Dashed curves calculated by integrating eqn. (15) using the constants listed in Table 5 (variant with temperature-independent constants KA, Kp) , solid lines are calculated similarly but using eqn. (17).
integration of kinetic equations. When eqn. (15)) with the values of contents shown in Table 5 (variant with temperature independent constants KA and KF) was used at a temperature of 85 aC a systematic deviation from the experimental results was found (dashed line in Fig. 2) * This is probably the consequence of the influence of a back reaction. We modified the kinetic equation by adding the simple term for a back reaction (eqn. 17 ) r=k(cA&-
(1/K)cncw)/(1+K2,~~+Kp~p+KW~w)4
(17)
where K is the equilibrium constant and cn is the concentration of bisphenol A. The value of K was found by an iteration procedure for which the sum of squared differences of experimental values of W/F and values obtained by integration of eqn. (17 ) was a minimum. Application of a similar procedure for the data obtained at lower temperatures was not possible as the expected effect of a back reaction was too small. Instead, it can.be supposed that the temperature dependence of K follows the Van ‘t Hoff equation. Reaction enthalpy of synthesis of bisphenol A is 90 kJ mol-l and the pre-exponential factor can be determined from the known value of K for 85°C. The resulting equation for K has the form Kz4.3.
lo-l5 exp (90 000/8.314 7’)
(18)
The solid lines in Fig. 2 are the result of integration of eqn. (17). It is evident that this kinetic equation satisfactorily describes the synthesis of bisphenol A within the whole range of conditions which appear in technological processes. Its successful application to the description of the behaviour of an industrial reactor will be reported elsewhere.
138 REFERENCES 1 2 3 4 5 6 7
R.A. Reinicker and B.C. Gates, AIChE J., 20 (1974) 933. F.N. Apel, L.B. Conte and H.L. Bender (Union Carbide), U.S. Patent 3 049 568 (1962). R.B. Wagner (Hercules Powder), U.S. Patent 3 172 916 (1965). B.W. McNutt and B.B. Gammill (Dow Chemical), U.S. Patent 3 394 089 (1968). D.W. Marquardt, J. Sot. Ind. Appl. Math., 11 (1963) 431. K. Je%ibek and K. Setinek, J. Mol. Catal., 39 (1987) 161. H.W. Heath and B.C. Gates, AIChE J., 18 (1972) 321.
129
Printed in The Netherlands
Kinetics of the Synthesis of Bisphenol A K. JERABEK*, J. ODNOHA* and K. SETINEK Institute of Chemical Process Fundamentals, CzechoslovakAcademy of Sciences, 165 02 Prague 6 (Czechoslovakia) (Received 15 April 1987, accepted 24 August 1987)
ABSTRACT The kinetics of the synthesis of bisphenol A from acetone and phenol on an ion-exchange catalyst promoted by partial neutralization of acid groups with 2-mercaptoethyl amine was investigated in the temperature range 50-85°C. A Langmuir-Hishelwood type rate equation was used for treatment of the experimental data. The rate constant and the adsorption coefficient of water as the reaction product are temperature dependent, while the adsorption coefficient of phenol and acetone can be regarded as constants within the given range of temperatures. The effect of adsorption of bisphenol A need not be included in the kinetic equation. The effect of the reverse reaction is apparent at temperatures higher than 7O’C. The equation thus obtained may be used in a description of integral data up to high conversions.
INTRODUCTION
2,2-Bis (4’ -hydroxyphenyl)propane (commercial names dian, bisphenol A) is one of the starting materials in the production of epoxy resins and polycarbonates. On an industrial scale it is produced by the acid-catalyzed condensation of acetone and phenol. Among others strongly acidic ion exchangers are used as the catalysts in this synthesis. Their advantages are low corrosion of the production equipment and the possibility of continuous production. The rate and selectivity of this reaction are greatly improved by the presence of compounds containing a mercapto group in the reaction mixture. There is very little information in the literature on the kinetics of this condensation; the only study of it was published by Reinicker and Gates [ 11.They used a stirred batch reactor and investigated the kinetics at one temperature only (91 “C ) , in the absence of a promoter containing a mercapto group. They tried to investigate a modified catalyst containing sulphhydryl grouns, but the chosen procedure, namely partial esterification of the acid groups with 2-mercaptoethanol, was unsuitable for this purpose. This procedure has been described in the patent literature [ 21, but it does not result in the assumedbonding *Present address: Research institute for Chemical Equipments, Prague, Czechoslovakia.
0166-9834/88/$03.50
0 1988 Elsevier Science Publishers B.V.
130
of the promoter to the ion exchanger. We have found, however, that instead of this, dehydration of 2-mercaptoethanol takes place and the product of this dehydration contaminates the polymer. Their results made Reinicker and Gates [l] come to the conclusion, which was in fact wrong, that compounds with sulphhydryl groups bound to the ion exchanger did not possess a promoting effect. It is known, however, that by employing other methods of modification, e.g. the partial reduction of sulpho groups [ 31 or the partial neutralization of the acidic groups with cysteamine (2-mercaptoethyl amine) [ 41 it is possible to significantly raise both the reaction rate (ca. five to ten times) and the selectivity. In the present work we have investigated the kinetics of the synthesis of bisphenol A using a catalyst modified with cysteamine. We took measurements within a sufficiently broad range of temperatures, so that the eventual description could also be employed for an industrial reactor operating in a non-isothermal regime. EXPERIMENTAL
Chemicals
Acetone, reagent grade (Lachema, Brno) was dried over a molecular sieve. Phenol, analytical grade (Reactivul, Rumania) was repurified by distillation. Gel-type ion exchanger Ostion KSC-1 (Spolchemie, astf n/L), declared divinylbenzene content 4%, was recycled between the Na+ and H+ form four times (the last recycling to the H+ form), washed with distilled water and eluted with a 10% aqueous solution of cysteamine hydrochloride (Cillag Chemie, Switzerland), in an amount corresponding to 15% of the capacity of the acid ion exchanger. The ion exchanger was left to stand overnight in contact with this solution. On the following day it was washed with distilled water and dried at 110°C for 12 h. Apparatus
and procedure
of measurement
Kinetic measurements were carried out in a glass flow microreactor, magnetically stirred, with a volume of about 5 cm3. The amount of catalyst varied between 0.05 and 1.5 g, according to the conditions of measurement. To accelerate the establishment of the stationary state the catalyst was pre-swollen in acetone for at least 1 h. Excess acetone was then removed, the reactor rinsed with the reaction mixture, and the reaction mixture fed at the required rate (15-45 cm3/h). The first sample for analysis was taken after 40 cm3 of the reaction mixture had passed through the reactor. After that, the rate of feeding was changed if necessary and further samples were taken after each pasage of 20 cm3 of the mixture. It was verified that such conditions were sufficient for
131 TABLE 1 Dependence of the reaction rate of bisphenol A synthesis on the particle size of the catalyst Molar ratio of acetone : phenol 1: 8,70 o C Particle size of the catalyst (mm)
r (mmol/g* h)
>0.8 0.63-0.80 0.35-0.63 0.22-0.35
19.2 30.3 28.2 31.8
the establishment of a stationary state of the reactor’s operation. Conversion was ascertained by measuring the ratio of the concentration of phenol and bisphenol A formed by liquid chromatography. A cartridge glass column ( CGC ) p?istroje, Praha) was used packed with Separon Six C&, 5 pm (Laboratorni in the measurements. The carrier phase was a water-methanol mixture (1: 2 ) . A variable-wavelength UV analyzer UVM-4 (V$vojove dfiny CSAV, Praha) , adjusted to the wavelength 290 nm, was used as the detector. In repeated measurements, using the same conditions, the statistical scatter of reaction rate values varied about 10%. RESULTS AND DISCUSSION
Control tests (Table 1) verified that with a catalyst of particle size 0.35-0.63 mm, the effect of ion-exchanger particle size on the results could be neglected. The first series of kinetic measurements were carried out at 70’ C. The effect of the acetone-to-phenol molar ratio on the reaction rate in the range between 1: 2 and 1: 32 was investigated. The conditions were chosen so that the conversion of acetone as the key component of the reaction mixture did not exceed 10%. For the initial stage of the data treatment the measured reaction rates (ratios of conversion to the space time W/F) were regarded as the initial reaction rates (r”) . The experimental data could then be correlated using simplified kinetic equations, derived with the supposition that the products had a negligible effect (Table 2 ) . In addition to the kinetic equations, in which various forms of interactions between the reactant and a single type of active’site were assumed (eqns. 1-11), equations corresponding to the views of Reinicker and Gates [l] were also included in the set (eqns. 12-14). According to these equations, the synthesis of bisphenol A on an ion-exchange catalyst proceeds with the participation of two interpenetrating phases. One of these phases, the polar one, contains acid sites and sorbs acetone only, while the other contains
132 TABLE 2 Set of kinetic equations Equation no.
Q(K)
Rate equation
(mmol’/g”- h’)
1240
(1)
r”=kcAcp/(l+KAcA+Kpcp)
(2)
r”=kcAc~/(l+KAcA+Kpcp)
2
(3)
r”=kcAcp/(l+KAcA+KPcp)
3
(4)
r”=kcAcp/(l+KAcA+KPcp)
(5)
r,,=kcAc~/(l+KAcA+Kpcp)
(6)
r”=kc,&/(1+K,\~~+K~~~)~
(7)
r”=k~~c~/(l+K~c~+K~c~)~
(8)
r”=kc,c~/(l+KAcA+Kpcp)
(9)
r”=kc,c2,/(1+K~c~+Kpcp)*
226
(10)
r”=kc~~~/(l+K~c~+K~c~)~
(11)
r~~=k~~c~/(l+I(ZAc~+K~c~)*
279 1056
4
544 1293 5297 885 417
4
296 246
(12)
~~‘=~~,c~/(~+K~c~)-(~+K~c~+K~c~)~
(13)
r~‘=kc,c~/(1+K~c,)~(1+K,~~+K~~~)~
296 246
(14)
r”=kcAc~/(l+K~c,)~(1+KAc,+Kp~p)4
219
r, reaction rate (mmol g- ’h-’ ) c, concentration (mol dm-“) k, rate constant (variable dimension) K, adsorption coefficient (dm” mol- ’)
su@cripts: A, acetone P, phenol
hydrocarbon chains on the polymer backbone and sorbs both acetone and phenol. The experimental data were processed using a non-linear regression method which utilizes Marquardt’s algorithm [ 5 1. The minimum sum of the squared deviations between the calculated and experimental reaction rate values Q(K) .zhieved with the individual models (the last column in Table 2 ) showed that the best description of the experimental data was obtained by using one of the equations of Reinicker and Gates (eqn. 14). However, a slightly higher Q(K) value corresponded to a considerably simpler equation (eqn. 9). Such a small difference between the adequacy of the description is probably due to the higher number of constants in eqn. (14) compared with eqn. (9) and does not justify a somewhat exotic view regarding the existence of two separate phases in an ion-exchange catalyst. Therefore, further development of the kinetic description was made using eqn. (9). The preferred high value of the exponent in the denominator (both eqn. 9 and 14 ) is in agreement with the observed dependence of ion-exchange catalyst activity on the change of concentration of active centres [ 61.
133 TABLE 3 Kinetic equation Equation
no.
including
the effect of water
Rate equation
Q(K).,,
(151 (16) subscript: W. water
The efects of the reaction products, water and bisphenol A on the reaction rate were examined separately by investigating changes in the reaction rate after the addition of one of the products to the reaction mixture of acetone and phenol in the molar ratio 1: 8. This ratio corresponded to the maximum in the dependence of the rate of bisphenol A synthesis on the ratio of the two reactants. The effect of wat.er was investigated for additions of 0.2-2.0 wt.% to the reaction mixture. The upper limit corresponds to almost 100% conversion of acetone in the reaction mixture used. Experimental data measured in the presence of water were correlated by eqn. (9) which was supplemented by two forms of the water term in the denominator (Table 3). With respect to large differences in the absolute values of the reaction rates in this assembly of data the optimal correlation was sought on the basis of the adjusted criterion Q(K) rel, Q
(K)
re,
=
‘=P
r
rtheor =P
The results showed (last column, Table 3) that eqn. (15) was much more adequate than eqn. (16). In Fig. 1, the dependence of the reaction rate on the concentration of water calculated using eqn. (15) is compared with the experimental data. The statistical scatter of the measured data from the original value is indicated at the particular experimental points. The fact that the starting compounds already contained a small quantity of water should however also be considered. According to the results of gas chromatographic analysis, the water content in the starting compounds after the purification procedures used was constant at about 0.11 wt.%. The reaction rate measured with a mixture without addition of water was therefore plotted twice. The solid line corresponds to zero water concentration in the reaction mixture, i.e. as estimated while determining the kinetic equation; the broken line indicates the reaction rate for real water concentration. Fig. 1 shows that in view of the high sensitivity of the reaction towards water concentration, the kinetic equation must contain the adsorption term reflecting the effect of water, and the actual concentration of water must be considered, even at low conversions. The results obtained in similar experiments with additions of bisphenol A
134
Fig. 1. Dependence of the reaction rate on water concentration in the reaction mixture calculated using eqn. (15); cw, molar water concentration in the mixture ( mol/dm3); r, reaction rate (mmol/g* h) ; o, experimental data.
are summarized in Table 4. They showed that the kinetic equation need not include the adsorption for b&phenol A. This reaction product is obviously not adsorbed by the catalyst. Table 5 shows the results of experiments performed at high values of the space time ( W/F), when relatively high conversions could be reached. A comparison between the reaction rates measured and calculated using eqn. (15) revealed that under these conditions a satisfactory description of the kinetics of synthesis of bisphenol A did not require a rate equation with the term for the reverse reaction. In a further series of measurements the data assembly was supplemented by data measured at 50, 60 and 85°C. The effect of both a change in the acetone-phenol ratio and the addition of water was evaluated. The final data set contained 101 experimental points. These data were treated by non-linear regression, using minimization of the relative sum of deviations squared, Q(K) rel.For all data in the set, the concentration of water was calculated as TABLE 4 Comparison of reaction rates measured after the addition of bisphenol A to the reaction mixture with values calculated from eqn. (15) Content of bisphenol A ( mol/dm” ) 0.207 0.443
P (mmol/g*h) Experimental
Calculated
23.3 19.3
21.8 19.3
135
TABLE 5 Comparison of reaction from eqn. (15)
rates calculated
at higher conversions
of acetone with values calculated
Molar ratio acetone: phenol 1: 8,7O”C Conversion acetone (%)
of
17.9 25.0 34.2 44.8
r (mmol/g.
h)
Experimental
Calculated
11.1 7.8 5.3 3.4
11.7 8.0 4.9 2.8
the sum of the initial water content (0.11wt.% ) , of the increase in the water content due to the reaction (on the basis of acetone conversion), and of the addition of water to the reaction mixture (if any). The acetone and phenol concentrations were also corrected for the effect of conversion. The whole set was treated at once; for all constants of eqn. (15) a temperature dependence was assumed according to the Arrhenius or Van ‘t Hoff equation, respectively. The results thus obtained are given in Table 6. A pronounced temperature dependence was found only for the rate constant, k, and for the adsorption coefficient of water, K,. This corresponds to the assumed physical meaning of constants of the kinetic equation. The constant k (a multiple of the rate constant of the surface reaction and of the adsorption coefficients of acetone and phenol) increased with temperature, while the adsorption coefficient of water, as the equilibrium constant of an exothermal reaction between water and -S03H groups, decreased with temperature. The energy of interaction of acetone and phenol with the catalyst was considerably weaker. The observed temTABLE 6 Two variants of constants determined for eqn. (15) in the treatment of experimental data for all temperatures together assuming that the temperature dependences of all constants obey the Arrhenius equation (see text) k
Q(K),,,
KV
KP
K.4
A’
E *t
A
E
A
E
A
E
11100 24600
17400 20500
0.102 0.479
- 4450 0
0.144 0.110
400 0
0.00244 0.00214
-21100 -21200
*A, pre-exponential factor in the Arrhenius **E, activation energy or heat of adsorption
or Van ‘t Hoff equation, (J mol-‘).
variable dimension.
4.48 4.77
136
perature dependence of KA and KF was so small, that within the measured temperature range a satisfactory description of the kinetics may be obtained also using values of the adsorption coefficients of acetone and phenol which are independent of temperature (second line in Table 6). For comparison, the complete data set was also treated by using an equation based on views forwarded by Reinicker and Gates [ 11 and derived from eqn. (14) by adding a term for water adsorption. The minimum Q(K) rel= 10.7 thus obtained, however, was much larger than in the case of eqn. (15) (Table 5). The isothermal data were best described by the original eqn. (14) (Table 2); however, the temperature dependence of constants of this equation obviously does not satisfy exponential relations, and the equation was therefore found to be unsatisfactory for a description of kinetic data measured at several temperature levels simultaneously. An important step in the verification of the validity of the kinetic equation was its comparison with integral data. The first series of measurements was performed with a reactor provided with a fixed catalyst bed consisting of a glass tube of inner diameter 8 mm. The catalyst packing (3 g in the dry state) occupied a bed height (in the working condition) of 17 cm. This is much less than in industrial reactors of this type, where the bed height is usually greater than 1.5 m. Due to this, the linear flow-rate of the reaction mixture through the reactor was very low, and the functioning of the catalyst was considerably affected by external diffusion. To obtain a fit between experiment and theory, the theoretical values had to be multiplied by an effectiveness factor fr0.4. Under laboratory conditions it was not possible to increase the linear flow-rate of the reaction mixture in the stationary catalyst bed high enough to guarantee elimination of the effect of external diffusion. Therefore, to obtain integral data, a stirred batch reactor had to be used. The transformation of an ion-exchange catalyst from the dry state in which it is stored into its working swollen state requires a considerable time [6,7]. The experiments were therefore arranged so that a glass microreactor was first used in the regime of a stirred flow reactor, similar to the preceeding experiments, until a stationary state was reached. After that, the feeding of the reaction mixture was stopped and the reactor then passed to the regime of a stirred batch reactor. Samples (about 10 mm3) were taken from the reactor at suitable time intervals. After the experiment had been completed, the amount of the reaction mixture in the reactor was determined by weighing. It was verified t.hat sampling during the experiment could cause a negligible error of 1 wt.% of the reactor charge at most. The value of W/J’was calculated from the mass of the reactor charge and the time of the reaction, starting from the cessation of flow of the reaction mixture. For the graphic presentation in Fig. 2 a correction was added for the W/F theoretically needed in an integral reactor for a conversion reached at the moment of transition from the regime of stirred flow reactor to that of batch reactor. The curves in Fig. 2 are the result of
137
Fig. 2. Comparison of experimental and theoretical dependences of conversion n ( % ) on the space time W/F (g cat* h/mol acetone) for various temperatures. Starting molar ratio phenol : acetone, F/A = 8. Dashed curves calculated by integrating eqn. (15) using the constants listed in Table 5 (variant with temperature-independent constants KA, Kp) , solid lines are calculated similarly but using eqn. (17).
integration of kinetic equations. When eqn. (15)) with the values of contents shown in Table 5 (variant with temperature independent constants KA and KF) was used at a temperature of 85 aC a systematic deviation from the experimental results was found (dashed line in Fig. 2) * This is probably the consequence of the influence of a back reaction. We modified the kinetic equation by adding the simple term for a back reaction (eqn. 17 ) r=k(cA&-
(1/K)cncw)/(1+K2,~~+Kp~p+KW~w)4
(17)
where K is the equilibrium constant and cn is the concentration of bisphenol A. The value of K was found by an iteration procedure for which the sum of squared differences of experimental values of W/F and values obtained by integration of eqn. (17 ) was a minimum. Application of a similar procedure for the data obtained at lower temperatures was not possible as the expected effect of a back reaction was too small. Instead, it can.be supposed that the temperature dependence of K follows the Van ‘t Hoff equation. Reaction enthalpy of synthesis of bisphenol A is 90 kJ mol-l and the pre-exponential factor can be determined from the known value of K for 85°C. The resulting equation for K has the form Kz4.3.
lo-l5 exp (90 000/8.314 7’)
(18)
The solid lines in Fig. 2 are the result of integration of eqn. (17). It is evident that this kinetic equation satisfactorily describes the synthesis of bisphenol A within the whole range of conditions which appear in technological processes. Its successful application to the description of the behaviour of an industrial reactor will be reported elsewhere.
138 REFERENCES 1 2 3 4 5 6 7
R.A. Reinicker and B.C. Gates, AIChE J., 20 (1974) 933. F.N. Apel, L.B. Conte and H.L. Bender (Union Carbide), U.S. Patent 3 049 568 (1962). R.B. Wagner (Hercules Powder), U.S. Patent 3 172 916 (1965). B.W. McNutt and B.B. Gammill (Dow Chemical), U.S. Patent 3 394 089 (1968). D.W. Marquardt, J. Sot. Ind. Appl. Math., 11 (1963) 431. K. Je%ibek and K. Setinek, J. Mol. Catal., 39 (1987) 161. H.W. Heath and B.C. Gates, AIChE J., 18 (1972) 321.