I n d . Eng. Chem, Res. 1988,27, 41-45
GA, PTBC, and TBHQ, regeneration was tried but no measurable rate of absorption of pure oxygen was observed. Thus, it seems unlikely on the basis of the above experiments as well as the chemistry of these oxidation reactions that a regenerative process for obtaining pure oxygen is possible with these PHBs. However, for removal of oxygen a t very low levels, these may have some practical applications.
Conclusions The absorption of oxygen in aqueous alkaline solutions of PTBC and TBHQ, under certain conditions, was found to conform to the fast pseudo-first-order reaction regime. The reaction was found to be first order in oxygen and PTBC or TBHQ. The absorption of oxygen in aqueous alkaline solutions of PG, GA, and TMHQ was found to conform to the instantaneous reaction regime. However, for very low alkali to PHB ratios, under certain conditions for PG, the absorption conformed to the pseudo-first-order reaction regime. The absorption of oxygen at very low partial pressures in aqueous PG containing potassium hydroxide can become gas phase controlled. Acknowledgment
A.V.P. is thankful to the University Grants Commission, New Delhi, for the award of Research Fellowship. Nomenclature [A*] = concentration of solute gas A at the gas-liquid interface, kmol/m3 [Bo] = concentration of liquid-phase reactant B in the bulk, kmol/m3 DA = diffusivity of dissolved solute gas in the liquid phase, mz/s DB = diffusivity of liquid-phase reactant, mz/s k = pseudo-first-order reaction rate constant, s-l k 2 = rate constant for second-order reaction, m3/(kmol.s) K , = physical mass-transfer coefficient on gas side, kmol/ (m2.s.atm) k L = physical mass-transfer coefficient on liquid side, m/s P A = partial pressure of gas A in the gas phase, atm R A = specific rate of absorption of gas A, kmol/(m2.s)
41
T = absolute temperature, K 2 = number of moles of the liquid-phase reactant reacting with 1 mole of dissolved gas Greek Symbol p =
viscosity of the reacting solution, P
Registry No. PG, 87-66-1; PTBC, 98-29-3; TBHC, 1948-33-0; TMHQ, 700-13-0; GA, 149-91-7; 0 2 , 7782-44-7.
Literature Cited Campbell, T. W. J. Am. Chem. SOC.1951, 73, 4190. Critchlow, A.; Haworth, R. D.; Pauson, P. L. J . Chem. SOC.1951, 1318. Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970; Chapter 1. Doraiswmv. L. K.: Sharma. M. M. Heteroeeneous Reactions., Wilev: " New Yoik, 1984; Vol. 11: Duncan, I. A,; Harriman, A,; Porter, G. Anal. Chem. 1979,51,2206. Hitchman, M. L. Measurement of Dissolved Oxygen; Wiley: New York, 1978; Chapter 8. Jhaveri, A. S.; Sharma, M. M. Chem. Eng. Sci. 1967,22, 1. Lange, T. Lange's Handbook of Chemistry, 13th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1985; Chapter 10. Langmaack, L. Ger. Offen 2000082, 1971; Chem. Abstr. 1971, 75, 119346f. Nierenstein, M. J. Chem. SOC.1915, 107, 1217. Pospisil, J.; Ettel, V. Collect. Czech. Chem. Commun. 1959,24, 729. Pratt, K. C.; Wakeham, W. A. Proc. R. SOC.London, Ser. A 1974, A336, 393. Rothe, A. G. Ger. Offen. DE 3 316 594,1984; Chem. Abstr. 1985,102, 64359f. Sharma, M. M.; Danckwerts, P. V. Chem. Eng. Sci. 1963, 18, 722. Sridharan, K.; Sharma, M. M. Chem. Eng. Sci. 1976, 31, 767. Takahashi, M.; Ito, H.; Takeuchi, H. Kagaku Kogaku Ronbunshu 1980, 6, 597; Chem. Abstr. 1980, 93, 222431a. Takeuchi, H.; Takahashi, K.; Hoshino, T.; Takahashi, M. Chem. Eng. Commun. 1980, 4, 181. Taylor, G. I. Proc. R. SOC.London, Ser. A 1953, A219, 186. Trivedi, R. N.; Vasudeva, K. Chem. Eng. Sci. 1975,30, 317. Vogel, A. I. A Text-Book of Quantitatiue Inorganic Analysis, 3rd ed.; The ELBS and Longman: London, 1975; Chapter 21. Wise, D. L.; Houghton, G. Chem. Eng. Sci. 1966,21, 999. Yadav, G. D. Ph.D. (Tech.) Thesis, University of Bombay, India, 1980. L
Received for review February 13, 1987 Revised manuscript received August 10, 1987 Accepted September 17, 1987
Activity Difference between the Internal and External Sulfonic Groups of Macroreticular Ion-Exchange Resin Catalysts in Isobutylene Hydration Son-Ki Ihm,* Moon-Jo Chung, and Kun-You Park Department of Chemical Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea
Two different types of macroreticular resin catalysts, Amberlyst XN-1010 and Amberlyst 15, were used in the hydration of isobutylene. T h e reaction was found to be diffusion-limited, and the experimental results were interpreted by a two-phase model. The internal active sites were believed t o be more active than the external ones. T h e intrinsic reaction rate constants and the activation energies were estimated for the internal and external active sites, respectively.
I. Introduction Cation-exchange resins are used in many organic synthesis processes. The macroreticular resins are usually more useful than the gel form resins, because even nonpolar and nonswelling reactants can easily diffuse in the macroreticular resins through the macropores and can be catalyzed on the macropore walls. The reaction and mass transfer in macroreticular resins were investigated by several authors, a two-phase model 0S88-5885/8S/2627-0041$01.50/0
was suggested by Ihm et al. (1982), and the model was applied by Ihm and Oh (1984) in the interpretation of the sucrose inversion catalyzed by macroreticular resins. The active sites of cation-exchange resin are sulfonic acid groups. Gates et al. (1972) observed that the activity became higher with the increase of the active site concentration in the dehydration of tert-butyl alcohol, and they proposed the mechanism of hydrogen bridge between the reactant and the network of sulfonic acid groups. Dooley 0 1988 American Chemical Society
42 hid. Eng. Chem. Res., Vol. 27, No. 1, 1988
et al. (1982) suggested that the active sites on the surface of gel microparticles are less active than those within the microparticles in the reesterification reaction. Chee and Ihm (1986) also suggested the possibility of higher activity of the internal sulfonic acid groups than the external ones in their study on the deactivation behavior of macroreticular resins for the ethanol dehydration. In the present work, the activity difference between the external and internal active sites of macroreticular catalysts was investigated in the hydration of isobutylene, using two different types of macroreticular cation-exchange resins.
11. Theoretical Consideration Macroreticular resins consist of very small randomly packed gel microparticles with continuous macropores. A fraction of the total active sites are distributed on the surface of the microparticles, and the rest are distributed within the gel microparticle. If the volume of the reactant solution is very large compared to the volume of the resin catalyst in it, the rate of concentration change of the bulk solution will be negligible in view of the residence time of the reactant inside the resin, and the resin particles can be assumed to be in a quasi-stationary state. On the assumptions that the resin particles are spherical and that the effective diffusivity of the reactant is constant, the material balances can be established for a first-order irreversible reaction. It should be noted that the rate constants for the internal and external active sites are discriminated to be ki and k,, respectively. microparticles
Table I. Properties of Resins properties capacity, mequiv/g % of surface, SOBH(XlOO) internal surface area, m2/g porosity, vol 70 av pore diameter, A cross-linkage, 70 % swelling in water
Amberlyst 15 4.7 4.39 50 36 240 20-25 60-70
Amberlyst XN-1010 3.3 52.8 540 50 51
-75 5-10
and the effectiveness factors, vi and aa, accounting for the diffusion in the gel microparticle and the macropore space, respectively, can be written as ai
"(
= mi
coth m i-
k)
The effectiveness factors can be regarded as the ratio of the observed reaction rates to the intrinsic reaction rates. Accordingly, the observed reaction rates can be expressed in terms of the effectiveness factor and the intrinsic reaction rates for the external and internal active sites. (10) oov = Ita[rea + (1 - ~ I o i r i i l If it is assumed that the intrinsic kinetics of the external and internal functional groups have the same dependence on the concentration, eq 10 becomes (11) BO" = k,,C = [ykaC + (1- r)kiC?ji]qa = hap,v,C where kapp
resin particles
reactor
where C is the concentration in the bulk. The appropriate boundary conditions are given as Ci = C, at ri = Ri and dCi/dri = 0 at ri = 0 C, = C at ra = R, and dC,/dra = 0 at r, = 0 Equations 1 and 2 can be solved with the boundary conditions to give
where
m i= Ri(
(1 - y)eki nViDi
)
= yka
+ (1 - Yhiki
(12)
111. Experimental Section Two types of macroreticular resins, Amberlyst 15 and Amberlyst XN-1010, were supplied by Rohm and Haas Co., and their properties are shown in Table I. The resins were washed with methanol and pure water in a packed column, activated by 10% sulfuric acid, and washed with pure water until the pH became 6. The activated resins were dried in air and classified by screening into two size groups of 25-30 and 35-40 mesh, and the broken beads were removed by rolling them on an inclined flat plate. About 10 g of resins of each group was weighed and dried at 100 "C for 1 2 h. The dried resins were weighed accurately and humidified to keep them from being cracked. A 1-L high-pressure reactor equipped with a variablespeed stirrer, electric band heater, and temperature controller was filled with 500 g of pure water and the prepared resin catalyst. The air in the reactor was removed by a vacuum pump, and 50-70 g of pure isobutylene (99.8%) was charged into the reactor through a nozzle on the top. The reactor was heated without stirring. In the mean time, the resins sank at the bottom of the reactor and the liquid isobutylene layer floated above the water layer. Once the desired temperature was reached, the reaction was initiated by stirring the mixture rigorously at above 800 rpm. About 2 mL of the reaction mixture was sampled 3-4 times at intervals of 20-30 min. The unreacted isobutylene was vaporized from the sample, and the product tert-butyl alcohol was analyzed by a gas chromatograph with a 80/100-mesh Porapak Q column. The average sizes of the resins were obtained from the photographs of the swollen resins magnified 10 times. The capacities of the resins used in each experiment were measured by exchanging the hydrogen ions with 5% NaCl
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 43 1
I
I
/
0
---- R = 0.3.49mm
R = 0 251 mm
0-
/
1
0
1
5 I
I
I
N
0
----
I
I
R = 0.336 mm
Time ( hr )
Figure 2. Effect of temperature on the reaction rate with Amberlyst XN-1010.
1
I
I
1
2
3
I
The average effectiveness factors accounting for the diffusion in the gel microparticles, qi, and qi2, can be assumed to be equal since the intrinsic rate constant, ki, and the diffusivity, Di, are constant at a given temperature and the size of gel microparticles, Ri, are not directly affected by the resin size, R,. Therefore eq 13 becomes
Time ( h r )
Figure 1. Effect of temperature on the reaction rate with Amberlyst 15.
solution and titrating them with 1 N NaOH solution.
IV. Results and Discussion The reaction rates could be limited by the external and internal mass-transfer rgsistances. Above 800 rpm, the reaction rate was not changed with the increase of the mixing intensity, and it can be assumed that the external mass-transfer resistance becomes negligible above 800 rpm. The internal mass transfer consists of the diffusion of reactants through the macropore space and their permeation into the swollen gel microparticles. Under the same reaction conditions, the reaction rate with larger resin beads was observed slower than that with smaller resins, and it can be assured that the internal mass-transfer resistance introduces appreciable influence on the overall reaction rate. The reaction rates at each temperature of 50,60,70, and 80 O C were measured by the slopes of the curves as shown in Figures 1 and 2. The solubility of isobutylene in water is very low and has a value of 1.32 X mol/L a t 80 "C under its own vapor pressure. Accordingly the reactant solution contains excess water, and the reaction could be assumed to be independent of the water concentration. The diffusivity of isobutylene can be assumed constant due to its low solubility in water. Gupta and Douglas (1967) investigated the reaction kinetics on the hydration of isobutylene with gel-form cation-exchange resins and showed that the reaction was first order with respect to the concentration of isobutylene and that the reaction was irreversible at low conversion level. This was also confirmed in the present study at different conversion levels. With these assumptions, the ratio of overall reaction rates with different sized resins can be written as oov, kov, v a l [ Y k a + (1 - ~ ) r i i , k i l -=-- (13) 00.2 kov, q a 2 [ y k a + (1- y)rii,kil
The ratio of the modified Thiele modulus, M1 and M2, can be represented by the ratio of the resin size from eq 7, and they can be obtained by solving eq 14 and 15 simultaneously.
M d M 2 = Ra1/Ra2
(15)
The effectiveness factor, qa, can be calculated from eq 9, and the apparent reaction rate constant, kapp, can be estimated from eq 11. The apparent reaction rate constants are summarized in Table I1 for the two different catalysts. The relations between kappof Amberlyst XN-1010 and that of Amberlyst 15 can be written as
where superscripts X N and 15 represent Amberlyst XN1010 and Amberlyst 15, respectively. It is assumed that ki and k, for both resins are the same, respectively, because the active sites are all the same. The association among the sulfonic groups which decides the intrinsic rate constant is different, mainly dependent upon their locations (i.e., external or internal). Equation 16 will show the maximum value when viXN = 1 and qiI5 = 0 and the minimum value when viXN = 0 and qi15 = 1. Therefore, the ratio will have the following range: YXN y15
kappXN
