36
I n d . Eng. C h e m . Res. 1988, 27, 36-41
Fiolitakis, E.; Schmid, M.; Hofmann, H.; Silveston, P. L. Can. J . Chem. Eng. 1983,61, 703. Hegedus, L. L.; Oh, S. H.; Baron, K. U.S. Patent 4222236, 1980. Herz, R. K. Ind. Eng. Chem. Prod. Res. Deu. 1981, 20, 451. Herz, R. K.; Kiela, J. B.; Sell, J. A. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 387. Jones, H. J.; Kummer, J. T.; Otto, K.; Shelef, M.; Weaver, E. E. Environ. Sci. Technol. 1971, 5, 790. Kuo, J. C. M.; Morgan, C. R.; Lassen, H. G. SAE Paper 710289, 1971. Monroe, D. R., General Motors Research Laboratories, Warren, MI, private communication, 1986. Muraki, H.; Shinjoh, H.; Fujitani, Y. Appl. Catal. 1986, 22, 325. Oh, S. H.; Cavendish, J. C. AIChE J . 1985, 31, 943. Schlatter, J. C.; Sinkevitch, R. M.; Mitchell, P. J. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 51.
Sell, J. A.; Herz, R. K.; Monroe, D. R. SAE Paper 800463, 1980. Silveston, P. L.; Hudgins, R. R.; Adesina, A. A.; Ross, G. S.; Feimer, J. L. Chem. Eng. Sci. 1986, 41, 923. Simanaitis, D. J. Auto. Eng. 1977, 85(8), 34. Summers, J. C.; Monroe, D. R. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 23. Taylor, K. C.; Sinkevitch, R. M. Ind. Eng. Chem. Prod. Res. Deu. 1983, 22, 45. Taylor, K. C. In Catalysis-Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: West Berlin, 1984; Vol. 5, p 119. Yokota, K.; Muraki, H.; Fujitani, Y. SAE Paper 850129, 1985.
Received for review February 4, 1987 Accepted September 24, 1987
Kinetics of Absorption of Oxygen in Aqueous Alkaline Solutions of Polyhydroxybenzenes Anand V. Patwardhan and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 029, India
Kinetics of oxygen absorption in aqueous alkaline solutions of polyhydroxybenzenes (PHBs) such as pyrogallol (PG), p-tert-butylcatechol (PTBC), tert-butylhydroquinone (TBHQ), 2,3,5-trimethylhydroquinone (TMHQ), and gallic acid (GA) was studied in a jet apparatus, a stirred cell, and a model stirred contactor, a t 29 f 1 “C. In the cases of PTBC and TBHQ, the reaction was found to be first order in oxygen as well as PTBC or TBHQ. The intrinsic second-order rate constants were in the range of 1 X 103-1 X lo5 m3/(kmol.s). In the case of PG, GA, and TMHQ, the system conformed to the instantaneous reaction regime. The theory of gas absorption with an instantaneous reaction was used to calculate the diffusivity of dissolved P G and GA. At very low partial pressures of oxygen, the oxygen absorption in alkaline P G can become predominantly gas film controlled. Polyhydroxybenzenes (PHBs) are phenols with two or more hydroxyl groups. These are also called benzenepolyols. The absorption of oxygen in the aqueous alkaline solutions of PHB and substituted PHB, such as 1,2,3benzenetriol or pyrogallol (PG), 3,4,5-trihydroxybenzoic acid or gallic acid (GA), p-tert-butylcatechol (PTBC), tert-butylhydroquinone (TBHQ), and 2,3,5-trimethylhydroquinone (TMHQ), was studied. The absorption of oxygen in aqueous alkaline solutions of PHBs and their substituted derivatives is relevant in several contexts with respect to the understanding of functioning of antioxidants, new methods of removing oxygen from certain streams, etc. The classic example is the use of alkaline PG in the Orsat apparatus for analysis of gaseous mixtures containing oxygen (Vogel, 1975). PG appears to be the strongest reducing agent among the benzenepolyols, as is evident from the fact that its aqueous alkaline solutions absorb oxygen from air and darken rapidly. PG, as an antioxidant, is useful in protecting decomposition of alkali cellulose (Langmaack, 1971). A mixture of defatted rice bran and alkalized PG is useful in protecting foodstuffs from oxygen. GA and its propyl ester are useful as antioxidants for various duties. The dihydroxybenzenes, namely PTBC and TBHQ, are used as antioxidants, antiozonants, monomer inhibitors, etc. There is scanty information in the literature on the kinetics of absorption of oxygen in aqueous alkaline solutions of PHBs and their derivatives. It is, however, apparent that these reactions are extremely fast and mass transfer is accompanied by chemical reaction in the diffusion film. It was, therefore, thought desirable to study the kinetics of absorption of oxygen in aqueous alkaline solutions of PG, GA, PTBC, TBHQ, and TMHQ.
Previous Studies Recently, Rothe (1985) has reported the production of nitrogen-rich inert gas from air by the absorption of oxygen in alkaline P G solution in a packed column. The spent PG solution has been claimed to be regenerated by heating for the desorption of oxygen. Takahashi et al. (1980) have reported the kinetics of absorption of oxygen in alkaline solution containing hydroquinone. The reaction was found to be first order in oxygen and first order in hydroquinone. Takeuchi et al. (1980) have reported the kinetics of absorption of oxygen in aqueous alkaline solutions containing the sodium salt of 1,4-naphthohydroquinone-2-sulfonic acid (NHQS). The reaction was found to be first order in oxygen and first order in NHQS. Catechol and hydroquinone are converted to the corresponding 0-and p-benzoquinonesby most oxidizing agents. The chemistry of oxidation of PG and substituted PG has been studied by various workers. Nierenstein (1915) observed that the oxidation of PG in potassium hydroxide solution gives, among other products, 2,3,2’,3‘,2”,3”hexahydroxyphenoquinone. Campbell (1951) has reported OH OH OH O H OH OH
o 0> 2,3, 2 ’ , 3 ,
f,3’-h e x a h y d r o x y p h e n o q u i n o n e
that the oxidation of 4,6-di-tert-butylpyrogallol in alkaline solution with air gives an orthoquinone, which then rearranges to give further products. Purpurogallin, a redbrown to black mordant dye, is obtained from electrolytic
0888-5885/8~/262~-0036$01.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 37 and other mild oxidations of PG. The reaction is believed to proceed through 3-hydroxy-o-benzoquinoneand 3hydroxy-6-(3,4,6-trihydroxyphenyl)-o-benzoquinone (Critchlow et al., 1951). Duncan et al. (1979) have described a procedure for the quantitative estimation of small amounts of oxygen produced by photochemical reaction. The concentration of oxygen was measured indirectly by absorption spectrometry of the highly colored oxidation product of PG. The reagent solution was stable over the experimental time scale and change in absorbance a t 450 nm gave a good measure of the concentration of oxygen present in the solution. Pospisil and Ettel (1959) have reported that silver oxide oxidation of 5-tert-octylpyrogallol gave the corresponding hydroxy-0-benzoquinone (6hydroxy-4-tert-octyl-1,2-benzoquinone). From the foregoing, it is clear that very limited data are available on the kinetics of absorption of oxygen in aqueous alkaline solutions of PHBs such as PG, GA, PTBC, TBHQ, and TMHQ. Further, the stoichiometry of the reaction between molecular oxygen and aqueous alkaline solutions of PG and GA is not known unequivocally. In addition, according to Rothe (1985), there is a possibility of regenerative absorption of oxygen in the case of aqueous alkaline solution of PG. This work was, therefore, undertaken to study the kinetics of absorption of oxygen in aqueous alkaline solutions of PG, GA, PTBC, TBHQ, and TMHQ and related problems.
Experimental Apparatus and Procedure Stirred Cell. Pure oxygen from an oxygen cylinder was used. Oxygen was stored in a balloon a t atmospheric pressure. Most of the experiments were carried out in a 9.5 X m i.d. glass stirred cell. The design of the stirred cell was similar to that used by Jhaveri and Sharma (1967). A glass stirrer with four blades, which just dipped into the liquid, was used. The gas phase in the stirred cell was also agitated by using a cruciform stirrer. The effective interfacial area was 59.8 X 10" m2. Experiments were also carried out in a glass stirred cell of 5.5 x m i.d. (effective interfacial area of 22 X lo-" m2). A known amount of a solution was taken, and the volumetric rate of absorption of pure oxygen, stored in a balloon at essentially atmospheric pressure, was noted. The solutions were made alkaline by the addition of potassium hydroxide. The mole ratios of potassium hydroxide to PHBs were 2, 2, 3, and 4 for PTBC, TBHQ, PG, and GA, respectively, for most of the experiments. In some experiments, aqueous polyethylene glycol 400 (PEG-400) solutions (35% w/v) of different PHB were used so that higher concentrations of some sparingly soluble materials could be used. The concentrations of PTBC and TBHQ were varied from 0.2 to 1 kmol/m3, the concentration of GA was varied from 0.1 to 0.5 kmol/m3,and the concentration of PG was varied from 0.1 to 2 kmol/m3. In a few experiments carried out in the glass stirred cell of 9.5 X m i.d., air was used instead of pure oxygen, so that the partial pressure of oxygen could be conveniently varied. The stirred cell was purged with fresh air for a sufficient time with the help of a small laboratory air blower. The purging was stopped, and the unit was connected to a balloon containing pure oxygen at essentially atmospheric pressure. After about 100 s, the volumetric rate of uptake of oxygen was noted. Jet Apparatus. The absorption of oxygen in aqueous alkaline solutions of PHBs was also studied in a laminar jet apparatus. The principal design features of the jet apparatus were akin to those employed by Sharma and Danckwerts (1963). Higher concentrations of PTBC and
TBHQ were used, so that the reaction could possibly be moved to the fast reaction regime, and values of rate constant can then be calculated (Doraiswamy and Sharma, 1984). Some experiments were also carried out with aqueous PEG-400 solutions (35% w/v) containing different PHBs. Stirred Contactor with Flat Gas-Liquid Interface. This type of contactor, which has independent stirrers for gas and liquid phases and where the interface is kept flat and no gas dispersion is allowed, was employed to study the effect of oxygen partial pressure on the specific rate of oxygen absorption in aqueous alkaline solutions of different PHBs. The design features of this contactor were akin to those employed by Sridharan and Sharma (1976). The model stirred contactor was operated a t a gas-side stirrer speed of 57 rev/s and liquid-side stirrer speed in the range 2.1-4.0 rev/s. The mode of operation of contactor was semicontinuous. To assess the effect of gas-side resistance, the gas-side stirrer speed was varied from 8 to 57 rev/s. All the experiments were conducted at 29 f 1 "C and essentially at atmospheric pressure by employing mixtures of oxygen and nitrogen. The specific rates of absorption were calculated on the basis of gas-phase analysis for oxygen by gas chromatography.
Molecular Diffusivity of PHBs in Aqueous Phase The values of diffusivity, D g , of different PHBs in water were determined by measuring the extent of dispersion of solute in the solvent (water) in a straight circular tube under the condition of laminar flow (Taylor, 1953; Pratt and Wakeham, 1974; Trivedi and Vasudeva, 1975). The extent of dispersion can be obtained from the measurement of concentration of solute as a function time a t the downstream end. The solute concentration in each sample was determined by measuring absorbance in the ultraviolet region. Physicochemical Data Solubility of Oxygen in Aqueous Solutions. The solubility data for oxygen in water have been reported by Lange (1985). The solubility of oxygen in a 35% (w/v) aqueous solution of PEG-400 was found by the Winkler method (Hitchman, 1978). The solubility of oxygen in electrolyte solutions was corrected for electrolyte concentration (Danckwerts, 1970). Diffusivity of Oxygen in Aqueous Solutions. The diffusivity of dissolved oxygen in water a t 29 f 1 "C is available from literature (Wise and Houghton, 1966). Physical Mass-Transfer Coefficient. Values of the physical mass-transfer coefficient, kL, are needed to check the conditions for different controlling regimes (Doraiswamy and Sharma, 1984). The kL values at different speeds of stirring in the stirred cell were determined by absorbing pure carbon dioxide in deionized water. The kL values at different speeds of stirring in the model stirred contactor were obtained from earlier work in this laboratory (Yadav, 1980). The value of diffusivity of oxygen in aqueous alkaline solutions was estimated from D A p / T = constant (1) Results and Discussion Absorption of Oxygen in Aqueous Alkaline Solutions of PTBC. The specific rate of absorption of pure oxygen in 0.2 M aqueous alkaline solution of PTBC in the stirred cell of 9.5 X m i.d. was found to depend on the hydrodynamic factors. The specific rate of absorption of oxygen from air was found to be independent of hydrodynamic factors in the range of stirring speed from 0.68 to 1.78 rev/s.
-
38 Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 xlo’,
CONTACT T I M E
1.0
I
2:o 1
1.5
,
,
2.4
14.8
4‘01
7 PTQC WITH
--C 0.6
3.6
//
3.21 3.3 -C 0.4 k m o l / m ’
PTBC WITH
+ 0.1 k m o l / m ’
TBHQ
+0.4 kmol/m’TEHQ
PEG-400
PEG
7 “E
WITH PEG \
I
4
0.5
0
2
4
6 8 10 CONTACT T I M E r l O a , I
1 2
14
1 6 1 7
Figure 1. Effect of contact time on the specific rate of absorption: plot of R A vs contact time for PTBC and TBHQ.
The specific rate of absorption of pure oxygen in aqueous alkaline solutions of PTBC containing 35% (Wjv) PEG-400 (in the 5.5 X m i.d. stirred cell) was found to depend on the hydrodynamic factors, while the specific rate of absorption of oxygen from air in aqueous solutions of PTBC containing 35% (w/v) PEG-400 was found to be independent of hydrodynamic factors in the range of stirring speed from 0.9 to 1.45 rev/s; the maximum variation was 7 % . The solubility of oxygen in aqueous solutions is very small (51 x kmol/(m3.atm)); hence, it is expected that the gas-side resistance would be practically absent when oxygen is absorbed from mixtures of oxygen and nitrogen. In the case of PTBC and TBHQ, the maximum concentration of alkaline solution that could be prepared without the addition of PEG-400 was 0.2 kmol/m3, while with the addition of PEG-400 (35% wjv in the reactant solution), the PTBC and TBHQ could be solubilized up to a concentration of 1 kmol/m3. The specific rate of absorption for PTBC (with and without PEG-400) in the laminar jet apparatus was found to be independent of the contact time in the range from 0.020 to 0.089 s (Figure 1). Also, the specific rate was proportional to (DA[Bo])1/2. To find out the effect of viscosity on the liquid-phase diffusivity of the reactive species, DB, diffusivity of aniline as a model compound was measured by the laminar dispersion technique, in water and in 35% (w/v) aqueous solution of PEG-400. It was found that the Stokes-Einstein relation is obeyed for the case under consideration; the ratio of viscosity of the 35% PEG solution to water is 4.8. The specific rate of absorption for PTBC (with and without PEG-400) in the stirred contactor was found to be directly proportional to the partial pressure of oxygen, which was varied from 0.05 to 0.20 atm (Figure 2 ) . The dissolved gas undergoes a pseudo-first-order reaction when the concentration of the reactant in the neighborhood of the gas-liquid interface is not very different from that in the bulk and the concentration of the dissolved gas is very small compared to the reactant concentration. The condition to be satisfied is
42 01
I
3
5
I 7
I
I
I
I
I
I
9
11
13
15
17
19
p* x 101, a t m
11 21
__c
Figure 2. Effect of partial pressure of oxygen on specific rate of absorption: plot of RA vs pAfor PTBC and TBHQ.
The specific rate of absorption, RA (kmol/(m%)), is then given as
R A = [A*](DAk)1/2
(3)
But since the specific rate was found to be proportional to (DA[Bo])1/2,eq 3 can be written as RA
= [A*](DAk2[B,J)1’2
(4)
Thus, the specific rate of absorption under certain conditions is independent of the hydrodynamics of the system; higher values of [Bo] and kL and lower values of [A*] are conducive to satisfy the condition given by expression 2. The absorption of oxygen in aqueous alkaline solutions of PTBC, therefore, appears to conform to the fast pseudofirst-order reaction regime. Table I11 gives the pertinent details. The average value of the second-order reaction rate constant, k2, at 29 “C, was estimated at 9.86 X lo4 and 3.81 X lo3 m3/(kmol.s) for PTBC without PEG-400 and PTBC with PEG-400, respectively. Absorption of Oxygen in Aqueous Alkaline Solutions of TBHQ. The specific rate of absorption of pure oxygen in aqueous alkaline solutions of TBHQ (with and without PEG-400) was found to depend on the hydrodynamic factors in the stirred cell. The specific rate of absorption of oxygen from air was found to depend on the hydrodynamic factors for TBHQ without PEG-400 and practically independent of the hydrodynamic factors for TBHQ with PEG-400, where substantially higher concentrations of TBHQ can be used, in the stirred cell. The specific rate of absorption for TBHQ (without PEG-400) in the laminar jet apparatus was found to depend on the hydrodynamic factors, while the specific rate of absorption (with PEG-400) in the laminar jet apparatus was found to be independent of the contact time (from 0.0082 to 0.089 s) for higher values of TBHQ concentration (Figure 1). Also, the specific rate was found to be proportional to (DA[B0])’/*. The specific rate of absorption for TBHQ (without PEG-490) was found to be a function of hydrodynamic factors for higher partial pressures of oxygen. The specific
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988 39
-
Table I. Oxygen Absorption in Aqueous Alkaline Solutions of PG and GA in Laminar Jet Apparatus at 29 OC (with and without PEG-400) PG (without PEG-400) PG (with PEG-400) 106RA, 103[Bol, contact time 106RA, W0DB, 103[~,1, contact time 101oDB? kmol/m3 x 103, s m2/s kmol/ (m2.s) kmol/m3 x 102, s kmol/(m2.s) m2/5 0.01 8.4 3.037 6.21 0.01 1.49 1.748 2.75 0.05 9.5 8.628 6.50 0.05 2.38 3.252 1.95 0.10 38.9 7.872 6.53 0.10 3.73 4.115 1.58 GA (without PEG-400) GA (with PEG-400) 106RA, 1O3[B0], contact time 106RA, W0DB, 1O3[Bol9 contact time 10'ODB? kmol/m3 x 103, s kmol/ (m2.s) m2/s kmol/m3 x 102, s kmol/ (m2.s) m2/5 0.01 7.1 3.138 6.20 0.01 1.50 1.739 2.29 0.05 12.8 6.804 5.00 0.05 2.27 3.122 1.56 0.10 38.5 7.646 6.07 0.10 5.69 3.016 1.24 'From eq 6.
Table 11. Oxygen Absorption in Aqueous Alkaline Solutions of PG in Stirred Model Contactor at 29 "C" liauid-side stirrer meed = 3.5 revis liauid-side stirrer meed = 4.0 rev/s 102PA, atm 0.50 0.98 1.03 1.78 3.01 4.12
106RA, kmol/ (m%) 3.846 7.764 7.764 13.55 19.00 18.82
1O4(R~/P~), kmol/(m2.s.atm) 7.692 7.923 7.538 7.615 6.312 4.568
102PA, atm 0.68 1.02 1.53 2.30 3.06 4.12
106RA, kmol/(m2.s) 5.241 7.768 11.76 17.78 20.95 21.20
~O*(RA/PA), kmol/ (ml-watm) 7.708 7.616 7.685 7.731 6.846 5.146
"Gas-side stirrer speed = 2.67 rev/s. Mode of operation = semicontinuous. [Bo] = 2.0 kmol/m3.
Table 111. AbsorDtion of Oxveen in Aaueous Alkaline Solutions of PTBC PEG, rate [KOH] / [PTBCI, kmol/m3 contactor [PTBC] 7c (w/v) PA? atm eq 0.2 2 stirred cell 0 0.96 0 4 0.2 0.20 stirred cell 2 35 0.2-1.0 0.96 stirred cell 2 35 0.2-1.0 0.20 stirred cell 2 jet apparatus 0 0.05-0.2 0.96 2 2 35 0.04-0.4 0.96 jet apparatus 0 0.2 0.055-0.204 stirred contactor 2 0.051-0.159 stirred contactor 35 0.2-0.6 2 stirred contactor 0 0.2 0.031-0.120 1.75 0 0.2 0.021-0.130 stirred contactor 1.50 0 0.011-0.140 stirred contactor 0.2 1.25
k2,
m3/ (kmo1.s)
remarks falls in intermediate regime
9.27 x 103 falls in intermediate regime 3.60 x 1.07 x 3.66 x 9.60 x 4.16 x 9.92 x 8.03 x 6.74 x
103 104 103 103
103 103 103 103
"Values corrected for vapor pressure of water.
Table IV. Absorption of Oxygen in Aqueous Alkaline Solutions of TBHQ" [KOHI/ [TBHQI
contactor stirred cell stirred cell stirred cell stirred cell jet apparatus jet apparatus stirred contactor stirred contactor
[TBHQI, kmol/m3 0.2 0.2 0.2-1.0 0.2-1.0 0.1-0.2 0.1-0.4 0.2 0.2-0.6
PEG,
% (w/v)
pa,b atm
0 0 35 35 0 35 0 35
0.96 0.20 0.96 0.20 0.96 0.96 0.029-0.193 0.029-0.131
rate eq
m3/ (kmol-s)
4
7.92 x 103
4 4
7.85 x 103 9.08 x 104 7.65 x 103
k2l
remarks falls in intermediate regime falls in intermediate regime falls in intermediate regime falls in intermediate regime
4
"Temperature: 29 "C. *Values corrected for vapor pressure of water.
rate of absorption (with PEG-400) was found to be independent of the hydrodynamic factors and directly proportional to the partial pressure of oxygen, pa (Figure 2). When the condition given by expression 2 is satisfied, the specific rate can be given by eq 4. Thus, the absorption of oxygen in aqueous alkaline solutions of TBHQ appears to conform to the fast pseudo-first-order reaction regime under certain conditions. The pertinent details are given in Table IV. The average value of second-order reaction rate constant lzz, at 29 "C, was estimated at 9.08 X lo4 and 7.81 X lo3 m3/(kmol.s), for TBHQ without and with PEG-400, respectively.
A few experiments were carried out in the 9.5 X m i.d. stirred cell with aqueous alkaline solutions of hydroquinone (1,4-benzenediol). Oxygen was absorbed from air. The pH of the reaction solution was varied from 9.8 to 13.0 by adding appropriate quantities of potassium hydroxide. The value of the second-order reaction rate constant at pH 13.0 and at 31 " C was found to be 1.71 X lo4 m3/(kmol-s), which is in reasonably good agreement with the value of 1.21 X lo4 m3/(kmol.s), at 25 "C, obtained by Takahashi et al. (1980). The value of 1.21 X lo4 m3/(kmol-s)for hydroquinone is considerably lower than the corresponding value for
40
Ind. Eng. Chem. Res., Vol. 27, No. 1, 1988
TBHQ (9.9 X lo4 m3/(kmol-s)) a t the same pH. Absorption of Oxygen in Aqueous Alkaline Solutions of PG and GA. The specific rate of absorption of pure oxygen in aqueous alkaline solutions of PG and GA in the 9.5 X low2m i.d. stirred cell was found to depend on the hydrodynamic factors in the range of stirring speed from 0.68 to 1.78 rev/s. Also, the specific rate was found to be directly proportional to the PG and GA concentrations. Thus, the absorption conforms to the instantaneous reaction regime. The condition to be satisfied is
-
-0- 0.25 0.50
The specific rate under this condition is given by
Here, the stoichiometric factor 2 was not known unequivocally. To find out 2, some experiments were carried out in the jet apparatus with very low concentrations of PG and GA (0.01-0.1 M solutions). Under these conditions, the absorption conformed to the instantaneous reaction regime and 2 could be calculated from eq 6. The diffusivity of dissolved oxygen, DA,was obtained from the literature (Wise and Houghton, 1966), and the diffusivity of liquid-phase reactant, DB,was obtained experimentally by the laminar dispersion technique (Taylor, 1953). The value of 2 was found out to be 2 for PG as well as GA. The values of the specific rate of oxygen absorption obtained in the laminar jet apparatus and values of DB obtained from these values of specific rates (using 2 = 2) are given in Table I. For purposes of comparison, the values determined by the laminar dispersion technique are 5.91 X and 7.50 X m2/s for PG and GA, respectively. The accuracy of these measurements is assessed to be *8%; this is based on measurement of diffusivity for solutes like phenol in water for which independent data are available. As may be seen, the values deduced from different techniques agree well within experimental limitations. The specific rate of absorption of oxygen in aqueous alkaline solutions of PG and GA in the stirred contactor was found to be dependent on the hydrodynamic factors and independent of partial pressure of oxygen (0.01-0.20 atm) at very high speeds of agitation in the gas phase. The concentration of PG was varied from 0.25 to 2.0 kmol/m3 and that of GA from 0.1 to 1.0 kmol/m3. At a very low speed of agitation in the gas phase (-2.67 rev/s) and with a higher concentration of PG (2.0 kmol/m3), a t very low partial pressures of oxygen, the absorption was predominantly gas film controlled (Table 11). The values of the true gas-side mass-transfer coefficient, k,, are comparable to the previously obtained values (Yadav, 1980); at low values of pA,k, is equal to R A / p A . Under conditions where gas film resistance is dominant, the effect of the concentration of PG should be negligible and experimental data for 1.0 kmol/m3 PG are comparable to those for 2.0 kmol/m3 PG. Absorption of Oxygen in Aqueous Alkaline Solution of TMHQ. A few experiments were carried out in the stirred contactor to study the absorption of oxygen in aqueous alkaline solutions of TMHQ; the mole ratio of potassium hydroxide to TMHQ was 2. The concentration of TMHQ was varied from 0.25 to 1.0 kmol/m3. The partial pressure of oxygen in the gas phase was varied from 0.03 to 0.12 atm. The specific rate of absorption was found to be dependent on the hydrodynamic factors and inde-
01
I 1
I
2 PA u 1 0 ~ , . t m
-
I
3
I
4
Figure 3. Effect of partial pressure of oxygen and alkali to PHB concentration ratio on specific rate of absorption: plot of RA vs p A for PG ([Bo] = 2.0 kmol/m3).
pendent of partial pressure of oxygen at very high speeds of stirring in the gas phase. Also, the specific rate was directly proportional to TMHQ concentration. Thus, the absorption of oxygen in aqueous alkaline solutions of TMHQ conforms to the instantaneous reaction regime.
Effect of Potassium Hydroxide to PHB Ratio on the Specific Rate of Oxygen Absorption in Aqueous Alkaline Solutions of PG and PTBC Some experiments were carried out in the stirred contactor, with the ratio of potassium hydroxide to PHB concentration being different from the previously mentioned values. For PG, the different ratios used were 1.0, 0.5, and 0.25. The specific rate of absorption of oxygen for PG under these conditions is practically independent of hydrodynamic factors for very low partial pressures of oxygen. Also, the specific rate is directly proportional to the partial pressure of oxygen (Figure 3). Hence, the absorption of oxygen conforms to the pseudo-first-order reaction regime. The values of second-order reaction rate constants are 1.03 X lo8, 3.78 X lo7, and 5.20 X lo6 m3/ (kmol-s) for alkali to PG ratios of 1.0, 0.5, and 0.25, respectively. Similarly, for PTBC, some experiments were carried out with alkali to PTBC ratios of 1.75, 1.5, and 1.25 (concentration of PTBC used was 0.2 kmol/m3 in the stirred contactor). The absorption of oxygen conformed to the pseudo-first-order reaction regime. The pertinent details are given in Table 111. The values of second-order rate constants obtained were 9.92 X lo3, 8.03 X lo3, and 6.74 X lo3 m3/(kmol.s) for the alkali to PTBC ratios of 1.75, 1.5, and 1.25, respectively. Trials for Regeneration As claimed by Rothe (19841, the regeneration of spent alkaline PG solution was tried. The spent solution was refluxed under nitrogen blanket for about 3 h in a round-bottom flask. Then the solution was cooled to ambient temperature in an ice bath under nitrogen blanket. This solution was checked for the absorption of pure oxygen in a stirred cell. No measurable rate of absorption was obtained. Similarly, for aqueous alkaline solutions of
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.
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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
