Znd. Eng. Chem. Res. 1991,30, 339-345 Savage, P. E.; Jacobs, G. E.; Javanmardian, M. Autocatalysis and Aryl-Alkyl Bond Cleavage in l-Dodecylpyrene Pyrolysis. Znd. Eng. Chem. Res. 1989,28,645-652. Speight, J. G. In Polynuclear Aromatic Compounds; Advances in Chemistry Series 217; American Chemical Society: Washington, DC, 1988; pp 201-215. Speight, J. G. Latest Thoughts on the Molecular Nature of Petroleum Asphaltenes. Prepr.-Am. Chem. Soc., Diu. Pet. Chem. 1989, 34, 321-328. Steiner, E. C.; Blau, G. E.; Agin, G . L. Introductory Guide SimuSolu Modeling and Simulation Software; The Dow Chemical Company: Midland, MI, 1986.
339
Vernon, L. W. Free Radical Chemistry of Coal Liquefaction: Role of Molecular Hydrogen. Fuel 1980,59, 102-107. Walker, J. A.; Tsang, W. Single-Pulse Shock Tube Studies on the Thermal Decomposition of n-Butyl Phenyl Ether, n-Pentylbenzene, and Phenotole and the Heat of Formation of Phenoxy and Benzyl Radicals. J. Phys. Chem. 1990,94, 3324-3327. Waller, P. R.; Williams, A.; Bartle, K. D. The Structural Nature and Solubility of Residual Fuel Oil fractions. Fuel 1989,68, 520-526. Received for review April 19, 1990 Revised manuscript received August 27, 1990 Accepted September 17, 1990
Absorption of Ethylene in Concentrated Sulfuric Acid Swades Kumar Chaudhuri and Man Mohan Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
The absorption of ethylene was studied in 90-98 w t 9’0 sulfuric acid. The specific rate of absorption in 98 wt %’ acid in a stirred cell conformed t o fast pseudo-first-order kinetics. Silver oxide was found to be the most effective catalyst, in intensifying the rate of absorption, among those used. T h e presence of products in the reaction mixture enhanced the specific reaction rate compared t o t h e fresh acid. High pressures favored the formation of diethyl sulfate, and the supercritical condition of ethylene increased the specific reaction rate substantially. Chlorobenzene was found to be the most effective solvent for the extraction of diethyl sulfate from the reaction mixture.
Introduction The reaction between ethylene and sulfuric acid results almost exclusively in the esterification of ethylene with the production of ethyl hydrogen sulfate and subsequently of diethyl sulfate. CH2=CH2 CHZ=CHZ
+ H2S04+ C2HSHS04
+ C2H5HS04 + (C2H,),SO4
2C2H5HS04
(C2H&S04 + H2SO4
(1)
(2)
(3)
Diethyl sulfate finds its largest use as an alkylating agent for different types of phenols and amines, which in turn are important intermediates in the manufacture of dyes and pharmaceuticals. It is also used as a cross-linking agent for furfural phenolic resins, as a softening agent for fibres, and as a vulcanizing agent for epoxidized butadiene rubber, as well as to prepare food-flavoring materials and perfumery ingredients such as l-ethoxy-4-ethylbenzene (Joachim et al., 1983). Though there are different processes for the preparation of diethyl sulfate (Miller, 1969), only the ethylenesulfuric acid process is industrially practiced. The kinetics and related aspects of this system have not been studied in spite of its long-time record of industrial usage. Therefore, the present work was concerned with the absorption of ethylene in concentrated sulfuric acid (>90 wt %) to explore the kinetics and related aspects of this system. Since the critical pressure and critical temperature of ethylene are 50.5 atm and 282.8 K, respectively, it was thought that this system would provide an interesting and potentially useful opportunity to study the effect of changeover from subcritical to supercritical conditions on the rate of absorption. Further, it was hoped that if the reaction conformed to the fast pseudo-first-order reaction regime, the system would provide an interesting case of a study of the effect of pressure on the effective interfacial area in a 0888-5885/91/2630-0339$02.50/0
mechanically agitated contactor, under otherwise uniform conditions.
Previous Studies There have been some qualitative and semiquantitative studies on the absorption of ethylene in concentrated sulfuric acid; quantitative data on the rates of reaction are scarce. Plant and Sidgwick (1921) carried out the rate measurements for this reaction by bubbling ethylene through sulfuric acid and noting the weight increase of the acid solution. They found that some diethyl sulfate was formed before all sulfuric acid had been converted to ethyl hydrogen sulfate. de-Loisy and Damiens (1923) investigated the effect of various factors, namely, concentration of acid, partial pressure of ethylene, and temperature, and found that, other factors being the same, the rate of absorption in 99.5 w t % acid was 3-5 times greater than that in 95.5 wt % acid. The effect of increasing pressure of ethylene in the reaction was investigated by Maimeri (1925). Experiments with 100 w t % sulfuric acid at ordinary temperatures showed that the amount of diethyl sulfate formed increased with an increase in pressure. At 3.5 atm the yields of diethyl sulfate and ethyl hydrogen sulfate were 35% and 51%,respectively (based on acid reacted), while at 10 atm the corresponding figures were 62% and 30%, respectively. Damiens et al. (1926a-c) reported a number of substances that catalyze the absorption of ethylene in sulfuric acid. These consist of lower oxides or salts of metal exhibiting more than one valency state. The use of silver or silver oxide or its salts as catalysts for the absorption of ethylene in 96 wt % sulfuric acid was suggested by Engelhardt et al. (1923). Davis and Schuler (1930) measured the rates of absorption of ethylene in sulfuric acid by noting the change in the ethylene pressure with time at 298 K. They reported that the rate of absorption is proportional to the partial pressure of ethylene and also proportional to the gas-liquid 0 1991 American Chemical Society
340 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991
contact area. Brooks (1935) studied the effect of pressure in promoting the absorption of ethylene in sulfuric acid. He reported that 98 wt % acid at 353 K can absorb 1.6 mol of ethylene/mol of acid in 50 min at 34 atm, as compared to 1.18 mol at 13.6 atm. Hellin and Jungers (1957) also noted the decrease in pressure with time of the ethylene atmosphere over sulfuric acid in a vibrating vessel. A kinetic study was made of the reactions in the ethylene-sulfuric acid-ethyl hydrogen sulfate-diethyl sulfate system by Harris and Himmelblau (1963, 1964). Concentration versus time data were obtained for the reaction of ethylene with concentrated sulfuric acid and diethyl sulfate to yield ethyl hydrogen sulfate, and these data were analyzed statistically to determine a kinetic model. The suggested controlling reaction mechanism is the formation of a carbonium ion and its reaction with bisulfate or ethyl sulfate ions. Recently, Schmidt (1983) has reported in a patent the reaction of 96 wt 90 sulfuric acid with a mixture of 18.5% ethylene and 81.5% nitrogen at 30.6 atm and 373 K in an autoclave. He has claimed that after 1.3 h the conversion of ethylene was 97% to a mixture of diethyl sulfate and ethyl hydrogen sulfate.
1.5 X m), equipped with a six-bladed turbine-type impeller. Ethylene was bubbled at the eye of the impeller at a superficial gas velocity of 0.23 cm/s. Ethylene gas was preheated by passing through a heating coil. Experiments under elevated pressures were conducted in a 1-L-capacity (7.5 X m id.) stainless steel, magnetically stirred autoclave (Autoclave Engineers, Inc. Erie, PA). The autoclave was baffled and equipped with a cooling coil. A 3.2 X m diameter six-bladed turbinetype impeller was placed 3 cm above the bottom. Ethylene was introduced at the eye of the impeller. Successive runs gave good reproducibility with an average deviation of less than 8% under identical operating conditions. The matching conversion was 10% with respect to ethylene for both the autoclave and mechanically agitated reactors. When diethyl sulfate was used to study the effect of product on the reaction rate, it was washed with 3% sodium carbonate solution to remove trace amount of any acid that may have been present in it. It was then washed with distilled water, dried over fused calcium chloride, and filtered.
Experimental Section The concentration of sulfuric acid is limited by two considerations for this reaction. At low acid concentration (e90 wt %), a number of undesirable products such as ethyl alcohol and diethyl ether are formed (Harris and Himmelblau, 1964). A t high acid concentration (>98 wt %), formation of carbyl sulfate, ethionic acid, and isethionic acid together with increased tar formation make the use of such concentrated acid unattractive (Plant and Sidgwick, 1921). So, the concentration of the acid in this work was varied from 90 to 98 w t 70;most of the measurements were made with 98 wt % sulfuric acid. Materials. The ethylene gas cylinder was obtained from B.O.C. Ltd., London, U.K. The purity of the gas was 99.990. Analytical grade sulfuric acid (98 wt 5%; specific gravity 1.84), diethyl sulfate (99%), silver oxide (>97%), ferrous sulfate (99%), and chlorobenzene (99.5%) were obtained from reputed firms. Cuprous oxide was Fluka grade (>97%). Apparatus a n d Procedure. Two stirred cells (9.5 X m i.d.) made of glass, with an m i.d. and 5.5 X effective gas-liquid interfacial area of 59.8 X 10" m2 and 22.5 X m2, respectively and having a flat gas-liquid interface and a cruciform stirrer, were used for the reaction. The design of the cells was similar to that used by Jhaveri and Sharma (1967). Stirred cell experiments were carried out in both a batch and semibatch manner. A known amount of acid solution of a predetermined concentration was taken in the reactor and kept in a constant temperature bath, the temperature of which was controlled within f l "C with a temperature controller. When the temperature of the acid solution attained the desired value, the reactor was purged with ethylene for about 60 s to drive out accumulated inerts and impurities. Then the specific rate of absorption of ethylene was measured. In the case of semibatch mode of operation, the inlet gas stream was preheated by passing through a copper heating coil immersed in the same temperature bath. For every run, the specific rate of absorption remained steady over a long period, exceeding at least 1000 s. Successive runs gave reasonable reproducibility, with an average deviation of less than 5%. Some experiments were carried out in two baffled mechanically agitated contactors (9.5 X m i.d., impeller m; 5.2 X m id., impeller diameter diameter 3 X
In the case of the batch mode of operation of stirred cells, the specific rate of ethylene absorption was obtained by the uptake method (Danckwerts, 1970). In the case of the semibatch mode of operation, the rate of absorption was measured by noting the increase in weight of the acid solution using a sensitive electronic balance and/or by gas-phase analysis of the ethylenenitrogen mixture on gas chromatograph when mixtures of ethylene with an inert gas were used. A Chemito Model 3800 GC connected with an SP 4270 integrator was used with hydrogen as the carrier gas. A 2 m X 3 mm stainless steel column packed with alumina (80-100 mesh) was used for analysis (oven temperature 423 K; bridge temperature 623 K; thermal conductivity detector (TCD) temperature 523 K; injector temperature 373 K; carrier gas flow rate 25 cm3/min). The rate of absorption in the case of the high-pressure reactor was studied by recording the fall of pressure and/or increase in weight of the acid solution. The material balance was checked by estimating the amount of ethyl alcohol in the distillate, which was obtained from the hydrolysis of a known amount of the reaction mixture. The aqueous ethanol solutions (distillate) were analyzed by gas-liquid chromatography using a 2 m X 3 mm stainless steel column packed with 10 wt % Carbowax 10 M on Chromosorb WHP (80-100 mesh). A Chemito Model 3800 GC was used with nitrogen as the carrier gas (bridge temperature 673 K; TCD temperature 575 K; initial oven temperature 343 K; final oven temperature 453 K; ramp rate 10 "C/min; carrier gas flow rate 25 cm3/min). The error in the material balance was less than 6%. Harris and Himmelblau (1961) have reported a method for the estimation of diethyl sulfate in mixtures of (C2H&304-C2H5HS04-H2S04,which was used in this work to estimate diethyl sulfate in the reaction mixture. The amount of diethyl sulfate in the extracted phase (chlorobenzene as solvent) was analyzed with nitrogen as carrier gas on a Perkin-Elmer (8500 Model) GC interfaced with a GP 100 Graphics Printer. A 2 m X 3 mm stainless steel column packed with 10 wt % OV-17 on Chromosorb L (80-100 mesh) was used for analysis (initial oven temperature 443 K; final oven temperature 523 K; ramp rate 15 "C/min; flame ionization detector and injector temperature 573 K; carrier gas flow rate 25 cm3/min). The above method was tested with a known mixture and was found to be very sound.
Methods of Analysis
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 341 Table I. Effect of Different Catalysts on the Specific Rate of Absorption of Ethylene" catalvst IBnl. kmol/m3 107RA, kmol/(m* s) 6 18.25b 1.98 1.09 17.52c silver oxide 18.25 71.28 36.0 silver oxide 17.52 65.36 60.0 cuprous oxide 18.25 10.10 5.0 ferrous sulfate 18.25 5.15 2.6 "Conditions: T = 303 f 1 K;P = 1 atm; V = 2.0 X lo-' m3; a = 59.8 x m*; speed of stirring = 1.166 rev/s; catalyst loading = 2% (g/lOOg of acid). b98wt % acid. c94.5wt % acid.
Table 11. Effect of Silver Oxide Catalyst on the Volumetric Rate of Absorption of Ethylenea wt reaction 103RAa, Ag20 loading, init acid incr, g time, s kmol/(m3 s) wt, g g/100 g of acid 1800 3.41 0 720 68.6 2 720 53.2 300 15.86 "Conditions: T = 333 f 1 K;P = 20.04 atm; V = 4 X lo-* m3; [Bo] = 18.25 kmol/m3 (98wt %); speed of stirring = 15 rev/s.
1
E
4
1
3
Pressure, atm
-
Figure 2. Effect of partial pressure of ethylene on the specific rate of absorption in a stirred cell. Reaction conditions: T = 353 f 1 K; V = 2.5 X m3; a = 59.8 X 10" m2; acid concentration = 98 w t 70;speed of stirring = 1.166 rev/s.
'
Y
'5.
E
Y c
3
a4
21 86
I
I
90
90
I 98
11
Acid concentration, w t %
Figure 1. Effect of concentration of sulfuric acid on the rate of ethylene absorption. Reaction conditions: (-@-) T = 303 f 1 K;P = 1 atm; V = 2.5 X lo4 m3; speed of stirring = 1.166 revolutions/s (rev/$; silver oxide loading = 1% (g/100 g of acid). (-k) T = 333 f 1 K;P = 20.04 atm; V = 4 X lo-' m3; speed of stirring = 23.33
121 0
I
I
200
400
I
600 Time, s
I
I
800
1000
IO
-c
rev/s.
Figure 3. Effect of partial pressure of ethylene on the volumetric rate of absorption in an autoclave. Reaction conditions: V = 4 X lo4 m3; acid concentration = 98 wt %; speed of stirring = 15 rev/s.
The concentration of sulfuric acid was estimated by a standard aqueous sodium hydroxide solution.
Table 111. Effect of Partial Pressure of Ethylene on the Selectivity of Diethyl Sulfate"
Results and Discussion Absorption of Ethylene in the Presence of Different Catalysts. The specific rate of absorption of ethylene in 98 wt % sulfuric acid was studied in the temperature range 303-353 K, in the presence of three different catalysts, namely, cuprous oxide, silver oxide, and ferrous sulfate. Some experiments were also carried out in the absence of any catalyst, and the effectiveness of the above catalysts was compared. Silver oxide was found to be the most effective catalyst among those used. This is probably due to the fact that only silver oxide is completely soluble in sulfuric acid at 303 K, whereas the other catalysts are practically insoluble in the acid solutions. Silver oxide with 2% (g/lOO g of acid) loading intensified the specific rate of absorption of ethylene by a factor of 36 and 60 in 98 and 94.5 wt % acid, respectively, whereas cuprous oxide and ferrous sulfate enhanced the specific rate of absorption by a factor of 5 and 2.6, respectively, in 98 wt % acid under identical operating conditions (Table I). Table I1 gives the effect of silver oxide catalyst
press., atm 20.04 5.08
time, s 2400 9000
ethylene abs, mol 7.34 6.82
%b
(C2H,),S0, 65.31 32.14
of C,H,HSO, 34.69 67.86
m3; [Bo] = 18.25 OConditions: T = 333 f 1 K; V = 4 X kmol/m3 (98 wt %); speed of stirring = 23.33 rev/s. "Based on ethylene reacted.
on the volumetric rate of absorption of ethylene under elevated pressure in 98 wt % acid a t 353 K. Effect of Acid Concentration. It has been found that the specific rate of absorption of ethylene in 98 wt % sulfuric acid satisfies the conditions for the fast pseudofirst-order reaction regime (regime 3; Doraiswamy and Sharma, 1984). Figure 1shows that the rate of absorption of ethylene increases with increase in the acid concentration in the range studied. Effect of Partial Pressure of Ethylene. The specific m i.d. stirred rate of absorption of ethylene in a 9.5 X cell a t 353 K in 98 wt % acid was found to be directly proportional to the partial pressure of ethylene (Figure 2).
342 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 -1:
t
-14
.-. a 5 T
-1
5
0
343
323
303
Temperature, -1 6.
Figure 4. Effect of temperature on the specific rate of ethylene absorption in a stirred cell a t 1 atm.
K
-
363
Figure 6. Effect of temperature on the activity of silver oxide catalyst. Reaction conditions: P = 1 atm; V = 9 X m3; a = 22.5 X m3; speed of stirring = 1.166 rev/s; silver oxide loading = 2% (g/lOO g of acid); acid concentration = 98 wt 70. 10
ta VI N
E
Y
6
-
1
-
r LD
W i t h l ' / . ( g / l W g a c ! d ) A g 2 0 loading, T= 3 0 3 K
- 4
s! K
4
a
--
2
W i t h o u t Ag20
1 .o 0
OO
0.5
Ag20 loading
1
15
, g / 1009 acid
2.0
---L
Figure 5. Effect of Ag20 loading on the specific rate of absorption of ethylene in stirred cell. Reaction conditions: T = 303 K; P = 1 atm; V = 2.5 X m3;a = 59.8 X m2; speed of stirring = 1.166 rev/s; (-O-) 94.5 wt % H2S04;(-A-) 98 wt % H2SO4.
The reaction under elevated pressure, 15-30 atm, and at different temperatures in a magnetically stirred autoclave also indicates that the rate of reaction is first order with respect to ethylene partial pressure (Figure 3). Table 111shows that high pressures favor the formation of diethyl sulfate. A t 5.08-atm pressure of ethylene, the yields of diethyl sulfate and ethyl hydrogen sulfate were 32.14% and 67.86%, respectively (based on ethylene reacted), while a t 20.04-atm pressure the corresponding values were 65.31 % and 34.6970, respectively. Effect of Temperature. The specific rate of absorption of ethylene in 98 w t '7~ acid was measured in the temperature range of 303-373 K, in the absence of catalyst, using a stirred cell. A semilogarithmic plot of the data obtained gives a straight line (Figure 4) from which the apparent activation energy was found to be 7.95 kcal/mol. Effect of Catalyst Loading. The effect of catalyst loading on the rate of absorption of ethylene in sulfuric acid was studied in the range 0.25-270 (g/100 g of acid) with silver oxide as the catalyst. Figure 5 shows that the
>
loading, T
303K
i
1
I
I
1.0 1.5 Speed of stirring,rev/s -+
:
Figure 7. Effect of speed of stirring on the specific rate of ethylene absorption in 9.5 X lo-* m i.d. stirred cell. Reaction conditions: P = 1 atm; V = 2.5 X m3;a = 59.8 X m2; acid concentration = 98 wt %.
specific rate of absorption is proportional to silver oxide loading in the range studied. Effect of Temperature on Catalyst Activity. Figure 6 shows the effect of temperature on the relative activity of silver oxide catalyst. It can be seen that the activity of silver oxide catalyst decreases by about 75% as the temperature was increased from 303 to 353 K. The catalytic effect of silver oxide may be due to the formation of a complex of the type Ag[C2H4],+and subsequent enhanced reactivity of this complex with sulfuric acid. This complex is, however, unstable at higher temperature and may be responsible for a relative decrease in the rate of absorption. Effect of Speed of Stirring. The effect of speed of stirring on the absorption of ethylene in 98 wt % sulfuric acid was studied. Figure 7 shows the effect of speed of stirring on the specific rate of absorption of ethylene in the 9.5 x m i.d. stirred cell. It can be seen that the speed of stirring had no effect on the specific rate of absorption in the absence of catalyst and also in the presence of catalyst at a loading of 170(g/loO g of acid). However, with 2% (g/lOO g of acid) catalyst loading, the speed of
Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 343
4 9
c
/ I
I
I
15
20
I
25 Speed of stirring, revls
1
30
1
35
Figure 8. Effect of speed of stirring on the volumetric rate of ethylene absorption in mechanically agitated contactors. Reaction conditions: T = 353 f 1 K; P = 1atm; acid concentration = 98 w t m3. (-k) 95-mm4.d. %. (-0-) 52-mm4.d. contactor, V = 9 X m3. contactor. V = 3 x Table IV. Effect of Diethyl Sulfate on the Specific Rate of Absorption of Ethylene i n a S t i r r e d Cell" diethyl sulfate Pol, 107~*, loading, wt 70 kmol/m3 kmol/(m2 s) 18.25 1.98 12 15.20 2.44 23 12.40 3.96 36 9.60 7.50
9
100
100
200 Time, s
300
400
0
-C
Figure 9. Effect of ethylene loading on the volumetric rate of absorption in an autoclave. Reaction conditions: T = 243 f 1 K; V = 4 x IO4 m3; speed of stirring = 15 rev/s; acid concentration = 98 wt %.
"
X
"Conditions: T = 303 K; P = 1atm; V = 2.5 X m2; speed of stirring = 1.166 rev/s.
m3; a = 59.8
7 t
t
4
Table V. Effect of Ethylene Loading o n the Specific Rate of Absorption of Ethylene i n a S t i r r e d Cell" 107RA,kmol/(m* s) ethylene loading, wt 7'0 1.98 3.11 20 4.26 32 m3; a = 59.8 a Conditions: T = 303 K; P = 1 atm; V = 2.5 X m2; [Bo] = 18.25 kmol/m3 (98 wt %); speed of stirring = x 1.166 revis.
1
0
8
16 Pressure, atm
stirring had a small effect on the specific rate of absorption. Figure 8 shows the effect of speed of stirring on the volumetric rate of absorption of ethylene in mechanically agitated contactors. In this case the speed of stirring has a strong effect on the rate in the range studied. Effect of Diethyl Sulfate and Ethylene Loading. It was found that the presence of diethyl sulfate in the reaction mixture enhanced the reaction rate compared to fresh sulfuric acid. Thus, the specific rate of absorption of ethylene in a synthetic mixture of diethyl sulfate and 98 wt '70sulfuric acid (36 w t % diethyl sulfate) was found to be 3.8 times higher than in 98 wt % acid under otherwise identical operating conditions as shown in Table IV. The higher rate of absorption in the mixture of diethyl sulfate-sulfuric acid may be due to higher solubility of ethylene in diethyl sulfate (Truchard and Harris, 1961). In the present study it was not possible to measure the solubility of ethylene due to conversion of diethyl sulfate to ethyl hydrogen sulfate. The specific rate of absorption of ethylene was found to be higher in sulfuric acid loaded with ethylene than that in fresh acid (Table V). The effect of ethylene loading on the volumetric rate of absorption was also studied at elevated pressures in the magnetically stirred autoclave.
-
24
32
Figure 10. Effect of pressure on interfacial area in the magnetically stirred autoclave. Reaction conditions: T = 333 f 1 K; V = 4 X 10"' m3; speed of stirring = 23.33 rev/s.
The autoclave was pressurized to a preset pressure. Then the rate of absorption was measured by noting the fall in pressure up to a certain value, the autoclave was repressurized to the initial pressure, and the process was repeated three times. Figure 9 shows the effect of ethylene loading on the volumetric rate of absorption. Effect of Pressure on Interfacial Area. The effect of elevated pressure on interfacial area is shown in Figure 10. It appears that the effect of pressure on effective interfacial area, above the critical speed, has a negligible effect. A similar observation has been reported in the case of absorption of COPin aqueous diethanolamine solutions in a mechanically agitated reactor (Oyevaar and Westerterp, 1989; Oyevaar et al., 1989). Effect of Supercritical Conditions. In some experiments, ethylene ( T , = 282.8 K; P, = 50.5 atm) was absorbed under subcritical (T, = 1.177; P, < 1)to supercritical ( T , = 1.177; P, > 1) conditions in 98 and 94.5 wt % sulfuric acid, respectively. Figures 11 and 12 show the effects of the supercritical state of ethylene on the rate of
344 Ind. Eng. Chem. Res., Vol. 30, No. 2, 1991 '-1
I
Table VI. Extraction of Diethyl Sulfate from a Mixture of Diethyl Sulfate and 98 w t % Sulfuric Acid with Chlorobenzene a s Solvent" composition of the mixture, % of diethyl sulfate wt% extracted in single diethyl sulfate sulfuric acid stage 70 30 95.6 60 40 91.0 50 50 66.7 a
I
40
I
I
4a Pressure, atm
44
-
I
52
56
Figure 11. Effect of supercritical condition of ethylene on volumetric rate of absorption in an autoclave. Reaction conditions: T = 333 1 K; V = 4 X lo-' m3; acid concentration = 98 wt %; speed of stirring = 15 rev/s.
*
I
/ I
Condition: so1vent:mixture = 2:l (v/v).
Conclusions The reaction between ethylene and concentrated sulfuric acid is first order with respect to ethylene, and the reaction conforms to fast pseudo-first-order reaction mechanism under certain conditions. High pressure favors the formation of diethyl sulfate. The presence of diethyl sulfate in the reaction mixture enhances the specific reaction rate of absorption compared to fresh acid under identical conditions. The specific rate of absorption in sulfuric acid loaded with ethylene was also found to be higher than that in the fresh acid. Silver oxide is a good catalyst for the reaction between ethylene and sulfuric acid. The effect of elevated pressures on effective interfacial area in the case of a mechanically agitated contactor, above the critical speed, is negligible. The changeover for the subcritical pressure (T, N 1) to supercritical pressure (T,N 1)gave a substantial rise in the volumetric rate of absorption of ethylene and thus has potential for industrial exploitation. Acknowledgment
I
S.K.C.is thankful to the University Grants Commission, New Delhi, India, for the award of a senior research fellowship.
/ 56 40 44 48 52
Nomenclature a = gas-liquid interfacial area, m2 a = specific interfacial area, m2/m3 [Bo]= concentration of B, sulfuric acid, kmol/m3 P = pressure, atm R A = specific rate of absorption of A, ethylene kmol/(m2 s) T = temperature, K V = volume of liquid, m3
236
Pressure, atm
Figure 12. Effect of supercritical condition of ethylene on volumetric rate of absorption in an autoclave. Reaction conditions: T = 333 f 1 K V = 4 X IO-' ms;acid concentration = 94.5 w t %; speed of stirring = 15 rev/s.
absorption in acid. It can be seen from Figure 11 that in 98 wt 70acid the rate of absorption increases by 8.0% in the subcritical region for a pressure increase of 4 atm, whereas for the same increase in pressure in the supercritical region the rate of absorption increases by "3.8%. Extractioh of Diethyl Sulfate from Reaction Mixture. The extraction of diethyl sulfate from the reaction mixture and reuse of the unconverted acid are the major problems associated with the ethylene-sulfuric acid process. In the present investigation it was found that it is possible to extract more than 90% of diethyl sulfate from the reaction mixture by using chlorobenzene as a solvent and that the unconverted acid can be recycled after mixing with fresh acid. It can be seen from Table VI that, for efficient extraction of diethyl sulfate from the reaction mixture, the mixture should contain 60% or more diethyl sulfate.
Greek Symbol 4 = enhancement factor, dimensionless
Subscripts c = critical r = reduced Registry No. H2C=CH2, 74-85-1; H2S04,7664-93-9; Ag20, 20667-12-3; diethyl sulfate, 64-67-5; ferrous sulfate, 7720-78-7; chlorobenzene, 108-90-7; cuprous oxide, 1317-39-1.
Literature Cited Brooks, B. T. Synthetic Alcohols and Related Products from Petroleum. Ind. Eng. Chem. 1935,27, 278-288. Damiens, A.; de-Loisy, E.; Pitter, 0. Fixing Ethylene by Sulfuric Acid. US.Patent 1574796,1926a; Chem. Abstr. 1926a, 20, 1415. Damiens, A.; de-Loisy, E.; Pitter, 0. Ethyl Sulfuric Acid. US.Patent 1589372, 1926b; Chem. Abstr. 1926b, 20, 3015. Damiens, A.; de-Loisy, E.; Pitter, 0. Fixing Ethylene by Sulfuric Acid. US.Patent 1599119,1926~;Chem. Abstr. 1926c, 20,3460. Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: New York, 1970. Davis, H. S.; Schuler, R. J. The Relative Rates of Absorption of the Gaseous Olefins into Sulfuric Acid. J.Am. Chem. SOC.1930,52, 721-738.
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Received for reoiew April 10, 1990 Revised manuscript received August 15, 1990 Accepted September 4, 1990
Kinetic Studies on the Catalytic Decomposition of Hydrogen Sulfide in a Tubular Reactor Vassilios E. Kaloidas and Nickos G. Papayannakos* Laboratory of Chemical Process Engineering, Department of Chemical Engineering, National Technical University of Athens, Iroon Polytechniou 9, GR-157 73 Zografou, Athens, Greece
The kinetics of hydrogen sulfide decomposition on catalyst MoS2is studied with the use of a tubular reactor. Data were obtained in the temperature range 1013-1133 K, pressure range 1.3-3.1 atm, mol/(s-cm2). A Hougen-Watson adsorption and feed specific flow rate range 2.8 X 10-4-3.5 X model is adopted to represent the reaction mechanism. Analysis of the data indicates that the rate-determining step of the decomposition reaction is the cleavage of hydrogen-sulfur bonds of the hydrogen sulfide adsorbed on the catalyst active sites. The kinetic parameters and their dependence on temperature are determined. The dependence of the catalyst irreversible deactivation on the reaction temperature is presented and discussed.
Introduction The catalytic decomposition of hydrogen sulfide toward hydrogen and sulfur production has become a subject of considerable investigation in the past fifteen years due to the effective utilization of H2S. This strong pollutant is either produced as an unavoidable by-product in processes of heavy oil hydrodesulfurization and coal gasification or obtained from sour natural gases. Research and development of hydrogen energy technology is also interested in the decomposition reaction of hydrogen sulfide. The advantage of the decomposition process compared to the conventional Clauss process is the production of hydrogen besides sulfur. Hydrogen is a valuable product used as raw material in the chemical industry and as a clean fuel to produce energy at high temperatures. In an early work, Kingman (1936) carried out H2Sdecomposition experiments over a heated molybdenum filament in the temperature range from 673 to 1138 K. A first-order reaction kinetics with respect to H2S partial pressure and an activation energy of 25 kcal/mol are mentioned. Searching for an effective catalyst for the H2S decomposition reaction, Kotera et al. (1974, 1977) and Fukuda et al. (1978) have compared the catalytic activities of the
* Author
to whom correspondence should be addressed. 0888-5885/91/2630-0345$02.50/0
compounds FeS, COS, NiS, MoS2, and WS2 a t temperatures and pressures ranging from 773 to 1073 K and from 0.08 to 0.13 atm respectively. FeS, COS, and NiS were sulfurized to FeS2,Cos2,and NiS2and presented negligible catalytic activity. An activation energy of 26.8 kcal/mol was calculated from the initial rates of H2S decomposition using MoS2 powder (specific area 4.1 m2/g) as a catalyst. A greater catalytic activity of MoS2 is reported compared to that of WS2. Using two different MoS2 charges, the first one at temperature 823 K for 180 h and the second one at 1073 K for 15.5 h, no significant changes of the catalytic activity a t each temperature have been observed. Raymont (1974,1975) carried out experiments using a commercial Co/Mo catalyst in a quartz reactor cell and reports a H2S catalytic decomposition activation energy of 8 kcal/mol. Chivers et al. (1980) compared the catalytic activities of some metal sulfide powders in a quartz reaction cell at temperatures ranging between 673 and 1073 K. They propose as active catalysts for H2S decomposition the compounds Cr2S3 (specific area 13.4 m2/g), MoS2 (3.7 m2/g), and WS2 (5.5 m2/g). In the up-to-date kinetic studies of the H2S catalytic decomposition no reaction mechanism has been suggested and the reverse reaction has not been taken into account. Various substances (such as Pt, Pd, A1203,red P) are referred to (Pascal, 1960) as catalysts of the H2Ssynthesis 0 1991 American Chemical Society
