Ind. Eng. Chem. Fundam. 1981, 20, 97-99
97
Possibility for Effective Production of Hydrogen from Hydrogen Sulfide by means of a Porous Vycor Glass Membrane The possibility of using a porous Vycor-type glass membrane was examined for the production of hydrogen from hydrogen sulfide. The porous Vycor-type glass tubing could be used up to 800 "C as a membrane for separating hydrogen and hydrogen sulfide. When this membrane was applied to the direct decomposition of hydrogen sulfide, the yield of hydrogen increased about two times the equilibrium yield calculated for the process without hydrogen removal.
Introduction A huge amount of hydrogen sulfide has been produced as an industrial byproduct in the fields of hydrodesulfurization of heavy oil and coal gasification. Nowadays increasing needs of hydrogen caused by the shortage of oil resources makes more attractive the method of production of hydrogen from hydrogen sulfide. The key to commercial use of hydrogen sulfide as a source of hydrogen lies in the developments of both good catalysts and the method of equilibrium shift because the direct decomposition of hydrogen sulfide is thermochemically unfavorable up to 1800 K. The present authors (Fukuda et al., 1978) have already found that molybdenum disulfide and tungsten disulfide were effective as catalysts in the direct decomposition of hydrogen sulfide. One of the methods for shifting equilibrium conditions industrially is the removal of products by condensation of sulfur and then intermittent removal of hydrogen from hydrogen sulfide by scrubbing with a selective solvent, for example, methanol used in the Rectisol process. It should be noted, however, that the process has an inevitable disadvantage of low heat efficiency, which results from the repeated cycles of heating and cooling a large quantity of reactants. Recently, the advantage of separating hydrogen directly from the reaction zone has been predicted as an alternative process (Raymont, 1975; Dokiya et al., 1977; Fukuda et al., 1978). Such a process cannot be established without finding a suitable selective diffusion membrane. Since hydrogen sulfide starts to decompose measurably above 500 "C, membranes should not be degradated above 500 "C. Palladium alloy membranes have been used in the industrial purification of hydrogen (Chem.Eng., 1965),and molecular sieves have also received much attention because of their ability to separate different chemical species. These membranes, however, could not be used above 500 "C because of their degradation in the presence of hydrogen sulfide at high temperatures. Some porous ceramic membranes may be used above 500 "C. Up to now, the porous Vycor glass has been used as a separation membrane for many gas mixtures below 500 "C in the absence of hydrogen sulfide (Huckins and Kammermeyer, 1953; Higashi et al., 1964; Sun-Tak Hwang and Kammermeyer, 1966);however, there are no reports on the use of it above 500 "C under hydrogen sulfide atmosphere. There is still a great possibility of using the porous Vycor glass above 500 "C in view of its thermal stability at high temperatures up to 800 "C (Nordberg, 1944). In this study, we report the results of examination of the possibility of using a microporous Vycor glass membrane for the effective production of hydrogen from hydrogen sulfide at temperatures up to 800 "C. Experimental Section A schematic diagram of the apparatus used is given in Figure 1. The diffusion cell (C) is made of stainless steel and its temperature is controlled by an electric furnace (D). 01 96-43 13/81/1020-0097$01.OO/O
A recrystallyzed nonporous alumina tubing (H) is placed close to the inner surface of the cell wall to prevent the sulfurization of the diffusion cell at high temperatures. Both ends of the porous glass tubing are sealed by carbon packing (I). The feed gas (A) is introduced into the high-pressure side in the diffusion cell and diffuses through the porous glass membrane (E) into the low-pressure side. The pressure of the feed gas is controlled by a pressure regulator (B). The undiffused stream is taken off at the bottom of the high-pressure side. The gas which diffused through the porous glass membrane is allowed to vent at atmospheric pressure from the bottom of the low-pressure side. The flow rates of the vent gases are measured by means of a soap bubble meter. Before the feed gas is introduced into the diffusion cell, both sides are flushed with nitrogen gas under 1 atm to remove the air. The composition of feed and diffused and undiffused gases is analyzed in the same manner as previously reported (Fukuda et al., 1978). The microporous Vycor-type glass tubing was supplied by Toshiba Kasei Co. (Japan); its specifications are 600 mm in length, 15 mm in outer diameter, and 3.0 mm in thickness. Its chemical composition is 96.60 w t 9 i Si02, 2.91 wt % Bz03, 0.42 wt % A1203,and 0.07 wt % Na20. The maximum distribution of the pore diameter was measured to be 45 A, and 86% of the total volume fell within f10 A. The porosity and the specific surface area of the porous glass were 0.79 and 191 m2/g, respectively. These specifications were similar to the so-called Vycor brand porous glass. Results and Discussion At fist, the feasibility test was made on the porous glass membrane under an atmosphere of hydrogen and hydrogen sulfide gas mixtures at temperatures up to 800 "C. The feasibility was checked by the separation factor (am), which is given in Table I for 4 runs on the same porous glass membrane. In these runs, the material balance was kept within f3% except for run 3. Mole fractions of hydrogen gas (X)in the feed gas, and pressures (P) on the high-pressure side were kept constant within &7% and 5%, respectively. Run 4 in Table I was carried out by using the same membrane as in runs 1, 2, and 3. The separation factor was well reproduced in run 4. This shows that no degradation of membrane performance has occurred during for at least 216 h operation at 600-800 "C in the presence of hydrogen sulfide. This phenomenon was confirmed from the observation that changes in the average pore diameters were not found after heating as shown in Table 11. It is well known that porous glasses shrink when heated and do not return to the original dimensions on cooling. The linear shrinkage on the porous Vycor-type glass was measured and shown in Figure 2. A remarkable shrinkage starts above 800 "C. Similar trends have been reported on Vycor brand porous glass (Nordberg, 1944; Corning 0 1981 American Chemical
Society
98
Ind. Eng. Chem. Fundam., Vol. 20, No. 1, 1981
Table I. Effect of Diffusion Cell Temperatures o n the Separation Factors of H,-H,S Separation Systema
--
lVd,
X 0.085 0.082 0.089 0.093
____run 1 2 3 4 ap =
P, atm 4.2 4.3 4.5 4.4
T , "C 10 600 800 10
t, h 24 144 72 24
mL/min 70 27 26 88
8
0.28 0.36 0.52 0.50
YU
yd
0.094 0.128 0.155 0.102
a'm
0.079 0.064 0.075 0.091
1.12 1.64 1.88 1.11
)r
,110
1.21 2.15 2.26 1.13
1.0 atm.
Table 11. Changes in Pore Characteristics of the Porous Glass after Heat Treatment
. ~ _ _ _ _ - _ _ _
material
av pore diam, A
raw heat treatment
41.4 41.2
pore density, cm3/A 31 x 34 x
loe3
surface area, m2/g 191 132
a Heat treatment of the porous glass was carried out at 780 " C for 24 h in air.
r ' IEMPLRLTUQE [ I
H
Figure 2. Shrinkage of the porous glass at various temperatures after 24 h heating in air. Table 111. Effect of the Use of the Porous Glass as a Separation Membrane in the Decomposition Reaction Temperatures of Hydrogen Sulfidea
P, T , mC/ run atm "C min 4.0 600 2.4 800 2 3 4.3 800 4 b 3.8 819 1
19 13 26 17
e 0.35 0.30 0.52 0.71
Yd
Yu
0.040 0.082 0.101 0.118
0.026 0.047 0.048 0.028
a"
1.56 1.81 2.32 4.60
X, 0.012 0.062 0.052 0.053
Figure 1. Schematic diagram of an apparatus used for the H2-H2S separation system: A, gas cylinder; B, pressure regulator; C , diffusion cell; D, electric furnace; E, membrane of porous Vycor-type glass tubing; F, sulfur condenser; G, gas chromatograph; H, nonporous alumina tubing; I, carbon packing.
MoS, is packed in the outer side of the t = 24 h. membrane as a catalyst (Dokiya e t al., 1977).
Glass Works, 1976). Thus, the microporous Vycor-type glass can be used as a separation membrane below 800 "C in the presence of hydrogen sulfide and sulfur without degradation. The separation factor was found to increase with rising cell temperatures as shown in Table I for runs 1, 2, and 3. This phenomenon can be explained by the increasing contribution of the Knudsen flow which occurs when the ratio of the pore radius to the mean free path is very small. At room temperature and under 1atm, the mean free path of hydrogen and hydrogen sulfide are 1230 and 430 A, respectively, whereas the pore diameter is 45 A. Since the mean free path is known to increase with temperature, the ratio can be expected to increase with temperature, which makes the Knudsen flow predominant. Moreover, the amounts of hydrogen and hydrogen sulfide adsorbed on the porous glass membrane were found to be negligible at high temperatures above 300 "C, where no surface diffusive flow contributes to the separation process (Kameyama et al., 1979). Thus, the gas flow diffusing through the membrane becomes closer to the ideal Knudsen flow and obeys Graham's law markedly with rising temperature. In run 3, the sum of the hydrogen in diffused and undiffused gases was detected to be 10 mol %. It is worth noting that the value was larger than 8.9 mol % hydrogen in the feed gas. This shows that the decomposition of
hydrogen sulfide has started remarkably at 800 " C as previously reported (Fukuda et al., 1978). On the basis of these results, the porous glass was used as separation membrane in the decomposition reaction of hydrogen sulfide at about 800 "C. In all cases, the yield of hydrogen produced (Yd)through the membrane became about two times higher than the equilibrium yield (X,) as shown in Table 111, which was calculated for the process without using the membrane. X,was calculated on the base of the thermodynamics about the decomposition of hydrogen sulfide. Sulfur was found to be concentrated mainly in undiffused gases, that is, in the outer side of the porous glass membrane. The catalytic activity of the porous glass particles on the decomposition of hydrogen sulfide was examined. Molybdenum disulfide has high catalytic activity as previously reported (Fukuda et al., 1978),but the porous Vycor-type glass particles shows no catalytic activity as shown in Table IV. Thus, the mechanism of the separation and concentration of hydrogen gas can be understood as follows. Hydrogen sulfide decomposes in the upstream side of the porous glass membrane and then the hydrogen produced diffuses selectively into the inner side through the porous glass membrane. The thermochemical equilibrium condition in the upstream side is continuously shifted by the separation of hydrogen from the reaction zone and the
Ind. Eng. Chem. Fundam. 1981, 20, 99-100
Table IV. Catalytic Decomposition of Hydrogen Sulfide at 750 Oca ~~
~
reaction
catalyst sur-
mate-
face area,
rial
m’/g
MoS,
-
blank porous glass
9.0
155
pres-
sure ticle of fed size, H,S, par-
mm 3-5
1-2
H, concn,b %
torr
20 min
120 120 142
27 5 5
75
120
min min 41 13 11
47 18 14
a Reaction apparatus was same as in previous report (Fukuda et al., 1978) and elementary sulfur was trapped out continuously from the reaction gas mixtures. Hydrogen concentration indicates the amount of hydrogen accumulated in the reaction system.
higher than equilibrium yield of hydrogen is obtained in the downstream side. This phenomenon is directly caused by the relatively easy diffusion of hydrogen and the rejection of hydrogen sulfide and sulfur by the membrane. The remarkable advantages of performing separation at reaction conditions became clear. Therefore, the microporous Vycor-type glass can be used as a separation membrane in the effective production of hydrogen from hydrogen sulfide. The separation factor, still low compared to the ideal one (4.1), may be mainly due to the nonseparative viscous flow under these high pressures. Further separation experiments will be carried out for the establishment of the practical hydrogen production system and when it is successfully finished, then even pure hydrogen will be obtained by a multistage cascade system. Nomenclature
p = total pressure on low-pressure side, atm
P = total pressure on high-pressure side, atm Nd = flow rate on low-pressure side, mL (STP)/min Nu = flow rate on high-pressure side, mL (STP)/min
99
T = diffusion cell temperature, “C t = period of run, h X = mole fraction of hydrogen gas in a feed gas mixture, dimensionless Yd = mole fraction of hydrogen gas in a diffused gas mixture, dimensionless Y, = mole fraction of hydrogen gas in an undiffused gas mixture, dimensionless X, = equilibrium mole fraction of hydrogen gas in the decomposition reaction of hydrogen sulfide under given conditions, dimensionless Greek Letters Yd (1 - X ) / X (1 - Yd), measured separation factor,
a, =
dimensionless a’, = Yd (1- Y,,)/Y, (1- Yd), measured separation factor,
dimensionless + Nu), ratio of flow rate of a diffused gas to that of a feed gas, dimensionless
8 = Nd/(Nd
Literature Cited Dokiya, M.; Kameyama, T.; Fukuda, K. Denski Kagasku 1977, 45(11), 701. Fukuda. K.; Dokiya, M.; Kameyama, T.; Kotera, Y. Ind. Ens. Chem. Fundam. 1976, i 7 , 243. Higashi, K.; Ito, H.; Oishi, J. J. Nud. Sci. Techno/. 1964, 7(6), 296. Huckins, H. E.; Kammermeyer, K. Chem. Eng. frog. 1953, 49(4), 180. Kameyama, T.; Ddtiya, M.; Fukuda, K.; Kotera, Y. Sep. Sci. Techno/. 1978, 141101. - , .953. --Nordberg, M. E. J. Am. Ceram. S O ~1944, . 27(10). 299. “Palladium Diffusion Yields High-Volume Hydrogen”, Chem. Eng. Mar I, 1865, 36. Raymont, M. E. D. Hydrocarbon Process. 1975, 54(7), 139. Sun-Tak Hwang; Kammermeyer, K. Sep. Sci. 1966, 7(5), 629. “Vycor Brand Porous (“Thirsty”) Glass No. 7930”, cited from the catalog on the Corning Glass Works, 1976. \
National Chemical Laboratory for Industry Tsukuba Research Center Yatabe, Ibaraki, 305, Japan
T. Kameyama* M. Dokiya M. Fujishige H.Yokokawa K. Fukuda
Received for review October 15, 1979 Accepted September 26, 1980
This work was done in the “Sun Shine Project” of the Ministry of International Trade and Industry of Japan.
Vapor-Liquid Equilibria for Stripper Design The infinite dilution activity coefficient for methanol in ldecanol at 55 OC was measured by a gas chromatography orocedure. The data (1-43. 1.44) C O ” x d favorablv with the value calculated from eauilibrium oressure-temperature measurements in the literature (1.45). The ihromatographic technique has direct application in stripper design.
Infinite dilution activity coefficients can be determined by gas-liquid chromatography (Everett and Stoddard, 1961; Martire, 1961; Snyder and Thomas, 1968). This method is limited to the case of a low-boiling solute dissolved in a relatively high-boiling solvent. This is generally the case in stripping columns, and thus this is an ideal method of determining vapor-liquid equilibria in such systems. There are many references in the literature to this method but few comparisons of experimental data to those obtained by more usual methods. Infinite dilution activity coefficients were obtained by the chromatographic procedure for the system, methanol/decanol at 55 “C. This system had been studied by Singh and Benson (1968) using the method of total pressure measurement over solutions of known concentration. Their data were correlated by the van Laar equation. This 0 196-4313181 11020-0099$01.0010
gave a value of 1.45 for the infinite dilution activity coefficient of methanol at 55 “C. In our work we used columns packed with 21.6% l-decanol on Chromosorb P support (30/60 mesh). (Chromosorb P is a product of Johns-Manville.) The chromatograph was a Hewlett-Packard model 5750 with thermal conductivity detectors. Two calibrated thermocouples were taped to the columns. Column inlet and outlet pressures were measured with strain gauges. Retention times were measured with a Hewlett-Packard Model 3370A electronic integrator. Helium carrier gas flowrates were measured with a soap bubble meter. Methanol and air were injected simultaneously using a 10-pL syringe. The amount of methanol injected was kept at or below 0.5 pL. The carrier gas rate was varied from 16 to 102 mL/min. Two columns were used, one with 1.04 0 1981 American Chemical Society
