Enqq Vol. 12. No. 12. PP. 1275 1279. 1987 Printed in Great Britain

0360-5442:87 $3.00 + 0.00 Pergamon Journals Ltd

SELECTIVE SEPARATION OF HYDROGEN SULFIDE BY PRESSURE-SWING ADSORPTION

0. A. SALMAN and A. I. BISHARA

Kuwait Institute for Scientific Research, P.O. Box 24885, 13109-Safat, Kuwait

(Receiwd 13 Nowmhrr 1986)

Abstract--A pressure-swing adsorption process using molecular sieves as adsorbents was tested for the effective separation of hydrogen (H,) from a gas stream containing 88 ~01% of hydrogen sulfide (H2S) and 12 ~01% H,. Molecular sieve type Y (sodium ion) exhibited the highest dynamic capacity. adsorption efficiency and saturation time.

INTRODUCTION

Hydrogen plays an important role in petrochemical industries and in oil refining. For example, H, is a necessary raw material for ammonia and methanol synthesis. In oil

refineries, large amounts of H, are consumed in the hydrodesulfurization of petroleum streams, while producing H,S. The most common process for treating H,S is the Claus process in which H,S is partially oxidized to produce sulfur, and the H, is wasted as water.

Thus, the possibility of recovering both H, and S by H,S decomposition is of great interest to the petroleum industry, since H, can be recycled for many refinery operations.

In a previous paper,’ we studied the kinetics of H,S thermochemical decomposition by solar energy using various catalysts. It was shown that an H, yield of 18 ~01% at a reaction temperature of 770°C and residence time of 0.3 set can be obtained using cobalt-~

molybdenum catalysts. Here we discuss H,-H,S separation when the concentration of H,S is 88 ~01%.

One way of selectively separating H,S is by using a solution of ethanolamine in a thermal swing process. In the process of desulfurization of petroleum fractions, this has

become an established technology, but the concentration of H,S in the gas stream is generally low (less than 10~01%). Kuichi et ~1.’ studied the absorption of H,S from a gas stream containing equimolar quantities of H,S and H, by using a solution of 20wt% monoethanolamine in water at 25°C. They showed that H,S was not detected in the

effluent gas. They also studied the regeneration of H,S by heating a solution containing 40mol% H,S in monoethanolamine at 100°C. About 75% of the H,S was released after 30 min of heating.

Another common technique for H,SH, separation is pressure-swing adsorption (PSA). PSA or heatless adsorption separates gases into two streams: product and purge. The purge stream contains a higher percentage of strongly adsorbable material than the product stream. Adsorbed gases are removed by pressure reduction, which allows for regeneration without the use of heat.

Bandermann and Harder3 studied the separation selectivity of two carbon molecular sieves with bulk densities of 0.64 and 0.7g-ml-’ and of two zeolites with 5 and 10 A pore diameters. They showed that all adsorbents, except for the 5A zeolite, had a constant adsorption capacity after 10 cycles. Whitely and Hamrin4 conducted experiments on the

continuous separation of H,S-H, mixtures with H,S concentrations of 3.01 and 6.32%. Their experimental system was similar to that constructed by Weaver and Hamrin5 for hydrogen isotope separation. The system consisted of two brass columns, 87.6cm long with 0.94cm diameter. Both columns were packed with molecular sieve type 4A, 8 x 12 mesh beads (Union Carbide). Feed gas was directed into one column and purged out of the other alternately by a four-way solenoid valve connected to a timer. The concentration of H,S in the product gas was determined by a thermal conductivity cell. It was reported that purge-to-feed ratio, cycle time, and total feed rate affect the separation efficiency.

1275

1276 0. A. SALMAN and A. I. BISHARA

Removal of H,S goes through a maximum with an increasing purge-to-feed ratio, and the optimum values lie between 2.88 and 4.19.

Fukui et al6 conducted a study to find the most suitable adsorbent for H,S removal by a dry process. Ten kinds of synthetic zeolites were evaluated, and the relationship between the amount adsorbed and the adsorbent physical properties were discussed. It was shown that molecular sieve type 13X and 5A had the highest adsorption power for H,S. The amounts of adsorbed H,S were affected by the pore sizes of adsorbents rather than by the surface pH.

We present results of testing various molecular sieves for the effective separation of H, using a pressure-swing adsorption process.

EXPERIMENTAL SECTION

Materials

H,S (98% purity), Ar (99% purity) and H, (99% purity) gas cylinders were purchased from the Kuwait Oxygen Company. Zeolite type 13X was purchased from Union Carbide (MS-1361) and had the following specifications: nominal pore diameter = 10 A; base = alumina silicate; cation = sodium; bulk density = 391bfte3. Molecular sieve type Y (sodiumion), type Y (rareearth) (14-8910), and type 4A were provided by the Strem Chemical Company.

Apparatus and method

A schematic of the apparatus used for pressure-swing adsorption is shown in Fig. 1 and consists of two 316 stainless steel columns, 30cm long with 4cm inside diameters; H,S and H, flow meters; a vacuum pump; a gas chromatograph (Shimadzu GC-9A) equipped

1

Vacuum pump

Fig. 1. Flow diagram for the pressure-swing adsorption apparatus for H&H, separation by molecular sieves.

with a thermal conductivity detector and Porapac-Q columns. Each adsorption column was filled with about 1OOg of adsorbent, giving an approximate bed length of 7.5 cm.

In a typical experiment, the flow rate and concentration of the feed gas were adjusted to the required value by varying the positions of the feed valves. The total flow rate was measured by a rotameter, and the feed concentration was measured by gas chromatography. Once the desired flow rate and concentration reached a constant value, the feed was introduced into column A through inlet valves.

The adsorption/desorption cycle consisted of the following four steps: (1) The gas-feed mixture flows into column A where most of the H,S is adsorbed, and Hz-enriched gas flows off the column to the gas chromatograph. (2)Column A is evacuated once it is saturated and column B is repressurized. (3)Gas feed mixture flows into column B where H,S is adsorbed. (4)Column B undergoes blowdown (it is regenerated) and column A is repressurized. In all experiments two flow rates (4.5 and 9cm3-set-‘) and two feed concentrations (12 and 30% H,) were used.

Pressure-swing adsorption 1177

RESULTS AND DISCUSSION

Four types of molecular sieves were evaluated as adsorbents for H,S: 4A, 13X, Y (sodium ion), and Y (rare earth). The zeolites were compared in terms of breakthrough curves

(Fig. 2). The equilibrium time, which is the time when the product gas concentration reaches

0” \ 06.- u-

04-

Time (mini

Fig. 2. Breakthrough curves for H,S adsorption over zeolites 4A, 13X, Y sodlun ion, and Y rare earth; at a feed flow rate of 4.5cm3/sec and an inlet concentration of 88% H,S.

the feed concentration, decreases in the following order: Y (sodium ion) > 4A > 13X > Y (rareearth). The rate of change in concentration is very high near the equilibrium time region, as may be seen by the steep slope of the curves.

Although zeolite type 4A had a slightly higher equilibrium time than type 13X, its stability was much less. The equilibrium time for zeolite type 4A dropped from 17 min in

cycle 1 to about 8 min in cycle 2 and continued to decline at a lower rate (Fig. 3). After

25

r ‘.

\

I I I

0 5 10

Cycle number

Fig. 3. Effect of cycle number on the breakthrough time for zeolites 4A, 13X sodium ion, and Y rare earth.

only five cycles, the equilibrium time decreased to approximately 5 min. Zeolite type 13X, however, had constant adsorption after five cycles. The equilibrium time after 11 cycles was about lOmin, a factor of 2 higher than that for zeolite 4A. Similarly, zeolites type Y (sodium ion and rare earth) reached a constant equilibrium time within five cycles.

Similar results were reported by Bandermann and Harder.3 Carbon molecular sieves with bulk densities of 0.64 and 0.7g-cme3 and zeolite type 13X showed a constant

equilibrium time after 10 cycles, and 4A zeolites had a continuously decreasing equilibrium time with an increase in the number of cycles.

The effect of feed flow rate and composition on separation was also studied. Figure 4 shows the adsorption breakthrough curves (cycle 1) for type Y (sodium ion) molecular sieves at two feed flow rates: 4.5 cm3 set- ’ and 9 cm3 set- ‘. The saturation time is seen to decrease with increasing feed rate. By increasing the flow rate from 4.5 to 9cm3 sect ‘,

1278 0. A. SALMAN and A. I. BISHARA

1.0 .I•----

08

0” 06 \ 5

I r---! 0 .

0 90 crr?/sec l 45 cm%ec

I-.-.~. . .4 -._ _._ _. I 0 10 20 30

Time (mm)

Fig. 4. Effect of feed flow rate on the breakthrough time for zeolite type Y sodium ion (cycle I).

the saturation time dropped from 23.5 to 11.5 min. This result is accounted for by the relation:’

t = (1 - 4) vr QULJ’~ (1)

where t = time required to saturate the bed completely, < = porosity, V, = bed volume, d, = particle density, qm = maximum adsorption capacity, C, = inlet concentration, and F = flow rate. Equation (l), which is based on a rectangular adsorption isotherm, shows that the equilibrium time is inversely proportional to the feed flow rate. Thus, doubling the flow rate should theoretically reduce t by a factor of l/2, which agrees with our findings.

The effect of inlet-feed concentration on adsorption stability is shown in Fig. 5. The

25

l 70% H*S

0 80% H,S

I 5 10

Cycle number

Fig. 5. Effect of inlet feed concentration on the adsorbtion stability of zeolite type Y (sodium ion)

zeolite adsorption behavior was more stable with a feed composite of 70% H2S. The saturation time reached a constant value of about 15 min after only three cycles. For the higher feed concentration, the saturation time continued to decline up to eight cycles and reached a steady value of about 11.5 min.

Finally, the characteristics of each zeolite were calculated in terms of dynamic capacity (q), length of working layer (I,,), and adsorption efficiency (7). These parameters are defined as

q = AFdC 09

where A = area above the breakthrough curve, F = feed tlow rate and

L, = L [

t’ - t

t’-(1 -f)(t'- t) 1 ;

here, L = height of adsorption layer, t = breakthrough time, t’ = saturation time,f = sym-

Pressure-swing adsorption I1179

metricity factor (0.5). Finally,

;’ = [L - (1 - ~~)L,]/L. (4)

The results of these calculations (Table 1) show that the highest dynamic capacity and

Table I. Data analysis for H2S adsorptiont ----- Adsorbent

type

13X

Y (Na+)

-------

4A

Y (rare earth)

CyclC NO.

.----_

1

11

--_

1

5

8

10

__--_

1

3

1

7

.---__

-T-

_ _

DynemiC

capacity

ts Q$Vloo 9 zeolite )

5.31

1.40

8.05

4.41

3.17

3.12

6.65

1.98

5.07

0.15

-

L

(m’

---

9.7

9.7 -__

7.5

7.5

7.5

7.5

--

9.7

9.7

--

7.5

7.5

---_

-r -----_

t’ (tin)

-__.

17.00

4.50

--

24.00

12.50

11.00

11.00

--

20.00

8.00

16.00

9.00

__-.

------

t (tin)

---___

14.00

3.50

---

22.50

11.40

8.00

9.50

----

17.00

6.00

-----

13.00

7.00

-----

Lo

(cm)

---

1.98

2.43

0.48

0.69

2.36

1.10

----

1.57

2.77

---_

1.55

1.88

-r-

c

_--_____--

Y (%)

_-_____._-

90

07

__------

97

95

84

93

_----_---

92

86

09

88

tlnlet H,S concentration = 88 ~01%: feed flow rate = 4.5cm’ set-l

adsorption efficiency were obtained for molecuIar sieve type Y (sodiumion). In addition, dynamic capacity declined steadily as a function of cycle number, implying loss in the stability of the zeolite. One common feature among the zeolites is high adsorption efficiency. For all experiments, values higher than 80% were obtained for 1’.

The results obtained in this study show that H,S--H, can be separated effectively with

molecular sieves in a pressure-swing adsorption process. Zeolite type Y (sodium ion) had the highest dynamic capacity, adsorption efficiency and saturation time.

Acknowleci~ement This work was supported, in part, by Kuwait University under Contract No. EC-014

REFERENCES

I. A. Bishara, 0. A. Salman, N. Kraishi and A. Marafi, Int. J. Hydrogen Energy In press. 2. H. Kiuchi, K. Funaki, Y. Nakai and T. Tanaka, Int. J. Hydrogen Energy 9, 701 (1984). 3. F. Bandermann and K. Harder, Int. J. Hydrogen Energy 7, 471 (1982). 4. M. D. Whitely and C. E. Hamrin, in ACS Symposium (Edited by M. J. Cornstock). Washington, D.C. (19801. 5. K. Weaver and C. E. Hamrin, Chem. Engng Sci. 29, 1873 (1974). 6. T. Fukui, Y. Ise, K. Boki and S. Tanada, J. Exp. Med 21, 21 (1974). 7 D. D. Do, AICHE 31, 1329 (1985).