MACRORETICULAR ION EXCHANGE RESINS AS HYDROGEN SULFIDE SORBENTS FRANK POLLlO AND ROBERT KUNIN
Rohm and Haas Co., Philadelphia, Pa,
Dry gas streams containing low volume percentages of H2S (0.5 to 4.0%) may be treated dynamically for HzS removal using Amberlyst XN-1007 or Amberlyst A-27, macroreticular quaternary ammonium anion exchange resins, operated
dry in the chloride form under ambient conditions. Increased HzS capacities are realized by conducting the adsorption in a pressurized system. Essentially complete hydrogen sulfide removal is achieved in either instance, hydrogen sulfide leakage being less than 0.01 volume YO.When the resin has become exhausted, regeneration is accomplished thermally, using a slow nitrogen purge which accelerates the release of HzS. The sorption operation is nonsensitive to COz and does not introduce moisture into the gas stream that has been treated.
REL,ATIVELY little
work has been directed toward the utilization of ion exchange resins as gas sorbents. The lack of intensive studies pertaining to the use of ion exchange resins for treatment of gas phases becomes understandable when one considers that the earlier ion exchange investigators were limited to the use of conventional geltype resin structures of low surface areas. For gas-phase reactions, especially under anhydrous conditions, these resins lack the necessary surface and pore attributes required of an effective adsorbent. Mills (1943) was one of the earliest ion exchange investigators to realize the possible utility of synthetic ion exchange resins for gas-phase applications. Robinson and Mills (1949) suggested the possible use of a polyamine exchanger based on a phenolic matrix for the removal of acidic or amine reactive substances from a gas phase. More recently, Cole and Schulman (1960) described the use of dry anion exchange resins, among other materials, for sorption of sulfur dioxide. With the recent availability of the relatively high surface and porous macroreticular resins (Kunin et al., 1962) many of the inherent disadvantages of utilizing synthetic ion exchange resins in an anhydrous condition have been overcome. Even in an anhydrous state, the macroreticular ion exchange resins retain the necessary pore and surface features essential for an adsorbent to be effective. As a result, favorable kinetic and adsorption properties are observed with their use for gas-phase applications. As the demand for natural gas has increased in recent years, the need has arisen t o process our natural gases of increasing hydrogen sulfide content, which in times past would have been wasted. Some idea as to the amount of natural gas consumed annually and future use has been stated (Oiland Gas Journal, 1958). I n the petroleum refining operations, removal of HzS from gas streams becomes imperative because of the deleterious effect of H2S upon many of the catalysts employed. Since the crude oils now being processed are increasing in sulfur, the necessity for removing H S from the recycle hydrogen streams is rapidly increasing. Furthermore, the recycle 62
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streams are also becoming possible sources of sulfur. The need for fuels of lower sulfur content has increased the use of hydrogen in petroleum refining and this in turn has increased the need t o remove HzS from the hydrogen recycle streams. An ion exchange resin process for treatment of gaseous streams containing HZS has advantages. Most conventional adsorption materials are sensitive to CO, and low molecular weight hydrocarbons, adsorbing these gases simultaneously with the HzS. A reduction in HZS capacity of the adsorbent unavoidably results. The ion exchange process under consideration, on the other hand, is insensitive to carbon dioxide and hydrocarbons. A disadvantage of most conventional aqueous purification systems, which cannot be minimized, is that moisture is introduced into the gas stream. This added water, plus the water normally found in the gas streams, must usually be removed, since it encourages corrosion and formation of gas hydrates. Experimental
The basic equipment used during the H2S sorption studies consisted of an adsorption and a desorption unit. In the adsorption unit, hydrogen sulfide and dry nitrogen are initially metered into a gas mixing tee-type tube, where hydrogen sulfide is diluted to a desired concentration in nitrogen. After mixing, a desired portion of the stream is passed upflow through the adsorption column and the excess gas throttled slowly to the atmosphere. Before entering the adsorption column, the gas stream is passed through a premixer column, which further assures homogeneity of the gas mixture. The adsorption column, constructed from a %-inch diameter straight glass pipe, contained the desired quantity of dry adsorbent being examined. I n the desorption unit, dry nitrogen is metered through the premixer column and then through the column containing the adsorbent previously exhausted with hydrogen sulfide. The premixer column and the adsorbent column
are thermostated by means of heating jackets, the temperature of the inside portion of the jackets in contact with the columns being conveniently measured using a pyrometer. Sorption Studies. Hydrogen sulfide sorption runs were carried out by passing dry nitrogen streams containing H,S upflow through the adsorbent column containing approximately 80 ml. of the dry adsorbent being tested, previously dried a t 60" C. for 16 hours a t reduced pressure ( < E mm. of Hg) or a t 110°C. (atmospheric pressure). Hydrogen sulfide loadings were performed a t ambient temperature, usually a t a flow rate of 100 cc. per minute, until influent and effluent were essentially equal in hydrogen sulfide concentration. The hydrogen sulfide leakage during exhaustion was followed quantitatively by gas chromatography (GLC) or qualitatively using moist lead acetate paper. During desorption, hydrogen sulfide was eluted from the exhausted adsorbent by passing dry preheated nitrogen upflow through the column a t a flow rate of 10 cc. per minute, the concentration of desorbed HIS being determined by GLC. Column temperature during desorption was 100" to 110" C. Total hydrogen sulfide leakage to breakthrough (adsorption phase) and total purged hydrogen sulfide (desorption phase) were determined iodometrically after the column effluent gas streams had passed through gas scrubbers containing standardized 0.1N iodine solutions. I n Table I , surface properties and anion exchange capacities (AEC) are listed for Amberlyst XN-1007 and Amberlyst A-27. Both resins are macroreticular quaternary ammonium anion exchangers based upon a styrenedivinylbenzene copolymer differing only in surface area and porosity. An adsorption isotherm was obtained for Amberlyst A-27 a t relatively low H,S concentrations. The degree of hydrogen sulfide saturation of the resin a t various concentrations was obtained dynamically using nitrogen streams containing 0.5, 1.0, 2.0, and 4.0 volume "i hydrogen sulfide passed a t atmospheric pressure through an 80-ml. anhydrous resin bed. Anhydrous resin, previously used during a six-cycle H ? S sorption study to process a nitrogen stream containing 2 6 H2S was employed to ensure that the resin's capacity for H a S had been fully stabilized. Using a flow rate of 100 cc. per minute, the adsorption runs were continued until effluent and influent HLS concentrations were equal. The adsorption isotherm prepared in this manner is represented in Figure 1. Figures 2 and 3 represent typical exhaustion and elution curves obtained in conjunction with the treatment of nitrogen streams containing H S . The curves are presented to illustrate the sharpness in breakthrough experienced during the exhaustion phase and relative ease with which H S may be thermally eluted during the desorption phase.
06,
-0
I
2
3
4
V O L U M E PERCENT H p S IN N I T R O G E N
Figure 1. Amberlyst A-27 hydrogen sulfide adsorption isotherm (dynamic) prepared at atmospheric pressure
L I T E R S OF 2 % H 2 S I N N I T R O G E N THROUGHPUT
Figure 2. Column performance of Amberlyst A-27 during hydrogen sulfide adsorption
Table I. Amberlyst Ion Exchange Resin Properties
Skeletal density, g./ml. Apparent density, g. iml. Porosity, ml. of poreiml. of bead Surface area, sq. m./g. Av. pore diameter, A. AEC. meq.ig.
A mberlyst
Amberlyst
XN-1007
A-27
1.14 0.81 0.29 30 480 4.2
1.14 0.55 0.51 46
810 2.5
V O L U M E ( c c ) OF NITROGEN ELUATE
Figure 3. Hydrogen sulfide desorption at 100" C. N i t r o g e n p u r g e g a s a t 10 cc. p e r minute flow r a t e
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exchange resin, has been used effectively in our laboratories for this purpose. Increased elution temperature has been found to accelerate the release of HzS during desorption a t a given nitrogen purge flow rate, although complete quantitative data are not available for comparative purposes. In practice, it has been found possible to desorb H2S from an exhausted resin a t the same temperature used during desorption. However, the volume of purge gas required is prohibitively large and is dependent on the desorption activation energy specific to the resin in question. During desorption, the maximum concentration of HrS that is achieved will also be dependent on the amount of H2S adsorbed per given volume of resin and the desorption purge flow rate used. Further H S sorption studies were conducted to determine the effect of pressure upon the H2S adsorption capacity of the resins. The pressurized adsorption unit consisted of a glass pipe column VJ inch in diameter and 18 inches long, containing approximately 58.5 ml. of dry chlorideform Amberlyst XN-1007 resin. The column was connected a t its lower extremity to a methane cylinder containing 1.5% (by volume) H2S and a t the other end to a back-pressure regulator equipped' with a pressure gage. The exit gas was passed through a gas scrubber containing standardized iodine solution (0.1N) to determine H r S leakage during exhaustion and H S concentration eluted during desorption. Exhaustion flow rate was 80 cc. per minute; the nitrogen purge flow rate during desorption (100" to 110"C.) was 8 cc. per minute. Amberlyst XN-1007 adsorption capacities measured a t various operating pressures are plotted in Figure 4. An examination of the adsorption isotherm indicates that by operating the HrS adsorption unit under pressurized conditions, increased adsorption capacities are realized. The maximum point a t which increased pressure would no longer exert an appreciable effect upon increasing the resin capacity for H2S was not determined because of the limitations of the equipment. T o operate at considerably higher pressures would have required a more elaborate apparatus. I n Table 111, the effect of flow rate upon the adsorption capacity of Amberlyst XN-1007 was demonstrated during a series of runs performed a t an operating pressure of 5 atm. Before making the runs a t 400 and 685 cc. per minute, a second run was performed using the resin a t the normal 80 cc. per minute flow rate. This was done
When the ion exchange resin is initially employed, the loading capacity for H2S usually exceeds its subsequent operating capacity. In addition, only 60 to 70% of the HzSinitially loaded is recovered during the first desorption, compared with 80 to 90% once the capacity of the ion exchange has been stabilized. The lower recovery of H2S during the initial desorption cycle can most likely be attributed to the irreversible (chemisorption) of a portion of the adsorbed H S by the new resin. The additional 10 to 20% H S unaccounted for during subsequent desorption cycles can partially be attributed to environmental physical losses of H2S during the desorption operation and, to a lesser degree, to certain inherent limitations of recovering and analyzing quantitatively the released H,S iodometrically. Possibly some H,S may have reacted with traces of oxygen leaking into the system and forming elemental sulfur. Amberlyst XN-1007 exhibits Hi3 adsorption capacities considerably greater than Amberlyst A-27. Comparative data (Table 11) illustrate that the differences in adsorption capacities become more pronounced when the adsorption capacities for the two resins are compared on a dry volume basis, since Amberlyst XN-1007 is denser than Amberlyst A-27. Carbon dioxide does not have an appreciable detrimental effect upon the adsorption process. To illustrate this point, Amberlyst XN-1007 was evaluated in performance during treatment of a methane stream containing both H,S and CO,. The stream was purposely prepared to contain CO, levels greater than would be encountered in actual practice, in order to determine the maximum loss in capacity that would be experienced. Two cycles were initially performed using a nitrogen stream containing l C c H?S, but without CO?, followed by three additional cycles made with a stream of the following composition: CH,, 65.5':; CO?, 33.5%; HzS, 1.0%. Though a small loss in capacity was observed during treatment of the stream containing CO2, the drop in capacity was not really significant, considering the very high level of CO, present in the stream. Unlike COZ, moisture present in the adsorption system will have a deactivating effect upon the ion exchanger's capacity for HZS because of the hydrophilic nature of anhydrous ion exchange resins. T o minimize or avoid moisture deactivation, an appropriate desiccant should be used t o dry the stream containing H,S before it is processed by the Amberlyst ion exchanger. Amberlite 200, a macroreticular sulfonic cation
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Table II. Hydrogen Sulfide Sorption Data on Arnberlyst XN-1007 vs. Arnberlyst A-27
Exhaustion temperature. + 24-26O C. 100-10" C. Elution temperature. Hi3 concentration. 2';
100 cc./min. 10 cc./min. 80 ml.
Exhaustion flow rate. Elution volumes. Bed volumes.
Adsorption Phase HgS Adsorbed Resin Amberlyst XN-1007 Amberlyst A-27
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Cycle
Desorption Phase
Temp. C.
Vol. treated, 1.
Meq.1 ml. d r y
Meq.1 g. dv
H2S desorbed, meq.;g. dry
Desorption, 4;. recovery
24 25 25 26 26 26
13.7 9.6 10.6 5.0 5.0 5.0
0.28 0.19 0.21
0.57 0.39 0.43 0.31 0.30 0.30
0.39 0.32 0.37 0.18 0.26 0.24
69 82 86 58 87 80
1 2 3 1
2 3
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0.10 0.09 0.09
Table 111. Effect of Flow Rate on H?SAdsorption Capacity of Amberlyst XN-1007 Operating pressure. 5 atm. H?S concentration in CH,. 1.5% (by volume)
Resin volume. 58.5 ml. Resin weight. 29 g. Operating Flou Rate Cycle
Cc.;min.
BVIBVIhr.
Vol. Treated, L .
1 2 3
80 80 400 685
82 82 410 703
33.2 28.0 28.0 27.4
4 a Corrected for leakage
z
1
v)
W
0 K
1 6 1
v
1 I
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2
1
3
4
1
5
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OPERATING PRESSURE ( a t m o s p h e r e s )
Figure 4. Amberlyst XN-1007 hydrogen sulfide adsorption isotherm as a function of pressure
so that a more realistic comparison could be made between the runs a t the normal flow rate (80 cc. per minute) and the increased flow rate runs, since a small drop in capacity is usually found during a second adsorption cycle relative to that observed using fresh resin. Discussion
The ability of dry macroreticular anion exchange resins t o adsorb H2S from dry gaseous streams has been demonstrated and is primarily due t o the favorable surface features still retained by the macroreticular ion exchange resins in an anhydrous condition. Conventional gel-type anion exchange resins, of similar chemical composition, lacking any appreciable surface area and porosity when anhydrous, show no tendency t o adsorb H,S under the experimental conditions. Though the mechanism of the sorption process has not been fully elucidated, more than a simple physical adsorption of H S appears to be involved-for instance, like the behavior of the conventional gel resins, high surface area copolymers, including the precursors of both Amberlyst XN-1007 and Amberlyst A-27, fail to adsorb H S . From this, one must conclude
Meq. H S I ml. resin 0.670 0.574 0.556 0.523
H,S Adsorbed" Meq. H L I g . resin
Lb. H& lb. resin
1.350 1.156 1.120 1.052
0.023 0.020 0.019 0.018
that the ion exchange functional groups possessed by the Amberlyst resins, b u t not by the copolymers, must perform an important function in the adsorption process. The anion exchange resin capacity (AEC) appears to assume more than a secondary role when some of the properties of the two resins (Table I) are used to explain the better H2S adsorption capacity obtained using Amberlyst XS-1007 than Amberlyst A-27 in processing a H2Sstream (Table 11). The H,S adsorption capacities of the macroreticular ion exchange resins are further improved by conducting the adsorption process under pressurized conditions. The amount of H S adsorbed by Amberlyst XX-1007 a t 7.3 atm. was more than twice the amount adsorbed under atmospheric conditions. The amount of H2S adsorbed a t the 7.3 atm. operating pressure was equivalent to approximately 40';; of the resin's ion exchange capacity. The Amberlyst XN-1007 adsorption isotherm (Figure 4) shows the dependence of the sorption capacity on pressure to be essentially linear, a t least for the pressure range investigated. The amount of H2S adsorbed would be expected to increase further with pressure, though not necessarily in a linear manner, since some degree of solvation and molecular association of H,S molecules could occur a t higher pressures. At more elevated pressures the theoretical total ion exchange capacity of the resin could be approached, and possibly exceeded, if the extent of solvation and molecular association of HZS became appreciable. literature Cited
Cole, R., Schulman, H. L., Ind. Eng. Chem. 52, 859 (1960). Kunin, R., Meitzner, E. F., Bortnick, N . M., J . A m . Chem. SOC.84, 305 (1962). Mills, G. F., U.S. Dept. Commerce, OTS, Washington, D. C., Rept. PB15608 (1943). Oil Gas J . 56, No. 11, 108 (1958). Robinson, D. A., Mills, G. F., Ind. Eng. Chem. 41, 2221 (1949).
RECEIVED for review August 21, 1967 ACCEPTED November 6, 1967
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