EP0359889A1

EP0359889A1 - Catalyst, apparatus and process for the desulfurization of low sulfur hydrocarbons

Info

Publication number: EP0359889A1
Application number: EP88630164A
Authority: EP
Inventors: Roger Raymond Lesieur
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1988-09-22
Filing date: 1988-09-22
Publication date: 1990-03-28

Abstract

A catalyst composition, highly active in the hydrogenolysis of sulfur containing compounds, and useful in the removal of sulfur from low sulfur content hydrocarbons is disclosed. The catalyst composition comprises a noble metal dispersed on an inert substrate. A desulfurization apparatus and a desulfurization process, each utilizing the catalyst material, are also disclosed.

Description

Technical Field

The field of art to which this invention pertains is catalytic material for treatment of hydrocarbon fluids.

Background Art

Fuel cell powerplants require a source of hydrogen in order to generate electrical power. A conventional method for the production of hydrogen is the reaction of hydrocarbon fluids with steam in the presence of a catalyst to produce a hydrogen containing gas. Generally, the hydrocarbon fluid that is subjected to the steam reforming process is first desulfurized to avoid poisoning of the reforming catalyst.
Sulfur containing compounds may be removed from hydrocarbon fluids by several processes. Inorganic sulfur compounds such as hydrogen sulfide may be removed by stripping or absorption. The less stable organic sulfur containing compounds, i.e. mercaptans and sulfides, may be thermally or catalytically decomposed to inorganic compounds and subsequently removed by absorption or stripping. The more stable organic sulfur containing compounds, i.e. disulfides and thiophenes are decomposed catalytically and are typically removed by a hydrodesulfurization process. In a hydrodesulfurization process the hydrocarbon fluid is treated catalytically in the presence of hydrogen to effect a conversion of the organic sulfur compounds to inorganic compounds which may be subsequently removed by absorption or by stripping.
Transition metal oxides, typically a mixture of cobalt oxide and molybdenum oxide dispersed on an inert support material, are commonly used as hydrodesulfurization catalysts. The transition metal oxides must be reduced to transition metal sulfides to become active in catalyzing the hydrodesulfurization reaction. The catalyst is sulfided by pretreatment with a sulfur containing compound, typically hydrogen sulfide. This pretreatment is a cumbersome and expensive process. Under hydrodesulfurization conditions the sulfur content of the catalyst is determined by the equilibrium that is established between the sulfur content of the catalyst and the sulfur content of the surrounding hydrocarbon stream. In general, the activity of the transition metal catalyst increases as the sulfur content of the catalyst increases. If the sulfur content of the hydrocarbon stream is very low, as is commonly the case with natural gas feedstocks, the sulfur content of the catalyst and the activity of such catalyst will also become very low. In the exteme case, a reversal of the pretreatment may result with sulfur being stripped off of the catalyst by the passing low sulfur content hydrocarbon stream, rendering the catalyst inactive.
Once sulfided, the transition metal catalysts are hazardous, carcinogenic materials and thus handling and disposing of the spent catalyst material requires great care and is subject to regulation.
Accordingly, what is needed in this art is an improved hydrodesulfurization catalyst, especially in the case of low sulfur content hydrocarbon fluids.

Disclosure of Invention

A catalyst composition, particularly adapted to the removal of sulfur from low sulfur content hydrocarbon fluids, is disclosed. The catalyst composition comprises a catalytically active noble metal dispersed on an inert support with a high specific surface area.
Another aspect of the invention concerns an apparatus for the removal of sulfur from low sulfur content hydrocabon fluids. The apparatus comprises a container surrounding the catalyst composition described above. An inlet and an outlet means are provided to permit a flow of hydrocarbon fluid through the container.
A further aspect of the invention concerns a process for removing sulfur from low sulfur content hydrocarbon fluids. The steps of the process comprise preheating the hydrocarbon fluid to a temperature between 300°F and 650°F, contacting the preheated hydrocarbon fluid with the catalyst composition described above and subsequently contacting the hydrocarbon fluid with a hydrogen sulfide absorbent.
A process for removing sulfur, involving contacting a hydrocarbon fluid with a mixture of a hydrogen sulfide absorbent and the catalyst composition described above is also disclosed.
The catalyst composition of the present invention exhibits a high activity in the hydrogenolysis of sulfur containing compounds without requiring sulfiding pretreatment, and is particularly well adapted to the hydrodesulfurization of low sulfur content hydrocarbons.
The foregoing and other features and advantages of the present invention will become more apparent from the following description and accompanying drawings.

Brief Description of Drawings

Figure 1 is a flow diagram of the fuel processing unit of a fuel cell powerplant.
Figure 2 is a vertical, cross-sectional view of a test apparatus.
Figure 3 demonstrates the relationship of the activity of the catalyst material of the present invention to temperature and to the ambient hydrogen sulfide concentration.

Best Mode for Carrying Out the Invention

The catalyst composition of the present invention is particularly adapted for use in the hydrogenolysis step of the hydrodesulfurization of low sulfur content hydrocarbon fluids, i.e. those hydrocarbon fluids having a sulfur content of less than or equal to about 50 parts per million.
The rate at which the organic sulfur containing compounds in a hydrocarbon fluid undergo hydrogenolysis is accelerated by contact with the catalytically active noble metal of the present invention. The catalyst composition of the present invention catalyzes the hydrogenolysis of organic sulfur containing compounds to yield hydrogen sulfide. For example, some typical hydrogenolysis reactions for a range of organic sulfur compounds are:
C₂H₅SH+H₂⇄C₂H₆+H₂S,
C₂H₅SSC₂H₅+3H₂⇄2C₂H₆+2H₂S, and
C₄H₄S+4H₂⇄C₄H₁₀+H₂S.
The noble metal catalyst of the present invention may be selected from the platinum group of noble metals, consisting of platinum, palladium and rhodium as well as iridium, osmium and ruthenium, or mixtures thereof. Platinum, palladium and rhodium are the more readily available members of the group, and are preferred for that reason. Platinum is particularly preferred.
It should be noted that the noble metal catalyst composition of the present invention differs from the transition metal catalysts of the prior art in that the noble metal catalyst composition accelerates the hydrogenolysis of sulfur containing compounds without requiring presulfiding treatment and the noble metal catalyst composition is not deactivated by exposure to low sulfur content hydrocarbon fluids.
In order to present a large catalytically active surface, the noble metal is dispersed in a thin layer on the surface of an inert support. The support must be stable and inert under the conditions of the hydrodesulfurization reaction and provide a high (greater than 50 square meters per gram) specific surface area. The substrate should also be physically robust to the extent necessary to resist abrasion and impact during handling, crushing from such stresses as the weight of surrounding catalyst material, and fracture from internal stresses such as might arise during temperature cycling.
Examples of materials which meet these criteria are the refractory metal oxides and activated charcoal. The refractory metal oxides are preferred. Refractory metal oxides found to be suitable for use as the substrate of the present invention are alumina and lanthanum stabilized alumina. Lanthanum stabilized alumina is commercially available in suitable form (1/8 inch - 1/2 inch diameter pellets, with a length to diameter ratio of one to two and a specific surface area of about 100 square meters per gram). One source of such material is W. R. Grace & Co.
The catalytically active noble metal is dispersed on the inert support by any conventional method in the art. Typically, metal salts are deposited on the support from solution and the solvent is evaporated to leave a finely dispersed film of the metal salt on the surface of the support. The amount of platinum dispersed may vary over a wide range, but is generally between 0.02% and 0.5% platinum by weight, based on the weight of the noble metal and substrate material.
The hydrogen sulfide product of the hydrogenolysis reaction must be removed from the hydrocarbon fluid to complete the hydrodesulfurization process. This may be accomplished by stripping the effluent hydrocarbon stream, as in a gas scrubber, or by passing the effluent stream through a bed of hydrogen sulfide absorbent material.
Particles of metal oxides, such as the oxides of iron, copper, nickel or zinc may be used as hydrogen sulfide absorbents. Zinc oxide is the preferred hydrogen sulfide absorbent material. The reaction of zinc oxide with hydrogen sulfide has a particularly high equilibrium constant, strongly favoring the desired forward reaction ZnO+H₂S⇄ZnS+H₂O. It should be noted that this characteristic is particularly important when water vapor is present in the reactant stream, as is the case in a fuel cell fuel processing unit. Zinc oxide is commercially available in suitable pellet, e.g. 3/16 inch diameter extrusions or spherical, e.g. 3/8 inch diameter, form.
The noble metal catalyst material of the present invention is poisoned by the hydrogen sulfide product of the hydrogenolysis reaction. Such poisoning lowers the activity of the catalyst material. The activity of the catalyst decreases as the concentration of hydrogen sulfide in the atmosphere surrounding the catalyst material increases. It should be noted that the situation is exactly the opposite in the case of the transition metal hydrodesulfurization catalysts of the prior art, and a superficial analysis would probably lead one to the conclusion that a noble metal would be a rather poor choice as a hydrodesulfurization catalyst. While the poisoning phenomenon does limit the usefulness of the noble metal catalyst to the treatment of hydrocarbon fluids having a low sulfur content, the noble metal catalyst is very effective within that limited range. Further, the activity of the noble metal catalyst composition may be enhanced if, rather than passing the effluent hydrocarbon steam through a separate scrubber or bed of hydrogen sulfide absorbent, a particulate hydrogen sulfide absorbent material is physically mixed or layered with the catalyst material in a single bed. The presence of the hydrogen sulfide absorbent enhances the activity of the catalyst material by absorbing the hydrogen sulfide reaction product as it is formed, thus maintaining the hydrogen sulfide concentration in the atmosphere surrounding the catalyst material at a low level.
The relative amount of absorbent to catalyst material may be varied over a wide range, with higher proportions of absorbent tending to further prolong the active life of the catalyst bed. A mixture of 1 part catalyst material to 10 parts zinc oxide pellets on a weight/weight basis has been shown to give a bed with an active life of at least 10,000 hours, given an incoming sulfur concentration of about 30 parts per million in the hydrocarbon stream and a space velocity of about 1 pound hydrocarbon/pound catalyst/hour.
A typical apparatus according to present invention comprises a container, surrounding a particulate bed with an inlet and outlet means suitable for the influx and efflux of the hydrocarbon stream. The particulate bed so contained comprises the catalyst material of the present invention, either alone or together with a particulate hydrogen sulfide absorbent as discussed above. The preferred embodiment of such an apparatus need be no more elaborate than a tubular shell of a length to diameter ratio, typically greater than one, that is sufficient to obtain uniform distribution of the hydrocarbon stream in the catalyst bed, and that is equipped with such internal baffles or packing as necessary to confine the particulate bed, yet permit influx and efflux of the hydrocarbon stream. Such apparatus is preferably constructed from stainless steel.
The process of the present invention comprises preheating a stream of hydrocarbon fluid, typically natural gas or possibly light naphtha, to a temperature in the range of about 300°F to about 650°F, and contacting the stream of preheated hydrocarbon fluid with a particulate bed containing catalytic material of the present invention in the presence of hydrogen. The hydrocarbon fluid is introduced at a space velocity which provides the contact time necessary to achieve a predetermined sulfur concentration in the effluent stream, typically less than or equal to about 0.5 part per million sulfur. A space velocity of 1 pound hydrocarbon fluid/pound catalyst material/hour has been shown to be effective in obtaining such a product.
The particulate bed comprises the catalytic material of the present invention, either alone or together with a particulate hydrogen sulfide absorbent as discussed above. If the bed comprises only the catalytic material of the present invention, the inorganic sulfur compounds must be removed from the hydrocarbon steam to complete the hydrodesulfurization. As discussed above, this may be accomplished by processes such as scrubbing or absorption. If the bed comprises a mixture of or alternating layers of the catalyst material and a hydrogen sulfide absorbent, the hydrodesulfurization process is completed in a single stage.

Example 1

A solution was made by adding 5.85 grams platinum diamino dinitrite, containing 61 percent by weight platinum, to a 50/50 solution (by volume) of concentrated nitric acid and distilled water and then stirring overnight. A lanthanum stabilized alumina support, designated as X-1/79-1, was purchased from W. R. Grace & Co. The lanthanum stabilized alumina was in the form of cylindrical pellets of about 1/8 inch diameter and ranging in length from about 1/8 inch to about 1/4 inch and had a nominal specific surface area of 100 square meters per gram. The platinum salt solution was slowly poured over the support material. The mixture was agitated ultrasonically for 5 minutes and then let stand for 30 minutes. The excess platinum salt solution was poured off and the damp particles were spread on several watch glasses and dried for four hours. The particles were gradually heated during the drying period and reached a final temperature of 240°F.

Example 2

The catalyst material of the present invention was used in the fuel processing unit of a fuel cell powerplant to desulfurize a natural gas feedstock. A flow diagram of the process is shown in Figure 1.
A stream of natural gas (1), doped to 30 parts per million sulfur with thiophane, was mixed with a stream of hydrogen gas (14) and the mixture (2) was fed to the preheater (3). The hydrogen gas was provided by recycling a portion of the product stream (13) of the shift converter (12). The ratio of hydrogen to hydrocarbon in the mixture (2) was 0.04:1 (on a weight to weight basis). The feed mixture (2) was preheated to a temperature in the range of about 300°F to 650°F in the preheater (3).
The preheated fuel mixture (4) was fed to the hydrogenolysis reactor (5). A longitudinal cross section of the reactor (5) is shown in Figure 2 and comprises a tubular wall (20), surrounding successive layers of particulate catalyst material (21) and packing material (22). The reactor was provided with temperature probes (23) and sample ports (28). The tubular wall was fabricated from 2 1/2 inch schedule 40 stainless steel pipe. A total of 190 grams of catalyst material was charged to the reactor. The catalyst material comprised 0.2% platinum by weight dispersed on lanthanum stabilized alumina as discussed in Example 1. Lanthanum stabilized alumina particles were used as a packing (22) to separate the catalyst material into three distinct reactor beds (21). The preheated mixture was introduced to the reactor (5) at a space velocity of 1 pound hydrocarbon/1 pound catalyst material/hour. The treatment of the natural gas with the catalyst material in the presence of hydrogen at an elevated temperature effects a conversion of organic sulfur containing compounds, i.e. thiophane, to an inorganic sulfur containing compound, i.e. hydrogen sulfide.
The effluent (6) of the reactor (5) was fed to a bed (7) of zinc oxide pellets. The hydrogen sulfide formed by the hydrogenolysis of the organic sulfur compounds reacts with the metal oxide according to the reaction H₂S + ZnO ⇄ZnS + H₂O. The sulfur containing compounds are thus removed from the hydrocarbon stream. The sulfur content of the desulfurized effluent (8) of the absorbent bed was monitored and recorded.
The desulfurized effluent (8) and steam (9) were fed to a steam reformer (10).
In a reformer, the desulfurized hydrocarbon is exposed to a reforming catalyst and undergoes reaction with the steam to yield carbon monoxide and hydrogen according to the reaction CH₄ + H₂O⇄CO + 3H₂. At the same time, the water gas shift equilibrium is established according to CO + H₂O⇄CO₂ + H₂. A mixture of H₂, CO, CO₂, H₂O and CH₄ is obtained. This mixture (11) is fed to the shift converter (12). The carbon monoxide in the mixture (11) is converted to carbon dioxide and hydrogen in the shift converter, acording to the reaction CO + H₂O⇄CO₂ + H₂. The hydrogen rich effluent (13) of the shift converter is split to provide a recycle stream of hydrogen (14) for the hydrodesulfurization process and a feed stream (15) for the fuel cell (16). The fuel cell uses the hydrogen stream as fuel for the generation of electrical energy.
The activity of the catalyst of the present invention is defined by: k = space velocity x ln
Activity data was gathered at several temperatures. This data is presented on a conventional Arrhenius graph in Figure 3. In this graph, the activity (k) is plotted against the reciprocal of the absolute temperature of the hydrogenolysis reaction. It can be seen that the activity of the catalyst increases with increasing temperature.
To demonstrate the improvement in catalyst activity that may be expected when the particles of catalyst material are physically mixed or layered with the hydrogen sulfide absorbent, the hydrogen sulfide content of the reaction mixture was varied. Activity data was gathered for each of the hydrogen sulfide levels. This data is also presented on a conventional Arrhenius graph in Figure 1. It can be seen that the activity of the catalyst increases dramatically as the hydrogen sulfide concentration of the reaction mixture is reduced. As discussed above, mixing the absorbent particles with the catalyst particles is a method by which the hydrogen sulfide is absorbed as soon as it is generated. The ambient concentration of hydrogen sulfide can then be maintained at a very low level, resulting in a very highly active catalyst bed.
As stated, the catalyst material according to the present invention provides high activity in catalyzing the hydrodesulfurization of low sulfur content hydrocarbon fluids. The catalyst need not be presulfided to become active and does not become a hazardous compound when used. The activity of the catalyst may be enhanced by mixing or layering the catalyst material with a particulate hydrogen sulfide absorbent. The use of such catalyst material, particularly in mixture with or in successive layers with the absorbent material, allows hydrodesulfurization of low sulfur content hydrocarbons in a more compact apparatus, by a simplified process, and at reduced expense.
Although this invention has been shown and described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail thereof may be made without departing from the spirit and scope of the claimed invention.

Claims

1. An apparatus for removing sulfur from low sulfur content hydrocarbon fluids comprising a container having an inlet and an outlet means to permit a flow of hydrocarbon fluid through the container, the container surrounding a catalyst composition comprising a catalytically active noble metal dispersed on an inert support, the support having a high specific surface area, and the catalyst composition exhibiting high activity in the hydrogenolysis of sulfur containing compounds without requiring a sulfiding pretreatment.

2. The apparatus of claim 1, wherein the catalyst composition additionally comprises a hydrogen sulfide absorbent, the absorbent enhancing the activity of the catalyst composition.

3. The apparatus of claim 1, wherein the catalyst composition comprises alternating layers of the dispersed noble metal and a hydrogen sulfide absorbent, said absorbent enhancing the activity of the catalyst composition.

4. The apparatus of claims 1 - 3 , wherein the noble metal is selected from the group consisting of platinum, palladium, rhodium and mixtures thereof.

5. The apparatus of claims 1 - 3 , wherein the inert support comprises alumina particles or lanthanum stabilized alumina particles.

6. The apparatus of claims 1or 2, wherein the hydrogen sulfide absorbent comprises zinc oxide particles.

7. A process for removing sulfur from low sulfur content hydrocarbon fluids, comprising:

a) preheating the hydrocarbon fluid to a temperature in the range of about 300°F to about 650°F,

b) contacting the preheated hydrocarbon fluid with a catalyst composition comprising a catalytically active noble metal dispersed upon an inert support, the support having a high specific surface area, and subsequently

c) contacting the hydrocarbon fluid with a hydrogen sulfide absorbent,
said catalyst composition exhibiting high activity in the hydrogenolysis of sulfur containing compounds without requiring a sulfiding pretreatment.

8. The process of claim 7 , wherein the catalyst composition and the hydrogen sulfide absorbent are disposed in a series of alternating layers, said absorbent enhancing the activity of the catalyst composition.

9. The process of claims 7 or 8, wherein the noble metal is selected from the group consisting of platinum, palladium, rhodium and mixtures thereof.

10. The process of claims 7 or 8 , wherein the inert support comprises alumina particles or lanthanum stabilized alumina particles.

11. The process of claim 7 or 8 wherein the hydrogen sulfide absorbent comprises zinc oxide particles.

12. A process for removing sulfur from low sulfur content hydrocarbon fluid, comprising:

a) preheating the hydrocarbon fluid to a temperature in the range of about 300°F to about 650°F, and

b) contacting the preheated hydrocarbon fluid with a catalyst composition comprising a catalytically active noble metal dispersed on an inert support in a mixture with a hydrogen sulfide absorbent, said support having a high specific surface area, said catalyst composition exhibiting high activity in the hydrogenolysis of sulfur containing compounds without requiring a presulfiding treatment, and said absorbent enhancing the activity of the catalyst composition.

13. The process of claim 12, wherein the noble metal is selected from the group consisting of platinum, palladium, rhodium and mixtures thereof.

14. The process of claim 12, wherein the support material comprises alumina particles or lanthanum stabilized alumina particles.

15. The process of claim 12, wherein the hydrogen sulfide absorbent comprises zinc oxide particles.