US 3485746 A
Abstract available in
Claims available in
Description (OCR text may contain errors)
Dec. 23, 1969 Filed Sept. 26, 1967 Dec. 23, 1969 H J, SETZER ET AL 3,485,746
SULFUR REMOVAL FROM HYDROCARBONS Filed Sept. 26, 1967 3 Sheets-Sheet 2 United States Patent O ILS. Cl. 208-244 2 Claims ABSTRACT OF THE DISCLOSURE A process is described wherein residual sulfur is removed from hydrocarbon fuels in a single step by contact with finely divided nickel at an elevated temperature in the presence of steam.
BACKGROUND OF THE INVENTION The present invention relates to an improved process for desulfurizing hydrocarbon fuels. It finds particular application in those systems, such as fuel cells, wherein even trace amounts of sulfur and sulfur-containing cornpounds may cause a serious degradation in either the performance or endurance of the system.
One of the more promising fuel cell concepts currently envisioned contemplates a hydrocarbon-air reaction, using a fuel such as JP-4 or natural gas, in an indirect system wherein hydrogen is first generated by steam reformation of the hydrocarbon fuel. Unfortunately, the reform catalyst and the palladium-silver membrane purifier of the reformer, as well as the typical fuel cell catalyst, are all susceptible to parasitic poisoning when the fuels utilized contain sulfur at the concentrations usually found in as-supplied JP-4, for example, typically in the range of 20D-500 parts per million sulfur. The necessity for removal of the sulfur is, accordingly, quite evident.
Several processes for sulfur removal from hydrocarbons are known in the petroleum industry, as evidenced by the patents to Houdry 2,259,469 and Watson 2,064,- 999. A typical process is hydrodesulfurization, wherein the hydrocarbon fuel is treated catalytically in the presence of hydrogen to effect a conversion of the organic sulfur compounds into an inorganic, easily-removed form, such as hydrogen sulfide, which is then removed in a subsequent refining operation. It is known that all types of organic sulfur compounds, including the thiophenes (Cid-14S) can be removed in this process provided conversion to hydrogen sulfide is first made and sufficient additional treatment is undertaken to substantially remove the hydrogen sulfide.
SUMMARY OF THE INVENTION Briefiy described, the present invention teaches a desulfurizing technique whereby the sulfur is removed in a single step. Accordingly, it satisfies the primary object of the invention which is to strip the residual sulfur from the as-supplied hydrocarbon fuels in a process which neither seriously affects the compactness or weight of the overall power system nor adds significantly to the parasitic power requirements thereof.
The basic technique involved provides desulfurization by reaction and/or adsorption of the sulfur-containing hydrocarbon in a bed of finely-divided nickel with the fuel in the vapor state and exposed to steam at an elevated temperature.
In the more preferred embodiment the reaction-adsorbent is effected at a temperature in excess of 500 F. but below that at which cracking of the fuel occurs. Hence, the desulfurization is effected at 50G-900 F. In the most preferred embodiment a small hydrogen addition is made to increase durability of the system.
It is another object of the present invention to provide a desulfurization process for hydrocarbon fuels which is essentially completely efficient in the sulfur removal sense.
In its more preferred manner of utilization, the techniques described herein are used to process those fuels having a sulfur content less than about 1000 parts per million sulfur or, even more preferably, less than about 500 parts per million sulfur although, as hereinafter dis cussed in greater detail, the more preferred utilization is not made because of any substantial reduction in the efficiency of sulfur removal with the higher sulfur contents. Hence, the present process is primarily adapted to supplement and not replace the currently-existing processes directed to the removal of sulfur in gross amounts.
BRiEF DESCRIPTION OF THE DRAWINGS FIG. l is an abbreviated schematic of a fuel cell system adapted to the practice of the instant process.
FIG. 2 is a graph illustrating the effect of temperature on the desulfurization performance of a typical system utilizing a hydrocarbon fuel doped with thiophene to 419 parts per million sulfur.
FIG. 3 is a graph plotting the effect of pressure on desulfurization performance with doped fuel.
FIG. 4 illustrates the effect of a hydrogen addition on the desulfurization process.
FIG. 5 illustrates the effect of varying thiophene percentages at a constant total sulfur level.
FIG. 6 plots sulfur loading of the reactant-adsorbent bed as a function of bed length.
DESCRIPTION OF THE PREFERRED EMBODIMENTS The various types of organic sulfur compounds usually found in the hydrocarbon fuels can generally be Classified as:
(a) mercaptans (b) sulfides (c) disulfides, and (d) thiophenes The mercaptans and disulfides are much less stable than the other organic sulfur compounds and, hence, much more amenable to removal. Generally, in fuels such as JP-4, these compounds have been removed at the refinery prior to distribution of the fuel. The primary concern, therefore, has been the removal of the more stable forms of sulfur compounds, namely the sulfides and, particularly, the thiophenes, which are both usually present in the as-supplied fuels. The organic sulfides can be represented by the general formula R-S-R, Where R and R are organic groups, either aliphatic or aromatic, and the molecule itself can also occur as a saturated ring structure. The thiophenes are aromatic sulfur compounds based on thiophene (Gli-14S).
In general, the detailed tests performed were directed toward fuels doped with thiophene since it represents the most diicult type of sulfur compound to remove. To insure controlled conditions with respect to the sulfur level in the various fuels, sulfur compounds were mixed with a sulfur-free hydrocarbon to bring the total sulfur level to about 400 parts per million, this level being chosen since it is representative of the average sulfur level of JP-4 marketed by the various manufacturers.
The basic reactant-adsorbent of the present invention is finely divided nickel, including the high nickel content alloys. The most preferred form of the reactant-adsorbent comprises 50 percent nickel supported on kieselguhr, such as Girdler G-49B nickel catalyst. The reactant-adsorbent is usually ground to 1%6 mesh, as the best balance between the desired high surface area and low pressure drop factors considered.
Desulfurization was, in general, practiced at temperatures between about 500-900 F., and pressures were varied between about 40-250 p.s.i.a., the effects of varying temperature and pressure in a specific system being set forth in FIGS. 2 and 3, respectively.
During the development process it was found that a small hydrogen addition to the reactant chamber, typically about 0.03 pound of hydrogen per pound of fuel, is advantageous. The experimental results clearly demonstrated that, although desulfurization could be accomplished without the addition of hydrogen to the fuelwater feed mixture, the breakthrough time for a given bed was 40 percent shorter without the hydrogen addition. Breakthrough was considered to have occurred when detectable sulfur in the fuel condensate was found, usually at about parts per million sulfur. The effect of a hydrogen addition on breakthrough time with varying quantities of hydrogen is illustrated graphically in FIG. 4.
As mentioned previously, the thiophenes are considered the most difficult organic sulfur compounds to remove in the fuel processing sequence. Tests with fuels containing 50-50 mixtures of diphenyl sulfide and benzothiophene indicated that the sulfide removal can be accomplished 5 or 7 times as readily as the thiophene removal when calculated in terms of sulfur retention in the reactant bed. It is thought that in the former case the sulfur is stripped from the organic molecule and retained at the bed as nickel sulfiide, while the thiophene molecule is adsorbed intact in the bed. The sulfur retention in a typical bed as a function of the thiophene fraction in the fuel is plotted in FIG. 5.
Exploratory tests were run with a variety of reactantadsorbents under a number of feed conditions. Tests run with fuel alone (no water or hydrogen feed) at 600 F. had to be terminated within ten minutes due to a plugged reactor resulting from coke formation in the bed.
` While the exact mechanism is not known, it is felt that the addition of steam provides an oxygen monolayer on the individual nickel particles which prevents catalytic cracking of the fuel and the consequent undesirable coke formation in the bed and presence of hydrogen sulfide in the eiuent from the bed. However, the monolayered oxygen does not interfere with the chemisorption of the sulfide and thiophene, probably through an oxygen displacement mechanism. On the other hand, a substantial conversion of the reactant-adsorbent to nickel oxide is to be avoided since nickel oxide is ineffective in providing the desired sulfur removal as clearly indicated with tests on nickel oxide reactant.
As suggested, a balance must be effected within the reactant chamber or bed in order to attain the desulfurization objective. It appears that the presence of hydrogen prevents the undesirable oxide buildup without destroying the desired blanketing monolayer on the nickel, thereby achieving the balance whereby catalytic fuel cracking is virtually prevented and yet the reactant remains sufiiciently active with respect to sulfur and the various sulfur corresponds to effect their complete removal.
It should be reiterated, however, that the hydrogen addition, while preferable, is yet only optional. Tests without the hydrogen addition indicated satisfactory sulfur removal, although the breakthrough time was measurably shortened. It is thought that a small amount of hydrogen is generated in the early stages of the reaction, this selfgenerated hydrogen serving to at least partially prevent the oxide buildup as previously mentioned.
1n the initial screening tests, fuel and water were fed to a small electrically heated boiler by positive displacement pumps, and hydrogen was supplied, as appropriate, from a pressurized cylinder. The mixture was introduced to a reactor containing the nickel reactant-adsorbent, the temperature of the reactor being maintained by external electrical heaters. A series of condensers downstream of `the reactor were used to collect the efiluent from the reactor for the purposes of analysis.
Various reactant-adsorbents were tested in addition to the G-49B mentioned above, including those identified as follows: two high nickel content hydrogenation types: one moderate nickel and one nickel-tungsten material, usually utilized as hydrogenation catalysts, and identified as G-60RS and Ni-4301, respectively; one nickel oxide sample; and one cobalt-molybdenum hydrodesulfurization catalyst: G-35. Both the nickel oxide bed and the cobalt-molybdenum materials were completely ineffective as reactant-adsorbents in the context of the present invention. It is not to be expected, therefore, that the usual desulfurization catalysts will necessarily work in the present process, emphasizing the 'basic difference between the reaction and/ or adsorption process herein described and the catalytic processes conventionally employed. The nickel base materials were effective in the single stage removal of sulfur in varying degrees of efiiciency, the nickel-tungsten material, for example, being most effective at 700 F.
In one demonstration, an existing system was adapted to incorporate the reactant-adsorbent desulfurizer described with the G-49B material. The reformer was operated without any loss in output for 200 hours using JP-4 fuel modified with a 50-50 mixture of thiophene and diphenyl sulfide to a total sulfur content of 470 parts per million by weight. Two 1GO-hour replaceable cartridges were utilized at an hourly space velocity, grams sulfur fed/ hr.)/ (gram of reactant-adsorbent), and a pressure of 240 p.s.i.a. Temperatures in the bed varied from 500 to 800 F., depending on location.
The efficiency of the sulfur removal was demonstrated during the course of a program designed to provide a means for detecting an incipient breakthrough. Prior to the actual breakthrough sulfur contents in the effluent stream were generally below those amounts detectable by all but the -more refined methods. Moreover, in the work with the supported nickel reactant-adsorbents, no hydrogen sulfide was eveer found in the effluent, even at periods well after breakthrough While this invention has been described in connection with several preferred embodiments and particular examples, these will be understood to be illustrative only, no limitation being intended thereby. The invention in its true `scope and spirit will be measured by the appended claims.
What is claimed is:
1. In a fuel cell system utilizing a low-boiling, hydrocarbon fuel feed having a residual sulfur content of up to 500 parts per million by weight wherein the hydrocarbon is passed through a catalystic reformer to convert a substantial portion of the hydrocarbon to hydro-gen and carbon oxides prior to admission of the fuel to the fuel cell, the method of stripping residual sulfur from the hydrocarbon to prevent poisoning of the reformer catalyst while avoiding substantial cracking of the hydrocarbon with the formation of coke in the sulfur-removal process, which comprises the steps of:
prior to admission of the hydrocarbon to the reformer,
vaporizing the hydrocarbon and mixing it `with steam in a ratio of about 2-5 moles of steam per mole of carbon in the hydrocarbon,
and passing the steam-hydrocarbon mixture through a reactant-adsorbent granular bed consisting of elemental nickel or alloy of high nickel content supported on an inert carrier at a temperature of 500- 900 F., and adsorbing the sulfur and sulfur-containing compounds thereon while preventing substantial cracking of the hydrocarbon;
recovering the desulfurized hydrocarbon from the reactant-adsorbent bed;
and feeding the efliuent to the reformer.
2. The method according to claim 1 in which:
hydrogen is added to the vaporized hydrocarbon-steam a 6 mixture admitted to the reactant-adsorbent bed in a FOREIGN PATENTS ratio of about CD2-0.1 pounds of hydrogen per 511398 8/1939 Great Britain` pound of hydrocarbon.
References Cited PAUL M. COUGHLAN, JR., Primary Examiner UNITED STATES PATENTS D J. M. NELSON, Assistant Examiner 3,379,505 4/1968 Holmes et a1. 23--212 8,367,862 2/1968 Mason et a1. 208-244 U-S- C1- X-R- 2,516,876 8/1950 Home et a1. 208417 136-86? 208-217