Mineral oils: processes and products – Refining – Sulfur removal
Reexamination Certificate
2001-04-13
2003-05-06
Griffin, Walter D. (Department: 1764)
Mineral oils: processes and products
Refining
Sulfur removal
C208S20800M, C585S274000, C585S276000, C585S900000, C585S904000, C502S025000, C502S028000, C502S033000, C502S517000
Reexamination Certificate
active
06558533
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to an improved process capable of providing product streams of light and heavy gasoline fractions, which are substantially free of sulfur containing compounds.
BACKGROUND OF THE INVENTION
Catalytic cracking is a petroleum refining process, which is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produced using a fluidized catalytic cracking (FCC) process. In the process, heavy hydrocarbon feedstocks are converted into lighter products by reactions taking place at elevated temperatures in the presence of a catalyst, with the majority taking place in the vapor phase. The feedstock is thereby converted into gasoline, distillates and other liquid fraction product streams as well as lighter gaseous cracking products having four or less carbon atoms per molecule. The three characteristic steps of a catalytic cracking process comprises: a cracking step in which the heavy hydrocarbon feed stream is converted into lighter products, a stripping step to remove adsorbed hydrocarbons from the catalyst material, and a regeneration step to burn off coke formations from the catalyst material which is then recirculated and reused in the cracking step.
Petroleum feedstocks normally contain organic sulfur compounds, such as mercaptans, sulfides and thiophenes. The products of the cracking process correspondingly tend to contain sulfur impurities even though about half of the sulfur compounds are converted to hydrogen sulfide during the cracking process, mainly by catalytic decomposition of non-thiophenic sulfur compounds. The thiophenic and other organic sulfur containing compounds have been found most difficult to remove. The specific distribution of sulfur in the cracking products is dependent on a number of factors including feed, catalyst type, additives present, conversion and other operating conditions, but, in any event in a certain proportion of the sulfur tends to enter the light or heavy gasoline fractions and passes over into the product pool. Although petroleum feedstock normally contains a variety of sulfur born contaminants, one of the chief concerns is the presence of unsubstituted and hydrocarbyl substituted thiophenes and its derivatives, such as thiophene, methylthiophene, ethylthiophene, propylthiophene, benzothiophene, and tetrahydrothiophene, as well as thiophenols, in the heavy and light gasoline fraction product streams of the refining process (e.g. fluid cracking catalyst process). These compounds generally have boiling points within the range of the light and heavy gasoline fractions and, thus, become concentrated in these product streams.
In response to concerns about emission of sulfur oxides and other sulfur compounds into the atmosphere following combustion, various governmental agencies have promulgated regulations on the amount of sulfur contained in these petroleum-refining products. For example, the U.S. Government has issued Reformulated Gasoline (RFG) regulations, as well as Amendments to the Clean Air Act. In addition, the California Air Resources Board has set a limit on the concentration of sulfur in gasoline to about 40 parts per million (ppm). Since the current sulfur levels in gasoline are about 385 ppm, these new lower goals require significant resources by most petroleum refiners to meet the new level.
Several approaches have been developed to remove sulfur from gasoline. One approach has been the removal of sulfur containing compounds from feedstock by hydrotreating the stock prior to cracking. While highly effective, this approach tends to be expensive in terms of capital costs of the equipment required as well as operationally, since large amounts of hydrogen are consumed.
From the economic point of view, it would be desirable to achieve thiophenic sulfur removal in the cracking process itself since this would effectively desulfurize the major components of the gasoline blending pool without additional treatment. Various catalytic materials have been developed for the removal of sulfur during the FCC process cycle. For example, a FCC catalyst impregnated with vanadium and nickel metal has been shown to reduce the level of product sulfur. See Myrstad et al., Effect of Nickel and Vanadium on Sulfur Reduction of FCC Naptha, Applied Catalyst A: General 192 (2000) pages 299-305. This reference also showed that a sulfur reduction additive based on a zinc-impregnated alumina is effective to reduce product sulfur in FCC products. However, when these materials are mixed with metal impregnated FCC catalyst, the effect of sulfur reduction was lessened and became economically inefficient.
Other developments for reducing product sulfur have centered on the removal of sulfur from the regenerator stack gases. An early approach developed by Chevron used alumina compounds as additives to the inventory of cracking catalyst to adsorb sulfur oxides in the FCC regenerator; the adsorbed sulfur compounds which entered the process in the feed were released as hydrogen sulfide during the cracking portion of the cycle and passed to the product recovery section of the unit where they were removed. See Krishna et al., Additives Improved FCC Process, Hydrocarbon Processing, November 1991, pages 59-66. Although sulfur is removed from the stack gases of the regenerator, product sulfur levels are not greatly effected, if at all.
An alternative technology for the removal of sulfur oxides from regenerator stack gases is based on the use of magnesium-aluminum spinels as additives to the circulating catalyst inventory in the FCC unit. Exemplary patents disclosing this type of sulfur removal additives include U.S. Pat. Nos. 4,963,520; 4,957,892; 4,957,718; 4,790,982 and others. Again, however, product sulfur levels are not greatly reduced.
A catalyst composition to reduce sulfur levels in liquid cracking products has been described in Wormsbecher and Kim in U.S. Pat. Nos. 5,376,608 and 5,525,210. The composition proposes the addition of a cracking catalyst additive, composed of an alumina-supported Lewis acid, with conventional zeolite molecular sieves. Although this system has the advantages of causing sulfur reduction in the cracking process, the composition has not achieved significant commercial success. It has been found that the compositions proposed by Wormsbecher et al. do not provide significant reduction of the levels of thiophenes and its derivatives, even when high levels of the alumina-supported Lewis acid additives are included in the composition. The use of greater than about 10 weight percent of additives in their composition does not provide a benefit equal to the cost of the additive.
The possibility of adsorbing thiophenes directly from gasoline has been explored briefly in the scientific literature. A. B. Salem in
Ind. Eng. Chem. Res
., 33, page 336 (1964) and Garcia et al. in
J. Phys. Chem
. 96 page 2669 (1991) have shown that certain zeolite materials can be used to adsorb thiophenes from olefin-free gasoline. Exchanged zeolites, such as Ag-exchanged Zeolite Y and Cu-exchanged Zeolite Y have been shown to adsorb sulfur from standard gasoline (See U.S. Pat. No. 4,188,285 and EP 0,275,855). However, in each case, the absorption capacity of the adsorbent material is insufficient for commercial application.
It is also recognized that raw fuels, such as gasoline, diesel fuels and the like may not be useful as a fuel source for a fuel cell power plant due to the presents of organo-sulfur compounds in the fuel source. Hydrogen generation in the presence of sulfur and sulfur compounds results in a poisoning effect on all of the catalysts used in a hydrogenation generating system, including fuel cell anode catalysts. Conventional fuel processing systems used with fuel cell power plants include a thermal steam reformer, such as that described in U.S. Pat. No. 5,5 16,334. In such a system, the sulfur is removed from the fuel by conventional hydrodesulfurization techniques. The resultant hydrogen sulfide is then removed using a zinc oxide bed. While
Harding Robert Hibbard
Schmidt Stephen Raymond
Wormsbecher Richard Franklin
Griffin Walter D.
Nguyen Tam M.
Troffkin H.
W.R. Grace & Co.-Conn
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