Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
1999-09-20
2003-10-21
Norton, Nadine G. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C208S120010, C208S120050, C208S120100, C208S120350, C208S120250, C208S243000, C208S247000, C208S248000, C208S249000
Reexamination Certificate
active
06635169
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the reduction of sulfur in gasoline and other petroleum products produced by the catalytic cracking process. In particular, the invention relates to an improved method which employs catalytic compositions for reducing product sulfur content.
Catalytic cracking is a petroleum refining process which is applied commercially on a very large scale, especially in the United States where the majority of the refinery gasoline blending pool is produced by catalytic cracking, with almost all of this coming from the fluid catalytic cracking (FCC) process. In the catalytic cracking process, heavy hydrocarbon fractions are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking products of four or less carbon atoms per molecule. The gas partly consists of olefins and partly of saturated hydrocarbons.
During the cracking reactions some heavy material, known as coke, is deposited onto the catalyst. This reduces its catalytic activity and regeneration is desired. After removal of occluded hydrocarbons from the spent cracking catalyst, regeneration is accomplished by burning off the coke to restore the catalyst activity. The three characteristic steps of a typical catalytic cracking process can be identified as follows: a cracking step in which the hydrocarbons are converted into lighter products, a stripping step to remove hydrocarbons adsorbed on the catalyst and a regeneration step to burn off coke from the catalyst. The regenerated catalyst is then reused in the cracking step.
Catalytic cracking feedstocks normally contain sulfur in the form of 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 is converted to hydrogen sulfide during the cracking process, mainly by catalytic decomposition of non-thiophenic sulfur compounds. Although the amount and type of sulfur in the cracking products are influenced by the feed, catalyst type, additives present, conversion and other operating conditions, a significant portion of the sulfur generally remains in the product pool. With increasing environmental regulation being applied to petroleum products, for example in the Reformulated Gasoline (RFG) regulations, the allowable sulfur content of the products has generally been decreased in response to concerns about the emissions of sulfur oxides and other sulfur compounds into the air following combustion processes.
One approach has been to remove the sulfur from the FCC feed by hydrotreating before cracking is initiated. While highly effective, this approach tends to be expensive in terms of the capital cost of the equipment as well as operationally since hydrogen consumption is high. Another approach has been to remove the sulfur from the cracked products by hydrotreating. Again, while effective, this solution has the drawback that valuable product octane may be lost when the high octane olefins are saturated.
From an economic point of view, it would be desirable to achieve sulfur removal in the cracking process itself since this would effectively desulfurize the major component of the gasoline blending pool without additional treatment. Various catalytic materials have been developed for the removal of sulfur during the FCC process cycle but, so far, most developments 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 Improve FCC Process
, Hydrocarbon Processing, November 1991, pages 59-66. The sulfur is removed from the stack gases from the regenerator but product sulfur levels are not greatly affected, if at all.
An alternative technology for the removal of sulfur oxides from regenerator removal is based on the use of magnesium-aluminum spinels as additives to the circulating catalyst inventory in the FCCU. Under the designation DESOX™ used for the additives in this process, the technology has achieved a notable commercial success. Exemplary patents on this type of sulfur removal additive 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 additive for the reduction of sulfur levels in the liquid cracking products is proposed by Wormsbecher and Kim in U.S. Pat. Nos. 5,376,608 and 5,525,210, using a cracking catalyst additive of an alumina-supported Lewis acid for the production of reduced-sulfur gasoline but this system has not achieved significant commercial success. The need for an effective additive for reducing the sulfur content of liquid catalytic cracking products has therefore persisted.
In application Ser. No. 09/144,607, filed Aug. 31, 1998, catalytic materials are described for use in the catalytic cracking process which are capable of reducing the sulfur content of the liquid products of the cracking process. These sulfur reduction catalysts comprise, in addition to a porous molecular sieve component, a metal in an oxidation state above zero within the interior of the pore structure of the sieve. The molecular sieve is in most cases a zeolite and it may be a zeolite having characteristics consistent with the large pore zeolites such as zeolite beta or zeolite USY or with the intermediate pore size zeolites such as ZSM-5. Non-zeolitic molecular sieves such as MeAPO-5, MeAPSO-5, as well as the mesoporous crystalline materials such as MCM-41 may be used as the sieve component of the catalyst. Metals such as vanadium, zinc, iron, cobalt, and gallium were found to be effective for the reduction of sulfur in the gasoline, with vanadium being the preferred metal. When used as a separate particle additive catalyst, these materials are used in combination with an active catalytic cracking catalyst (normally a faujasite such as zeolite Y and REY, especially as zeolite USY and REUSY) to process hydrocarbon feedstocks in the fluid catalytic cracking (FCC) unit to produce low-sulfur products. Since the sieve component of the sulfur reduction catalyst may itself be an active cracking catalyst, for instance, zeolite Y, REY, USY, and REUSY, it is also possible to use the sulfur reduction catalyst in the form of an integrated cracking/sulfur reduction catalyst system, for example, comprising USY as the active cracking component and the sieve component of the sulfur reduction system together with added matrix material such as silica, clay and the metal, e.g. vanadium, which provides the sulfur reduction functionality.
In application Ser. Nos. 09/221,539 and 09/221,540, both filed Dec. 28, 1998, sulfur reduction catalysts similar to the ones described in application Ser. No. 09/144,607 were described, however, the catalyst compositions in those applications also comprise at least one rare earth metal component (e.g. lanthanum) and a cerium component, respectively.
SUMMARY OF THE INVENTION
An improved catalytic cracking process has now been developed which is capable of improving the reduction in the sulfur content of the liquid products of the cracking process, including the gasoline and middle distillate cracking fractions. The present process employs sulfur reduction catalysts similar to the ones described in application Ser. Nos. 09/144,607, 09/221,539 and 09/221,540, each of which is incorporated herein by reference in their entirety, in that the cracking catalyst employed in the present invention contains a product sulfur reduc
Bhore Nazeer A.
Chester Arthur W.
Liu Ke
Timken Hye Kyung Cho
Cross Charles A.
Mobil Oil Corporation
Norton Nadine G.
W.R. Grace & Co.-Conn.
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