Hydrocracking processes are used in the petroleum industry to convert gas oil range and heavier petroleum feedstocks into lighter petroleum products. Fluid catalytic cracking is also used to perform a similar function. The process selected to convert gas oil range petroleum feedstocks into lighter petroleum products will vary according to a number of factors such as the desired product mix from the process, the type of feedstock available for the process and the like. Such considerations are well known to those skilled in the art.
Hydrocracking
In general, hydrocracking processes comprise charging the gas oil range petroleum feedstock to one or more fixed bed hydrocracking zones containing a hydrocracking catalyst at hydrocracking conditions to produce a first hydrocracking zone product stream. The feed to the hydrocracker is generally hydrotreated to reduce the concentrations of sulfur nitrogen and aromatics, prior to its hydrocracking. The product stream from the first hydrocracking zone is typically charged to a fractionation zone where it is separated into a product stream which is recovered and passed to further processing to produce a variety of products such as gasoline, kerosene, jet fuel, diesel fuel and the like and a bottoms stream which is passed to a second fixed bed hydrocracking zone which contains a hydrocracking catalyst at hydrocracking conditions to produce a second hydrocracking zone product stream. The second hydrocracking zone product stream is then passed to cooling and charged to a fractionating zone which may be the same fractionating zone used to fractionate the product stream from the first hydrocracking zone. The separation of products in the fractionating zone is as discussed previously. It is necessary to cool the product stream from the second hydrocracking zone to a temperature suitable for fractionation and in typical processes of this type quantities of heavy hydrocarbonaceous materials accumulate in the bottoms stream from the second hydrocracking zone as a result of the recycling of the bottoms stream from the fractionating zone to the second hydrocracking zone, and eventually accumulate in amounts large enough that the heavy materials precipitate in the cooling zone. Previously, such heavy materials have been eliminated by withdrawing a portion of the product stream from the second hydrocracking zone and shipping it to a refinery with a fluid catalytic cracking unit for processing, or the like. The portion of the second hydrocracking zone product stream withdrawn is fixed by the amount of heavy material which must be removed to prevent precipitation of the heavy material in the cooling zone. Such remedies for this problem result in a reduction in the amount of lighter, more valuable hydrocarbon products recovered from the process and in considerable expense when the material removed is shipped to another refinery for additional processing. Accordingly, a continuing effort has been directed to the development of processes whereby substantially all the gas oil petroleum feedstocks charged to hydrocracking processes can be converted to lighter, more valuable petroleum products in the process.
FCC
In most modern FCC units the hot regenerated catalyst is added to the feed at the base of the riser reactor. The fluidization of the solid catalyst particles may be promoted with a lift gas.
Steam can be used in an amount equal to about 1-5 wt % of the hydrocarbon feed to promote mixing and atomization of the feedstock. Preheated charge stock (150.degree.-375.degree. C.) is mixed with hot catalyst (&gt;650.degree. C.) from the regenerator. The catalyst vaporizes and super heats the feed to the desired cracking temperature, usually 450.degree.-600.degree. C. During the upward passage of the catalyst and feed, the feed is cracked and coke deposits on the catalyst. The cracked products and coked catalyst exit the riser and enter a solid-gas separation system, e.g., a series of cyclones, at the top of the reactor vessel. The cracked hydrocarbon products are typically fractionated into a series of products, including gas, gasoline, light gas oil and heavy cycle gas oil. Some heavy cycle gas oil may be recycled to the reactor. The bottoms product, a "slurry oil", is conventionally allowed to settle. The solids portion of the settled product rich in catalyst particles may be recycled to the reactor.
The following references, which contain good overviews of FCC processes are incorporated herein by reference: U.S. Pat. No. 3,152,065 (Sharp et al.); U.S. Pat. No. 3,261,776 (Banman et al.); U.S. Pat. No. 3,654,140 (Griffel et al.); U.S. Pat. No. 3,812,029 (Snyder); U.S. Pat. No. 4,093,537; U.S. Pat. Nos. 4,118,337; 4,118,338; 4,218,306 (Gross et al.); U.S. Pat. No. 4,444,722 (Owen); U.S. Pat. No. 4,459,203 (Beech et al.); U.S. Pat. 4,639,308 (Lee); U.S. Pat. No. 4,675,099; 4,681,743 (Skraba) as well as in Venuto et al., Fluid Catalytic Cracking With Zeolite Catalysts, Marcel Dekker, Inc. (1979).
Hydrotreating
It is also known to treat hydrocarbon feedstocks employing hydrotreating catalysts to achieve hydrodesulfurization (HDS), hydrodenitrification or hydrodenitrogenation (HDN), carbon residue reduction (CRR), hydrogenation (HYD), hydrodeoxygenation (HDO), and hydrodemetallation (HDM). Some mild hydrocracking might also take place during these hydrotreating processes.
Hydrocracking catalysts are generally silica-alumina or zeolite based, and therefore are acidic. The nitrogen compounds and aromatics in the hydrocarbon feed can temporarily, as well as permanently deactivate these hydrocracking catalysts if they are not removed or reduced significantly prior to the hydrocracking operation, and therefore it is a general practice to hydrotreat the hydrocarbon feed prior to hydrocracking operations.
FCC catalysts are also generally silica-alumina or zeolite based, and are acidic. These catalysts are also deactivated or poisoned by nitrogen compounds and by some aromatics in the hydrocarbon feed.
The hydrocracking process and FCC process can be improved significantly by using a hydrotreating catalyst having high hydrodearomatization (HDAr) and HDN activities, to hydrotreat the feed prior to hydrocracking and FCC processes. One type of catalyst which is often used in the field of petroleum processing is a catalyst comprising Group VIB and Group VIII metal components on a refractory inorganic oxide, such as alumina. Hydrotreating catalysts within this description are described in U.S. Pat. No. 4,188,284 (Quick et al.); U.S. Pat. No. 4,224,144 (Hensley, Jr., et al.); U.S. Pat. No. 4,278,566 (Hensley, Jr., et al.); U.S. Pat. No. 4,306,965 (Hensley, Jr., et al.); U.S. Pat. No. 4,530,911 (Ryan, et al.); and U.S. Pat. No. 4,758,544 (Plesko, et al.).
Some hydrotreating catalysts include a zeolite component in addition to Group VIB and VIII metals. See, for example, U.S. Pat. Nos. 3,671,425; 3,716,475; 3,954,671; and U.S. Pat. No. 3,983,029 (White).
Many of the above-described hydrotreating catalysts saturate or convert some aromatics. However, a large amount of these aromatics still reaches the hydrocracker or FCCU catalysts, including polynuclear aromatics which are somewhat insoluble, and their accumulation results in the fouling of process equipment, such as heat exchangers and process lines, and also contribute to shortened catalyst life and higher processing temperatures, because in these reactors a major contributor to exothermic reactions is the saturation of the aromatics rings which must take place before the rings can be cracked.
Catalysts which have been disclosed which are supposed to possess improvements by allowing a greater level of denitrification are described in, for example, U.S. Pat. No. 4,548,920 (Thompson et al.); U.S. Pat. No. 5,071,805 (Winslow, et al.). In U.S. Pat. No. 4,716,142, (Laine et al.) written before the recent stricter environmental regulations, the process provides enhanced denitrification, but selectively produces aromatics.
U.S. Pat. No. 4,990,243 (Winslow, et al.) provides a layered catalyst system for hydrodenitrification of hydrocarbons.
In the art there have been attempts to further reduce aromatics in the hydrocarbon feed prior to its entry into the hydrocracker or FCCU. This is a particularly desirable goal recently, because aromatic hydrocarbons in fuels represent a source of atmospheric pollution and the laws governing the permissible amounts in gasoline and diesel oil have become stricter.
Problems associated with the formation of polycyclic aromatic compounds within hydrocracking reactions zones are addressed in U.S. Pat. No. 4,775,460 (Reno) by a feed pretreating sequence which comprises first contacting the feed with a metal-free alumina to produce the polycyclic compounds or their precursors, followed by contacting the feed with a bed of adsorbent.
In U.S. Pat. No. 4,961,839 (Stine et al.) the problems with the formation of polynuclear aromatic compounds are addressed by operating at high conversion rates with a high hydrogen concentration followed by a unique separation method. See also U.S. Pat. No. 5,120,427 (Stine, et al.).
In U.S. Pat. No. 5,110,444 (Haun et al.) middle distillate petroleum streams are hydrotreated to produce a low sulfur and low aromatic product in a process employing three reaction zones in series. Hydrogen flows between the reaction zones countercurrent to the hydrocarbons. Hydrogen sulfide is removed from effluent of the first two reaction zones by hydrogen stripping. The second and third reaction zones employ a sulfur-sensitive noble metal hydrogenation catalyst. Operating pressure increases and temperature decreases from the first to third reaction zones.
In U.S. Pat. No. 5,116,484 (Smegal) there is disclosed a process for the hydrogenation of nitrogen-containing hydrocarbons in a hydrocarbon feedstock by passing the hydrocarbons through a first catalyst bed containing a hydrotreating catalyst containing nickel and tungsten on an alumina support and thereafter passing hydrocarbons to a second catalyst bed.
There is a need in the art for a relatively simple, efficient process for hydrogenating a greater percentage of aromatics in the hydrocarbon feedstock before the hydrocarbons are fed into a hydrocracker or FCCU. If it were possible to obtain a significantly higher level of hydrogenation of all types of aromatics prior to hydrocracking or FCCU operations, there would be a number of important benefits with respect to hydrocracker and FCCU conversion and product quality. In addition, it would be extremely valuable if a catalyst which performed better with respect to hydrodearomatization were capable of simultaneously eliminating sulfur and nitrogen to a significant extent.