PATENT
United States Patent Application 20120031811
Inventors:
Cowan, Timothy M. (Sudbury, MA, US)
Murty, Vedula K. (Willowbrook, IL, US)
Application Number: 12/852984
Publication Date: 02/09/2012
Assignee: UOP LLC (Des Plaines, IL, US)
Abstract:
FIELD OF THE INVENTION
This invention generally relates to a process for the selective production of either naphtha or middle distillate from a hydrocarbon feedstock introduced into a hydrocracking zone having a fixed catalyst.
BACKGROUND OF THE INVENTION
Petroleum refiners often produce desirable products such as turbine fuel, diesel fuel and other products known as middle distillates, as well as lower boiling hydrocarbonaceous liquids such as naphtha and gasoline, by hydrocracking a hydrocarbon feedstock derived from crude oil. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered or derived from crude oil by distillation or by thermal or catalytic processes. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371° C. (700° F.), usually at least about 50% by weight boiling above 371° C. (700° F.). A typical vacuum gas oil normally has a boiling point range between about 315° C. (600° F.) and about 565° C. (1050° F.).
Hydrocracking is generally accomplished by contacting the gas oil or other feedstock with a suitable hydrocracking catalyst under conditions of elevated temperature and pressure in the presence of hydrogen so as to yield a product containing a distribution of hydrocarbon products desired by the refiner. The operating conditions and the hydrocracking catalysts chosen within a hydrocracking reactor influence the yield of the hydrocracked products.
Additionally, the hydrocarbon feedstock can first be introduced into a hydrotreating zone to remove various impurities carried in the feedstock, such as nitrogen and sulfur, prior to entering the hydrocracking zone. The term “hydrotreating” can refer to processes wherein a hydrogen-containing treat gas is used in the presence of suitable catalysts which are primarily active for the removal of heteroatoms, such as sulfur and nitrogen and for some hydrogenation of aromatics. The hydrocarbon feedstock often is introduced into the hydrotreating zone along with an additional hydrogen stream in the presence of a hydrotreating catalyst to reform the nitrogen components of the feedstock into ammonia and the sulfur components into hydrogen sulfide.
The hydrotreated effluent stream typically is introduced into a hydrocracking zone over an appropriate catalyst at a temperature and pressure sufficient to cause conversion of the heavy boiling material into lower boiling material.
Alternatively, the hydrotreated effluent stream can first be introduced into a stripping zone at a temperature and pressure sufficient to remove the ammonia and hydrogen sulfide constituents from the stream. The hydrotreated effluent stream then typically is introduced into the hydrocracking zone over an appropriate catalyst at a temperature and pressure sufficient to convert the feedstock into components with lower boiling points.
If the ammonia content of the hydrotreated stream is not adequately reduced in the stripper, then the ammonia impurities can reduce the hydrocracking zone catalyst activity, greatly increasing the temperature required to affect a given level of conversion in the hydrocracking zone. One such adjustment, for example, is to greatly increase the processing temperature to affect a given level of conversion in the hydrocracking zone.
The hydrocracking catalyst and process conditions typically are selected to crack the hydrocarbon feed to a specific, desired product, range of products, and/or product constituents. Once the hydrocracking process is started, the resulting catalyst activity and product selectivity are difficult to modify during the duration of the life of the catalyst, and thus modifying the products or product constituents produced during that hydrocracking run is difficult as well. If a change is desired in the products, range of products and/or product constituents during a hydrocracking run, then production normally must be stopped to change out catalysts or make other similar process changes.
For example, if the operation of the hydrocracker is set up to preferentially yield middle distillate products (e.g., with a boiling point range of about 121° C. (250° F.) to about 399° C. (750° F.)), the hydrocracker would then contain a catalyst appropriate for the production of such products at the required operating conditions. Changing the product output to favor naphtha production (e.g., with a boiling point range of about 10° C. (50° F.) to about 204° C. (400° F.)) would require halting the hydrocracker operation and changing out the catalyst and modifying the process condition accordingly, at considerable expense and loss of production time. One alternative is to adjust the operating conditions, such as temperature and pressure conditions. This typically does not shift the final product yield enough to provide significant amounts of the newly desired products without a complete change out of the hydrocracking catalyst.
SUMMARY OF THE INVENTION
The process disclosed herein uses the ammonia content present in or added to a feed to a hydrocracking zone to influence the catalyst activity and efficiency thereof to crack a hydrocarbonaceous feedstock to a desired hydrocarbon product, range of products, and/or mix of hydrocarbon constituents, such as those found in middle distillates or naphthas. In one aspect, the ammonia used in the hydrocracking zone is obtained from reacting nitrogen in the hydrocarbonaceous feedstock with a hydrogen stream under hydrotreating conditions and in the presence of hydrotreating catalysts. Therefore, in this aspect, it is not necessary to add ammonia from an external source. However, if desired, an external source of ammonia may be used to supplement, or instead of, the ammonia obtained from the feedstock. This external source of ammonia may be in the form of aqueous ammonia, anhydrous ammonia, or another hydrocarbonaceous feedstock containing nitrogen.
In one aspect, the process (and related apparatus) provide for selective production of a hydrocarbon product stream, from a hydrocarbonaceous feed stream supplied to a hydrocracking zone having a fixed catalyst system. In another aspect, the hydrocarbon product stream may comprise either primarily a naphtha or primarily a middle distillate product stream. The desired products and/or product range are selected by controlling the ammonia concentration introduced into the hydrocracking zone with the hydrocarbonaceous stream. The hydrocracking zone, as a result, is subject to relatively continuous operation without a halt in operations to change the catalyst systems already in use.
The desired product stream can be changed between preferred products during operation of the hydrocracking zone without substantial changes in the initial operating conditions of the hydrocracking zone. The hydrocracking zone may operate at conditions including a temperature from about 204° C. (400° F.) to about 482° C. (900° F.) and a pressure from about 3.4 MPa (500 psig) to about 20.7 MPa (3000 psig), with any number of catalyst systems that are typically used for the production of naphtha or diesel constituents.
Modifying the ammonia concentration in the feed to the hydrocracking zone provides for conversion of the feedstock into a variety of products without requiring a changeover of the existing fixed catalyst system. The activity of the catalyst can be modified depending on the amount of ammonia introduced into the system, and thus the yield of the desired product may be modified according to the activity change in the catalyst that results.
In one aspect, where the ammonia concentration is present from about 0 to about 50 wppm ammonia, primarily a naphtha stream comprising from about 35 to about 70 wt-% naphtha is produced. Where the ammonia concentration is present from about 10 to about 200 wppm, primarily a middle distillate stream is produced comprising from about 20 to about 80 wt-% middle distillate or diesel.
Generally, a high ammonia concentration favors the production of middle distillate, and a low ammonia concentration favors the production of a naphtha product. The ammonia concentration affects the catalyst by slowing down the catalyst activity at high ammonia concentrations (e.g., to yield middle distillate in one aspect) or by having a minimal impact upon catalyst activity with low ammonia concentrations (e.g., to yield naphtha in another aspect). In one aspect, a stripping zone, such as an enhanced hot separator (“EHS”), can drive the ammonia concentration depending upon its process conditions, such as a temperature that ranges from about 148° C. (300° F.) to about 343° C. (650° F.). In the case of naphtha, a lower ammonia concentration is desired and hence a higher temperature in the EHS, i.e., at the higher end of the range, drives the ammonia lower, such that it separates out the ammonia into an overhead stream that does not directly feed into the process. On the other hand, where the middle distillate is desired a lower temperature in the EHS, i.e., a temperature at the lower end of the range, can drive the ammonia into the bottoms liquid product, thus keeping the ammonia concentration high and feeding the effluent containing ammonia into the hydrocracker along with the hydrocarbonaceous feed stream. The hydrocracking zone may operate at conditions including a temperature from about 204° C. (400° F.) to about 482° C. (900° F.) and a pressure from about 3.4 MPa (500 psig) to about 20.7 MPa (3000 psig). The catalyst LHSV ranges from about 0.5 to about 4.0 hr−1.
Prior to entering the hydrocracking zone, the hydrocarbonaceous feedstock may initially be introduced into a hydrotreating zone to treat the feedstock stream with hydrogen to reform any nitrogen components present in the feedstock into ammonia in addition to reforming sulfur components into hydrogen sulfide. This reaction may account for at least a portion of the ammonia source to the hydrocracking zone. The effluent from the hydrotreating zone is then introduced into a stripping zone to remove hydrogen sulfide from the hydrocarbon stream and to reduce the ammonia content of the hydrocarbon stream, as appropriate, to yield the desired final product from the hydrocracker. In another aspect, an external source of ammonia may be used to supplement the ammonia provided from the hydrotreating zone, or as an alternative ammonia source, if needed.
The hydrotreated feedstock also may be directly introduced to a hydrocracking zone prior to stripping. The partially hydrocracked effluent may then be sent to a stripping zone to remove hydrogen sulfide from the hydrocarbon stream and adjust the ammonia content of the hydrocarbon stream, as appropriate, to yield the desired final product from a second hydrocracking zone. The hydrocarbon stream from the stripping zone is introduced into the second hydrocracking zone. In the second hydrocracking zone, the hydrocarbon product stream is cracked to the desired product (e.g., either primarily naphtha or middle distillate products) according to the effect of the ammonia upon the catalyst system and the hydrocarbon zone conditions.
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