Improved hydrocracking process

Abstract: A catalytic hydrocracking process wherein a hydrocarbonaceous feedstock and a liquid recycle stream is contacted with hydrogen in a hydrocracking reaction zone at elevated temperature and pressure to obtain conversion to lower boiling hydrocarbons. A liquid hydrocarbonaceous stream produced from the effluent of the hydrocracking reaction zone is fractionated to produce at least one liquid hydrocarbonaceous product stream and a liquid hydrocarbonaceous stream containing hydrocarbons boiling at a temperature in the boiling range of the feedstock and heavy polynuclear aromatic compounds. At least a portion of the liquid hydrocarbonaceous stream containing heavy polynuclear aromatic compounds is introduced into a zone of the divided-wall fractionation zone to produce a stream rich in polynuclear aromatic compounds. At least another portion of the liquid hydrocarbonaceous stream containing hydrocarbons boiling at a temperature in the boiling range of the feedstock is recycled to the hydrocracking reaction zone.

Description: BACKGROUND OF THE INVENTION The field of art to which this invention pertains is the hydrocracking of a hydrocarbonaceous feedstock. 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, for example. Feedstocks most often subjected to hydrocracking are gas oils and heavy gas oils recovered from crude oil by distillation. A typical heavy gas oil comprises a substantial portion of hydrocarbon components boiling above about 371°C (700°F), usually at least about 50 percent by weight boiling above 371°C (700°F). A typical vacuum gas oil normally has a boiling point range between 315°C (600°F) and 565°C (1050°F). Hydrocracking is generally accomplished by contacting in a hydrocracking reaction vessel or zone the gas oil or other feedstock to be treated 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 within a hydrocracking reactor influence the yield of the hydrocracked products. Although a wide variety of process flow schemes, operating conditions and catalysts have been used in commercial activities, there is always a demand for new hydrocracking methods which provide lower costs and higher liquid product yields. It is generally known that enhanced product selectivity can be achieved at lower conversion per pass (60% to 90% conversion of fresh feed) through the catalytic hydrocracking zone. However, it was previously believed that any advantage of operating at below about 60% conversion per pass was negligible or would only see diminishing returns. Low conversion per pass is generally more expensive, however, the present invention greatly improves the economic benefits of a low conversion per pass process and demonstrates the unexpected advantages. INFORMATION DISCLOSURE US-A-5,720,872 discloses a process for hydroprocessing liquid feedstocks in two or more hydroprocessing stages which are in separate reaction vessels and wherein each reaction stage contains a bed of hydroprocessing catalyst. The liquid product from the first reaction stage is sent to a low pressure stripping stage and stripped of hydrogen sulfide, ammonia and other dissolved gases. The stripped product stream is then sent to the next downstream reaction stage, the product from which is also stripped of dissolved gases and sent to the next downstream reaction stage until the last reaction stage, the liquid product of which is stripped of dissolved gases and collected or passed on for further processing. The flow of treat gas is in a direction opposite the direction in which the reaction stages are staged for the flow of liquid. Each stripping stage is a separate stage, but all stages are contained in the same stripper vessel. International Publication No. WO 97/38066 (PCT/US 97/04270) discloses a process for reverse staging in hydroprocessing reactor systems. US-A-3,328,290 discloses a two-stage process for the hydrocracking of hydrocarbons in which the feed is pretreated in the first stage. US-A-5,980,729 discloses a hydrocracking process utilizing reverse staging in hydroprocessing reactor systems and a hot, high-pressure stripping zone. US-A-3,437,584 discloses a hydrocracking process, wherein in a first step a hydrocarbonaceous input stream and hydrogen are fed to a hydrocracking zone. The effluent from this hydrocracking zone is separated in a separation zone into a first stream containing hydrogen and hydrocarbons boiling at a temperature below the boiling range of the hydrocarbonaceous input stream and a second stream comprising hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous input stream and heavy polynuclear aromatic compounds. In a following process step, one portion of the second stream is transferred to a divided-wall fractionation zone and another portion is recycled to the hydrocracking zone. BRIEF SUMMARY OF THE INVENTION The present invention is a catalytic hydrocracking process which uses a divided-wall fractionator to recover lower boiling hydrocarbon product streams, a liquid recycle stream and a drag stream containing a high concentration of heavy polynuclear aromatic compounds. The process of the present invention benefits from the ability to achieve a lower capital cost, lower operating expense and simplified operation. Specific embodiments of the invention may provide higher liquid product yields, specifically higher yields of turbine fuel and diesel oil with a low conversion per pass operation. Other benefits of a low conversion per pass operation include the minimization or elimination of the need for inter-bed hydrogen quench and the minimization of the fresh feed pre-heat since the higher flow rate of recycle liquid will provide additional process heat to initiate the catalytic reaction and an additional heat sink to absorb the heat of reaction. An overall reduction in fuel gas and hydrogen consumption, and light ends production may also be obtained. Finally, the low conversion per pass operation requires less catalyst volume. In accordance with one embodiment the present invention relates to a process for hydrocracking a hydrocarbonaceous feedstock that passes a hydrocarbonaceous input stream and hydrogen to a hydrocracking zone containing hydrocracking catalyst to produce a hydrocracking effluent; combines a hydrocarbonaceous feedstock with at least one of the hydrocarboneous input streams or the hydrocracking effluent; separates the effluent from said hydrocracking zone in a first separation zone to produce a first stream containing hydrogen and hydrocarbons boiling at a temperature below the boiling range of said hydrocarboneous input stream and a second stream comprising hydrocarbons boiling at a temperature in the boiling range of said hydrocarbonaceous input stream and heavy polynuclear aromatic compounds; introduces at least a portion of the second stream into a second separation zone to produce a third stream comprising hydrocarbons boiling at a temperature in the boiling range of said hydrocarbonaceous input stream and heavy polynuclear aromatic compounds and a fourth stream comprising hydrocarbons boiling at a temperature equal to or below the boiling range of said hydrocarboneous input stream and having a lower concentration of heavy polynuclear aromatic compounds than the third stream; introduces at least a portion of said third stream into a first divided zone located in the bottom end of a divided-wall fractionation zone to produce a fifth stream rich in polynuclear aromatic compounds; recycles at least another portion of said second stream to said hydrocracking zone to provide at least a portion of said hydrocarbonaceous input stream; and recovers a liquid hydrocarbonaceous product stream from at least a portion of at least one of the first stream or the fourth stream. In accordance with a more limited embodiment, the second separation zone comprises a second divided zone located in the bottom of the divided wall fractionation zone. According to a further more limited embodiment the undesirable production of polynuclear aromatic compounds is controlled by removing a small dragstream of high pressure product stripper bottoms to reject polynuclear aromatic compounds and recovering valuable diesel boiling range hydrocarbons and unconverted feedstock by routing the dragstream to a hot flash separator and subsequently to a divided wall fractionation zone to produce a concentrated stream of polynuclear aromatic compounds while recovering the valuable hydrocarbon compounds. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a simplified process flow diagram of a hydrocracking process arranged in accordance with this invention. Fig. 2 is a simplified process flow diagram of an alternate arrangement for a hydrocracking process of this invention. DETAILED DESCRIPTION OF THE INVENTION It has been discovered that a divided-wall fractionation zone may be successfully utilized to produce various product streams from a hydrocracking reaction zone including, for example, naphtha, kerosene and diesel hydrocarbon streams while simultaneously preparing a liquid hydrocarbonaceous recycle stream having a reduced concentration of heavy polynuclear aromatic compounds and a small hydrocarbon slip stream containing an enhanced concentration of heavy polynuclear aromatics. The process of the present invention is particularly useful for hydrocracking a hydrocarbonaceous oil containing hydrocarbons and/or other organic materials to produce a product containing hydrocarbons and/or other organic materials of lower average boiling point and lower average molecular weight. The hydrocarbonaceous feedstocks that may be subjected to hydrocracking by the method of the invention include all mineral oils and synthetic oils (e.g., shale oil, tar sand products, etc.) and fractions thereof. Preferred hydrocarbonaceous feedstocks according to the present invention boil in the range from 232°C to 565°C. Illustrative hydrocarbon feedstocks include those containing components boiling above 288°C, such as atmospheric gas oils, vacuum gas oils, deasphalted, vacuum, and atmospheric residua, hydrotreated or mildly hydrocracked residual oils, coker distillates, straight run distillates, solvent-deasphalted oils, pyrolysis-derived oils, high boiling synthetic oils, cycle oils and cat cracker distilllates. A preferred hydrocracking feedstock is a gas oil or other hydrocarbon fraction having at least 50% by weight, and most usually at least 75% by weight, of its components boiling at temperatures above the end point of the desired product, which end point, in the case of heavy gasoline, is generally in the range from 193°C to 215°C. One of the most preferred gas oil feedstocks will contain hydrocarbon components which boil above 288°C with best results being achieved with feeds containing at least 25 percent by volume of the components boiling between 315°C and 538°C. Also included are petroleum distillates wherein at least 90 percent of the components boil in the range from 149°C to 426°C. The petroleum distillates may be treated to produce both light gasoline fractions (boiling range, for example, from 10°C to 85°C and heavy gasoline fractions (boiling range, for example, from 85°C to 204°C. The present invention is particularly suited for the production of increased amounts of middle distillate products. In one embodiment the selected feedstock may be first introduced into a denitrification and desulfurization reaction zone together with a hot hydrocracking zone effluent at hydrotreating reaction conditions. Preferred denitrification and desulfurization reaction conditions or hydrotreating reaction conditions include a temperature from 204°C to 482°C, a pressure from 3.34 kPa to 17.1 kPa, a liquid hourly space velocity of the fresh hydrocarbonaceous feedstock from 0.1 hr -1 to 10 hr -1 with a hydrotreating catalyst or a combination of hydrotreating catalysts. The term "hydrotreating" as used herein refers 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. Suitable hydrotreating catalysts for use in the present invention are any known conventional hydrotreating catalysts and include those which are comprised of at least one Group VIII metal, preferably iron, cobalt and nickel, more preferably cobalt and/or nickel and at least one Group VI metal, preferably molybdenum and tungsten, on a high surface area support material, preferably alumina. Other suitable hydrotreating catalysts include zeolitic catalysts, as well as noble metal catalysts where the noble metal is selected from palladium and platinum. In another embodiment of the present invention the resulting effluent from the denitrification and desulfurization reaction zone or the selected feedstock may be introduced into a hydrocracking zone. The hydrocracking zone may contain one or more beds of the same or different catalyst. In one embodiment, when the preferred products are middle distillates, the preferred hydrocracking catalysts utilize amorphous bases or low-level zeolite bases combined with one or more Group VIII or Group VIB metal hydrogenating components. In another embodiment, when the preferred products are in the gasoline boiling range, the hydrocracking zone contains a catalyst which comprises, in general, any crystalline zeolite cracking base upon which is deposited a minor proportion of a Group VIII metal hydrogenating component. Additional hydrogenating components may be selected from Group VIB for incorporation with the zeolite base. The zeolite cracking bases are sometimes referred to in the art as molecular sieves and are usually composed of silica, alumina and one or more exchangeable cations such as sodium, magnesium, calcium, rare earth metals, etc. They are further characterized by crystal pores of relatively uniform diameter between 4 and 14 Angstroms (10 -10 meters). It is preferred to employ zeolites having a relatively high silica/alumina mole ratio between 3 and 12. Suitable zeolites found in nature include, for example, mordenite, stilbite, heulandite, ferrierite, dachiardite, chabazite, erionite and faujasite. Suitable synthetic zeolites include, for example, the B, X, Y and L crystal types, e.g., synthetic faujasite and mordenite. The preferred zeolites are those having crystal pore diameters between 8-12 Angstroms (10 -10 meters), wherein the silica/alumina mole ratio is 4 to 6. A prime example of a zeolite falling in the preferred group is synthetic Y molecular sieve. The natural occurring zeolites are normally found in a sodium form, an alkaline earth metal form, or mixed forms. The synthetic zeolites are nearly always prepared first in the sodium form. In any case, for use as a cracking base it is preferred that most or all of the original zeolitic monovalent metals be ion-exchanged with a polyvalent metal and/or with an ammonium salt followed by heating to decompose the ammonium ions associated with the zeolite, leaving in their place hydrogen ions and/or exchange sites which have actually been decationized by further removal of water. Hydrogen or "decationized" Y zeolites of this nature are more particularly described in US-A-3,130,006. Mixed polyvalent metal-hydrogen zeolites may be prepared by ion-exchanging first with an ammonium salt, then partially back exchanging with a polyvalent metal salt and then calcining. In some cases, as in the case of synthetic mordenite, the hydrogen forms can be prepared by direct acid treatment of the alkali metal zeolites. The preferred cracking bases are those which are at least 10 percent, and preferably at least 20 percent, metal-cation-deficient, based on the initial ion-exchange capacity. A specifically desirable and stable class of zeolites are those wherein at least 20 percent of the ion exchange capacity is satisfied by hydrogen ions. The active metals employed in the preferred hydrocracking catalysts of the present invention as hydrogenation components are those of Group VIII, i.e., iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. In addition to these metals, other promoters may also be employed in conjunction therewith, including the metals of Group VIB, e.g., molybdenum and tungsten. The amount of hydrogenating metal in the catalyst can vary within wide ranges. Broadly speaking, any amount between 0.05 percent and 30 percent by weight may be used. In the case of the noble metals, it is normally preferred to use 0.05 to 2 weight percent. The preferred method for incorporating the hydrogenating metal is to contact the zeolite base material with an aqueous solution of a suitable compound of the desired metal wherein the metal is present in a cationic form. Following addition of the selected hydrogenating metal or metals, the resulting catalyst powder is then filtered, dried, pelleted with added lubricants, binders or the like if desired, and calcined in air at temperatures of, e.g., 371°-648°C (700°-1200°F) in order to activate the catalyst and decompose ammonium ions. Alternatively, the zeolite component may first be pelleted, followed by the addition of the hydrogenating component and activation by calcining. The foregoing catalysts may be employed in undiluted form, or the powdered zeolite catalyst may be mixed and copelleted with other relatively less active catalysts, diluents or binders such as alumina, silica gel, silica-alumina cogels, activated clays and the like in proportions ranging between 5 and 90 weight percent. These diluents may be employed as such or they may contain a minor proportion of an added hydrogenating metal such as a Group VIB and/or Group VIII metal. Additional metal promoted hydrocracking catalysts may also be utilized in the process of the present invention which comprises, for example, aluminophosphate molecular sieves, crystalline chromosilicates and other crystalline silicates. Crystalline chromosilicates are more fully described in US-A-4,363,718. The hydrocracking of the hydrocarbonaceous feedstock in contact with a hydrocracking catalyst is conducted in the presence of hydrogen and preferably at hydrocracking reactor conditions which include a temperature from 232°C (450°F) to 468°C (875°F), a pressure from 3.3 MPa (500 psig) to 20.6 MPa (3000 psig), a liquid hourly space velocity (LHSV) from 0.1 to 30 hr -1 , and a hydrogen circulation rate from 337 normal m 3 /m 3 to 4200 normal m 3 /m 3 (2000 to 25,000 standard cubic feet per barrel). In accordance with the present invention, the term "substantial conversion to lower boiling products" is meant to connote the conversion of at least 5 volume percent of the fresh feedstock. In one embodiment, the per pass conversion in the hydrocracking zone is in the range from 15% to 60%, preferably in a range of from 15% to 45% and more preferably in a range of from 20% to 40%. The resulting effluent from the hydrocracking reaction zone may be contacted with an aqueous stream and partially condensed, and then introduced into a high pressure vapor-liquid separator operated at a pressure substantially equal to the hydrocracking zone and a temperature in the range from 38°C (100°F) to 71°C (160°F). A hydrogen-rich gaseous stream is removed from the vapor-liquid separator to provide at least a portion of the hydrogen introduced into the denitrification and desulfurization reaction zone as hereinabove described. In a preferred embodiment of the present invention the first stream passes to an aromatic saturation zone containing hydrogenation catalyst to produce a sixth stream comprising hydrocarbonaceous compounds boiling at a temperature below the boiling range of said hydrocarbonaceous feedstock and having a reduced concentration of aromatic compounds and at least a portion of the sixth stream and fourth stream pass to a second divided zone of the divided wall fractionation zone to recover at least a portion of said hydrocarbonaceous product stream. Fresh make-up hydrogen may be introduced into the process at any suitable and convenient location. Before the hydrogen-rich gaseous steam from the vapor-liquid separator is introduced into the denitrification and desulfurization reaction zone, it is preferred that at least a significant amount of the hydrogen sulfide is removed and recovered by means of known, conventional methods. In a preferred embodiment, the hydrogen-rich gaseous stream introduced into the denitrification and desulfurization reaction zone contains less than about 50 wppm hydrogen sulfide. A liquid hydrocarbonaceous stream is recovered from the vapor-liquid separator and may be passed to a second vapor-liquid separator having a lower pressure to produce a gaseous stream containing hydrogen and normally gaseous hydrocarbons and another liquid hydrocarbonaceous stream which is passed to a stripper column to produce a gaseous stream containing normally gaseous hydrocarbons and a liquid hydrocarbonaceous stream containing trace quantities of heavy polynuclear aromatic compounds which is passed to a zone on one side of a divided-wall in a divided-wall fractionation zone to produce at least one hydrocracked hydrocarbonaceous product stream and a bottoms liquid hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds. At least a portion of the bottoms liquid hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds is recycled to the denitrification and desulfurization reaction zone as described hereinabove. At least a portion of the bottoms liquid hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds which stream is removed from one side of the divided-wall fractionation zone may be introduced into the opposing side of the divided-wall fractionation zone which is located in the bottom end of the fractionation zone and preferably stripped with steam to flash off hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstocks and to produce a heavy bottoms stream rich in heavy polynuclear aromatic compounds. In order to achieve the maximum advantage of the process of the present invention, it is preferred that the heavy bottoms stream rich in heavy polynuclear aromatic compounds is in an amount less than about 1 weight percent of the hydrocarbonaceous feedstock. In accordance with the present invention, the divided-wall fractionation zone may accept a heated stream containing hydrocarbons boiling at a temperature below the boiling range of said hydrocarbonaceous feedstock, hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds to produce at least one liquid hydrocarbonaceous product stream and a liquid hydrocarbonaceous stream comprising hydrocarbons boiling at a temperature in the boiling range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds. Preferably the divided-wall fractionation zone produces one or more product streams including naphtha, kerosene and diesel, for example. The divided-wall fractionation zone is preferably constructed with a solid dividing wall located in the lower end of the fractionation zone to partition the lower end to provide two separate zones which contain and maintain two separate liquids. The dividing wall is necessarily constructed to prevent the admixture of the two liquids while permitting the movement of vapor from each zone to the upper end of the fractionation zone. Since the liquid volumetric flow rates are expected to be unequal in the two zones, it is preferred that the zone having the lower flow rate be proportionally smaller than the other zone in order to efficiently utilize the total volume available in the lower end of the fractionation zone. The heated feed to the divided-wall fractionation zone may be introduced at any convenient place or elevation including either above or below the upper end of the dividing wall in order to effect the desired fractionation and product generation. The introduction of the liquid stream into the fractionation zone to produce a stream rich in heavy polynuclear aromatic compounds is preferably made at a location below the upper end of the dividing wall in order to prevent cross-contamination by heavy polynuclear aromatic compounds between the two zones defined by the dividing wall. In another embodiment of the present invention, the hydrocracking process may be performed without a denitrification and desulfurization reaction zone and with one or more hydrocracking zones as long as at least a portion of an effluent from at least one hydrocracking zone is introduced into a divided-wall fractionation zone as herein described. Accordingly, the resulting effluent from the denitrification and desulfurization reaction zone or the hydrocracking zone may be transferred without intentional heat-exchange (uncooled) and introduced into a hot, high pressure stripping zone maintained at essentially the same pressure as the preceding reaction zone, and contacted and countercurrently stripped with a hydrogen-rich gaseous stream to produce a first gaseous hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling at a temperature less than 371°C, hydrogen sulfide and ammonia, and a first liquid hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling at a temperature greater than 371°C. The stripping zone is preferably maintained at a temperature in the range from 232°C to about 486°C. The effluent from the preceding reaction zone is not substantially cooled prior to stripping and would only be lower in temperature due to unavoidable heat loss during transport from the reaction zone to the stripping zone. It is preferred that any cooling of the preceding reaction zone effluent prior to stripping is less than about 38°C. Maintaining the pressure of the stripping zone at essentially the same pressure as the preceding reaction zone means that any difference in pressure is due to the pressure drop required to flow the effluent stream from the reaction zone to the stripping zone. It is preferred that the pressure drop is less than 589 kPa. The hydrogen-rich gaseous stream is preferably supplied to the stripping zone in an amount greater than about 1 weight percent of the hydrocarbonaceous feedstock. At least a portion of the first liquid hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling at a temperature greater than about 371°C recovered from the stripping zone is introduced into a hydrocracking zone along with added hydrogen. The resulting first gaseous hydrocarbonaceous stream containing hydrocarbonaceous compounds boiling at a temperature less than 371°C (700°F), hydrogen, hydrogen sulfide and ammonia from the stripping zone may be introduced in an all vapor phase into a post-treat hydrogenation reaction zone to hydrogenate at least a portion of the aromatic compounds in order to improve the quality of the middle distillate, particularly the jet fuel. The post-treat hydrogenation reaction zone may be conducted in a downflow, upflow or radial flow mode of operation and may utilize any known hydrogenation catalyst. The effluent from the post-treat hydrogenation reaction zone is preferably cooled to a temperature in the range from 4°C (40°F) to 60°C (140°F) and at least partially condensed to produce a second liquid hydrocarbonaceous stream which is recovered and fractionated to produce desired hydrocarbon product streams and to produce a second hydrogen-rich gaseous stream which is bifurcated to provide at least a portion of the added hydrogen introduced into the hydrocracking zone as hereinabove described and at least a portion of the first hydrogen-rich gaseous stream introduced in the stripping zone. DETAILED DESCRIPTION OF THE DRAWING With reference now to Fig. 1, a feed stream comprising vacuum gas oil and heavy coker gas oil is introduced into the process via line 1 and admixed with a hydrogen-rich recycle gas transported via line 35. The resulting admixture is carried via line 2 and admixed with a hereinafter-described recycle oil transported via line 24. This resulting admixture is then transported via line 3 into combination reaction zone 4 and is contacted with a denitrification and desulfurization catalyst. A resulting effluent from the denitrification and desulfurization catalyst is passed into a hydrocracking catalyst which is also contained in combination reaction zone 4. A resulting hydrocracked effluent from combination reaction zone 4 is carried via line 5 and is admixed with a water wash stream introduced via line 6 and the resulting admixture is transported via line 7 and introduced into heat-exchanger 8. A resulting cooled effluent from heat-exchanger 8 is transported via line 9 and introduced into vapor-liquid separator 10. A spent water wash stream is removed from vapor-liquid separator 10 via line 11. A hydrogen-rich gaseous stream containing hydrogen sulfide is removed from vapor-liquid separator 10 via line 27 and introduced into gas recovery zone 28. A lean solvent is introduced via line 29 into acid gas recovery zone 28 and contacts the hydrogen-rich gaseous stream in order to adsorb an acid gas. A rich solvent containing acid gas is removed from acid gas recovery zone 28 via line 30 and recovered. A hydrogen-rich gaseous stream containing a reduced concentration of acid gas is removed from acid gas recovery zone 28 via line 31, compressed in compressor 32. A compressed hydrogen-rich gaseous recycle stream is transported via line 33 and is admixed with a make-up hydrogen gaseous stream carried via line 34 and the resulting admixture is transported via line 35 and is admixed with the fresh feedstock as hereinabove described. A liquid hydrocarbonaceous stream is removed from vapor-liquid separator 10 via line 12 and is introduced into low pressure flash zone 13. A vaporous stream containing hydrogen and normally gaseous hydrocarbons is removed from low pressure flash zone 13 via line 14 and recovered. A liquid hydrocarbonaceous stream is removed from low pressure flash zone 13 via line 15 and introduced into stripper 16. A gaseous stream containing normally gaseous hydrocarbon compounds is removed from stripper 16 via line 17 and recovered. A liquid hydrocarbonaceous stream is removed from stripper 16 via line 18 and introduced into divided-wall fractionation zone 19. A naphtha boiling range hydrocarbon stream is removed from divided-wall fractionation zone 19 via line 20 and recovered. A kerosene boiling range hydrocarbonaceous stream is removed from divided-wall fractionation zone 19 via line 21 and recovered. A diesel boiling range hydrocarbonaceous stream is removed from divided-wall fractionation zone 19 via line 22 and recovered. A bottoms stream containing hydrocarbons boiling in the range of the fresh feedstock and containing heavy polynuclear aromatic compounds is removed from zone 37 located in the lower portion of divided-wall fractionation zone 19 via line 23. At least a portion of the hydrocarbonaceous stream carried via line 23 is transported via line 24 and recycled as hereinabove described. Another portion of the hydrocarbonaceous stream carried via line 23 is transported via line 25 and introduced into zone 38 located in the lower portion of divided-wall fractionation zone 19. Zone 38 of divided-wall fractionation zone 19 is stripped with steam which is introduced via line 36. A heavy hydrocarbonaceous stream containing an enhanced level of heavy polynuclear aromatic compounds is removed from zone 38 of divided-wall fractionation zone 19 via line 26 and recovered. With reference now to Fig. 2, a feed stream comprising vacuum gas oil and heavy coker gas oil is introduced into the process via line 51 and admixed with a hereinafter-described recycle stream provided via line 145 and the resulting admixture is transported via line 52 and is admixed with a hereinafter-described effluent from hydrocracking zone 127 transported via line 128. The resulting admixture is transported via line 53 into hydrotreating zone 54. The resulting effluent from hydrotreating zone 54 is transported via line 55 and introduced into stripping zone 56. A vaporous stream containing hydrocarbons and hydrogen passes upward in stripping zone 56 and is removed from stripping zone 56 via line 60 and introduced into aromatic saturation zone 111. A resulting effluent from aromatic saturation zone 111 is transported via line 112, admixed with a water wash stream introduced by line 113 and introduced into heat-exchanger 115 via line 114. A resulting cooled effluent from heat-exchanger 115 is transported via line 116 and introduced into vapor-liquid separator 117. A hydrogen-rich gaseous stream is removed from vapor-liquid separator 117 via line 118 and introduced into acid gas recovery zone 119. A lean solvent is introduced via line 120 into acid gas recovery zone 119 and contacts the hydrogen-rich gaseous stream in order to dissolve an acid gas. A rich solvent containing acid gas is removed from acid gas recovery zone 119 via line 121 and recovered. A hydrogen-rich gaseous stream containing a reduced concentration of acid gas is removed from acid gas recovery zone 119 via line 122, compressed in compresor 123, transported via line 124 and admixed with fresh make-up hydrogen which is introduced via line 149. The resulting admixture is transported via line 150 and at least a portion thereof is subsequently transported via lines 125 and 126 and is introduced into hydrocracking zone 127. Another portion of the hydrogen-rich gas is transported via line 151 and introduced into heat-exchanger 146. A resulting heated hydrogen-rich gaseous stream is removed from heat-exchanger 146 and is transported via line 152 and introduced into stripping zone 56. An aqueous stream containing dissolved salt compounds is removed from vapor-liquid separator 117 via line 131 and introduced into cold flash zone 132. A liquid hydrocarbonaceous stream is removed from vapor-liquid separator 117 via line 147 and is admixed with a gaseous stream provided via line 130 and the resulting admixture is transported via line 148 and introduced into cold flash zone 132. A gaseous stream is removed from cold flash zone 132 via line 133 and recovered. An aqueous stream containing dissolved salt compounds is removed from cold flash zone 132 via line 134 and recovered. A liquid hydrocarbonaceous stream is removed from cold flash zone 132 via line 135 and introduced into stripper 136. Stripping steam is provided via line 153 and introduced into stripper 136 to produce a stream containing normally gaseous hydrocarbons and transported via line 137. A liquid hydrocarbonaceous stream is removed from stripper 136 via line 138 and introduced into divided wall fractionator 139. A naphtha stream, a kerosene stream and a diesel stream are removed from divided wall fractionator 139 via lines 140, 141 and 142, respectively. A liquid hydrocarbonaceous stream containing compounds boiling in the range of the hydrocarbon feedstock is removed from divided wall fractionator 139 via line 145 and is transported and admixed with the fresh feedstock provided by line 51 as hereinabove described. A liquid hydrocarbonaceous stream containing compounds boiling in the range of the hydrocarbon feedstock is removed from stripping zone 56 via line 57 and a portion is transported via line 58 and line 126 and is introduced into hydrocracking zone 127 and another portion is transported via line 59 and introduced into hot flash zone 129. A vapor stream is removed from hot flash zone 129 via line 130 and is introduced into cold flash zone 132 via line 148. A liquid hydrocarbonaceous stream is removed from hot flash zone 129 via line 144 and transported and introduced into an isolated section of divided walled fractionator 139. A stream containing heavy polynuclear aromatic compounds is removed from divided wall fractionator 139 via line 143 and recovered. ILLUSTRATIVE EMBODIMENT The following are illustrations of the hydrocracking process of the present invention while hydrocracking a well-known feedstock whose pertinent characteristics are presented in Table 1. HYDROCRACKER FEEDSTOCK ANALYSIS 80% Vacuum Gas Oil/20% Coker Gas Oil from Arabian Crude Specific Gravity @ 16°C 0.928 Distillation, Volume Percent IBP, °C 351 10 379 50 436 90 518 EP 565 Sulfur, weight percent 3.0 Nitrogen, weight ppm 1250 Conradson Carbon, weight percent 0.36 Bromine Number 7.5 The goal of these examples is to maximize selectivity to middle distillate hydrocarbons boiling in the range of 127°C to 387°C. Diesel fuel, one of the components of middle distillate, also requires a maximum of 50 ppm sulfur, a minimum cetane index of 50 and a 95 volume percent boiling point of 350°C. EXAMPLE 1 Forty thousand volume units of the hereinabove-described feedstock is admixed with a hot hydrocracking catalyst zone effluent in an amount of 80,000 volume units of hydrocarbon and hydrogen is introduced into a hydrotreating catalyst zone operated at hydrotreating conditions including a pressure of 13 MPa, a hydrogen circulation rate of 1348 n m 3 /m 3 and a temperature of 399°C. The resulting effluent from the hydrotreating catalyst zone is passed to a hot, high-pressure stripper maintained at essentially the same temperature and pressure as the hydrotreating catalyst zone utilizing a hot, hydrogen-rich stripping gas to produce a vapor stream containing hydrogen and hydrocarbonaceous compounds boiling below and in the boiling range of the hydrocarbonaceous feedstock, and a liquid hydrocarbonaceous stream comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock in an amount of 72,000 volume units which is introduced into the hydrocracking catalyst zone along with hydrogen in an amount of 2022 n m 3 /m 3 (based on feed to the hydrocracking catalyst zone) and a hereinafter-described liquid hydrocarbonaceous recycle stream in an amount of 8,000 volume units. The overhead vapor stream from the hot, high-pressure stripper is introduced into a post treat hydrogenation reactor at a temperature of 382°C to saturate at least a portion of the aromatic hydrocarbon compounds. The resulting effluent from the post treat hydrogenation reactor is cooled to a temperature of 54°C and introduced into a high pressure separator wherein a hydrogen-rich vapor stream is produced and subsequently, after acid gas scrubbing, is recycled, in part, to the hydrocracking catalyst zone. A liquid hydrocarbonaceous stream is removed from the high-pressure separator and introduced into a cold flash zone. A liquid hydrocarbonaceous stream in an amount of 1200 volume units and comprising hydrocarbonaceous compounds boiling in the range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds in an amount of 50 weight ppm is removed from the hot, high pressure stripper and introduced into a hot flash drum operated at a temperature of 399°C and a pressure of 1.7 MPa. A hot gaseous stream is removed from the hot flash drum, cooled and introduced into the previously described cold flash zone. A liquid hydrocarbonaceous stream is removed from the cold flash zone and introduced into a divided wall fractionation zone to produce products listed in Table 2. PRODUCT YIELDS Volume Units Butane 1,150 Light Naphtha 3,100 Heavy Naphtha 3,000 Turbine Fuel 17,000 Diesel Fuel 20,000 A liquid hydrocarbonaceous stream containing heavy polynuclear aromatic compounds is removed from the hot flash drum and introduced into the divided wall fractionation zone to recover vaporous hydrocarbons and a heavy liquid hydrocarbonaceous stream in an amount of 200 volume units and rich in heavy polynuclear aromatic compounds. Another liquid hydrocarbonaceous stream in an amount of 8,000 volume units and lean in heavy polynuclear aromatic compounds is removed from the divided wall fractionation zone and introduced into the hydrocracking zone as the liquid hydrocarbonaceous recycle stream described hereinabove. EXAMPLE 2 One hundred volume units of the hereinabove-described feedstock is admixed with 200 volume units of a hereinafter-described recycle stream and recycle hydrogen, and is introduced into a hydrotreating catalyst zone operated at hydrotreating conditions including a pressure of 6.8 MPa, a hydrogen circulation rate of 674 n m 3 /m 3 and a temperature of 399°C. The effluent from the hydrotreating catalyst zone is directly introduced into a hydrocracking catalyst zone operated at a temperature of 410°C. The resulting effluent from the hydrocracking catalyst zone is partially condensed and introduced into a high pressure vapor-liquid separator. A hydrogen-rich gaseous stream is removed from the high pressure vapor-liquid separator and at least a portion after acid gas scrubbing is recycled to the hydrotreating catalyst zone. A liquid hydrocarbonaceous stream is removed from the high pressure vapor-liquid separator and introduced into a low pressure vapor-liquid separator to produce a vapor stream containing hydrogen and normally gaseous hydrocarbons, and a liquid hydrocarbonaceous stream which is introduced into a stripper column. A stripped liquid hydrocarbonaceous stream is removed from the stripper column and introduced into a divided-wall fractionation zone to produce the products listed in Table 3. A heavy liquid hydrocarbonaceous stream containing hydrocarbon compounds boiling in the range of the hydrocarbonaceous feedstock and heavy polynuclear aromatic compounds in an amount of 50 weight ppm is removed from a first isolated section in the bottom of the divided-wall fractionation zone and 200 volume units are recycled and admixed with the fresh feedstock and 3 volume units are introduced into a second isolated section in the bottom of the divided-wall fractionation zone and stripped with steam. A heavy liquid hydrocarbonaceous stream in an amount of 0.5 volume units and rich in heavy polynuclear aromatic compounds is removed from the second isolated section in the bottom of the divided-wall fractionation zone and recovered. PRODUCT YIELDS Volume Units Butane 3.2 Light Naphtha 7.8 Heavy Naphtha 9.4 Turbine Fuel 45.3 Diesel Fuel 48.2 The foregoing description, drawing and illustrative embodiments clearly illustrate the advantages encompassed by the process of the present invention and the benefits to be afforded with the use thereof.

Claims: A process for hydrocracking a hydrocarbonaceous feedstock which process comprises: (a) passing a hydrocarbonaceous input stream and hydrogen to a hydrocracking zone containing hydrocracking catalyst to produce a hydrocracking effluent; (b) combining a hydrocarbonaceous feedstock with at least one of the hydrocarboneous input streams or the hydrocracking effluent; (c) separating the effluent from said hydrocracking zone in a first separation zone to produce a first stream containing hydrogen and hydrocarbons boiling at a temperature below the boiling range of said hydrocarbonaceous input stream and a second stream comprising hydrocarbons boiling at a temperature in the boiling range of said hydrocarbonaceous input stream and heavy polynuclear aromatic compounds; (d) introducing at least a portion of the second stream into a second separation zone to produce a third stream comprising hydrocarbons boiling at a temperature in the boiling range of said hydrocarbonaceous input stream and heavy polynuclear aromatic compounds and a fourth stream comprising hydrocarbons boiling at a temperature equal to or below the boiling range of said hydrocarboneous input stream and having a lower concentration of heavy polynuclear aromatic compounds than the third stream; (e) introducing at least a portion of said third stream into a first divided zone located in the bottom end of a divided-wall fractionation zone to produce a fifth stream rich in polynuclear aromatic compounds; (f) recycling at least another portion of said second stream to said hydrocracking zone to provide at least a portion of said hydrocarbonaceous input stream; and (g) recovering a liquid hydrocarbonaceous product stream from at least a portion of at least one of the first stream or the fourth stream. The process of Claim 1 wherein prior to separation in the first separation zone the effluent from said hydrocracking zone and the hydrocarbonaceous feedstock pass to a denitrification and desulfurization reaction zone containing a catalyst and the denitrification and desulfurization reaction zone effluent undergoes separation to produce the first and second stream. The process of Claims 1 or 2 wherein the denitrification and desulfurization reaction zone effluent or the hydrocracking effluent passes directly to the first separation zone which comprises a hot, high pressure stripper utilizing a hot hydrogen-rich stripping gas to produce the first stream as a first vapor stream comprising hydrogen and hydrocarbonaceous compounds boiling at a temperature below the boiling range of said hydrocarbonaceous feedstock, and to produce the second stream comprising hydrocarbonaceous compounds boiling in the range of said hydrocarbonaceous feedstock. The process of Claim 3 wherein the effluent from the hydrocracking zone passes to the denitrification and desulfurization zone and at least a portion of said second stream passes to the hydrocracking zone as the hydrocarbonaceous input stream. The process of Claim 4 wherein the first stream passes to an aromatic saturation zone containing hydrogenation catalyst to produce a sixth stream comprising hydrocarbonaceous compounds boiling at a temperature below the boiling range of said hydrocarbonaceous feedstock and having a reduced concentration of aromatic compounds and at least a portion of the sixth stream and fourth stream pass to a second divided zone of the divided wall fractionation zone to recover at least a portion of said hydrocarbonaceous product stream. The process of Claim 5 wherein a liquid stream comprising hydrocarbonaceous compounds boiling in the range of said hydrocarbonaceous feedstock is recovered from said second divided zone and recycled to said denitrification and desulfurization reaction zone. The process of any of Claims 1-6 wherein said hydrocarbonaceous feedstock boils in the range from 232°C to 565°C. The process of any of Claims 3-7 wherein said hot, high pressure stripper is operated at a temperature no less than 38°C below the outlet temperature of said denitrification and desulfurization reaction zone and at a pressure no less than about 589 kPa below the outlet pressure of said denitrification and desulfurization reaction zone. The process of any of Claims 1-8 wherein said hydrocracking zone is operated at a conversion per pass in the range from 15% to 60%. The process of Claim 1 wherein the second separation zone comprises a second divided zone located in the bottom of the divided wall fractionation zone.

Inventors: BJORKLUND BRADFORD L, HOEHN RICHARD K, BJORKLUND, BRADFORD L., HOEHN, RICHARD K.
Applicants: UOP LLC
Application year: 2001
Main classification: C10G47
IPC codes: C10G49, C10G47