HEAVY OIL CATALYTIC CRACKING

Abstract: A fluid catalytic cracking process and apparatus is described which includes a high temperature stripper (hot stripper) to control the carbon level and sulfur on spent catalyst, followed by catalyst cooling to control the regeneration inlet temperature. The high temperature stripper operates at a temperature between 55 DEG C (100 DEG F) above the temperature of a catalyst-hydrocarbon mixture exiting a riser and 816 DEG C (1500 DEG F). The regenerator inlet temperature is controlled to obtain the desired regeneration temperature, regenerator outlet temperature, and degree of regeneration. The regenerator is maintained at a temperature between 55 DEG C (100 DEG F) above that of the catalyst in the high temperature stripper and 871 DEG C (1600 DEG F). The present invention has the advantage that it separates hydrogen from catalyst to eliminate hydrothermal degradation, and separates sulfur from catalyst as hydrogen sulfide and mercaptants which are easy to scrub. The catalyst cooler enables the regenerator and high temperature stripper to be run independently at respective desired temperatures.

Description: HEAVY OIL CATALYTIC CRACKING This invention is concerned with a fluidized catalytic cracking process wherein coked deactivated catalyst is subject to high temperature stripping to control the carbon level on spent catalyst. More particularly, the concept employs a high temperature stripper to control the carbon level on the spent catalyst, followed by catalyst cooling to control the temperature of the catalyst to regeneration. The field of catalytic cracking has undergone progressive development since 1940. The trend of development of the fluid catalytic cracking process has been to all riser cracking, use of zeolite-containing catalysts and heat balanced operation. Other major trends in fluid catalytic cracking processing have been modifications to the process to permit it to accommodate a wider range of feedstocks, in particular, feedstocks that contain more metals and sulfur than had previously been permitted in the feed to a fluid catalytic cracking unit. Along with the development of process modifications and ' catalysts, which could accommodate these heavier, dirtier feeds, there has been a growing concern about the amount of sulfur contained in the feed that ends up as SO in the regenerator flue gas. Higher sulfur levels in the feed, combined with a more complete regeneration of the catalyst in the fluid catalytic cracking regenerator tends to increase the amount of SO contained in the regenerator flue gas. Some attempts have been made to minimize the amount of S0 χ discharged to the atmosphere through the flue gas by providing agents to react with the S0 χ in the flue gas. These agents pass along with the regenerated catalyst back to the fluid catalytic cracking reactor, and then the reducing atmosphere releases the sulfur compounds as FLS. Suitable agents for this purpose have been described in U. S. Patent K s. 4,071,436 and 3,834,031. Use of a cerium oxide agent for this purpose is shown in U. S. Patent No. 4,001,375. Unfortunately, the conditions in most fluid catalytic cracking regenerators are not the best for SO adsorption. The high temperatures encountered in modern fluid catalytic cracking regenerators (up to 870°C (1600°F)) tend to discourage S0 χ adsorption. Che approach to overcome the problem of SO in flue gas is to pass catalyst from a fluid catalytic cracking reactor to a long residence time steam stripper. After the long residence time steam stripping, the catalyst passes to the regenerator, as disclosed by U. S. Patent No. 4,481,103 to Kra beck et al. However, this process preferably steam strips spent catalyst at 5Q0°-550°C (932° to 1022°F), which is not sufficient to remove some undesirable sulfur- or hydrogen-containing components. Furthermore, catalyst passing from a fluid catalytic cracking stripper to a fluid catalytic cracking regenerator contains hydrogen-containing components, such as coke, adhering thereto. This causes hydrothermal degradation when the hydrogen reacts with oxygen in the regenerator to form water. U. S. Patent No. 4,336,160 to Dean et al attempts to reduce hydrotheππal degradation by staged regeneration. However, the flue pas from both stages of regeneration contains SO which is difficult to clean. Another need of the prior art is to provide improved means for controlling fluid catalytic cracking regeneration temperature. Improved regenerator temperature control is desirable, because regenerator temperatures above 871°C (1600°F) can deactivate fluid cracking catalyst. Typically, the temperature is controlled by adjusting the CO/CO- ratio produced in the regenerator. This control works on the principle that production of CO produces less heat than production of C0-. However, in some cases, this control is insufficient. It would be desirable to separate hydrogen from catalyst to eliminate hydrothermal degradation. It would be further advantageous to remove sulfur-containing compounds prior to regeneration to prevent S0 χ from passing into the regenerator flue gas. Also, it would be advantageous to better control regenerator temperature. U. S. Patent No. 4,353,812 to Lomas et al discloses cooling catalyst from a regenerator by passing it through the shell side of a heat-exchanger with a cooling medium through the tube side. The cooled catalyst is recycled to the regeneration zone. This 'process is disadvantageous, in that it does not control the temperature of catalyst from the reactor to the regenerator. The prior art also includes fluid catalytic cracking processes which utilize dense or dilute phase regenerated fluid catalyst heat removal zones or heat-exchangers that are remote from, and external to, the regenerator vessel to cool hot regenerated catalyst for return to the regenerator. Fxamples of such processes are found in U. S. Patent Nos. 2,970,117 to Harper; 2,873,175 to Owens; 2,862,798 to McKinney; 2,596,748 to Watson et al; 2,515,156 to Jahnig et al; 2,492,948 to Eerger; and 2,506,123 to Watson. The processes disclosed in these patents have the disadvantage that the regenerator operating temperature is affected with the temperature of catalyst from the stripper to the regenerator. Accordingly, the present invention comprises a fluid catalytic cracking process and apparatus which employs a high temperature stripper, followed by cooling of the stripped catalyst to control a regenerator inlet temperature. The present invention provides a process for controlling the fluid catalytic cracking of a feedstock containing hydrocarbons, comprising the steps of: passing a mixture comprising catalyst and the feedstock through a riser conversion zone under fluid catalytic cracking conditions to crack the feedstock; passing the mixture, having a riser exit temperature, from the riser into a fluid catalytic cracking reactor vessel; separating a portion of catalyst from the mixture, with the remainder of the mixture forming a reactor vessel gaseous stream; heating the separated catalyst portion by combining the separated catalyst portion with a portion of regenerated catalyst from a fluid catalytic cracking regenerator vessel to form combined catalyst; stripping the combined catalyst, by contact with a stripping gas stream, at a stripping temperature between 55°C (100°F) above the riser exit temperature and 816°C (1500°F), the regenerated catalyst portion having a temperature between 55°C (100°F) above the stripping temperature and 871°C (1600°F) prior to heating the separated catalyst; cooling the stripped catalyst, prior to passing it into the regenerator vessel, to a temperature sufficient to cause the regenerator vessel to be maintained at a temperature between 55°C (100°F) above the stripping temperature and 871°r (1600°F); and regenerating the cooled catalyst stream in the fluid catalytic cracking regenerator vessel by contact with an oxygen-containing stream at fluid catalytic cracking regeneration conditions. The riser exit temperature is defined as the temperature of the catalyst-hydrocarbon mixture exiting from the riser. The riser exit temperature may be at any suitable temperature. However, a riser exit temperature of 482°-593°C (900° to 1100°F) is preferred, and 538°-566°C (1000° to 1050°F) is most preferred. More particularly the present invention provides a process for controlling the fluid catalytic cracking of a feedstock containing hydrocarbons and sulfur-containing compounds, comprising the steps of: passing a mixture comprising catalyst and the feedstock through a riser conversion zone at fluid catalytic cracking conditions to crack the feedstock; passing the mixture, having a riser exit temperature between 538°-566°C (1000° and 1050°F), from the riser conversion zone to a closed cyclone system located within a fluid catalytic cracking reactor vessel; separating a portion of catalyst from the mixture in the closed cyclone system, with the remainder of the mixture forming a reactor vessel gaseous stream;heating the separated catalyst portion by combining the separated catalyst portion in the reactor vessel, with a portion of regenerated catalyst from a fluid catalytic cracking regenerator vessel to form combined catalyst; stripping the combined catalyst, by contact with a stripping gas stream in the reactor vessel, under stripping conditions comprising a stripping temperature between 83°C (150°F) above the riser exit temperature and 760°C (1400°F) and a residence time of a gaseous stream from 0.5 to 5 seconds, the regenerated catalyst portion having a temperature between 83°C (150°F) above the stripping temperature and 871°C (1600°F) prior to heating the separated catalyst, wherein the separated catalyst portion comprises sulfur-containing compounds and hydrocarbons derived from the feedstock, the stripping conditions are sufficient to separate 45 to 55% of the sulfur-containing compounds and 70 to 0% of hydrogen from the hydrocarbons in the separated catalyst portion of the combined catalyst to produce the gaseous stream, and the gaseous stream comprises stripping gas and molecular hydrogen, hydrocarbons and the sulfur-containing hydrocarbons separated from the separated catalyst; cooling the stripped catalyst stream to between 28°-83°C (50° and 150°F) below the stripping temperature by indirect heat-exchange with a heat-exchange medium in a heat-exchanger located outside the reactor vessel, causing the regenerator vessel to be maintained at a temperature between 83°C (150°F) above the stripping temperature and 871°C (1600°F), thereby maintaining said regenerator vessel temperature independently of the stripping step temperature; and regenerating the cooled catalyst stream in the fluid catalytic cracking regenerator vessel, by contact with an oxygen-containing stream under fluid catalytic cracking regeneration conditions. In its apparatus respects, the present invention provides an apparatus for controlling the fluid catalytic cracking of a feedstock comprising hydrocarbons, comprising: means defining a riser conversion zone through which a mixture comprising catalyst and the feedstock passes at fluid catalytic cracking conditions to crack the feedstock; a fluid catalytic cracking reactor vessel; means for passing the mixture from the riser into the fluid catalytic cracking reactor vessel, the mixture having a riser exit temperature as it passes into the reactor vessel; means for separating a portion of catalyst from the mixture, with the remainder of the mixture forming a reactor vessel gaseous stream; means for heating the separated catalyst portion, comprising means for combining the separated catalyst portion with a portion of regenerated catalyst to form combined catalyst; means for stripping the combined catalyst by contact with a stripping gas stream to form a stripped catalyst stream; a fluid catalytic cracking regenerator vessel for producing the portion of regenerated catalyst; and a heat-exchanger for cooling the stripped catalyst stream, the heat-exchanger being located outside the reactor vessel, the fluid catalytic cracking regenerator vessel thereby regenerating the cooled catalyst stream by contact with an oxygen-containing stream at fluid catalytic cracking regenerator conditions. In its more particular apparatus aspects, the present invention provides an apparatus for controlling the fluid catalytic cracking of a feedstock comprising hydrocarbons and sulfur-containing compounds, comprising: means defining a riser conversion zone through which a mixture comprising catalyst and the feedstock passes at fluid catalytic cracking conditions to crack the feedstock; a fluid catalytic cracking reactor vessel; means for passing the mixture from the riser conversion zone to a closed cyclone system located within the fluid catalytic cracking reactor vessel, the mixture having a riser exit temperature between 538°-566°C (1000° and 1050°F) as it passes from the riser to the closed cyclone system, the closed cyclone system including means for separating a portion of catalyst from the mixture and forming a reactor vessel gaseous stream from the remainder of the mixture; means for heating the separated portion of catalyst, comprising means for combining a portion of regenerated catalyst with the separated catalyst portion to form a combined catalyst in the reactor vessel; means for stripping the combined catalyst by contact with a stripping gas in the reactor vessel, thereby maintaining the combined catalyst in the means for stripping at a stripping temperature between 83°C (150°F) above the temperature of the mixture exiting the riser and 760°C (1400°F) and a residence time of gas in the means for stripping from 0.5 to 5 seconds, the separated catalyst portion comprising hydrocarbons and sulfur-containing compounds derived from the feedstock, the means for stripping thereby separating 45 to 55% of the sulfur-containing compounds and 70 to 80% of hydrogen from the hydrocarbons in the separated catalyst portion; a stripped catalyst effluent conduit, attached to the reactor vessel for passing the stripped catalyst stream therethrough; a fluid catalytic cracking regenerator vessel for producing the portion of regenerated catalyst at a temperature between 83°C (150°F) above the stripping temperature and 871°C (1600°F); and an indirect heat-exchanger attached to the reactor effluent conduit, whereby the indirect heat-exchanger is sufficiently sized for cooling the stripped catalyst stream to a temperature between 28°-83°C (50° and 150°F) below the stripping temperature, thereby causing the catalyst in the regenerator vessel to be maintained at a temperature between 83°C (150°F) above the stripping temperature and 871°C (1600°F), causing the temperature of the catalyst in the regenerator vessel to be maintained independently of the stripping temperature, the regenerator vessel regenerating the cooled catalyst stream by contacting it with an oxygen-containing stream under fluid catalytic cracking regeneration conditions. The present invention strips catalyst at a temperature higher than the riser exit temperature to separate hydrogen, as molecular hydrogen or hydrocarbons from the coke which adheres to catalyst, to eliminate hydrothermal degradation, which typically occurs when hydrogen reacts with oxygen in a fluid catalytic cracking regenerator to form water. The high temperature stripper (hot stripper) also removes sulfur from coked catalyst as hydrogen sulfide and ercaptans, which are easy to scrub. In contrast, removing sulfur from coked catalyst in a regenerator produces S0 χ , which passes into the regenerator flue gas and is more difficult to scrub. Furthermore, the high temperature stripper removes additional valuable hydrocarbon products to prevent burning these hydrocarbons in the regenerator. An additional advantage of the high temperature stripper is that it quickly separates hydrocarbons from catalyst. If catalyst contacts hydrocarbons for too long a time at a temperature greater than or equal to 538°C (1000°F), then diolefins are produced which are undesirable for downstream processing, such as alkylation. However, the present invention allows a precisely controlled, short contact time at 538°C (1000°F) or greater to produce premium, unleaded gasoline with high selectivity. The heat-exchanger (catalyst cooler) controls regenerator temperature. This allows the hot stripper to run at a desired temperature to control sulfur and hydrogen without interfering with a desired regenerator temperature. It is desired to run the regenerator at least 55°C (100°F) hotter than the hot stripper. However, the regenerator temperature should be kept below 871°C (1600°F) to prevent deactivation of the catalyst. The drawing is a schematic representation of a high temperature stripper and catalyst cooler of the present invention. The figure illustrates a fluid catalytic cracking system of the present invention. In the figure, a hydrocarbon feed passes from a hydrocarbon feeder 1 to the lower end ,of a riser conversion zone d . Regenerated catalyst from a standpipe 102, having a control valve 104, is combined with the hydrocarbon feed in the riser 4, such that a hydrocarbon-catalyst mixture rises in an ascending dispersed stream and passes through a riser effluent conduit 6 into a first reactor cyclone 8. The riser exit temperature, defined/ as the temperature at which the mixture passes from the riser 4 to conduit 6, ranges between 482° and 593°C (900° and 1100°F), and preferably between 538° and 566°C (1000° and 1050°F). The riser exit temperature is controlled by monitoring and adjusting the rates and temperatures of hydrocarbons and regenerated catalyst into the riser 4. Riser effluent conduit 6 is attached at one end to the riser 4 and at its other end to the cyclone 8. The first reactor cyclone separates a portion of catalyst from the catalyst-hydrocarbon mixture and passes this catalyst down a first reactor cyclone dipleg 12 to a stripping zone 30 located therebelow. The remaining gas and catalyst pass from the first reactor cyclone 8 through a gas effluent conduit 10. The conduit 10 is provided with a connector 24 to allow for thermal expansion. The catalyst passes through the conduit 10, then through a second reactor cyclone inlet conduit 22, and into a second reactor cyclone 14. The second cyclone 14 separates the stream to form a catalyst stream, which passes through a second reactor cyclone dipleg 18 to the stripping zone 30 located therebelow, and an overhead stream .The second cyclone overhead stream, which contains the remaining gas and catalyst, passes through a second cyclone gaseous effluent conduit 16 to a reactor overhead port 20. Gases from the atmosphere of the reactor vessel 2 may pass through a reactor overhead conduit 22 into the reactor overhead port 20. The gases which exit the reactor 2 through the second cyclone gaseous effluent conduit 16 and the reactor overhead conduit 22 are combined and exit through the reactor overhead port 20. It will be apparent to those skilled in the art that although only one series connection of cyclones 8, 14 is shown in the embodiment, more than one series connection and/or more or less than two consecutive cyclones in series connection could be employed. The mixture of catalyst and hydrocarbons passes through the first reactor cyclone overhead conduit 10 and the second reactor cyclone inlet conduit 22 without entering the reactor vessel 2 atmosphere. However, the connector 24 may provide an annular port to admit stripping gas from the reactor vessel 2 into the conduit 10 to aid in separating catalyst from hydrocarbons adhering thereto. The closed cyclone system and annular port is described more fully in U. S. Patent No. 4,502,947 to Haddad et al. The separated catalyst from cyclones 8, 14 pass through respective diplegs 12, 18 and are discharged therefrom after a suitable pressure is generated within the diplegs by the buildup of the catalyst. The catalyst falls from the diplegs into a bed of catalyst 31 located in the stripping zone 30. The first dipleg 12 is sealed by being extended into the catalyst bed 31. The second dipleg 18 is sealed by a trickle valve 19. The separated catalyst is contacted and combined x-άth hot regenerated catalyst from the regenerator 80 in the stripping zone 30. The regenerated catalyst has a temperature in the range between 55°C (100°F) above that of the stripping zone 30 and 871°C (1600°F) to heat the separated catalyst in bed 31. The regenerated catalyst passes from the regenerator 80 to the reactor vessel 2 through a transfer line 106 attached at one end to the regenerator vessel 80 and at another end to the reactor vessel 2. The transfer line 106 is provided with a slide valve 108. Combining the separated catalyst with the regenerated catalyst promotes the stripping at a temperature in the range between 55°C (100°F) above the riser exit temperature and 816°C (1500°F). Preferably, the catalyst stripping zone operates at a temperature between 83°C (150°F) above the riser exit temperature and 760°C (1400°F). The catalyst 31 in the stripping zone 30 is contacted at high temperature, discussed above, with a stripping gas, such as steam, flowing countercurrently to the direction of flow of the catalyst. The stripping gas is introduced into the lower portion of the stripping zone 30 by one or more conduits 34 attached to a stripping gas header 36. The catalyst residence time in the stripping zone 30 ranges from 2.5 to minutes. The vapor residence time in the catalyst stripping zone 30 ranges from 0.5 to 30 seconds, and preferably 0.5 to 5 seconds. The stripping zone 30 removes coke, sulfur and hydrogen from the separated catalyst which has been combined with the regenerated catalyst. The sulfur is removed as hydrogen sulfide and mercaptans. The hydrogen is removed as molecular hydrogen, hydrocarbons, and hydrogen sulfide. Most preferably, the stripping zone 30 is maintained at temperatures between 83°C (150°F) above the riser exit temperature, which are sufficient to reduce coke load to the regenerator by at least 50%, remove ^0-80 °f the hydrogen as molecular hydrogen, light hydrocarbons and other hydrogen-containing compounds, and remove 45 to 55% of the sulfur as hydrogen sulfide and mercaptans, as well as a portion of nitrogen as ammonia and cyanides. The catalyst stripping zone 30 may also be provided with trays (baffles) 32. The trays may be disc- and doughnut-shaped and may be perforated or unperforated. Stripped catalyst passes through a stripped catalyst effluent conduit 38 to a catalyst cooler 40. The catalyst cooler 40 is a heat-exchanger which cools the stripped catalyst from the reactor vessel 2 to a temperature sufficient to maintain the regenerator vessel 80 at a temperature between 55°C (100°F) above the temperature of the stripping zone 30 and 871°C (1600°F). Preferably, the catalyst cooler 40 cools the stripped catalyst stream to a temperature sufficient to control the regenerator vessel 80 at a temperature to between 83°C (150°F) above the temperature of the stripping zone 30 and 871°C (1600°F). Most preferably, the stripped catalyst stream, is cooled between 28° and 83°C (50° and 150°F) below the stripping zone temperature, so long as the cooled catalyst temperature is at least 593°C (1100°F). The catalyst cooler 40 is preferably an indirect heat-exchanger located outside the reactor vessel 2. A heat-exchange medium, such as liquid water (boiler feed water), passes through a conduit 50, provided with a valve 54, into a set of tubes 48 within the catalyst cooler 40. The catalyst passes through the shell side 46 of the catalyst cooler 40. The catalyst cooler 40 is attached to an effluent conduit 42 provided with a slide valve 44. The cooled catalyst passes through the conduit 42 into a regenerator inlet conduit 60. In the regenerator riser 60, air and cooled catalyst combine and pass upwardly through an air catalyst disperser 74 into a fast fluid bed 62. The fast fluid bed 62 is part of the regenerator vessel 80. In the fast fluid bed 62, combustible materials, such as coke which adheres to the cooled catalyst, are burned off the catalyst by contact with lift air. Air passes through an air supply line 66 through a control valve 68 and an air transfer line 68 to the regenerator inlet conduit 60. Optionally, if the temperature of the cooled catalyst from the conduit 42 is less than 593°C (1100°F), a portion of hot regenerated catalyst from the standpipe 102 passes through a conduit 101, provided with a control valve 103, into the fast fluid bed 62. The fast fluid bed 62 contains a relatively dense catalyst bed 76. The air fluidizes the catalyst in bed 76, and subsequently transports the catalyst continuously as a dilute phase through the regenerator riser 83. The dilute phase passes upwardly through the riser 83, through a radial arm 84 attached to the riser 83, and then passes downwardly to a second relatively dense bed of catalyst 82 located within the regenerator vessel 80. The major portion of catalyst passes downwardly through the radial arms 84, while the gases and remaining catalyst pass into the atmosphere of the regenerator vessel 80. The gases and remaining catalyst then pass through an inlet conduit 89 and into the first regenerator cyclone 86. The first cyclone 86 separates a portion of catalyst and passes it through a first dipleg 90, while remaining catalyst and gases pass through an overhead conduit 88 into a second regenerator cyclone 92. The second cyclone 92 separates a portion of catalyst and passes the separated portion through a second dipleg 96 having a trickle valve 97, with the remaining gas and catalyst passing through a second overhead conduit 94 into a regenerator vessel plenum chamber 98. A flue gas stream 110 exits from the regenerator plenum chamber 98 through a regenerator flue gas conduit 100. The regenerated catalyst settles to form the bed 82, which is dense compared to the dilute catalyst passing through the riser 83. The regenerated catalyst bed 82 is at a substantially higher temperature than the stripped catalyst from the stripping zone 30, due to the coke burning which occurs in the regenerator 80. The catalyst in bed 82 is at least 55°C (100°F) hotter than the temperature of the stripping zone 30, preferably at least 83°C (150°F) hotter than the temperature of the stripping zone 30. The regenerator temperature is, at most, 871°C (1600°F) to prevent deactivating the catalyst. Coke burning occurs in the regenerator inlet conduit 60, as well as the fast fluid bed 62 and riser 83. Optionally, air may also be passed from the air supply line 64 to an air transfer line 70, provided with a control valve 72, to an air header 78 located in the regenerator 80. The regenerated catalyst then passes from the relatively dense bed 82 through the conduit 106 to the stripping zone 30 to provide heated catalyst for the stripping zone 30. Any conventional fluid catalytic cracking catalyst can be used in the present invention. Use of zeolite catalysts in an amorphous base is preferred. Many suitable catalysts are discussed in U. S. Patent No. 3,926,778 to Owen et al. Che example of a process which can be conducted in accordance with the present invention begins with a 343° to 593°C (650° to 1100°F) boiling point hydrocarbon feedstock which passes into a riser conversion zone 4, where it combines with hot regenerated catalyst at a temperature of about 815°C (1500°F) from a catalyst standpipe 102 to form a catalyst-hydrocarbon mixture. The catalyst-hydrocarbon mixture passes upwardly through the riser conversion zone 4 and into a riser effluent conduit 6 at a riser exit temperature of about 538°C (100 °F). The catalyst passes from the conduit 6 into the first reactor cyclone 8, where a portion of catalyst is separated from the mixture and drops through a dipleg 12 to a bed of catalyst 31 contained within a stripping zone 30 therebelow. The stripping zone 30 operates at about 704°C (1300°F). The remainder of the mixture passes upwardly through the first overhead conduit 10 into a second reactor cyclone 14. The second cyclone 14 separates a portion of catalyst from the first cyclone overhead stream and passes the separated catalyst down the second dipleg 18. The remaining solids and gases pass upwardly as a second cyclone overhead stream through conduit 16 into the reactor vessel overhead port 20. In the stripping zone 30, the catalyst from diplegs 12, 18 combines with catalyst from regenerator 80, which passes through a conduit 106 and is stripped by contact with steam from a steam header 36. The regenerated catalyst from the conduit 106 is at a temperature of about 815°C (1500°F) and provides heat to maintain the stripping zone 30 at about 704°C (1300°F). The stripped catalyst passes through a conduit 38 into a catalyst cooler 40 at a temperature of about 704°C (130 °F). The catalyst cooler 40 cools the 704°C (1300°F) catalyst to about 621°C (1150°F). The cooling occurs by indirect heat-exchange of the hot stripped catalyst with boiler feed water, which passes through a conduit 50 to form steam which exits through a conduit 52. The cooled catalyst, at a temperature of about 621°C (1150°F), combines with lift air from a conduit 66 in a regenerator inlet conduit 60 to form an air-catalyst mixture. The mixture passes upwardly through the conduit 60 into fast fluid bed 76. The catalyst continues upwardly from fast fluid bed 76 through the regenerator riser 83 and into a regenerator vessel 80. The catalyst is then separated from gases by the radial arm 84, as well as cyclones 86 and 92, and passes downwardly through the regenerator to form a relatively dense bed 82. The coke adhering to the stripped catalyst burns in the conduit 60, the fast fluid bed 62, the riser 83, and the regenerator vessel 80. Pue to the coke burning, the catalyst in bed 82 is heated to a temperature of about 815°C (1500°F). Catalyst bed 82 then supplies catalyst for the standpipe 102, which combines with the hydrocarbon feedstock. Bed 82 also provides catalyst for conduit 106 which passes to the stripping zone 30. Gaseous effluents pass through a first cyclone 86 and second cyclone 92 and leave the regenerator 80 as a flue gas stream 110 through a flue gas conduit 100. Operating the stripping zone as a high temperature (hot) stripper, at a temperature between 55°C (100°F) above a riser exit temperature and 816°C (1500°F), has the advantage that it separates hydrogen, as molecular hydrogen as well as hydrocarbons, from catalyst. Hydrogen removal eliminates hydrothermal degradation, which typically occurs when hydrogen reacts with oxygen in a fluid catalytic cracking regenerator to form water. The hot stripper also removes sulfur from coked catalyst as hydrogen sulfide and mercaptans , which are easy to scrub . By removing sulfur from coked catalyst in the hot stripper, the hot stripper prevents formation of S0 χ in the regenerator. It is more difficult to remove SO from regenerator flue gas than to remove hydrogen sulfide and mercaptans from a hot stripper effluent. The hot stripper enhances removal of hydrocarbons from spent catalyst, and thus prevents burning of valuable hydrocarbons in the regenerator. Furthermore, the hot stripper quickly separates hydrocarbons from catalyst to avoid overcracking. Preferably the hot stripper is maintained at a temperature between 83°C (150°F) above a riser exit temperature and 760°C (1400°F) to reduce coke load to the regenerator by at least 50%, and strip away 70 to 80% of the hydrogen as molecular hydrogen, light hydrocarbons and other hydrogen-containing compounds. The hot stripper is also maintained within the desired temperature conditions to remove 45 to 55% of the sulfur as hydrogen sulfide and mercaptans, as well as a portion of nitrogen as ammonia and cyanides. This concept advances the development of a heavy oil (residual oil) catalytic cracker and high temperature cracking unit for conventional gas oils. The process combines the control of catalyst deactivation with controlled catalyst carbon-contamination level and control of temperature levels in the stripper and regenerator. The hot stripper temperature controls the amount of carbon removed from the catalyst in the hot stripper. Accordingly, the hot stripper controls the amount of carbon (and hydrogen, sulfur) remaining on the catalyst to the regenerator. This residual carbon level controls the temperature rise between the reactor stripper and the regenerator. The hot stripper also controls the hydrogen content of the spent catalyst sent to the regenerator as a function of residual carbon. Thus, the hot stripper controls the temperature and amount of hydrothermal deactivation of catalyst in the regenerator. This concept may be practiced in a multi-stage, multi-temperature stripper or a single stage stripper. Bnploying a hot stripper, to remove carbon on the catalyst, rather than a regeneration stage reduces air pollution, and allows all of the carbon made in the reaction to be burned to CO-, if desired. The stripped catalyst is cooled (as a function of its carbon level) to a desired regenerator inlet temperature to control the degree of regeneration desired, in combination with the other variables of CO/CO2 ratio desired, the amount of carbon burn-off desired, the catalyst recirculation rate from the regenerator to the hot stripper, and the degree of desulfurization/ denitrification/decarbonization desired in the hot stripper. Increasing CO/CC2 ratio decreases the heat generated in the regenerator, and accordingly decreases the regenerator temperature. Burning the coke, adhering to the catalyst in the regenerator, to CO removes the coke, as would burning coke to CO2, but burning to CO produces less heat than burning to CO-, . The amount of carbon (coke) burn-off affects regenerator temperature, because greater carbon burn-off generates greater heat. The catalyst recirculation rate from the regenerator to the hot stripper affects regenerator temperature, because increasing the amount of hot catalyst from the regenerator to the hot stripper increases hot stripper temperature. Accordingly, the increased hot stripper temperature removes increased amounts of coke so less coke need burn in the regenerator; thus, regenerator temperature can decrease. The catalyst cooler controls regenerator temperature , thereby allowing the hot stripper to be run at temperatures between 55°C (100°F) above a riser exit temperature to 816°C (1500°F), which facilitate controlling sulfur and hydrogen, while allowing the regenerator to be run independently at temperatures at least 100°F hotter than the stripper, while preventing regenerator temperatures greater than 871°C (1600°F) which deactivate catalyst. Use of the catalyst cooler on catalyst exiting the stripper also allows circulation of catalyst to the regenerator riser to increase catalyst density in the regenerator riser, while controlling the regenerator temperature. This reduces catalyst deactivation and provides additional control. While specific embodiments of the method and apparatus aspects of the invention have been shown and described, it should be apparent that the many modifications can be made thereto without departing from the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description, but is only limited by the scope of the claims appended thereto.

Claims: CLAIMS 1. A process for controlling the fluid catalytic cracking of a feedstock containing hydrocarbons, comprising the steps of: passing a mixture comprising catalyst and the feedstock through a riser conversion zone under fluid catalytic cracking conditions to crack the feedstock; passing the mixture, having a riser exit temperature, from the riser into a fluid catalytic cracking reactor vessel; separating a portion of catalyst from the mixture, with the remainder of the mixture forming a reactor vessel gaseous stream; heating the separated catalyst portion by a heat step consisting essentially of combining the separated catalyst portion with a portion of regenerated catalyst from a fluid catalytic cracking regenerator vessel to form combined catalyst; stripping the combined catalyst, by contact with a stripping gas stream, consisting essentially of stream at a stripping temperature between 55°C above the riser exit temperature and 815°C, the regenerated catalyst portion having a temperature between 55°C above the stripping temperature and 871°C prior to heating the separated catalyst to produce a stripped catalyst; cooling the stripped catalyst, prior to passing it into the regenerator vessel, to a temperature sufficient to cause the regenerator vessel to be maintained at a temperature between 55°C above the stripping temperature and 871°C wherein the cooling step comprises passing the stripped catalyst stream to a heat exchanger located outside the reactor vessel; and regenerating the cooled catalyst stream in the fluid catalytic cracking regenerator vessel by contact with an oxygen-containing stream at fluid catalytic cracking regeneration conditions. 2. The process of claim 1, wherein the stripped catalyst stream is indirectly heat-exchanged with a heat-exchange medium in the heat exchanger. 3. The process of claim 1 or 2, wherein the riser exit temperature ranges between 482° and 593 °C, and the he t -exch nger cools the stripped catalyst stream to cause the catalyst in the regenerator vessel to be maintained at a temperature between 83 °C above the stripping step temperature and 871°C. 4. The process of claim 1 , 2 and 3 wherein the heating step and the stripping step occur within the reactor vessel and the stripping step occurs at a stripping temperature between 83°C above the riser exit temperature and 760 °C and a residence time for the gaseous stream from 0.5 to 5 seconds . 5. The process of any one of the preceding claims , wherein the separating step comprises separating the mixture from the riser conversion zone in a closed cyclone system in communication with the riser conversion zone. 6 . The process of any one of the preceding claims , wherein the riser exit temperature ranges from 538° to 565 °C and the stripped catalyst stream is cooled in the heat-exchanger to between 28° and 83 °C below the stripping temperature , the heat -exchanger thereby causing the regenerator vessel temperature to be maintained independently of the stripping temperature. 7 . The process of any one of the preceding claims , wherein the separated catalyst portion of the combined catalyst contains sulfur-containing compounds and hydrogen-containing compounds derived from the feedstock , and the stripping step removes 45 to 55% of the sul ur -containing compounds and 70 to 80% of the hydrogen-containing compounds in the separated catalyst portion. 8 . The process of any one of the preceding claims , wherein the combined catalyst passes countercurrently to the stripping gas during the stripping step. 9. An apparatus for controlling the fluid catalytic cracking of a feedstock comprising hydrocarbons , compris ing: means defining a riser conversion zone through which a mixture comprising catalyst and the feedstock passes at fluid catalytic cracking conditions to crack the feedstock; a fluid catalytic cracking reactor vessel; means for passing the mixture from the riser into the fluid catalytic cracking reactor vessel, the mixture having a riser exit temperature as it passes into the reactor vessel; means for separating a portion of catalyst from the mixture, with the remainder of the mixture forming a reactor vessel gaseous stream; means for heating the separated catalyst portion, by a heating step consisting essentially of combining the separated catalyst portion with a portion of regenerated -catalyst to form combined catalyst; means for stripping the combined catalyst by contact with a stripping gas stream to form a stripped catalyst stream; a fluid catalytic cracking regenerator vessel for producing the portion of regenerated catalyst; and a heat-exchanger for cooling the stripped catalyst stream, the catalyst cooler being located outside the reactor vessel, the fluid catalytic cracking regenerator vessel thereby regenerating the cooled catalyst stream by contact with an oxygen-containing stream at fluid catalytic cracking regenerator conditions. a stripped catalyst effluent conduit, attached to the means for stripping catalyst stream from the means for stripping to the heat-exchanger. 10. The apparatus of claim 9, wherein the heat exchanger is upstream of the regenerator vessel. 11. The apparatus of claim 9 or 10, wherein the riser conversion zone accommodates the feedstock which further comprises sulfur-containing compounds , and the means for stripping accomodates a residence time of gas in the means for stripping from 0.5 to 30 seconds, the means for stripping maintaining the combined catalyst therein at a temperature between 55°C above the riser exit temperature and 815°C, thereby removing molecular hydrogen, hydrocarbons and sulfur-containing compounds derived from components of the feedstock in the separated catalyst portion of the combined catalyst, wherein said removed sulfur-containing compounds consist essentially of hydrogen sulfide and mercaptans.. 12. The apparatus of Claim 11, wherein the catalyst cooler is an indirect heat-exchanger for cooling the stripped catalyst stream to a temperature sufficient to cause the regenerator vessel to be maintained at a temperature between 55°C above the stripping temperature and 871°C, thereby producing the regenerated catalyst portion having a temperature between 55°C above the stripping temperature and 871°C. 13. The apparatus of claim 12, whereby the riser conversion zone maintains a temperature of mixture exiting the riser between 538° and 565°C, and the heat-exchanger is sized to cool the stripped catalyst stream sufficiently to thereby cause the catalyst in the regenerator vessel to be maintained at a temperature between 83°C above that of the means for stripping and 871°C. 14. The apparatus of claim 13, wherein the stripping gas consists essentially of stream, wherein the means for heating and the means for stripping are located in the reactor vessel, and the means for stripping allows a residence time for the gas in the means for stripping from 0.5 to 5 seconds, thereby causing the stripping temperature to be maintained at a temperature between 83°C above that of the riser exit temperature and 760°C. 15. The apparatus of claim 14, wherein the means for separating the mixture from the riser conversion zone comprises a closed cyclone system in communication with the riser conversion zone. 16. The apparatus of claim 15, whereby the riser conversion zone maintains the temperature of mixture exiting the riser between 538° and 565°C, wherein the catalyst cooler is sized to cool the reactor vessel catalyst stream to a temperature between 28° and 83°C below the stripping temperature, the catalyst cooler thereby maintaining the regenerator vessel temperature independently of stripping temperature. 17. The apparatus of claim 16 , wherein the separated catalyst portion contains the sulfur-containing compounds and hydrogen-containing compounds derived from the feedstock , and whereby the means for stripping removes 45 to 55% of the sulfur-containing compounds and 70 to 80% of the hydrogen-containing compounds in the separated catalyst portion of the combined catalyst. 18. The apparatus of claim 17 , wherein the means for stripping comprises means for passing the combined catalyst countercurrently to the stripping gas .

Inventors: OWEN HARTLEY, OWEN, HARTLEY
Applicants: MOBIL OIL CORP, MOBIL OIL CORPORATION
Application year: 1989
Main classification: B01J29
IPC codes: C10G11