Catalyst regeneration process

Abstract: A hydrocarbon cracking catalyst having coke deposits thereon is regenerated by the combustion of the deposits in a regenerator 2 having a lower combustion zone 12 and an upper reducing zone 14. A mixture of oxygen and an inert gas (e.g. nitrogen or carbon dioxide) is introduced into the combustion zone 12 through a distributor 18. The combustion zone 12 is at least 24% by volume of the total flow rate of all gases into the zone 12. At least part of the coke is combusted. In addition nitrogen in the coke is converted to nitrogen oxides (NO x ). A substantially oxygen-free combustion gas comprising carbon dioxide, nitrogen oxides and at least 1% by volume of carbon monoxide flows into the reducing zone 14 with the result that at least part of the nitrogen oxides are reduced to nitrogen.

Description: This invention relates to a process for regenerating a hydrocarbon cracking catalyst having coke deposits thereon. In catalytic cracking processes hydrocarbon feedstock is injected into the riser section of a hydrocarbon cracking reactor, where it cracks into lighter, valuable products on contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator vessel. As the endothermic cracking reactions take place, the catalyst gets covered with coke deposits. The catalyst and hydrocarbon vapours are carried up the riser to the disengagement section of the reactor, where they are separated. Subsequently, the catalyst flows into the stripping section, where the hydrocarbon vapours entrained with the catalyst are stripped by steam injection, and the stripped catalyst flows through a spent catalyst standpipe and into the catalyst regenerator vessel. Typically, the catalyst is regenerated by introducing air into the regenerator vessel to burn coke off the catalyst, thereby rejuvenating it. The coke combustion reactions are highly exothermic and heat the catalyst. The hot, reactivated catalyst flows through the regenerated catalyst standpipe back to the riser to complete the catalyst cycle. The coke combustion exhaust gas stream rises to the top of the regenerator and leaves the regenerator through the regenerator flue. The exhaust gas contains nitrogen and carbon dioxide (CO 2 ), and generally also contains carbon monoxide (CO), oxygen, sulphur oxides (SO x ), nitrogen oxides (NO x ) and reduced nitrogen species, such as ammonia. The catalyst regenerator may be operated in complete combustion mode, which has now become the standard combustion mode, or in partial CO combustion mode. In complete combustion operation, the coke on the catalyst is completely burned to CO 2 . This is typically accomplished by conducting the regeneration in the presence of excess oxygen, provided in the form of excess air. The exhaust gas from complete combustion operations comprises primarily CO 2 , nitrogen and excess oxygen, but also contains NO x and SO x . In partial carbon monoxide combustion mode operation, the catalyst regenerator is operated with insufficient air to burn all of the coke in the catalyst to CO 2 , consequently the coke is combusted to a mixture of CO and CO 2 . The CO is oxidised to CO 2 in a downstream CO boiler. The effluent from the CO boiler comprises primarily CO 2 and nitrogen, but also contains NO x and SO x . Recently, there has been considerable concern about the amount of NO x and SO x being released to the environment in refinery flue gases. It is now the accepted view that most of the NO x present in catalyst regenerator exhaust comes from coke nitrogen, i.e. nitrogen contained in the coke in the form of heterocompounds such as condensed cyclic compounds, and that little or none of the, NO x contained in the exhaust gas is derived from the nitrogen contained in the air feed to the regenerator. The mechanism by which the coke nitrogen ends up as NO x differs, depending on whether the regenerator is operated in complete combustion mode or in partial combustion mode. In complete combustion mode regenerator operation, coke nitrogen is converted to a mixture of NO x and elemental nitrogen. In this operational mode, the amount of NO x in catalyst regenerator flue gas tends to increase as the excess oxygen concentration from the regenerator increases. When the regenerator is operated in partial CO combustion mode, very little NO x is produced in the regenerator, and coke nitrogen leaves the regenerator as reduced nitrogen species, such as ammonia. The reduced nitrogen species are unstable in the CO boiler, and they are easily converted to NO x and elemental nitrogen. Several approaches have been used in industry to reduce NO x in cracking catalyst regenerator exhaust gases. These include capital-intensive and expensive options, such as pre-treatment of reactor feed with hydrogen and flue gas post-treatment options; intermediate cost options, such as split-feed injection to the hydrocarbon reactor, and less expensive options, such as the use of catalysts and catalyst additives. Efforts to reduce NO x from boiler stacks downstream of FCC units operated in partial combustion mode are centred on the reduction of ammonia and other NO x precursors in regenerator flue gas. US Patent No. 4,755,282 discloses the use of a noble metal on an inorganic support to reduce the ammonia content of flue gas from regenerators. US Patent No. 4,744,962 teaches the addition of NO x either to the regenerator or to the downstream flue gas line. US Patent No. 5,021,144 teaches the reduction of ammonia from a regenerator operated in partial CO combustion, by the addition of excess amounts of CO promoter. US Pat. No 5,268,089 teaches that NO x can be reduced by regenerator operation "on the brink", i.e. in a region between conventional partial CO combustion operation and complete combustion operation with less than 0.05 mol% excess CO. The patent indicates that by operating in this mode, the reduced nitrogen species, such as ammonia, which are formed in partial CO combustion operation, are oxidised to nitrogen oxides and elemental nitrogen, and because of the prevailing reducing environment in the regenerator, the nitrogen oxides are reduced to elemental nitrogen prior to leaving the regenerator. Drawbacks associated with operating in the above-described mode are the existence of very high regenerator temperatures and afterburn, in addition to the difficulties associated with regenerator controllability. Several patents disclose the reduction of NO x in FCC regenerators by such means as promoters, segregated feed cracking and post treatment of exhaust gas. These process are discussed in detail in US 5,268,089. Because of a need to avoid polluting the atmosphere with noxious gases, efforts are continuously underway to find new and improved methods of reducing the concentration of NO x and SO x in industrial flue gases, such as FCC regenerator exhaust gases. This invention provides a method of taking advantage of the peculiar nitrogen chemistry in partial CO combustion to reduce the amount of effluent NO x by enriching selected zones in the regenerator with oxygen. The invention is carried out by regenerating a coked hydrocarbon processing catalyst by burning most of the coke off the catalyst under partial combustion conditions in a highly oxidising zone, thereby converting at least part of the coke nitrogen to nitrogen oxides, and then converting the nitrogen oxides to elemental nitrogen by passing them through a highly reducing zone. According to the present invention there is provided a process for regenerating a hydrocarbon cracking catalyst having coke deposits thereon by combusting said coke deposits, comprising the steps of: (a) contacting said catalyst with an oxygen-inert gas mixture in a combustion zone at an average temperature in the range of about 650 to about 815°C, the total rate of oxygen introduction into the combustion zone being at least 24 per cent by volume of the total flow rate of all gases into the combustion zone, thereby combusting at least part of said coke and converting nitrogen contained in said coke to nitrogen oxides to produce a substantially oxygen-free combustion gas comprising carbon dioxide, nitrogen oxides and at least 3% by volume of carbon monoxide; and b) converting most or all of nitrogen oxides to elemental nitrogen, by passing said combustion gas through a reducing zone above the combustion zone, wherein said combustion zone and said reducing zone are located in a catalyst regeneration vessel. The invention uses an apparatus for regenerating coked hydrocarbon cracking catalyst comprising: a) a catalyst regeneration vessel; (b) conduit means for introducing coked catalyst into said regenerator and for maintaining said coked catalyst in the Fluidised state in said regenerator; (c) first means for introducing an oxygen-containing gas into the lower half of said regeneration vessel; (d) second means for introducing an oxygen-containing gas into said lower half of said regeneration vessel but above the location of said first means; (e) conduit means located below said first and said means, for removing regenerated catalyst from said regeneration vessel; and (f) means located above said second means, for removing combustion gas from said regeneration vessel. The gases entering the combustion zone include oxygen and one or more inert gases, such as nitrogen, argon, carbon dioxide or mixtures of these. Typical feed gas mixtures entering the combustion zone include air, oxygen-enriched air, oxygen-carbon dioxide mixture, oxygen-argon mixture and oxygen-enriched air-carbon monoxide mixture. Preferred gas mixtures include oxygen-enriched air and oxygen-carbon dioxide gas mixture in which the oxygen content is at least 25.5% by volume. The maximum concentration of oxygen entering the combustion zone is generally such that the average temperature prevailing in the combustion zone is in the range of 650 to 815°C; and is preferably such that the average temperature in the combustion zone is in the range of 670 to 790°C. The process is particularly useful for regenerating a hydrocarbon cracking catalyst, especially a hydrocarbon cracking catalyst which is in the form of a Fluidised bed. In the invention the combustion zone and the reducing zone are each situated within a catalyst regeneration vessel. The combustion zone is located in the lower part of the regeneration vessel and the reducing zone is located in the upper part of the regeneration vessel, i.e. above the combustion zone. In one embodiment of the invention the gases entering the combustion zone are combined before they enter the combustion zone. In another and more preferred embodiment, the gases are introduced into the combustion zone in two or more streams. In the latter embodiment, the first gas, comprising oxygen and an inert gas is introduced into the lower part of the combustion zone, and a second gas, comprising oxygen or an oxygen-inert gas mixture is introduced into the portion of the combustion zone downstream of the point of entry of the first gas. In a most preferred aspect of the second embodiment, the first gas is air, oxygen-enriched air or oxygen-carbon dioxide gas mixture, and the second gas is oxygen-enriched air or high purity air. The amount of oxygen introduced into the combustion zone is such that the combustion gas contains at least 3% by volume carbon monoxide, and most preferably limited to the extent the combustion gas contains at least 3.5% by volume carbon monoxide. In a preferred embodiment of the invention, the regeneration process comprises a part of an FCC operation in which hot regenerated catalyst is transferred from the catalyst regeneration vessel to a hydrocarbon cracking reactor, where it contacts fresh hydrocarbon and cracks the hydrocarbon into lower molecular weight hydrocarbons, and in the process, becomes coated with coke. The coked catalyst is then separated from the cracked hydrocarbons and returned to the regenerator for rejuvenation. Preferably, an apparatus according to the invention includes a means for combusting carbon monoxide contained in the combustion gas to carbon dioxide. The invention provides a method of considerably reducing nitrogen oxide emissions from FCC catalyst regeneration units. The coke present on the catalyst is burned in a combustion zone of the regenerator with an oxygen-inert gas mixture in which the oxygen is at a higher concentration than its normal concentration in air, i.e. 20.9% by volume. This environment reduces the amount of coke nitrogen that is converted to reduced nitrogen compounds, such as ammonia, and increases the amount of nitrogen oxides that are produced in the combustion zone from the coke nitrogen. The total amount of oxygen in the combustion zone is limited so that the regenerator is operated in partial combustion mode; consequently the exhaust gases leaving the combustion zone contain enough carbon monoxide to create a reducing zone downstream of the combustion zone. The result is that the nitrogen oxides are reduced to harmless elemental nitrogen. The presence of the reducing environment downstream from the combustion zone is important for successful operation of the invention. Thus, the invention will only result in a reduction of nitrogen oxides in the combustion gas when the regenerator is operated in partial combustion mode with at least 1% carbon monoxide present in the exhaust gas leaving the combustion zone of the regenerator. Furthermore, to effect substantial conversion of reduced nitrogen species in the coke to nitrogen oxides in the local "enriched" oxygen zone, i.e. the combustion zone, it is necessary that the oxygen level in the combustion zone be at least 24 mol% (24% by volume), and preferably 25.5 mol% (25% by volume) of the total gas entering this zone. To enhance the sequential conversion of a portion of the reduced nitrogen species to nitrogen oxides and nitrogen, and the simultaneous reduction of the formed nitrogen oxides to elemental nitrogen, the regenerator should be operated in the temperature range of 650 to 815°C, and preferably in the range of 670 to 790°C. In a preferred embodiment of the invention the supplemental oxygen will be introduced into the bed at an elevation slightly above the main combustion gas distribution system. This ensures maximum selectivity for the conversion of reduced nitrogen species to nitrogen oxides, which will be ultimately reduced before the gas leaves a regenerator vessel operated in the mode specified above. Operating in partial CO combustion mode, with large amounts of enriched oxygen runs would be expected to raise the temperature in the regenerator. It is well established that nitrogen oxides levels tend to increase with increasing excess oxygen level for "standard" (complete combustion) operation. Enriching a regenerator with oxygen, to reduce nitrogen oxides might thus appear counter-intuitive. However, this logic is not directly transferable to partial CO combustion operation, where the downstream boiler is the principal source of nitrogen oxides. It is not believed that using enriched air for a single-stage regenerator operated in complete combustion mode will lower regenerator nitrogen oxides emissions. The presence of higher levels of oxygen in the regenerator is also expected to enhance the effectiveness of de-SO x additives, which pick up SO x as SO 3 and not SO 2 in the regenerator. The increasing level of SO x removal in the regenerator will result in a higher amount of hydrogen sulphide discharged by the catalyst along with reactor products, which can be recovered by several well-known processes. The process and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic side view of a catalyst regeneration vessel of an FCC plant illustrating one embodiment of the invention. Figure 2 is a cross-sectional view of the catalyst regeneration vessel of Figure 1 taken through the line II-II of Figure 1. Figure 3 is a schematic side view of an alternative catalyst regeneration vessel to that shown in Figure 1. Figure 4 is an enlarged view of the combustion zone illustrated in Figure 3. Except as otherwise stated, in the ensuing description all percentages are by volume. The hydrocarbon cracking reactor and associated processing equipment such as, valves and gauges, that are not directly related to the invention and which are not necessary for an understanding of the invention have been omitted from the Figure for the sake of simplicity. The same reference numerals have been used to represent the same or similar parts in the various drawings. Turning now to the embodiment illustrated in Figures 1 and 2, hydrocarbon cracking catalyst regenerator 2 is provided externally with spent catalyst transport line 4, regenerated catalyst transport line 6, oxygen-containing gas supply line 8, exhaust gas line 10. A combustion zone, designated generally as 12, and a reducing zone, designated generally as 14, are located in the lower and upper regions, respectively, of the interior of regenerator 2. Feed line 16 connects supply line 8 to the centre of gas distributor 18. Distributor 18 is provided with nozzles 20. In the embodiment illustrated in the drawings, distributor 18 comprises of two arms disposed horizontally in the lower part of regenerator 2 at right angles to the line 16; however, distributor 18 can have any desired configuration, such as a circular ring extending around the periphery of vessel 2. Combustion zone 12 is the area within regenerator 2 where combustion of the coke takes place. Since combustion is effected by contact of the oxygen with the hot coke, combustion zone 12 is defined by the oxygen entering regenerator 2. In the design depicted in Figure 1, nozzles 20 are disposed downwardly to direct the flow of oxygen-containing gas below distributor 18, and thus enlarge the combustion zone 12. Exhaust gas line 10 connects the upper region of regenerator 2 with an optional carbon monoxide boiler 22. Boiler 22 is provided on its outlet end with vent line 24. In cases where the carbon monoxide boiler 22 is not used, the carbon monoxide-containing exhaust gas is transferred to downstream operations for further treatment, such as a shift reaction. In the process performed in the system illustrated in Figures 1 and 2, spent hydrocarbon cracking catalyst is transferred from a hydrocarbon cracking reactor (not shown) to regenerator 2 via spent catalyst line 4. The spent catalyst swirls around the interior of regenerator 2 in a tangential motion. A dilute Fluidised bed forms in the upper part of regenerator 2 and a dense Fluidised bed forms in the lower region of the regenerator. As the catalyst comes into contact with the oxygen in the incoming feed gas, the coke on the surfaces of the catalyst ignites and burns, thus producing combustion gases containing carbon dioxide, carbon monoxide, water vapour, nitrogen oxides, and probably sulphur oxides. The total oxygen content of the gas entering regenerator 2 is insufficient to convert all of the coke to carbon dioxide, i.e. regenerator 2 is operated in a partial combustion mode. However, the incoming feed gas is rich enough in oxygen to cause substantial quantities of the reduced nitrogen in the coke compounds to convert to nitrogen oxides. If the incoming feed gas were less concentrated in oxygen, some of the coke nitrogen would be converted to reduced nitrogen compounds, such as ammonia. The reduced nitrogen compounds would be converted to nitrogen oxides in the highly oxidising environment of the carbon monoxide boiler located downstream of regenerator 2. The combustion gases rapidly rise to the top of regenerator 2 and enter reducing zone 14, wherein the concentration of carbon monoxide is sufficiently high to create a reducing environment. Most or all of the nitrogen oxides in the exhaust gas are converted to elemental nitrogen as the gas passes through reducing zone 14. The combustion gases next enter carbon monoxide boiler 22, if this equipment is used in the system. The exhaust gas leaving boiler 22, now comprising carbon dioxide and, perhaps nitrogen, leaves the system through vent line 24. As the gases pass through boiler 22, the carbon monoxide in the exhaust gas is converted to carbon dioxide, but little if any further oxidation of the nitrogen in the exhaust gas takes place. Thus, by virtue of the invention, some of the reduced nitrogen compounds that would otherwise be converted to nitrogen oxides in the system are converted to the more stable elemental nitrogen. Figures 3 and 4 illustrate a preferred embodiment of the invention. In this embodiment, auxiliary oxygen distributor 26 is positioned in the upper part of combustion zone 12a, above oxygen distributor 18. Distributor 26 is provided with oxygen supply line 28 and nozzles 30. In the operation of the process of the invention in the embodiment of Figures 3 and 4, spent catalyst enters regenerator 2 in the manner described above. A supply of oxygen-containing gas enters regenerator 2 through line 16. This gas is an oxygen-inert gas mixture. It may be air, oxygen-enriched air, oxygen-carbon dioxide or a mixture of oxygen and other inert gas(es), such as oxygen-argon gas mixture or oxygen-enriched air-carbon dioxide gas mixture. A second supply of oxygen-containing gas is introduced into combustion zone 12a through line 28, distributor 26 and nozzles 30. This gas may be substantially pure oxygen, i.e. oxygen containing no more than about 1% of other gases; or high purity oxygen, i.e. gas containing at least about 80%, and preferably at least about 90% oxygen; or oxygen-enriched air. The purpose of the gas entering combustion zone 12a through distributor 26 is to enhance the oxidising environment in combustion zone 12a. This will ensure a more complete conversion of coke nitrogen to intermediate nitrogen oxides, and thus more ultimate conversion of coke nitrogen to elemental nitrogen. The combustion gases leaving zone 12a pass through reducing zone 14 and optional carbon monoxide boiler 22 in the manner described above. It will be appreciated that it is within the scope of the present invention to utilise conventional equipment to monitor and automatically regulate the flow of gases within the system so that it can be fully automated to run continuously in an efficient manner. Although the invention has been described with particular reference to a specific experiment, this experiment is merely exemplary of the invention and variations are contemplated. For example, the process of the invention may be practised in equipment arrangements other than those illustrated in the drawings, and the invention may be applied to systems other than FCC systems, such as fixed bed systems.

Claims: A process for regenerating a hydrocarbon cracking catalyst having coke deposits thereon by combusting said coke deposits, comprising the steps of: (a) contacting said catalyst with an oxygen-inert gas mixture in a combustion zone at an average temperature in the range of 650 to 815°C, the total volumetric flow rate of oxygen into the combustion zone being at least 24 per cent of the total volumetric flow rate of all gases into the combustion zone, thereby combusting at least part of said coke and converting nitrogen contained in said coke to nitrogen oxides to produce a substantially oxygen-free combustion gas comprising carbon dioxide, nitrogen oxides and at least 3% by volume of carbon monoxide; and (b) converting most or all of nitrogen oxides to elemental nitrogen, by passing said combustion gas through a reducing zone above the combustion zone, wherein said combustion zone and said reducing zone are located in a catalyst regeneration vessel. A process as.claimed in claim 1 in which the oxygen introduced into said combustion zone constitutes at least 24 mole per cent of the mixture. A process as claimed in any one of the preceding claims, wherein said combustion zone is located in the lower part of said regeneration vessel and said reducing zone is located downstream of said combustion zone. A process as claimed in any one of the preceding claims, wherein said oxygen-inert gas mixture comprises oxygen and a gas selected from nitrogen, carbon dioxide and mixtures of these. A process as claimed in any one of the preceding claims, wherein said oxygen-inert gas mixture contains at least 25.5 mole % oxygen. A process as claimed in any one of the preceding claims, wherein oxygen is introduced into said combustion zone in at least two feed streams. A process as claimed in claim 6, in which one of the feed streams, consisting of oxygen enriched air or substantially pure oxygen, is introduced into the regeneration vessel at one or more points in the upper region of the combustion zone. A process as claimed in any one of the preceding claims, wherein downstream of the reduction zone substantially all of the carbon monoxide in said combustion gas is converted by combustion to carbon dioxide.

Inventors: MENON RAGHU K, RAMACHANDRAN RAMAKRISHNAN, MENON, RAGHU K., RAMACHANDRAN, RAMAKRISHNAN
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1995
Main classification: C10G11
IPC codes: B01J38, B01D53, B01J29, C10G11, B01J35