The present invention relates to a method for reducing pollutant gas levels in flue gases generated in catalyst regenerators in hydrocarbon catalytic cracking systems.
Modern hydrocarbon catalytic cracking systems use a moving bed or fluidized bed of a particulate catalyst. It is carried out in the absence of externally supplied molecular hydrogen, and is thereby distinguished from hydrocracking. The cracking catalyst is subjected to a continuous cyclic cracking reaction and catalyst regeneration procedure. In a fluidized catalytic cracking (FCC) system, a stream of hydrocarbon feed is contacted with fluidized catalyst particles in a hydrocarbon cracking zone, or reactor, usually at a temperature of about 425.degree.-600.degree. C. The reactions of hydrocarbons in the hydrocarbon stream at this temperature result in deposition of carbonaceous coke on the catalyst particles. The resulting fluid products are thereafter separated from the coked catalyst and are withdrawn from the cracking zone. The coked catalyst is then stripped of volatiles and is cycled to a catalyst regeneration zone. In the catalyst regenerator, the coked catalyst is contacted with a gas, such as air, which contains a predetermined concentration of molecular oxygen to burn off a desired portion of the coke from the catalyst and simultaneously to heat the catalyst to a high temperature desired when the catalyst is again contacted with the hydrocarbon stream in the cracking zone. After regeneration, the catalyst is cycled to the cracking zone, where it is used to vaporize the hydrocarbons and to catalyze hydrocarbon cracking. The flue gas formed by combustion of coke in the catalyst regenerator is removed from the regenerator. It may be treated to remove particulates and carbon monoxide from it, after which it is normally passed into the atmosphere. Concern with control of pollutants in flue gas has resulted in a search for improved methods for controlling such pollutants. In the past, concern has centered on sulfur oxides and carbon monoxide. More recently, concern over pollutants has been extended to the level of nitrogen oxides in some cracking systems, particularly in systems using complete combustion-type regeneration.
The amount of conversion obtained in an FCC cracking operation is the volume percent of fresh hydrocarbon feed changed to gasoline and lighter products during the conversion step. The end boiling point of gasoline for the purpose of determining conversion is conventionally defined as 221.degree. C. Conversion is often used as a measure of the severity of a commercial FCC operation. At a given set of operating conditions, a more active catalyst gives a greater conversion than does a less active catalyst. The ability to provide higher conversion in a given FCC unit is desirable in that it allows the FCC unit to be operated in a more flexible manner. Feed throughput in the unit can be increased, or alternatively a higher degree of conversion can be maintained with a constant feed throughput rate. The type of conversion, i.e., selectivity, is also important in that poor selectivity results in less naphtha, the desired cracked product, and higher gas and coke makes.
One conventional mode of FCC catalyst regeneration currently used in many systems is an incomplete combustion mode. In such systems, referred to herein as "standard regeneration" systems, a substantial amount of coke carbon is left on regenerated catalyst passed from the FCC regeneration zone to the cracking zone after regeneration, i.e., more than 0.2 weight percent carbon, usually about 0.25 to 0.45 weight percent carbon. The flue gas removed from an FCC regenerator operating in a standard regeneration mode is characterized by a relatively high carbon monoxide/carbon dioxide concentration ratio. The atmosphere in much or all of the regeneration zone is a reducing atmosphere because of the presence of substantial amounts of unburned coke carbon and carbon monoxide.
In general, reducing the level of carbon on regenerated catalyst below about 0.2 weight percent has been difficult. Prior to the introduction of zeolite catalyst, there was little incentive to attempt to remove substantially all coke carbon from the catalyst, since even a fairly high carbon content had little adverse effect on the activity and selectivity of commercial amorphous silica-alumina catalysts. Most of the FCC cracking catalysts now used, however, contain crystalline aluminosilicate zeolites, or molecular sieves. Zeolite-containing catalysts have usually been found to have relatively higher activity and selectivity when their coke carbon content, after regeneration, is relatively low. An incentive has thus arisen for attempting to reduce the coke content of regenerated FCC catalyst to a very low level, e.g., below 0.2 weight percent.
Carbon monoxide is one of the pollutant gases found in FCC regenerator flue gas. Several methods have been suggested for burning substantially all carbon monoxide to carbon dioxide during regeneration, in order to avoid air pollution by the carbon monoxide, recover heat and prevent afterburning. Among the procedures suggested for use in obtaining complete carbon monoxide combustion in an FCC regenerator have been: (1) increasing the amount of oxygen introduced into the regenerator relative to standard regeneration; and either (2) substantially increasing the average operating temperature in the regenerator or (3) including various carbon monoxide oxidation promoting metals in the system to promote carbon monoxide burning in the regenerator. Combustion-promoting metals have been employed in two ways: (a) in low concentration on essentially all the particulate solids circulating in the cracking system, i.e., on the catalyst; or (b) in high concentrations on only a very small fraction (less than 1%) of the particulate solids in the cracking system, often with the promoter metal supported on low-acidity, essentially noncatalytic solids. Various other solutions have also been directed to the problem of afterburning of carbon monoxide, such as addition of extraneous combustibles or use of water or heat-accepting solids to absorb the heat generated by combustion of carbon monoxide in an afterburning mode. Because of their expense and activity, the promoting metals are used at very low concentration on promoted particles when associated with essentially all the particulate solids in a cracking system. Promoter concentrations for platinum are typically 0.1 to 10 ppm (weight) in promoted catalysts, or when the promoting metal is supplied to the system as an additive in the feed, e.g., as a feed-soluble compound.
Complete combustion regeneration systems using a high temperature in the catalyst regenerator, rather than oxidation-promoting metals, to accomplish complete carbon monoxide combustion have not been found as entirely satisfactory as promoted systems. Some of the heat generated by carbon monoxide combustion is usually lost in the flue gas. Much of the CO combustion takes place in a dilute catalyst phase in an afterburning mode, and the resulting high temperature in the regenerator dilute phase can permanently adversely affect the activity and selectivity of the FCC catalyst.
Because of activity limitations, combustion promoting metals, such as platinum, must be incorporated into particulate solids in relatively higher concentrations, e.g., 0.01 to 1 weight percent, when the promoted particles constitute a very small fraction (less than 1%) of the total solids inventory in a cracking system. When using carbon monoxide combustion promoting metals associated with a very small fraction of the total particulate solids inventory in a cracking system (including both particulate catalyst and any other solids in the system), essentially complete carbon monoxide combustion has been obtained commercially. Low levels of coke on regenerated catalyst, another desirable result, have also been obtained. On the other hand, the amount of undesirable nitrogen oxides has increased quite substantially in the flue gas from catalyst regenerators using promoting metals contained on less than 1% of the circulating particulate solids. This has created a serious air pollution problem in disposing of the flue gas.
As mentioned above, the art has suggested various modes of addition of Group VIII noble metals and other carbon monoxide combustion promoting metals to FCC systems. In U.S. Pat. No. 2,647,860 it is proposed to add 0.1-1 weight percent chromic oxide to an FCC catalyst to promote combustion of carbon monoxide to carbon dioxide and to prevent afterburning. U.S. Pat. No. 3,364,136 proposes to employ particles containing a small pore (3-5 A.) molecular sieve with which is associated a transition metal from Groups IB, IIB, VIB, VIIB and VIII of the Periodic Table, or compounds thereof, such as a sulfide or oxide. Representative metals disclosed include chromium, nickel, iron, molybdenum, cobalt, platinum, palladium, copper and zinc. The metal-loaded, small-pore zeolite may be used in an FCC system in physical mixture with cracking catalysts containing larger-pore zeolites active for cracking, or the small-pore zeolite may be included in the same matrix with zeolites active for cracking. The small-pore, metal-loaded zeolites are supplied for the purpose of increasing the CO.sub.2 /CO ratio in the flue gas produced in the FCC regenerator. In U.S. Pat. No. 3,788,977, it is proposed to add uranium or platinum impregnated on an inorganic oxide such as alumina to a FCC system, either in physical mixture with FCC catalyst or incorporated into the same amorphous matrix as a zeolite used for cracking. Uranium or platinum is added for the purpose of producing gasoline fractions having high aromatics contents, and no effect on carbon monoxide combustion when using the additive is discussed in the patent. In U.S. Pat. No. 3,808,121 it is proposed to supply large-size particles containing a carbon monoxide combustion promoter metal in an FCC regenerator. The smaller-size catalyst particles are cycled between the FCC cracking reactor and the catalyst regenerator, while, because of their size, the larger promoter particles remain in the regenerator. Carbon monoxide oxidation promoters such as cobalt, copper, nickel, manganese, copper, chromite, etc., impregnated on an inorganic oxide such as alumina are disclosed for use. Belgian patent publication No. 820,181 suggests using catalyst particles containing platinum, palladium, iridium, rhodium, osmium, ruthenium or rhenium or mixtures or compounds thereof to promote carbon monoxide oxidation in an FCC catalyst regenerator. An amount between a trace and 100 ppm of the active metal is added to FCC catalyst particles by incorporation during catalyst manufacture or by addition of a compound of the metal to the hydrocarbon feed to an FCC unit using the catalyst. The publication asserts that addition of the promoter metal increases coke and hydrogen formation during cracking. The catalyst containing the promoter metal can be used as such or can be added in physical mixture with unpromoted FCC cracking catalyst.
U.S. Pat. Nos. 4,072,600 and 4,093,535 disclose the use of combustion-promoting metals in catalytic cracking systems in concentrations of 0.01 to 50 ppm, based on total catalyst inventory. The combustion-promoting metals are disposed on the catalyst particles.
The hydrocarbon feeds processed in commercial FCC units normally contain sulfur, usually termed "feed sulfur." It has been found that about 2-10% or more of the feed sulfur in a hydrocarbon feedstream processed in an FCC system is invariably transferred from the feed to the catalyst particles as a part of the coke formed on the catalyst particles during cracking. The sulfur deposited on the catalyst, herein termed "coke sulfur," is passed from the cracking zone on the coked catalyst into the catalyst regenerator. About 2-10% or more of the feed sulfur is continuously passed from the cracking zone into the catalyst regeneration zone in the coked catalyst. In an FCC catalyst generator, sulfur contained in the coke is burned along with the coke carbon and hydrogen, forming gaseous sulfur dioxide and sulfur trioxide, which are conventionally removed from the regenerator in the flue gas.
Most of the feed sulfur does not become coke sulfur in the cracking reactor. Instead, it is converted either to normally gaseous sulfur compounds such as hydrogen sulfide and carbon oxysulfide, or to normally liquid organic sulfur compounds. All these sulfur compounds are carried along with the vapor cracked hydrocarbon products recovered from the cracking reactor. About 90% or more of the feed sulfur is continuously removed from the cracking reactor in the stream of processed, cracked hydrocarbons, with about 40-60% of this sulfur being in the form of hydrogen sulfide. Provisions are conventionally made to recover hydrogen sulfide from the effluent from the cracking reactor. Typically, a very-low-molecular-weight off-gas vapor stream is separated from the C.sub.3 + liquid hydrocarbons in a gas recovery unit, and the off-gas is treated, as by scrubbing it with an amine solution, to remove the hydrogen sulfide. Removal of sulfur compounds such as hydrogen sulfide from the fluid effluent from an FCC cracking reactor is relatively simple and inexpensive compared to removal of sulfur oxides from an FCC regenerator flue gas by conventional methods. Moreover, if all the sulfur which must be recovered from an FCC operation could be recovered in a single recovery operation performed on the reactor off-gas, the use of two separate sulfur recovery operations in an FCC unit could be obviated.
It has been suggested to diminish the amount of sulfur oxides in FCC regenerator flue gas by desulfurizing a hydrocarbon feed in a separate desulfurization unit prior to cracking or to desulfurize the regenerator flue gas itself, by a conventional flue gas desulfurization procedure, after its removal from the FCC regenerator. Clearly, either of the foregoing alternatives requires an elaborate, extraneous processing operation and entails large capital and utilities expenses.
If sulfur normally removed from the FCC unit in the regenerator flue gas as sulfur oxides is instead removed from the cracking reactor as hydrogen sulfide along with the processed cracked hydrocarbons, the sulfur thus shifted to the reactor effluent constitutes simply a small increment to the large amount of hydrogen sulfide and organic sulfur invariably present in the reactor effluent. The small added expense, if any, of removing even as much as 5-15% more hydrogen sulfide from an FCC reactor off-gas by available means is substantially less than the expense of reducing the flue gas sulfur oxides level by separate feed desulfurization. Present commercial off-gas hydrogen sulfide recovery facilities can, in most if not all cases, handle any additional hydrogen sulfide which would be added to the off-gas if the sulfur normally in the regenerator flue gas were substantially all converted to hydrogen sulfide in the FCC reactor off-gas. It is accordingly desirable to direct substantially all feed sulfur into the fluid cracked products removal pathway from the cracking reactor and thereby reduce the amount of sulfur oxides in the regenerator flue gas.
It has been suggested, e.g., in U.S. Pat. No. 3,699,037, to reduce the amount of sulfur oxides in FCC regenerator flue gas by adding particles of Group IIA metal oxides and/or carbonates, such as dolomite, MgO or CaCO.sub.3, to the circulating catalyst in an FCC unit. The Group IIA metals react with sulfur oxides in the flue gas to form solid sulfur-containing compounds. The Group IIA metal oxides lack physical strength. Regardless of the size of the particles introduced, they are rapidly reduced to fines by attrition and rapidly pass out of the FCC unit with the catalyst fines. Thus, addition of dolomite and the like Group IIA materials is essentially a once-through process, and relatively large amounts of material must be continuously added in order to reduce the level of flue gas sulfur oxides.
It has also been suggested, e.g., in U.S. Pat. No. 3,835,931, to reduce the amount of sulfur oxides in an FCC regenerator flue gas by impregnating a Group IIA metal oxide onto a conventional silica-alumina cracking catalyst. The attrition problem encountered when using unsupported Group IIA metals is thereby reduced. However, it has been found that Group IIA metal oxides, such as magnesia, when used as a component of cracking catalysts, have a rather pronounced undesirable effect on the activity and selectivity of the cracking catalysts. The addition of a Group IIA metal to a cracking catalyst results in two particularly noticeable adverse consequences relative to the results obtained in cracking without the presence of the Group IIA metals: (1) the yield of the liquid hydrocarbon fraction is substantially reduced, typically by greater than 1 volume percent of the feed volume; and (2) the octane rating of the gasoline or naphtha fraction (24.degree.-221.degree. C. boiling range) is substantially reduced. Both of the above-noted adverse consequences are seriously detrimental to the economic viability of an FCC cracking operation, so that even complete removal of sulfur oxides from regenerator flue gas would not normally compensate for the simultaneous losses in yield and octane which result from adding Group IIA metals to an FCC catalyst.
Alumina has been a component of many FCC and moving-bed cracking catalysts, but normally in intimate chemical combination with silica. Alumina itself has low acidity and is generally considered to be undesirable for use as a cracking catalyst. The art has taught that alumina is not selective, i.e., the cracking hydrocarbon products recovered from an FCC or other cracking unit using an alumina catalyst would not be desired valuable products, but would include, for example, relatively large amounts of C.sub.2 and lighter hydrocarbon gases.
U.S. Pat. No. 4,071,436 discloses the use of alumina for reducing the amount of sulfur oxides in the flue gas formed during cracking catalyst regeneration. The alumina can be used in the form of a particulate solid mixed with cracking catalyst particles. In some cases, alumina contained in the cracking catalyst particles is also suitable; however, alumina contained in conventional cracking catalyst is usually not very active, since it is intimately mixed with a large fraction of silica.
U.S. Pat. No. 4,115,250 and No. 4,115,251 disclose the synergistic use of oxidation-promoting metals for carbon monoxide burning in combination with the use of alumina for reducing the amount of sulfur oxides in cracking catalyst regenerator flue gas. When alumina and highly active oxidation-promoting metals are both included in the same particle, alumina in the particle is ineffective for removing sulfur oxides from the regenerator flue gas, especially in the presence of even a small amount of carbon monoxide. On the other hand, when the alumina and combustion-promoting metal are used on separate particles circulated together in a cracking system in physical admixture, the ability of the alumina to reduce the level of sulfur oxides in the flue gas can be considerably enhanced.
In carrying out the method for reducing the level of sulfur oxides in catalyst regenerator flue gas using alumina, in general as disclosed in U.S. Pat. No. 4,017,436, No. 4,115,250 and No. 4,115,251 under commercial conditions, we have now noted that the overall concentration of silica in the particulate solids inventory in a catalytic cracking system exerts an unexpected effect on the activity and stability of alumina in the inventory with respect to the capacity of the alumina to form sulfur-containing solids in a catalyst regenerator, regardless of the type of association with alumina or other materials with which the silica is present in the catalyst inventory, except for silica in the form of zeolitic crystalline aluminosilicates. Previously, it was believed that contamination of alumina in the system by silica presented a problem only if the silica were introduced into the circulating particulate solids inventory already chemically combined with alumina or, at least, only if it were introduced in the same particles as the alumina. We have now found that, under commercial catalytic cracking and regeneration conditions, silica can migrate from particles of high silica concentration to particles of low or zero silica concentration during circulation of particles having different silica concentrations in physical admixture in a cracking system. Silica which is subject to such migration may be termed "amorphous" or "non-crystalline" silica, to distinguish it from silica in the form of zeolitic crystalline aluminosilicates, which is relatively stable and is subject to little or no migration under commercial conditions. The present invention is directed, in part, to overcoming the deactivation of alumina resulting from silica migration from one particle to another in the particulate solids inventory in a catalytic cracking system.