GB2081597A - Particulate catalyst regeneration including control of flue gas pollutants - Google Patents

Abstract

Flue gas pollutants including sulfur oxides and nitrogen oxides are removed from catalyst regeneration flue gas by burning sulfur- and nitrogen-containing coke off coke-contaminated catalyst particles and burning carbon monoxide in a regeneration zone, reacting sulfur trioxide with alumina in the regeneration zone to form a solid, and heating the flue gas after removing the flue gas from the regeneration zone to reduce the nitrogen oxide content of the flue gas.

Description

SPECIFICATION Particulate catalyst regeneration including control of flue gas pollutants This invention relates to the art of catalyst regeneration. More specifically, the present invention is concerned with a method of burning sulfurcontaminated and nitrogen-contaminated coke off particulate catalyst, while decreasing gaseous pol- lutants contamination of flue gas formed in the coke burning.

Catalytic cracking is a well-known commercial process using particulate catalyst in a moving bed or a fluidized bed. Cracking conditions include the absence of externally supplied molecular hydrogen, whereas hydro-cracking is carried out in the presence of molecular hydrogen. In catalytic cracking, an inventory of particulate catalyst is continuously cycled between a cracking reactor and a catalyst regenerator. In a fluidized catalytic cracking (FCC) system, hydrocarbon feed is contacted with catalyst particles in a hydrocarbon cracking zone, or reactor, at a temperature of about 425"C-600"C, usually 460"C-560"C. The reactions of hydrocarbons at the elevated operating temperature result in deposition of carbonaceous coke on the catalyst particles.The resulting fluid products are separated from the cokedeactivated, spent catalyst and are withdrawn from the reactor. Coke-containing catalyst particles are stripped of volatiles, usually by means of steam, and passed to the catalyst regeneration zone. In the catalyst regenerator, the spent catalyst is contacted with a predetermined amount of molecular oxygen.

A desired portion of the coke is burned off the catalyst. The coke burning step restores catalyst activity and simultaneously heats the catalyst to, e.g., 540"C-815"C, usually 590"C-730"C. Flue gas formed by combustion of coke in the conventional catalyst regenerator may be treated for removal of particulates and conversion of carbon monoxide, after which the flue gas is normally discharged into the atmosphere.

Most fluidized catalytic cracking systems now use zeolite-containing catalyst having high cracking activity and selectivity. Zeolite-type catalysts have a particularly high cracking activity when the concentration of carbon on the catalyst after regeneration is relatively low. It is therefore generally desirable to burn as much coke off zeolite-containing catalysts as is possible, so as to obtain a relatively high activity and selectivity in carrying out hydrocarbon cracking.

It is also often desirable to burn carbon monoxide as completely as possible during catalyst regeneration to retain as much heat energy as possible in the cracking system. Retention of heat energy is especially important when high catalyst cracking selectivity leads to low concentration of coke on the spent catalyst. Among the ways suggested to enhance coke burning to increase catalyst cracking selectivity and to burn more carbon monoxide to provide process heat, is enhanced carbon monoxide combustion in a dense-phase fluidized catalyst bed in the catalyst regenerator. Carbon monoxide burning may, for example, be promoted by use of an active, combustion promoting metal.Metals have been used commercially for this purpose, either as an integral component of the cracking catalyst particles or as a component of a discrete particulate additive, in which the active metal is associated with a support other than the catalyst particles. Additive particles are mixed with catalyst particles in the circulating particulate solids inventory. Various ways of employing carbon monoxide combustion promoting metals in cracking systems have been suggested in the patent art. In U.S. Patent No. 2,647,860, it is proposed to add 0.1-1 weight percent chromic oxide to a cracking catalyst used in a moving bed system to promote combustion of carbon monoxide to carbon dioxide and to prevent afterburning. In U.S.

Patent No.3,808,121, it is proposed to introduce relatively large sized particles containing a carbon monoxide combustion promoting metal into a cracking catalyst regenerator. The circulating particulate solids inventory, comprised of relatively small-sized catalyst particles, is cycled between the cracking reactor and the catalyst regenerator, while the combustion promoting particles remain in the regenerator because of their size. Oxidation promoting metals such as cobalt, copper, nickel, manganese, copper-chromite, etc., impregnated on an inorganic oxide such as alumina, are disclosed.

Belgian Patent Publication No.820,181 discloses the use of cracking catalyst particles containing a Group VIII noble metal or rhenium to provide CO combustion during regeneration of the catalyst. The combustion promoter is used in an amount of a trace to 100 ppm. No. 820,181 teaches that including such combustion promoting metals in the catalyst results in an increase in formation of coke and hydrogen when the catalyst is employed for hydrocarbon cracking.

U.S. Patents No. 4,072,600 and No.4,093,535 disclose the addition of trace amounts of Group Vl ll noble metals or rhenium to a cracking catalyst inventory in order to enhance combustion of CO in a catalyst regenerator. These patents teach that it is well known that the presence of a hydrogena tionidehydrogenation component in a cracking catalyst is detrimental to the cracking process because of the resulting undesirable increase in coke and hydrogen formation in cracking. The patents teach that detrimental results are caused by the reaction conditions employed in a cracking reactor, which favor undesirable reactions.According to these patents, ideally, the least possible amount of the carbon monoxide combustion promoting metal necessary to convert the desired amount of CO to CO2 should be used, since this ideal least possible amount would have the least adverse effect on the hydrocarbon cracking step. According to these patents, minimization of the concentration of the carbon monoxide combustion promoter in a catalytic cracking system can be accomplished by monitoring the flue gas leaving the catalyst regenerator and adjusting the rate of addition of combustion promoter such that the least amount of promoter necessary to completely burn carbon monoxide is added.

U.S. Patent No.4,153,535 discloses a cyclic fluidized cracking process using a zeolite-type cracking catalyst, a metallic reactant (which reacts with sulfur oxides) selected from sodium, magnesium, calcium, strontium, barium, scandium, titanium, chromium, molybdenum, manganese, cobalt, nickel, antimony, copper, zinc, cadmium, lead, the rare earth metals (more than 20 elements), their compounds and mixtures thereof, and a metallic oxidation promoter selected from ruthenium, rhodium, palladium, osmium, iridium, platinum, vanadium, tungsten, uranium, zirconium and silver. A similar system is disclosed in U.S. Patent No.4,240,899.

Representative catalyst regeneration patent literature is shown in the following patents: U.S. Patent No.3,909,392 describes a scheme for enhancing carbon monoxide combustion by thermal means.

Catalyst is used to provide a heat sink in the dilute phase above a fluidized bed for the increased heat production. British Patent Publication 2,001,545 describes a two-stage system for a regenerating catalyst, with partial catalyst regeneration being carried out in the first stage and secondary, more complete regeneration carried out in the second stage with a separate regeneration gas. U.S. Patent No.

3,767,566 describes a two-stage regeneration scheme in which partial regeneration takes place in an entrained catalyst bed, and secondary, more complete regeneration takes place in a dense fluidized catalyst bed. A somewhat similar regeneration operation is described in U.S. Patent No.3,902,990, which discusses the use of several stages of regeneration, with dilute and dense-phase beds of catalysts being employed, and with the use of plural streams of regeneration gas. U.S. Patent No.

3,926,843 describes a plural-stage regeneration scheme in which dilute phase and dense-phase coke burning are performed. British Patent Publication No.1,499,682 discloses use of a combustion promoting metal for enhancing carbon monoxide combustion.

It has been suggested, e.g., in U.S. Patent No.

3,699,037, to reduce the amount of sulfur oxides in FCC regenerator flue gas adding particles of Group IIA metal oxides andtor carbonates, such as dolomite, MgO or CaCO3, 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 IlA 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 the U.S. Patent 3,835,031, to reduce the amount of sulfur oxides in an FCC regenerator flue gas by impregnating a Group IIA metal oxide onto a conventional silicaalumina 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 catalyst, have a rather pronounced undesirable effect on the activity and selectivity of the cracking catalysts.

Such undesirable effects are particularly apparent when the magnesium compound is used in an amount sufficient to appreciably diminish the amount of sulfur oxides in the flue gas. 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 (759430"F boiling range) is substantially reduced.Both of the abovenoted 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 normai!y compensate for the simultaneous losses in yield and octane which result from adding Group IIA metals to an FCC catalyst. The same can generally be said for the numerous reactant metals disclosed as sulfur oxides reactants in U.S. Patent No.4,153,535, discussed above.

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 cracked 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 C3 and lighter hydrocarbon gases.

U.S. Patent 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. Patents 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 oxidationpromoting metals are both included in the same particle, alumina in the particle is relatively 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.

One problem encountered in some cracking operations using metal-promoted complete carbon monoxide combustion-type regeneration has been the generation of undesirable nitrogen oxides (NO,) in the flue gas formed by burning coke. For example, U.S. Patent No.4,235,704 discloses a method for controlling nitrogen oxides by adjusting the concentration of carbon monoxide combustion promoter in relation to the level of nitrogen oxides in flue gas exiting a catalyst regenerator. In many cases, however, it is desired to have a larger amount of combustion promoter present, than could be feasible in the '704 patent method.The present invention is directed, in part, toward providing a catalyst regeneration system which accomplishes a high degree of coke removal and complete carbon monoxide combustion with a catalyst regeneration system, while substantially decreasing the concentration of nitrogen oxide present in flue gas formed by burning the coke, without recourse to overly restrictive limitations on the use of combustion promoters.

We have found that a particulate catalyst which has been contaminated by sulfur-containing and nitrogen-containing coke deposition can be regenerated by burning off sufficient coke to provide an essentially carbon-free, regenerated catalyst, and a flue gas free from carbon monoxide, nitrogen oxides and sulfur oxides can be formed in carrying out the regeneration by (a) passing a regeneration gas including free oxygen upwardly through a densephase fluidized bed of particulate solids comprising substantially coke-free particles of the catalyst in a regeneration zone, and removing the regeneration gas from the regeneration zone; (b) introducing the coke-contaminated catalyst particles into the bed and generating carbon oxides, nitrogen oxides and sulfur oxides in the bed by burning off the coke; (c) burning sufficient carbon monoxide within the bed to maintain the regeneration gas above the bed at a temperature below 735"C and not higher than 27"C above the maximum temperature of the regeneration gas within the bed and to provide a carbon monoxide concentration of less than 0.5 volume percent in the regeneration gas removed from the regeneration zone, and introducing sufficient free oxygen into the bed to provide at least 1 volume percent free oxygen in the regeneration gas removed from the regeneration zone; (d) including in the particulate solids an amount of a sulfur dioxide combustion promoter sufficient to provide substantial incremental conversion of sulfur dioxide to sulfur trioxide within the bed; (e) including reactive alumina in the particulate solids, and decreasing the amount of sulfur oxides in regeneration gas removed from the regeneration zone by forming a sulfur-containing and aluminum-containing solid from the sulfur trioxide and the reactive alumina; and (f) recovering substantially coke-free particles of said catalyst from said fluidized bed and decreasing the amount of nitrogen oxides in the regeneration gas removed from said regeneration zone by maintaining the regeneration gas outside said zone at a temperature above760'C for at least 1 second.

In accordance with another aspect of the invention, we have found that a particulate catalyst which has been contaminated with sulfur-containing and nitrogen-containing coke can be regenerated, and a flue gas with decreased levels of carbon monoxide, nitrogen oxides and sulfur oxides can be formed in carrying out the regeneration by (a) forming carbon monoxide, gaseous sulfur oxides and gaseous nitrogen oxides by reacting free oxygen with the coke in a regeneration zone; (b) decreasing the amount of the carbon monoxide by reacting the carbon monoxide with free oxygen in the regeneration zone in the presence of a carbon monoxide combustion promoter; (c) decreasing the amount of the gaseous sulfur oxides and forming a sulfur-containing solid in the regeneration zone by reacting the sulfur oxides with reactive alumina contained in a substantially silicafree particulate solid otherthan the catalyst present in the regeneration zone in physical admixture with the catalyst; and (d) withdrawing gas including the nitrogen oxides from the regeneration zone at a temperature below 735"C and decreasing the amount of the nitrogen oxides by maintaining the gas at a temperature above 760"C for at least one second outside the regeneration zone.

The accompanying drawing is a schematic representation of one preferred embodiment of the present invention.

Referring to the drawing, there is shown a regeneration vessel 1. Coke-containing catalyst is introduced into the vessel 1 through a conduit 3, and substantially coke-free catalyst is removed by way of a conduit 5. A dense-phase fluidized bed 7 of substantially coke-free catalyst is maintained in the vessel 1. Regeneration gas is introduced into the lower end of the vessel through a conduit 9 and discharged into the regeneration zone through a distributor 11.

The regeneration gas passes upwardly through a distribution grid 13, through the fluidized bed 7, and out of the upper end of the dense-phase bed, indicated by a line at 15 in the drawing. Substantially all the coke in the spent and partially regenerated catalyst present in the fluidized bed is preferably burned with free oxygen introduced in the regeneration gas.

Regeneration gas (flue gas) leaving the top 15 of the bed 7 passes into a cyclone separator 17. Entrained solids are separated from the flue gas in the cyclone and returned to the dense-phase catalyst bed through a dipleg 19. The flue gas is then withdrawn from the vessel 1 at a temperature below 735"C through a conduit 21 and passed into a furnace (not shown) to decrease its nitrogen oxides and concentration. Fuel gas and oxygen are introduced into the furnace and burned in admixture with the flue gas to heat the flue gas to a temperature above 760 C. After an appropriate residence time at a temperature above 760"C in the furnace, flue gas with a reduced nitrogen oxides content is removed.Conventional elements of the embodiment depicted, such as controlling means, pumping and valve means, and the like, are not shown in the drawing and are not described in order to simplify the explanation. The use and disposition of such elements will be clear to those skilled in the art.

As used herein, the term "substantially coke-free catalyst" refers to catalyst which contains less than 0.2 weight percent carbon.

As used herein, the term "dense-phase fluidized bed" means a fluidized bed of particulate solids having a density of at least 12 pounds per cubic foot.

As used herein, the term "regeneration gas" includes the gas mixtures (e.g., air) introduced to the catalyst regenerator, the gas mixtures present in the regenerator, and the gas mixtures removed from the regenerator (i.e., flue gases).

Catalysts that are best adapted for regeneration according to this invention are those in the form of particulate solids. Preferably, catalyst to be regenerated is sized appropriately for catalytic use in an entrained bed orfluidized bed catalytic conversion system. With reference to the types of catalytic hydrocarbon conversion operations presently practiced commercially, this invention is especially advantageous for regeneration of FCC catalysts; however, use of the invention is not limited to FCC catalyst regeneration operations, and can be used for treating any coke-contaminated particulate catalyst which can be wholly or partially reactivated by coke burnoff.

A regeneration zone employed in carrying out the invention may be supplied by a suitable vessel or chamber, capable of containing regeneration gas and catalyst particles at the desired temperature and pressure. Suitable vessels and chambers will be readily apparent to those skilled in the art from the description herein. Conventional FCC cracking catalyst regeneration vessels and chambers, for example, can suitably be employed.

The regeneration gas or gas mixture introduced into the catalyst regenerator must provide an appropriate free oxygen (molecular oxygen) supply to the regenerator. Normally, air is quite suitable for use in supplying free oxygen, but use of air is not essential. For example, pure oxygen or oxygenenriched air can also be used, if desired. Conventional gases present in commercial FCC operations, such as free nitrogen (molecular nitrogen), carbon dioxide, steam, and the like, may suitably be present in fluidizing and entrainment gases.

In general, regeneration conditions maintained in the regenerator include a combination of temperature and pressure sufficient to permit the specified or desired degree of coke combustion, carbon monoxide combustion and sulfur dioxide combustion to take place, in the manner discussed herein. The temperature of the regeneration gas is maintained below 735"C. Temperatures of 540"C-735"C are normally quite suitable. Temperatures of 590"C-730"C are preferred. Preferably, the rates of flow of regeneration gases, entrainment gases and particulate solids such as catalyst particles through the regeneration system are maintained at levels which provide a dense-phase fluidized bed of particulate catalyst in the regeneration zone.Fluid bed operation can be accomplished in a conventional manner by maintaining upward superficial gas velocities appropriate to the size and density of catalyst particles undergoing regeneration and by maintaining catalyst introduction and withdrawal rates at proper levels. The operating pressure in the regenerator is usually not particularly critical. Absolute pressures of 1-20 atmospheres are generally quite suitable. Pressures of 1.5-5 atmospheres are preferred.

It will be apparent to those skilled in the art that the amount of coke contained in spent, deactivated catalyst, as well as the concentrations of sulfur and nitrogen in the coke, will vary widely depending on such factors as the composition and boiling range of the hydrocarbon feed with which the catalyst has previously been contacted, the composition of the catalyst, the type of hydrocarbon conversion or treatment system in which the catalyst is used (e.g., moving bed, fluid bed, entrained bed), etc. The benefits of regeneration according to the invention can be obtained with coke-containing catalysts which have a coke concentration varying over a broad range and for catalysts contaminated with coke having sulfur and nitrogen contents which can vary over a broad range.

In one embodiment of the invention, a densephase fluidized bed of substantially coke-free particulate catalyst is preferably maintained in a regeneration zone, such as an appropriate vessel, chamber, or the like. The average carbon content of the catalyst particles in a dense-phase bed as a whole, when a dense-bed regeneration is employed, is preferably less than 0.2 weight percent. Particularly preferably, the average concentration of coke carbon present in the catalyst particles in a densephase bed as a whole is less than 0.1 weight percent.

In carrying out regeneration in a dense-phase bed system, coke-containing, deactivated catalyst is preferably introduced into the dense-phase fluidized bed at a controlled rate, and substantially coke-free catalyst is removed from the fluidized bed at a rate appropriate to maintain the bed at the desired size, density, and average particle coke content. Preferably, coke-containing, spent catalyst particles are introduced into a relatively lower level of the dense-phase fluidized bed, and the substantially coke-free catalyst particles are removed from a relatively higher level of the bed.Because ofthe high turbulence and good overall mixing inherent in the fluidized bed system, however, catalyst present in any portion of the fluidized bed includes a minor proportion of partially regenerated catalyst particles mixed with a major proportion of substantially completely coke-free particles. Coke-containing catalyst is preferably introduced into the regeneration zone at a rate low enough so that the average coke content of catalyst particles in the bed as a whole is not raised above 0.2 weight percent, and preferably not above 0.1 weight percent. That is, catalyst withdrawn from any part of the bed is, as a whole, substantially coke-free.

In the embodiment using a dense-phase fluidized bed, a regeneration and fluidizing gas including free oxygen is preferably introduced into the lower end of the dense-phase fluidized bed. Regeneration gas is passed upwardly through the bed, removed from the upper end of the bed and withdrawn from the regeneration zone. Enough free oxygen is preferably introduced into the bed to provide an oxidizing atmosphere containing at least one volume percent free oxygen in all parts of the dense-phase fluidized bed, as well as in the flue gas removed from the regeneration zone. Of course, the lowest average free oxygen concentration will normally be found in the flue gas portion of the regeneration gas, since it is furthest downstream in the regeneration gas pathway. Likewise, the maximum temperature is usually found in the regeneration gas downstream from the dense-phase bed.Preferably, the regenera tion gas introduced to the regenerator contains enough free oxygen to provide at least 2 volume percent free oxygen in the regeneration gas in all parts of the dense-phase bed and in the regeneration gas downstream of the dense-phase bed (flue gas).

A carbon monoxide combustion promoter is pref erably included in admixture with the catalyst parti cles, e.g., in the dense-phase fluidized bed in prefer ably an amount necessary to provide combustion, preferably within a dense-phase bed, of substantially all carbon monoxide generated by coke burning.

Sufficient carbon monoxide is preferably burned within the regeneration zone so that regeneration gas removed from the regeneration zone (i.e., flue gas) contains less than 0.5 volume percent carbon monoxide. Preferably, sufficient carbon monoxide is burned within the dense-phase bed in the regenerator to provide less than 0.05 volume percent carbon monoxide in the flue gas.

Co ventional carbon monoxide combustion promoters can be used, if necessary, to provide ade quate combustion of carbon monoxide. Most transi tional metals and their oxides are to some extent active for promoting carbon monoxide combustion.

Preferred carbon monoxide combustion promoters are metals or compounds of metals selected from ruthenium, rhodium, palladium, iridium, platinum, copper, chromium and manganese. Platinum is a particularly preferred carbon monoxide combustion promoter.

When a carbon monoxide combustion promoter is used in the embodiment carried out with a dense phase fluidized bed, the amount of promoter emp loyed is preferably an amount sufficient to provide complete carbon monoxide combustion within the dense-phase bed. Complete carbon monoxide con version within a dense-phase fluidized bed is indi cated, for example, by (1) the substantial absence of carbon monoxide in flue gas withdrawn from the regeneration zone, and (2) a maximum temperature in regeneration gas above (downstream from) the dense-phase catalyst bed which is not more than 27"C above the maximum temperature of the regen eration gas in the dense phase bed.Preferably, the maximum temperature of the regeneration gas downstream from the dense-phase bed does not exceed 15"C above the maximum temperature of regeneration gas in the dense-phase catalyst bed. Of course, the maximum temperature of the r.egenera tion gas above the dense-phase bed may suitably be lower than the maximum dense-phase gas tempera ture, although this is not a typical mode of operation unless extraneous cooling is employed.Since these concurrent conditions, i.e., (1) low flue gas CO con centration and (2) relatively low maximum down stream regeneration gas temperature, indicate essentially complete carbon monoxide burning in a dense-phase bed, the prior art teachings as to the use of carbon monoxide combustion promoter pro vide no incentive to use any combustion promoter if use of a promoter is not necessary to obtain com plete combustion. Likewise, the prior art provides no incentive to employ a greater concentration of combustion promoter than necessary to obtain complete, dense-phase bed combustion when a combustion promoter is used, and, in fact, the art teaches that use of more than the necessary amount of promoter is detrimental because of the adverse effect on the hydrocarbon conversion step.

According to another aspect of the invention, sulfur oxides formed by coke burning are removed from the regeneration gas within the regeneration zone by reacting at least the sulfur trioxide with reactive alumina. The alumina useful for the reaction may be included in all or a portion of the catalyst present in the regeneration zone or may be included in essentially catalytically inert particles present in the regeneration zone in physical mixture with the catalyst. Suitable alumina is not in intimate combination with more than 40 weight percent silica and is preferably substantially free from silica in intimate combination. Suitable alumina has an average surface area of at least 50 square meters per gram, preferably at least 100 square meters per gram.Alumina is suitable for use in carrying out the invention if it contains an average of more than about 50 parts per million (weight) of"reactive alumina", as determined by treating particles containing the alumina by the following steps: {1) passing a stream of a gas mixture containing, by volume, 10% water, 1% hydrogen sulfide, 10% hydrogen and 79% nitrogen over the aluminacontaining particle continuously at a temperature of 1200 F and atmospheric pressure until the weight of the particle is substantially constant;; (2) passing a stream of a gas mixture containing, by volume, 10% water, 15% carbon dioxide, 2% oxygen and 73% nitrogen over the particle resulting from step (1) at a temperature of 1200 F and atmospheric pressure until the weight ofthe particle is substantially constant, the weight of the particle at this time being designated "Wa"; and (3) passing a stream of a gas mixture containing, by volume, 0.05% sulfur dioxide, and, in addition, the same gases in the same proportions as used in step (2), over the particle resulting from step (2) at a temperature of 1200"F and atmospheric pressure until the weight of the particle is substantially constant, the weight of the particle at this time being designated "Ws".

The weight fraction of reactive alumina in the solid particle, designated "Xa", is determined by the formula Xa = Ws - Wa x Molecular Wt. Alumina Wa 3 x MolecularWt. SulfurTrioxide Various known and commercially available cracking catalysts include a sufficient concentration of reactive alumina to be at least somewhat useful in carrying out some embodiments of the invention, particularly those catalysts which include a preponderance of alumina in their overall composition. On the other hand, many conventional aluminacontaining catalysts contain substantially no reactive alumina. Most, if not all, conventional catalysts include both silica and alumina.It is felt that the sub stantial absence of reactive alumina in many alumina-containing catalysts is the result of intimate combination of silica and alumina in the catalysts, especially cracking catalysts containing 50 weight percent or more silica, which tends to combine intimately with alumina in a manner that renders the alumina relatively inactive for reaction with sulfur oxides.

Catalysts containing a relatively large amount of alumina present as a discrete phase (free alumina) can be prepared by employing starting materials containing 50%-60% or more of alumina or an alumina precursor, as well as by forming catalyst from materials such as clays known to contain at least some discrete, free alumina. See for example, U.S. Patent No.4,166,787. A discrete alumina phase, including reactive alumina, can be added to a previously made catalyst by impregnation as described in U.S. Patent No.4,115,249.

When discrete, alumina-containing particles are mixed with the catalyst particles in order to supply reactive alumina, the alumina-containing particles are preferably substantially free from silica. Discrete alumina-containing particles may contain more than 50 ppm (weight) of reactive alumina. The alumina content of the particles is generally at least 60 weight percent and preferably at least 90 weight percent. A preferred form of alumina is gamma alumina. Alpha alumina is unsuitable, because of its low surface area and lack of reactivity.

According to one preferred embodiment of the invention, a sulfur dioxide combustion promoter is included in a dense-phase fluidized bed of the catalyst in an amount sufficient to provide substantial incremental conversion of sulfur dioxide to sulfur trioxide within the dense-phase fluidized bed. The prior art provides no incentive to include a combustion promoter of any kind in a regenerator in which substantially all carbon monoxide is already being burned in a dense-phase fluidized bed. For example, when a carbon monoxide combustion promoter, such as platinum, is employed, the art teaches the desirability of adding as small an amount of promoter as necessary to obtain complete carbon monoxide conversion. From the standpoint of the prior art, use of as little promoter as necessary is justified on both technical and economic grounds.Technically, addition of more hydrogenationldehydrogenation activity to a catalyst is widely taught to be undesirable because of the expected increase in coke and hydrogen formation in the cracking step. Economically, the art provides no justification for using more combustion promoter than necessary to accomplish the goal recognized by the art, i.e., complete burning of carbon monoxide, so that use of additional combustion promoterwould be merely an unjustified expense to the refiner.

Regenerator conditions sufficient to provide complete combustion of carbon monoxide in a cracking catalyst regenerator, whether or not burning is aided by a carbon monoxide combustion promoter, will normally result in some degree of combustion of sulfur dioxide to form sulfur trioxide. Since the total amount of sulfur dioxide formed in the regeneration zone is extremely small relative to the amount of carbon monoxide formed, it is reasonable to expect that regeneration conditions sufficient to cause burning of substantially all carbon monoxide in a regeneratorwould be inherently sufficient to burn most of the sulfur dioxide to form sulfur trioxide.

Contrary to the teachings qfthe art as to the effect of carbon monoxide combustion promoters, and contrary to expectations regarding the degree of sulfur dioxide combustion in a complete COcombustion system, I have found that inclusion of a sulfur dioxide combustion promoter in a regenerator, in conjunction with, and in addition to, regeneration conditions (with or without carbon monoxide combustion promoters) sufficient to achieve substantially complete combustion of carbon monoxide within the dense-phase bed, results in a striking enhancement of sulfur dioxide combustion.Moreover, when free alumina is included in the particulate solids in the regenerator, inclusion of the sulfur dioxide combustion promoter also results in a striking and unexpected decrease in the sulfur oxides content of flue gas removed from the catalyst regenerator, as compared to the same regeneration system without the sulfur dioxide combustion promoter.

A sulfur dioxide combustion promoter may be included in the particulate solids in the regenerator in an amount sufficient to provide a substantially increased conversion of sulfur dioxide to sulfur trioxide and a substantial decrease in the amount of sulfur oxides in flue gas leaving the regenerator, relative to the sulfur dioxide combustion flue gas and sulfur oxides concentration achieved with regeneration conditions (with or without use of a carbon monoxide combustion promoter) sufficient to provide complete carbon monoxide combustion in the dense-phase bed.A sufficient amount of the sulfur dioxide combustion promoter is preferably included in the regenerator to incrementally decrease the sulfur oxides concentration in the flue gas leaving the regeneration zone by at least 10 percent (calculated as SO2), relative to SO, removal in a conventional complete CO combustion mode of operation. Particularly preferably, a sufficient amount of the sulfur dioxide combustion promoter is included with the solids in the regenerator to decrease the sulfur oxides content of the flue gas leaving the regenerator by at least 50 percent.

Conventional sulfur dioxide oxidation promoters can be used. Most transition metals and metal com- pounds such as the oxides are to some extent active for promoting combustion (or oxidation) of sulfurdioxide to sulfur trioxide. Preferred sulfur dioxide combustion promoters are metals or compounds of metals selected from ruthenium, rhodium, palladium, iridium, platinum, copper, chromium, manganese and vanadium. Platinum is a particularly preferred sulfur dioxide combustion promoter.

The sulfur dioxide oxidation promoter may be present in a portion of the catalyst particles. It may also be present in essentially catalytically inert particles physically mixed with the catalyst particles. The SO2 combustion promoter may also be present in the bed in fixed or combined form such that it remains within the regeneration zone, rather than being introduced and withdrawn along with the catalyst particles and alumina particles.

Further according to the present invention, the amount of nitrogen oxides present in regeneration (flue) gas removed from the catalyst regeneration zone is decreased by maintaining the regeneration gas at a temperature above 760"C for at least 1 second. Preferably, the flue gas, after removal from the regenerator, is heated to a temperature of 800"C to 1050"C. Preferably, flue gas residence time in the heat treatment is maintained between 2 and 10 seconds.

The heating step may be performed by direct or indirect heat introduction. Preferably, heating is carried out by direct heat introduction, as by mixing a fuel (and free oxygen, if necessary) with the flue gas and combusting the fuel to increase the temperature of the flue gas to the desired level. Combustion may, for example, be carried out in a boiler of conventional design, by mixing the flue gas with fuel and air or by combusting fuel gas and air in a conventional burner and mixing the resulting hot mixture of combusted fuel and air with the flue gas.

The invention can best be further explained by way of example by reference to the preferred embodiment shown in the accompanying drawing.

The invention is preferably employed in a system for burning coke off spent FCC catalyst. The catalyst is preferably one containing 0.1 to 60 weight percent of a zeolite component and a matrix or binder containing silica and alumina, particularly preferably a matrix with more than 50 weight percent alumina, and having a substantial concentration of reactive alumina. A dense-phase fluidized bed of substantially coke-free (averaging less than 0.1 weight percent coke) FCC catalyst particles is maintained in the regeneration vessel 1. The particles in the fluidized bed have an average carbon concentration of less than 0.1 weight percent, so that the concentration of carbon on regenerated catalyst removed through the conduit5 is less than 0.1 weight percent.Spent catalyst containing about 0.5 weight percent coke is introduced into the bed through the conduit 3 and regenerated catalyst is withdrawn through the conduit 5 at essentially the same rate. Coke in the spent catalyst has a sulfur concentration of about 1.0 weight percent and a nitrogen concentration of about 1.0 weight percent. Regeneration conditions are first adjusted to provide essentially complete combustion of coke in the dense-phase fluidized bed and carbon monoxide in the regeneration gas, with flue gas leaving the regenerator at a temperature substantially below 735"C. Sufficient free oxygen is introduced into the bed through the distributor 11 to burn essentially all the coke off the spent catalyst and to maintain the average carbon content of catalyst in the bed at less than 0.1 weight percent, i.e., substantially coke-free catalyst.Essentially all the carbon monoxide generated in the bed 7 is burned within the dense-phase bed, so that the temperature difference between the gas in the bed 7 and the flue gas entering the cyclone 17 is less than 10 C and the flue gas removed from the regenerator through the conduit 21 contains less than 0.05 volume percent carbon monoxide. Sufficient excess free oxygen is introduced into the bed 7 so that the flue gas in the conduit 21 contains at least 2 volume percent free oxygen.A carbon monoxide combustion-promoting additive comprising 0.1 weight percent platinum on alumina particles is mixed with the catalyst particles in the bed 7 in an amount sufficient to provide substantially complete combustion of carbon monoxide within the dense-phase bed 7 and to maintain the maximum temperature of the regeneration gas in the dilute-phase region of the regenerator at below 27"C above the temperature of the regeneration gas in the bed 7. Preferably, the maximum temperature in the dilute-phase region is maintained within 10"C above or below the maximum temperature in the dense-phase bed 7. Preferably, the temperature in the bed 7 is maintained at about 677 C. Preferably, the temperature of the regeneration gas (flue gas) above the bed 7 is maintained below704 C, especially preferably below 687"C.Typically, the above conditions are maintained by a nominal catalytically active platinum concentration, based on total circulating solids inventory, of about 0.1-1 ppm (weight). Under these regeneration conditions, the concentration of sulfur oxides in the regeneration gas in the conduit 21 is found to be about 300 ppm (volume). The rate of flow of nitrogen oxides in the conduit 21 is found to be about 160 pounds per hour, calculated as NO2, equivalent to about 400 ppm (vol.) concentration. A sulfur dioxide combustion promoter is next added to the catalyst particles in the bed 7 in an amount sufficient to cause substantial conversion of sulfur dioxide to sulfur trioxide and to decrease substantially the concentration of sulfur oxides in the regeneration gas.A sufficient amount of a sulfur dioxide combustion promoter comprising 0.1 weight percent platinum on alumina is added to the catalyst in the bed 7 to maintain a nominal platinum concentration of 10 ppm (weight) in the solids in the bed 7. Spray-dried particles of alumina are also added to the fluidized bed in an amount sufficient to maintain 10 weight percent alumina in the bed. After addition of the sulfur dioxide combustion promoter and alumina particles, the sulfur oxides concentration in the flue gas in the conduit 21 is found to be only about 65 ppm (volume).

The rate of flow of nitrogen oxides in the flue gas in the conduit 21 is again measured and found to be about 200 pounds per hour calculated as NO2 (equivalent to about 500 ppm (vol.) concentration). The flue gas is then passed into a boiler and heated to a temperature of about 870"C by firing fuel gas. The flue gas is held at the elevated temperature in the boiler for about 5 seconds and then released. The rate of flow of nitrogen oxides in the gas in the conduit 21 is measured and found to be about 50 pounds per hour above that for the boiler operating without flue gas addition (equivalent to only about 125 ppm (vol.) concentration with respect to the flue gas withdrawn from the regenerator through the conduit 21).

A preferred embodiment of the invention having been described, a number of equivalents and mod ifications of the illustrated embodiment will be apparent to those skilled in the art. These alternatives are intended to be within the scope of the invention, as defined in the appended claims.

Claims (14)

1. A method of regenerating a particulate catalyst contaminated by sulfur- and nitrogen-containing coke deposition, which comprises: (a) passing a regeneration gas including free oxygen upwardly through a dense-phase fluidized bed of particulate solids comprising substantially coke-free particles of said catalyst in a regeneration zone, and removing said regeneration gas from said regeneration zone; (b) introducing said coke-contaminated catalyst particles into said bed and generating carbon oxides, nitrogen oxide and sulfur oxides in said bed by burning off said coke;; (c) burning sufficient carbon monoxide within said bed to maintain the regeneration gas above said bed at a maximum temperature below 735"C and not higherthan 27"C above the maximum temperature of the regeneration gas within said bed and to provide a carbon monoxide concentration of less than 0.5 volume percent in the regeneration gas removed from said regeneration zone, and introducing sufficient free oxygen into said bed to provide at least 1 volume percent free oxygen in the regeneration gas removed from said regeneration zone; (d) including in said particulate solids an amount of sulfur dioxide combustion promoter sufficient to provide substantial incremental conversion of sulfur dioxide to sulfur trioxide within said bed;; (e) including reactive alumina in said particulate solids, and decreasing the amount of sulfur oxides in the regeneration gas removed from said regeneration zone by forming a sulfur-containing and alumina-containing solid from said sulfur trioxide and said reactive alumina; and (f) recovering substantially coke-free particles of said catalyst from said fluidized bed and decreasing the amount of nitrogen oxides in the regeneration gas removed from said regeneration zone by maintaining the regeneration gas outside said regeneration zone at a temperature above 760"C for at least 1 second.

2. A method according to Claim 1, wherein the particulate solids in said bed contain an average carbon concentration of less than 0.1 weight percent.

3. A method according to Claim 1 or 2, wherein the regeneration gas removed from said regeneration zone includes less than 0.05 volume percent carbon monoxide.

4. A method according to Claim 1,2 or 3, wherein sufficient free oxygen is introduced into said bed to provide at least 2 volume percent free oxygen in the regeneration gas removed from said regeneration zone.

5. A method according to Claim 1,2,3 or 4, wherein a carbon monoxide combustion promoter comprising at least one metal or compound of a metal selected from ruthenium, rhodium, palladium, iridium, platinum, copper, chromium and manganese is included in said particulate solids in an amount sufficient to provide said temperature differential.

6. A method according to Claim 1,2,3, or4 5, wherein said sulfur dioxide combustion promoter comprises at least one metal or compound of a metal selected from ruthenium, rhodium, palladium, iridium, platinum, copper, chromium, manganese and vanadium.

7. A method according to any preceding claim, wherein the sulfur- and nitrogen containing coke contaminated catalyst particles are derived from the cracking of hydrocarbons with said catalyst particles in the absence of added hydrogen.

8. A method according to Claim 7, wherein said catalyst particles include a zeolitic crystalline alumino-silicate component.

9. A method according to any preceding claim, wherein not more than 5 weight percent of particles in said fluidized bed include a substantial concentration of said sulfur dioxide combustion promoter. -

10. A method according to any preceding claim, wherein said catalyst particles contain less than 50 weight percent silica, excluding silica included inany zeolitic crystalline aluminosilicate component present.

11. A method of burning sulfur- and nitrogencontaining coke off coke-containing particles of a catalyst, which comprises: (a) forming carbon monoxide, gaseous sulfur oxides and gaseous nitrogen oxides by reacting free oxygen with said coke in a regeneration zone; (b) decreasing the amount of said carbon monoxide by reacting said carbon monoxide with free oxygen in said regeneration zone in the presence of a carbon monoxide combustion promoter; (c) decreasing the amount of said gaseous sulfur oxides and forming a sulfur-containing solid in said regeneration zone by reacting said sulfur oxides with reactive alumina contained in a substantially silicafree particulate solid other than said catalyst present in said regeneration zone in physical admixture with said catalyst; and (d) withdrawing gas including said nitrogen oxides from said regeneration zone at a temperature below 735"C and decreasing the amount of said nitrogen oxides by maintaining said gas at a temperature above 760"C for at least one second outside said regeneration zone.

12. A method according to Claim 11, wherein said carbon monoxide combustion promoter is platinum.

13. A method according to Claim 11 or12, wherein said catalyst particles include a zeolitic crys- talline aluminosilicate component.

14. A method of regenerating a particulate catalyst contaminated by sulfur- and nitrogen-containing coke deposition, substantially as hereinbefore described with reference to the accompanying drawing.