GB2032947A

GB2032947A - Catalytic Cracking with Reduced Emissions of Sulfur Oxides

Info

Publication number: GB2032947A
Application number: GB7931381A
Authority: GB
Original assignee: Atlantic Richfield Co
Current assignee: Atlantic Richfield Co
Priority date: 1978-09-11
Filing date: 1979-09-10
Publication date: 1980-05-14
Also published as: GB2032947B; CA1154735A; FR2435518A1; FR2435518B3; DE2936718A1; DE2936718C2; IT1123113B; IT7925611A0

Abstract

Sulfur oxides are removed at least partially from the gaseous effluent from a catalyst regenerator of a fluid catalytic cracking unit operated with feedstock containing sulfur compounds by associating the sulfur oxides in the effluent gas with discrete particles of alumina having at least one rare earth compound, preferably cerium or a rare earth mixture rich in cerium, supported thereon, the particles then being returned, in admixture with regenerated catalyst, to the cracking zone, and the sulphur- containing particulate material reacting in the cracking zone to form hydrogen sulphide (which is removed from the cracking zone with the cracked fractions). The alumina particles may be a component of particles of a composite fluid cracking catalyst or separate fluidizable entities other than cracking catalyst and physically admixed with the catalyst particles.

Description

SPECIFICATION Catalytic Cracking With Reduced Emissions of Sulfur Oxides The invention is concerned with the catalytic cracking of sulfur-containing hydrocarbon feedstock in a manner such as to effect a significant decrease in the emission of noxious oxides of sulfur in the gases emitted from the regeneration zone of a fluid catalytic cracking (FCC) unit. Carbon monoxide emissions may also be reduced.

Considerable recent research effort has been directed to the reduction of sulfur oxide emissions in stack gases from the regenerators of cyclic FCC units. One technique that has been proposed involves circulating one or more metal oxides capable of associating with oxides of sulfur with the cracking catalyst inventory in the regeneration zone. When the particles containing associated oxides of sulfur are circulated to the reducing atmosphere of the cracking zone, the associated sulfur compounds are released as gaseous sulfurbearing material such as hydrogen sulfide which is discharged with the products from the cracking zone and are in a form readily handled in FCC units. The metal reactant is regenerated to an active form, and is capable of further associating with sulfur oxides when cycled to the regenerator.

Incorporation of Group II metal oxides on particles of cracking catalyst in such a process has been proposed (U.S. 3,835,031 to Bertolacini). In a related process described in U.S.

4,071,436 to Blanton et al, discrete fluidizable alumina-containing particles are circulated through the cracking and regenerator zones along with physically separate particles of the active zeolitic cracking catalyst. The alumina particles pick up oxides of sulfur in the regenerator, forming at least one solid compound, including both sulfur and aluminum atoms. The sulfur atoms are released as volatiles, including hydrogen sulfide, in the cracking unit. U.S.

4,071,436 further discloses that 0.1 to 10 weight percent MgO and/or 0.1 to 5 weight percent Cr203 are preferably present in the aluminacontaining particles. Chromium is used to promote coke burnoff. Similarly, a metallic component, either incorporated into catalyst particles or present on any one of a variety of "inert" supports, is exposed alternately to the oxidizing atmosphere of the regeneration zone of an FCCU and the reducing atmosphere of the cracking zone to reduce sulfur oxide emissions from regenerator gases in accordance with the teachings of Belgian Patents 849,635,849,636 and 849,637 (1977). In Belgian 849,637, a metallic oxidation promoter such as platinum is also present when carbon monoxide emissions are to be reduced. The aforementioned Belgian and related U.S.Patents (4,153,534 and 4,1 53,535) disclose nineteen different metallic components, including materials as diverse as alkaline earths, sodium, heavy metals and rare earth, as being suitable reactants for reducing emissions of oxides of sulfur. The metallic reactants that are especially preferred are sodium, magnesium, manganese and copper. When used as the carrier for the metallic reactant, the supports that are used preferably have a surface area at least 50 square meters per gram.

Examples of allegedly "inert" supports are silica, alumina and silica-alumina. The aforementioned patents further disclose that when certain metallic reactants (exemplified by oxides of iron, manganese or cerium) are employed to capture oxides of sulfur, such metallic components can be in the form of a finely divided fluidizable powder.

Many commercial zeolitic FCC catalysts contain up to 4% rare earth oxide, the rare earth being used to stabilize the zeolite and provide increased activity. It has been found that the mere presence of rare earth in a zeolitic cracking catalyst will not necessarily reduce SOx emissions to an appreciable extent.

Prior to the present invention, a versatile effective technique for using a metallic compound to pick up and then release SOx in a FCC unit without impairing the effectiveness of the active zeolitic cracking catalyst has not met general acceptance in refineries. Many of the proposed metallic reactants lose effectiveness when subjected to repeated cycling. Thus when Group II metal oxides are impregnated on FCC catalysts or various supports, the activity of the Group II metals is rapidly deactivated under the influence of steam. Discrete alumina particles, when combined with silica-containing catalyst particles and subjected to steam at elevated temperatures, e.g., those present in FCC unit regenerators, are of limited effectiveness in reducing SOx emissions.

Incorporation of sufficient chromium on an alumina support to improve SOx sorption results in undesirably increased coke and gas production.

It has been found that members of the allegedly "inert" supports for metallic reactants mentioned in the Belgian and related U.S. patents (supra) are not capable of stabilizing metallic compounds theoretically capable of picking up SOx in a regenerator and releasing sorbed sulfur in the cracking zone.

Accordingly, an object of the instant invention is the provision of improved means for reducing emissions of sulfur oxides, and optionally carbon monoxide, from FCC units by circulating a reagent for associating with SOx in a regenerator and disassociating sulfur compounds in the cracking unit reaction zone and/or the stripping zone.

This invention results from the discovery, unpredictable from the prior art, that the combination of rare earth, preferably cerium oxide, and certain forms of alumina function to reduce SOx emissions in FCC units and these materials, used in combination, maintain their effectiveness under conditions that render other proposed metallic SOx sorbents ineffective without impairing the yield of valuable hydrocarbon products achieved during hydrocarbon cracking.

The present invention involves the use of fluidizable attrition-resistant particles comprising at least one rare earth compound, preferably cerium, rare earth mixture rich in cerium, lanthanum or rare earth mixture rich in lanthanum, most preferably cerium or rare earth mixture rich in cerium, supported or deposited on discrete particles of alumina to reduce emissions of oxides of sulfur in the gaseous effluents of fluid catalytic cracking units operating with sulfurcontaining feedstocks. The particles associate at least partially with oxides of sulfur in the regenerator flue gas to form one or more solid sulfur-containing compounds and these compounds are released as hydrogen sulfide in the cracking zone of the unit.

The discrete particles of alumina used as a support for the rare earth compound may be present as a component of particles of a fluidizable acidic catalytic cracking catalyst or, preferably, the alumina supporting the rare earth compounds(s) may be present in fluidizable particles other than cracking catalyst particles and physically admixed with the cracking catalyst particles when the cracking catalyst particles are cycled through the cracking and regeneration zones of a fluid catalytic cracking unit. Most preferably the discrete particles of alumina supporting the rare earth compound are fluidizable particles of high purity alumina. A given amount of rare earth treated alumina is considerably more effective when the alumina is present in discrete entities other than particles of cracking catalyst as compared to discrete alumina present in catalyst particles.

At least a portion of the discrete alumina used as a support for the rare earth must be "free" alumina (present as a simple oxide), and in an "active" form. By "active" form is meant alumina capable in the absence of rare earth supported thereon of associating with oxides of sulfur in the gaseous regenerator effluent and releasing them as hydrogen sulfide in the cracking zone at least during initial cycling of the catalyst particles between the cracking zone, regeneration zone and recycle to the cracking zone of the fluid catalytic cracking unit. When all or most of the alumina is in combined form, for example present as a binary oxide such as silica-alumina or a ternary oxide, positive interaction between rare earth and alumina, resulting in improved reduction of SOx emissions, is not observed.Thus when the discrete alumina particles are a component of the cracking catalyst particles, the particles of equilibrium cracking catalyst must be capable in the absence of deposited rare earth of associating with at least a portion of oxides of sulfur in the regeneration zone and releasing them as hydrogen sulfide in the regenerator zone. The term "equilibrium" refers to the state or condition of the catalyst particles after repeated regeneration and reuse in cracking.Similarly, when the rare earth-treated alumina is present as discrete entities other than cracking catalyst and such entities are in physical admixture with cracking catalyst, the equilibrium aluminacontaining entities are capable, in the absence of rare earth deposited thereon, of decreasing the amounts of oxides of sulfur present in the flue gas as compared to the levels of oxides of sulfur that would be produced in the absence of the aluminacontaining entities. Discrete entities of silicaalumina wherein all of the alumina is in combined form will not suffice as a support for rare earth and entities wherein most of the alumina is in combined form will be inefficient as compared to entities composed solely or predominantly of free high purity alumina.

In practice of the invention, the rare earth compound(s) must be supported on the alumina per se although one or more rare earth materials may be present with one or more constituents of the solid entities, of which the discrete alumina may be a component. For example, many presentday commercial composite zeolitic cracking catalyst contain rare earth such as cerium or a rare earth mixture associated with the zeolite component as a result of ion-exchange with cations originally associated with the zeolite.

When such composite catalysts also contain discrete free alumina as a matrix component, the rare earth will normally not be supported or deposited to an appreciable extent on the discrete alumina in the matrix when conventional ionexchange techniques are practiced to prepare the catalyst particles. For the most part, the rare earth will be present with the zeolitic component and, in this state or condition, the rare earth will not synergistically act with the alumina in the same catalyst particles to reduce sulfur oxide emissions as it will when the rare earth is supported on the alumina particles.Therefore, unless exchange with rare earth is carried out under conditions such that additional rare earth is deposited on at least a portion of the discrete alumina particles in the matrix of composite catalyst particles, it will be necessary to deposit rare earth on discrete alumina components on such catalyst particles by additional processing. This may be accomplished, for example, by impregnating the finished catalyst particles with rare earth or by impregnating the alumina component prior to incorporation of the alumina particles into the catalyst matrix.

In one preferred embodiment of the invention, fluidizable particles of cerium supported or deposited on alumina are used in a continuous cyclic FCC process wherein the regenerator operates in a partially oxidizing mode, i.e., in a regenerator where excessive heat due to complete CO oxidation cannot be tolerated and the regenerator operates at about 1100 to 1 2500F.

One advantage of the rare earth supported on alumina particles in accordance with the present invention is the capability of substantial SOx pickup without operating the FCC unit in full oxidizing, e.g., high temperature, mode.

However, the present particles of rare earth deposited on alumina are also advantageously used in a continuous cyclic FCC process wherein the regenerator operates in a complete or substantially complete oxidizing mode. Optionally, a strong carbon monoxide oxidation promoter (e.g., platinum), is circulated with catalyst particles and the rare earth supported alumina particles in practice of this embodiment. The oxidation promoter can be added to the catalyst particles or to the hydrocarbon feedstream or regenerator as a liquid. Alternatively the oxidation promoter can be used as discrete entities on a solid support which is not necessarily the same alumina support on which the rare earth is present.

As noted previously, the present invention involves the use, as a support for rare earth, of alumina that is preferably substantially free from oxides of other metals, such as silica and alkali metals, e.g., sodium. Contrary to the prior art teaching of equivalence between silica-alumina, silica and alumina as supports for metallic oxide reactants for oxides of sulfur, high purity alumina is advantageously suited for the purposes of the present invention. When other so-called "inert" particles, such as silica or silica-alumina, are used as the support for the rare earth, the discrete fluidizable entities are effective in some cases in reducing SOx emissions when the supported particles are initially used (fresh condition).

However, the presence of appreciable silica in association with alumina does not provide a stable rare earth oxide supported catalyst and the material is of limited, if any, practical use in a cyclic FCC process in which such particles are repeatedly recycled with catalyst inventory and subjected to the action of steam; e.g., in the stripping and regeneration zones. Similarly, when about 10% cerium oxide was supported on a widely used zeolitic cracking catalyst, containing substantially no free alumina, the cerium was effective in the fresh condition but after hydrothermal treatment, which simulated conditions found in commercial units, the catalyst had virtually no effect on SOx reduction.However, cerium was quite effective when impregnated on another zeolitic cracking catalyst that contained appreciable free gamma-alumina even after the cerium impregnated catalyst underwent severe hydrothermal treatment.

A correlatice feature of the present invention resides in the use of rare earth compound as a deposited agent, e.g., impregnant, on the aforesaid alumina. Cerium oxide (or cerium rich rare earth oxide) supported on alumina is significantly more effective for removal of sulfur oxides than is either pure alumina or magnesia or a variety of other metal oxides on pure alumina.

Widen chromium oxide supported on pure alumina was used for SOx emissions, results of testing indicated that there was an undesirable increase in the coke producing tendencies of the zeolitic cracking catalyst. This problem was not experienced when cerium or cerium-rich rare earth mixture was supported on high purity alumina. In addition, the use of particulate cerium (or cerium rich rare earth) on alumina is effective in reducing carbon monoxide emissions in a controlled manner from FCC units without requiring the use of an expensive platinum group metal oxidation promoter.

In practice of the especially preferred embodiment of the invention, the alumina support material is in the form of fluidizable attrition resistant particles having a tapped density similar to that of typical FCC catalysts (for example, about 0.7 to 1.0 g./cc. and preferably about 0.80 to 1.0 g./cc.). The alumina should have a surface area of at least about 10 m2/g., preferably at least about 50 m2/g. (determined by the well-known B.E.T. method using nitrogen as absorbate). The alumina in the support can be gamma, eta, chi, delta, alpha, kappa, theta and the like and mixtures thereof. The alumina preferably contains less than about 1% by weight (anhydrous basis) of silica and less than about 0.5% by weight of alkali metal oxides.

Cerium or other suitable rare earth or rare earth mixture may be deposited on the alumina by contacting the alumina particles with a solution, preferably aqueous, of rare earth; for example, a solution containing cerium ions (preferably Ce+3, Ce+4 or mixtures thereof) or a mixture of rare earth cations containing a substantial amount (for example, at least 40%) of cerium ions. Watersoluble sources of rare earth include the nitrate and chloride. Solutions having a concentration (weight) in the range of 3 to 30% are useful.

Sufficient rare earth salt is added to incorporate from about 0.05 to 20% (wt.), preferably about 0.1 to 10% rare earth, and most preferably about 0.5 to 10% rare earth, by weight, calculated as oxide, on the particles.

It is not necessary to wash alumina particles after certain soluble rare earth salts (such as nitrate or acetate) are added. After impregnation with rare earth salt, the particles can be dried and calcined to decompose the salt, forming an oxide in the case of nitrate or acetate. Alternatively the particles can be charged to a FCC unit with the rare earth in salt form. In this case a rare earth salt with a thermally decomposable anion can decompose to the oxide in the reactor and be available to associate with SOx in the regenerator.

Especially good results were achieved using particles of alumina impregnated with 7% cerium oxide, by weight, calculated as CeO2 and mixed with commercial zeolitic FCC catalyst in amount such that the cerium impregnated alumina particles were present in amount in the range of about 5 to 25% by weight of the mixture.

When too much rare earth such as cerium is present and/or an excessive quantity of the rare earth supported particles are circulated with catalyst inventory, the cracking properties of the overall catalyst system may be adversely affected.

For example, the catalyst may produce too much coke and/or gases. On the other hand, when insufficient rare earth such as cerium is employed and/or an insufficient quantity of supported particles are used, the reduction in levels of SOx emissions may be inadequate. In general, high rare earth concentrations are indicated when the feedstocks are highly contaminated with sulfur.

In practice of the invention, the rare earth treated alumina particles are circulated as discrete entities with the catalyst inventory at a rate which can be flexibly altered to reduce SOx emissions to a desired level. Preferably the rare earth supported alumina particles are employed in an amount within the range of about 0.5 to 25%, more preferably about 3 to 20%, based on the weight of the particles of cracking catalyst in the circulating cracking catalyst inventory. Preferably the circulating inventory (cracking catalyst particles and rare earth supported alumina) will analyze rare earth (as oxide) in amount within the range of about 0.002 to 4.0%, more preferably about 0.01 to 1.0% in addition to rare earth ionexchanged onto particles of cracking catalyst, based on the total weight of the mixture.The rare earth supported alumina particles can be steamed or calcined before they are introduced into the reaction system. However, steaming or calcining is not necessary.

When the rare earth is to be deposited on particles of cracking catalyst, preferably a zeolitic catalyst, the catalyst must contain appreciable alumina present as a simple oxide; for example, gamma alumina or other transitional forms of alumina enumerated above. The catalyst should contain at least 5%, preferably at least 10% and most preferably at least 20% by weight free alumina. Alumina may be incorporated into the matrix as discrete particles during catalyst manufacture; for example by incorporating finely divided gamma alumina with the zeolite and matrix components before particle formation (e.g., spray drying). Alternatively alumina may be formed during catalyst manufacture; an example is alumina formed in carrying out the process described in U.S. 3,647,718 to Haden et al.

Rare earth salt, preferably cerium, rare earth rich in cerium (e.g., 40% or more cerium), lanthanum, or rare earth rich in lanthanum (e.g., 40% or more lanthanum) is deposited on catalyst particles containing the alumina in amount sufficient to deposit from about 1 to 1 5% (wt.), preferably 2 to 10% rare earth, and most preferably 5 to 10% on the catalyst particles.

Methods used for impregnating alumina entities other than catalyst particles described above can be used to deposit rare earth on catalyst particles.

The invention is particularly useful in any of the many catalytic cracking unit designs used in the fluid catalytic cracking of petroleum feedstock such as gas-oils and heavier stocks containing sulfur, preferably about 0.1 to 5%, more typically about 0.5 to 2.5% by weight of sulfur. Preferably, substantially no hydrogen is added to the reactor in these processes.

Exemplary of a useful reactor-regenerator system that can be employed in practice of our invention is the riser reactor illustrated in U.S.

3,944,482 to Mitchell.

The operating conditions of FCC unit regeneration zones are conventional and well known in that art. Regeneration zone temperatures may be in the range, for example, of about 1000 to about 1 6000 F., preferably about 11 000F., to about 1 5000F.

The following examples are given for illustrative purposes.

Example I In accordance with the present invention, the following procedure was used to produce an attrition-resistant, high bulk density gamma and alumina support. Catapal SB (alpha alumina monohydrate) was used as the starting material.

Catapal SB was dispersed by addition to a dilute nitric acid solution to form a viscous slurry which was spray dried to atomize the slurry. The spray dried beads were classified and a fraction in the range of 45 to 1 50 microns was recovered and calcined at 11 000F. for two hours in air.

Portions of the calcined alumina particles were impregnated with cerium nitrate (Ce(NO3)3) solutions of 4.8 to 22% concentration (weight) to deposit from 2 to 10% by weight cerium (reported as CeO2) on the particles.

The impregnated particles were then calcined in air at 11 000F. to eliminate oxides of nitrogen.

The procedure was repeated with a commercial lanthanum-rich rare earth nitrate solution for purposes of comparison. The mixed rare earth was reported to contain 60% La203, 6% CeO2, 8% PreOi1, 25%Nd203and1%others.

SOx pickup was measured by a fixed fluidized test unit in which 10-14 grams of test sample were contacted at 12000 F. with 200 ccimin. of a gas mixture typically consisting of 2000 p.p.m.

SO2, 4.5% CO, 6.5% CO2 and 3.0% 02 in N2 for twelve minutes. This gas mixture was selected to resemble a typical FCC regenerator environment.

Flue gas composition was monitored continuously for CO, CO2 and SO, by individual IR cells and for O, by an oxygen analyzer. Rate as well as capacity for SOx absorption could thus be evaluated. The percent reduction of SO, indicated by IR was also compared to percent SOx picked up determined by an analytical method (LECO).

All samples were steamed for four hours at 14000 F., prior to testing to simulate an equilibrium condition.

For purposes of comparison the SOx capacity of HFZ-20 zeolitic cracking catalyst was measured. This high alumina catalyst was selected because of its higher capacity for reducing SOx emissions compared to other commercial cracking catalysts. For further purposes of comparison, sample of the spray dried alumina were impregnated, as described above, with aqueous solutions of other metal salts, including salts of chromium and magnesium, followed by calcination at 11 000F.

For further purposes of comparison, an unimpregnated samples of the gamma alumina support (prepared from Catapal) was tested.

Results of the SOx screening test show that rare earth oxides, particularly CeO2, supported on discrete particles of gamma alumina, when mixed with HFZ catalyst exhibited excellent SOx pickup.

(CrO3 supported on alumina similarly showed good SOx pickup.) However, when admixed with HFZ-20 at a 14% level, up to 7.3% MgO supported on alumina, which was described in the prior art, showed no increase in SOx pickup as compared with a pure gamma alumina control sample run under identical conditions.

Samples of alumina containing metal additives described above were evaluated for their effects on catalytic cracking. The testing was carried out using representative gas-oil feedstock in a laboratory unit (MAT) with HFZ-20 catalyst and various amounts of alumina supported metal additives found to be effective for SOx pickup. It was found that while CrO3 supported on alumina had little effect on weight percent conversion of feedstock, gasoline, LCO and gas (i.e., C4 minus) when compared to an appropriate reference only calcined alumina and CeO2 and CeO2-containing rare earth oxide mixture on gamma alumina had minimal effect on the coke-producing tendency of HFZ catalyst as seen in the weight percent coke and even more dramatically in the H2/CH4 mole ratio values.

It is important to note that, unlike CrO3 on alumina, which the prior art describes for the promotion of coke burnoff, up to 8% CeO2 on alumina did not significantly increase the cokeproducing tendencies of HFZ catalysts.

Further studies indicated that the addition of 8.2% CeO2 to alumina particles significantly increased SOx pickup over that for the pure alumina additive, i.e., 64% for 1 gram of 8.2% Ce0#y-Al203 versus 14% for 1 gram of y-AI203 added to 12 grams of cracking catalyst. It was also observed that the level of rare earth oxides, as well as the amount of cerium in the rare earth mixture supported on the alumina, affected both the SOx reduction capability and the degree of CO oxidation of the additive, i.e., increased amount of CeO2 increases both SOx reduction and CO oxidation.

It was further found that the increased SOx reduction ability of rare earth oxide(s) on alumina was not due to a stabilization of the fresh alumina as described in U.S. 3,993,573. Under the hydrothermal deactivation conditions used,5.4% CeO2 on y-Al203 having a surface area less than that of steamed pure alumina, was found to be much more active towards SOx pickup than an equivalent amount of fresh (unsteamed) alumina.

Other tests were carried out to determine the effect of using cerium supported alumina in physical mixture with a solid platinum oxidation promoter. The promoter was produced by impregnating a solution of chloroplatinic acid on fluidizable particles of a silica-alumina support having a B.E.T. surface area of about 12 m2/g.

These particles contained 60 p.p.m. platinum expressed as the metal. Data on the relative effectiveness of CeO2 supported on alumina and alumina plus a CO oxidation promoter (platinum) showed that both systems were substantially more active for SOx reduction than was a pure alumina additive. The data also showed that 5.4% CeO2 on alumina was more active for SOx reduction than an equivalent amount of alumina plus particles of the supported platinum CO oxidation promoter. In addition of a platinum CO oxidation promoter to CeO2 on alumina further increased its SOx pickup. Such an increase could be important in meeting environmental control specification.

Other tests showed that 6% cerium oxide supported on the gamma alumina particles did not lose much of its SOx pickup ability when temperature was raised from 1100 to 1 300C F.

This is very important since this additive can therefore be used under a wide range of regenerator conditions without loss of effectiveness. In contrast the ability of pure alumina to pick up SOx was significantly less at 1 3000F. than at 11000F.

Example II This example illustrates practice of the embodiment of the invention wherein the alumina support for rare earth is a component of particles of a fluidizable cracking catalyst (HFZ & 33 catalyst). HFZ-33 catalyst was selected because it is unusually high in content of transitional alumina (gamma) compared to other commercial cracking catalysts.

While the catalyst analyzes about 60% Al2O3, part of the alumina is in the zeolite and thus not present in free form. X-ray analysis, using conventional procedures, showed that the catalyst contained approximately 27% zeolite.

Examination of the X-ray diffraction pattern of HFZ-33 catalyst indicated that the catalyst contained about 28% by weight gamma alumina.

(Representative samples of those other commercial FCC catalysts analyzed 0.2%, 2.8% and 7.5% gamma alumina by the X-ray technique.) In accordance with this invention HFZ-33 was modified by impregnation with a cerium nitrate solution and calcined in the presence of oxygen to deposit about 6% cerium oxide, calculated as CeO2, on the catalyst particles. In similar manner, about 8% cerium oxide was deposited on another sample of HFZ-33. The procedure was repeated with a commercial lanthanum-rich rare earth nitrate solution for purposes of comparison. The mixed rare earth was reported to contain 60% La203, 6% CeO2, 8% Pr8O11, 25% Nd2O3 and 1% others. As mentioned, HFZ-33 as supplied does not contain rare earth.

The impregnation of dry HFZ-33 was carried out by contacting the catalyst particles with cerium nitrate or mixed rare earth nitrate solutions, adding 0.63 ml. of solution per gram of HFZ-33. The impregnated support was dried at about 2000 F. and then calcined at 11 000F. and in the presence of oxygen.

As a control, HFZ-33 without rare earth or cerium addition was tested.

SOx absorption was measured by the fixed fluidized test unit described in Example I. Test sample was contacted with 200 ccimin. of a gas mixture consisting of 2300 p.p.m. SO2,2.5% CO, 3.6% CO2 and 2.0% 02 in N2 for twelve minutes at 12000 F.

All samples were steamed for four hours at 14000F. prior to testing to simulate an equilibrium condition.

It was found that the presence of rare earth improved SOx removal by the HFZ catalyst and that pure cerium was superior to mixed rare earth at all levels of addition. Therefore the use of pure cerium or cerium-rich rare earth mixtures is preferred in the preparation of modified FCC catalysts that can provide lowered SOx emissions.

The data show that 7.41% CeO2 was markedly superior to 5.93% in terms of SOx pickup.

A more thorough evaluation of both SOx pickup and regeneration was carried out on a sample of HFZ-20 catalyst impregnated with 8.7% CeO2 in the manner described above.

Significant reduction of SOx emissions resulted when the sample was steamed and then circulated through a laboratory FCC unit including a reactor and regenerator. This unit was described in a paper titled "Laboratory Circulating Fluid Bed Unit for Evaluating Carbon Effects on Cracking Catalyst Selectivity" presented by S. J. Wachtal et al at the American Chemical Society meeting of September 12-17,1971.

Example III Similar tests were carried out with a commercial product, HEZ#5 5 cracking catalyst.

This catalyst has a chemical composition similar to that of HFZ-33 but HEZ-55 contains about 2% rare earth, whereas HFZ-33 is free of rare earth.

Although HEZ-55 contains about the same amount of gamma alumina as HFZ-33 and also contains rare earth introduced during catalyst manufacture, the two catalysts exhibited comparable capacity to associate with SOx under regeneration conditions and release the SOx as H2S under cracking conditions. This finding shows that the mere presence of both rare earth and gamma alumina in FCC catalyst particles will not in itself result in a cracking catalyst having outstanding ability to associate with oxides of sulfur in a FCC regenerator and release them as H25 in the cracking zone. Thus, it was necessary to have a sufficient quantity of rare earth deposited on the alumina in the particles of cracking catalyst, a result not necessarily achieved when rare earth is introduced during catalyst manufacture.

Example IV Other tests were conducted with CBZ & catalyst, a commercial zeolitic catalyst containing rare earth (above 3% by weight) and less than 1% gamma alumina as determined by X-ray diffraction. When cerium was impregnated on this catalyst, the fresh catalyst was considerably more effective than the unimpregnated catalyst in reducing SOx emissions. However, when the impregnated catalyst was steamed, the improvements achieved by impregnation with cerium were negated. This indicates that the combination of rare earth and alumina was necessary.

When CBZ-1 was mixed with 5 to 23% by weight of separate entities containing CeO2 supported on gamma alumina (7% CeO2) and prepared as described in Example I, there was outstanding reduction of SOx at 1100 and 1 3000F. when the mixture was steamed and then circulated through a laboratory FCC unit including a reactor and regenerator. However when unimpregnated gamma alumina was mixed with the catalyst and the mixture steamed and evaluated in the circulating unit, it was found that alumina was significantly deactivated and did not exhibit the outstanding ability to reduce SOx emissions achieved when cerium was supported on the alumina entities circulated with the catalyst particles.

From analysis of CO2/CO ratios in the flue gas it was also observed that the zeolitic catalyst particles could be circulated with the cerium supported alumina particles to reduce SOx emissions without causing excessive temperature increases in the regenerator.

This invention has been described with respect to specific embodiments and examples. It will be understood that the invention is not limited thereto and that it can be variously practiced within the scope of the following claims.

Claims

1. A process for continuous cyclic fluid catalytic cracking which comprises (a) contacting a hydrocarbon feedstock containing sulfur with a circulating inventory of particles of cracking catalyst in a reaction zone under hydrocarbon cracking conditions to produce lower boiling hydrocarbons, and to cause deposition on said catalyst particles of solid deactivating sulfurcontaining carbonaceous material; (b) removing said catalyst particles containing said deposited materials from the reaction zone; (c) regenerating said particles by oxidation at elevated temperature in a regeneration zone to combust at least a portion of said sulfur-containing carbonaceous deposit material, producing a gaseous effluent from said regeneration zone; (d) removing regenerated catalyst particles from said regeneration zone and recycling at least a portion of said catalyst particles to the reaction zone, and (e) reducing emissions of oxides of sulfur in said gaseous regeneration zone effluent by cycling with said cracking catalyst particles discrete particles of alumina other than said cracking catalyst particles, said alumina being present as a simple oxide and being capable of associating with sulfur oxides in said regeneration zone and of disassociating with said associated sulfur components in said reaction zone when first subjected to steps (a), (b) and (c), at least a portion of said discrete particles of alumina having supported thereon at least one rare earth compound in amount sufficient to enhance the reduction of emissions of sulfur oxide in said gaseous regeneration zone effluent.

2. The process of claim 1 carried out in the absence of a precious metal combustion promoter.

3. The process of claim 1 carried out in the presence of a precious metal combustion promoter.

4. The process of any one of claims 1 to 3 wherein said rare earth is selected from cerium, rare earth mixtures rich in cerium, lanthanum and rare earth mixtures rich in lanthanum.

5. The process of any one of claims 1 to 4 wherein said rare earth compound is present in amount in the range of about 0.5 to 25% by weight, based on the weight of said discrete particles of alumina.

6. The process of any one of claims 1 to 5 wherein said rare earth compound, expressed as the oxide, is present in amount in the range of about 2 to 10% by weight, based on the weight of said discrete particles of alumina.

7: The process of any one of claims 1 to 6 wherein said rare earth compound comprises a cerium salt and said discrete particles of alumina are present as a component of only a portion of particles of said cracking catalyst.

8. The process of any one of claims 1 to 6 wherein said rare earth compound comprises a cerium salt and said discrete particles of alumina are present as a component of only a portion of particles of a cracking catalyst also containing a zeolitic molecular sieve component.

9. The process of claim 8 wherein said discrete particles of alumina are present in said portion of particles of cracking catalyst that contain said alumina in amount of at least about 5% by weight as determined by X-ray diffraction analysis, said rare earth compound being present as a deposit on said portion of cracking catalyst particles in amount, expressed as the oxide, within the range of about 1 to 15% based on the anhydrous weight of said portion of cracking catalyst particles.

10. A process for continuous cyclic fluid catalytic cracking which comprises (a) contacting a hydocarbon feedstock containing sulfur with a circulating inventory of particles of cracking catalyst in a reaction zone under hydrocarbon cracking conditions to produce lower boiling hydrocarbons, and to cause deposition on said catalyst particles of solid deactivating sulfur containing carbonaceous material; (b) removing said catalyst particles containing said deposit materials from the reaction zone; (c) regenerating said particles by oxidation at elevated temperature in a regeneration zone to combust at least a portion of said sulfur-containing carbonaceous deposit material, producing a gaseous regeneration zone effluent containing at least one oxide of sulfur (d) removing regenerated catalyst particles from said regeneration zone and recycling at least a portion of said catalyst particles to the reaction zone, and (e) reducing emissions of oxides of sulfur in said gaseous regeneration zone effluent by cycling said cracking catalyst through said zones in physical admixture with fluidizable entities other than said particles of cracking catalyst, said fluidizable entities comprising alumina having a surface area of at least 10 m2/g., said entities being capable of associating with sulfur oxides in said regeneration zone and of disassociating with said associated sulfur components in said reaction zone and being present in amount sufficient to reduce emissions of sulfur oxide in said gaseous regeneration zone effluent when first subjected to steps (a), (b) and (c), at least a portion of said discrete fluidizable alumina entities having supported thereon a minor amount of at least one rare earth component, said amount of rare earth component being sufficient to enhance the reduction of sulfur oxides in said gaseous regeneration zone effluent.

11. The process of claim 10 carried out in the absence of a precious metal combustion promoter.

12. The process of claim 10 carried out in the presence of a precious metal combustion promoter.

13. The process of any one of claims 10 to 12 wherein said rare earth comprises cerium and said entities contain cerium expressed as oxide in amount in the range of about 0.1 to 25% by weight.

14. The process of claim 13 wherein said entities contain cerium, expressed as the oxide, in amount in the range of about 2 to 10% by weight and alumina in said entities is substantially free from silica.

15. The process of claim 13 or claim 14 wherein said entities containing cerium are circulated with said catalyst inventory in amount within the range of about 0.5 to 20% based on the weight of the cracking catalyst particles.

16. The process of claim 1 5 wherein said entities containing cerium are circulated with said catalyst inventory in amount within the range of about 3 to 10% based on the weight of the cracking catalyst particles.

17. The process of any one of claims 13 to 1 6 wherein said entities containing cerium have a density of at least about 0.7 g./cc.

18. A process for continuous cyclic fluid catalytic cracking which comprises (a) contacting a hydrocarbon feedstock containing sulfur with a circulating inventory of particles of cracking catalyst in the substantial absence of added hydrogen in a reaction zone under cracking conditions to produce lower boiling hydrocarbons and to cause deposition on said catalyst particles of solid deactivating sulfur-containing carbonaceous material; (b) removing said catalyst particles containing said deposit from the reaction zone; (c) regenerating said particles by oxidation at elevated temperature in a regeneration zone to combust at least a portion of said sulfurcontaining carbonaceous deposit, producing a gaseous regeneration zone effluent containing at least one oxide of sulfur; (d) removing regenerated catalyst particles from said regeneration zone and recycling them to the reaction zone, and (e) reducing emissions of oxides of sulfur in said gaseous regeneration zone effluent by cycling said cracking catalyst through said zones in physical admixture with discrete fluidizable attrition-resistant particles of high purity anhydrous alumina having a surface area of at least about 50 m2/g., said alumina particles being capable of associating with sulfur oxides in said regeneration zone to form a nongaseous reaction product and releasing associated sulfur oxides as hydrogen sulfide in the cracking zone and being present in amount sufficient to reduce emissions of sulfur oxide in said gaseous regeneration zone effluent when first subjected to steps (a), (b) and (c), and wherein in place of at least a portion of said alumina particles of high purity there is used anhydrous alumina having a density in the range of about 0.8 to 1.0 g./cc. and containing less than about 1% by weight silica and less than about 0.5% by weight alkali metal oxide, said anhydrous alumina particles having impregnated thereon a compound of cerium or a compound of rare earth mixture rich in cerium, said impregnated particles containing from 2 to 10% by weight of cerium oxide and being capable of enhancing the reduction of oxides of sulfur in said regeneration zone effluent.

19. The process of claim 18 carried out in the absence of a precious metal combustion promoter.

20. The process of claim 18 carried out in the presence of a precious metal combustion promoter.

21. The process of any one of claims 18 to 20 wherein said entities containing cerium are present in amount such that the circulating inventory contains about 0.002 to 4% cerium oxide, weight basis, in addition to cerium ionexchanged onto cracking catalyst particles.

22. The process of claim 21 wherein said entities containing cerium are present in amount such that the circulating inventory contains about 0.1 to 1.0% cerium oxide, weight basis, in addition to cerium ion-exchanged onto cracking catalyst particles.

23. The process of any one of claims 18 to 22 wherein said entities containing cerium are circulated with said catalyst inventory in amount within the range of 5 to 10% based on the weight of the cracking catalyst particles.

24. The process of any one of claims 18 to 23 wherein said entities contain cerium, expressed as oxide, in amount of about 7% by weight and are present in amount in the range of about 5 to 25% by weight of the mixture with said cracking catalyst particles.

25. The process of any one of claims 18 to 24 wherein said high purity alumina particles are composed of gamma alumina.

26. The process of claim 25 wherein said high purity alumina particles are produced by slurrying alpha alumina monohydrate in an acid solution, spray drying the slurry and calcining the resulting spray dried product to convert the alumina to gamma form.

27. A process of continuous cyclic fluid catalytic cracking with reduced emission of sulfur oxides from regenerator gaseous effluent which comprises (a) contacting a hydrocarbon feedstock containing organic sulfur compounds in the absence of added hydrogen with a circulating inventory of cracking catalyst particles comprising a crystalline zeolitic aluminosilicate and an inorganic oxide matrix and separate discrete particles of cerium or cerium-rich rare earth uniformly impregnated in amount from about 5 to 10% by weight expressed as cerium oxide on fluidizable particles of high purity alumina having a surface area of at least 50 m2/g. and a density in the range of 0.8 to 1.0 g/cc. in a reaction zone in a riser cracker at about 900 to 11 000F. to produce lower boiling hydrocarbons and to cause deposition on said catalyst particles of solid deactivating sulfur-containing carbonaceous material; (b) removing said catalyst particles and said particles of cerium impregnated on alumina containing said deposit from the reaction zone and stripping volatiles from said particles with steam in a stripping zone: (c) regenerating said stripped particles by oxidation at elevated temperature in a regeneration zone at a temperature in the range of about 1100 to 13000 F. with combustion of carbon monoxide to carbon dioxide to burn off residual sulfurcontaining carbonaceous deposit, producing a gaseous regeneration zone effluent containing oxides of sulfur and (d) removing regenerated catalyst particles and cerium impregnated alumina particles from said regeneration zone and recycling them to the reaction zone, whereby emissions of oxides of sulfur in said gaseous regeneration zone effluent are reduced substantially as a result of association of oxides of sulfur with cerium impregnated alumina during step (c) and disassociation of associated oxides of sulfur as hydrogen sulfide during steps (a) and (b).

28. The process of claim 27 wherein a platinum-containing oxidation promoter is present.

29. The process of any one of claims 1 to 9 wherein said discrete particles of alumina are composed of gamma alumina.

30. A process as claimed in claim 1, claim 10, claim 18 or claim 27, substantially as hereinbefore described with particular reference to the Examples.

31. A process as claimed in claim 1, claim 10, claim 18 or claim 27, substantially as illustrated in any one of the Examples.

32. Catalytically cracked hydrocarbon feedstock obtained by the process claimed in any one of the preceding claims.