FIELD OF THE INVENTION
This application is based on U.S. Provisional Application Ser. No. 60/505,805, filed Sep. 25, 2003.
- BACKGROUND OF THE INVENTION
This invention relates to regeneration of spent catalyst in a fluid catalytic cracking (FCC) process and the reduction of NOx formed during regeneration of the cracking catalyst in the presence of a carbon monoxide combustion promoter.
Catalytic cracking of heavy petroleum fractions is one of the major refining operations employed in the conversion of crude petroleum oils to useful products such as the fuels utilized by internal combustion engines. In fluidized catalytic cracking processes, high molecular weight hydrocarbon liquids and vapors are contacted with hot, finely-divided, solid catalyst particles, either in a fluidized bed reactor or in an elongated transfer line reactor, and maintained at an elevated temperature in a fluidized or dispersed state for a period of time sufficient to effect the desired degree of cracking to lower molecular weight hydrocarbons of the kind typically present in motor gasoline and distillate fuels.
In the catalytic cracking of hydrocarbons, some non-volatile carbonaceous material or coke is deposited on the catalyst particles. Coke comprises highly condensed aromatic hydrocarbons and generally contains from about 4 to about 10 weight percent hydrogen. When the hydrocarbon feedstock contains organic sulfur and nitrogen compounds, the coke also contains sulfur and nitrogen species. As coke accumulates on the cracking catalyst, the activity of the catalyst for cracking and the selectivity of the catalyst for producing gasoline-blending stocks diminishes.
Catalyst which has become substantially deactivated through the deposit of coke is continuously withdrawn from the reaction zone. This deactivated catalyst is conveyed to a stripping zone where volatile deposits are removed with an inert gas at elevated temperatures. The catalyst particles are then reactivated to essentially their original capabilities by substantial removal of the coke deposits in a suitable regeneration process. Regenerated catalyst is then continuously returned to the reaction zone to repeat the cycle.
Catalyst regeneration is accomplished by burning the coke deposits from the catalyst surfaces with an oxygen containing gas such as air in a regenerator separate from the fluidized reactor used in catalytic cracking. In the catalyst regenerator, the coke burns off, restoring catalyst activity and heating the catalyst to, e.g., 500-900° C., usually 600-750° C. Flue gas formed by burning coke in the regenerator may be treated to remove particulates and convert carbon monoxide, after which the flue gas is normally discharged into the atmosphere.
The removal of carbon monoxide from the waste gas produced during the regeneration of deactivated cracking catalyst can be accomplished by conversion of the carbon monoxide to carbon dioxide in the regenerator or carbon monoxide boiler after separation of the regeneration zone effluent gas from the catalyst. When sulfur and nitrogen containing feedstocks are utilized in catalytic cracking process, the coke deposited on the catalyst contains sulfur and nitrogen. During regeneration of coked deactivated catalyst, the coke is burned from the catalyst surface that then results in the conversion of sulfur to sulfur oxides and nitrogen to nitrogen oxides (NOx).
Initially, there was little incentive to attempt to remove substantially all coke carbon from the catalyst, since even a fairly high carbon content had little adverse effect on the activity and selectivity of amorphous silica-alumina catalysts. Most of the FCC cracking catalysts now used, however, contain zeolites, or molecular sieves. Zeolite-containing catalysts have usually been found to have relatively higher activity and selectivity when their coke carbon content after regeneration is relatively low. An incentive arose for attempting to reduce the coke content of regenerated FCC catalyst to a very low level.
When the regenerators operate in a complete CO combustion mode, the mole ratio of CO2/CO is at least 10 in the regenerator flue gas. During regeneration operated at complete combustion mode, several methods have been suggested for burning substantially all carbon monoxide to carbon dioxide to avoid air pollution, recover heat, and prevent afterburning. Among the procedures suggested for use in obtaining complete carbon monoxide combustion in an FCC regeneration have been: (1) increasing the amount of oxygen introduced into the regenerator relative to standard regeneration; and either (2) increasing the average operating temperature in the regenerator or (3) including various carbon monoxide oxidation promoters in the cracking catalyst to promote carbon monoxide burning. Various solutions have also been suggested for the problem of afterburning of carbon monoxide, such as addition of extraneous combustibles or use of water or heat-accepting solids to absorb the heat of combustion of carbon monoxide.
Specific examples of treatments applied to regeneration operated in the complete combustion mode include the addition of a CO combustion promoter metal to the catalyst or to the regenerator. For example, U.S. Pat. No. 2,647,860 proposed adding 0.1 to 1 weight percent chromic oxide to a cracking catalyst to promote combustion of CO. U.S. Pat. No. 3,808,121 taught using relatively large-sized particles containing CO combustion-promoting metal into a regenerator. The small-sized catalyst is cycled between the cracking reactor and the catalyst regenerator while the combustion-promoting particles remain in the regenerator. Also, U.S. Pat. Nos. 4,072,600 and 4,093,535 teach the use of Pt, Pd, Ir, Rh, Os, Ru, and Re in cracking catalysts in concentrations of 0.01 to 50 ppm, based on total catalyst inventory to promote CO combustion in a complete burn unit. Most FCC units now use a platinum-containing CO combustion promoter.
When using carbon monoxide combustion-promoting metals, such as platinum, associated with a small fraction of the total particulate solids inventory, essentially complete carbon monoxide combustion has been obtained commercially. Low levels of coke on regenerated catalyst, another desirable result, have also been obtained. On the other hand, the amount of undesirable nitrogen oxides formed in the regenerator flue gas has substantially increased in catalyst regenerators using combustion-promoting promoting metals contained on a small fraction of the circulating particulate solids. This has created an air pollution problem in disposing of the regenerator flue gas. Use of combustion promoters comprising only a small fraction of the total solids inventory in a cracking system is nevertheless often preferable to use of a small amount of promoting metal on a large fraction of the catalyst solids. This is because of the operating flexibility obtainable when using a small amount of combustion-promoting additive particles. For example, use of the additive can be discontinued rapidly without removing a large portion of the catalyst inventory from circulation in a unit.
Accordingly, it is difficult in a catalyst regenerator to completely burn coke and CO without increasing the NOx content of the regenerator flue gas. Many jurisdictions restrict the amount of NOx that can be in a flue gas stream discharged to the atmosphere. In response to environmental concerns, much effort has been spent on finding ways to reduce NOx emissions.
For example, NOx is controlled in the presence of a platinum-promoted complete combustion regenerator in U.S. Pat. No. 4,290,878, issued to Blanton. Recognition is made of the fact that the CO promoters result in a flue gas having an increased content of nitrogen oxides. The excessive amounts of undesirable NOx was suppressed by using in addition to Pt, a small amount of Rh or Ir on the same additive particle.
- SUMMARY OF THE INVENTION
U.S. Pat. No. 4,300,997 to Meguerian et. al discloses the use of a promoter comprising palladium and ruthenium to promote the combustion of CO in a complete CO combustion regenerator without simultaneously causing the formation of excess amounts of NOx. The ratio of palladium to ruthenium is from 0.1 to about 10.
- DETAILED DESCRIPTION OF THE INVENTION
In the present invention, a method is provided for restricting the formation of nitrogen oxides formed in a hydrocarbon cracking catalyst regeneration zone wherein carbon monoxide is combusted with a molecular oxygen-containing gas in contact with a carbon monoxide combustion promoter including a combustion-promoting metal or compound of a metal selected from rhodium, iridium, or mixtures thereof associated with at least one particulate porous inorganic solid.
The present invention is used in connection with a fluid catalyst cracking process for cracking hydrocarbon feeds. The same hydrocarbon feeds normally processed in commercial FCC systems may be processed in a cracking system employing the present invention. Suitable feedstocks include, for example, petroleum distillates or residuals, either virgin or partially refined. So-called synthetic feeds such as coal oils, bitumen and shale oils are also suitable. Suitable feedstocks normally boil in the range from about 200°-600° C. or higher. A suitable feed may include recycled hydrocarbons which have already been subjected to cracking.
The cracking catalyst employed may be a conventional particulate acidic racking catalyst, such as silica-alumina. The catalyst may, for example, be a conventional non-zeolitic cracking catalyst containing at least one porous inorganic oxide, such as silica, alumina, magnesia, zirconia, titania, etc., or a mixture of silica and alumina or silica and magnesia, etc, or a clay or acid-treated clay or the like. The catalyst may also be a conventional zeolite-containing cracking catalyst including a zeolitic crystalline aluminosilicate associated with a porous refractory matrix which may be, for example, silica-alumina. The matrix generally constitutes 70-95 weight % of the cracking catalyst, with the remaining 5-30 weight % being a zeolite component dispersed on or embedded in the matrix. The zeolite may be rare earth-exchanged or hydrogen-exchanged. Conventional zeolite-containing cracking catalysts often include an X-type zeolite or a Y-type zeolite. Low (less than 1%) sodium content Y-type zeolites are particularly good. As will be apparent to those skilled in the art, the composition of the acidic cracking component in the catalyst particles employed in the system is not a critical feature of the present method. Thus, the catalyst particles may be either completely amorphous or partly amorphous and partly crystalline.
Cracking conditions employed in the cracking or conversion step in an FCC system are frequently provided in part by pre-heating or heat-exchanging hydrocarbon feeds to bring them to a temperature of about 315°-400° C. before introducing them into the cracking zone; however, pre-heating of the feed is not essential. Cracking conditions normally include a catalyst/hydrocarbon weight ratio of about 3-10. A hydrocarbon weight space velocity in the cracking zone of about 5-50 per hour is preferably used. The average amount of coke contained in the catalyst after contact with the hydrocarbons in the cracking zone, when the catalyst is passed to the regenerator, is preferably between about 0.5 weight % and about 2.5 weight %, depending in part on the carbon content of regenerated catalyst in the particular system, as well as the heat balance of the particular system.
The catalyst regeneration zone used in an FCC system employing an embodiment of the present invention may be of conventional design. The gaseous atmosphere within the regeneration zone normally includes a mixture of gases in concentrations which vary according to the locus within the regenerator. The concentrations of gases also vary according to the coke concentration on catalyst particles entering the regenerator and according to the amount of molecular oxygen and steam passed into the regenerator. Generally, the gaseous atmosphere in a regenerator contains 5-25% steam, varying amounts of oxygen, carbon monoxide, and carbon dioxide. Nitrogen and nitrogen oxides are also present such as from the use of air as the coke combustion source and/or from the combustion of nitrogen-containing coke. The present invention is applicable in cases in which oxygen (O2) or air is employed for combustion of coke in the catalyst regenerator. As will be appreciated by those skilled in the art, air is almost invariably employed to provide some or all of the oxygen needed for combustion in FCC regenerators.
A combustion-promoting metal is employed in carrying out the method of the present invention. The combustion-promoting metals which are suitable for use include one or more of the metals rhodium or iridium, or compounds thereof, such as the oxides, sulfides, sulfates, etc. At least one of these metals or metal compounds is used, and mixtures of two of the metals are also suitable.
The promoting metal or metal compound is associated with a particulate solid inorganic oxide which may be a particulate solid other than the catalyst, e.g., a finely divided, porous inorganic oxide, such as alumina, silica, etc., sized suitably for circulation in an FCC system, or a particulate solid which remains in the catalyst regenerator rather than circulating through the cracking system with the particulate solids inventory.
The total concentration of the combustion-promoting metal, or metals, or compounds thereof used in the cracking system, with respect to the circulating catalyst inventory, is sufficient to promote the desired amount of combustion of coke on the catalyst and to promote the desired amount of combustion of carbon monoxide.
The promoting metal, metals, or compound thereof, are preferably employed in an FCC system in association with discrete, promoted particulate solids, which are physically admixed with, and circulated in the particulate solids inventory in an FCC system with unpromoted catalyst particles. The promoted particulate solids may be formed from any material which is suitable for circulation in an FCC system in admixture with the catalyst. Particularly suitable materials are the porous inorganic oxides, such as alumina, silica, zirconia, etc., or mixtures of two or more inorganic oxides, which may be amorphous, crystalline, or both, such as silica-alumina, natural and synthetic clays and the like. Crystalline aluminosilicate zeolites are not used as the supports for the CO combustion promoters of this invention. Gamma-alumina is particularly useful. The combustion-promoting metal or metal compound can be added to a particulate solid to form a promoted particulate solid in any suitable manner, as by impregnation or ion exchange, or can be added to a precursor of a particulate solid, as, for example, by co-precipitation from an aqueous solution with an inorganic oxide precursor sol. The promoted particulate solids can then be formed into particles of a size suitable for use in an FCC system by conventional means, such as spray-drying, crushing of larger particles to the desired size, etc.
Rhodium and/or iridium are the metals for use in the present method. When the metal is used in association with circulating particulate solids, the total amount of rhodium and/or iridium used in an FCC system with respect to the circulating particulate solids inventory is between about 0.05 to 200 parts per million, by weight, with an amount between about 0.2 and 100 parts per million being particularly preferred. It will be apparent that the concentration of rhodium and/or iridium in promoted particles will be relatively greater when a relatively small proportion of promoted particles is used. The concentration of rhodium and/or iridium in discrete promoted particles used in carrying out the invention is usually within the range from 0.01 weight percent to 1 weight percent. Preferably, the concentration of rhodium and/or iridium in promoted particles is between 0.2 and 0.5 weight percent.
Sintering of iridium is well known under oxygen atmospheres at elevated temperatures. McVicker et. al taught an approach for preventing sintering and maintaining high metal dispersion of Ir/Al2O3 catalysts. (G. B. McVicker, R. L. Garten, and R. T. K. Baker, J. Catal., (1978), 54, 129). Group IIA-oxides of Ca, Sr, and Ba have been reported to stabilize the Ir surface area of Ir/Al2O3 in the presence of oxygen at elevated temperatures. Oxidative stabilization is believed to result from the formation of an immobile surface iridate via the reaction of a mobile, molecular iridium oxide species with a well-dispersed Group IIA-oxide. While the stabilization of supported iridium as described above is known for automotive catalysis to remove hydrocarbon and NOx pollutants, such stabilization is not believed to have been used in FCC regenerators. Accordingly, alumina, silica, silica-alumina, and other oxidic supports containing TiO2, ZrO2, alkaline earth metal oxides or lanthanide oxides can effectively be used to support, in particular, the Ir metal for regenerator NOx removal.
A fresh promoted particulate solid which contains at least one metal or metal compound of the type specified above can, for example, be physically admixed with unpromoted FCC catalyst and the mixture can then be charged to an FCC system. The fresh promoted particulate solids can optionally be added separately in the desired amount to an FCC unit already containing a substantial inventory of unpromoted or promoted FCC catalyst.
Substantially complete combustion or carbon monoxide and coke is preferably carried out in the cracking catalyst regenerator. Sufficient coke is preferably burned off the catalyst during regeneration to provide an average level of coke on regenerated catalyst of less than 0.2 weight %, and preferably less than 0.1 weight %. The carbon monoxide produced in the catalyst regenerator is preferably substantially all burned to carbon dioxide. The flue gas removed from the regenerator preferably has not more than 1000 parts per million, by volume, of CO therein, particularly preferably not more than 500 parts per million, by volume.
The amount of oxygen must be sufficient to burn the desired amount of coke and carbon monoxide, but must not substantially exceed that required to carry out the combustion step in the regenerator. Thus, sufficient oxygen must be introduced into the regeneration zone so that flue gas removed from the regeneration zone contains at least 1 volume % molecular oxygen. This oxygen in the flue gas is termed “excess” oxygen. At least 1 volume % excess oxygen is required in order to provide the high degree of coke and carbon monoxide burning required in the process.
Preferably, the catalyst regeneration zone includes at least one dense-phase bed of fluidized particulate solids (density greater than 10 pounds per cubic foot). Two or more dense beds may be employed if a plurality of regeneration chambers is used, as in staged regeneration. Preferably, substantially all the carbon monoxide generated in a dense-phase catalyst bed is burned to carbon dioxide in the dense-phase bed. It is also preferred to control the average temperature of dense-phase beds of solids in a regeneration zone so that the average temperature does not exceed 675° C. Dense-phase burning of the carbon monoxide generated in an FCC catalyst regenerator is indicated when the average temperature in a dilute phase above a dense-phase catalyst bed is only slightly different, or lower than, the average temperature in the dense phase.
- EXAMPLE 2
Rhodium is impregnated onto alumina support particles to a level of 500 ppm from an aqueous solution of rhodium nitrate. The dried material is calcined at 500° C. for 2 h.
- EXAMPLE 3
Iridium is impregnated onto alumina support particles to a level of 500 ppm from an aqueous solution of iridium chloride. The dried material is calcined at 500° C. for 2 h.
- EXAMPLE 4
Alumina support particles are impregnated with an aqueous solution of barium acetate, dried, and calcined at 650° C. for 2 h. The product contains 10% BaO by weight.
- EXAMPLE 5
Iridium is impregnated onto the product made in Example 3 to a level of 500 ppm from an aqueous solution of iridium chloride. The dried material is calcined at 500° C. for 2 h.
- EXAMPLE 6
Rhodium is impregnated onto alumina support particles to a level of 250 ppm from an aqueous solution of rhodium nitrate. The dried material is calcined at 500° C. for 2 h. The material is then impregnated with iridium to a level of 250 ppm from an aqueous solution of iridium chloride. The dried material is recalcined at 500° C. for 2 h.
- COMPARATIVE EXAMPLE A
Alumina support particles are calcined to 1200° C. for 2 h to convert the transitional alumina substantially to α-alumina. Rhodium is impregnated onto this alumina support to a level of 500 ppm. The dried material is calcined at 500° C. for 2 h.
- EXAMPLE 7
Platinum is impregnated onto alumina support particles to a level of 500 ppm from an aqueous solution of a monoethanol amine complex. The dried material is calcined at 500° C. for 2 h.
- EXAMPLE 8
CO oxidation testing - Experiments were carried out in a fluid bed reactor. 6 g of fresh steamed FCC catalyst was blended with 0.05 to 1.0 wt. % of the low NOx CO promoters prepared in the previous examples. CO combustion was tested using the following molar gas composition: 5% CO, 3% O2, 5% CO2, balance nitrogen. The flow rate of gas over the catalyst and co promoter solids was 400 cc/min. The reactor temperature was 593° C. The relative CO promotion rate constant was determined by measuring the slope of the activity (defined as -In(1 -CO conversion)) vs. space time. These slopes are shown in Table 1.
| ||TABLE 1 |
| || |
| || |
| || ||Comparative || || |
| || ||Example A ||Example 1 ||Example 2 |
| || |
| ||Fresh Promoter ||788 ||720 ||434 |
| ||Steamed ||196 ||132 ||154 |
| ||Promoter |
| || |
It was found that Pt based promoter was more active for CO oxidation than either Rh or Ir based promoters as has been shown in prior art.
- EXAMPLE 9
Ammonia Decomposition Testing—Experiments were carried out with a fixed bed reactor using the following gas composition: 450 ppm NH3
, 15% steam, 2%-6% CO, and Ar as balance. A total gas flow rate of 260 cc/min (STP), GHSV (39,000 h-1) was used which would be similar to that experienced by the additive in a commercial FCC regenerator. A 0.4 g precious metal on alumina sample was used as the NH3
reducing additive along with 1.6 g of a kaolin microsphere as an inert diluent. Activity data for NH3
decomposition at different CO concentrations using 500 ppm alumina-supported precious metals are shown in Table 2 below. It was found that the supported Rh and Ir promoters were very active for NH3
decomposition; whereas Pt had no activity at 700° C. in the presence of 15% steam and 2-6% CO.
|TABLE 2 |
|% Conversion of NH3 |
| ||CO Concentration |
| ||Sample ||2% ||4% ||6% |
| || |
| ||Comparative A || 0 ||/ ||/ |
| ||Example 1 ||100% ||90% ||75% |
| ||Example 2 || 32% ||— ||— |
| || |
- EXAMPLE 10
HCN Removal Testing Similar experiments to NH3
decomposition were carried out with the replacement of NH3
by 450 ppm HCN. Interestingly, HCN behaved similarly to NH3
over the precious metal catalysts. As shown in Table 3, Pt had no activity for HCN removal. Once again, Rh had significantly more activity than Pt for HCN decomposition.
|TABLE 3 |
|% Conversion of HCN |
| ||CO Concentration |
| ||Sample ||2% ||4% ||6% |
| || |
| ||Comparative A || 0 || 0 ||/ |
| ||Example 1 ||100% ||87% ||70% |
| || |
Ammonia and CO oxidation testing—Simultaneous ammonia and carbon monoxide testing was carried out in a fixed bed reactor using the following gas composition: 2% CO, 2% O2, 8% CO2, 500 ppm NH3, 15% H2O and balance Ar at flow and temperature conditions similar to those described in Examples 8 and 9. Results are shown in Table 4.
| ||TABLE 4 |
| || |
| || |
| || ||Comparative Example A ||Example 1 |
| || |
| ||CO conversion (%) ||100% ||100% |
| ||NH3 conversion (%) || 96 || 96 |
| ||Selectivity to N2 || 2 || 43 |
| ||Selectivity to NOx || 98 || 57 |
| || |
Under these conditions of testing although both Pt and Rh based materials show identical conversions for CO and NH3, the selectivity's are very different. Pt based materials almost quantitatively convert NH3 to NOx whereas Rh based materials will convert significant quantities of NH3 to nitrogen.