GB2114147A - Production of high octane gasoline from catalytic cracking unit - Google Patents

Abstract

This invention provides an improvement in the operation of an FCC unit such as to maintain the octane rating of the gasoline fraction from the cracker at a high level over repeated cycles of cracking charge and regeneration by using fresh zeolitic catalyst particles having an alkali metal oxide less than about 1.5% (based on the zeolite content) and controlling the amount of alkali metal oxide that comes into contact with catalyst inventory throughout cracking, stripping and regeneration so as to maintain alkali metal oxide content of equilibrium catalyst below 2.0% based on the weight of zeolite in the fresh zeolitic catalyst

Description

SPECIFICATION Process for production of high octane gasoline from catalytic cracking unit This invention is concerned with improving the octane rating of gasoline produced by fluid catalytic cracking of gas oil feedstock with a zeolitic cracking catalyst having a low sodium content.

Fluid catalytic cracking (FCC) of hydrocarbon charge to produce gasoline involves cyclic contact of the charge at cracking temperature with a particle form solid cracking catalyst, whereby components of the charge are converted by cracking to lower boiling hydrocarbons including a gasoline fraction with concurrent deposition on the catalyst on an inactivating carbonaceous contaminant. Gasoline is recovered from the products of conversion. Optionally hydrocarbon is stripped by steam from the catalyst particles before activity of the contaminated catalyst is restored by burning the carbonaceous deposit. Catalyst so regenerated is contacted with additional hydrocarbon charge, whereby the catalyst declines in regenerated activity over repeated cycles of charge contact and regeneration.The average activity of the catalyst inventory is maintained at substantially constant equilibrium values by replacing a portion of the catalyst inventory with fresh catalyst of activity above equilibrium values.

It has been proposed to improve the octane rating of gasoline from an FCC unit by employing an ultrastable zeolite promoter in the cracking catalyst composition, the zeolite preferably being free of rare earth metal. Reference is made to British Patent No. 2,022,439. The term "ultrastable" as used in this patent refers to a family of synthetic crystalline aluminosilicate zeolite having a low sodium content.

The effect of exchangeable cations on the catalytic cracking activity is described in a paper "lon- Exchanged Ultrastable Y Zeolites. 3. Gas Oil Cracking over Rare Earth-Exchanged Ultrastable Y Zeolites" by Julius Scherzer and Ronald E. Ritter, W. R. Grace, Ind. Eng. Chem. Prod. Res. Dev.; 17; 3, 21 9 (1 978). Rare earth exchanged ultrastable zeolites (Re H-USY) are shown to be more active for cracking than are H-ultrastable zeolites (H-USY). However, even the Re-H-USY is less active than typical Re-H-Y zeolite. Re-H-Y is shown to have a higher concentration of Bronsted acid sites than do the H-USY zeolites.The authors suggest that the lower density of acid sites in USY zeolites reduces the rate of conversion of olefins into paraffins and of aromatics into condensed polycyclics (coke), thus allowing the olefins and aromatics to diffuse out of the zeolite and to desorb. Exchange of rare earth into H-USY zeolites tends to increase the rate of these hydrogen-transfer reactions, resulting in more coke and higher conversions. The authors do not discuss the effects of sodium exchange or contamination which would be expected to decrease hydrogen-transfer reactions and lower conversion.

Until the present invention, it was not possible utilizing low sodium containing catalysts which promise to enhance octane to achieve desired results in commercial FCC units.

The present invention relates to a process for catalytic cracking of a hydrocarbon charge to produce gasoline by contacting the charge at cracking temperature with a particle form solid cracking catalyst containing a zeolite whereby components of the charge are converted by cracking to lower boiling hydrocarbons including a gasoline fraction with concurrent deposition on the catalyst of an inactivating carbonaceous contaminant, recovering gasoline from said products of conversion, optionally steam stripping hydrocarbon from catalyst, regenerating catalytic cracking activity of the contaminated catalyst by burning carbonaceous deposit therefrom, and contacting catalyst so regenerated with additional such charge, whereby the catalyst declines in activity over repeated cycles of charge contact and regeneration, the average activity of the catalyst inventory being maintained at substantially constant equilibrium values by replacing a portion of the catalyst inventory with fresh catalyst of activity above said equilibrium values; the improvement whereby the octane rating of said gasoline fraction is maintained at a high level over repeated cycles of charge contact and regeneration, which comprises using fresh catalyst particles containing alkali metal oxide of less than about 1.5% by weight of zeolite content, maintaining the alkali content of said hydrocarbon charge below 0.5 ppm and controlling the amount of alkali metal oxide that comes into contact with catalyst inventory throughout cracking, stripping and regeneration so as to maintain alkali metal oxide content of equilibrium catalyst below 2.0% based on the weight of the zeolite of said catalyst in fresh condition.

This invention provides an improvement in the operation of an FCC unit such as to maintain the octane rating of the gasoline fraction from the cracker at a high level over repeated cycles of cracking charge and regeneration by using fresh zeolitic catalyst particles having an alkali metal oxide less than about 1.5% (based on the zeolite content) and controlling the amount of alkali metal oxide that comes into contact with catalyst inventory throughout cracking, stripping and regeneration so as to maintain alkali metal oxide content of equilibrium catalyst below 2.0%, based on the weight of zeolite in the fresh zeolitic catalyst.

In an especially preferred embodiment of the invention the fresh catalyst is prepared by (a) ionexchanging synthetic sodium faujasite with ammonium ions, leaving residual sodium therein, (b) calcining to facilitate subsequent exchange of residual sodium in the zeolite and (c) ion-exchanging the calcined zeolite with ammonium ions to further reduce sodium, the catalyst also containing an inorganic matrix component which may be mixed with the faujasite before or after steps (a), (b), and (c).

The prior art lacks recognition that sodium poisoning of cracking catalyst during use has any influence on the octane rating of FCC unit gasoline, and fails to appreciate the necessity of controlling contact of a low sodium content zeolitic cracking catalyst with sodium or other alkali metal during use in an FCC unit in order to maintain the octane enhancing ability of low sodium content zeolitic catalysts.

Our findings are unexpected when viewed in light of statements appearing in the publication of Scherzer and Ritter (supra). Contrary to the various allegations in the publication we found that while sodium contamination had in fact caused a decrease in coke formations and had reduced conversion, the sodium contamination had also lowered olefin yield and thereby gasoline octane.

Catalyst particles useful in practice of the instant invention embrace fluidizable particles comprising a zeolitic cracking component selected from the group consisting of H-zeolite, NH4-zeolite, ReO-zeolite and mixtures thereof, the catalyst containing a weight of rare earth oxide less than about 5% based on the weight of the zeolite in the fresh catalyst. in a specially preferred embodiment such zeolite is synthetic faujasite having in fresh condition a unit cell size (a) in the range of about 24.30 to 24.75 A as determined by X-ray diffraction. The catalyst particles, in fresh (unused) condition contain less than 1.5% Na2O (or equivalent other alkali metal oxide) based on the weight of the zeolite component, the amount of zeolite component being estimated by X-ray and typically being in the range of 5 to 30% by weight of the catalyst particles.Most preferably, the fresh catalyst particles contain less than 1.0% by weight Na2O or equivalent of other alkali metal oxide and preferably they contain less than 0.5% by weight Na2O based on the weight of the zeolite component. Thus, both the zeolite and the nonzeolite (matrix) component(s) should be very low in sodium and other alkali metal oxides.

The inorganic oxide component of the catalyst particles may be, for example, synthetic silicaalumina, naturally occurring clay, processed clay, as well as mixtures of the aforementioned with inert additives known in the art and utilized as components as cracking catalysts to enhance activity, selectivity, etc. Representative of catalysts that may be used are those described in British Patent 2,022,439, U.S. 3,944,800, U.S. 4,058,484. Catalysts prepared by the process of U.S. 3,506,594 and having low levels of sodium are recommended.

Catalyst of the invention may be used in conventional FCC units using conventional operating conditions. The invention may also be practiced under cracking and regeneration conditions that represent departures from conventional conditions. Typical conditions for FCC are described in U.S.

3,944,482.

Practice of our invention preferably involves preventing deposits on fresh low sodium-content zeolitic cracking catalyst inventory of more than about 0.5% by weight sodium oxide based on the weight of the zeolite component of the fresh catalyst and/or other alkali metal oxide during all stages of use. Sources of alkali metal that may contact and deposit on recirculating catalyst during recycling through reactor (cracker), stripper and regenerator include salt transported into the refinery associated with crude oil. Salt in crude at its source is usually higher than salt content entering the refinery due to salting during transport, etc.For purposes of this invention alkali metal, and hence salt content, must be controlled, by desalting if necessary, such that alkali metal from all sources, including alkali metal in processing water contacting catalyst (e.g., steam introduced with feedstock, stripping or during regeneration) does not exceed that which results in the presence of more than 2.0% by weight total alkali metal oxide based on the weight of zeolite in the ti esh catalyst. If necessary, conventional crude desalting methods may be used or the FCC feedstock may be desalted. Well known methods for desalting are described in Nelson "Petroleum Refinery Engineering, McGraw/Hill, Fourth Edition, 1958, on pages 266-288". Other desalting techniques may be employed.Processing that introduces caustic soda in feedstock for the FCC unit should be avoided or controlled to minimize the amount of sodium that comes into contact with circulating catalyst inventory. Similarly other alkali metals (potassium and lithium) should be controlled. Potassium hydroxide used in alkylation unit should not be permitted to contaminate hydrocarbon feedstocks or water introduced to the reactor, etc.

This invention will be more fully understood and the benefits appreciated from the following illustrative examples.

Five FCC catalysts, all free from rare earth, were tested to determine sodium effects on octane.

The catalyst used in test 1 was prepared by the general procedure described in U.S. 3,506,594 using repeated contact with an ammonium salt solution to exchange readily exchangeable sodium ions, followed by calcination and re-exchange with ammonium salt solution to reduce further the alkali metal oxide content. The fluid cracking catalyst contained 21 % of hydrogen faujasite (24.62 A cell size) as determined by X-ray diffraction, and 0.20 wt.% Na20. Matrix was amorphous silica-alumina derived from kaolin clay. In tests 2 to 4 the sodium oxide content of this catalyst was increased as described below. The catalyst used in test 5 contained an ammonium zeolite also prepared by procedures described in U.S. 3,506,594, using repeated contact with an ammonium salt solution to reduce Na20 without subsequent calcination and re-exchange to reduce further the sodium oxide content. The fresh catalyst used in test 5 had a zeolite content of 25%, as determined by X-ray diffraction, the zeolite having a unit cell size of 24.75 A.

Test 1. A sample of the above described cracking catalyst was steam treated (100% steam) at 14500 F for four hours and used to crack gas oil in an FCC pilot unit.

Test 2. 3259 g of another unsteamed sample of the same batch of fluid cracking catalyst used in Test 1 (21 % of a hydrogen faujasite, 24.62 A cell size, and 0.20 wt.% Na2O) was slurried with a solution containing 4980 ml of H2O and 1 75 ml of 2M NaOH. The slurry was filtered after stirring for 45 minutes at 1 800 F. The solids were then washed and dried. The final catalyst had an Na2O content of 0.34% volatile free (V. F.) weight basis. This catalyst was then steamed at 1 4500F for four hours in 100% steam at atmospheric pressure and used to crack gas oil in an FCC pilot unit.

Test 3. 3398 g of the same unsteamed batch of fluid catalyst used on Test 1 was mixed to incipient wetness with a solution consisting of 2653 ml of H2O and 1 5 9 NaOH. The impregnated catalyst was dried and found to contain 0.44 wt% Na2O. The catalyst was steamed at 1 4500F for four hours and used to crack gas oil on an FCC pilot unit.

Test 4. Another sample of the same batch of cracking catalyst used in Test 1 was steamed at 1 4500F for four hours. The steamed catalyst (3405 g) was taken to incipient wetness by impregnation with a solution of 201 8 ml of H2O and 1 5 g of NaOH. This sample was then dried and found to contain 0.50 wt% Na20. The catalyst was used to crack gas oil on an FCC pilot unit.

Test 5. A sample of non-rare earth containing FCC catalyst described above and containing about 25% zeolite and 0.58% Na20 was steamed for four hours at 14500 F.

Conditions and gasoline octane numbers for the five samples mentioned above are given in the accompanying table. Pilot unit conditions were maintained similar as was conversion so as to influence gasoline octane as little as possible.

Effect of quantity of Na2O in cracking catalyst on gasoline octane Reactor Wt.% Gasoline Octane Ex. wt.% Na2O* Cat/Oil WHSV temp. cony. RON MON 1 0.95 5.92 17.93 9300F 71.7 93.6 81.7 2 1.62 6.31 15.96 9290F 72.4 92.5 80.4 3 2.10 6.75 15.90 9310F 72.1 92.2 80.4 4 2.38 6.63 15.85 9320F 70.1 91.7 80.3 5 2.32 6.34 18.49 9300F 71.9 91.6 80.7 *Based on weight of zeolite component.

Results in the table show that gasoline RON and MON were adversely affected in all cases after sodium poisoning. Reference is made to examples 2, 3 and 4. In fact the higher octane of the gasoline produced with the catalyst containing 0.95% Na2O based on zeolite content as compared to example 5, 2.32% Na2O based on zeolite content, disappeared when sodium contamination of the former occurred such that the Na2O level exceeded 1.62% by weight.

Data in the table demonstrate that in order to maintain the octane enhancing ability of low sodium H-faujasite catalysts, sodium must be prevented from depositing on the catalyst during use.

Our findings reveal that deposition of even small amounts of Na2O (0.5 wt% based on zeolite component) have a deleterious effect on octane. It is estimated that a level of 0.5 ppm Na2O in the gas oil feed will result in sodium poisoning (Na2O of catalyst 0.1 wt%) severe enough to cause an octane loss. Thus, the sodium input to the FCC unit must be kept below 0.5 ppm of feed.

Claims (6)

Claims 1. A process for catalytic cracking of a hydrocarbon charge to produce gasoline by contacting the charge at cracking temperature with a particle form solid cracking catalyst containing a zeolite whereby components of the charge are converted by cracking to lower boiling hydrocarbons including a gasoline fraction with concurrent deposition on the catalyst of an inactivating carbonaceous contaminant, recovering gasoline from said products of conversion, optionally steam stripping hydrocarbon from catalyst, regenerating catalytic cracking activity of the contaminated catalyst by burning carbonaceous deposit therefrom, and contacting catalyst so regenerated with additional such charge, whereby the catalyst declines in activity over repeated cycles of charge contact and regeneration, the average activity of the catalyst inventory being maintained at substantially constant equilibrium values by replacing a portion of the catalyst inventory with fresh catalyst of activity above said equilibium values; characterized by using fresh catalyst particles containing alkali metal oxide of less than about

1.5% by weight of zeolite content, maintaining the alkali content of said hydrocarbon charge below 0.5 ppm and controlling the amount of alkali metal oxide that comes into contact with catalyst inventory throughout cracking, stripping and regeneration so as to maintain alkali metal oxide content of equilibrium catalyst below 2.0% based on the weight of the zeolite of said catalyst in fresh condition.

2. A process according to Claim 1 wherein said fresh catalyst comprises a zeolite component selected from the group consisting of H-zeolite, NH4-zeolite, and mixtures thereof.

3. A process according to Claim 2 wherein said zeolite is synthetic faujasite having a unit cell size in the range of about 24.30 to 24.75 A

4. A process according to any of Claims 1 to 3 wherein said fresh catalyst is prepared by (a) ionexchanging synthetic sodium faujasite with ammonium ions, leaving residual sodium therein, (b) calcining to facilitate subsequent exchange of residual sodium, and (c) ion-exchanging with ammonium ions to reduce further sodium therein, said catalyst also containing an inorganic oxide matrix component, said matrix component being mixed with said faujasite before or after steps (a), (b), and (c).

5. The process according to any of Claims 1 to 4 wherein all steam used during cracking, steaming and regeneration that comes into contact with cracking catalyst particles is substantially free from alkali metal.

6. A process for the production of gasoline, the process being substantially as hereinbefore described.