US3801493A

US3801493A - Slack wax cracking in an fccu with a satellite reactor

Info

Publication number: US3801493A
Application number: US00300522A
Authority: US
Inventors: D Youngblood; R Kuhlmann; R Pratt
Original assignee: Texaco Inc
Current assignee: Texaco Inc
Priority date: 1972-10-25
Filing date: 1972-10-25
Publication date: 1974-04-02
Anticipated expiration: 1991-04-02

Abstract

A FLUID CATALYTIC CRACKING PROCESS WHEREIN A GAS-OIL AND SLACK WAX ARE CONVERTED INTO HYDROCARBONS BOILING BELOW 430* F. THE PROCESS EMPLOYS TWO REACTORS SUCH THAT HIGH POUR POINT TEMPERATURE COMPONENTS OF SLACK WAX DO NOT CONTAMINATE LIGHT CYCLE OIL PRODUCTS OF GAS-OIL CRACKING.

Description

p i 1974 D.J. YOUNGBL .OOD ETAL 3,801,493

SLACK WAX CRACKING IN AN FCCU WITH A SATELLITE REACTOR Filed Oct. 25, 1972 REACTOR C3 C4 vv34 v 1 SATELLITE PRODUCTS 37 I FLUE I K 4 GAS STEAM WAX RECYCLE J GAS U/L 7 VIRGIN 10 GAS O/L United States Patent O US. Cl. 208-78 6 Claims ABSTRACT OF THE DISCLOSURE A fluid catalytic cracking process wherein a gas-oil and slack wax are converted into hydrocarbons boiling below 430 F. The process employs two reactors such that high pour point temperature components of slack wax do not contaminate light cycle oil products of gas-oil cracking.

BACKGROUND OF THE INVENTION The present invention is related to fluid catalytic cracking of slack wax for conversion into high octane gasoline and low molecular weight hydrocarbons suitable for alkylation. More particularly, the present invention relates to a fluid cracking process wherein a cracking charge stock, i.e., virgin gas-oil, is catalytically cracked in a first reaction zone and wherein slack wax is catalytically cracked in a second, separate cracking zone. Used catalysts from the first and second cracking zones are regenerated in a common regenerator for use in cracking additional gas-oil and slack wax.

In the production of lubricating oils, selected petroleum fractions, such as vacuum gas-oils, are treated to obtain lube stocks having superior lubricating properties. Generally, such selected petroleum fractions are treated to remove substantial portions of aromatic components and waxy paraffin components therefrom. The selection of petroleum fractions for production of lube stocks is made based upon the quality of lube stocks which can be obtained therefrom. A discussion of the principles for selecting petroleum fractions is beyond the scope of this disclosure. Also, such principles are Well known in the prior art and will not be discussed further here.

Aromatic components of a lube oil range petroleum fraction generally have lower viscosities than nonaromatic hydrocarbons of similar molecular weights. Such lower viscosities are generally undesirable in lube stocks. Consequently, petroleum fractions are generally treated to remove a substantial portion of the aromatic components to improve lube stock viscosity characteristics. Such aromatic components, if of a relatively low molecular weight, may be disposed of by blending directly into furnace oil or similar petroleum products. Alternatively, such aromatic components may be disposed of in a conversion process such as fluid catalytic cracking, thermal cracking, etc., wherein the aromatic components are converted into useful products such as gasoline and furnace oil. Aromatic components are generally removed from petroleum fractions by means well known in the art such as, for example, furfural extraction.

Waxy paraffin components of a petroleum fraction comprise relatively high molecular weight normal paraflin hydrocarbons having melting points substantially higher than other hydrocarbon components of the petroleum fraction of similar molecular weight. The presence of such 3,801,493 Patented Apr. 2, 1974 waxy paraflin hydrocarbons increases pour point temperatures of petroleum fractions. In order to produce lube stocks having a desirably low pour point temperature, petroleum fractions are treated to remove substantial amounts of waxy parafiin hydrocarbons therefrom. Treating methods, such as urea dewaxing, solvent dewaxing, etc. are well known in the art of producing lube stocks, and will not be discussed further herein. Waxy parafiin hydrocarbons separated from a petroleum fraction in a dewaxing process are generally referred to as slack wax. Slack wax comprises a major portion of waxy paraflin hydrocarbons in admixture with a minor portion of nonwaxy hydrocarbons. Although slack wax may be refined to produce a paraflin wax product, the amount of slack wax separated from petroleum fractions in the production of lube stocks far exceeds the amount required to supply the paraffin wax market. Consequently, substantial amounts of slack wax must be disposed of by other means.

One means for disposing of slack wax is by conversion in a fluidized catalytic cracking process. The parafiin wax hydrocarbons are relatively easy to catalytically crack into valuable naphtha and C -C hydrocarbons. However, when slack wax is included in the charge stock to a fluidized catalytic cracking process, only a portion of the slack wax is converted into naphtha and lighter hydrocarbons. A substantial portion of slack wax remains unconverted and is recovered from a fluidized catalytic cracking process as a component of the light cycle gas-oil product. As the melting point of waxy paraffin hydrocarbons is substantially higher than the melting point of similar molecular weight, nonwaxy hydrocarbons, the presence of any substantial amount of unconverted slack Wax increases the pour point temperature of light cycle gas-oil. Light cycle gas-oil is employed as a component of diesel fuels and furnace oils for which a low pour point temperature is desirable, particularly if such products are to be used in cold climates. Consequently, the presence of unconverted slack wax in a light cycle gas-oil product from a fluid catalytic cracking unit is undesirable.

Generally, the production of lube stocks is a relatively low volume process in a petroleum refinery and slack wax is not available in suflicient amount for the sole charge stock to a fluidized catalytic cracking process. Additionally, as slack wax is easily cracked, deposition of coke upon cracking catalyst is relatively small and may not be sufficient to provide the heat necessary to maintain desired high cracking temperatures within a fluidized cracking unit.

SUMMARY OF THE INVENTION Now, according to the method of the present invention, an improved catalytic cracking process for conversion of slack wax into valuable naphtha and C -C hydrocarbons is disclosed. In a preferred embodiment of the present invention, a charge stock, e.g., virgin gas-oil, is contacted with hot regenerated catalyst from a regeneration zone in a first riser for conversion of gas-oil into catalytic cracking products. From the first riser, hydrocarbon vapor and catalyst discharges into a first reaction vessel wherein hydrocarbon vapors disengage the catalyst. Hydrocarbon vapors are recovered overhead from the first reaction vessel for separation in a first fractionation zone, into catalytic cracking products, including light gases, a C -C fraction comprising propane, propylene, butylene, n-butane, and isobutane, a naphtha fraction, a light cycle gasoil fraction having a low pour point suitable for use in furnace oil, an intermediate cycle gas-oil fraction, and a heavy cycle oil fraction. Slack wax is converted by contact with hot regenerated catalyst from the regeneration zone in the lower portion of a second riser. Hydrocarbon vapors and catalysts discharge from the upper end of the second riser into a second reaction vessel wherein hydrocarbon vapors and used catalyst disengage. Hydrocarbon vapors from the second reaction vessel are recovered and separated in a second fractionation zone into light gases, a C -C fraction comprising propane, propylene, butylene, n-butane, and isobutane, a high octane naphtha fraction suitable for use in motor fuel and a bottoms fraction. The bottoms fraction from the second fractionation column comprises unconverted slack wax of substantially the same nature as slack wax charged to the second riser. This bottoms fraction has a high pour point temperature and is unsuitable for use in refinery products such as diesel fuel and furnace oil. Such bottoms fraction from the second fractionation zone is recycled to the second riser along with additional amounts of fresh slack wax for conversion into valuable C C hydrocarbons and naphtha. Net conversion of bottoms fraction and slack wax is maintained at 100% of fresh slack wax charge rate.

Spent catalyst from the first and second reactor vessels 1s stripped of volatile hydrocarbons in a stripping zone and stnpped catalyst from the strpiping zone is transferred into a regeneration zone for regeneration by burnmg coke therefrom with air. Regenerated catalyst from the regeneration zone is withdrawn for contact with addit onal virgin gas-oil and slack wax in the first and second users as hereinabove described.

Advantages of the process of the present invention inelude means for converting excess quantities of slack wax into useful C -C hydrocarbons and naphtha by fluidized catalytic cracking in an economical manner and without increasing the pour point temperature of light cycle gas-oil product from the fluidized catalytic cracking process. Addrtionally, as the coke yield from cracking slack wax is low and may not be sufiicient to maintain desired crackmg temperatures for wax conversion, additional coke which can be generated in the cracking of gas-oil is burned in the common regenerator, providing additional heat required for efficient conversion of slack wax. These advantages and others will be more fully described in the detailed description of the invention, which follows.

BRIEF DESCRIPTION OF THE DRAWING The figure of the attached drawing is a schematic representation of a fluidized catalytic cracking um't employing one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION In the process of the present invention, slack wax is converted into valuable C -C hydrocarbons and naphtha in a fluidized catalytic cracking unit. Slack wax comprises a crude waxy paraifin stream obtained from a solvent dewaxing process. A solvent dewaxing process generally comprises treating a petroleum fraction which contains paraflin wax components with a solvent in which nonwax hydrocarbons are preferentially soluble; cooling the solvent-petroleum fraction mixture to a temperature at which paraffin wax components solidify and crystallize and at which the nonparaffin wax components in solution with solvent remain liquid; and separating, by such means as filtration, the solid parafiin wax component from the solvent-nonparafiin wax mixture. Thus, the slack wax comprises a major portion of waxy normal paraflin hydrocarbons and a minor portion of nonwaxy hydrocarbons which adhere to the filtered solid wax. Such solvent dewaxing processes which are commonly employed in the preparation of lube stocks from petroleum fractions are well known in the prior art. For examples see Petroleum Refinery Refining Process Handbook, September 1960, pp. 240 and 243.

Catalytic cracking charge stocks in addition to the slack wax, which are contemplated for use in the present invention, include virgin gas-oil, vacuum gas-oil, topped crude, residuum, etc. Preferably, the catalytic cracking charge stock employed is low in content of metals such as iron, nickel, vanadium, copper, etc. and low in content of asphaltenes. Particularly desirable catalytic cracking charge stocks are virgin atmosphere gas-oil, virgin vacuum gas-oil and mixtures thereof.

Catalysts contemplated for the present invention include those composed of inorganic metal oxides having catalytic cracking activity such as silica-alumina, silicathoria, silica-magnesia, silica-zirconia, etc. The metal oxides may be present in an amorphous gel,'a crystalline alumino-silicate zeolitic molecular sieve, or as mixtures thereof. Preferably, the catalyst comprises an active metal oxide as exemplified by silica-alumina gel and a large pore diameter zeolitic crystalline alumino-silicate. The zeolites preferred as cracking catalysts herein possess ordered rigid three-dimensional structures having uniform pore diameters within the range of from about 5 to about 15 angstroms. The preferred composite catalyst employed herein comprises from about 1 to about 25 weight percent zeolitic molecular sieves, about 10 to 65 weight percent alumina and the remainder silica. In general, the zeolitic molecular sieves which form the high activity components of the catalyst are alkali metal crystalline alumino-silicates which have been treated to replace all or at least a substantial portion of the original alkali metal ions with other cations such as hydrogen and/or a metal or a combination of metals such as barium, calcium, magnesium, manganese, or rare earths such as cerium, lanthanum, neodymium, praseodymium, Samarium, and yttrium. Preferably, the zeolitic portion of the cracking catalyst is represented by zeolite X and/or zeolite Y in which the alkali metal has been replaced with cerium or lanthanum.

As contemplated herein, gas-oil charge is converted in the presence of a composite cracking catalyst comprising zeolite and amorphous silica-alumina in a first elongated reaction zone. Conversion is undertaken at a temperature in the range of from about 840 to 1150 F. and preferably at a temperature in the range of from about 880 to about 975 P. such that the gas-oil undergoes conversion within the range of from about 40 to about and preferably from 50 to 70. Conversion herein is defined as minus volume percent of product boiling above 430 F. Conversions within the range stated above are obtained by operating at a catalyst to oil ratio within the range of from about 4.5 to 15.0 and preferably from 5.5 to 10.0. Other conditions within the first elongated riser reaction zone include, for example, residence time of hydrocarbons from 2 to 8 seconds and preferably from 3.5 to 6.0 seconds, superficial vapor velocities of from 15 to 50 feet per second, and preferably from 20 to 40 feet per second, weight hourly space velocities of from 10 to 100, and preferably from 40 to 65, and equilibrium catalyst activity of from 25 to 60. Catalyst activity may be determined employing 400 grams of catalyst under the following test conditions: reactor temperature 920 F.; space velocity (pounds of oil per hour per pound of catalyst) 2.0; weight ratio (catalyst to oil) 1.0; reaction time (hours) 0.5. Catalyst activity is measured by fractionating cracked liquid product to a 390 F. cut point. The activity is calculated as 100 minus volume percent gas-oil boiling above 390 F. (basis fractionator feed) recovered from the fractionator.

Effluent from the first riser, comprising hydrocarbon vapors and catalysts, is discharged into a disengaging space above a dense phase fluidized bed of cracking catalysts in a first reaction vessel. By discharging into such a disengaging space, the velocity of the riser effluent stream is reduced to a vapor velocity of less than 4.0 feet per second such that the vapor product separate from the used catalysts. Vapor products are recovered from the disengaging space and fractionated in a first fractionation zone into desired product fractions, e.g., a C -C fraction, a naphtha fraction boiling in the (l -430 F. range, a light cycle gas-oil fraction, an intermediate cycle gas-oil fraction, and a heavy cycle gas-oil fraction. The C -C fraction, the naphtha fraction, light cycle gas-oil fraction, and heavy cycle gas-oil fraction, are removed from the cracking process as products. Intermediate cycle gas-oil fraction may be removed as a product, or all, or a portion may be recycled for further cracking. In the event intermediate cycle gas-oil is recycled, conversion is undertaken preferably in a second riser which discharges into the same first reaction vessel as the riser employed for cracking virgin gas-oil. Conversion of intermediate cycle gasoil is undertaken at temperatures in the range of 840 to 1100" F. and preferably in the range of from 900 to 1000 F. and the intermediate cycle gas-oil undergoes conversion, as hereinabove defined, in the range of from about to 60 and preferably in the range of 25 to 50.

The slack wax, according to the method of the present invention, is converted in the presence of cracking catalyst in a third elongated riser which discharges into a separate disengaging space in a second reactor vessel. Conversion of slack wax is undertaken at temperatures ranging from about 840 to 1100 F., preferably at temperatures ranging from about 975 to 1025 F. or higher, slack wax in the third riser undergoes conversion within the range of from about 40 to 95, and preferably from 80 to 90, wherein conversion is as hereinabove defined. Conversion of slack wax in the third riser is accomplished employing a catalyst to oil ratio within the range of from about 4.5 to 15.0 and preferably from 5.5 to 10.0. Other conditions within the slack wax riser include, for example, residence times of the slack wax of from 2 to 8 seconds, preferably from 3.5 to 6.0 seconds, superificial vapor velocities of from 15 to 50 feet per second, preferably to 40 feet per second, weight hourly space velocities of from 10 to 100 and preferably from 40 to 65 and an equilibrium catalyst activity of from to 60, wherein the catalyst activity is as hereinabove defined. Effluent from the slack wax riser comprising hydrocarbon vapors and catalyst is discharged into a slack wax disengaging space in a second reaction vessel. By discharging into such disengaging space, velocity of the slack wax riser efiluent stream is reduced to a vapor velocity of less than 4.0 feet per second such that vaporous hydrocarbons separate from used catalyst. Hydrocarbon vapors recovered from the slack wax disengaging space are fractionated in a second fractionation zone into desired fractions, e.g., a gas fraction, a C -C hydrocarbon fraction, a C -430 F. naphtha fraction, and a Wax cycle oil fraction boiling above 430 F. The wax cycle oil fraction, preferably comprising from about 20 to about 10 volume percent of the slack wax charged to the slack wax riser, primarily comprises hydrocarbon compounds similar to compounds contained in uncracked slack wax. That is, the wax cycle oil is highly paraflinic in nature and has a very high pour point temperature. As this wax cycle oil is highly parafiinic, it is relatively easily converted into additional naphtha and C -C hydrocarbon products. Preferably therefore, the wax cycle oil fraction is totally recycled to the slack wax riser for additional conversion.

Used catalyst from the slack wax reaction vessel and from the gas-oil reaction vessel contain volatile liquid hydrocarbons and coke thereon. Consequently, all used catalyst is preferably passed into a common stripping zone wherein volatile hydrocarbons are stripped from the used catalysts employing a hot stripping medium, preferably steam. From the stripping zone the stripped catalyst having coke deposited thereon is passed into a regeneration zone wherein coke is burned from the catalyst employing air as a source of oxygen. Hot, regenerated catalyst, substantially reduced in coke content, is transferred from the regeneration zone for contact with additional amounts of slack wax and gas-oil in the respective slack wax riser and gas-oil risers.

The accompanying figure illustrates an apparatus by which one embodiment of the method of the present invention may be practiced. The figure is a schematic illustration and many components normally employed in such a process, such as pumps, instrumentation, valves, etc. which are unnecessary to describe the present invention have been omitted for purposes of clarity. One skilled in the catalytic cracking art may supply such omitted items. The accompanying figure is for illustration of the invention only and is not intended to restrict the invention thereby since modifications may be made within the scope of the claims without departing from the spirit thereof.

In the drawing, virgin gas-oil in line 10 is contacted with hot regenerated catalyst from line 11 in the inlet portion of gas-oil riser 12. The resulting suspension of catalyst in oil vapor passes upwardly through gas-oil riser 12 into a gas-oil reaction vessel 13. In gas-oil reaction vessel 13, cracked gas-oil vapors disengage used catalyst with the vapors flowing upward and the catalyst passing downward into a dense phase fiudized bed of used cracking catalyst having an upper surface 14.

An intermediate gas-oil fraction separated from the cracked product of gas-oil cracking in fractionation equipment, not shown, is introduced through line 15 into the inlet section of cycle gas-oil riser 16 wherein it is contacted with hot, regenerated catalyst from standpipe 17. The resulting catalyst vapor mixture passes upwardly through cycle gas-oil riser 16 and discharges through the outlet of riser 16 into the lower portion of gas-oil reaction vessel 13. Vapor effluent from cycle gas-oil riser 16 passes upward through the dense phase catalyst bed offecting further conversion of cycle gas-oil. Cracked cycle gas-oil vapors disengage dense phase catalysts at bed surface 14 and mix with vapors of cracked virgin gas-oil. Cracked product disengages from the catalyst above dense phase bed surface 14. Cracked vapors and any entrained catalyst pass through cyclone separator 18 wherein entrained catalyst is separated and returned to the catalyst dense phase through dip leg 19. Although a single cyclone separator is shown for clarity it will be understood that several cyclones may be assembed in parallel and in series to achieve substantially complete separation of entrained catalyst and hydrocarbon vapor. Effiuent hydrocarbon vapors pass from cyclone separator 18 through line 20 into plenum chamber 21, and are discharged from the gas-oil reaction vessel 13 through line 22. Vapor line 22 conveys cracked product vapors to a first fractionation zone, not shown, wherein conversion products are recovered and separated into desired product fractions and an intermediate cycle oil recycle fraction by fractionation methods well known in the art.

Slack wax from a solvent dewaxing unit, not shown, in line 23 and wax cycle oil in line 24 pass into the inlet of slack wax riser 25 wherein slack wax and wax cycle oil are contacted with hot regenerated catalyst from standpipe 26. The resulting catalyst-hydrocarbon vapor mixture at a temperature of about 1000 F. passes upward through slack wax riser 25 at an average residence time of about 5.0 seconds. Other conditions in slack wax riser 25 include catalyst to oil ratio of about 10 and a weight hourly space velocity of about 50. Preferably, about of slack wax is converted in slack wax riser 25. The catalyst-hydrocarbon vapor mixture from slack wax riser 25 discharges into the lower portion of slack wax reaction vessel 26. Catalyst emerging from riser 25 enters a dense phase catalyst bed having an upper surface 27 and hydrocarbon vapor eflluent from riser 25 passes upward through said dense phase catalyst bed. Preferably, combined slack wax riser cracking, wax cycle oil riser cracking, and additional dense phase bed cracking of wax hydrocarbons provide an overall conversion, basis fresh slack wax, of

volume percent. Vapor velocities in the slack wax reaction vessel 26 are in the range of from about 2 to about 6 feet per second at the point of vapor disengagement from the dense phase bed at bed surface 27. Cracked hydrocarbon vapors and any entrained catalyst above dense phase bed surface 27 pass through cyclone 28 wherein entrained catalyst is separated and returned to bed 27 via dip leg 29. Although a single cyclone is shown for clarity it will be understood that several cyclones is shown for clarity it will be understood that several cyclones in series and parallel may be employed to achieve substantially complete separation of hydrocarbon vapors and entrained catalyst. Effluent hydrocarbon vapors pass from cyclone 28 through line 30 into plenum chamber 31 and are discharged from slack wax reaction vessel 26 through line 32. Vapor line 32 conveys cracked wax hydrocarbons to fractionation zone 33, wherein the cracked wax hydrocarbons are separated into desired fractions including a C -C gas fraction, a C -C hydrocarbon fraction rich in propylene, butylene, and isobutane, a C -430 F. naphtha fraction, and a wax cycle oil fraction. From fractionator 35 the C -C fraction is recovered for further use, such as fuel, via line 36, C -C fraction is recovered via line 37, the naphtha fraction is recovered via line 38, and the wax cycle oil fraction is recovered via line 24. As hereinabove described, wax cycle oil from line 24 is returned to slack wax riser for additional conversion. Propylenes, butylenes and isobutane which may be recovered from the C -C fraction in line 37 are good charge stocks for an alkylation process wherein they may be converted into high octane gasoline.

Dense phase catalyst in the lower portion of slack wax reaction vessel 26 passes downward through line 39 and slide valve 40 into stripping zone 41. Dense phase catalyst in the lower portion of gas-oil reaction vessel 13 passes downward through standpipes 42 and 43 and

slide valves

44 and 45 into stripping zone 41. Stripping zone 41 is provided with bafiies 46 attached to the wall of the stripper 41. Stream rising through stripper 41 displaces and removes adsorbed and entrained volatile hydrocarbons from the catalyst, which vapors pass upwardly through stripper vent line 49 disengaging into the upper portion of gas-oil reaction vessel 13.

Stripped catalyst, containing coke, is withdrawn from the bottom of stripping zone 41 through spent catalyst standpipe 50 at a rate controlled by slide valve 51 and discharged through standpipe 52 into regenerator 53. In regenerator 53 coke burns from the surface of the catalyst and the resulting flue gas passes upwardly and enters cyclone 57 wherein entrained catalyst is separated and returned to the regenerator dense phase bed through dip leg 58. Cyclone 57, although represented as a single separator, may of course comprise an assembly of cyclones in parallel and in series to effect substantially complete separation of entrained catalyst from the flue gas. Efiluent flue gas from cyclone 57 passes through line 59 into plenum chamber 60 and outwardly through flue line 61 to vent facilities, not shown. Regenerated catalyst is withdrawn from the bottom of regenerator 53 through

lines

62, 63 and 64 at rates controlled by

slide valves

65, 66, and 67 to supply hot regenerated catalyst to

standpipes

17, 11 and 26 as hereinabove described.

By following the method of the present invention, slack wax may be catalytically cracked to produce valuable naphtha and C C hydrocarbons which may be converted into high octane gasoline in a volume yield exceeding 100% of the volume of slack wax charged to the cracking process. Additionally, slack wax may be cracked economically in a process wherein a substantially large volume of gas-oil is also cracked without combining cracked products of the slack wax and gas-oil and without adversely affecting the pour point temperature of a light cycle oil product of the catalytic cracking of gas-oil.

8 EXAMPLE To show the advantage of cracking slack Wax, a slack wax stream was obtained from the urea dewaxing of an Amna Vacuum gas-oil and the slack wax was catalytically cracked in a fluidized bed of zeolite-containing cracking catalyst at a reactor temperature of 1000" F., a catalyst to weight ratio of 4.0. Conversion of slack wax was on a once through basis. Product yields obtained in this cracking reaction were as follows:

Component:

C and lighter 1 3.3 Propane 4.4 Propylene 2 19.9 Isobutane 7.8

n-Butane 3.9 Butylenes 2 19.6 C -43O F. naphtha 42.2 Wax cycle oil 20.0 Coke 1 1.4

1 W13. percent.

2 Vol. percent, basis slack wax charge.

Propylenes, butylenes and isobutane produced above, along with additional isobutane from an outside source, are good alkylation charge stock. The potential alkylate from these C C hydrocarbons amounts to 68.1 volume percent of the slack wax charge to cracking. Consequently total potenial gasoline from this cracking process and an alkylation process, represented by the sum of C -430 F. naphtha and potential alkylate, is 110.3 vol. percent of slack wax charge. Calculated octane (Research-H cc. TEL) for this gasoline is 101.2.

As can be seen, the coke yield from cracking slack wax is only 1.4 wt. percent of the slack wax charge, This amount of coke is not sufiicient to provide enough heat to maintain a 1000 F. reaction temperature in a fluidized cracking process. Consequently, additional heat must be obtained from some other source. According to the method of the present invention this additional heat is obtained by burning coke from spent catalyst employed to crack gas-oil in the same regenerator with spent slack wax cracking catalyst.

This high yield of high octane gasoline can be obtained by catalytically cracking a low cost slack wax. By em ploying the process of the present invention, slack wax may be economically cracked into useful products without contaminating cracked light cycle oil products from other cracking stocks, such as virgin gas-oil, with the high pour point temperature wax cycle oil from slack wax cracking. Also, higher coke yields obtainable from cracking gasoils may be employed to provide necessary heat for maintaining the slack wax cracking temperature at a desired high value (1000 F.).

We claim:

1. In a fluid catalytic cracking process for conversion of slack wax and a cracking charge stock wherein cracking charge stock is cracked in a first riser in the presence of cracking catalyst under cracking conditions, wherein catalyst and hydrocarbon vapors from said first riser are sep arated in a first catalyst disengaging zone, wherein hydrocarbon vapors from said first catalyst disengaging zone are fractionated in a first fractionation zone into at least a gas fraction containing C -C hydrocarbons, a C naphtha fraction, a light cycle oil fraction, and a heavy cycle oil fraction, wherein used catalyst from said first catalyst disengaging zone is regenerated in a catalyst regeneration zone, and wherein regenerated catalyst from the regeneration zone is contacted with additional cracking charge stock in said first riser; the improvement which comprises:

(a) cracking slack wax in a second riser in the presence of regenerated catalyst from said regeneration zone;

(b) separating catalyst and cracked wax vapor from said second riser in a second catalyst disengaging zone;

(c) fractionating cracked wax from said second catalyst disengaging zone into a C -C gas fraction, a C -C hydrocarbon fraction, a C -430" F. naphtha fraction, and a wax cycle oil fraction; and

(d) recycling said wax cycle oil fraction from said second fractionation zone to said second riser.

2. The method of claim 1 wherein used catalyst from said second catalyst disengaging zone is regenerated in said regeneration zone, wherein coke is burned from used catalyst in said regeneration zone to provide heat for cracking additional cracking charge stock and slack wax, and wherein cracking charge stock is cracked under conditions to yield sufiicient coke for heating regenerated catalyst to a temperature in the range of from ll50 to 1250 F.

3. The method of claim 1 wherein used catalyst from said first catalyst disengaging zone and used catalyst from second catalyst disengaging zone are combined in a stripping zone, wherein volatile hydrocarbons are vaporized and stripped from said combined used catalyst with hot stripping vapor, wherein hydrocarbon vapors and strip ping vapor from the stripping zone are passed into the first catalyst disengaging zone, and wherein stripped catalyst is passed for regeneration into the regeneration zone.

4. The method of claim 1 wherein slack wax is cracked major portion of alkali metal cations exchanged with cations selected from barium, lanthanum, and mixtures thereof which catalyst contains from about 5 to about 20 percent zeolite, from 10% to alumina and the remainder silica.

6. The method of claim 5 wherein conversion of slack wax and wax cycle oil into naphtha and lighter compounds is essentially equal to the charge of slack wax to the second riser.

References Cited UNITED STATES PATENTS 2,379,159 6/1945 Kanhofer 20874 2,409,353 10/1946 Giulian et al 208120 2,767,126 10/ 1956 Rice 20874 2,921,014 1/1960 Marshall 20874 2,981,674 4/1961 Good 208 3,143,491 8/ 1964 Bergstrom 20874 3,617,496 11/1971 Bryson et a1. 208 3,661,799 5/1972 Cartmell 208164 3,689,402 9/ 1972 Youngblood et al 20893 3,714,024 1/1973 Youngblood et a1 20878 DELBERT E. GANTZ, Primary Examiner G. E. SOHMITKONS, Assistant Examiner US. Cl. X.R.