CA1105406A - Catalytic cracking of metal-contaminated oils - Google Patents

Abstract

CATALYTIC CRACKING OF METAL-CONTAMINATED OILS

ABSTRACT OF THE DISCLOSURE
Metal-contaminated oils, including mildly hydrotreated residual oils, are catalytically cracked in the absence of added hydrogen in a fluid catalytic cracking process wherein the regenerated catalyst has less than about 0.05 wt.%
residual carbon.

Description

5~6 BACKGROUND OF THE INVENTION

Field of the Invention ~his invention is concerned with the catalytic cracking o~ metal-contarninated oils in the absence of added hydrogen.
In particular, it is concerned with the fluid catalytic cracking o~ heavy hydrocarbon oils, such as residual oils, that contain substantial quantities of metal. Partial demetal-lation of such metal-contaminated oils followed by ~luid catalytic cracking is another aspect of this invention.

Background of the Invention Fluid catalytic cracking of hydrocarbon oils is a major ref~nery process. The installed plants are character-istically large, and are usually designed to process from about - 5,000 to 135,000 bbls/day of fresh feed. Briefly, the catalyst l~ section of the plant consists of a cracking section where a heavy char~estock is cracked in contact with fluidized cracking catalyst, and a regenerator section where fluldized catalyst coked in the cracking operation is regenerated by burning with air. All of the plants utilize a~\large ~entory of cracking catalyst which is continuously circulating between the cracking and regenerator sections. The size o~ this circulatin~ inver,-tory in existing plants is within the range of 50 to 600 tons.
Because the catalytic activity of the circulating inventory of catalyst tends to decrease with age, fresh makeup catalyst usually amounting to about one to two percent of the circulating ~S~6 inventory, which corresponds to about 0.1 to 0.25 lbs. per bbl.
o~ fresh feed, is added per day ~o maintain optimal catalyst activity, with daily withdrawal plus losses of about like amount of aged circulating inventory, commonly referred to as "equilibrium" catalyst.
In general, the oils fed to this process are principally the petroleum distillates commonly known as gas oils, which boil in the temperature range of about 650F to 1000F, supplemented at times by coker gas oil, vacuum tower overhead, etc. These ; lO oils generally have an API gravity in the range of about 15 to 45 - and are substantially free of metal contaminants.
The chargestock, which term herein is used to refer to the total fresh feed made up of one or more oils, is cracked in the reactor section in a reaction zone maintained at a tempera-ture of about 800F to 1200~F~ a pressure of about l to 5 atmospheres, and with a usual residence time for the oil of ~rom about one to ten seconds with a modern short contact time riser design. The catalyst residence time is from about one to fifteen seconds. The cracked products are separated from the coked catalyst and passed to a main distillation tower where separation of gasses and recovery of gasoline, fuel oil~ and recycle stock is effected.
Petroleum reflners usually pay close attention in the fluid catalytic cracking process (hereinafter re~erred to as the FCC process) to supplying feedstocks substantially free of metal contaminants. The reason for this is that the metals present in the chargestock are deposited along with the coke on the cracking catalyst. Unlike the coke~ however, they are not removed by regeneration and thus they accumulate on the circulating inventory. ~he metals so deposited act as a catalys~ poison and, depending on khe concentration o~ metals on the catalys~, more or less ad~ersely af'f'ect the e~ficiency o~ the process by decreasing the catalyst activi~y and increasing the production of coke, hydrogen and dry gas at the expense of gasoline and/or fuel oil. Excessive accumulat~on of metals can cause serious problems in the usual FCC operation. For example, the amount of gas produced may exceed the capacity of the down-stream gas plant, or excessive coke loads may result in regenerator temperatures above the metallurgical limits. In such cases the re~iner must resort to reducing the feed rate with attendant economic penalty. Thus, a catalyst inven~ory that contains excessive deposits of me~al is normally regarded as highly undesirable.
The principal metal contaminants in crude petroleum oils are nickel and vanadium, although iron and small amounts o~ copper also may be present. Additionally, trace amounts of zinc and sodium are sometimes found. It is known that almost all o~ the nickel and vanadium in crude oils is associated with very large nonvolatile hydrocarbon molecules, such as metal porphyrins and asphaltenes. Crude oils, of course, vary in metal content, but usually this content is substantial.
An Arab light whole crude, for example a may assay 3.2 ppm (i.e. parts by weight of' metal per million parts of crude) of' nickel and 13 ppm of vanadium. A typical Kuwait whole crude, generally considered o~ average metals content, may assay 6.3 ppm of nickel and 22.5 ppm of vanadium. Regardless ~ '6 of ~he crude scurce, however, it is known that ~istillates produced from the crude are almost free of the metal contaminants which concentrate in the residual oil fractions.
Petroleum engineers concerned with the FCC process have several ways for referring to the metal content of a chargestock. One o~ these is by re~erence to a "metals factor", designated Fm~ The factor may be expressed in equation form as follows:
Fm = ppm ~e ~ ppm V ~ 10 (ppm Ni + ppm Cu) - 10 A chargestock having a metals factor greater khan

2.5 is considered indicative of one which will poison cracking catalys~ to a significant degree. This factor takes into account that the adverse effect of nickel is substantially more than that o~ vanadium and iron present in equal concentra-tions with the nickel.
Another way of expressing the metals content of a chargestock ls as "ppm Nickel Equivalent", which is defined as ppm Nickel Equivalent = ppm nickel ~ 0.25 ppm ~ vanadium For the purpose of this specification, we shall use the ppm Nickel Equivalent designation in discussing metals content of metal-contaminated oils, distillate stocks, and catalysts.
As shown above, no mention is made of copper because this metal usually is not present to any signi~icant extent. How-ever, it is to be understood herein thak i~ it is present in significant concentration~ it is to be included in the computation of Nickel Equivalent and welghted as nickel.

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It is current practice in FCC technology to control the metals content of the char~estock so that it does not exceed about 0.25 ppm Nickel Equivalent. Ca~alyst makeup is managed to control the activity of the circulating inventory.
~ith this practice, for exarnple, in a plant utilizing 50,000 bbls/day of` fresh feed, and an equilibrium catalyst withdrawal ; of 9 tons ~er day, the wi~hdrawn catalyst under steady state conditions will contain abou~ 300 ppm Nickel Equivalent of metals, taking into account that the fresh catalyst contributes ` 10 70 ppm to this value. Thus, the circulating inventory is maintained at about 300 ppm Nickel Equivalents of metal, which is considered tolerable, the usual range being at about 200 to 600 ppm, with preferred operation being at about 200 to 400 ppm.
It is to be understood, of course, that the metals content o~
the chargestock may vary from day to day without serious dis-ruption, provided that the weighted average of the metals content does not exceed about 0.25 ppm nickel equivalent of metal.
It is important, for the purpose of the present invention, to understand that all references to the metals content of an oil~ or of a chargestock, refer to the time-weighted average taken over a substantial period of time such as one month, for example. Because of the large inventory of catalyst relative to the total metals introduced into the system by the chargestock in one day, for example, the metals content Or the catalyst changes little each day with fluctuations in the quality of the chargestock. However, a persistent increase in the metals content of the latter will in time result in a well-defined, calculatable increase in the metals content of the circulating inventory of` catalyst, which determines the performance of the FCC unit. In fact, ~t is evident ` -6-that the circulating inventory of catalys~, by its me~als . content, provides a time-average value of the metals con~en~ of the chargestock. It is in this context, then, that the phrase "metals content o~ the chargestock~ is used herein.
For ~he purpose of this invention, chargestocks to the FCC process that contain up to about 0.40 ppm Nickel Equivalent of metal contaminants will be regarded as sub-stantially ~ree of metal contaminants. Chargestocks that contain at least about 0.50 ppm Nickel Equivalents o~ metal will include those chargestocks referred to as metal-contaminated.
With very limited exceptions, residual oils have not been successfully included in the chargestocks to the FCC
process. The reasons for this are not fully understoodg although from the foregoing discussion it is apparent that their high metals content is certainly a major contributing ~actor. There has been interest in using ~hem~ however. The reason for this interest becomes apparent when we consider, for example, that typically only about 26 volume % of an Arab light whole crude is the 650-1000F gas oil ~raction3 while the total 650F plus resid constitutes about 43 volume %. Thus, were it feasible to efficiently operate with residual oil fractions, a very substantial increase in the amount o~ gasoline plus fuel oil derivable from a barrel of crude could be obtained. In some refineries, the vacuum resid remaining after the distillation of the gas oil is coked and the coker gas oil is included in the FCC chargestock. However, it is generally recognized that coker gas oil, because of its high unsaturated and high aromatics content, is a poor quality feed.

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It has been proposed in the prior art to hydro-treat residual oils under such conditions that the metals content is brought into the range commonly associated with gas oils. Such hydrotreated residual oils, substantially free of metal contaminants, may then be used as charge-stock or a component thereof to the FCC process~ Processes ` to achieve such metals and sulfur reduction are disclosed in U.S. Patent 3,891,541, issued June 24, 1975 and U.S.
Patent 3,876,523, issued April 8, 1975, for example. The combination of hydrotreating to reduce metals and sulfur content followed by cracking also is disclosed in a publication by Hildebrand et al. in The Oil and Gas Journal, pp 112-124, December 10, 1973. However, no installation is known which has adopted the proposed scheme, probably because the cost and severity associated with the operation involves a heavy economic penalty.
It is one object of this invention to provide an improved process for the fluid catalytic cracking of metal-contaminated hydrocarbon oils. It is a further object of this invention to provide a method for the Eluid catalytic cracking of residual petroleum oils which is highly selective for the production of liquids in the motor uel and heating oil boiling ranges. These and other objects of this invention will become evident to those skilled in the art from reading this entire specification including the claims thereof.

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SUMMARY OF TH~ INV~NTION
It has now been discovered that metal-contaminated hydrocarbon chargestocks are e~ficiently cracked in the FCC
process in the absence of added hydrogen when contacted with regenerated catalyst containing less than about 0.10 wt~%
residual carbon. As more fully described hereinafter, proper regeneration o~ the catalyst to low values of residual carbon is a critical requirement in the practice of this invention.
In fact, it is preferred that the metal-contamina~ed oil be con-tacted with regenerated catalyst containing less than about 0.05 wt.% residual carbon~ the particularly preferred value being less than about O.Q25 wt.%. In the practice of this invention, the chargestock preferably contains from at least about 0.50 to about 5.~ ppm Nickel Equivalents of metals.
In certain circumstances, chargestocks that contain higher levels of metals, up to lO or even 15 ppm Nickel Equivalents, may be use~. The metals content of the cir-culating inventory of catalyst is maintained in the range o~ about 700 to 5,000 ppm Nickel Equivalents of metals, and preferably in the range of about 800 to 2,000 ppm. Residual or other oils that have metals content in excess of bhe preferred range may be economically brought into that range by known hydrodemetallation processes operated under relatively-mild conditions. This invention permits the use of residual oils as part or all of the FCC chargestock without prior demetallation in some instances or with only partial demetalla-tion in others.

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It will be recognized by those ski~led in the art that the speci~ied levels o~ residual carbon are achievable with regenerators that operate with excess air and in which the flue gas is substan~ially free of unburned carbon monoxide.
The use of pIatinum or other CO-combustion promoter in the catalyst inventory is an adjunct in the practice of the present inven~ion, as more fully described hereinafter.

DETAILED DESCRIPTION OF THE INVENTION
The term chargestock as used herein re~ers to the total ~resh feed supplied to the process of ~his invention, i.e.
to the oil or blends of oils that have not had prior contact with cracking catalyst. In actual practice, recycle streams may be mixed and introduced with the chargestock to be cracked in the reactor section, but it is to be understood that a limitation referring to metal content of the chargestock refers to the metal content of the fresh feed prior to blending with such recycle streams.
Any metal~contaminated hydrocarbon oil is contemplated as useful in the present invention. The preferred oils are those of petroleum origin, such as crude petroleum, topped crude petroleum, atmospheric residua and vacuum residua.
However, metal-contaminated hydrocarbon boils derived from shale, coal, tar sands or other sources may be used. Mixtures of petroleum distillate oils and residua derived from petroleum are within the scope of this invention, as are blends of such residua with other hydrocarbon oils~ In general, metal-contaminated chargestocks that contain residua components are ; preferred in the process of this invention.

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`'''' -The chargestocXs suitable for the process of this invention are the higher boiling or the residual fractions separated at a distillation cut point o~ 410F, and pre~erably those separated at a cut point of about 650F. The charge-stocks preferably also should not contain more than 60 percent by weight of aromatic hydrocarbons.
Although metal-contaminated hydrocarbon chargestocks that contain at least about 0.50 ppm to about 15 ppm Nickel Equivalents of me~al are suitable for the proce~s of this invention, the pre~erred chargestocks are those that contain from at least about 0.50 ppm to 5.0 ppm. The particularly preferred chargestocks contain from 1.0 to about 3.0 ppm Nickel Equivalents of metal. It is of course to be understood that the individual oils making up the chargestock may contain metals contamination substantially greater or less than those specified, and such oils are usable, of course, provided that the finàl chargestock is within the limits specified. For example, a residuum that contains 5 ppm Nickel Equivalents of metal may be blended with an equal volume of distillate o~l to form the chargestock to the process, said chargestock then bein~
characterized by a metals content o~ about 2.5 ppm Nickel Equivalents, whlch is in the particularly preferred range.
Alternatively, the residual oil may be hydrotreated to demetalize it ~o a metals content o~ about 2.0 ppm Nickel Equivalents of metal, for example, and the demetalized resldual oil utilized as the sole oil in the chargestock to the process of this invention. Typically, demetalization conditions comprise a hydrogen pressure of about 500 to 3~000 psig, a hydrogen .~

circulation rate of about 1,000 to 15,000 scf/bbl of feed, a temperature o~ about 600F to 850F, a space velocity of 0.1 to 5.0 LHSV~ and the presence of a catalyst comprising a Group VI B metal and an iron group metal on an alumina support.
It will be recognized by those skilled in the art that FCC operation with chargestocks of the types specified for this invention normally leads to impractical levels of metal contamination of the circulating inventory of cracking catalyst. However, it has been discovered ~hat by conducting the regeneration of the catalyst in such a manner as to leave less than about 0.10 wt.% residual carbon on the regenerated catalyst, the effect of the metal poisons on the selectivity of the catalyst is markedly suppressed. This phenomenon is illustrated by the data shown in Table I, for example, which reports the results o~ cracking a typical gas oil on a catalyst poisoned with about 1200 ~ickel Equivalents of metal and regen-erated to the usual level o~ 0.2 wt.% residual carbon, and the cracking of a hydrotreated residual oil on the same catalyst regenerated under conditions such that only 0.02 wt.% residual ; carbon is present. Both cracking operations are for 75%
conversion of ~eed.
` TABLEI
Gas Oil, HDT Resid, Normal Operation Crackin~ at Iiow CR
% Loss in Gasoline 10 0 % Increase in Coke300 13 % Increase in H21000 220 ' It has furthermore been discovered that cracking metal-contaminated oils such as atmospheric residua with severely metal poisoned catalyst regenerated in the usual ma~ner to leave about 0.2 w~.% of carbon on catalyst results in very low catalytic activity when this feedstock is compared with gas oil. This discovery is a possible explanation for the lack of commercial utilization of residual oils in FCC. This loss of activity is surprisingly very mùch smaller when the catalyst is regenerated to contain less than about 0.25 wt.% çarbon, as illustrated in Table II.

p~, o ~o I ~ , i _ ~, o Q ,_1 ~ , ~i r~ ~ O ~ ' o ~a V ~ ~ O O ` co rl ~ ~ O (n ~D O L~~O

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m ~y I
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Columns (A) and (B) of Table II show the observed marked effect of the wt.% residua]. carbon (CReg). Column (C) is computed for a cat-to-oil ratio selected to give the same conversion with both high and low levels of residual carbon.
The reduction in dry gas and coke achieved with the 10W residual carbon catalyst is very subs~antial, i.e. the selectivity is markedly enhanced. The sensitivity of the residual oil (HDT residua) is unexpectedly large, as shown in the last line of the table. ~hese effects, while present, '~h'ave been~repo'rted to ~ç ~ery much smaller with a t' i al ~ ''' iï~ g st k, a shown in columns (D) and (E).
It is a necessary condition in the process of this invention that the catalyst be regenerated under such conditions as to leave less than about 0.10 wt.% residual carbon on the catalyst, and most preferably less than about 0.025 wt.%. It will be recognized by those skilled in the art that the common regeneration practices leave 0.1 to about 0.3 wt.% residual carbon on catalysts. However, regenerators have been designed which, unlike the usual regenerators, utilize excess air and convert substantially all of the carbon monoxide normally formed in regeneration to carbon dioxide. With such regenerators, it is characteristic for the carbon on regenerated catalyst to have values less than about 0.05 wt.%. All known regenerators have what may be characterized as a dense fluid bed; those ; 25 operating with excess oxygen are characteristically operatedat high dense fluid bed temperatures, usually above 1300F
but within metallurgical constraints of the equipment, currently about 1400F. These high temperaturès are required in order to ~f~ 6 completely combust the carbon monoxide. The actual level of residual carbon in this operation depends on the temperature of the dense bed as well as the residence time of the catalyst in the regenerator. Thus, temperatures, of at least about 1300F for the dense bed in the regenerator is a requirement for the process of the present invention. Typically, such regenerators operate with about 2~ excess oxygen in the flue gas, and the flue gas contains typically less than about 2000 ppm of carbon mono~ide. All o~ the foregoing remarks apply to catalysts that do not have a CO-combustion promoter present.
It has recently been discovered that the combustion of carbon monoxide in the regenerator section may be promoted by trace amounts up to about 5 ppm of an oxidation catalyst comprising at least one metal selected from the group consisting of periods 6 and 7 of Group VIII
of the periodic table and rhenium, as described in U.S.
Patent 4,072,600. Such promoter is effective to achieve complete CO-combustion and low levels of residual carbon on catalyst without encountering excessivly high temperatures either in the dense bed or in the cyclones of the regenerator. Furthermore, the use of such combustion promoter makes it possible to achieve complete CO-combustion in regenerators designed for partial CO-combustion, such as swirl regenerators, and promotes a more uniform regeneration therein. It is preferred to utilize cracking catalyst that contains a trace amount of CO-combustion promoter in the process of this invention.

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~ 4~6 Although any fluid cracking catalyst may be used in the proce~s ol this invention, it is preferred to use cracking ca~alyst o~ high ac~ivity and selectivity such as those containing crystalline alum~nosilicate zeoli~es, ~or example, zeolite X
or ~eolite Y, having pore ~iameters grea~er than about 6A.
Suitable catalysts are those described, for example, in U.S.
Patent 3~140,249. Where the metal-contaminated chargestock of the process of this invention contains a substantial fraction of residual oil, or is entirely composed of residual oil, the coke load on the regenerator will tend to be high by virtue o~
the deposition of what is commonly called "additive coke" on the catalyst. Thus, there will be more heat available from regneration than is required to heat the chargestock fed to the cracking section. It is contemplated in such situations that a catalyst cooler will be incorporated in the regenerator in order to sustain a heat-balanced operation. The steam generated by the catalyst cooler is useful as an adjunct in the cracking operation or in other parts of the refinery.
` Where the metal-contamina~ed chargestock fed to the cracking section comprises a large component of hydrotreated residual oil, it is preferred to introduce the feedstock with i an amount of dispersion steam in the range of about 1 to 15 wt.%
of the fresh feed. ~his dispersion steam is effective in reducing the contact time of the oil and catalyst in the cracking section and further serves to improve the selectivity of the cracking operation.

Those skilled in the art will recognize that the steady-state, or equilibrium concentration of metals in the circulating inventory of cracking catalyst can be affected and controlled by selection o~ the fresh catalyst makeup rate. It is contemplated, in the present inven~ion, in some instances to use larger than usual makeup rateS for thiS purpose, as circums~ances dictate.
~hus, makeup rates more ~han 2 percent per day and up to about 10 percent per day are contemplated. ~hus, the advantages of the process of this invention may be maximized by adjusting the degree 10 of hydrotreating and the catalyst makeup rate over wide ranges, depending on circumstances such as the cost o~ catalyst and Of hydrogen, ~or example, and the characteristics of the available oil-.

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Claims (12)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A process for the catalytic cracking of a metal-contaminated feedstock in a fluidized-bed cracking instal-lation in which catalyst circulates between a reactor where in fluidized form it contacts the charge and a regenerator in which carbon deposited on the catalyst in consequence of the contact is removed by combustion, a portion of the circulating inventory of catalyst undergoing periodic replacement by fresh catalyst, characterized by the facts that said combustion is so conducted as to leave no more than 0.1% by weight of carbon on catalyst returned there-from to the reactor and that said replacement is so conducted as to maintain from 700 to 5000 ppm of Nickel Equivalents of Metal on said circulating catalyst inventory.

2. A process acording to Claim 1 wherein the catalyst returned to the reactor contains less than 0.05%
by weight of carbon.

3. A process according to Claim 1 wherein the catalyst returned to the reactor contains less than 0.025%
by weight of carbon.

4. A process according to Claim 1, 2 or 3 wherein gas supplied to said regenerator for the purpose of effecting said combustion contains sufficient oxygen to result in the presence in the exhaust gas from the regenerator of 2% by weight of oxygen.

5. A process according to Claim 1, 2 or 3 wherein from 800 to 2000 ppm of Nickel Equivalents of Metal is maintained on said circulating catalyst inventory.

6. A process according to Claim 1, 2 or 3 wherein said replacement is conducted with respect to 2 to 10 weight percent of circulating catalyst inventory per day.

7. A process according to Claim 1, 2 or 3, the fresh feedstock to which contains 0.5 to 15 ppm Nickel Equivalents of Metal.

8. A process according to Claim 1, the feedstock to which comprises residual oil.

9. A process according to Claim 8 wherein the residual oil has been subjected to a hydrotreating.

10. A process according to Claim 1, 2 or 3 wherein the contacting of catalyst and feedstock is performed in the presence of 1 to 15% by weight, with reference to said feedstock, of steam.

11. A process according to Claim 1, wherein the catalyst comprises a zeolite.

12. A process according to Claim 11 wherein the catalyst contains up to 50 ppm of a metal of period 6 or 7 of Group VIII of the Periodic Table, or of rhenium.