Note: Descriptions are shown in the official language in which they were submitted.
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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.
5~
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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s~
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.
: ~8-
, ~ .
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.
~,~q~
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
.~
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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.
-13-
p~, o ~o I ~ , i
_ ~, o
Q ,_1
~ , ~i
r~ ~ O ~ ' o
~a V ~ ~ O O ` co
rl ~ ~ O (n ~D O L~~O
E~ o o~ o
H ~ m ~ O ~ ~O ~O L~
~ ~ O
m ~y I
~ ~d -- o o . ~ ~ ~ . ~
~ ¢ ~o O ~ O ~ ~J
,_
o
, ~ o ~ C~
.~ ~, ~ V
,; V ~ o U~
. ~ . ~ o ~ ~o
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3 ~ Q.
,0 ~ ~ O .
~ J~ ~
a~ ~ ~ o ~ ~ ~ ~) O v
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-
~5~
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.
-16~
~ 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.
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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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