Note: Descriptions are shown in the official language in which they were submitted.
~ ~3~ 3
PROCESS FOR THE FLUID CAT~LYTIC
CRACKING OF ~ HYDROCARBON FEEDSI'O~K
~ he present invention relates to a proeess for
the fluid catalytie eracking of a hydrocarbon feed-
stoek in a reaetor system comprising a cracking zone
and a regeneration zone in eommunication with eaeh
other.
The fluid eatalytie eraeking proeess has for many
years been eonsidered one of the major gasoline pro-
ducers for the refining industry. Typically the feed-
stocks employed have been distillates, e.g. gas oils
and operating conditions have been selected for
relatively high eonversion of the gas oil feedstocks
to permit maximum yields of gasoline. The cracking
of gas oils to gasoline is fairly well understood and
the limitations imposed on the ~eedstock gas oils are
quite well defined. The major limitations currently
placed on gas oil feedstocks are the amount of carbon
or "eoke" preeursors and the amount of metallie eom-
pounds contained in the feedstocks.
These eoke formers or precursors, which are
typically high molecular weight condensed ring hydro-
,~
....
37'93
carbons, are primarily a function o~ the crude typeand the boiling range of the material. Results ob-
tained by Conradson or Ramsbottom carbon residue
analyses represent approximate measures o~ these
compounds and of the coking propensity o~ the raw
oil. Normally it is desirable to limit the amounts
of these compounds in the gas oil feedstocks to 0.5%
by weight or less by the Conradson method. The reason
is that if the coke precursor co~pounds in the gas oil
are permitted to increase much, more coke may be de-
posited on the catalyst in the hydrocarbon reaction
zone than is required for the process heat balance.
Temperatures in the regeneration zonej where the coke
is oxidized to C0 and/or C02, can become excessive
and, equally important, the catalyst particle temper-
atures can rise to the point that the catalytic struct-
ure is damaged or destroyed with a resulting loss in
activity.
The second major limitation imposed on gas oils
is the content of organic nickel(Ni)-, vanadium(V)-,
and iron(~e)-containing compounds therein. These com-
pounds, commonly called "porphyrins", are distilled
into the high-boiling fractions of vacuum gas oils.
Addltlonally, inorganic metal compounds may be present.
Typically, the metal content should be limited to the
extent that the Nickel Equivalent defined by the
~i~3,793
equation NE = Ni + 0.3 x V, and expressed in ppm by
weigh-t, is les.s than 0.4. The circulati.ng catalyst
adsorbs the metals mentioned almost completely and
becomes poisoned. These metals~ in their active
state on the catalyst~ depress the yield of primary
gasoline product, promote various dehydrogenation
reactions, and can produce large quantities of hydrogen
and coke. This can lead to tremendous increases in
volumes of unwanted gases and very quickly overload
gas compressors and gas recovery facilities.
When one compares residual oils, such as whole
cru~es or atmospheric reduced crudes to typical gas
oils, it is apparent that in addition to being more
difficult if not impossible to vapori e completely,
the residual oils contain higher amounts of "coke"
precursors (as determined by Conradson or Ramsbottom
carbon residue) and larger quantities of metals Ni,
V and Fe. In spite of the potential processing diffi-
culties imposed by the presence of such contaminants,
the refiner has been prompted by the tigh.tening of
crude supplies to expand the characteristics of the
feedstock charged to the fluid cakalytic cracking
process beyond tho:se of the relatively clean distil-
lates s.uch as gas oils. In order to increase charge
stock availability to meet gasoline and fuel oil
demands higher-boiling feeds which were previously
~1~13793
considerecl only marginal or unsuitable because of sueh
contaminants are receiving increasing considerations.
The reaction conditions of the fluid catalytic
cracking process typically comprise a temperature in
the range of from L~25O to 550C, a pressure in the
range of from atmospheric to 4 bar abs., and a
catalyst-to-oil ratio ~elow 7. The catalyst-to-oil
ratio is the quotient of the circulation rate of the
catalyst and the feed rate of the hydrocarbon oil~
expressed in the same mass units.
In fluid catalytic cracking units, it is neces~
sary to remove heat at a controllable rate from the
regeneration ~one where spent catalyst is regener-
ated by burning off carbonaceous deposits, with an
o~ygen-containing gas, such as air, in order to maintain
equilibrium cracking conditions since the exothermic
heat of regeneration imparted to the eatalyst is trans-
mitted to the fresh oil feed to the eraeking reactor.
It is also necessary to remove heat eontinuously to
prevent undue regeneration temperature levels tending
to sinter and deaetivate the eatalyst by surface area
reduetion. When heavy hydrocarbon feeds, sueh as
atmospherie residues, vaeuum residues~ and heavy crude
oils are catalytically eraeked, a greater amount of
earbon deposits on the eatalyst partieles than in the
eraeking of feeds such as gas oil. When the spent
1~37~3
catalyst from the catalytic crackin~ of such heaYy
hydrocarbon feeds is regenerated by combustion
with an oxygen-containing gas- in the regeneration
zone, the heat removal problem is further aggravated
since more heat is released in the regenerator than
that which can be utili~ed in the process.
With the introduction of more active zeolite-
based cracking catalysts the above signalled problems
with respect to the heat balance, in particular at
maximum yield conditions, have become more serious,
since now the maximum product yield is achieved at a
lower coke make, and therefore the heat balance is
disturbed again. To avoid overcracking of the feed-
stock, when using the more se:Lective and active zeolite-
based cracking catalysts, a reduction in crackingseverity is necessary. The cracking severity is
usually expressed by the "severity parameter"
(C/O~Sy), wherein C is the catalyst circulation
rate, O is the oil feed rate and Sv is the space
velocity. C/O is commonly called the catalyst-to-oil
ratio. All rates are expressed in ~ass units,
usually tons/day. The severity reduction may be
effected in several ways. One method of lowering the
cracking severity is to reduce the catalyst
circulation rate C~ However, this results in a rise
of the temperature of the catalyst ;n the regeneration
11~37~3
zone because a reduced quantlty of catalyst has to
pick up about the sa~e quàntity o~ heat released by
the same amount of coke in the exothermic regeneration
reaction in order to provide the necessary heat for
the endothermic cracking reaction. As already ob-
served excessive regenerator temperatures cause
damage to the catalyst because of th.e resulting higher
catalyst particle temperatures, and may also cause
damage to the regenerator itself. Therefore, the further
lowering of the cracking severity will be constrained
by the maximum allowable regeneration temperature.
On.the other hand, increasing. the oil feed rate O
will cause problems with respect to the mixing and
vaporization of the feedstock, which is effected by
direct contact with regenerated, hot catalyst and
which is usually carried out in a so-called liftpot
and a so-called ri.ser zone. Since the hot regener-
ated catalyst coming from the regeneration zone is
used for supplying the re.quired heat for feedstock
vaporization, a r.eduction of, in .general, th.e catalyst-
to-oil ratio wl.:ll adversely affect the temperature in
the regeneration zone and the said feedstock vapor-
ization. Parti.cularl~ with the heavy hydrocarbon feed-
stocks mention.ed this causes probl`ems, as an in-
sufficient vaporization of the feedstock leads to anincreased coke~make in the cracking zone. Although a
~ ~ ~;375~3
certain coke-make is required in the fluid catalytic
cracking pro.cess b.ecau.se o~ the desired heat for feed-
stock vapori.zation, an excessive coke-make causes a
too high catalyst temperature during regeneration.
Another possible method to lower the cracking
severity is to increase the space velocity Sv, i.e.
the oil ~eed rate divided by the reactor catalyst
inventory, for instance by decreasing the reactor
catalyst inventory. However, this results in the same
problems as mentioned above with respect to reducing
the catalyst-to-oil ratio. Moreover, there is a limit
to the reduction of catalyst inventory, imposed by the
necessity to keep the upper level of the fluidized
bed abo~e the draw-off standpipes of the reactor~
In catalytic cracking reactor systems it is of
course desirable to regenerate the catalyst particles
as completely as possible and also to utili~e all the
heat that can be freed from the coke deposit by com-
busting it completely. In practice, howe~er, the com-
bustion is often .performed incompletely3 i.e. carbon
is burnt to carbon monoxide in the regeneration zone.
The carbon monoxide is then oxidized to carbon. dioxide
in a separate flue gas combustor, coupled to a waste
heat boiler, to reco~er the chemical and the sensible
heat present :in the fl.ue gas. The .reason for this
complicated scheme is that a complete combustion
~1~3'7S~3
directly in th.e regeneration zone would releas.e so
much heat that that would damage th.e cataly~st and/or
the regenerator materials. If it were possible to
keep the temperature in the regeneration zone below
725C, a generally accepted upper limit, while prac-
tisin~ the so-called "complete C0-combustion", then
thi.s would mean a tremendous simplification of the
catalytic cracking process. Unfortunately, most
presently used heavier feedstocks produce so much
coke deposit on the catalyst that complete C0-com-
bustion is not possible, when using those feedstocks.
From the above it appears ~at the operation of
heat-balanced, fluid catalytic cracking units is
constrained by several operational conditions which
adversely affect each other and that there is little
flexibility left to vary feedstocks and/or catalysts.
As it is moreover desirable to operate the process
such that dependent on the market demands it yields
either an optimum amount of middle distillates or an
optimum amount of gasoline, there is clearly a need
for an impro~ed fluid catalytic cracking process in
which the flexibility of the operation is .not con-
strained by th.e overall heat balance requirement of
the process, as dekermined by such parameters as the
maximum regenerator temperature, the minimum regener-
ator temperature, the capacity o~ the equipment
~ ~3793
usedg the acti.vity of the catal~st, a~d the coking
tendency of the feedstock.
It is thus contemplated to provide a process
for the ~luid catalytic cracking of hydrocarbon oils,
and in particular to process hereby more difficult-to-
crack and/or more coking feedstocks. Such feedstocks
include "residual oils", meaning all hydrocarbons,
regardless of their initial boiling points, which
contain heavy bottoms, such as tars, asphalts
(bitumens), asphaltenes, resins, etc. Accordingly,
a residual oil can be a whole crude, an atmospheric
reduced crude, or even the bottoms fraction, boiling
above about 570-600C, remaining after vacuum column
distillation. But more unconventional feedstocks, such
as propane and butane deasphalted oils, extracts and
thermally cracked flashed distillates are also included.
In general, feedstocks and/or feedstock mixtures with
an average carbon residue (as determined by the Con-
radson Carbon Residue of.Petroleum Products test,
ASTM designation D 18965) up to 2.0 mass. %, and a
Nickel Equivalent (as defined hereinabove) up to
1.4 ppm can be processed according to th.e process of
the present application. Like it is con.templated to
process a wide ran.ge of.feedstocksg it is likewise
intended to: use a wide range of catalysts. Conventional
cracking catalystsj e.g. amorphous catalysts based on
.3793
natural or sy~th.e~ic clays. or on si.lica-alumina, can be
us.ed, hut preferably more active and/or ~ore s-e.lective
cracking catalys;ts comprising crystalline alumino-
silicate or ironsilicate zeolites~ usually composited
with a siliceous matrix, such as silica-alumina, silica-
gel, etc., are considered for use. The process of the
invention therefore allows for a rather great variation
in feedstocks, catalysts, and products, consequently.
The activity/selectivity of a particular catalyst com-
position is conveniently expressed by its zeolitecontent. The zeolite content can be determined con-
veniently for instance in the way as described in
French patent specification 2,330~756. The zeolite-
containin~ catalysts preferably contain 1-20% of
zeolite, in particular a natural or synthetic alkali-
metal aluminosilicate zeolite, such as type X, type Y
or type L, wherein at least a substantial portion of
the ~kali metal has been replaced by hydrogen or other
metal ions, the remainder being silica-alumina. Slnce
the application of too much zeolite could result in
o~ercracking, i..e. an excessive gas make, the ~eolite
content pre.~erahly li:es between 1 and lQ% b.y mass,
based on the total catalyst composition.
It is therefore an object of the pre.sent invention
to provi.de a process for the fIuid catalyt:ic: cracking
of a hydrocarb.on feedstock, in particular a he.avy
93
res.idual one, in w~hich.t.he. heat balance o~ a reactor
system as defined. he.re.inafter is. not solely~ dependent
on the heat t,ransfer from the regeneration zone to the
cracking zone by the circulating catalyst.
It is further an object to provide a pro.cess by
which it will be possible to produce an optimum yield
of either gasoline or of middle distillates dependent
on market demands from heavy hydrocarbon feedstocks by
applying iIl particular zeolite-based catalysts.
It is another object to provide a process for the
fluid catalytic cracking which may be operated with
complete C0-combustion when regenerating the spent
coke-containing catalyst.
A general object of the present invention is to
increase the operational flexibility of the process,
independent of the type of feedstock processed or the
catalyst used.
These objects are accomplished according to the
invention, by keeping the temperature in the regener-
ation zone below 725C through removal of heat fromthe bed of the catalyst being regenerated through in-
direct heat exchange with the fresh.h~drocarbon f.eed-
stock to b,e: pro.ces:~ed.
The present invention thus reIate:s. to a process
for th.e fluid c.atalytic cracking of a hydrocarbon fe.ed-
stock in.a reactor sys.tem comprising at least a cracking
;3793
zo.ne and a ~e.generat.ion zone in.communicati,on w.i,th each
other, by contacting th.e said hydrocarbon fe.edstock in
the crackïng zone w;,th a hot cracking catalyst, there-
by caus.ing at .least part of the feedstock to be vapor-
ized and subsequently to be cracked to lower-boiling
hydrocarbon products under cracking conditions com-
prising a temperature in the range of from Ll25 to
550C, a pressure in the range of from atmospheric to
4 bar abs. and a catalyst-to-oil ratio below 7, after
which the lower boiling hydrocarbon products are
separated from spent catalyst and recovered while the
spent catalyst containing coke deposits is passed to
the regeneration zone where an oxygen-conkaining gas is
psssed through a bed of the catalyst, thereby oxidizing
the coke deposits with the production of heat, thus
regenerating and heating the catalyst, after which the
hot, regenerated catalyst is returned to the cracki.ng
zone, in wh.ich process the temperature in the regener-
ation zone is kept below 725C by removing heat ~rom
the bed of the catalyst being regenerated through in-
direct heat exchange with the fres.h hydrocarbon feed-
stock~ thereby heating said feedstock to a temperature
of at leal~.t 2~0C, ~hereafter said ~eedstock.is
introduced into .th.e cracking zone and evaporated at a
temperature:aboYe 2~0C by directly.contactin~.it with
the hot, regenerated catalyst.
11~3~ 3
The main advanta~e. of the process. in .accardance
with the inventi`on is that the heat balance will no
longer depend solely on the heat transfer from the
regeneration zone to the cracking zone by means of the
catalyst circulation. By pre-heating the feedstock in
the regeneration zone a possibility is b.eing offered
for withdrawing heat from the circulating catalyst,
but meanwhile still supplying this heat to the endo-
thermic cracking reaction. There.fore, the present
process will allow an increa.sed flexibility in cracking
severity without exceeding regeneration zone temper-
ature limitations. The vaporization upon contact of
preheated feedstock and hot catalyst takes place now
substantially faster, thereby minimizing the amount
of liquid hydrocarbons in the catalyst pores. The
liquid hydrocarbons remaining in the catalyst pores
are primarily the cause of the excessive coke deposits
when using heavy feedstocks.
It is observed that it is kno~n as s.uch.to with-
draw heat liberated during catalyst regeneration, for
instance, it is known to generate h.igh pres:sure steam
through indirect heat exchange between ~ater or
steam suppl.ied to. coils, i.e. a plurali:ty o~ in.ter-
connected tub.es, in the: catalyst .b.ed. In .such an
arrangement, how.e~`e.r, the heat is irr.e~ers.i.bly wi.th-
drawn from the rea`ctor system. In the pre.sent in-
37~3
~4
vention s.uch :heat liherated is o~ us.e in th.e saidreactor syste~ b.y pre.heating the feeds:tock to be
processed in the same system.
The products of the cracking process according
to the invention comprise in general lower molecular
weight and lower boiling compounds than ~ound in the
original feedstock. These products find particular
utility as feedstreams for petrochemical, polymer,
gasoline, and alkylate manufacture. The cracking
process can be optimized for a maximal production of
middle distillates, i.e. kerosine and gas oils,
approximate boiling ranges140~300C, and 180-370C,
respectively, or for a maximal gasoline and naphtha
production, boiling range approximately 30-200C,
but some gaseous compounds, such as the lower olefins,
will always be formed too.
It is possible to effect the indirect heat ex-
change of the bed of the catalyst being rege~erated
with the fre.sh hydrocarbon feedstock in several ways.
One may, for instance~ circulate a heat-exchange fluid
through heat-exchange pipes located in th.e regeneration
zone, and s.ubs.equently through a h.eat exchanger through
which th.e ~.e.edst.oc~ is led too. This. solution, however,
requires a .s.ubs.~antlal investment, and.i~ in general
not pre~erred:. Pre~erab.ly~ .the indi.rect h.eat exchange
of the catalyst being regenerated with the fresh
793
hydrocarbon feedsto.ck i.s, ef~ected b.y paasing s~id
feedstock th~ough one or more he.at exchange pi,pes
located in th.e. b.ed o~ the catalys-t .being .regenerated.
It has surprisingly been found that the presence of
the pipes in the fluid bed(occupying about Z0% of the
bed area) has moreover a positive influence upon the
bubble size and the bubble distribution of the air in
the bed. The h.orizontal tubes disrupt the bubble growth
by dividing the rising bubbles into a larger number
of smaller bubbles, and this in turn results in an
increase in the mass transfer coefficient from the air
to the catalyst particles, meaning that more coke can
be burnt at the same throughput of air.
The size and the thickness of such pipes, as well
as the kind of material to be used will be obvious to
experts in the art of designing heat exchange equipment
for the petroleum processing industry.
It has been found that during normal operation,
while the average temperature of the feedstock rises
to a temperature above 200 C, the velocity adjustment
of th.e feed~tock in the heat e~change pipes and the
.. . .
heat trans.~er w.ïthin th.e hydrocarbon,.f.eeds,tock are
such that the temperature of the :fe,eds:t.ock no,where
rises, too hi.gh..' Maximum, film temperatures. at .th,e inner
pipe'surface' of abo:ut 400C occas~i.onall~,oc.cur~ but
s.uch..temperatures do not cause any not:i:ceab,l'e thermal
~379
cracking of the h~drocar.b.ons., s.ince:.th.e...heat i.s im-
mediately trans~ferred to the bulk o~ the oi.l, and the
temperature .cons.eq.uently drops.
The initi.al temperature of khe fe.edstock to be
processed in the catalytic cracking unit, i.e. the
temperature befo.re the preheating according to the in-
vention, depends on its source and the degree of cool-
ing it underwent during its production. In refineries
it is common practice to minimize heat lo~ses, and
therefore the cracking feedstock, e.g. the residues of
an atmospheric or a vacuum distillation unit, are
transferred as quickly as they become available to the
fluid catalytic cracking unit, preferably via insulated
lines. The (initial) feedstock temperature would then
be about 200 or 250C, respeckively. The same applies,
mutatis mutandis, to other feedstocks such as gas oils,
deasphalted oils. and the like.
Sometimes oil-~ired charge-preheaters. may have to
be used, .for ins.tan.ce when th.e .initial :te~perature o~
the ~eeds.tock is; S.Q low that kh.e feedst.ock -. eYen when
preh.eated according to th.e ;nvention i.s not he~ated to
vaporization .temperatures. by the direct .contact:with.
th.e hot, freshly regenerated catalyst mas;s. On :the
other hand, c~argb pre-coolers ma~ haYe.to..~e. ~s.ed too
sometimes~ for ins:tance.~when the .f.eedstQck.i.s di.~ficult
to crack and highly::coke-~producing, whi;ch:would re.sult
in a too ~igh temperature of the catal~st particles.
3~793
17
Preheating the charge is costly, and precooling the
charge, apart fr~m being laborious, decreases the
VaporizatiQn of th.e feed upon contact with the hot
catalyst. As mentioned before, poor vaporization
5 leads to an increased coke-make and a decreased product
yield. Some very high coke-making hydrocarbon feeds
therefore are not suited for catalytic cracking ac-
cording to the prior art methods, but th.ey can be
cracked by preheating the charge in the regeneration
lO zone according to the invention.
According to a preferred embodiment o:f the process
of the invention the fresh hydrocarbon feedstock prior
to the indirect heat exchange with the catalyst being
regenerated may have a temperature of at least 80 and
15 not higher than 250 C. Preferably, the fresh feedstock
is heated by said heat exchange to a temperature in
the range of from 250-425C. Feedstocks of 80-150C
are called relatively cold, and those o:f 150 250C are
called relatively hot. The fresh feedstock is heated
20 preferably to a temperature in the range of from
325-400C in accordance with the process of the in-
vention. Temperatures. in the range of from 350-400C
should not be sustain.ed for too long a period, sin.ce
then thermal cracking could start to occur. ~he danger
25 3f this is ~inimal h.oweve'r, as long as. khe pre.~heated
feedstock is: fed di.rectly- to the' re.act'i.on zone.
J 1~3793
As e.xplained hereinb.e.fore,.it ~.ia des:irable but
often i.mpos.s.ible.to operate a fluid catalytic cracki.ng
process with.co~ple.te CO-combustion. This is parti-
cularly difficult .when one wants.to pro.cess heavi.er
coke-making feedstocks in reactor systems that were
designed for ligh.ter feedstocks~ such as gas oils. Due
to the pres.ent method of h.eat with.drawal from the
regeneration zone, o.peration with comp.lete CO-combustion
is possible, usin~ a wider range of` feedstocks. The
application therefore also relates to a process, where-
in the temperature in the regeneration zone is kept
below 725C while oxidizing any coke deposit on the
spent catalyst completely to carbon dioxide.
Although a temperature of 725C is considered to
be the upper limit for the temperature in the regener-
ation zone, it lS obvious that it is safer to operate
at temperatures below 725C, e.g., to maintain an
upper limit of 700C. On the other hand, the temper-
ature in the regeneration zone should.not fall too low,
not below e.g. 610C, for in that .case th.e comb.ustion
of the coke deposits would take.pla.ce too s.lowly, and,
moreover, the' catalyst mass would not .b.e hot. enough to
be able to. ~aporï.ze..the feed in the' cracking:zone.
It is conte~plated ~ithin the :scope :of.the pre.sent
invention to.cool part of t.he preh.eated.fe.eds:t'ock by
means of ind;rec't h.eat exchange with 'a :suitable cooling
medium s~uch. as. w~ate.r, to remoye any~.ex:cess. heat .which
cannot he accommodated in the reactor system. This may
be the case with.~ery high coke-making fe.eds and
particularly w.here t.he present process. is operated
wikh complete C0-combustion duri.ng spent catalyst
regeneration. In such cases so much h.eat may have to
be withdrawn from the catalyst bed to prevent th.e
temperature from rising above 725C that at least
part of the pre-heated hydrocarbon feedstock will
have to be cooled again externally by indirect heat
exchange with a cooling medium like water. The heat
thus contained in the cooling medium may be used in
other units of the refinery in which the catalytic
cracking unit is located. In other words, in a special
embodiment of the present process, at least part of
the feedstock after having been heated through in-
direct heat exchange with the catalyst beIng regener-
ated is cooled outsi.de the regeneration zon~ by in-
direct heat exchange with a cooling medi~m.
Thus pre-heate:d and subsequently partially re-
coo.led feeds.t.o:ck. may: he f'ed to the reaction.zone
directly, and in ca.se only part of the .f.eedst:ock was
re-cooled, after blending with the other porti.on of
the feedstock.,:.tha't was not re~cool.ed
~he f.leYi.bi.lity~ of th~e proce~s. and t~ te~per-
ature stability of the 'f'eedstock.'enter;.ng :the reaction
37~3
zone i.s. inc.reas~ed.how.eYer, when a certain amount. of
recycle is applied. Pr,efera~ly at .leas:t part of the
feedstock heated and.coo.led in the ab.ove-.described
way is recombined with.fresh feeds.tock to b.e heated
through indirect heat exchange with the catalyst being
regenerated.
If (some of~ t.he feedstock is h.eated and cooled
again, it turns out to be uneconomical if the feedstock
that is to be cooled~ is cooled to a temperature higher
than the temperature of the fresh hydrocarbon feed-
stock, for then one could have easier cooled with the
feedstock itself, e.g. simply by blending fresh and
heated feedstocks. Therefore, it is preferred that at
least part of the heated feedstock is cooled to a
temperature not higher than the temperature of the
fresh feedstock to be heated.
In the latter case at least part of the heat.ed
feedsto.ck is cooled to a temperature preferably in the
range of from 80 to 150C depending on the particular
temperature of .the .fe.edst:ock to be: heated.
As menti:oned ahove, all or part of the' heated
feedstock may .b.e:cool.ed after the h.e,ating in the
re.generati,on.zo~é.'.~or process economical :re.asons it is
. . .
preferred h.ow~e:~er' *hat a thi.rd to a tenth.of the said
he.ated .feedsto.ck is, cooled and b:lended wi~th f,re.s,h.
.
feed~tock, Thls. amounts to a recycle- ratio -of 0.11
3'793
to 0.50, when the .recycle ratio is: de~ined as the
ratio of rec~cle.d feed to fresh feed.
In some instances it is advantageous to burn a
so-called "torch oil" in th.e regenerat.ion zone. This
oil is a portion of the reaction product which is very
hard to crack and which is less suited for recycling
with fresh feed, in contradistinction to the.so-called
cycle oils. It is advantageously used t.h.en as a heating
oil, e.g. in the regeneration zone, in particular if one
aims at an optimum yield of middle distillates. The
added extra heat is easily transferred to the feed ac-
cording to the process of the invention. The application
therefore also relates to a process wherein a hydro-
carbon fuel is burnt in the regeneration zone.
The invention will now be illustrated with the aid
of the figures. Figure l depicts a simplified flow
scheme o~ a fluid catalytic cracking reactor system.
Figure 2 is a graphic illustration of the effect of pre-
heating'the feedstock on the catalyst~to oil ratio and
the regenerator temperature, and will be discussed in
detail in Example 2.
Referring.now :to figure l, fresh:.fe.eds.tock is
supplied through a line lg and via rege.nerator coils
16 and a line '17, i;ncluding a h~eat exchan.ger 18, the
fe.edstock.is. pr.ehe:a:t.e.d and introduc.ed into'li.ne 3.
It i.s possib.l'é to rec:~cle part of th.e :feedstock via a
line 1~ to pas;s thr~ugh.regenerator coils, 16 again~
resulting in a ~ore s:ta:ble temperature of the feedstock
in line 17 and a more stable temperature of fluid ~ed
13. The regenerator coils 16 are sub~erged in the
fluidized bed 13 of catalyst particles being.re-
generated.
In some ins.tances it may be advantageous.to install
another heat exchanger in line 1, either before or after
branch 2, in order to cool or heat excessively hot or
cold fresh feedstocks. It is also possible to install
the heat exchanger 18 in the recycle line 19, in
particular when processing very heavy, coke-making
feedstocks, for then a high temperature of the feed in
lines 3 or 17 (to facilitate the vaporization of the
heavy feedstock in the liftpot) has to he combined
with :a large cooling effort in the regenerator, because
of the extra coke being combusted. The feeds.t~o:ck is
passed through line 3 to a liftpot 4~ wherein the feed-
stock.i.s contact:ed with.hot, re.generat.ed catalyst sup-
plied through.'a s.tandpipe;5. Th.e feedstock'evàporatesand travels wi.th.th.e' catalyst parti.cles. through a riser :
reactor 6~ in up~ard p.lug-flow fashi:on,.to a fluid
bed 7 in a reactor .~ss.sel ~8. The gass.ous and vapor-
ized liqui.d crack:ed products are remoYed throu~h.the
top of th.e'.~ssssl' 8. via a con~ui`t ~..Conduit 2 l:S con-
nected to cyclonss (not~shown) remoYing sntrained solid
..,
733
catalys.t parti.c;les~ ~rom th.e gas stre.a~ and returning
those particles. to th.e: fluid bed 7 vîa dî.p.legs (also
not shown). The spent catalyst particles -are stripped
from any adhering hydrocarbons in a stripper vessel 10,
located under th.e rea.ctor vessel 8. Th.en the stripped,
spent catalyst particles are fed via a standpipe 11 to
a regenerator vessel 12. In thîs vessel a fluidized
bed 13 of catalyst particles is maintained by an
oxygen-containing gas stream, usually air, supplied
via a nozzled pipe 14. The carbon deposlts on the
catalyst particles are burnt off in the bed 13 and
through the standpipe 5 the regenerated catalyst
particles return to the liftpot 4. The combustion
gases are removed at the top of the regenerator vessel
12 through a conduit 15.
In the process. accor~ing to the invention, line 2
is not used, at least not necessarily. If may, of
course, be and generally is present as a remainder
of a con.ventional reactor system~ in case of a revamp
of such conventional system for implementing the
present proces.s, and it will be used with advantage
for start-up.!purposes,. or as a s~a-fety relief line. To
these ends .the:re is: p.rovided a ~alve (not :shD~n) at
th.e junction of lines: 1 and 2, enabli:ng one:to direct
the ~eedst.ock:.w.holly or parti.ally- into line.:2 as
desired~
~ ~3~733
24
EXAMPLE 1
The fluid catal~tic cracking unit of a refinery
is first operated conventionally, but with complete
C0-combustion. HoweYer, in this mode o~ operation
it is only poss.ible to process cold feed (in t.he range
of 50-80C), as any increase of the feed temperature
results in an unacceptable increase :of the regener-
ator temperature. In the regenerator a high oxygen
slip (excess oxygen) has to be maintained to cool the
regenerator bed.
In order to apply the process of the present in-
vention this unit is revamped by installing preheating
coils. in the regenerator. Because of the particular
size and shape of this regenerator, two parallel sets
of three coils in series are installed. The coils
occupy the cross-section of the vessel between the top
of the manhole and the lowest level of the draw-off
bin. Each coil has its own feed inlet and outlet and
each coil consti:tutes a self-supporting .beam requiring
only end s.upports. Furthermore, each coil is con-
structed s.uch.that .it is ab.le to pass through the
manhole. Th~ .t.ube: all~oy .selected permits operation at
! a maximum .re.generator .temperature of 700.C and a .design
press.ure of 25. har ahs. In th.e desi:gn th;e necessary
attenti.on.is gi.Yen.~to .thermal ex:pan$ion as:.pect:s:. The
total length of pîping inside th.e re.generator is
9~
sufficient .to. heat up the feed fro~ .200 to .370C at
a feed rate of 3000 tons/day.
The reaction conditions and the .re.sults; of the
measures taken can b.e .seen in the following Table:
conven- oper-
tional ation
oper- with
ation _ preheating
.. .. , . _ _
reactor temperature C 510 510
" pressure bar 1.3 1.3
catalyst-to-oil ratio 9 6
max. regenerator
temperature C 660 660
initial temperature
of feed C 50-80 200
liftpot entrance
temperature of the feed C50-80 370
regenerator oxygen
slip %vol 5 2
regenerator air/coke
ratio 19.5 15
. _
Conradson number
of feed %mass 0.4 2.0 004 0.4
Ni-equivalent of
feed ppm 0.4 0.4. 1.4 0.4
catalyst zeolite
~ c-ontent ~ %mass 3 3 3 5
x determined as described in French pat. spec. 2.,330,756.
1~3793
26
Obviously the operational flexibility increases as
a result of the preheating according to the invention.
The table illustrates three cases according to the in-
vention:
1) more coke-producing feedstocks can be processed,
2) more metal-containing feedætocks can be processed,
and
3) more active/selective catalyæt can be used.
Moreover less oxygen is used in the regenerator
for cooling purposes, and warmer feeds can be processed,
thus obviating the need to pre-cool the feedstock.
EXAMPLE 2
In figure 2~ the effect of preheating the feed
in regenerator coils 16 according to the invention is
shown. The re~eneration zone temperature Treg is
plotted as a function of the catalyst-to-oil ratio,
C/O. Clearly, a reduction of the catalyst feed rate C
results in a rise of Treg, which is to be kept below
72~C. The reduction of C is necessary, however, to
lower the cracking severity~ e.g. when using more
active/selective catalysts.
Two cases are illustrated for a fluid catalytic
cracking reactor system according to figure 1.
Series A 1n conventional heat-balanced catalytic
cracking operation~ using line 2, and series B in
accordance with the invention, by-passing line 2 and
11~3~793
preheating the feed in the regeneration zone 12,
without heat exchange in exchanger 18 or recirculation
through line 19. In all measurements the feed was
introduced in line 1 at a temperature of 200C, and
the reactor/riser outlet temperature was kept at
520C. Curve A shows the results of the heat-balanced
operation of serles A, and curve B shows the results
of the heat-balanced operation of series B, while pre-
heating the feed in the regenerator coils to 1l00C.
Curve B is shifted towards lower C/0 ratios and lower
regenerator temperatures, proving that in the process
according to the invention more selective/active
catalysts and/or more coke-producing feedstocks can
be used, without exceeding the maximum regenerator
temperature of 725C.
