U.S. patent application number 09/811165 was filed with the patent office on 2002-02-07 for recracking mixture of cycle oil and cat naphtha for maximizing light olefins yields.
Invention is credited to Swan, George A. III.
Application Number | 20020014438 09/811165 |
Document ID | / |
Family ID | 26893285 |
Filed Date | 2002-02-07 |
United States Patent
Application |
20020014438 |
Kind Code |
A1 |
Swan, George A. III |
February 7, 2002 |
Recracking mixture of cycle oil and cat naphtha for maximizing
light olefins yields
Abstract
A process for increasing the yield of C.sub.3 and C.sub.4
olefins by injecting light cat naphtha and cat cycle oil together
with steam into an upstream reaction zone of a FCC riser reactor.
The products of the upstream reaction zone are conducted to a
downstream reaction zone and combined with fresh feed in the
downstream reaction zone.
Inventors: |
Swan, George A. III; (Baton
Rouge, LA) |
Correspondence
Address: |
EXXONMOBIL RESEARCH AND ENGINEERING COMPANY
P.O. BOX 900
1545 ROUTE 22 EAST
ANNANDALE
NJ
08801-0900
US
|
Family ID: |
26893285 |
Appl. No.: |
09/811165 |
Filed: |
March 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60197920 |
Apr 17, 2000 |
|
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|
Current U.S.
Class: |
208/113 ;
208/120.01; 585/648; 585/649 |
Current CPC
Class: |
C10G 11/18 20130101;
C10G 51/02 20130101 |
Class at
Publication: |
208/113 ;
208/120.01; 585/648; 585/649 |
International
Class: |
C10G 011/00 |
Claims
What is claimed is:
1. A fluid catalytic cracking process to increase yields of C.sub.3
and C.sub.4 olefins which comprises: (a) conducting hot regenerated
catalyst to an FCC unit having at least one riser reactor
containing a downstream and an upstream reaction zone, (b)
contacting hot catalyst with a cycle oil, a light cat naphtha, and
steam in the upstream reaction zone at a temperature of from about
620 to 775.degree. C. and a vapor residence time of cycle oil,
naphtha, and steam of less than 1.5 sec. wherein at least a portion
of the C.sub.5 to C.sub.9 olefins present in the light cat naphtha
is cracked to C.sub.3 and C.sub.4 olefins, and wherein at least a
portion of the saturates in the cycle oil is converted to lower
boiling point products including C.sub.3 and C.sub.4 olefins, (c)
contacting at least the catalyst products of cycle oil and naphtha
cracking, and steam from the upstream reaction zone with a heavy
feedstock in the downstream reaction zone at an initial temperature
of from about 600 to 750.degree. C. with vapor residence times of
less than about 20 seconds, (d) conducting spent catalyst, cracked
products and steam from the first and second reaction zones to a
separation zone, (e) separating from the cracked products a cycle
oil fraction, a light cat naphtha fraction, and steam from spent
catalyst and recycling at least a portion of the cycle oil fraction
and light cat naphtha fraction to the upstream reaction zone in
step (b), (f) conducting spent catalyst to a stripping zone and
stripping spent catalyst under stripping conditions, and (g)
conducting stripped spent catalyst to a regeneration zone and
regenerating spent catalyst under regeneration conditions.
2. The process of claim 1 wherein the amount of steam in the
upstream reaction zone is from 2 to 50 wt. %, based on total weight
of light cat naphtha and cycle oil.
3. The process of claim 1 wherein the residence time of cycle oil,
naphtha, and steam in the upstream reaction zone is less than about
1 sec.
4. The process of claim 1 wherein process conditions in step (b)
include catalyst/oil ratios of 75-150 (wt/wt) at pressures of
100-400 kPa.
5. The process of claim 1 wherein process conditions in step (c)
include catalyst/oil ratios of 4-10 at pressures of 100-400 kPa and
vapor residence times of 2-20 sec.
6. The process of claim 1 wherein the feedstock in step (c)
includes from 1 to 15 wt. %, based on feedstock, of a higher
boiling fraction with initial boiling point greater than
565.degree. C.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This patent application claims benefit of U.S. provisional
patent application 60/197,920 filed Apr. 17, 2000.
FIELD OF THE INVENTION
[0002] This invention relates to a fluid catalytic cracking
process. More particularly, a mixture of cycle oil, light cat
naphtha, and steam are added to the reaction zone to improve yields
of light olefins.
BACKGROUND OF THE INVENTION
[0003] Fluid catalytic cracking (FCC) is a well-known method for
converting high boiling hydrocarbon feedstocks to lower boiling,
more valuable products. In the FCC process, the high boiling
feedstock is contacted with fluidized catalyst particles in the
substantial absence of hydrogen at elevated temperatures. The
cracking reaction typically occurs in the riser portion of the
catalytic cracking reactor. Cracked products are separated from
catalyst by means of cyclones and coked catalyst particles are
steam-stripped and sent to a regenerator where coke is burned off
the catalyst. The regenerated catalyst is then recycled to contact
more high-boiling feed at the beginning of the riser.
[0004] Typical FCC catalysts contain active crystalline
aluminosilicates such as zeolites and active inorganic oxide
components such as clays of the kaolin type dispersed within an
inorganic metal oxide matrix formed from amorphous gels or sols
that bind the components together on drying. It is desirable that
the matrix be active, attrition resistant, selective with regard to
the production of hydrocarbons without excessive coke make and not
readily deactivated by metals. Current FCC catalysts may contain in
excess of 40 wt. % zeolites.
[0005] There is a growing need to utilize heavy streams as feeds to
FCC units because such streams are lower cost as compared to more
conventional FCC feeds such as gas oils and vacuum gas oils.
However, these types of heavy feeds have not been considered
desirable because of their high Conradson Carbon (con carbon)
content together with high levels of metals such as sodium, iron,
nickel and vanadium. Nickel and vanadium may lead to excessive "dry
gas" production during catalytic cracking. Vanadium, when deposited
on zeolite catalysts can migrate to and destroy zeolite catalytic
sites. High con carbon feeds lead to excessive coke formation.
These factors result in FCC unit operators having to withdraw
excessive amounts of catalyst to maintain catalyst activity. This,
in turn, leads to higher costs from fresh catalyst make-up and
deactivated catalyst disposal.
[0006] U.S. Pat. No. 4,051,013 describes a cat cracking process for
simultaneously cracking a gas oil feed and upgrading a
gasoline-range feed to produce high quality motor fuel. The
gasoline-range feed is contacted with freshly regenerated catalyst
in a relatively upstream portion of a short-time dilute-phase riser
reactor zone maintained at first catalytic cracking conditions and
the gas oil feed is contacted with used catalyst in a relatively
downstream portion of the riser reaction zone which is maintained
at second catalytic cracking conditions. U.S. Pat. No. 5,043,522
relates to the conversion of paraffinic hydrocarbons to olefins. A
saturated paraffin feed is combined with an olefin feed and the
mixture contacted with a zeolite catalyst. The feed mixture may
also contain steam. U.S. Pat. No. 4,892,643 discloses a cat
cracking operation utilizing a single riser reactor in which a
relatively high boiling feed is introduced into the riser at a
lower level in the presence of a first catalytic cracking catalyst
and a naphtha charge is introduced at a higher level in the
presence of a second catalytic cracking catalyst. U.S. Pat. No.
5,846,403 discloses an FCC reaction wherein a mixture of light
catalytically cracked naphtha ("light cat naphtha" or "LCN") and
steam is injected into an FCC riser at a point upstream of gas oil
or residual oil injection. Such LCN and steam coinjection results
in augmented light olefin production in the FCC unit.
[0007] It would be desirable to have improved FCC processes capable
of increasing light olefin yield while at the same time reducing
dry gas.
SUMMARY OF THE INVENTION
[0008] It has been discovered that adding a mixture of cycle oil,
light cat naphtha, and steam to an upstream reaction zone in an FCC
process results in improved light olefin yields compared to base
operation and a decrease in dry gas compared to neat light cat
naphtha recycle. Accordingly, the present invention relates to a
fluid catalytic cracking process for upgrading feedstocks to
increase yields of C.sub.3 and C.sub.4 olefins, the process
comprising:
[0009] conducting hot regenerated catalyst to an FCC unit having at
least one riser reactor containing a downstream and an upstream
reaction zone,
[0010] contacting hot catalyst with a cycle oil, a light cat
naphtha, and steam in the upstream reaction zone at a temperature
of from about 620 to 775.degree. C. and a vapor residence time of
cycle oil, naphtha, and steam of less than 1.5 sec. wherein at
least a portion of the C.sub.5 to C.sub.9 olefins present in the
light cat naphtha is cracked to C.sub.3 and C.sub.4 olefins and
wherein at least a portion of the cycle oil's saturated species are
converted into lower boiling point products including C.sub.2 to
C.sub.5 olefins.
[0011] contacting the catalyst, cracked cycle oil products, and
cracked naphtha products, and steam from the upstream reaction zone
with a heavy feedstock in the downstream reaction zone at an
initial temperature of from about 600 to 750.degree. C. with vapor
residence times of less than about 20 seconds,
[0012] conducting spent catalyst, cracked products and steam from
the first and second reaction zones to a separation zone,
[0013] separating from the cracked products a cycle oil fraction, a
light cat naphtha fraction, and steam from spent catalyst and
recycling at least a portion of the cycle oil fraction and light
cat naphtha fraction to the upstream reaction zone in step (b),
[0014] conducting spent catalyst to a stripping zone and stripping
spent catalyst under stripping conditions, and
[0015] conducting stripped spent catalyst to a regeneration zone
and regenerating spent catalyst under regeneration conditions.
[0016] In another embodiment, the invention is related to a product
formed in accord with such a process.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a flow diagram showing the two zone feed injection
system in the riser reactor.
[0018] FIG. 2 shows the selectivity for olefins compared to dry gas
for various LCN:cycle oil ratios.
DETAILED DESCRIPTION OF THE INVENTION
[0019] The invention is based on the discovery that increased
C.sub.3 and C.sub.4 olefin production over conventional (i.e.,
base) FCC operation and decreased dry gas production compared to
neat LCN recycle in an FCC process may be obtained by injecting
cycle oil, LCN, and steam at a point upstream of heavy feed
injection. More particularly, the invention is related to an FCC
riser reactor having at least one two-zone riser reactor wherein
cycle oil, LCN, and steam are injected into a second zone upstream
of a first zone, the heavy feed being injected into the first
zone.
[0020] The riser reactor of a typical FCC unit receives hot
regenerated catalyst from the regenerator. Fresh catalyst may be
included in the catalyst feed to the riser reactor. A lift gas such
as light hydrocarbon vapors, or steam may be added to the riser
reactor to assist in fluidizing the hot catalyst particles. In the
present process, cycle oil, light cat naphtha, and steam are added
in an upstream zone of the riser reactor. The cycle oil may include
heavy cycle oil, light cycle oil, and mixtures thereof. Heavy cycle
oil refers to a hydrocarbon stream boiling in the range of
240.degree. C. to 370.degree. C. (about 465.degree. F. to about
700.degree. F.). Light cycle oil refers to a hydrocarbon stream
boiling in the range of 190.degree. C. to 240.degree. C. (about
375.degree. F. to about 465.degree. F.) . Light cat naphtha refers
to a hydrocarbon stream having a final boiling point less than
about 150.degree. C. (300.degree. F.) and containing olefins in the
C.sub.5 to C.sub.9 range, single ring aromatics (C.sub.6-C.sub.9)
and paraffins in the C.sub.5 to C.sub.9 range. Cycle oil and light
cat naphtha ("LCN") is injected into the upstream reactor zone
together with 2 to 50 wt. % of steam, based on total weight of
cycle oil and LCN. The cycle oil, LCN, and steam have a vapor
residence time in the upstream zone of less than about 1.5 sec.,
preferably less than about 1.0 sec, and more preferably less than
0.5 seconds. Cat/oil ratios range from 75-150 (wt/wt) at pressures
of 100 to 400 kPa and temperatures in the range of 620-775.degree.
C. The addition of cycle oil, steam, and LCN in this upstream zone
results in increased C.sub.3 and C.sub.4 olefins yields by cracking
C.sub.5 to C.sub.9 olefins in the LCN feed and cracking principally
saturated species in cycle oil to produce naphtha and lighter
products.
[0021] Conventional heavy FCC feedstocks having a boiling point in
the 220-575.degree. C. range such as gas oils and vacuum gas oils
are injected in the downstream riser reaction zone. Small amounts
(1-15 wt. %) of higher boiling fractions such as vacuum resids may
be blended into the conventional feedstocks. Reaction conditions in
the downstream reaction zone include initial temperatures of from
600-750.degree. C. and average temperatures of 525-575.degree. C.
at pressures of from 100-400 kPa and cat/oil ratios of 4-10 (wt/wt)
and vapor residence times of 2-20 seconds, preferably less than 6
seconds.
[0022] Suitable catalysts include any catalyst typically used to
catalytically "crack" hydrocarbon feeds. It is preferred that the
catalytic cracking catalyst comprise a crystalline tetrahedral
framework oxide component. This component is used to catalyze the
breakdown of primary products from the catalytic cracking reaction
into clean products such as naphtha for fuels and olefins for
chemical feedstocks. Preferably, the crystalline tetrahedral
framework oxide component is selected from the group consisting of
zeolites, tectosilicates, tetrahedral aluminophosphates (ALPOs) and
tetrahedral silicoaluminophosphates (SAPOs). More preferably, the
crystalline framework oxide component is a zeolite.
[0023] Zeolites which can be employed in accordance with this
invention include both natural and synthetic zeolites. These
zeolites include gmelinite, chabazite, dachiardite, clinoptilolite,
faujasite, heulandite, analcite, levynite, erionite, sodalite,
cancrinite, nepheline, lazurite, scolecite, natrolite, offretite,
mesolite, mordenite, brewsterite, and ferrierite. Included among
the synthetic zeolites are zeolites X, Y, A, L. ZK-4, ZK-5, B, E,
F, H, J, M, Q, T, W, Z, alpha and beta, ZSM-types and omega.
[0024] In general, aluminosilicate zeolites are effectively used in
this invention. However, the aluminum as well as the silicon
component can be substituted for other framework components. For
example, the aluminum portion can be replaced by boron, gallium,
titanium or trivalent metal compositions which are heavier than
aluminum. Germanium can be used to replace the silicon portion.
[0025] The catalytic cracking catalyst used in this invention can
further comprise an active porous inorganic oxide catalyst
framework component and an inert catalyst framework component.
Preferably, each component of the catalyst is held together by
attachment with an inorganic oxide matrix component.
[0026] The active porous inorganic oxide catalyst framework
component catalyzes the formation of primary products by cracking
hydrocarbon molecules that are too large to fit inside the
tetrahedral oxide component. The active porous inorganic oxide
catalyst framework component of this invention is preferably a
porous inorganic oxide that cracks a relatively large amount of
hydrocarbons into lower molecular weight hydrocarbons as compared
to an acceptable thermal blank. A low surface area silica (e.g.,
quartz) is one type of acceptable thermal blank. The extent of
cracking can be measured in any of various ASTM tests such as the
MAT (microactivity test, ASTM #D3907-8). Compounds such as those
disclosed in Greensfelder, B. S., et al., Industrial and
Engineering Chemistry, pp. 2573-83, November 1949, are desirable.
Alumina, silica-alumina and silica-alumina-zirconia compounds are
preferred.
[0027] The inert catalyst framework component densities,
strengthens and acts as a protective thermal sink. The inert
catalyst framework component used in this invention preferably has
a cracking activity that is not significantly greater than the
acceptable thermal blank. Kaolin and other clays as well as
.alpha.-alumina, titania, zirconia, quartz and silica are examples
of preferred inert components.
[0028] The inorganic oxide matrix component binds the catalyst
components together so that the catalyst product is hard enough to
survive interparticle and reactor wall collisions. The inorganic
oxide matrix can be made from an inorganic oxide sol or gel which
is dried to "glue" the catalyst components together. Preferably,
the inorganic oxide matrix will be comprised of oxides of silicon
and aluminum. It is also preferred that separate alumina phases be
incorporated into the inorganic oxide matrix. Species of aluminum
oxyhydroxides .gamma.-alumina, boehmite, diaspore, and transitional
aluminas such as .alpha.-alumina, .beta.-alumina, .gamma.-alumina,
.delta.-alumina, .epsilon.-alumina, .kappa.-alumina, and
.rho.-alumina can be employed. Preferably, the alumina species is
an aluminum trihydroxide such as gibbsite, bayerite, nordstrandite,
or doyelite.
[0029] Coked catalyst particles and cracked hydrocarbon products
from the upstream and downstream reaction zones in the riser
reactor are conducted from the riser reactor into the main reactor
vessel which contains cyclones. The cracked hydrocarbon products
are separated from coked catalyst particles by the cyclone(s).
Coked catalyst particles from the cyclones are conducted to a
stripping zone where strippable hydrocarbons are stripped from
coked catalyst particles under stripping conditions. In the
stripping zone, coked catalyst is typically contacted with steam.
Stripped hydrocarbons may be combined with cracked hydrocarbon
products and recovered for storage or further processing.
[0030] After the coked catalyst is stripped of strippable
hydrocarbon, the catalyst is then conducted to a regenerator.
Suitable regeneration temperatures include a temperature ranging
from about 1100 to about 1500.degree. F. (593 to about 816.degree.
C.), and a pressure ranging from about 0 to about 150 psig (101 to
about 1136 kPa). The oxidizing agent used to contact the coked
catalyst will generally be an oxygen-containing gas such as air,
oxygen and mixtures thereof. The coked catalyst is contacted with
the oxidizing agent for a time sufficient to remove, by combustion,
at least a portion of the carbonaceous deposit and thereby
regenerate the catalyst.
[0031] Referring now to FIG. 1, hot catalyst 10 from the
regenerator (not shown) is conducted through regenerated catalyst
standpipe 12 and slide valve 14 into the "J" bend pipe 16 which
connects the regenerator standpipe 12 to the riser reactor 32. Lift
gas 20 is injected into pipe 16 through injection nozzle 18 thereby
fluidizing hot catalyst particles 10. Cycle oil and light cat
naphtha 22 together with steam 24 are injected into upstream
reaction zone 34 through nozzle 26; multiple injection nozzles may
be employed. In reaction zone 34, C.sub.5 to C.sub.9 olefins in the
LCN are cracked to C.sub.3 and C.sub.4 olefins. Moreover, at least
a portion of the saturated species present in the cycle oil is
converted to lower boiling point products including light olefins.
This reaction is favored by short residence times and high
temperatures. Cracked hydrocarbon products, partially deactivated
catalyst and steam from reaction zone 34 are conducted to
downstream reaction zone 36. In reaction zone 36, conventional
heavy FCC feedstocks 28 are injected through multiple injection
nozzles 30 and combined with the cracked hydrocarbon products,
catalyst and steam from reaction zone. Residence times in zone 36
are longer which favor conversion of feed 28. Cracked products from
zone 34 and 36 together with coked catalyst and steam are then
conducted to the reactor vessel containing cyclones (not shown)
where cracked products are separated from coked catalyst
particles.
[0032] The LCN:cycle oil ratio at injection should range from 0.1
to 0.75, based on the combined weight of cycle oil and LCN.
Preferably the ratio ranges from about 0.1 to about 0.6, and more
preferably from about 0.2 to about 0.5.
[0033] The invention will now be further understood by reference to
the following examples.
EXAMPLES
[0034] Comparative recycle options for short contact time FCC units
were evaluated using a process model based on an existing FCC unit.
Accordingly, the calculation directly compared existing unit
performance with calculated results reflecting LCN and cycle oil
injection in admixture with the heavy feed and approximately two
meters upstream of the primary feed injectors. A cat cycle oil
("CCO") with boiling range of 240/370.degree. C., light cat naphtha
(LCN) with 10/100.degree. C. boiling range, a constant fresh feed
rate of 172 m.sup.3 hr, and nominal recycle rate of 10 m.sup.3/hr
were used in this example. The heavy feed employed contained VGO
and about 4 wt. % vacuum resid.
[0035] Feed properties are summarized in Table I.
1TABLE I FEEDSTOCK PROPERTIES VGO VAC RESID Gravity, API 23.8 11.4
Sulfur, wt. % 1.10 1.40 Thiophenic sulfur, wt. % 0.88 1.12
Nitrogen, wppm 1369 4111 Basic nitrogen, wppm 413 1247 Conradson
carbon, wt. % N.A. 15.3
[0036] Catalyst properties are set forth in Table II:
2TABLE II CATALYST PROPERTIES Unit Cell, .sym. 24.27 Surface area,
m.sup.2/gm 0.80 ABD, gm/cc 0.40 Pore Vol., cc/gm 1.52 REO, wt. %
1930 V, wppm 4150 Ni, wppm 61
[0037] When recycle was employed, air blower rate ranged about 4%
above the base case. Maximum catalyst circulation rate was 19
tons/min. Table III summarizes simulation results for neat LCN,
neat CCO, and several examples of their blends.
3TABLE III 2 3 4 5 6 7 1 LCN CCO LCN & CCO LCN & CCO LCN
& CCO LCN & CCO CASE # BASE RECYCLE RECYCLE RECYCLE RECYCLE
RECYCLE RECYCLE FRESH FEED RATE, T/HR 157.1 157.1 157.1 157.1 157.1
157.1 157.1 TOTAL FEED RATE, T/HR 157.1 164.2 166.2 164.8 164.5
164.3 164 FEED TEMP, DEG C. 270 270 270 270 270 269 269 REACTOR
TEMP, DEG C. 525 525 525 525 525 525 525 REGEN TEMP, DEC C. 706 698
702 700 700 699 699 CAT CIRC, T/M 17.27 18.86 18.47 18.6 18.6 18.7
18.8 CAT/OIL WT/WT (TOTAL FD) 6.6 6.89 6.67 6.76 6.8 6.84 6.86
TOTAL AIR, KNM3/HR 84.63 87.91 88.48 87.97 87.97 87.97 87.97 LCN
RECYCLE, T/HR 0 7.1 0 2.3 3.5 4.6 5.2 CCO RECYCLE, T/HR 0 0 9.1 5.4
3.9 2.6 1.8 PRE-INJ VAP RES TIME, SEC N/A 0.35 0.4 0.39 0.38 0.37
0.37 PRE-INJ TEMP, DEG C. N/A 691 698 695 694 693 693 430 F.
CONVERSION, WT % 72 70 71.87 71.24 70.96 70.67 70.5 02 - DRY GAS
3.63 4.49 3.94 4.1 4.2 4.29 4.34 PROPYLENE, WT % FF 3.94 4.38 4.08
4.18 4.23 4.28 4.3 PROPANE, WT % FF 1.28 1.43 1.41 1.42 1.42 1.43
1.43 BUTYLENES, WT % FF 5.41 6.06 5.67 5.8 5.87 5.94 5.96 BUTANES,
WT % FF 2.83 2.93 2.89 2.9 2.91 2.92 2.92 LCN (C5/100 C.) WT % FF
25.04 20.02 24.35 22.96 22.22 21.52 21.17 CCO (240/370 C.) WT % FF
16.39 17.25 14.93 15.77 16.13 16.49 16.7 BTMS (370 c+), WT % FF
9.37 10.55 10.82 10.69 10.64 10.59 10.56 COKE, WT % EF 4.83 4.98
5.02 4.99 4.99 4.99 4.99
[0038] Feed pre-heat and riser outlet temperatures are constant for
each example. At approximately constant total recycle rate, the
highest light olefin yields and the highest olefin to dry gas
selectivity are achieved with LCN and CCO recycle. Cases with
recycle of LCN and CCO streams in admixture with base gas oil feed
results in improvements that are much less pronounced. Dry gas
yields increase with increasing LCN recycle. There is a 2 wt. %
430.degree. F. conversion penalty for the neat LCN recycle case
(and large LCN volume reduction), whereas the neat CCO recycle
option gives a minimal conversion debit. In essence, examples 2 and
3 bracket the ideal situation wherein light olefins yields are
increased without a large dry gas penalty and conversion of fresh
feed is maximized.
[0039] FIG. 2 shows that the ratio of light olefin yield increase
to dry gas yield increase may be adjusted by including cycle oil
with LCN recycle, in accord with the invention. The ordinate in
FIG. 2 shows the increase in light olefin yield divided by the
increase in dry gas yield plotted for various LCN:cycle oil ratios
on the abscissa. For the blend of LCN and CCO, the preferred blend
composition contains about 30 wt. % LCN.
[0040] The calculated pre-injection vapor residence time for all
examples is approximately constant at only 0.35-0.4 second.
Extremely high (120-160) cat/oil ratios are realized at these
elevated temperatures, and both catalytic and thermal reactions
occur. While not wishing to be bound by any theory, it is believed
that CCO injected into the upstream zone may provide an in situ
quench for LCN cracking at this extraordinary intensity.
[0041] It should be noted that 430.degree. F. conversion decreases
resulting from catalyst "pre-coking" prior to base feed injection.
However, the conversion decrease is smaller for CCO compared to LCN
at virtually identical coke yield. While not wishing to be bound,
it is believed that recycling CCO drives bottoms yield up slightly
more than recycling LCN at the same volumetric flow rate, but cycle
oil conversion is enhanced.
* * * * *