Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A NEVJ OPERATING METHOD
FOR FLUID CATALYTIC CRACKING
FIELD OF THE INVENTION
This invention relates to Fluid Catalytic Cracking (FCC) for producing
( liquid fuels and light olefins from hydrocarbon mixtures such as petroleum
fractions.
More particularly, it relates to a nonlinear characteristic of the FCC process
that leads
to a novel FCC operating strategy for converting hydrocarbon mixtures.
BACKGROUND OF THE INVENTION
FCC has been, and will remain for quite some time, the primary
conversion process in oil refining. In a typical present-day FCC process, a
liquid feed
mixture is atomized through a nozzle to form small droplets at the bottom of a
riser.
The droplets contact hot regenerated catalyst and are vaporized and cracked to
lighter
products and coke. The vaporized products rise through the riser. The catalyst
is
( separated out from the hydrocarbon stream through cyclones. Once separated,
the
catalyst is stripped in a steam stripper of adsorbed hydrocarbons and then fed
to a
regenerator where coke is burned off. The products are sent to a fractionator
for
fractionation into several products. The catalyst, once regenerated, is then
fed back into
the riser. The riser-regenerator assembly is heat balanced in that heat
generated by the
coke burn is used for feed vaporization and cracking. The most common FCC
feeds by
far are gas oils or vacuum gas oils (VGO) which are hydrocarbon mi5,'tures
boiling
above about 650°F(343.3°C). . When -refiners need to convert
heavy, or highly
contaminated oils such as resids, they usually blend a small amount of such
heavy oils
with the gas oil feeds. Due to a dwindling supply of high-quality crudes, the
trend in
the petroleum industry is that FCC will have to convert more and more heavy,
dirty
feeds. Such feeds contain a high level of contaminants such as nitrogen,
sulfur, metals,
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polynuclear aromatics, and Conradson Carbon Residue (CCR, a measure of
asphaltene
content). Hereafter, the term heavy component is used to include such highly
contaminated hydrocarbons as resids, deasphalted oils, lube extracts, tar
sands, coal
liquids, and the like. Such heavy components are added to other feeds
containing less
heavy components to obtain an FCC feed. These heavy components will become a
significant portion of FCC feeds in years to come.
The technical problems encountered with FCC feeds containing heavy
components have been reviewed by Otterstedt et al. (Otterstedt, J. E., Gevert,
S. B.,
Jaras, S. G., and Menon, P. G., Applied Catalysis, 22, 159, 1986). Chief among
them
are high coke and gas yields, catalyst deactivation, and SOX in flue gas. The
coke
forming tendency of such heavy component-containing feeds has traditionally
been
gauged by their CCR content. VGO feeds typically contain less than 0.5 wt%
CCR,
whereas atmospheric and vacuum resids typically contain 1 to 15 wt% and 4 to
25 wt%
CCR, respectively. Since cracking of such heavy components can produce coke
levels
far higher than that required by existing FCC units, the maximum permissible
Level of
the heavy component in the FCC. feed is often limited by the unit's coke
burning
capacity. Many FCC units today are capable of cracking only 5 to 15 wt% resid,
or
heavy component, in the feed. Due to feed cost considerations, there is a
strong need
for economical methods that can expand the FCC's operating envelope, that is,
to be
- able to increase the heavy component limit within existing hardware
constraints.
What is needed in the art is an FCC method which allows for increased
use of alternative feeds and yield improvements for desired products via
stretching the
operating Limits of existing hardware.
SLTIyIMARY OF THE INVENTION
Applicants have found that the liquid yield in FCC does not degrade
linearly, nor does the coke yield increase linearly, as the amount of heavy
component in
the feed increases. This means that the damaging marginal effect of feed
contaminants
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on the FCC catalyst becomes increasingly weaker with increasing amounts of
heavy
components. Accordingly, the present invention discloses a new, improved FCC
operating method for cracking feeds of differing quality.
Thus, the present invention is directed to a Fluid Catalytic Cracking
process conducted under fluid catalytic cracking conditions comprising
injecting into at
least one reaction zone of a fluid catalytic cracking unit (FCCU) having one
or more
risers, a plurality of feeds wherein said plurality of feeds comprises ax
least one feed (a)
and at least another feed ((3) wherein said feeds (a) and ((3) (a) differ in
Conradson
Carbon Residue by at least about 2 wt% points; or (b) differ in hydrogen
content by at
least about 0.2 wt%; or (c) differ in API gravities by at least about 2
points; or (d) differ
in nitrogen content by at least about 50 ppm; or (e) differ in carbon-to-
hydrogen ratio
by at least about 0.3; or (f) differ in mean boiling point by at least about
200°F(93.3°C);
and wherein said feeds (a) and ((3) are alternately injected and wherein said
alternate
injection maintains said risers in a cyclic steady state, while the rest of
the FCC unit is
in a steady state. The cycle period for alternate injection is judiciously
selected to
maintain said risers in a cyclic steady state. Such cyclic operation can
result in a higher
time-average conversion and a lower coke selectivity compared to prior art,
noncyclic
operation. The benefit can translate into a higher heavy-component feed
cracking
( capacity at constant liquid yield.
BRIEF DESCRIPTION OF THE FIGURES
Figure la: Conversion as a function of wt% resid in total feed.
Figure 1b: Coke yield as a function of wt% resid in total feed.
Figure 2a: Coke-free kinetic conversion to <430°F (221.1 °C)
products
vs. wt% resid in feed; S 15°C, 8 C/O.
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Figure 2b: Coke-free kinetic conversion to <6~0°F (343.3°C)
products
vs. wt% resid in feed; 515°C, 8 C/O_
Figure 2c: Coke selectivity vs. wt% resid in feed; X15°C, 8 C/O.
Figure 3a: Conversion to <430°F (221.1°C) products vs. wt%
feed
hydrogen; 496°C, 6.5 C/O; catalyst A.
Figure 3b: Conversion to <430°F (221.1°C)products vs. wt% feed
hydrogen; 496°C, 6.5 CIO; catalyst B.
Figure 3c: Coke yield vs. wt% feed hydrogen; 496°C, 6.5 CIO; catalyst
C.
Figure 3d: Propylene yield vs. wt% feed hydrogen; 496°C, 6.5 CIO;
catalyst B.
Figure 3e: Distillate yield vs. wt% feed hydrogen; 496°C, 6.5 CIO;
catalyst C.
Figure 3f Naphtha yield vs. wt% feed hydrogen; 496°C, 6.5 CIO;
catalyst C.
Figure 3g: Bottoms yield vs. wt% feed hydrogen; 496°C, 6.5 C/O;
catalyst C.
Figure 3h: Butylene yield vs. wt% feed hydrogen; 496°C, 6.5 CIO;
catalyst C.
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DETAILED DESCRIPTION OF THE INVENTION
The invention is more easily understood from the Figures that can be
readily obtained through routine laboratory and/or pilot plant
experimentation. Fiwre 1
depicts qualitatively the nonlinear dependencies of conversion and coke yield
on the
concentration of the resid in the feed. The curve for conversion is convex,
whereas that
for coke yield is concave. For instance, if an FCC unit's coke burning
capacity is such
that the maximum permissible concentration of the resid is 10 wt%, then the
prior art
teaches that it is cost effective to charge the unit with a feed containing 10
wt% resid in
VGO, point C in Figure la. For the above, the instant invention teaches a FCC
operation that is entirely different from that taught by the prior art.
Instead of keeping
the heavy component at 10 wt% at all times, the instant invention calls for
alternating
the concentration of the heavy component between two levels: one is higher
than 10
wt% resid and the other is lower. The cycle period (total combined time for
injection
of the two alternating feeds) is selected in such a way that it is Iong enough
to maintain
the FCCU riser in a cyclic steady state. Such a cycle period is necessarily
short enough
that the operation of other subsystems (fractionator, regenerator, and
stripper) of the
FCC unit are not disturbed. Thus, the other subsystems of the FCC unit are not
affected
to a degree that would impact the unit or process.
Those skilled in the art would know, with reference to the instant
invention, how to select the feeds utilizable in the instant invention.
Essentially, the
feeds are selected from the nonlinear curves of conversion and coke make
versus a feed
quality index such as wt% resid as shown in Figures la and lb, or wt% feed
hydrogen
as shown in Figure 3b. As stated earlier, such plots can be obtained a priori
in small
scale routine experiments. Knowing the FCC unit's resid capacity then helps
the skilled
artisan to select two feeds (a) and ((3) for utilization in the instant
invention. For
example, if one predetermined that a 3% increase in liquid yield was desired,
any two
feeds which give the 3 % increase [see, e.g., (D minus F) on Figure 1 a (D
minus F)
being the predetermined increase desired) would be selected. Preferably, the
increase
in liquid yield will be at least about 0.5 wt% on feed, and/or the decrease in
coke make
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will be at least about 0.2 wt% on feed. The wt% decrease in coke make would be
represented by G minus E on Figure Ib. By selecting two such feeds, the blend
of the
liquid products from alternately cracking the two feeds (D) is higher than
that which
could be achieved if the two feeds were first mixed and then cracked (F). Note
that any
feed quality index can be used to generate the plots, e.g., % resid, hydrogen
content,
API gravity, nitrogen content, C/H ratio, boiling point, to name a few.
Typically, at
least three feeds will be used to generate the plots.
Referring to Figure 1 a, one example of the invention is to cycle the
concentration of the heavy component between 0 and 20 wt% (points A and B in
Figure
1) with equal time interval. In another embodiment, the concentration can be
cycled
between 5 and 15 wt%. In either case, the time average resid concentration is
10 wt%.
However, as Figure 1 shows, the alternating operation gives a higher time
average
conversion (point D in Figure la) and a lower time average coke yield (point E
in
Figure Ib) th n the prior art, nonalternating (uniform feed injection)
operation (points F
and G) with a feed containing 10 wt% of the heavy component. Figures 1 a and 1
b also
imply that the greater the difference in the quality of the two feed
components (for
instance, gas oil vs. vacuum resid), the larger the benefit (lower coke make
and
increased liquid yield). The benefit stems from the non-linearity shown in
Figure 1.
That is, the loss caused by the heavy component-containing feed is more than
offset by
the gain caused by the other feed. The heavy component-containing feed is -
highly
contaminated with CCR, nitrogen, polynuclear aromatics, and/or metals. They
are also
characterized by low hydrogen content or low API gravity.
Applicants believe that the reason the instant invention can maintain the
FCC operation in a cyclic steady state is due to the wide disparity in the
response times
of various FCC subsystems to external disturbances. Owing to its short contact
time
and near plug flow, the riser has a very short response time, typically on the
order of 5
seconds. 'The regenerator is much more sluggish, with response time typically
on the
order of 30 minutes. The response times of stripper and fractionator are also
orders of
magnitude longer than that of the riser. If, for example, each of the two
feeds is
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injected for 20 seconds (that is, the cycle period is 40 seconds); then the
riser can
quickly equilibrate itself to a new steady state long before the subsequent
feed switch.
Thus, the riser is essentially operated between two steady states. The riser
is referrcd to
as being in a cyclic steady state. On the other hand, the 40 second cycle
period is too
short for the sluggish regenerator to respond. The fluctuations caused by feed
cycling
will be quickly smoothed out, and the regenerator basically is in a steady
state. The
same is true for the stripper and fia.ctionator. For instance, the liquid
holdup, heavy
vapor-liquid tragic, and reflex in the fractionator would quickly damp out any
high
frequency fluctuations.
Hence, one skilled in the art could readily select a cycle period at which
the FCC unit operates as if there were two risers for individual cracking of
two feeds of
different quality. The feed switching for practical purposes is imperceptible
to the
regenerator, stripper, and fractionator.
The preferred feed cycle period may be symmetrical where each feed is
fed for the same amount of time, or asymmetrical where the feeds are fed for
different
periods of time. The feed cycle times are readily selected by the skilled
artisan based
upon the response times of the risers, regenerator, and fractionator.
Selection should
preferably be based upon the longest time permitted by the regenerator
operation and
product recovery considerations. Thus, the instant invention offers many
choices in
both feed considerations. While the above example alternates two feeds with
equal
time intervals, this symmetric mode of feed switching may not necessarily give
the
maximum benefit. In some cases, asymmetric switching may be preferred; that
is, each
feed is injected for a different amount of time. For instance, in the above
example
where the cycle period is 40 seconds, the individual periods for the straight
VGO and
20 wt%-resid-in-VGO feeds may be 15 and 25 seconds, respectively. The feed
concentrations of the heavy component used in the instant operation may also
be
chosen for maximum benefit. One may also use different flow rates for the two
feeds.
Thus, the instant operation offers many degrees of freedom for process
optimization.
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Typical cycle times can range from 10 seconds to 3 minutes, preferably, 20
seconds to
2 minutes. The FCCU is operated by continuously repeating each cycle.
Those skilled in the art would immediately see that for a given
conversion or coke yield, the instant operation translates into a higher
capacity for the
FCC unit to convert the heavy component of the feed. The process of the
instant
invention is run at FCC conditions known to those skilled in the art.
Although the foregoing is discussed in the context of heavy feed
cracking, those skilled in the art would also immediately see that the instant
operation
can be applied to any feed pair whenever the feed properties are sufficiently
different.
For instance, for maximum olefin production, the feed pair may comprise a
naphtha-
rich stock and naphtha-lean stock. Nonlimiting examples of feed property
yardsticks
for suitable feeds are (a) hydrogen content (differing by at least about 0.2
wt%), (b)
carbon-to-hydrogen ratio {differing by at least about 0.3), (c) API gravity
(differing by
at Least about 2 points), (d) nitrogen content (differing by at least about 50
ppm), (e)
mean boiling point (differing by at least about 200°F) (93.3°C),
(f) a CCR (differing by
at least about 2 wt%), etc. Preferably, only two feeds will be utilized.
Applicants believe that the benefits of the instant invention originate
from the convex and concave behaviors illustrated in Figures la and lb.
Accordingly,
the following illustrative, nonlimiting examples were obtained in experiments
aimed at
establishing the convex and concave responses to changes in heavy feed
component
level for various feedstocks, catalysts, and cracking conditions. It should be
noted that
while Figure 1 uses the wt% resid-in-feed as the measure of the feed heavy
component
level, other measures can also be used, for instance, CCR, hydrogen, nitrogen,
polars
plus-multi-ring aromatics, to name a few.
While the instant invention method can be used for any two feeds whose
qualities [(a) to (fjJ are sufficiently different, it is particularly suited
for converting
heavy, low quality hydrocarbon mixtures. It gives a higher time-average 3iquid
yield
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and a lower time average coke make than those obtained from prior art,
nonalternating
operations. Additionally, in many cases, a higher time average propylene yield
than
that obtained in nonalternating operation can be obtained. The present method
can be
implemented in different cracking reactor configurations, including but not
limited to
short contact time risers, fluidized reactors, and downflow reactors.
In the case where an FCC unit is equipped with two risers or one riser
having segregated reaction zones, the invention cau also be practiced with
greater than
two feeds. By segregated is meant physically separated or spatially separated
at a
distance effectively yielding two separate reaction zones. For example, when
three
feeds of decreasing quality (as defined by (a) to (f), for example) or
crackability a, (3
and y, respectively, are at the refiner's disposal, feeds a and (3 can be
alternately
injected into the first riser and feeds a and y alternately injected into the
second riser in
accordance with the feed selection criteria [(a) to (f)] hereinbefore
discussed. The
products from each riser may then be combined. Additionally, any combination
of the
three feeds where two feeds are alternately injected into each riser can be
utilized. For
example, in one riser with two reaction zones, a and (3 can be alternately
injected into
one reaction zone and a and y into the second reaction zone. Additionally, a
and /3 can
be alternately injected into one reaction zone of a first riser and a and y
can be injected
into separate reaction zones of the same riser or into a second riser as
follows: (i)
simultaneously injecting into a single reaction zone of a single riser feed
(a) from at
least one injection nozzle of said riser and feed (y) from the remaining
nozzles of the
riser; or (ii) simultaneously injecting feed (a) into at least one reaction
zone of a second
riser and feed (y) into another reaction zone of the second riser of the FCCU.
As can be
seen, many possible combinations are possible. Preferably, in such a case, the
cleanest,
most crackable feed will be injected into each riser along with one of the two
remaining
feeds in each alternating riser. By cleanest, most crackable feed is meant
that feed
having the highest hydrogen content, or the highest API or the lowest nitrogen
content,
the lowest carbon-to-hydrogen ratio or the lowest mean boiling point or lowest
CCR as
compared to the other two feeds. The criteria for the feeds are that the two
feeds
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injected into the same riser must meet the criteria previously described
herein [(a) to
(f)]. Namely, the feeds injected into the same riser must (a) have CCR
differing by at
least 2 wt% points; or (b) differ in hydrogen content by at least about 0.2
wt%; or (c)
differ in API gravities by at least about 2 points; or (d) differ in nitrogen
content by at
least about SO ppm; or (e) differ in carbon-to-hydrogen ratio by ax least
about 0.3; or (fj
differ in mean boiling point by at least about 200°F (93.3°C).
In all the examples given below, the desired nonlinear behaviors were
observed.
EXAMPLE 1
For this series of experiments, a pure VGO and two feed blends
comprising a V GO and a vacuum resid (VR) were prepared, one containing 16 wt%
resid, the other 32 wt%. Table 1 lists the properties of the feed blends in
terms of their
CCR (wt%) acrd indigenous nitrogen (wppm) levels. An equilibrium catalyst
impregnated with 3500 ppm Ni was used.
TABLE 1 - PROPERTIES OF FEED BLENDS
VRlVGO. wt% / wt% CCR N. ppm
0/100 0.26 1181
16/84 2 1524
32168 4.2 1852
The cracking experiments were conducted in an FCC pilot unit at S I
S°C
and a catalyst-to-oil (C/O) ratio of 8. During the- run, the catalyst is
metered from a
regenerated catalyst hopper into a riser using a screw feeder. The hot
catalyst contacts
incoming oil and gaseous nitrogen and is carried up the riser where the oil is
cracked.
At the end of the riser, the spent catalyst and reactor products enter a
separation zone.
Here the gases continue overhead to a product recovery system and the catalyst
drops
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down a stripper and into a spent catalyst hopper. The gaseous products are
cooled to
produce a C$+ liquid product and a CS- product gas.
Since cracking follows second-order kinetics, a measure of the extent of
cracking is the so-called kinetic conversion ~. Denoting X430 as the weight
percent
conversion to the <430°F (221.1 °C) product on a coke-free
basis, then X430 y.y ° X430
~zzi.i~ 1(100 - X430) ~m.y The coke selectivity S is calculated by S = YI~43~
y.y where
Y is the weight percent coke yield on feed. Let the percent conversions of the
straight
VGO and 32% VR-in-VGO feeds be Xl and X2, respectively. Their time-average
kinetic conversion is then ~ _ (Xl + XZ)/2/[I00 - (Xl + XZ)/2], and the
corresponding
time-average coke selectivity is S = (Yl + Y2)I2I~.
Figures Za and 2b show, respectively, the coke-free kinetic conversions
to <430°F and <650°F products as functions of the resid content
of the total feed.
Figure 2c depicts a similar plot for coke yield. From these plots one can
deteanine the
time average kinetic conversion and coke selectivity. It follows from Figures
2a to 2c
that ~ (for conversions to <430°F (221.1°C) and <650°F
(343.3°C) products) are
higher than those obtained from the 16% VR-in-VGO feed, while S is Iower. Each
t data point is the average of two or three runs. Specifically, the 430
(221.1) and 650
{343.3) coke-free kinetic conversions were improved by 5.3% and 7.5%,
respectively.
That is, in the case of 430 (221.1) coke-free conversion, the ratio of ~ to ~
(for the
16% VR-in-VGO feed) is 1.053. And the coke selectivity is lowered by 12.2%.
EXAMPLE 2
The above experiment was repeated at' a. C/O of 5. It was observed that
the 430 (221.1) and 650 (343.3) kinetic conversions increased by 10.2% and
11.7%,
respectively. Moreover, the coke selectivity is lowered by 9.3%.'
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EXAMPLE 3
The experiment described in Example 2 was repeated at 560°C and a
C/O of 5. In this case, the 430 (221.1 ) and 650 (343.3) kinetic conversions
were
improved by 3.7% and 4.9%, respectively. And the coke selectivity is lowered
by
21.5%.
EXAiVIPLE 4
In this case, the catalyst was the same as in Example 1 except that it was
not impregnated with Ni. Cracking conditions are 5 C/O and 515°C. The
430 (221.1)
and 650 (343.3) kinetic conversions were improved by 8.9% and 10.7%,
respectively,
with the coke selectivity being decreased by 4.4%. The propylene yield was
improved
by 6.5%.
EXAMPLE 5
The feed components used in this example are a hydrotreated VGO
(HTGO) and a butane-deasphalted resid (DAO). Table 2 lists the compositions
and
properties of the feed blends.
i
TABLE 2 - PROPERTIES OF FEED BLENDS
DAO/HTGO. wt% / wt% CCR N, ppm
0/100 0.17 541
20/80 1.6 1030
40/60 3.0 . 1519
The cracking experiments were run at 530°C and 8 C/O over an
equilibrium catalyst
different from that used in Example 4. The 430 (221.1) and 650 (343.3) kinetic
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conversions were increased by 4.9% and 10.8%, respectively. The coke
selectivity is
decreased by 7.4%.
EXAMPLE 6
A vacuum gas oil was separated into different fractions having varying
hydrogen contents via solvent extraction. These resulting fractions were each
cracked
at 496°C, 6.5 C/O, and 80 g/m oil rate over several commercial
catalysts, designated as
catalysts A, B, and C. Table 3 lists the properties of these catalysts. The
hydrogen
content of the feed was used as the feed quality measure. The dat,~. shown in
Figures 3a
to 3h were obtained for feeds whose hydrogen contents are 10.4, 12.1, 13.6,
and 13.8
wt%. The results shown in the Figures clearly show the desired nonlinear
effects.
TABLE 3 - CATALYST PROPERTIES
SURFACE
AREA, UNIT CELL
CATALYST m2/g- A
A 154 24.24
B 84 24.34
C 80 24.38
