Note: Descriptions are shown in the official language in which they were submitted.
~21;35~.
F-1984 -1-
CATALYTIC CONVERSION OF LIGHT OLEFINIC
FEEDSTOCKS IN A FCC GAS PLANT
This invention relates to the catalytic conversion of
olefinic feedstocks in a FCC gas plant to gasoline and fuel oil
using a ZSM-5 type zeolite catalyst.
The present invention provides a process for producing a
gasoline fraction and/or a fuel oil fraction by contacting a
feedstock comprising C~ to C5 olefins with a crystalline
aluminosilicate zeolite having a pore diameter greater than 5
Angstrom Units, a silica-to-alumina ratio of at least 12 and a
constraint index of from 1 to 12 under reaction conditions effective
to convert the olefins to a gasoline fraction and/or a fuel oil
fraction, characterized by employing as the feedstock the discharge
stream from the final stage of the wet gas compressor or the
overhead from the high pressure receiver in the gas plant of a fluid
catalytic cracking unit and passing the effluent from the
zeolite-catalyzed conversion through the separation and recovery
facilities of the gas plant to recover the gasoline fraction and/or
the fuel oil fraction.
Hydrocarbon mixtures containing significant quantities of
light olefins are frequently encountered in petrochemical plants and
petroleum refineries. Because of the ease with which olefins react,
these streams serve as feedstocks in a variety of hydrocarbon
conversion processes. Many olefinic conversion processes require
that the olefinic feed be provided in a highly purified condition.
However, processes which may utilize the olefinic feedstocks without
the need for further separation and puri~ication are highly
desirable.
Although the main purpose of catalytic cracking is to
convert gas oil to compounds of lower molecular weight in the
gasoline and middle distillate boiling ranges, significant
F-1984 -2-
quantities of Cl-C~ hydrocarbons are also produced. These light
hydrocarbon gases are rich in olefins which heretofore have made
them prime candidates for conversion to gasoline blending stocks by
means of polymerization and/or alkylation. Fractionation of the
effluent from the fluid catalytic cracking reactor has been employed
to effect an initial separation of this stream. The gaseous
overhead from the main fractionator is collected and processed in
the FCC gas plant. ~ere the gases are compressed, contacted with a
naphtha stream, scrubbed, where necessary, with an amine solution to
remove sulfur and then fractionated to provide, for examplet light
olefins and isobutane for alkylation~ light olefins for
polymerization, n-butane for gasoline blending and propane for LPG.
Light gases are recovered for use as fuel.
Since alkylation units were more costly to build and
operate than polymerization units, olefin polymerization was
initially favored as the route for providing blending stocks.
Increased gasoline demand and rising octane requirements soon
favored the use of alkylat~on because it provided gasoline blending
stocks at a higher yield and with a higher octane rating than the
comparable polymerized product. However, catalytic alkylation can
present some safety and disposal problems. In addition, feedstock
purification is often required to prevent catalyst contamination.
Further, sometimes there is insufficient isobutane available in a
refinery to permit all the olefins from the FCC to be catalytically
~5 alkylated.
The broad concept of contacting an olefinic charge stock
with the special class of zeolites with which this invention is
concerned is known in the art and is the subject of various U.S.
patents. Thus, for example, U.S. Patent No. 3,960,978 teaches
~'~) conversion of olefins to olefinic gasolines. U.S. Patent No.
4,021,502 discloses conversion of olefins over ZSM-12. U.S. patent
No. 3,760,024 discloses contacting olefins with ZSM-5 type
~3~
F-1984 _3_
zeolites. U.S. Patent No. 3,7759501 discloses preparation of
aromatics by contacting olefins over ZSM-5 type catalysts. U.S.
Patent No. 3,827 7 968 discloses a two-step aromatization process
wherein in the first step an olefin is contacted over a ZSM-5 type
zeolite. U.S. Patent No. 4~11,640 discloses a process for
contacting a highly olefinic gasoline with a ZSM-5 type zeolite to
produce fuel oil and gasoline having enhanced gum stability. U.S.
Patent No. 4,2~7,992 discloses a process for separating ethylene
from a mixture of C2-C5 olefins by contacting the mixture with a
ZSM-5 type zeolite under conditions effective to convert at least
80~ of the C3+ olefins and no more than 20% of the ethylene.
However, none of the prior art is directed toward the conversion of
olefinic gas streams in a fluid catalytic cracking (FCC) unit gas
plant to gasoline and fuel oil.
This invention relates to an improvement in the process for
producing a gasoline fraction and/or a fuel oil fraction by
contacting a feedstock comprising C2 to C5 olefins with a
crystalline aluminosilicate zeolite having a pore diameter greater
than 5 Angstrom Units, a silica-to-alumina ratio of at least 12 and
2() a constraint index of from 1 to 12 under reaction conditions
effective to convert the olefins to a gasoline fraction and/or a
fuel oil fraction, the improvement comprising employing as the
feedstock the discharge stream from the final stage of the wet gas
compressor or the overhead from the high pressure receiver in the
gas plant of a fluid catalytic cracking unit and passing the
effluent from the zeolite catalyzed conversion through the
separation and recovery facilities of the gas plant to recover the
gasoline fraction and/or the fuel oil fraction.
In general, the catalysts used in accordance with this
~) invention are crystalline zeolites having a silica/alumina ratio
greater than 12 and a Constraint Index ~C.I.) of from 1 to 12. The
zeolites are generally termed ZSM-5 type zeolites. These zeolites
5~-~
F-1984 ~4~
and their use as conversion catalysts ~or ole~ins are described in
the U.S. patents referred to above, particularly U.S. Patent Nos.
3,760,024, 3,960~978, 4,021,502, 4,211,640 and 4,227,992.
rhe preferred class of zeolites defined herein are ZS~-5
type zeolites as exemplified by ZSM-5, ZSM-ll, ZSM-l~, ZSM-35,
ZSM-~89 with ZSM-5 being particularly preferred.
ZSM~5 is described in U.S. Patent No. 3,702,886, ZSM-ll in
U.S. Patent No. 3,709,979~ ZSM-12 in U.S. Patent No. 3,832,449,
ZSM-35 in U.S. Patent No. 4,016?245 and ZSM-38 in U.S. Patent No.
4,0~6,859.
By utilizing the olefinic feedstreams of a FCC gas plant as
feedstocks in the prescnt invention, the need for separate processes
for the polymerization or alkylation of the olefins in this stream
is obviated or reduced. The concept of utilizing a ZSM-5 type
zeolite catalyzed conversion instead of the prior art processes for
providing C5+ gasoline andior fuel oil blending stocks has many
advantages~ Since the ZSM-5 type zeolites can tolerate the
poisoning effects of the impurities normally found in raw FCC
product streams, extreme purification procedures often encountered
2~ in polymerization and alkylation do not have to be employed. Also,
locating the process of this invention in the FCC gas plant reduces
the capital investment and operating costs required for a conversion
process of this nature since no new product recovery facilities have
to be provided~ The effluent from the catalytic conversion can
continue through the gas plant which has the facilities required to
separate the gasoline fraction and/or fuel oil fraction as blending
stocks from the lighter materials which are recovered as LPG and
fuel gas. The only new equipment which must be provided to practice
the olefin conversion in accordance with this invention is a
catalytic reactor, feed/effluent heat exchangers and a preheat
furnace.
i5~
~-1984 -5-
Although there are a number of streams in the gas plant of
a FCC unit which contain lignt olefins which might be usefully
employed in this invention, two of them are preferred The
discharge stream From the final stage of the wet gas compressor and
the overhead from the high pressure receiver are these preferred
streamsO The multi-stage wet gas compressor in the gas plant is
required to increase the pressure of the gaseous overhead from the
FCC main fractionator so that it may be effectively processed in the
gas plant. The discharge from the final stage is usually from 1308
1~ to 1653 kPa (175 tG 225 psig) and 149 to 177C (300 to 350F).
Where the sulfur content of the FCC light gases is significant, an
amine scrubber, employing mono-, di- or triethanol amine or mixtures
thereof, may be located between t~e stages of the wet gas compressor
or immediately upstream of ~he conversion facilities which are the
subject of this invention to reduce the sulfur content to acceptable
levels. Where the levels of ammonia are significant, a water wash
stage might also be incorporated to reduce ammonia content and to
remove traces of entrained amine solution.
The overhead from the high pressure receiver associated
with the wet gas compressor is the other preferred feeds-tock for the
present process. This gaseous stream does not have the same
composition as the discharge from the compressor since other
streams, such as the liquid from the interstage receiver and the
rich liquid from the primary absorber, as well as the stripper
~5 overhead~ discharge into this vessel. The gas stream passing from
the high pressure receiver is usually at from 1136 to 1480 kPa (150
to 200 psig) and from 32 to 43C (90 to 110F).
In general~ the process of this invention is carried out at
a pressure from 1136 to 1825 kPa (150 to 250 psig), a temperature
from 149 to 399C (300 to 750F) and a space velocity of from 0~1 to
10 WHSV, based on the C2 to C5 olefins.
~L~ 3 ~'J~
F-1984 -6-
As has been stated hereinbefore, the broad concept of
contacting olefins, alone or in admixture with each other or other
hydrocarbons, over the identified catalyst with which this invention
is concerned is not per se novel. The key to this invention resides
in selecting a C2 to C5 olefins-containing gas stream in a FCC
gas plant as the feedstock for the desired catalytic conversion and
the use of the separation and recovery facilities of the gas plant
to process the effluent from the catalytic conversion such that
gasoline and/or fuel oil fractions useful in gasoline and fuel oil
blending may be produced without the necessity of providing product
recovery facilities, thereby minimizing the capital investment and
operating costs for the subject process.
The improvements described herein can be illustrated by
reference to Figures 1 and 2 which present, respectively, a flowplan
of a FCC gas plant and a flowplan of an embodiment of this
invention. The dotted squares labeled A and B in Figure 1 indicate
the alternate locations for the reactor and attendant equipment
depicted in Figure 2.
Referring to Figure 1, the condensed overhead from the FCC
main fractionator flows through line 2 into FCC main fractionator
overhead accumulator 4 for separation into a gaseous phase and a
liquid phase. The gaseous portion of the column overhead flows from
accumulator 4 to the FCC gas plant through line 6 to the suction of
the first stage of wet gas compressor 8 for the initial increase in
~S pressure. The wet gas discharges from the first stage of the
compressor through line 10 to interstage receiver 12 and line 1~ and
then through the second stage of the wet compressor from which it is
discharged through line 16 at about 163C (325F) and about 1480 kPa
(2ûO psig). Where the sulfur level of the gaseous stream is above
-~ acceptable levels, an ethanolamine scrubber (not shown) may be
located in line 10 to reduce the sulfur content to acceptable
levels. Alternatively the scrubber may be located upstream of the
reactor and equipment depicted in Figure 2, i.e., in lines 16 or 32.
F-1984 7-
As a first option9 the gas stream in line 16 may be
employed in the practice of this invention. This stream typically
has a composition as sh~wn in Table I. Referring to Figure 2, which
may be considered as being located at position A of Figure 1, the
S olefinic stream in line 1~ passes through heat exchanger 102 where
it is pre-hea~ed by indirect heat exchange with the reactor
e~fluent, described hereinafter. The partially
F-1984 -8- 12135~
TA13LE I
__
USEFUL ~EEDSTREAMS
HIGH PRESSURE
2ND STAGE WET RECEIVER
GAS COMPRESSOR OUTLET OVERHEAD
With No With No
Component, A~ine (1) Amine Amine (1) Amine
mol. % _ Scrubber Scrubber Scrubber Scrubber
H2 11.7 10.7 12.6 11.5
Cl 18.8 17.3 22.3 20.4
N2 9.1 8.4 g.9 9.2
2 1.9 -- 2.0
H2S -- 6.3 0.5 6.6
C2= 7.3 6.7 12.2 11.2
1~ C2 9.8 9.0 21.1 19.3
~3= 11.8 10.8 10.~ 9.7
C3 4.7 4.3 3.5 3.2
iC4 2.2 2.0 0.7 0.7
C4= 10.2 9.4 300 2.8
2(:) nC4 1.3 1.2 0.4 0.4
~asoline 11.2 10.3 2.7 2.5
H20 _1.9 1.7 0.5 0.5
100.0 100.0 100.0 100.0
(1) Amine absorber located between first and
second stage o~ wet gas compressor.
F-1984 1 2 1 3 ~
heated ole~in stream then flows through line 104 to preheat furnace
106 where it is heated to the reaction temperature in the range of
149-399C (300-750~C). The thus heated stream flows through line
108 to reactor 110 which contains a fixed bed 112 of ZSM-5 zeolite
catalyst. While passing through the reactor at a space velocity of
~rom 0.1 to 10 WHSV, the C2 ko C~ olefins in the gaseous mixture
are converted to hydrocarbons boiling in the gasoline and ~uel oil
range. The reaction mixture leaves reactor 110 through line 114 and
flows to the shell side of heat exchanger 102 where this effluent
lU stream provides some of the preheat to the olefinic feedstream.
Referring to both Figures 1 and 2, the reaction mixture ~hen passes
to lines 18, 20 and 22 and enters high pressure receiver 24.
Several other streams are also passed into this receiver where they
are separated into a liquid phase and a gaseous phase. These
include the liquid phase from compressor interstage receiver 12
which flows from receiver 12 through lines 26, 28 and 22 to receiver
24, the rich liquid from the primary absorber, described
hereinafter~ which passes from the absorber through lines 30, 2& and
22 to the receiver and the overhead from the stripper, also
described hereinafter, which passes through lines 31, 20 and 22 to
the receiver. The temperature of these combined streams in receiver
24 is about 38C 1;100F)o
The streams entering high pressure receiver 24 are
separated therein into a gaseous phase and a liquid phase. The
?~S gaseous phase passes through lines 32 and 34 to primary absorber
36. Dotted square ~ represents an alternate location for the Figure
2 flowplan and this embodiment will be described hereinafter. for
purposes of the present description where the Figure 2 ~lowplan is
located at dotted square A, the gaseous phase from high pressure
3() receiver 24 passes directly to primary absorber 36 where a C5+
liquid stream passing in countercurrent flow to the gas absorbs
heavy hydrocarbons from the gas stream. The C5+ liquids employed
F-1984 -10- 12135~1
include the liquid phase ~rom FCC main column overhead accumulator 4
and a portion of the final liquid product from the gas plant. These
streams are passed to primary absorber 36 through lines 38 and 40,
respectively. The rich liquid from absorber 36 flows from the
column through lines 30, 28 and 22 and enters high pressure receiver
24, as described above.
The unabsorbed gases pass from the top of absorber 36
through line 42 where they are combined with coker gas supplied
through line 44. The combined gaseous stream passes through line 46
into sponge absorber 48 where they are contacted in countercurrent
fashion with sponge oil which is a stripped heavy naphtha or light
fuel oil boiling in the 177-260C (350-500F) range. In this
absorber, the C3+ gases are absorbed by the sponge oil which
passes from sponge absorber 48 through line 50 and discharges into
FCC main column (not shown) by means of line 2. The unabsorbed
C2- gases pass from the absorber through line 52 and are
eventually burned as fuel gas.
Returning to the liquid phase in high pressure receiver 24,
which contains the gasoline and/or fuel oil fractions obtained by
2~ the ZSM-5 catalytic conversion of the light olefin gases, this
liquid passes from the receiver through line 54 to stripper ~6 where
steam is employed to remove the light gases from this stream. The
steam and the light gases pass from the top of the stripper through
line 31 and eventually discharge into high pressure receiver ~4 from
~5 which the useful light gases are recovered.
The stripped C3+ liquid passes ~rom stripper 56 through
line 58 to debutanizer 6û where a C4- fraction is separated and
passes from the column as the overhead through line 62 where it is
recovered as LPG product. The gasoline and/or fuel oil fraction is
removed from debutanizer 60 as the bottoms fraction through line
64. A portion of this fraction is recycled through line 40 to the
primary absorber as a portion of the absorbing liquid as described
F-1984
above. The remaining portisn of the C5~ bottoms is recovered as
product through line 66 and is employed as blending stock for
gasoline and/or fuel oil following futher fractionation, as required.
As described above, this invention may optionally be
practiced employing the overhead from high presure receiver 24 as
the feed for the conversion to gasoline and/or fuel oil. Typically,
this stream has a composition as shown in Table I. In this
embodiment, the flowplan of Figure 2 is located at position B of
Figure 1 with the feed passing through line ~2 and the reaction
lU mixture continuing on to the separation equipment of the gas plant
through line 34. Obviously, when practicing this embodiment the
discharge from the wet gas compressor flows directly from line 16 to
line 18. In other regards this embodiment is practiced in
substantially the same manner as the embodiment descrioed above
where the Figure 2 flowplan is located at position A of Figure 1.
3y practicing the invention as described herein ~hereby the
olefinic feedstreams in the FCC gas plant are catalytically
converted in the presence of a ZSM-5 type zeolite to gasoline and/or
fuel oil fractions and the existing facilities are employed to
separate and recover these fractions, the capital investment and
operating costs required for this ~onversion are minimized.
