Note: Descriptions are shown in the official language in which they were submitted.
CRR101794 PATENT
DOCKET N0. 94A219-1
21 3 48 2 1
CATALYTIC CRACKING PROCESS
FIELD OF THE INVENTION
This invention relates to the cracking of hydrocarbons and more particularly
to the recovery of olefins from the off-gas from a catalytic cracking
operation.
BACKGROUND OF THE INVENTION
The effluent from a hydrocarbon cracking unit contains a wide spectrum of
hydrocarbons. To recover the hydrocarbons the effluent is cooled and subjected
to a series of separation steps, such as condensation and distillation to
recover the
heavy and light liquid components. After removal of these components, the
remaining light gas stream can be compressed and cooled, thereby condensing
most of the remaining hydrocarbons from the stream. The noncondensable gas
remaining after the light gas compression and condensation step, generally
referred
to as off-gas, is comprised substantially of hydrogen and small amounts of C,
to
C3 hydrocarbons, and perhaps some other ga:~eous components, such as nitrogen
CRR101794 213 48 ~ ~ PATENT
DOCKET NO. 94A219-1
and carbon dioxide. The off-gas is usually sent to flare or used as fuel. To
minimize the amount of hydrocarbons remaining in the off-gas, the light gas
stream
is compressed to as high a pressure and cooled to as low a temperature as is
practicable. Consequently, the energy expended in cooling and compressing the
condensable light gases is considerable.
It is desirable to reduce the overall cost of recovering cracked hydrocarbon
products and maximize the amount of valuable C2 and C3 alkenes recovered from
the hydrocarbon cracking unit off-gas. This objective could be attained if an
efficient and cost effective method of recovering lower alkenes from gas
streams
were available. The present invention provides an alkene adsorption method
which
reduces the energy requirements of hydrocarbon cracking processes and provides
substantially complete recovery of the lower alkenes contained in cracking
unit off-
gas.
According to the invention, a hydrocarbon feed stock is cracked to yield a
product comprising a mixture of lower hydrocarbons. Easily condensable
hydrocarbon components are first separated from the cracked product and the
remaining gaseous effluent is compressed and cooled, thereby producing a
condensate containing additional hydrocarbons and leaving an off-gas comprised
predominantly of hydrogen and C, to C3 hydrocarbons, and perhaps other gases,
such as nitrogen. The off-gas stream is subjected to a pressure swing
adsorption
(PSA) process or a temperature swing adsorption (TSA) process at an elevated
temperature in a bed of adsorbent which preferentially adsorbs alkenes from a
gas
stream contain the alkenes and one or more alkanes. The adsorption process is
operated under conditions which result in the production of a nonadsorbed gas
component containing most of the hydrogen and alkane components (and nitrogen,
if present? contained in the off-gas, and an .adsorbed component containing
most
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2134821
of the alkene components in the stream. Tl~e process is desirably operated to
retain
substantially all of the alkene in the gas stream.
The adsorption step is typically carried out at a temperature in the range of
about
0°C to about 250°C., and is preferably carried out at a
temperature above about 50°C.
The adsorption step is generally carried out at an absolute pressure in the
range of about
0.2 to 100 bar, and is preferably carried out at: an absolute pressure of
about 1 to 50 bar.
The adsorbent is selected from alumina" type 4A zeolite, type SA. zeolite,
type 13X
zeolite, type Y zeolite and mixtures of these.
In a preferred embodiment of the invention, the adsorbent is a type A zeolite,
and
1 o in the most preferred embodiment, it is type 4~A zeolite.
When the adsorption process is PSA, tlhe pressure during the regeneration step
is
reduce, usually to an absolute pressure in the range of about 100 to about
5000 millibar,
and preferably to an absolute pressure in the range of about 100 to about 2000
millibar.
When the adsorption process is TSA, the bed. temperature is usually raised
during bed
regeneration to a value in the range of about 100 to about 350°C, and
is preferably raised
to a value in the range of about 150 to 300°C.
In other preferred embodiments of the invention the adsorption bed
regeneration
step is effected by vacuum means or by purging the bed with one or more of an
inert gas,
the nonadsorbed gas product from the adsorption system or the adsorbed product
gas from
2 o the adsorption system, or by combinations of vacuum and purge
regeneration; and bed
repressurization is at least partly effected using the alkene-enriched
desorbed gas from the
adsorption system.
3
21 348 2 1
BRIEF DESCRIPTION OF THE DRAWING
The drawing illustrates, in a block diagram, a system for cracking
hydrocarbons
in accordance with a principal embodiment of the present invention.
3a
213821
CRR101794 PATENT
DOCKET NO. 94A219-1
In the principal aspect of the invention a hydrocarbon stream is cracked,
thereby producing a gaseous product comprised mainly of hydrogen and a wide
spectrum of hydrocarbons. This product is cooled and fractionated, thereby
separating out the heavy and intermediate hydrocarbons in the product. The
condensed hydrocarbon mixture is generally further processed to recover
various
hydrocarbon cuts and high purity hydrocarbons from the stream. The gas phase
remaining after the condensation step, typically containing hydrogen and C4
and
lighter hydrocarbons is compressed, cooled! and fractionated or flashed to
separate
the condensable gases from the stream. The noncondensables, comprised
predominantly of hydrogen, methane and a small amount of C2 and C3
hydrocarbons
is subjected to a pressure swing adsorption process or a temperature swing
adsorption process to produce an adsorbed phase rich in ethylene and propylene
and a nonadsorbed phase rich in hydrogen and the alkanes (and nitrogen, if
present)
present in the gas stream. After being desorbed from the adsorption system the
ethylene-propylene mixture is discharged from the system for further
purification
or combined with the condensable gas stream.
The invention can be better understood from the accompanying drawing.
Auxiliary equipment not necessary for an uinderstanding of the invention,
including
compressors, heat exchangers and valves, has been omitted from the drawing to
simplify discussion of the invention.
In the drawing, A is a hydrocarbon cracking plant, B is a fractionator, C is
a gas compressor, D is heat exchanger, E is. a demethanizer or a flash chamber
and
F is an adsorbent-based gas separation syatem.
Plant A may be any hydrocarbon cracking system typically used in petroleum
refining operations. The particular cracking method employed in the process of
the
invention forms no part of the invention and any of the commonly used thermal
and
4
:~
CRR101794 ~ 13 48 21 PATENT
DOCKET NO. 94A219-1
catalytic cracking processes can be used in the practice of the invention.
Cracking
unit A is typically equipped on its inlet end with hydrocarbon feed line 2 and
its
cracked gas outlet is connected to the inlet of fractionator B via line 4.
Fractionator B is a conventional fractionating column designed to produce an
overhead stream comprised of C4 and lighter hydrocarbons, a side stream
comprised of C5 and heavier liquid hydrocarbons and a bottoms stream comprised
of heavy residual components. The overhead stream, the C5 and heavier product
stream and residual product stream are discharged from column B through lines
6,
8 and 10, respectively. Line 10 is connected to the inlet of unit A through
line 12.
Line 6 joins the overhead outlet of column B with the inlet of unit E.
Compressor
C and cooler D are located in line 6. Compressor C and cooler D are any
typical
gas compressor and heat exchanger usable for compressing and cooling
hydrocarbon gases. Unit E is any conventional flash chamber or fractionating
column, and it is designed to separate tlhe noncondensable off-gas from the
condensable light hydrocarbon components contained in the feed stream to this
unit. The condensed light hydrocarbons are discharged from unit E through line
14.
Line 16 connects the off-gas outlet of unit E to the inlet of separator F.
Separator F is an adsorption system whose principal function is to separate
the alkenes contained in the off-gas from unit E (mainly ethylene or
propylene) from
the other gases contained in this stream. 'This unit is typically a pressure
swing
adsorption or temperature swing adsorption system, generally comprising two or
more stationary beds arranged in parallel and adapted to be operated in a
cyclic
process comprising adsorption and desorption. In such systems the beds are
cycled out of phase to assure a pseudo-continuous flow of alkene-enriched gas
from the adsorption system.
The beds of separator F are packed with an adsorbent which selectively
adsorbs alkenes from a gas mixture containing the alkenes and one or more
alkanes. In general, the adsorbent may be alumina, silica, zeolites, carbon
molecular sieves, etc. Typical adsorbent:; include alumina, silica gel, carbon
5
CRR 101794 21 3 4 8 2 ~ PATENT
DOCKET NO. 94A219-1
molecular sieves, zeolites, such as type A and type X zeolite, type Y zeolite,
etc.
The preferred adsorbents are type A zeolite;s, and the most preferred
adsorbent is
type 4A zeolite.
Type 4A zeolite, i.e. the sodium form of type A zeolite, has an apparent pore
size of about 3.6 to 4 Angstrom units. This adsorbent provides enhanced
selectivity and capacity in adsorbing ethylene from ethylene-ethane mixtures
and
propylene from propylene-propane mixtures at elevated temperatures. This
adsorbent is most effective for use in the invention when it is substantially
unmodified, i.e. when it has only sodium ions as its exchangeable cations.
However, certain properties of the adsorbent, such as thermal and light
stability,
may be improved by partly exchanging some of the sodium ions with other
cations.
Accordingly. it is within the scope of the preferred embodiment of the
invention to
use a type 4A zeolite in which some of the :;odium ions attached to the
adsorbent
are replaced with other metal ions, provided 'that the percentage of ions
exchanged
is not so great that the adsorbent loses its type 4A character. Among the
properties that define type 4A character are the ability of the adsorbent to
selectively adsorb ethylene from ethylene-ethane mixtures and propylene from
propylene-propane gas mixtures at elevated temperatures, and to accomplish
this
result without causing significant oligomerization or polymerization of the
alkenes
present in the mixtures. In general, it has Ibeen determined that up to about
25
percent (on an equivalent basis) of the sodium ions in 4A zeolite can be
replaced
by ion exchange with other cations without .divesting the adsorbent of its
type 4A
character. Cations that may be ion exchanged with the 4A zeolite used in the
alkene-alkane separation include, among others, potassium, calcium, magnesium,
strontium, zinc, cobalt, silver, copper, manclanese, cadmium, aluminum,
cerium,
etc. When exchanging other cations for sodium ions it is preferred that less
than
about 10 percent of the sodium ions (on an equivalent basis) be replaced with
such
other cations. The replacement of sodium ions may modify the properties of the
adsorbent. For example, substituting some of the sodium ions with other
cations
may improve the stability of the adsorbent.
6
CRR101794 21 3 4 8 Z ~ PATENT
DOCKET N0. 94A219-1
Another class of preferred adsorbents are those which contain certain
oxidizable metal cations, such as copper-containing adsorbents, which possess
enhanced adsorptive capacity and selectivity with respect to the preferential
adsorption of alkenes from gaseous alkene-a~lkane mixtures. Suitable adsorbent
substrates for manufacturing copper-modified adsorbents include silica gel,
and
zeolite molecular sieves, such as zeolite type; 4A, zeolite type 5A, zeolite
type X
and zeolite type Y. The manufacture and use of copper-modified adsorbents and
examples of suitable copper-containing adsorbents are set forth in U.S. Patent
No.
4,917,711,
Separator F is provided with waste gas. discharge line 18, purge gas line 20
and alkene discharge line 22, which, in the ernbodiment illustrated in the
drawing,
is connected to condensed light hydrocarbon discharge line 14. Purged gas
recycle
line 24 connects line 22 to the inlet to separator F.
According to the process of the invention practiced in the system illustrated
in the drawing, a hydrocarbon cracker feed stream, such as gas oil, is
introduced
into cracking unit A. The hydrocarbon feed is typically cracked into a hot
gaseous
product comprised of mixed hydrocarbons, e"g. hydrocarbons having up to about
12 carbon atoms, and a heavy hydrocarbon residual product. The hot gaseous
product leaves unit A and is next separated in fractionator B into a heavy
residual
stream, which is removed through line 10 and discharged from the system or
recycled to unit A through line 12; a intermediate hydrocarbon stream
comprised
mostly of liquid hydrocarbons having 5 or more carbon atoms, which is removed
through line 8; and a light hydrocarbon gas stream comprised substantially of
hydrogen, hydrocarbons having up to 4 carbon atoms, and perhaps nitrogen,
which
leaves column B via line 6. The light hydrocarbon gas stream passing through
line
6 is compressed in unit C to the desired presaure, cooled in heat exchanger D
to
the temperature at which most of the C2 to C4 hydrocarbons in the stream are
condensed and introduced into unit E. A prodluct stream comprised of the
readily
7
CRR 101794 21 3 4 8 2 ~~ PATENT
DOCKET NO. 94A219-1
condensable components of the feed to unit E is removed from this unit through
line 14 and sent to downstream processing units for further hydrocarbon
separation. A gas stream comprised predominantly of hydrogen and C~ to C3
hydrocarbons is discharged from unit E through line 16 and is introduced into
separator F.
As the off-gas passes through the adsorption beds of separator F the alkene
components of the stream are adsorbed onto l:he adsorbent while the hydrogen
and
alkanes (and any nitrogen present) in the gas stream pass through the
adsorbent
and exit separator F through line 18 as nonadsorbed gas. Separator F is
preferably
operated in a manner which results in the adsorption of substantially all of
the
alkene and rejection of most of the hydrogen and alkane present in the feed to
this
unit.
The temperature at which the adsorptiion step is carried out depends upon
a number of factors, such as the particular adsorbent being used, e.g.
unmodified
4A zeolite, a particular metal-exchanged 4A zeolite or another adsorbent which
selectively adsorbs alkenes from alkene-alkane mixtures, and the pressure at
which
the adsorption is carried out. In general, thE: adsorption step is carried out
at a
minimum temperature of about 0°C and is preferably: carried out at a
minimum
temperature of about 50° C. and is most preferably carried out at a
temperature
of at least about 70° C. The upper temperature limit at which the
adsorption step
in unit B is carried out is determined mostly by economics. In general the
adsorption step can be carried out at a temperature below the temperature at
which the alkene undergoes chemical reaction, such as polymerization. The
upper
adsorption temperature limit is about 250°C. When unmodified 4A zeolite
is used
as the adsorbent the reaction is generally carried out at or below 200°
C., and is
preferably carried out at a temperature at or below 170° C. Oxidizable
metal-
containing adsorbents, such as copper modified adsorbents, are particularly
effective at temperatures above about 100°C, for example at
temperatures
between about 100° C. and 250° C. They are preferably used at
temperatures in
8
CRR101794 PATENT
2134821
DOCKET NO. 94A219-1
the range of about 110 to 200° C., and most preferably at temperatures
in the
range of about 125 to about 175°C.
The pressures at which the adsorption step is carried out generally ranges
from about 0.2 to about 100 bar, and preferably from about 1 to 50 bar for
pressure swing adsorption cycles, and is usually about atmospheric or above
for
temperature swing adsorption cycles.
When the adsorption process is PS~A the regeneration step is generally
carried out a temperature in the neighborhood of the temperature at which the
adsorption step is carried out and at an absolute pressure lower than the
adsorption
pressure. The pressure during the regeneratiion step of PSA cycles is usually
in the
range of about 20 to about 5000 millibar, and preferably in the range of about
100
to about 2000 millibar. When the adsorption process is TSA, bed regeneration
is
carried out at a temperature higher than the adsorption temperature, usually
in the
range of about 100 to about 350° C, and preferably in the range of
about 150 to
300° C. In the TSA embodiment, the pre:aure is generally the same
during the
adsorption and regeneration steps, and it is often preferred to conduct both
steps
at about atmospheric pressure or above. When a combination of PSA and TSA is
used the temperature and pressure during the bed regeneration step are higher
and
lower, respectively, than they are during the adsorption step.
When the adsorbed alkene front traveling through the vessels) of separator
F in which the adsorption step is being carried out reaches the desired point
in the
vessel(s), the adsorption process in these vessels) is terminated and these
vessels
enter the regeneration mode. During regeneration, the alkene-loaded vessels
are
depressurized, if the adsorption cycle is pressure swing adsorption, or
heated, if a
temperature swing adsorption cycle is employed. As the regeneration proceeds,
alkene-enriched gas is discharged from separator F through line 20. This
stream
can be combined with the light hydrocarbon stream in line 14, as illustrated
in the
drawing, or discharged from the system for further processing.
9
PATENT
CRR101794 21 3 4 8 Z 1
DOCKET NO. 94A219-1
The method of regeneration of the adsorption beds depends upon the type
of adsorption process employed. In the case of pressure swing adsorption, the
regeneration phase generally includes a countercurrent depressurization step
during
which the beds are vented countercurrently until they attain the desired lower
pressure. If desired the pressure in the beds may be reduced to subatmospheric
pressure by means of a vacuum inducing device, such as a vacuum pump (not
shown).
In some cases, in addition to the countercurrent depressurization step(s), it
may be desirable to purge the bed with an inert gas or one of the gas streams
exiting separator F. In this event the purge step is usually initiated towards
the end
of the countercurrent depressurization step, or subsequent thereto. During the
purge step, a nonadsorbable purge gas can be introduced into separator F via
line
and passed countercurrently through ,the adsorbent beds, thereby forcing
15 desorbed alkene out of out of separator F through line 22. The purge gas
may be
nonadsorbed product gas exiting separator IF through line 18, or a
nonadsorbable
gas obtained from a different source, such a;s an inert permanent gas like
nitrogen.
In a preferred method of operation of 'the system of the drawing, the alkene
20 desorbed from separator F during the countercurrent depressurization
stepls) is
discharged into line 14, and all or a portion of the purge gas and alkene
desorbed
from the bed during the purge step is recycled to separator F through line 24
for
reprocessing. The advantage of this embodiment is that it permits the amount
of
purge gas that is transferred to line 14 to be minimized.
The adsorption cycle may contain steps other than the fundamental steps of
adsorption and regeneration. For example, it may be advantageous to
depressurize
the adsorption bed in multiple steps, with the first depressurization product
being
used to partially pressurize another bed in the adsorption system. This will
further
reduce the amount of gaseous impurities transferred to line 14. It may also be
desirable to include a cocurrent purge step between the adsorption phase and
the
CRR101794
21 3 4 8 21 DOCKET N0. 94A219-1
regeneration phase. The cocurrent purge its effected by terminating the flow
of
feed gas into separator F and passing high purity alkene cocurrently into the
adsorption bed at adsorption pressure. This. has the effect of forcing
nonadsorbed
gas in the void spaces in separator F toward the nonadsorbed gas outlet,
thereby
ensuring that the alkene produced during the countercurrent depressurization
will
be of high purity. The high purity alkene used for the cocurrent purge can be
obtained from an intermediate storage facility in line 22 (not shown), when
separator F comprises a single adsorber; or from another adsorber that is in
the
adsorption phase, when separator F comprises multiple adsorbers arranged in
parallel and operated out of phase.
It will be appreciated that it is within the scope of the present invention to
utilize conventional equipment to monitor and automatically regulate the flow
of
gases within the system so that it can be fully automated to run continuously
in an
efficient manner.
An important advantage of the invention is that it permits removal of
valuable alkenes from a hydrocarbon cracking unit off-gas stream without also
removing substantial amounts of the low value alkanes contained in the off-
gas.
It will be appreciated that a system that achieves enhanced selectivity, and
hence
increased overall recovery of alkenes from a cracking operation is highly
beneficial.
The invention is further illustrated by the following hypothetical example in
which, unless otherwise indicated, parts, percentages and ratios are on a
volume
basis. The example illustrates the process of the invention as it applies to
the
catalytic cracking of a gas oil.
EXA.m.Pl.~1
A gaseous gas oil stream is processed in a fluid catalytic cracker containing
a catalyst based on type Y zeolite and other' active components at a
temperature
11
CRR101794 PATENT
21 3 4 8 2 1 DOCKET NO. 94A219-1
of about 400° C., thereby producing a gaseous product stream. The
gaseous
product is fractionated into a viscous bottoms product, which is combined with
the
gas oil feed to the catalytic cracking unit; a condensed mixed hydrocarbons
side
stream containing mostly C6 and higher hydrocarbons, which is removed as a
liquid
product; and a gaseous overhead stream comprised mostly of CQ and lighter
hydrocarbons. The overhead stream is compressed to a pressure of 33 bar,
cooled
to a temperature of 15 ° C and introduced into a light hydrocarbon
fractional
distillation unit, wherein the overhead stream is split into a bottoms stream
comprising most of the hydrocarbons and an overhead noncondensable gas stream
having the concentration listed in the Table as stream 1.
The noncondensable gas stream is subjected to a pressure swing adsorption
process having a two minute cycle in an adsorption system comprised of a pair
of
adsorption vessels packed with type 4A zeolite. The adsorption vessels are
arranged in parallel and operated out of phase. During the adsorption step the
beds
are maintained at a temperature of 100° C and an absolute pressure of 8
bar, and
during bed regeneration the beds are depressurized to an absolute pressure of
1.2
bar. Desorbed and nonadsorbed gas streams having the compositions listed in
the
Table as streams 2 and 3, respectively, are obtained.
12
CRR101794 PATENT
21 3 4 8 2 1 DOCKET N0. 94A219-1
STREAM 1 STREAM 2 STREAM 3
COMPONENTS
Ibmoles/hr Ibmoles/hr Ibmoles/hr
hydrogen 178.8 17.9 160.9
methane 955.8 372.7 583.0
ethane 402.7 169.2 233.6
ethylene 209.6 167.7 41.9
propylene 248.7 156.7 92.0
propane 32.9 11.8 21.0
isobutane 2.0 0.0 2.1
1-butene 2.0 0.0 2.0
cis 2-butene 0.0 0.0 0.0
normal butane 2.0 0.0 2.0
isopentene 2.0 0.0 2.0
normal pentane 2.0 0.0 2.0
hexane 3.8 0.0 3.8
TOTAL 2,043.4 898.0 1,149.3
Although the invention has been described with particular reference to a
specific experiment, this experiment is merely exemplary of the invention and
variations are contemplated. For example,, the process of the invention may be
practiced in equipment arrangements other than those illustrated in the
drawings.
The scope of the invention is limited only by the breadth of the appended
claims.
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