Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
The present invention relates to the separation of olefins from a
stream of mixed cracked gases. More particularly the present invention
relates to the separation of Gower olefins and particularly C2_3 olefins from
cracked gases from an ethylene cracker.
BACKGROUND OF THE INVENTION
Cryogenic separation systems of the prior art have suffered from
various drawbacks. In conventional cryogenic recovery systems, the
cracked gas is typically compressed to about 450-550 prig, requiring 4-6
stages of compression. Additionally, in conventional cryogenic recovery
systems, three fractionators (distillation tower systems) are required to
separate the ethylene from the other components of the cracked gas
stream: demethanizer, deethanizer, and C2 splitter. Because the
separation of ethane from ethylene involves close boiling compounds, the
splitters generally require very high reflux ratios and a large number of
trays, typically on the order of 100 to 250 trays each. The conventional
cryogenic separation technology also requires multi-level cascaded
propylene and ethylene refrigeration systems, as well as complicated
hydrogen and methane turboexpanders and recompressors or a methane
refrigeration system, adding to the capital and operation cost and
complexity of the conventional cryogenic separation technology.
Much work has been done with solutions of silver (I), and to a
lesser degree copper (I), compounds as alternatives to cryogenic
separation of olefins and paraffins. Silver and copper in their +1 valent
states are known to selectively and reversibly bind olefins. While these
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materials have had some success in simple separation of olefins from
paraffins, their application has been limited due to their reactivity with the
hydrogen, carbon monoxide, and acetylenes normally found in olefin-
containing cracked gas streams. To date, n~o known commercial
application of this phenomenon has been developed.
Recently, Wang and Steifel (U.S. Patent 6,120,692, WO 00/61528)
reported that dithiolene nickel complexes can selectively and reversibly
bind olefins. It was further observed that these complexes were not
reactive with coproducts found in commercial olefin streams, specifically
parafFins, hydrogen, carbon monoxide, and acetylene. Further, the metal
dithiolenes were observed to be unreactive to low concentrations of
hydrogen sulfide. It was found that the bound olefins could be readily
released either physically, through a change in temperature or pressure,
or electrochemically by altering the oxidation state of the metal complex.
The present invention seeks to provide a simple process for the
recovery of high purity olefins, including ethylene, from a cracked gas
stream, preferably without the need for distillation separation of close
boiling olefins and paraffins.
The present invention further seeks to provide a process for the
recovery of C3 and higher olefins and diolefins with low levels of non-
olefinic impurities.
Additionally, the present invention seeks to separate olefins and
diolefins from paraffins, acetylenes, hydrogen, and carbon monoxide in a
single processing step.
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The present invention also seeks to provide a process for the
recovery of high purity olefins that reduces refrigeration requirements.
SUMMARY OF THE IN'1~ENTION
The present invention provides a process foir the separation of C2_6 olefins
and C4_6 diolefins from a mixed gas stream also containing one or more
members selected from the group consisting of paraffins, acetylenes,
hydrogen, and carbon monoxide comprising:
(i) compressing said mixed gas stream to a pressure from 100
to 450 psig;
(ii) caustic washing said compressed mixed gas stream to
remove acidic gases including hydrogen sulphide to a level of less than
5,000 ppm;
(iii) drying said washed compressed mixed gas stream to a dew
point of from -100°C to -130°C;
(iv) contacting said dried washed compressed mixed gas stream
with reversible chemical binding agent which preferentially complexes one
or more olefins, diofefins or both to form complexed olefins diolefins or
both;
(v) separating said dried washed compressed mixed gas stream
from said stream of complexed olefins, diolefins or both; and
(vi) treating said complexed olefins, diolefins or both to release
said olefins, diolefins or both and regenerate said reversible chemical
binding agent.
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BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic flow chart of a cryogenic process of the
prior art.
Figure 2 is a schematic flow chart of the process of the present
invention.
DETAILED DESCRIPTION
The present invention uses selective chemical binding reactions to
separate high purity streams of olefins and dioiefins from mixed streams
through a reversible chemical reaction.
The present invention may be used in the separation of olefins and
diolefins from paraffins, acetylenes, hydrogen, and carbon monoxide
without resorting to distillation or cryogenic liquefaction. The olefins and
diolefins are first separated from the other cracked gas components in a
chemical binding process. The olefins are then relatively easily separated
from each other using conventional distillation due to their relatively wide
boiling point differences.
Typically the olefins may be C2_6 olefins, preferably linear olefins
preferably C2_4, most preferably Cz_3 olefins, most preferably alpha olefins.
(i.e. ethylene and propylene).
The diolefins may be C4_6 diolefins, preferably C4_5 diolefins which
may be straight chained or branched and unsubstituted or substituted by
one or more C~_4 alkyl radicals. The diolefins may include butadiene and
isoprene.
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The mixed gases may also include paraffins such as C~-10
hydrocarbons that are straight chained or branched and which may be
unsubstituted or substituted by one or more C~_4 alkyl radicals.
In a typical conventional cryogenic separation process, as shown in
Figure 1, a mixed gas stream from a source,. preferably cracked gas from
a cracker such as an ethylene cracker is fed by line 2 to a compressor 4.
The compressed gas leaves compressor 4 by line 6 and is fed to a caustic
washer 8. In caustic washer 8 acidic gases including hydrogen sulphide
are reduced to a level of less than 5,000 ppm, preferably less than 500,
most preferably less than 100 ppm. The washed compressed gas is fed
by line 10 to dryer 12 where the gas is dried by a conventional drying
means such as a molecular sieve. The compressed dried gas is then fed
by line 14 to the chilling train 16. Hydrogen and methane are separated
from the cracked gas by liquefying a portion of the methane and
essentially all of the heavier components in the chilling train 16 typically
using propylene and ethylene refrigeration. The gaseous components
(e.g. hydrogen and some of the methane) are removed from the chilling
train 16 by line 20 which feeds into line 28 and ultimately into
turboexpander 30.
The liquids from the chilling train 16 are removed via a line 18 and
fed to a demethanizer tower 22. The methane is removed from the top of
the demethanizer tower 22 in a line 24, and is combined with the hydrogen
and methane in a line 20 to form a single stream in a line 28. The
hydrogen and methane in a line 28 is fed to expander 30 and returned to
the chilling train 16 as a refrigerant via a line 32 and removed from the
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chilling train 16 by line 34 to be recompressed in a compressor 36 and
recovered in a line 38.
The C2+ components are removed from the bottom of the
demethanizer tower 22 by line 26 and fed to a deethanizer tower 40. The
C2 components are removed from the top of the deethanizer tower 40 by
line 42 and passed to an acetylene hydrogenation reactor 46 for selective
hydrogenation of acetylenes. The effluent from the reactor 46 is then fed
by a line 48 to a C2 splitter 50 for separation of the ethylene, removed
from the top of splitter 50 in line 52, and ethane, removed from the bottom
of splitter 50 in a line 54.
The C3+ components are removed from the bottom of the
deethanizer tower 40 in line 44 and may be subjected to further separation
and purification steps which are well known to those skilled in the art.
In the invention process depicted in Figure 2 the cracked gas in line
102 is compressed in a compressor 104. The compressed gas is fed by
line 106 to a caustic washer 108 and fed via a line 110 to dryer 112. The
dried gas is fed by line 114 chemical separating unit 116 which separates
a stream containing ethylene and higher olefins and diolefins in line 120
from a stream consisting primarily of non-olefins (e.g. paraffins and
acetylenes, C02, CO and other products in the mixed gas stream) in a line
118. The primarily non-olefins stream in a line 118 may be subjected to
further separation and purification using methods well known to those
skilled in the art.
The olefin-containing stream in line 120 is fed to a deethylenizer
tower 122. The ethylene is removed from the top of the deethylenizer
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tower 122 in a line 124. The C3+ olefins and diolefins are removed from
the bottom of the deethylenizer tower 122 in line 126 and may be
subjected to further separation and purification.
The present invention provides a novel process for the recovery of
olefins and diolefins from cracked gases cornprising the steps of (a)
contacting the cracked gas stream with a metal complex capable of
selectively reacting and binding with olefins and diolefins to produce a
stripped paraffin-rich gaseous stream, (b) recovering the bound olefins
and diolefins from the metallic complex by reversing the binding reaction,
and (c) separating the resulting olefin and diolefin stream into an ethylene
and a mixed-olefin and diolefin stream.
The cracked gas streams useful as feedstocks in the process of the
present invention can typically be any gas stream which contains light
olefins, namely ethylene and propylene, in combination with other gases,
particularly hydrogen and saturated hydrocarbons. Typically, cracked gas
streams for use in accordance with the practice of the present invention
will comprise a mixture of butane, butenes, propane, propylene, ethane,
ethylene, acetylene, methyl acetylene, propadiene, butadienes, methane,
hydrogen, and carbon monoxide.
The cracked gas stream is preferably first compressed to a
pressure ranging from about 100 psig to about 450 psig, preferably from
about 200 psig to about 400 psig, in the compressing step to produce a
compressed cracked gas stream. The compression may be effected in
any compressor or compression system known to those skilled in the art.
This relatively low compression requirement represents a significant
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improvement over the prior art cryogenic processes. In the prior art
cryogenic process, the cracked gas is typically required to be compressed
to about 450-550 psig and requires 4-6 stages of compression. In the
present process, the compression requirements are significantly reduced
thereby representing a significant savings.
The compressed gas is then caustic washed to remove hydrogen
sulfide and other acid gases, as is well knov~rn to those skilled in the art.
Any of the caustic washing processes known to those skilled in the art
may be employed in the practice of the present invention. However, in the
practice of the present invention which includes a chemical binding step,
complete removal of hydrogen sulfide is not necessary because the
olefins and diolefins will be selectively removed from the hydrogen sulfide
in the selective chemical binding system and because traces of hydrogen
sulfide are not known to adversely affect the metal complex used in the
selective chemical binding process.
The washed and compressed gas is then dried, such as over a
water-absorbing molecular sieve to a dew paint of from about -150°F
(about -100°C) to about -200°F (about -128°C) to produce
a dried
stream.
The dried process stream may then be passed directly to the
selective chemical binding system of the present invention.
In a preferred embodiment of the invention, the dried process
stream may then be purified to remove some higher hydrocarbons prior to
introduction in the selective chemical binding unit.
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More preferably, the dried process stream may be debutanized to
recover C5+ and heavier components from the stream prior to removal of
olefins and diolefins. This process is well known to those skilled in the art.
The bottoms from the debutanizer comprises substantially all of the C5+
hydrocarbons and may be separated into its component parts for pentene
recovery, and recycling of paraffins to the steam cracker, as desired. The
overhead from the debutanizer comprises substantially all of the C4 and
lighter hydrocarbons and is then passed to the selective chemical binding
system of the present invention.
In the selective chemical binding section the olefin-containing
vapour stream is contacted with a metal-containing complex to separate
the olefins and diolefins from the bulk of the stream. Through such
contacting, the olefins and diolefins are chemically reacted with the metal
complex and are selectively separated (removed) from the paraffinic
components (typically C~_1o paraffins which are straight chained or
branched and are unsubstituted or substituted by one or more C,_4 alkyl
radicals including methane, ethane, propane and butane). The scrubbed
gases, mainly paraffins, hydrogen, acetylenes, and carbon monoxide, are
removed from the top of the binding reactor. The olefins and diolefins
bound with the metal complex are then recovered through a reversal of
the binding chemical reaction to generate a stream containing essentially
olefins and diolefins.
This contacting system may be chosen from any of several
methods well known in the art, including but not limited to liquid absorption
(e.g. passing through a liquid pool or co or counter current adsorption in
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for example a tower), solid-phase adsorption passing through a fixed
adsorption bed), and contacting through polymeric membranes.
(e.g. membranes which may be doped with the reversible chemical
binding agent).
Olefin andlor diolefin recovery (and regeneration of the reversible
chemical binding agent) may be effected by a physical method, such as
reduction in temperature or pressure of the stream, or by an
electrochemical method whereby the oxidation state of the metal complex
is changed to release the bound olefins or diolefins.
The metal complex used may be selected from any number of
compounds that possess the following properties:
1 ) the metal complex reacts with olefins and diolefins to bind
the olefin or diolefin in a single molecule;
2) the binding of olefin or diolefin to the metal complex can be
readily reversed by one or more of a) a reduction in pressure, b) increase
in temperature, andlor c) a change in the oxidation state of the metal-
complex; and
3) the metal complex does not react with paraffins, hydrogen,
carbon monoxide, or acetylenic compounds not containing a carbon-
carbon double bond.
Examples of this type of metal complex include but are not limited
to the metal dithiolene complexes discussed by Wang and Steifel (U.S.
Patent No. 6,120,692).
Dithiolene is a commonly used name for 1,2-enedithiolate or
benzene-1,2-dithiolate and related dithiolates.
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The transition metal dithiolene complexes which may be useful in
accordance with the present invention rnay be selected from the group of
complexes of the formulae:
MLS2 ~2 (R1 R2)~2
Rl
S
M\
R2
L - z
and
M~S2 ~6 (R3 R4 R6 R7~~2
R3
R4
\s Rs
R6
2
The preparation of complexes of the formulae (i) and (ii) is
disclosed in United States Patent 6,120,692 referred to above.
In the formulae M is a transition metal, preferably a Group VIII
metal, R' and R2 may be the same or different, and are independently
selected from a hydrogen atom, electron-withdrawing groups including
those that are or contain heterocyclic, cyano, carboxylate, carboxylic ester,
keto, nitro, and sulfonyl groups, and hydrocarbyl groups, including C~_6,
preferably C~_4 alkyl groups, C5_$, preferably C6_$ cyclo alkyl groups, C2_$,
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preferably C2_4 alkenyl groups and C6_$ aryl groups, unsubstituted or fully
or partly substituted, preferably those substituted with electron-
withdrawing groups. Preferably the groups are cyano groups or halo
substituted C~~. alkyl groups, more preferably the halo substituents on the
carbon atoms are fluoro groups. Most preferably R~ and R2 are CF3 or
CN.
The benzene dithiolato compounds, represented by the structure in
the formula (ii) above. In the formula (ii), M also is a transition metal,
preferably a Group VIII metal, R3, R4, R5, and R6 may be the same or
different and are independently selected from the group consisting of a
hydrogen atom; electron-withdrawing groups as described above, and
hydrocarbyl groups, including C~_~, preferably C~_4 alkyl groups, C5_$,
preferably C6_$ cyclo alkyl groups, C2_$, preferably C2_4 alkenyl groups and
C6_$ aryl groups, unsubstituted or fully or partly substituted, preferably
those substituted at the carbon atoms of the hydrocarbyl group that are
electron-withdrawing groups such as a halide atom, preferably a fluorine
atom.
Mueller-Westerhoff, U. T., "Dithiolene and Related Species",
Comprehensive Coordination Chemistry, Vol. 2, 595-631 (1987);
McCleverty, J. A., "Metal 1,2-Dithiolene and Related Complexes", Prog.
Inorg. Chem., Vol. 10, 49-221 (1968)) disclose more complex forms of the
dithiolenes that also may be used.
The transition metals are preferably Fe, Co, Ni, Cu, Pd and Pt,
preferably Ni. Thus the complex can be any metal bis(1,2-enedithiolate),
preferably a group VIII metal bis(1,2-enedithiolate), more preferably
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substituted 1,2-enedithiolate with electron withdrawing groups, and most
preferably the bis[1,2-bis(trifluoromethyl)ethylene-1,2-dithiolato] metal
complex.including but not limited to bis-cis(1,2-perfluoromethylethylene-
1,2-dithiolato)nickel and bis-cis(1,2-cyanoethylene-1,2-dithiolato)nickel.
The recovered olefins and diolefins from the selective chemical
binding unit may then be further purified in a deethylenizer tower to
separate the ethylene from the C3 and higher olefins and diolefins. This
stream may require compression to a pressure ranging from about 250
psig to about 300 psig, preferably about 300 psig, if a reduction in
pressure was used to reverse the selective chemical binding with the
metal complex. Any means of compression known to those skilled in the
art may be employed. It is further understood that this stream may require
a solvent removal step if the bound metal complex was dissolved in a
volatile solvent. If a temperature change is required the temperature
differential between the complexed and free reversible chemical bonding
agent (complex) should be at least about 25°C, preferably at least
about
30°C, most preferably at least about 50°C.
The mixed separated olefins would be fed to a deethylenizer tower
that operates at a pressure ranging from about 250 psig to about 300 psig,
generally about 275 psig. Typically, Bow level propylene refrigeration is
sufficient for feed chilling and to condense the overheads in the
deethylenizer. High purity ethylene is taken at or near the top of the
deethylenizer. A mixed olefins and diolefins stream is removed from the
bottom of the deethylenizer.
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Conventionally, the recovery of high-purity ethylene by distillation is
a very expensive proposition due to the difficulty of separating low- and
close-boiling compounds via distillation. In the demethanizer, methane is
separated from C2 and heavier components.. In the deethanizer, ethane,
ethylene, and acetylene are separated from C3 and heavier components.
In the C2 splitter, ethylene is separated from ethane. A large number of
trays and high reflux ratios are required for these separations, particularly
for the separation of ethylene from ethane, vvhere about 100-250 trays are
required. Additionally, large quantities of energy in the form of steam, hat
water, refrigeration and cooling water are required for the operation of
these towers. Further, the capital required for these towers as well as the
required complex ethylene, propylene, and turbo-expanded hydrogen and
methane refrigeration systems is extremely high.
However, the present invention employing the selective chemical
binding system, enables the separation of olefins and diolefins from
paraffins, acetylenes, hydrogen, and carbon monoxide without resorting to
distillation or cryogenic liquefaction. Thus, the olefins and diolefins are
first separated from the other cracked gas components in a chemical
binding process. The olefins are then relatively easily separated from
each other using conventional distillation due to their relatively wide
boiling
point differences. Further, the level of refrigeration required within the
condensers of these distillation towers are significantly lower due to the
absence of difficult to condense components such as hydrogen, methane,
and carbon monoxide. Thus, low reflux ratios, small number of trays, and
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minimal and low-level refrigeration are sufficient to produce high-purity
ethylene and higher olefin products.
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