U.S. patent application number 12/436480 was filed with the patent office on 2010-06-03 for absorption method for recovering gas contaminants at high purity.
Invention is credited to David A. BAHR, Lamar A. Davis.
Application Number | 20100135892 12/436480 |
Document ID | / |
Family ID | 42223000 |
Filed Date | 2010-06-03 |
United States Patent
Application |
20100135892 |
Kind Code |
A1 |
BAHR; David A. ; et
al. |
June 3, 2010 |
ABSORPTION METHOD FOR RECOVERING GAS CONTAMINANTS AT HIGH
PURITY
Abstract
Solvent absorption processes for separating components of an
impure feed gas are disclosed. The processes involve two stages of
gas-liquid contacting, namely a first, absorption stage and a
second, stripping stage. In the case of a carbon dioxide
(CO.sub.2)-- containing methane gas as an impure feed gas,
contacting, in the stripping stage, the solvent effluent from the
absorption stage with a recycled vapor fraction of the solvent
effluent from the stripping stage can improve the recovery and
purity of not only the methane (and/or other light hydrocarbons in
the impure feed gas), but also that of the CO.sub.2 contaminant
gas.
Inventors: |
BAHR; David A.; (Chicago,
IL) ; Davis; Lamar A.; (West Dundee, IL) |
Correspondence
Address: |
HONEYWELL/UOP;PATENT SERVICES
101 COLUMBIA DRIVE, P O BOX 2245 MAIL STOP AB/2B
MORRISTOWN
NJ
07962
US
|
Family ID: |
42223000 |
Appl. No.: |
12/436480 |
Filed: |
May 6, 2009 |
Current U.S.
Class: |
423/437.1 ;
48/127.3 |
Current CPC
Class: |
Y02C 20/40 20200801;
B01D 2256/22 20130101; B01D 2257/7022 20130101; B01D 53/1487
20130101; Y02C 10/06 20130101 |
Class at
Publication: |
423/437.1 ;
48/127.3 |
International
Class: |
B01D 53/62 20060101
B01D053/62; C01B 31/20 20060101 C01B031/20 |
Claims
1. A gas purification method comprising: (a) contacting an impure
feed gas comprising methane and carbon dioxide (CO.sub.2) with a
solvent to provide a methane-enriched gas and a first stage solvent
effluent comprising absorbed methane and absorbed CO.sub.2; and (b)
contacting the first stage solvent effluent with a recycled vapor
fraction of a second stage solvent effluent, wherein the second
stage solvent effluent is obtained by contacting the first stage
solvent effluent with the recycled vapor fraction in step (b).
2. The method of claim 1, wherein the impure feed gas comprises
predominantly methane and further comprises CO.sub.2 in an amount
from about 1% to about 10% by volume.
3. The method of claim 1, wherein the solvent is a physical
solvent.
4. The method of claim 1, further comprising: (c) recovering a
purified CO.sub.2 product gas from a gas-depleted liquid fraction
of the second stage solvent effluent.
5. The method of claim 4, wherein the purified CO.sub.2 product gas
is recovered after heating at least a portion of the gas-depleted
liquid fraction of the second stage solvent effluent to regenerate
the solvent.
6. The method of claim 5, wherein the purified CO.sub.2 product gas
comprises CO.sub.2 in an amount of at least about 90% by volume on
a water-free basis.
7. The method of claim 1, wherein step (b) provides a second
methane-enriched gas.
8. The method of claim 7, wherein the methane-enriched gas provided
in step (a) is combined with the second methane-enriched gas to
provide a purified methane product gas.
9. The method of claim 8, wherein the purified methane product gas
comprises methane in an amount of at least about 95% by volume on a
water-free basis.
10. The method of claim 1, wherein (i) the recycled vapor fraction
and (ii) the gas-depleted liquid fraction are obtained from one or
more flash separations of the second stage solvent effluent at one
or more flash separation pressures lower than a second stage
contacting pressure in step (b).
11. The method of claim 10, wherein (i) and (ii) are obtained from
two flash separations of the second stage solvent effluent at two
successively lower flash separation pressures.
12. A two-stage gas purification process comprising: (a) contacting
an impure feed gas stream comprising methane and CO.sub.2 and a
solvent stream in a counter-current absorber to provide a
methane-enriched overhead gas stream and a first stage solvent
effluent bottoms stream comprising absorbed methane and absorbed
CO.sub.2; and (b) contacting the first stage solvent effluent
bottoms stream and a recycled vapor fraction of a second stage
solvent effluent bottoms stream in a counter-current stripper to
provide a second methane-enriched overhead gas stream and the
second stage solvent effluent bottoms stream; and (c) flash
separating the second stage solvent effluent bottoms stream in one
or more flash separators to provide (i) one or more vapor fractions
of the second stage solvent effluent bottoms stream and (ii) a
gas-depleted liquid fraction of the second stage solvent effluent
bottoms stream; and (d) recovering a purified CO.sub.2 product gas
stream from the gas-depleted liquid fraction of the second stage
solvent effluent bottoms stream.
13. The two-stage gas purification process of claim 12, wherein
step (c) comprises flash separating the second stage solvent
effluent bottoms stream in two flash separators to provide high and
medium pressure vapor fractions, and wherein the recycled vapor
fraction comprises at least a portion of each of the high and
medium pressure vapor fractions.
14. The two-stage gas purification process of claim 12, wherein
step (d) comprises flash separating the gas-depleted liquid
fraction to provide a fuel gas stream as a vapor fraction and a
CO.sub.2-rich solvent stream as a liquid fraction, and wherein the
purified CO.sub.2 product gas stream is recovered after heating at
least a portion of the CO.sub.2-rich solvent stream to recover the
solvent.
15. The two-stage gas purification process of claim 14, wherein the
purified CO.sub.2 product gas stream comprises CO.sub.2 in an
amount of at least about 90% by volume on a water-free basis and is
recovered after heating at least a portion of the CO.sub.2-rich
solvent stream to a temperature of at least about 150.degree. C.
(302.degree. F.) to regenerate the solvent.
16. The method of claim 12, further comprising combining the first
methane-enriched overhead gas stream from the absorber and the
second methane-enriched overhead gas stream from the stripper to
provide a purified methane product gas stream comprising methane in
an amount of at least about 95% by volume on a water-free
basis.
17. The two-stage gas purification process of claim 17, wherein at
least about 99% of the methane and at least about 65% of the
CO.sub.2 present in the impure feed gas stream are recovered in the
purified methane product gas stream and the purified CO.sub.2
product gas stream, respectively.
18. The method of claim 12, wherein the absorber and the stripper
are maintained at a pressure from about 41 barg (600 psig) to about
83 barg (1200 psig).
19. The method of claim 18, wherein the one or more flash
separators are maintained at pressures of less than about 27 barg
(400 psig).
20. A method for recovering a purified CO.sub.2 product gas and a
purified methane product gas from an impure feed gas comprising
methane and CO.sub.2, the method comprising contacting the impure
feed gas with a solvent in an absorption stage and a stripping
stage and recycling a vapor fraction of a solvent effluent from the
stripping stage, back to the stripping stage.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to gas separation methods, for
example the separation of contaminant carbon dioxide (CO.sub.2)
from methane using a solvent absorption process. In a two-stage,
absorption/stripping operation, recycling a vapor fraction of the
liquid solvent-containing stream exiting the stripper bottoms
improves the purity of the recovered carbon dioxide.
DESCRIPTION OF RELATED ART
[0002] The removal of carbon dioxide (CO.sub.2) from predominantly
light hydrocarbon- (e.g., methane-) containing, impure gas streams
is desired, among other reasons, to improve the heating value of
the purified gas product for pipeline transmission. Differences in
a number of properties between CO.sub.2 and light hydrocarbons can
serve as potential bases for gas separations. These differences
include solubility, acidity in aqueous solution, and molecular size
and structure. Possible separations can therefore rely on physical
or chemical absorption into liquid solvents, pressure swing
adsorption with solid adsorbents, and membrane systems.
[0003] Liquid solvent absorption (i.e., "wet") systems, for
example, are commonly used for natural gas purification to remove
minor amounts of CO.sub.2. This contaminant can be preferentially
absorbed in physical solvents such as a dimethylethers of
polyethylene glycol or chemical solvents such as alkanolamines or
alkali metal salts. The resulting CO.sub.2-rich (i.e., "loaded")
solvent is subsequently regenerated by heating to recover both
CO.sub.2 and a regenerated solvent that can be recycled for further
use in absorption. Solvent regeneration is also normally conducted
at a reduced pressure relative to the upstream absorption pressure,
to promote vaporization of absorbed CO.sub.2 from the solvent. The
solvent absorption and solvent regeneration by heating are usually
carried out in different columns containing packing, bubble plates,
or other vapor-liquid contacting devices to improve the efficiency
of mass transfer between phases. The CO.sub.2 may be recovered in
more than one stream, including vapor fractions of flash separators
and regenerator column vapor effluents.
[0004] Chemical solvents, and particularly amines and other basic
compounds, react with contaminant CO.sub.2, an acid gas, to form a
contaminant-solvent chemical bond. Considerable energy release is
associated with this bond formation during the
thermodynamically-favored, acid-base reaction. Due to the
substantial heat input required to break the bonds of the
heat-stable salts formed as chemical reaction products, chemical
solvents are not economically regenerated. Physical solvents, on
the other hand, do not react chemically with gas contaminants, but
instead promote physical absorption based on a higher contaminant
equilibrium solubility at its partial pressure in the impure gas
(i.e., a higher Henry's law constant).
[0005] Physical solvents that remain chemically non-reactive with
the impure gas stream are therefore desirable in absorption systems
due to the ease of solvent regeneration. However, the removal of a
given contaminant gas using a physical solvent is limited by its
partial pressure driving force. Also, while the contaminant may be
effectively removed from a desired gas such as methane, the
recovery of the contaminant itself in a purified form remains
problematic. For example, natural gas streams and coalbed methane
both contain high quantities of methane and can be purified by the
selective physical absorption of contaminant CO.sub.2. However, the
CO.sub.2 contaminant is not easily recovered at high purity using
conventional absorption systems, without significant additional
costs. Yet obtaining purified CO.sub.2 as a separated contaminant
of an impure methane gas stream would enhance its value for
industrial applications. One use of particular interest is enhanced
oil recovery, in which CO.sub.2 is injected into an oil reservoir
where it dissolves the oil in place, thereby reducing its viscosity
and making it more mobile for movement toward the producing
well.
[0006] There is consequently a need in the art for gas purification
methods in which contaminants that are initially present in only
minor amounts are recovered at high purity levels.
SUMMARY OF THE INVENTION
[0007] The present invention is associated with the discovery of
solvent absorption processes for separating a carbon dioxide
(CO.sub.2) contaminant of methane and/or other light hydrocarbons
at high purity. The processes involve two stages of gas-liquid
contacting, namely a first, absorption stage and a second,
stripping stage. The absorption stage provides nearly complete
removal of CO.sub.2, by preferential absorption of this component
from an impure light hydrocarbon-containing feed gas, into a
physical solvent. In the case of a methane-containing feed gas, for
example, the resulting solvent exiting the absorption stage
contains absorbed quantities of both CO.sub.2 and methane, with the
relative absorbed CO.sub.2: absorbed methane ratio exceeding the
CO.sub.2: methane ratio in the impure feed gas, due to the
absorption selectivity of the physical solvent for CO.sub.2.
Consequently, the gas or vapor phase exiting the first stage is
enriched in methane, compared to the feed gas.
[0008] The liquid effluent from the absorption stage nevertheless
contains a significant quantity of methane, despite the relatively
lower quantity of this desired, non-preferentially absorbed
component. The second, stripping stage beneficially removes much of
this absorbed methane into a second methane-enriched gas, which may
be combined with the methane-enriched gas exiting the absorption
stage. Importantly, it has been discovered that contacting, in the
stripping stage, the solvent effluent from the absorption stage
with a recycled vapor fraction of the solvent effluent from the
stripping stage can improve the recovery and purity of not only the
methane (and/or other light hydrocarbons in the impure feed gas),
but also that of the CO.sub.2 contaminant gas.
[0009] The solvent effluent from the absorption stage can therefore
be stripped using, for example, all or part of a flash separator
vapor fraction obtained from reducing the pressure of the solvent
effluent from the stripping stage. This pressure reduction, which
may be performed in one or more stages to provide one or more vapor
fractions (portions of which, alone or in any combination, can be
recycled to the stripping stage), can be used in conjunction with
subsequent heating of the resulting gas-depleted liquid fraction of
the stripping stage solvent effluent to regenerate the solvent.
[0010] Overall, therefore, aspects of the invention are associated
with the significant benefits derived from "backstripping" of the
first, absorption stage solvent effluent with a recycled vapor
fraction (e.g., from flash separation) of the liquid effluent from
the stripping stage. The benefits of conducting the second
stripping stage in this manner include not only an enhanced
recovery of methane, initially absorbed in the solvent after the
first absorption stage, but also effective purification of the
CO.sub.2 that remains absorbed in the solvent effluent after the
second stage. This absorbed CO.sub.2 is subsequently recovered as
CO.sub.2 product at high purity, upon regenerating the solvent at
elevated temperature. In a representative embodiment, the CO.sub.2
product gas has a purity such that it is directly applicable in a
number of industrial applications, including enhanced oil
recovery.
[0011] These and other aspects and features relating to the present
invention are apparent from the following Detailed Description.
BRIEF DESCRIPTION OF THE DRAWING
[0012] FIG. 1 depicts a representative two-stage process for
purifying an impure, methane-containing feed gas to provide the
recovered, carbon dioxide (CO.sub.2) as a purified product gas.
[0013] The drawing is to be understood to present an illustration
of the invention and/or principles involved. Details including
pumps, compressors, heaters, chillers, reboilers, condensers, solid
filtration and liquid knockout devices, instrumentation and control
loops, and other items not essential to the understanding of the
invention are not shown. As is readily apparent to one of skill in
the art having knowledge of the present disclosure, gas
purification methods according to various other embodiments of the
invention, will have configurations and components determined, in
part, by their specific use.
DETAILED DESCRIPTION
[0014] The invention is directed to gas purification methods in
which a contaminant, present as a minor component of an impure feed
gas, is selectively absorbed into a solvent. The methods
advantageously recover significant portions of the impure feed gas
components, including the contaminant, in purified product gas
streams. Representative impure gas streams include those comprising
light hydrocarbons (e.g., C.sub.1-C.sub.3 hydrocarbons such as
methane), and non-hydrocarbon gas contaminants, such as carbon
dioxide (CO.sub.2). Examples of such gas streams include natural
gas and coalbed methane, which comprise predominantly methane and
also comprise CO.sub.2 in an amount from about 1% to about 10% by
volume. For simplicity, the invention is described hereinafter with
respect to such methane and CO.sub.2 systems with the latter
component being present at contaminant levels, although it will be
appreciated that the invention is broadly applicable to the
purification of impure gas feeds in which a contaminant, present in
an minor amount, is preferentially absorbed into a liquid solvent,
and particularly a physical solvent.
[0015] Representative embodiments of the invention are therefore
directed to gas purification methods involving two contacting
stages. In the first, absorption contacting stage, an impure feed
gas is contacted with a solvent. In the second contacting stage,
the "loaded" solvent from the first stage, comprising absorbed
components of the impure feed gas, is subjected to stripping,
meaning that a vapor is introduced to strip or drive off at least
some of these absorbed components, into a gas effluent from the
stripping stage. The absorption and stripping stages are normally
each carried out in two separate absorber and stripper vessels,
with liquid solvent streams flowing downwardly toward a solvent
effluent outlet, counter-current to gas streams flowing upwardly
toward a gas effluent outlet. It is, however, possible for
absorption and stripping to be performed in a single vessel, such
as in a stacked arrangement in upper and lower sections of the
vessel, respectively.
[0016] According to the present invention, the vapor used for the
stripping stage is a recycled vapor fraction of the solvent
effluent from this stripping stage. The significant commercial
advantages associated with the use of this recycled vapor in the
stripping stage are discussed above.
[0017] A gas purification method according to an exemplary
embodiment therefore comprises contacting an impure feed gas
comprising methane and CO.sub.2 with a solvent, and particularly a
physical solvent that selectively (or preferentially) absorbs
CO.sub.2 over methane.
[0018] Representative physical solvents include propylene
carbonate, tributyl phosphate, methanol, tetrahydrothiophene
dioxide (or tetramethylene sulfone). Others include alkyl- and
alkanol-substituted heterocyclic hydrocarbons such as
alkanolpyridines (e.g., 3-(pyridin-4-yl)-propan-1-ol) and
alkylpyrrolidones (e.g., n-methyl pyrrolidone), as well as
dialkylethers of polyethylene glycol, with polyethylene glycol
dimethyl ether being a preferred physical solvent. The feed
gas-solvent contacting in a first contacting stage therefore
provides (i) a gas with a higher methane content than the feed gas
(i.e., a methane-enriched gas) and (ii) a solvent effluent
comprising the absorbed CO.sub.2 as well as some absorbed methane,
with (i) and (ii) exiting the first contacting stage as vapor and
liquid phases, respectively.
[0019] The first stage solvent effluent is then contacted, in a
second stage of contacting (a stripping stage), with a recycled
vapor fraction of the solvent effluent of this contacting stage.
The second stage solvent effluent is obtained as a liquid phase
exiting this stage, generally having a reduced quantity of absorbed
methane relative to the first stage solvent effluent, as a result
of a portion of this absorbed methane being stripped with the
recycled vapor fraction. The second stage of contacting also
generally provides a second methane-enriched gas (i.e., having a
higher methane content than the recycled vapor fraction), that
exits the second stage of contacting as a vapor phase. The vapor
phases or methane-enriched gases exiting both the first and second
contacting stages may be combined to provide a purified methane
product gas, which, according to representative embodiments of the
invention comprises methane in an amount of generally at least
about 90% (e.g., in the range from about 90% to about 99%) by
volume, typically at least about 93% (e.g., in the range from about
93% to about 99%) by volume, and often at least about 95% (e.g., in
the range from about 90% to about 99%) by volume, on a water-free
basis. According to representative embodiments of the invention,
generally at least about 95%, typically at least about 98%, and
often at least about 99% or substantially all (e.g., at least about
99.5%) of the methane originally present in the impure feed gas is
recovered in the purified methane product gas.
[0020] The recycled vapor fraction of the second stage solvent
effluent is therefore used, in the second contacting stage, to
strip a portion of the methane that is initially absorbed into the
liquid solvent after the first contacting stage. This portion of
methane, stripped into the vapor phase exiting the second
contacting stage, represents an additionally recovered amount,
beyond that recovered in the vapor phase exiting the first stage.
The recycled vapor fraction is obtained by vaporization of some
portion of the gases absorbed into the liquid solvent effluent
exiting the second contacting stage as a liquid phase. This
vaporization may be achieved by depressurization and/or heating of
the second stage solvent effluent. Generally, the vapor fraction
that is recycled to the second contacting stage is obtained from
one or more flash separations of the second stage solvent effluent.
In the case of a plurality of flash separations (e.g., the use of
two flash separations in series), successively lower pressure flash
separations (e.g., at high pressure followed by medium pressure)
may be utilized to provide a plurality of vapor fractions, all or a
portion of which may be combined to provide the recycled vapor
fraction. The flash separation pressures are generally all lower
than the pressure used in the second, stripping stage.
[0021] A plurality of flash separations are therefore often
performed in separate flash separators, although multiple stages of
vapor-liquid flashing or equilibrium stages may be combined in a
single vessel, albeit at an essentially constant pressure, such as
a distillation or fractionation column having a section containing
packing, trays, or other vapor-liquid contacting devices.
Distillation at one pressure, preceded or followed by flash
separation at a different pressure, is also possible. In any event,
one or more vapor fractions of the second stage solvent effluent
are obtained, at least a portion of which are recycled, as the
recycled vapor fraction, back to the second stage of contacting as
described above.
[0022] The vaporization of a fraction (or fractions) of the second
stage solvent effluent, provides an associated, gas-depleted liquid
fraction of the second stage solvent effluent. Because CO.sub.2 is
preferentially absorbed in the solvent over methane, the absorbed
gases remaining in the gas-depleted liquid fraction or solvent
phase, after the removal of the vapor fraction(s), are enriched in
CO.sub.2. Therefore, the purified CO.sub.2 remaining in the
gas-depleted liquid fraction is advantageously recovered as a
product gas, in addition to the purified methane product gas as
discussed above. Often, the purified CO.sub.2 product gas is
recovered after heating at least a portion of the gas-depleted
liquid fraction of the second stage solvent effluent, as necessary
to regenerate the solvent. The purified CO.sub.2 product gas
recovered from all or a portion of the gas-depleted liquid
fraction, according to representative embodiments of the invention,
comprises CO.sub.2 in an amount of generally at least about 90%
(e.g., in the range from about 90% to about 99%) by volume,
typically at least about 92% (e.g., in the range from about 92% to
about 99%) by volume, and often at least about 94% (e.g., in the
range from about 94% to about 99%) by volume, on a water-free
basis. In a specific embodiment, for example, the portion of the
gas-depleted liquid fraction that is regenerated is obtained as a
CO.sub.2-rich solvent stream, which is a liquid fraction, after a
flash separation (e.g., at low pressure) to provide a fuel gas as a
vapor fraction that is not recycled to the second contacting stage.
According to representative embodiments of the invention, generally
at least about 50% (e.g., in the range from about 50% to about
95%), typically at least about 60% (e.g., in the range from about
60% to about 90%), and often at least about 65% (e.g., in the range
from about 65% to about 80%) of the CO.sub.2 originally present in
the impure feed gas is recovered in the purified CO.sub.2 product
gas.
[0023] An exemplary, two-stage, gas purification process according
to the invention as described herein, is illustrated in FIG. 1. As
shown, impure feed gas stream 2 comprising methane and CO.sub.2 is
contacted with solvent stream 4 in counter-current absorber 100 to
provide, as exiting vapor and liquid phases, methane-enriched
overhead gas stream 6 and first stage solvent effluent bottoms
stream 8 comprising absorbed portions of methane and CO.sub.2. This
first stage solvent effluent 8 is then passed to counter-current
stripper 200 where it is contacted with recycled vapor fraction 10
of second stage solvent effluent bottoms stream 12. Vapor and
liquid phases exiting the counter-current stripper 200 are,
respectively, second methane-enriched overhead gas stream 14 and
the solvent effluent bottoms stream 12. It is generally desired to
combine methane-enriched overhead gas stream 6 from absorber 100
and second methane-enriched overhead gas stream 14 from stripper
200, with the combined stream being purified methane product gas
stream 38 having a methane purity as described above and suitable
for pipeline transmission.
[0024] Flash separation of solvent effluent bottoms stream 12 in
high pressure and medium pressure flash separators 300a, 300b
provides high and medium pressure vapor fractions 16a, 16b,
respectively, which are combined and recycled to a lower section
counter-current stripper 200 as recycled vapor fraction 10.
Appropriate gas compressors (not shown) are used to flow vapor
fractions 16a, 16b, and recycled vapor fraction 10 from both high
and medium pressure flash separators 300a, 300b, to the
counter-current stripper 200, operating at a higher pressure. In
representative embodiments, counter-current stripper 200, as well
as counter-current absorber 100, operate generally at a pressure in
the range from about 41 barg (600 psig) to about 83 barg (1200
psig), and normally in the range from about 55 barg (800 psig) to
about 69 barg (1000 psig). The high and medium pressure flash
separators 300a, 300b operate generally at a pressure of less than
about 27 barg (400 psig) (e.g., in the range from about 2 barg (30
psig) to about 27 barg (400 psig)). For example, high pressure
flash separator 300a may be maintained at a pressure in the range
from about 21 barg (300 psig) to about 27 barg (400 psig), and low
pressure flash separator 300b may be maintained at a pressure in
the range from about 2 barg (30 psig) to about 14 barg (200
psig).
[0025] According to other embodiments, recycled vapor fraction 10
may comprise only a portion of high pressure vapor fraction 16a, a
portion of medium pressure vapor fraction 16b, or portions of both
of these vapor fractions 16a, 16b. For example, optional purge
stream 18 may be used in some embodiments to reduce the
steady-state content of one or more components of impure feed gas
accumulating in vapor fractions 16a, 16b. Optional purge stream may
remove portions of vapor fractions 16a, 16b, or may remove a
portion of the combination of these vapor fractions 16a, 16b, with
the portion that is not purged being passed to counter-current
stripper 200 as recycled vapor fraction 10.
[0026] As a result of flash separations to provide vapor fractions
16a, 16b, corresponding liquid fractions 20a, 20b are also
provided, generally at the successively lower pressures of high and
medium pressure flash separators 300a, 300b. Corresponding liquid
fractions 20a, 20b also have remaining absorbed gases with
successively increasing contents of CO.sub.2 relative to methane
and other components. The last liquid fraction 20b, having a
corresponding vapor fraction 16b (resulting from the last flash
separation in flash separator 300b), with at least a portion
thereof being recycled to counter-current stripper 200, is namely
gas-depleted liquid fraction 20b of the second stage solvent
effluent bottoms stream 12.
[0027] Purified CO.sub.2 product gas stream 22 is recovered from
gas-depleted liquid fraction 20b, for example by heating at least a
portion of this stream to regenerate the solvent. In the specific
embodiment illustrated in FIG. 1, for example, gas-depleted liquid
fraction 20b is passed to an low pressure flash separator 400,
normally operating at a lower pressure than medium pressure flash
separator 300b. Flash separation of gas-depleted liquid fraction
20b provides fuel gas stream 24 as a vapor fraction and
CO.sub.2-rich solvent stream 26 as a liquid fraction. Fuel gas
stream 24 is often a relatively small vapor stream having a
significant amount of methane, for example in the range from about
20% to about 50% by volume, with this vapor stream being in
equilibrium with CO.sub.2-rich solvent stream 26 at a typical
pressure of low pressure flash separator 400 in the range from
about 0.5 barg (7 psig) to about 5 barg (75 psig). CO.sub.2-rich
solvent stream 26 is then heated to recover the remaining gas
absorbed therein and having a significantly higher CO.sub.2
content.
[0028] As shown in FIG. 1, CO.sub.2-rich solvent stream 26 is
divided into portions 26a, 26b, with one portion 26b being recycled
to an upper section of counter-current stripper 200 and another
portion 26a being passed to feed/effluent heat exchanger 50 of
solvent regenerator 500. Feed/effluent heat exchanger 50 is used in
conjunction with solvent reboiler (not shown) and optionally one or
more heaters of solvent regenerator 500, to heat the portion 26a of
the CO.sub.2-rich solvent stream from which CO.sub.2 is recovered.
This CO.sub.2-rich solvent portion 26a, as an inlet to solvent
regenerator 500, is typically heated to a temperature in the range
from about 38.degree. C. (100.degree. F.) to about 204.degree. C.
(400.degree. F.), and often in the range from about 66.degree. C.
(150.degree. F.) to about (149.degree. C.) 300.degree. F.,
depending on the particular solvent and component(s) of impure gas
stream 2 to be recovered. In an exemplary embodiment in which the
solvent comprises dimethyl ethers of polyethylene glycol, the
solvent is generally heated to a temperature of at least about 150
C (302.degree. F.) (e.g., from about 150.degree. C. (302.degree.
F.) to about 204.degree. C. (400.degree. F.)) to regenerate the
solvent and recover purified CO.sub.2 product gas stream 22.
[0029] In the exemplary embodiment depicted in FIG. 1, purified
CO.sub.2 product gas stream 22 is recovered from overhead vapor
stream 28 of solvent regenerator 500, generally after this stream
is passed through an overhead condenser (not shown) and overhead
reflux drum 600 to which a make-up water stream 30 may be added and
from which (i) purge water stream 34 for impurity removal and/or
(ii) overhead reflux liquid stream 32 for return to solvent
regenerator 500, may be withdrawn. Purified CO.sub.2 product gas
stream 22 advantageously has a purity level as discussed above,
which is suitable for a number of industrial applications,
including enhanced oil recovery.
[0030] Regenerated solvent stream 36 is recovered from a bottom
section of solvent regenerator 500, substantially depleted of all
absorbed gases, and may be cooled by heat exchange against
CO.sub.2-rich solvent portion 26a using heat exchanger 50 and
further cooled using a chiller (not shown) prior to introduction
into counter-current absorber 100. In general, regenerated solvent
stream 36 is combined with optional make-up solvent stream 5 to
provide solvent stream 4 that is introduced to counter-current
absorber 100 as described above. Optional make-up solvent stream 5
replaces the total solvent losses throughout the two-stage gas
purification process.
[0031] Overall, aspects of the invention are associated with
processes for purifying impure feed gas streams which
advantageously allow the recovery of components of these gas
streams at high purity. The processes comprise contacting the
impure feed gas with a solvent in an absorption stage and a
stripping stage and recycling a vapor fraction of a solvent
effluent from the stripping stage, back to the stripping stage. An
exemplary impure feed gas stream is a predominantly
methane-containing gas stream having CO.sub.2 as an impurity or
contaminant in an amount from about 1% to about 10% by volume.
Those having skill in the art will recognize the applicability of
the methods disclosed herein to any of a number of gas purification
processes, and particularly those utilizing a physical solvent that
preferentially absorbs the contaminant that is to be recovered at
high purity. It will also become apparent that embodiments of the
invention disclosed herein will use a smaller or larger number of
flash separations and/or other types of separations, to provide the
recycled vapor fraction, relative to the specific embodiment
illustrated in FIG. 1.
[0032] Those having skill in the art, with the knowledge gained
from the present disclosure, will recognize that various changes
could be made in the above processes without departing from the
scope of the present disclosure. Mechanisms used to explain
theoretical or observed phenomena or results, shall be interpreted
as illustrative only and not limiting in any way the scope of the
appended claims.
[0033] The following example is set forth as representative of the
present invention. This example is not to be construed as limiting
the scope of the invention as other equivalent embodiments will be
apparent in view of the present disclosure and appended claims.
EXAMPLE 1
[0034] The two-stage gas purification process illustrated in FIG. 1
above was simulated using chemical process simulation software,
namely ASPEN PLUS.RTM. (Aspen Technology, Inc., Burlington, Mass.,
USA) with a proprietary properties package. The model impure feed
gas chosen for this simulation contained 95% and 5% by volume of
methane and CO.sub.2, respectively. The physical solvent chosen was
a commercial solvent used in the SELEXOL.RTM. process licensed by
UOP, LLC (Des Plaines, Ill.), containing dimethyl ethers of
propylene glycol.
[0035] The results of the simulation showed that more than 99.5% of
the methane present in the impure feed gas could be recovered, in
the combined, methane-enriched gas streams exiting the counter
current absorber and stripper columns as vapor phases (i.e., the
purified methane product gas). The combined stream, namely the
methane product gas stream, had a methane purity of greater than
96.8% by volume on a water-free basis. Importantly, using the
flowscheme in FIG. 1, in which vapor fractions of the solvent
effluent from the counter-current stripper were recycled back to
the stripper, allowed more than 67% of the CO.sub.2 present in the
impure gas feed to be recovered as a purified CO.sub.2 product gas
stream exiting the solvent regenerator as a vapor phase. This
product gas stream had a CO.sub.2 purity of greater than 97% by
volume on a water-free basis.
[0036] This high CO.sub.2 purity would be acceptable, without
further purification, for use in enhanced oil recovery.
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