U.S. patent application number 13/002966 was filed with the patent office on 2011-06-09 for two stage process for producing purified gas.
Invention is credited to Andrew Malcolm Beaumont, Henricus Abraham Geers.
Application Number | 20110132034 13/002966 |
Document ID | / |
Family ID | 40481839 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110132034 |
Kind Code |
A1 |
Beaumont; Andrew Malcolm ;
et al. |
June 9, 2011 |
TWO STAGE PROCESS FOR PRODUCING PURIFIED GAS
Abstract
The present invention provides a process for producing purified
gas from a feed gas stream comprising methane and gaseous
contaminants, the process comprising the steps of: 1) providing the
feed gas stream; 2) cooling the feed gas stream to a temperature at
which at least part of the feed gas stream is present in the liquid
phase; 3) separating the cooled feed gas stream by means of
cryogenic distillation into a bottom stream rich in contaminants
and lean in methane, and into a top stream rich in methane and lean
in gaseous contaminants, in which the bottom stream contains
between 0.5 and 15% of the methane present in the feed gas stream;
4) cooling the stream rich in methane to a temperature at which
solid and/or liquid phase contaminants are formed; 5) introducing
the cooled stream of step 4) into a gas/liquid/solids separation
vessel, and 6) removing from the gas/liquid/solids separation
vessel the purified gas and a stream rich in contaminants.
Inventors: |
Beaumont; Andrew Malcolm;
(Rijswijk, NL) ; Geers; Henricus Abraham;
(Rijswijk, NL) |
Family ID: |
40481839 |
Appl. No.: |
13/002966 |
Filed: |
July 3, 2009 |
PCT Filed: |
July 3, 2009 |
PCT NO: |
PCT/EP2009/058400 |
371 Date: |
February 21, 2011 |
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
F25J 2200/02 20130101;
F25J 2235/80 20130101; F25J 3/0233 20130101; C10L 3/106 20130101;
Y02C 10/12 20130101; F25J 2270/60 20130101; F25J 2205/02 20130101;
F25J 2270/904 20130101; F25J 2210/62 20130101; F25J 2270/66
20130101; F25J 2200/72 20130101; Y02C 20/40 20200801; C10L 3/10
20130101; F25J 3/0266 20130101; F25J 2240/02 20130101; F25J 2260/20
20130101; F25J 2270/12 20130101; F25J 2205/10 20130101; F25J 3/0209
20130101; F25J 2205/20 20130101 |
Class at
Publication: |
62/620 |
International
Class: |
F25J 3/02 20060101
F25J003/02; F25J 3/08 20060101 F25J003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 18, 2008 |
EP |
08160688.1 |
Claims
1. A process for producing purified gas from a feed gas stream
comprising methane and gaseous contaminants, the process comprising
the steps of: 1) providing the feed gas stream; 2) cooling the feed
gas stream to a temperature at which at least part of the feed gas
stream is present in the liquid phase; 3) separating the cooled
feed gas stream by means of cryogenic distillation into a bottom
stream rich in contaminants and lean in methane, and into a top
stream rich in methane and lean in gaseous contaminants, in which
the bottom stream contains between 0.5 and 15% of the methane
present in the feed gas stream; 4) cooling the stream rich in
methane to a temperature at which solid and/or liquid phase
contaminants are formed; 5) introducing the cooled stream of step
4) into a gas/liquid/solids separation vessel, and 6) removing from
the gas/liquid/solids separation vessel the purified gas and a
stream rich in contaminants.
2. A process according to claim 1, in which the feed gas contains
between 15 and 90 vol % of carbon dioxide.
3. A process according to claim 1, in which the feed gas contains
between 5 and 55 vol % of hydrogen sulphide, and between 5 and 80
vol % of methane.
4. A process according to claim 1, in which the feed gas stream has
a temperature between -10 and 70.degree. C., and a pressure between
20 and 150 bara.
5. A process according to claim 1, in which the cooling of the feed
gas stream is done by heat exchange against a cold fluidum,
comprising an external refrigerant.
6. A process according to claim 5, in which additional cooling of
the feed gas stream is done by nearly isentropic expansion or in
which additional cooling of the feed gas stream is done by
isenthalpic expansion.
7. A process according to claim 1, in which the feed gas stream is
cooled to a temperature between -10 and -50.degree. C.,
8. A process according to claim 1, in which the bottom temperature
of the cryogenic distillation section is between -5 and 30.degree.
C., or in which the top temperature of the cryogenic distillation
section is between -60 and -30.degree. C.
9. A process according to claim 1, in which the top stream of the
cryogenic distillation step contains between 10 and 25 wt % of the
gaseous contaminants present in the feed gas stream or, in which
the bottom stream of the cryogenic distillation step contains
between 0.5 and 15 wt % of the methane present in the feed gas
stream.
10. A process according to claim 1, in which the cooling of the
stream rich in methane in step (4) is done by isenthalpic
expansion, or in which the cooling of the stream rich in methane in
step (4) is done by nearly isentropic expansion.
11. A process according to claim 1, in which the cooling of the
stream rich in methane in step (4) is done by heat exchange against
a cold fluidum, comprising an external refrigerant.
12. A process according to claim 1, the process further comprises
the steps of: (7) introducing the stream rich in contaminants into
the intermediate or bottom part or both of the gas/liquid/solids
separation vessel to obtain a diluted slurry of acidic
contaminants; (8) introducing the diluted slurry of acidic
contaminants via an eductor into a heat exchanger in which solid
acidic contaminant present in the diluted slurry of contaminants is
melted into liquid acidic contaminant, wherein the heat exchanger
is positioned outside the separation vessel, and eductor is
arranged inside or outside the separation device or partly inside
and outside the separation vessel; (9) introducing at least a part
of the liquid contaminant obtained in step 8 into a gas-liquid
separator, wherein the gas-liquid separator is preferably the
bottom part of the gas/liquid/solids separation vessel; (10)
introducing part or all of the liquid contaminant obtained in step
9 into the gas/liquid/solid separation vessel as described above;
(11) removing from the gas-liquid separator a stream of liquid
acidic contaminant; and (12) separating the stream of liquid
contaminant obtained in step 11 into a liquid product stream and a
recirculation stream which is used as a motive fluid in the
eductor.
13. A process according to claim 1, the process further comprises
the steps of: (a) providing heat to the stream rich in contaminants
to melt at least part of the solid contaminants, yielding a heated
contaminant-rich stream; (b) withdrawing the heated
contaminant-rich stream from the vessel; (c) reheating at least a
part of the heated contaminant-rich stream to form a reheated
recycle stream; and (d) recycling at least a part of the reheated
recycle stream to the vessel
14. A process according to claim 1, wherein the feed gas stream is
a natural gas stream and the purified gas is purified natural gas,
the process further comprising the step of subjecting the purified
natural gas to a second cryogenic distillation, followed by
liquefying the natural gas to produce liquefied natural gas.
15. (canceled)
Description
[0001] The invention relates to a process for producing purified
gas from a feed gas stream comprising methane and gaseous
contaminants. The invention further relates to a device to carry
out the process, as well as to products made according to the
process.
[0002] The removal of acidic contaminants, especially carbon
dioxide and/or hydrogen sulphide, from methane containing gas
streams has been described in a number of publications.
[0003] In WO 2004/070297 a process for removing contaminants from a
natural gas stream has been described. In a first step, water is
removed from the feed gas stream. This is especially done by
cooling the feed gas stream resulting in methane hydrate formation,
followed by separation of the hydrates. Further cooling resulted in
the formation of solid acidic contaminants. After separation of the
solid acidic contaminants a cleaned natural gas stream was
obtained. It is preferred to convert the solid contaminants into a
liquid by heating the solids. When high amounts of gaseous
contaminants are present, only a part of the contaminants will be
removed, and extensive further purification of the methane rich
stream is required.
[0004] In WO 03/062725 a process is described for the removal of
freezable species from a natural gas stream by cooling a natural
gas stream to form a slurry of solid acidic contaminants in
compressed liquefied natural gas. The solids are separated from the
liquid by means of cyclone. It will be clear that a complete
separation of the liquid from the solids is not easily
achieved.
[0005] In WO 2007/030888 a process is described similar to the
process described in WO 2004/070297, followed by washing the
cleaned natural gas stream with methanol.
[0006] In U.S. Pat. No. 4,533,372 a cryogenic process is described
for the removal of carbon dioxide and/or natural gas by treating
the feed stream in a distillation zone and a controlled freezing
zone.
[0007] In U.S. Pat. No. 3,398,544 the removal of gaseous
contaminants from a natural gas stream is described by cooling to
liquefy the stream and partly solidification of the stream,
followed by expansion and separation of the cleaned gas stream and
the solids.
[0008] There is still a need for a further improved process to
produce a purified gas from a methane comprising feed gas stream.
It is especially desired to design a relatively simple process,
resulting in a methane rich stream which is almost pure methane and
a contaminants stream that only contains a minimum amount of
methane. Such a separation is difficult to achieve in a one-step
process.
[0009] It has now been found that a purified gas can be produced
from a feed gas stream comprising methane and gaseous contaminants
in a two stage process. The first stage is a cryogenic distillation
process in which the bulk of the gaseous contaminants is removed
from the feed gas stream, followed by further cooling, for example
by means of expansion, of the cleaned feed gas stream, leading to a
solid and/or liquid contaminants stream, and a relatively pure
methane stream. Optionally, a further cryogenic distillation stage
can be included which yields a liquid methane overhead product with
virtually no contaminants present and makes this process an
integral part of an LNG production process.
[0010] Thus, the present invention provides a process for producing
purified gas from a feed gas stream comprising methane and gaseous
contaminants, the process comprising the steps of: [0011] 1)
providing the feed gas stream; [0012] 2) cooling the feed gas
stream to a temperature at which at least part of the feed gas
stream is present in the liquid phase; [0013] 3) separating the
cooled feed gas stream by means of cryogenic distillation into a
bottom stream rich in contaminants and lean in methane, and into a
top stream rich in methane and lean in gaseous contaminants, in
which the bottom stream contains between 0.5 and 15% of the methane
present in the feed gas stream; [0014] 4) cooling the stream rich
in methane to a temperature at which solid and/or liquid phase
contaminants are formed; [0015] 5) introducing the cooled stream of
step 4) into a gas/liquid/solids separation vessel, and [0016] 6)
removing from the gas/liquid/solids separation vessel the purified
gas and a stream rich in contaminants.
[0017] The process according to the present invention provides an
elegant way to produce purified gas. It actually only comprises the
cooling of a feed gas stream, followed by cryogenic distillation,
and a further purification step of the obtained methane enriched
stream, by separating contaminants from the methane enriched stream
as a solids and/or liquid stream. The two stage purification
process results in a relatively very pure methane stream, while
also the contaminants stream is relatively pure and only comprises
a small amount of methane. Optionally a further cryogenic
distillation step may be added to remove the remaining
contaminants. Thus a methane stream is obtained that allows
immediate production of LNG.
[0018] The feed gas stream containing methane and gaseous
contaminants may be any methane containing gas, for instance from
natural sources as natural gas, associated gas, coal bed methane or
from industrial sources as refinery streams or synthetic sources as
Fischer-Tropsch streams or from biological sources as anaerobic
waste or manure fermentation. Gas streams, such as natural gas
streams, may become available at a temperature of from -5 to
150.degree. C. and a pressure of from 10 to 700 bar, suitably from
20 to 200 bar. The feed gas stream comprises besides methane
suitably carbon dioxide and optionally hydrogen sulphide as acidic
contaminants. The amount of the hydrocarbon fraction in the feed
gas stream is suitably from 10 to 85 mol %, preferably from 25 to
80 mol %, based on the total gas stream. The hydrocarbon fraction
in the natural gas stream may suitably contain from 0 to 20 mol %,
suitably from 0.1 to 10 mol %, of C2-C6 compounds. The gas stream
may also comprise up to 20 mol %, suitably from 0.1 to 10 mol % of
nitrogen, based on the total gas stream.
The amount of methane present may vary over a wide range, e.g. from
3 to 90 vol %, preferably 5 and 80 vol % methane, more preferably
between 10 and 90 vol %
[0019] The acidic contaminants in the feedstream are especially
carbon dioxide and hydrogen sulphide, although also carbonyl
sulphide (COS), carbon disulphide (CS2), mercaptans, sulphides and
aromatic sulphur compounds may be present. Beside acidic
contaminants, also inerts may be present, for instance nitrogen and
noble gases as argon and helium. The amount of acidic contaminants
present in the feed gas may vary over a wide range. The amount of
hydrogen sulphide in the gas stream containing methane, if present,
is suitably in the range of from 5 to 40 volume % of the gas
stream, preferably from 20 to 35 volume %. The amount of carbon
dioxide is in the range of from 10 to 90 vol %, preferably from 20
to 75 vol %, based on the total gas stream. It is observed that the
present process is especially suitable for gas streams comprising
large amounts of contaminants, e.g. 10 vol % or more, suitably
between 15 and 90 vol %, and is especially suitable for gas stream
comprising carbon dioxide as contaminant.
[0020] The amount of C.sub.2+ hydrocarbons in the feed gas may vary
over a large range. Suitably the amount of C.sub.2+ hydrocarbons is
between 0.1 and 25 vol %. Preferably there is in the feed gas
between 0.2 and 22 vol % of C.sub.2-C.sub.8 hydrocarbons, more
preferably between 0.3 and 18 vol % of C.sub.2-C.sub.4
hydrocarbons, especially between 0.5 and 15 vol % of ethane. Very
suitably the feed gas is natural gas, associated gas, coal bed
methane gas or biogas comprising acidic contaminants and
C.sub.2+-hydrocarbons. The term C.sub.2+-hydrocarbons refers the
ethane and higher hydrocarbons. The hydrocarbons comprise in
principle all hydrocarbon compounds. Especially paraffins and
monocyclic aromatic compounds may be present in the feed gas
stream.
[0021] The raw feed gas stream may be pre-treated for partial or
complete removal of water and optionally some heavy hydrocarbons.
This can for instance be done by means of a pre-cooling cycle,
against an external cooling loop, a cold internal process stream,
or a cold LNG stream. Water may also be removed by means of
pre-treatment with molecular sieves, e.g. zeolites, aluminium oxide
or silica gel or other drying agents. Water may also be removed by
means washing with glycol, MEG, DEG or TEG, or glycerol. Other
processes for forming methane hydrates or for drying natural gas
are also possible. The amount of water in the gas feed stream is
suitably less than 1 vol %, preferably less than 0.1 vol %, more
preferably less than 0.01 vol %. Water may also be removed by
hydrate formation in the way as described in WO2004/070297.
Suitably, water is removed until the amount of water in the natural
gas stream comprises at most 50 ppmw, preferably at most 20 ppmw,
more preferably at most 1 ppmw of water, based on the total feed
gas stream.
[0022] The above-mentioned treatment step for water removal may
also be applied prior to step (5), i.e. water may be removed from
the stream rich in methane prior to introducing the stream rich in
methane into the gas/liquid/solids separation vessel.
[0023] The amount of acidic contaminants that is removed by the
process of the invention will depend on a number of factors. In
practice, when using optimum conditions, at least 70 vol % (based
on total acidic contaminants in the feed gas) of acidic
contaminants will be removed, preferably at least 80 vol %, more
preferably at least 90 vol %. The amount of methane that will
remain in the contaminants streams, when using optimum conditions,
will be between 1 and 15 vol % based on methane present in the feed
gas stream.
[0024] Suitably the feed gas stream has a temperature of from -5 to
150.degree. C. and a pressure of from 10 to 700 bara, suitably from
20 to 200 bara.
[0025] The cooling of the feed gas stream is suitably done by heat
exchange against a cold fluidum, especially an external
refrigerant, e.g. a propane cycle, an ethane/propane cascade or a
mixed refrigerant cycle, or an internal process loop, suitably a
carbon dioxide or hydrogen sulphide stream, a cold methane enriched
stream or a cold LNG stream. In a preferred embodiment, the bottom
stream obtained in the gas/liquid/solid separation vessel,
optionally after liquefaction, may be used as an internal cooling
stream.
[0026] In a preferred embodiment, additional cooling of the feed
gas stream is done by nearly isentropic expansion of the feed gas
stream, especially by means of an expander, preferably a turbo
expander or laval nozzle. In another preferred embodiment,
additional cooling of the feed gas stream is done by isenthalpic
expansion, preferably isenthalpic expansion over an orifice or a
valve, especially over a Joule-Thomson valve.
[0027] Suitably the feed gas stream is cooled to a temperature
between -10 and -50.degree. C., preferably between -20 and
-40.degree. C. before performing the cryogenic distillation
step.
[0028] The cryogenic distillation is suitable performed in a
cryogenic distillation column. Such columns are known in the art.
Suitably the bottom temperature of the cryogenic distillation
section is between -15 and 35.degree. C., preferably between -5 and
30.degree. C. A reboiler may be present to supply heat to the
column. Suitably the top temperature of the cryogenic distillation
section is between -70 and -40.degree. C., preferably between -60
and -30.degree. C. In the top of the cryogenic distillation column
a condenser may be present, to introduce cold into the column.
[0029] Preferably the cryogenic distillation is carried out in such
a way that the amount of gaseous contaminants in the top stream of
the cryogenic distillation step contains between 5 and 40% of the
gaseous contaminants present in the feed gas stream, preferably
between 10 and 25%. Further, the bottom stream of the cryogenic
distillation step contains between 0.5 and 10% of the methane
present in the feed gas stream, preferably between 1 and 5%. Using
an optimum design and optimum process conditions the methane loss
in the process can be less than 3 vol %, or even less than 2 vol %.
The remaining part of the bottom stream are the contaminants, and,
when present, C2+, especially C3+, species. Usually most of the C3+
species will leave the cryogenic distillation unit via the bottom
stream, the C2+ species will leave the column partly via the top,
partly via the bottom.
[0030] Further cooling of the top stream rich in methane in step
(4) is suitably done by expansion. Preferably the expansion is done
by isenthalpic expansion, especially isenthalpic expansion over an
orifice or a valve, in particular a Joule-Thomson valve.
Alternatively, the expansion is done by nearly isentropic
expansion, especially by means of an expander, preferably a turbo
expander, or a laval nozzle.
[0031] In another embodiment of the invention the cooling of the
stream rich in methane in step (4) is done by heat exchange against
a cold fluidum, especially an external refrigerant, e.g. a propane
cycle, an ethane/propane cascade or a mixed refrigerant cycle, or
an internal process loop, suitably a carbon dioxide of hydrogen
sulphide stream, a cold methane stream, or a cold LNG stream.
[0032] At such cooling hydrocarbons may condense and such liquid
condensate may be recovered before the gas stream is cooled further
to the temperature at which acidic contaminants solidify.
Preferably, the cooling stage of the natural gas stream comprises
one or more expansion steps. For this purpose conventional
equipment may be used. Conventional equipment includes
turbo-expanders, so-called Joule-Thomson valves and venturi tubes.
It is preferred to at least partly cool the gas stream over a
turbo-expander, releasing energy. One advantageous effect of using
the turbo-expander is that the energy that is released in the
turbo-expander can suitably be used for compressing at least part
of the purified gas. Since the stream of the purified gas is
smaller than the feed gas stream now that acidic contaminants have
been removed, the energy is suitably such that the purified gas may
be compressed to an elevated pressure that makes it suitable for
transport in a pipeline.
[0033] The cooling steps eventually lead to the desired temperature
at which acidic contaminants solidify. However, since the feed gas
stream also may comprise hydrocarbons other than methane it is
preferred to cool the feed gas stream, suitably by expansion, to a
temperature below the dew point of propane. In this way the
vaporous gas stream will develop liquid hydrocarbons, including
propane, which can subsequently be recovered easily from the
vapour.
Combinations of expansion and cooling are also possible.
[0034] It will be clear that also a combination of the
above-described cooling techniques may be used.
[0035] The solids collected at the bottom of the gas/liquid/solid
separation vessel may be removed by methods known in the art. In a
very suitable way to remove the solids at least part the solid
contaminants obtained in step (6) are liquefied by heat exchange
against a suitable heating stream. The heating stream may be an
external heating stream, but is preferably an internal process
stream, e.g. the dehydrated feed gas stream. In a preferred
embodiment only solids are obtained, similar to the process as
described in WO 2004/070297.
[0036] The methane rich stream leaving the gas/liquid/solids
separation vessel usually contains at least 90 vol % methane,
preferably at least 95 vol %. Using an optimum design and under
optimum process conditions the methane content can be over 97 vol
%, or even more than 98 vol %. Thus, the two stage process of the
present invention is a very efficient separation process as most of
the gaseous contaminants in the feed stream are removed.
[0037] The contaminants stream leaving the gas/solids separation
vessel usually contains less than 10 wt % methane, preferably less
than 5 wt %. Using an optimum design and optimum process conditions
the methane loss in the process can be less than 3 vol %, or even
less than 2 vol %.
[0038] In the event that the feed gas is natural gas, the purified
natural gas can be processed further in known manners, for example
by catalytic or non-catalytic combustion to produce synthesis gas,
to generate electricity, heat or power, or for the production of
liquefied natural gas (LNG), or for residential use.
[0039] Further process steps may be required in order to reach LNG
specifications. In this case, preferably the methane enriched
gaseous phase is further purified in a second cryogenic
distillation process. The cryogenic distillation column is known in
the art. Suitably the bottom temperature of the second cryogenic
distillation section is between -30 and 10.degree. C., preferably
between -10 and 5.degree. C. A reboiler may be present to supply
heat to the column. Suitably the top temperature of the second
cryogenic distillation section is between -110 and -80.degree. C.,
preferably between -100 and -90.degree. C. In the top of the second
cryogenic distillation column a condenser may be present, to
provide reflux and a liquefied (LNG) product.
[0040] As an alternative, further purification of the methane
enriched gaseous phase may be accomplished by absorption with a
suitable absorption liquid. Suitable absorbing liquids may comprise
chemical solvents or physical solvents or mixtures thereof.
[0041] A preferred absorbing liquid comprises a chemical solvent
and/or a physical solvent, suitably as an aqueous solution.
[0042] Suitable chemical solvents are primary, secondary and/or
tertiary amines, including sterically hindered amines.
[0043] A preferred chemical solvent comprises a secondary or
tertiary amine, preferably an amine compound derived from
ethanolamine, more especially DIPA, DEA, MMEA
(monomethyl-ethanolamine), MDEA (methyldiethanolamine) TEA
(triethanolamine), or DEMEA (diethyl-monoethanolamine), preferably
DIPA or MDEA. It is believed that these chemical solvents react
with acidic compounds such as CO2 and H2S.
[0044] Suitable physical solvents include tetramethylene sulphone
(sulpholane) and derivatives, amides of aliphatic carboxylic acids,
N-alkyl pyrrolidone, in particular N-methyl pyrrolidine, N-alkyl
piperidones, in particular N-methyl piperidone, methanol, ethanol,
ethylene glycol, polyethylene glycols, mono- or di(C1-C4)alkyl
ethers of ethylene glycol or polyethylene glycols, suitably having
a molecular weight from 50 to 800, and mixtures thereof. The
preferred physical solvent is sulfolane. It is believed that CO2
and/or H2S are taken up in the physical solvent and thereby
removed.
[0045] Other treatments of the methane enriched gaseous phase may
include a further compression, when the purified gas is wanted at a
higher pressure. If the amounts of acidic contaminants in the
purified gas are undesirably high, the purified gas may be
subjected to one or more repetitions of the present process.
[0046] It is an advantage of the present process enables
purification of natural gas comprising substantial amounts of
acidic contaminants, resulting in purified natural gas comprising
low levels of contaminants, especially of sulphur contaminants. The
production of LNG from such natural gas, which would be very
difficult if not impossible by conventional processes, is made
possible. Thus, the invention also provides LNG obtained from
liquefying purified natural gas obtained by the process. The LNG
thus-obtained typically has very low concentrations of contaminants
other than natural gas.
[0047] Since it is easier to transport liquids than to transport
solids, it is preferred to melt at least partly the solid
contaminants in the stream rich in contaminants. Therefore, it has
been proposed to heat at least a part of the stream rich in
contaminants to cause melting of the solid contaminants, thereby
yielding a heated liquid contaminant-rich stream that is withdrawn
from the bottom of the vessel, suitably by pumping.
[0048] Thus, in one embodiment where melting of solid contaminants
is desired, the process further comprises the steps of: [0049] (7)
introducing the stream rich in contaminants into the intermediate
or bottom part or both of the gas/liquid/solids separation vessel
to obtain a diluted slurry of acidic contaminants; [0050] (8)
introducing the diluted slurry of acidic contaminants via an
eductor into a heat exchanger in which solid acidic contaminant
present in the diluted slurry of contaminants is melted into liquid
acidic contaminant, wherein the heat exchanger is positioned
outside the separation vessel, and eductor is arranged inside or
outside the separation device or partly inside and outside the
separation vessel; [0051] (9) introducing part or all of the liquid
contaminant obtained in step 8 into a gas-liquid separator, wherein
the gas-liquid separator is preferably the bottom part of the
gas/liquid/solids separation vessel; [0052] (10) introducing part
or all of the liquid contaminant obtained in step 9 into the
gas/liquid/solid separation vessel as described above; [0053] (11)
removing from the gas-liquid separator a stream of liquid acidic
contaminant and [0054] (12) separating the stream of liquid
contaminant obtained in step 11 into a liquid product stream and a
recirculation stream which is used as a motive fluid in the
eductor.
[0055] In this solid-melting embodiment, a continuously moving
slurry phase is obtained, minimizing the risk of any blockages in
the cryogenic separation vessel or in the pipelines and other
pieces of equipment. Further, when a fully liquid stream is
withdrawn from the heat exchanger, the absence of solid contaminant
reduces the risk of blockages or erosion in subsequent pipelines or
other equipment, and no damages will occur to any devices having
moving parts, such as pumps. Moreover, when a pure liquid stream is
withdrawn from the heat exchanger, a relatively cold liquid stream
is obtained, thus minimizing the heat requirements of the
separation device, and maintaining a high amount of exchangeable
cold in the product stream.
[0056] Eductors, also referred to as siphons, exhausters, ejectors
or jet pumps, are as such well-known and have extensively been
described in the prior art. Reference herein to an eductor is to a
device to pump produced solid and liquid CO2 slurry from the
separator to the heat exchanger. The eductor is suitably designed
for use in operations in which the head pumped against is low and
is less than the head of the fluid used for pumping. Suitably, the
eductor is a liquid jet pump. For a description of suitable
eductors, also referred to as ejectors or jet pumps, reference is
made to Perry's Handbook for Chemical Engineering, 8th edition,
chapter 10.2. In accordance with the present invention any type of
eductor can be used. Also a configuration may be used in which
multiple eductors are uses.
[0057] Preferably, the eductor is arranged inside the separation
device or partly inside and outside the separation device.
[0058] Suitably, a housing can be positioned around the eductor,
enabling the eductor to be removed from the separation device. Such
a housing can, for instance, be a vessel like containment, e.g. a
pipe, that can be isolated from the process through valves.
[0059] In another embodiment of the present invention the eductor
is arranged outside the separation device. Such an embodiment can
be useful in situations in which the eductor in use needs to be
repaired or replaced.
[0060] The eductor can be of such a size that it fits completely in
the separation device or it may cover the entire diameter of the
separation device, usually a vessel. However, it may also extend at
two locations through the internal wall of the separation
device.
[0061] In an alternative embodiment to melt solid contaminants, the
process comprises the steps of: [0062] (a) providing heat to the
stream rich in contaminants to melt at least part of the solid
contaminants, yielding a heated contaminant-rich stream; [0063] (b)
withdrawing the heated contaminant-rich stream from the vessel;
[0064] (c) reheating at least a part of the heated contaminant-rich
stream to form a reheated recycle stream; and [0065] (d) recycling
at least a part of the reheated recycle stream to the vessel.
[0066] The reheated recycle stream is recycled to the
gas/liquid/solids separation vessel, to provide heat to the solid
and/or liquid contaminants to melt at least part of the solid
acidic contaminants. In this way the benefits of direct heat
exchange are obtained whilst no alien species are introduced into
the mixture. Further, the melting step can be carried out also when
no condensates are available. Moreover, there is no need to provide
for a complex heat exchanger in the lower part of the
gas/liquid/solids separation vessel.
[0067] The recycle of part of the reheated recycle stream is
intended to melt at least part of the solid acidic contaminants in
the vessel so that blocking is prevented and removal of the acidic
contaminants is facilitated. Preferably, the heat that is being
provided by the recycled reheated recycle stream is such that it
causes the melting of all solid acidic contaminants. The skilled
person may achieve this by selecting the desired temperature of the
reheated recycle stream and/or the amount of reheated recycle
stream. Therefore, the part of the contaminant-rich stream that is
reheated to form the reheated recycle stream is preferably heated
to form a liquid stream, more preferably without any solid acidic
contaminant. Suitably the heating up is done to a temperature well
above the melting point of the solid acidic contaminants, such as
at least 5.degree. C. above the highest melting point. The heat of
the relatively warm liquid will melt at least part of the solid
acidic contaminants in the vessel. It is even more preferred that
the part of the heated contaminant-rich stream that is reheated to
form the reheated recycle stream is heated to such a temperature
that the stream becomes at least partly vaporous. Not only will
more energy be recycled to the vessel so that the melting of solid
acidic contaminants is conducted more smoothly, but also any light
hydrocarbon that may be entrained in the heated contaminant-rich
stream will be freed up and can be included in the purified
hydrocarbon gas that is withdrawn from the vessel. In this way the
recovery of purified hydrocarbon gas is enhanced.
[0068] In the event that the contaminant-rich stream mainly
comprises carbon dioxide and is therefore a CO2-rich stream,
preferably CO2-rich stream is further pressurised and injected into
a subterranean formation, preferably for use in enhanced oil
recovery or for storage into an aquifer reservoir or for storage
into an empty oil reservoir. It is an advantage that a liquid
CO2-rich stream is obtained, as this liquid stream requires less
compression equipment to be injected into a subterranean formation.
Preferably, at least 90%, more preferably at least 95% and most
preferably at least 98% of the solid acidic contaminants are
melted. In this way a liquid stream of contaminants is obtained,
which can be easily transported further.
[0069] The invention further relates to a plant for carrying out
the process as described above, as well as to purified natural gas
obtained by a process as described above. More especially, the
invention also concerns a process for liquefying natural gas
comprising purifying the natural gas as described above, followed
by liquefying the natural gas by methods known in the art.
[0070] The process of the present invention is usually carried out
in a continuous mode.
[0071] The invention is further explained by means of FIGS. 1 and
2. It will be understood that the Figures exemplify possible ways
to apply the process and that the Figures are not limiting. In FIG.
1, a dry feed gas stream 1 (water content 20 ppmw., 70 bara, 30 wt
% methane, 70 wt % carbon dioxide, 25.degree. C.) is cooled down in
heat exchanger 2 (this heat exchanger in reality represents a
sequence of expansion, cooling via cold integration and cooling via
external refrigeration) to a temperature of -15 to -35.degree. C.
The cooled stream is introduced in cryogenic distillation column 3.
A gaseous top stream 4 is removed from the column and cooled down
in condenser heat exchanger 5 to -40 to -55.degree. C. Part of the
stream is reintroduced as reflux into the distillation column via
line 6. A liquid bottom stream 7 is removed from the distillation
column. Part of the stream is heated in reboiler heat exchanger 8
and reintroduced in the column via line 9. The remaining part,
comprising about 96 wt % carbon dioxide is removed from the process
via line 10. The remaining stream from heat exchanger 5 is removed
via line 11 and cooled down further, first in a heat exchanger (not
shown), and subsequently over expansion valve 12. The stream is
then introduced in gas/liquid/solids separation vessel 13. The
stream comprises between 20 and 40 wt % carbon dioxide and between
60 and 80 wt % methane. A top stream 14 is removed from the
gas/solid separator 14. This stream comprises 90 to 95 wt %
methane. The solids in separator 13 are partly melted, for example
by means of heating coil 16, and a slurry is removed form the
separator via line 15. The slurry comprises about 98 wt % carbon
dioxide. The heat in coil 16 may be provided by the warm feed gas
stream 1.
[0072] In FIG. 2, a preferred embodiment of the process depicted in
FIG. 1 is shown. The numbers and description of FIG. 1 apply to
FIG. 2 also. In FIG. 2, a dry raw feed gas stream comprising
methane and carbon dioxide is led via line 16a to expander 25,
where it is expanded. The expanded feed gas stream is cooled in
heat exchanger 24 and the resulting cooled stream is led to an LNG
heat exchanger 23. Resulting further cooled stream la comprising
methane and carbon dioxide is cooled down in heat exchanger 2 and
processed as described in FIG. 1. The second heat exchanger to cool
down stream 11 is depicted here as 35. Stream 18 comprising methane
and carbon dioxide may be subjected to further purification such as
a second cryogenic distillation in a second distillation column
(not shown). A third stream 20 comprising methane and carbon
dioxide emanating from the LNG heat exchanger 23 is combined as
stream 21 with the stream enriched in carbon dioxide and led from
the process via line 22. In the heat exchanger 24 solid contaminant
present is melted into liquid phase contaminant. Part of this
liquid phase contaminant is passed via a conduit 26b as a diluted
slurry of contaminants to an intermediate position of separation
vessel 13, whereas the main part of liquid phase contaminant is
introduced into the bottom part of the separation vessel 13 by
means of a conduit 26a. The diluted slurry of contaminants is
directed towards the top opening of an eductor 34. In the eductor
34 the diluted slurry is used as a suction fluid and via the
eductor 34 it is passed into a heat exchanger 24 via a conduit 27.
Liquid phase contaminant is subsequently withdrawn from the
separation vessel 13 by means of a conduit 15 using a pump 33. Part
of the withdrawn liquid phase contaminant is recovered as a product
stream and part of said liquid phase contaminant is recycled via a
conduit 15a to the eductor 34. As an alternative, pump 33 may also
be located in conduit 15a. A funnel (not shown) is present to guide
the slurry stream into the direction of conduit 27. Another part of
said liquid phase contaminant is led via pump 19 to heat exchanger
34. In vessel 13 the solids and liquids, if any, will gather at the
bottom of the vessel whereas the vapour, i.e., the purified gas is
removed from the top of the vessel via a line 14.
* * * * *