U.S. patent application number 12/995411 was filed with the patent office on 2011-04-28 for producing purified hydrocarbon gas from a gas stream comprising hydrocarbons and acidic contaminants.
Invention is credited to Henricus Abraham Geers, William David Prince.
Application Number | 20110094264 12/995411 |
Document ID | / |
Family ID | 39951590 |
Filed Date | 2011-04-28 |
United States Patent
Application |
20110094264 |
Kind Code |
A1 |
Geers; Henricus Abraham ; et
al. |
April 28, 2011 |
PRODUCING PURIFIED HYDROCARBON GAS FROM A GAS STREAM COMPRISING
HYDROCARBONS AND ACIDIC CONTAMINANTS
Abstract
Process for producing purified hydrocarbon gas from a gas stream
comprising hydrocarbons and acidic contaminants, which process
comprises the steps: (a) cooling the gas stream to a temperature to
form a mixture comprising solid and optionally liquid acidic
contaminants and a vapour containing gaseous hydrocarbons; (b)
separating the solid and optionally liquid acidic contaminants from
the mixture in a vessel, yielding the purified hydrocarbon gas; (c)
providing heat to at least a part of the solid and optionally
liquid acidic contaminants to melt at least part of the solid
acidic contaminants, yielding a heated contaminant-rich stream; (d)
withdrawing the heated contaminant-rich stream from the vessel;
wherein the process further comprises: (e) reheating at least a
part of the heated contaminant-rich stream to form a reheated
recycle stream; and (f) recycling at least a part of the reheated
recycle stream to the vessel.
Inventors: |
Geers; Henricus Abraham;
(Rijswijk, NL) ; Prince; William David;
(Aberdeenshire, GB) |
Family ID: |
39951590 |
Appl. No.: |
12/995411 |
Filed: |
May 28, 2009 |
PCT Filed: |
May 28, 2009 |
PCT NO: |
PCT/EP09/56539 |
371 Date: |
November 30, 2010 |
Current U.S.
Class: |
62/618 |
Current CPC
Class: |
B01D 2257/304 20130101;
C10L 3/10 20130101; F25J 3/0209 20130101; Y02C 10/12 20130101; F25J
2220/64 20130101; F25J 2220/68 20130101; F25J 2200/02 20130101;
F25J 3/061 20130101; F25J 2220/66 20130101; F25J 3/0635 20130101;
F25J 2270/90 20130101; Y02C 20/40 20200801; F25J 3/067 20130101;
B01D 53/002 20130101; B01D 2257/504 20130101; B01D 2256/24
20130101; F25J 2200/70 20130101; F25J 3/0233 20130101; F25J 2205/20
20130101; F25J 3/0266 20130101; F25J 2240/02 20130101; C10L 3/102
20130101 |
Class at
Publication: |
62/618 |
International
Class: |
F25J 3/08 20060101
F25J003/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 30, 2008 |
EP |
08157282.8 |
Claims
1. Process for producing purified hydrocarbon gas from a gas stream
comprising hydrocarbons and acidic contaminants, which process
comprises the steps: (a) cooling the gas stream to a temperature to
form a mixture comprising solid acidic contaminants and a vapour
containing gaseous hydrocarbons; (b) separating the solid acidic
contaminants from the mixture in a vessel, yielding the purified
hydrocarbon gas; (c) providing heat to at least a part of the solid
acidic contaminants to melt at least part of the solid acidic
contaminants, yielding a heated contaminant-rich stream; (d)
withdrawing the heated contaminant-rich stream from the vessel; (e)
reheating at least a part of the heated contaminant-rich stream to
form a reheated recycle stream; and (f) recycling at least a part
of the reheated recycle stream to the vessel.
2. Process as claimed in claim 1 in which the part of the heated
contaminant-rich stream that is reheated to form the reheated
recycle stream is heated to form a liquid stream.
3. Process as claimed in claim 1, in which 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.
4. Process as claimed in claim 1, in which a part of the heated
contaminant-rich stream is separated and this part of the heated
contaminant-rich stream is reheated to form the reheated recycle
stream.
5. Process as claimed in claim 4, in which the part of the heated
contaminant-rich stream that is separated is selected such that the
reflux ratio ranges from 0.5 to 10.
6. Process as claimed in claim 1, in which substantially the entire
heated contaminant-rich stream that is withdrawn from the vessel is
reheated to form the reheated recycle stream, and a part of the
thus obtained reheated recycle stream is recycled to the
mixture.
7. Process as claimed in claim 6, in which the part of the reheated
recycle stream that is recycled to the mixture is selected such
that the reflux ratio ranges from 0.5 to 10.
8. Process as claimed in claim 1, in which the reheating of at
least part of the heated contaminant-rich stream is conducted via
heat exchange, in which the heat exchange is preferably conducted
with at least part of the gas stream.
9. Process as claimed in claim 1, in which the reheating of at
least part of the heated contaminant-rich stream is conducted by
adding thereto a warm fluid, preferably natural gas condensate.
10. Process as claimed in claim 1, in which the cooling of the gas
stream utilises one or more heat exchange steps.
11. Process as claimed in claim 1, in which the cooling of the gas
stream utilises one or more expansion steps.
12. Process as claimed in claim 1, in which the gas stream has been
dehydrated, preferably to a water content of less than 50 ppmw,
based on the total gas stream.
13. Process as claimed in claim 12, in which gas stream is expanded
from a pressure ranging from 40 bar to 200 bar, to a pressure of 10
bar to 40 bar.
14. Process as claimed in claim 1, wherein the purified gas is
purified natural gas and the process further comprises the steps of
cooling the purified natural gas to obtained liquefied natural
gas.
15. (canceled)
Description
[0001] The present invention relates to a process for the removal
of acidic contaminants from a gas stream comprising hydrocarbons
and acidic contaminants. The invention especially relates to a
process in which carbon dioxide and hydrogen sulphide are removed
from natural gas that contains hydrocarbons and acidic
contaminants.
[0002] Such a process is known from WO-A 2004/070297. This document
discloses a process in which a natural gas stream comprising
hydrocarbons and acidic contaminants is first cooled in a first
vessel to remove water from the natural gas, and subsequently the
natural gas is further cooled in a second vessel to solidify acidic
contaminants or dissolve such contaminants in a liquid, which
contaminants are removed so that a purified natural gas is
recovered. In the specification it is acknowledged that solid
acidic contaminants may block the outlet of the second vessel. To
prevent solid acidic contaminants from blocking such outlet a warm
liquid comprising natural gas condensates may be introduced into
the lower part of the vessel so that at least part of the solid
acidic contaminants melts.
[0003] In WO-A 2007/030888 a similar process for the removal of
acidic contaminants from natural gas is described. In this process
the formed solid acidic contaminants are heated to a temperature
above the melting point temperature of the contaminants by means of
a heat exchanger in the form of a bundle coil. The fluid that is
passed through the bundle coil can be the natural gas or any other
process stream. Alternatively, a liquid process stream derived from
another part of the process can be mixed with the solid acidic
contaminants to melt these contaminants. The addition of a
relatively warm stream to the solid acidic contaminants has the
advantage that it provides a more efficient and direct heat
transfer than the indirect heat exchange via a bundle coil.
However, by adding either a condensate stream or another process
stream to the solid acidic contaminants, it may become necessary to
separate these from the contaminants since otherwise a significant
loss of valuable hydrocarbons could be incurred. Such separation
needlessly complicates the process. The present invention has as
objective to avoid such complications.
[0004] Accordingly, the invention provides a process for producing
purified hydrocarbon gas from a gas stream comprising hydrocarbons
and acidic contaminants, which process comprises the steps: [0005]
(a) cooling the gas stream to a temperature to form a mixture
comprising solid and optionally liquid acidic contaminants and a
vapour containing gaseous hydrocarbons; [0006] (b) separating the
solid and optionally liquid acidic contaminants from the mixture in
a vessel, yielding the purified hydrocarbon gas; [0007] (c)
providing heat to at least a part of the solid and optionally
liquid acidic contaminants to melt at least part of the solid
acidic contaminants, yielding a heated contaminant-rich stream;
[0008] (d) withdrawing the heated contaminant-rich stream from the
vessel; wherein the process further comprises: [0009] (e) reheating
at least a part of the heated contaminant-rich stream to form a
reheated recycle stream; and [0010] (f) recycling at least a part
of the reheated recycle stream to the vessel.
[0011] In the present process the reheated recycle stream is
recycled to the vessel, to provide heat to the solid and optionally
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 process can be carried out also when no condensates
are available. Moreover, the present process avoids the need to
provide for a complex heat exchanger in the lower part of he
vessel.
[0012] The gas stream can be any stream of gas that comprises
acidic contaminants and hydrocarbons. In particular the process
according to the present invention can be applied to a natural gas
stream, i.e., a gas stream that contains significant amounts of
methane and that has been produced from a subsurface reservoir. It
includes a methane natural gas stream, an associated gas stream or
a coal bed methane stream. The amount of the hydrocarbon fraction
in such a gas stream is suitably from 10 to 85 mol % of the gas
stream, preferably from 25 to 80 mol %. Especially, the hydrocarbon
fraction of the natural gas stream comprises at least 75 mol % of
methane, preferably 90 mol %. The hydrocarbon fraction in the
natural gas stream may suitably contain from 0 to 20 mol %,
suitably from 0.1 to 10 mol %, of C.sub.2.sup.-C.sub.6 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.
[0013] 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. In the
process of the present invention the gas stream comprises suitably
hydrogen sulphide and/or carbon dioxide as acidic contaminants. It
is observed that also minor amounts of other contaminants may be
present, e.g. carbon oxysulphide, mercaptans, alkyl sulphides and
aromatic sulphur-containing compounds. The major part of these
components will also be removed in the process of the present
invention.
[0014] The amount of hydrogen sulphide in the gas stream containing
methane is suitably in the range of from 5 to 40 volume % of the
gas stream, preferably from 20 to 35 volume % and/or 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 %.
[0015] Gas stream containing the large amounts of contaminants as
described above cannot be processed using conventional techniques
as amine extraction techniques as they will become extremely
expensive, especially due to the large amounts of heat needed for
the regeneration of loaded amine solvent.
[0016] The gas stream, and in particular natural gas streams
produced from a subsurface formation, may typically contain water.
In order to prevent the formation of gas hydrates in the present
process, at least part of the water is suitably removed. Therefore,
the natural gas stream that is used in the present process has
preferably been dehydrated. This can be done by conventional
processes. A suitable process is the one described in WO-A
2004/070297. Other processes for forming methane hydrates or for
drying natural gas are also possible. Other dehydration processes
include treatment with molecular sieves or drying processes with
glycol or methanol. 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 natural gas stream.
[0017] As indicated above, acidic contaminants that are usually
present in natural gas streams include hydrogen sulphide and carbon
dioxide. It is also possible that the natural gas stream contains
other components, including ethane, propane and hydrocarbons with
four or more carbon atoms, even after an optional earlier recovery
of condensates. It will be appreciated that when a portion of
acidic contaminants, e.g., carbon dioxide, solidifies in the
cooling stage, other components, e.g., hydrogen sulphide and
hydrocarbons other than methane, may liquefy. The liquid components
are suitably removed together with the solid acidic contaminants
from the vapour.
[0018] In a first step of the present process the gas stream is
cooled. The cooling may be effected by any known method, such as
indirect heat exchange and expansion. Alternatively, a direct heat
exchange, e.g., by spraying with a cold liquid, as shown in WO-A
2004/070297, is also possible. The skilled person will appreciate
that expansion causes a lowering of temperature, so that cooling
may be achieved by expansion and adapting pressure. Preferably the
expansion is done by isenthalpic expansion, preferably isenthalpic
expansion over an orifice or a valve, especially a Joule-Thomson
valve or a series of Joule-Thomson valves. In another preferred
embodiment the expansion is done by nearly isentropic expansion,
especially by means of an expander, preferably a turbo expander, or
a laval nozzle. The cooling may be conducted in several steps. It
is preferred that the gas stream is subjected to heat exchange with
one or more other cold process streams or external streams. Cold
external streams may be suitably streams from an LNG (liquefied
natural gas) plant, such a cold LNG stream or a refrigerant stream,
or from an air separation unit. One suitable stream comprises the
purified hydrocarbon gas. 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 hydrocarbon gas. Since the stream of the purified
hydrocarbon gas is smaller than the gas stream now that acidic
contaminants have been removed, the energy is suitably such that
the purified hydrocarbon gas may be compressed to an elevated
pressure that makes it suitable for transport in a pipeline.
[0019] The cooling steps eventually lead to the desired temperature
at which acidic contaminants solidify. However, since the natural
gas stream also may comprise hydrocarbons other than methane it is
preferred to cool the natural gas stream, suitably by expansion, to
a temperature below the dew point of propane. In this way the
vaporous natural gas stream will develop liquid hydrocarbons,
including propane, which can subsequently be recovered easily from
the vapour.
[0020] It is preferred to achieve the cooling in several steps,
e.g., by indirect heat exchange and/or expansion. It is also
possible to solidify by spraying with a cold liquid, as shown in
WO-A 2004/070297. Suitably, solid acidic contaminants are obtained
in a final expansion step. The final expansion step is preferably
achieved over a Joule-Thomson valve. Therefore, preferably, in a
first step, which may be achieved by various intermediate steps and
various methods, the gas stream is cooled to a temperature ranging
from 1 to 40.degree. C. above the freeze out temperature of the
first acidic contaminant to freeze out, the freeze out temperature
being the temperature at which solid contaminants are formed.
Preferably, the cooling is effected till from 2 to 10.degree. C.
above the freeze out temperature. In a final step the gas stream is
preferably cooled to the temperature at which a mixture of solid
and/or liquid acidic contaminants and a vapour comprising gaseous
hydrocarbons are formed by expansion over a valve. Preferably, the
gas stream is partly or completely liquid before being expanded
over the valve, and solid contaminants are formed upon expansion.
This ensures better separation performance in the vessel. Suitably,
the gas stream is expanded from a pressure ranging from 40 to 200
bar to a pressure of 10 to 40 bar. Expansion over this pressure
range suitably causes that solid acidic contaminants are formed. It
will be appreciated by the person skilled in the art that at the
formation of solid acidic contaminants also liquid acidic
contaminants may be formed and/or hydrocarbons may condense. These
liquid components are suitably separated together with the solid
acidic contaminants.
[0021] The solidification of acidic contaminants may take place
very rapidly, especially upon expansion over a valve, thereby
forming the mixture comprising solid and optionally liquid acidic
contaminants and a vapour comprising gaseous hydrocarbons. To
facilitate the separation the mixture is passed into a vessel
wherein the separation between solid acidic contaminants and vapour
takes place. By gravity the solid acidic contaminants, and any
liquid that is formed, drop to the bottom of the vessel. After such
separation the solid acidic contaminants can be removed from the
process.
[0022] After separation of solid and optionally liquid acidic
contaminants, the purified hydrocarbon gas that is being recovered
after the separation step can be used as product. The recovered
purified hydrocarbon gas may also be subjected to further treatment
and/or purification. For instance, the purified hydrocarbon gas may
be subjected to fractionation. In the event that the purified
hydrocarbon gas is natural gas intended for pipeline transportation
or for producing liquefied natural gas (LNG),in order to reach
pipeline specifications or LNG specifications the purified natural
gas may further purified. Further purification can for example be
done in an additional cryogenic distillation column, suitably with
a bottom temperature 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 column is between
-110 and -80.degree. C., preferably between -100 and -90.degree. C.
In the top of the cryogenic distillation column a condenser may be
present, to provide reflux and a liquefied (LNG) product.
[0023] As an alternative, further purification may be accomplished
by absorption with a suitable absorption liquid. Suitable absorbing
liquids may comprise chemical solvents or physical solvents or
mixtures thereof.
[0024] A preferred absorbing liquid comprises a chemical solvent
and/or a physical solvent, suitably as an aqueous solution.
[0025] Suitable chemical solvents are primary, secondary and/or
tertiary amines, including sterically hindered amines.
[0026] 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 CO.sub.2 and H.sub.2S.
[0027] 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(C.sub.1-C.sub.4)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 CO.sub.2 and/or H.sub.2S are taken up in the
physical solvent and thereby removed.
[0028] Other treatments may include a further compression, when the
purified hydrocarbon gas is wanted at a higher pressure. If the
amounts of acidic contaminants in the purified hydrocarbon gas are
undesirably high, the purified hydrocarbon gas may be subjected to
one or more repetitions of the present process.
[0029] In the event that the hydrocarbon 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.
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.
[0030] Since it is easier to transport liquids than to transport
solids, it is preferred to melt at least partly the solid acidic
contaminants. Therefore, it has been proposed to heat at least a
part of the solid acidic contaminants to cause melting, thereby
yielding the heated contaminant-rich stream that is withdrawn from
the vessel bottom, suitably by pumping.
[0031] According to the present process at least a part of the
heated contaminant-rich stream is reheated to yield a reheated
recycle stream. The recycle of part of the reheated recycle stream
is meant 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.
[0032] In order to improve the heat transfer between the warm
fluid, i.e. either liquid or vaporous, and the cold solid and
optionally liquid contaminants, the vessel is preferably provided
with internals. These internals will increase the contacting
surface between cold solids and warm fluid as well as provide
residence time to the components in the vessel so that acidic
contaminants may condense and/or solidify and liquid hydrocarbons
may be evaporated. The skilled person may select the internals from
a variety of possibilities. Very suitable are sieve plates,
perforated plates or bubble trays. Their construction is relatively
easy in the cryogenic environment of the vessel, whereas the
contacting performance is very good.
[0033] An even more preferred embodiment comprises a vessel that
has been provided with at least one deflecting means that has been
arranged in the interior of the vessel. Downwards-falling solid and
liquid acidic contaminants are distributed more homogeneously over
the cross-section of the vessel, thereby improving the separation
between solid and acidic contaminants on the one hand and the
gaseous hydrocarbons on the other. The shape of the deflecting
means can be selected from a variety of shapes; the deflecting
means may, e.g., be square, circular or of a ring-shape. Preferably
the deflecting means has downwards-directed slopes to avoid build
up of solid material on the deflecting means. A very suitable shape
is a cone or a combination of a cone and an inverted cone. Whereas
the cone ensures a smooth distribution of solid and liquid
material, the inverted cone provides for a suitable passage for
upwards-flowing gases. The deflecting means suitably covers from 5
to 75% of the cross-section of the vessel.
[0034] It is evident that the reheating of part of the heated
contaminant-rich stream requires energy. In a preferred embodiment
a part of the heated contaminant-rich stream is separated and this
part of the heated contaminant-rich stream is reheated to form the
reheated recycle stream. In this way only a portion of the heated
contaminant-rich stream requires to be heated up. The size of the
part of the heated contaminant-rich stream that is separated can be
selected by the skilled person depending on conditions such as the
temperature of the reheated recycle stream and the amount and
nature of the solid acidic contaminants. Suitably, the part of the
heated contaminant-rich stream that is separated is selected such
that the reflux ratio ranges from 0.5 to 10. This embodiment is
especially advantageous when the part of the heated
contaminant-rich stream is heated up to form a vaporous recycle
stream.
[0035] In another embodiment of the present invention substantially
the entire heated contaminant-rich stream that is withdrawn from
the vessel is reheated to form the reheated recycle stream, and a
part of the thus obtained reheated recycle stream is recycled to
the mixture. This embodiment is especially useful when the heated
contaminant-rich stream is heated up to a liquid. A part of the
reheated recycle stream is recycled, whereas the other part is
withdrawn, optionally after recovery of entrained hydrocarbons. The
size of the part that is being recycled can be determined by the
skilled person, based on the conditions, also indicated above.
Suitably, the part of the reheated recycle stream that is recycled
to the mixture is selected such that the reflux ratio ranges from
0.5 to 10.
[0036] The way in which the contaminant-rich stream is reheated can
be done in any feasible way. External, e.g., electrical, heaters
are possible. However, preferably, the reheating of at least part
of the contaminant-rich stream is conducted via heat exchange. Any
process stream with a sufficiently higher temperature can be used
for this. This includes any condensate stream or hydrate-stream.
Preferably, the heat exchange is conducted with at least part of
the gas stream. Alternatively, a warm fluid may be added to the
contaminant-rich stream. Suitable warm fluids include a natural gas
condensate.
[0037] In the event that the contaminant-rich stream mainly
comprises carbon dioxide and is therefore a CO.sub.2-rich stream,
preferably CO.sub.2-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
CO.sub.2-rich stream is obtained, as this liquid stream requires
less compression equipment to be injected into a subterranean
formation.
[0038] The process will be explained in more detail by means of the
following figures.
[0039] FIG. 1 shows a schematic embodiment of a vessel wherein a
recycle stream is being applied.
[0040] FIG. 2 shows another embodiment of such a vessel.
[0041] FIG. 3 shows a schematic flow scheme of a natural gas
purification unit using the process of the present invention.
[0042] FIG. 1 shows a vessel wherein a dehydrated natural gas
stream is cooled by expansion in a Joule-Thomson valve 2.
Alternatively, instead of a Joule-Thomson valve a venturi tube or a
turbo-expander may be used. The thus cooled mixture of solids and
vapour is passed through a line 3 to a vessel 4. Line 3 that
connects the valve 2 with the vessel 4 is short so that the solids
will not block the entry of the mixture to the vessel 4. It is also
possible to do away with the line 3 altogether and connect the
Joule Thomson valve directly to the wall of vessel 4. The cooled
mixture is separated in vessel 4 to a purified hydrocarbon gas that
exits the vessel 4 via an outlet 5. Solid acidic contaminants, and
any liquid components, fall down via a deflecting means 15 to the
lower part of the vessel 4 forming a slurry containing solid acidic
contaminants 6. Deflecting means 15 has the shape of a cone. Via a
line 9 a warm recycle stream is recycled into the vessel 4.
Optionally an additional electrical immersion heater or bundle coil
14 through which a warm fluid is passed, may be provided to provide
additional energy. Due to the recycle of warm fluids solid acidic
contaminants melt and the remaining liquid, i.e., the heated
contaminant-rich stream, is withdrawn from the vessel 4 via a line
7, optionally by pumping. The entire heated contaminant-rich stream
in line 7 is subjected to heat exchange in heat exchanger 10,
through which a warm fluid, e.g. the natural gas stream, is being
passed to reheat the contaminant-rich stream. A part of the thus
reheated stream is withdrawn from the process via a line 11.
Another part is recycled to the vessel via line 9. The part that is
being recycled can be fed into the vessel 4 in any known way.
Hence, it is possible to use a single nozzle, or a plurality of
nozzles, one or more spargers, or nozzles arranged in a supply line
that extends into the vessel, which supply line may be ring-shaped.
The lines 9 and 11 have been provided with valves 12 and 13 to
control the flow of recycle stream to the vessel 4 and the liquid
level inside vessel 4. The line-up of FIG. 1 is especially suitable
for the situations in which the reheated recycle stream is
maintained in the liquid phase.
[0043] In FIG. 2 a situation is shown that is especially suitable
for situations wherein recycle streams are reheated to become
vaporous. Similar to the embodiment of FIG. 1, a natural gas stream
is passed via a line 21 and a Joule-Thomson valve 22 and another
short line 23 to a vessel 24. Alternatively, a venturi tube may be
used instead of a Joule-Thomson valve. Purified hydrocarbon gas is
removed via an outlet 25. Solid acidic contaminants and optionally
also some liquids fall down via a deflecting means 36 and are
collected as layer 26 in the bottom part of vessel 24. The
deflecting means 36 in this embodiment has been executed as a
combination of a cone and an inverted cone. The layer 26 comprising
solid acidic contaminants can be heated to melt at least part of
the solid acidic contaminants by the recycle of a reheated recycle
stream via a nozzle 32. An additional heater 35 may optionally be
provided. Due to the energy input of the recycle and/or heating
solid acidic contaminants melt, and the remaining heated
contaminants-rich stream is withdrawn from the vessel via line 27,
optionally via pumping. The heated contaminant-rich stream is split
into stream 29, which is discarded or is sent to another part of
the process, and stream 28, which is subjected to heat exchange in
a heat exchanger 30. The heat exchanger is similar to heat
exchanger 10 in FIG. 1. A reheated recycle stream exits heat
exchanger 30 via line 31. Line 31 debouches into the nozzle 32
through which the reheated recycle stream enters the vessel 24.
Both lines 31 and 29 have been provided with valves 33 and 34,
respectively, to control the flow of recycle stream to the vessel
24 and the liquid level inside vessel 24.
[0044] FIG. 3 shows a more extensive flow scheme of a unit wherein
the present process can be carried out.
[0045] A natural gas stream is introduced via a line 101 into a
dehydrating unit 118. In the dehydration unit 118 water is being
removed from the natural gas stream, e.g., by means of molecular
sieves. The water is eventually removed via a line 102. The
dehydrated natural gas is passed via a line 103 to a turbo-expander
119 where it is cooled, and subsequently forwarded via a line 104.
The natural gas in line 104 is cooled further via a heat exchanger
122. Subsequently, the natural gas stream is passed via a line 105
for further heat exchange. To recover as much energy as possible
the natural gas stream may be passed to a heat exchanger 124
wherein it exchanges heat with purified hydrocarbon gas and heated
contaminant-rich stream. If desired further heat exchange may
optionally be established in heat exchanger 125. Via a line 106 and
la line 107 the further cooled natural gas stream is passed to a
Joule-Thomson valve 126 where it is cooled to a temperature at
which acidic contaminants solidify so that a mixture of solid
acidic contaminants and vaporous hydrocarbons enter vessel 120 via
a line 108. Alternatively, a venturi tube may be used instead of a
Joule-Thomson valve. There separation takes place so that purified
hydrocarbon gas is withdrawn at the top via a line 109. The figure
shows that short line 108 connects the Joule Thomson valve 130 with
the vessel 120. This line is typically short so that blocking of
the line by solids is prevented. It is also possible to do away
with the line altogether and connect the Joule Thomson valve
directly to the wall of vessel 120.
[0046] The solid and optionally liquid acidic contaminants and
optionally liquid hydrocarbons are falling down, preferably along a
deflecting means (not shown), towards the bottom of vessel 120
where they are collected and heated by means of a warm reheated
recycle stream entering the vessel 120 via a line 117, thereby
melting solid acidic contaminants. The thus obtained heated
contaminant-rich stream is withdrawn from the vessel via a line 112
which is pumped further using pump 121. The heated contaminant-rich
stream is divided into a part that is withdrawn via a line 113 and
a part that is forwarded to the heat exchanger 122 via a line 115.
In heat exchanger 122 the part of the heated contaminant-rich
stream is reheated by means of heat exchange with the natural gas
stream provided via line 104, to form a reheated recycle stream.
The reheated recycle stream is forwarded via line 116 to a valve
123 which controls the flow of the reheated recycle stream. Via the
line 117, which may be provided with a nozzle (not shown) the
reheated recycle stream is introduced into vessel 120.
[0047] The line 113 with the heated contaminant-rich stream leads
the molten contaminants to the heat exchanger 124, and
subsequently, the contaminants are withdrawn via line 114. In heat
exchanger 124 the molten contaminants in line 113 and cold purified
hydrocarbon gas in line 109 are subjected to heat exchange with the
natural gas stream in line 105. The streams are shown in co-current
fashion. It is evident to the skilled person that the streams may
also be arranged in a counter-current way, e.g., the relatively
warm natural gas stream is counter-current with the two other
streams. It will be appreciated that it is also feasible to use
only one of the other streams or use a stream from another process,
such as a stream from an LNG plant and/or an air separation
plant.
[0048] From the heat exchanger 124 the purified hydrocarbon gas is
passed via a line 110 to a compressor 127. The compression energy
for compressor 127 is suitably provided by the expander 119. The
compressed gas may be recovered as product in line 111 or used for
further treatment.
* * * * *