U.S. patent application number 12/614027 was filed with the patent office on 2010-05-06 for process for removing gaseous contaminants from a feed gas stream comprising methane and gaseous contaminants.
Invention is credited to Diki Andrian, Rick Van Der Vaart.
Application Number | 20100107687 12/614027 |
Document ID | / |
Family ID | 40497673 |
Filed Date | 2010-05-06 |
United States Patent
Application |
20100107687 |
Kind Code |
A1 |
Andrian; Diki ; et
al. |
May 6, 2010 |
PROCESS FOR REMOVING GASEOUS CONTAMINANTS FROM A FEED GAS STREAM
COMPRISING METHANE AND GASEOUS CONTAMINANTS
Abstract
A process for removing gaseous contaminants from a feed gas
stream that comprises a gaseous product and gaseous contaminants,
comprising: providing the feed gas stream, cooling the feed gas
stream to a temperature at which liquid phase contaminant is formed
as well as a gaseous phase rich in gaseous product, separating the
two phases by means of a gas/liquid separator, and introducing the
gaseous phase rich in gaseous product into a cryogenic separation
device that comprises a freezing zone and a distillation zone
positioned below the freezing zone, and removing from the cryogenic
separation device a bottom stream rich in liquid phase contaminant
and lean in gaseous product, and a top stream rich in gaseous
product and lean in gaseous contaminant. The invention further
includes a device for carrying out the present process, the
purified gas stream, and a process for liquefying a feed gas
stream.
Inventors: |
Andrian; Diki; (Rijswijk,
NL) ; Vaart; Rick Van Der; (Rijswijk, NL) |
Correspondence
Address: |
SHELL OIL COMPANY
P O BOX 2463
HOUSTON
TX
772522463
US
|
Family ID: |
40497673 |
Appl. No.: |
12/614027 |
Filed: |
November 6, 2009 |
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
B01D 46/0031 20130101;
B01D 50/002 20130101; C10L 3/10 20130101; F25J 2205/20 20130101;
F25J 2200/02 20130101; F25J 2205/10 20130101; F25J 2220/66
20130101; B01D 2256/24 20130101; F25J 2270/60 20130101; Y02C 10/12
20130101; B01D 2257/304 20130101; F25J 3/0266 20130101; F25J
2205/04 20130101; B01D 2257/308 20130101; F25J 2270/12 20130101;
B01D 2257/306 20130101; F25J 2240/02 20130101; B01D 2258/06
20130101; F25J 2215/04 20130101; B01D 53/002 20130101; F25J 3/0209
20130101; F25J 3/0233 20130101; B01D 45/02 20130101; B01D 2257/504
20130101; C10L 3/102 20130101; F25J 2280/40 20130101; Y02C 20/40
20200801; F25J 2200/74 20130101; B01D 45/14 20130101 |
Class at
Publication: |
62/620 |
International
Class: |
F25J 3/08 20060101
F25J003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 6, 2008 |
EP |
08168502.6 |
Claims
1. A process for removing gaseous contaminants from a feed gas
stream that comprises a gaseous product and gaseous contaminants,
the process comprising: 1) providing the feed gas stream; 2)
cooling the feed gas stream to a temperature at which liquid phase
contaminant is formed as well as a gaseous phase rich in gaseous
product; 3) separating the two phases as obtained in step 2) by
means of a gas/liquid separator; and 4) introducing the methane
enriched gaseous phase as obtained in step 3) into a cryogenic
separation device which comprises a freezing zone and a
distillation zone which is positioned below the freezing zone; and
5) removing from the cryogenic separation section a bottom stream
rich in liquid phase contaminant and lean in gaseous product, and a
top stream rich in gaseous product and lean in gaseous
contaminant.
2. The process according claim 1 wherein the gas/liquid separator
in step 3) comprises a gas/liquid inlet at an intermediate level, a
liquid outlet arranged below the gas/liquid inlet and a gas outlet
arranged above the gas/liquid inlet, in which vessel a normally
horizontal coalescer is present above the gas/liquid inlet and over
the whole cross-section of the vessel and in which vessel a
centrifugal liquid separator is arranged above the coalescer and
over the whole cross-section of the vessel, the liquid separator
comprising one or more swirl tubes.
3. The process according to claim 1 wherein the gas/liquid
separator in step 3) comprises a centrifugal separator which
comprises a bundle of parallel channels that are arranged within a
spinning tube parallel to an axis of rotation of the spinning
tube.
4. The process according claim 3 wherein the gas/liquid separator
in step 3) comprises: a) a housing comprising a first, second and
third separation section for separating liquid from the mixture,
wherein the second separation section is arranged below the first
separation section and above the third separation section, the
respective separation sections are in communication with each
other, and the second separation section comprises a rotating
coalescer element; b) tangentially arranged inlet means to
introduce the mixture into the first separation section; c) means
to remove liquid from the first separation section; d) means to
remove liquid from the third separation section; and e) means to
remove a gaseous stream, lean in liquid, from the third separation
section.
5. The process according to claim 1 wherein the gas/liquid
separator in step 3) comprises a housing with a gas inlet for
contaminated gas at one end of the vessel, a separating body, a gas
outlet for purified gas at the opposite end of the housing and a
contaminants outlet downstream of the separating body or upstream
and downstream of the separating body, wherein the separating body
comprises a plurality of ducts over a part of the length of the
axis of the housing, which ducts have been arranged around a
central axis of rotation, in which apparatus the separating body
has been composed of a plurality of perforated discs wherein the
perforations of the discs form the ducts.
6. The process according to claim 1 wherein the gaseous
contaminants are carbon dioxide, and/or hydrogen sulphide, wherein
the carbon dioxide, if present, is present in the range of from 1
to 90 vol %, based on the total feed gas stream, and wherein which
the hydrogen sulphide, if present, is present stream in the range
of from 0.1 to 60 vol % based on the total feed gas stream.
7. The process according to claim 1 wherein the feed gas stream is
a natural gas which comprises between 20 and 80 vol % of
methane.
8. The process according to claim 1 wherein the feed gas stream in
step 1) has a temperature between -20 and 150.degree. C. and a
pressure between 10 and 150 bara.
9. The process according to claim 1 wherein the cooling in step 2)
is performed using at least one technique selected from the group
consisting of: expansion over an orifice or a valve; an expander; a
turbo expander; and a laval nozzle, and wherein the feed gas stream
is pre-cooled to a temperature between 15 and -35.degree. C.
10. The process according to claim 1 wherein the feed gas stream is
cooled in step 2) to a temperature between -30 and -80.degree.
C.
11. The process according to claim 1 wherein the freezing zone in
step 4) is designed to control the formation and melting of solid
contaminant and to prevent the introduction of solid contaminant
into the distillation zone.
12. The process according to claim 1 wherein step 4) is carried out
as follows: a) the gaseous phase rich in gaseous product is cooled
to a temperature above the freeze out temperature of any
contaminant present in the feed gas stream to obtain a cooled
gaseous phase rich in gaseous product, b) the cooled gaseous phase
rich in gaseous product as obtained in step a) is introduced into
the distillation zone of the cryogenic separation device; c)
gaseous phase rich in gaseous product inside the top section of the
distillation is introduced into the freezing zone; d) the gaseous
phase rich in gaseous product that is introduced in the freezing
zone is contacted in the freezing zone with a stream of cold liquid
at a temperature lower than the freeze out temperature of any
contaminant present in the gaseous phase rich in product, for
forming solid contaminant and a further gaseous product enriched
gaseous phase; e) the solid contaminant obtained in step d) is
melted and a stream of melted solid contaminant is introduced into
the distillation zone; and at least part of the further gaseous
product enriched gaseous phase obtained in step d) is condensed to
form liquid phase.
13. The process according to claim 1 wherein the gaseous phase rich
in gaseous product obtained in step 3) is cooled in a cooling step
to a temperature at which at least part of the gaseous phase rich
in gaseous product is condensed, and the fluid so obtained is
introduced into the cryogenic separation device in step 4).
14. The process according to claim 1 wherein the gaseous phase rich
in gaseous product obtained in step 3) is recompressed in one or
more compression steps and the recompressed gaseous phase rich in
gaseous product so obtained is cooled by means of expansion to a
temperature above the freeze out temperature of any contaminant
present in the feed gas stream before it is introduced into the
cryogenic separation device in step 4), wherein the cooling between
steps 3) and 4) is at least partly be done by means of an external
refrigerant.
15. The process according to claim 14 wherein the external
refrigerant has a higher molecular weight than the gaseous phase
rich in gaseous product to be cooled.
16. The process according to claim 14 wherein the external
refrigerant comprises a propane cycle, an ethane/propane mixed
refrigerant or an ethane/propane cascade.
Description
RELATED CASES
[0001] This case claims priority to European application
08168502.6, filed 6 Nov. 9, 2008, which is incorporated herein by
reference.
FIELD OF THE INVENTION
[0002] The present invention relates to a process for the removal
of gaseous contaminants from a feed gas stream which comprises a
gaseous product and gaseous contaminants, in particular the removal
of gaseous contaminants such as carbon dioxide and/or hydrogen
sulphide from a natural gas or a gas stream from partial or
complete oxidation processes, like syngas or flue gas.
BACKGROUND OF THE INVENTION
[0003] Gas streams produced from subsurface reservoirs such as
natural gas, associated gas and coal bed methane or from
(partial)oxidation processes, usually contain in addition to the
gaseous product concerned such as methane, hydrogen and/or nitrogen
contaminants such as carbon dioxide, hydrogen sulphide, carbon
oxysulphide, mercaptans, sulphides and aromatic sulphur containing
compounds in varying amounts. For most of the applications of these
gas streams, the contaminants need to be removed, either partly or
almost completely, depending on the specific contaminant and/or the
use. Often, the sulphur compounds need to be removed into the ppm
level, carbon dioxide sometimes into the ppm level, e.g. LNG
applications, or down to 2 or 3 vol. percent, e.g. for use as
heating gas. Higher hydrocarbons may be present, which, depending
on the use, may be recovered.
[0004] Processes for the removal of carbon dioxide and sulphur
compounds are know in the art. These processes include absorption
processes using e.g. aqueous amine solutions or adsorption
processes using e.g. molecular sieves. These processes are
especially suitable for the removal of contaminants, especially
carbon dioxide and hydrogen sulphide, that are present in
relatively low amounts, e.g. up till several vol %.
[0005] In WO 2006/087332, a method has been described for removing
contaminating gaseous components, such as carbon dioxide and
hydrogen sulphide, from a natural gas stream. In this method a
contaminated natural gas stream is cooled in a first expander to
obtain an expanded gas stream having a temperature and pressure at
which the dewpointing conditions of the phases containing a
preponderance of contaminating components, such a carbon dioxide
and/or hydrogen sulphide are achieved. The expanded gas stream is
then supplied to a first segmented centrifugal separator to
establish the separation of a contaminants-enriched liquid phase
and a contaminants-depleted gaseous phase. The
contaminants-depleted gaseous phase is then passed via a
recompressor, an interstage cooler, and a second expander into a
second centrifugal separator. The interstage cooler and the second
expander are used to cool the contaminants-depleted gaseous phase
to such an extent that again a contaminants-enriched liquid phase
and a further contaminates-depleted gaseous phase are obtained
which are subsequently separated from each other by means of the
second centrifugal separator. In such a method energy recovered
from the first expansion step is used in the compression step, air,
water and/or and an internal natural gas loop is used in the
interstage cooler.
[0006] A disadvantage of this known method is that there is still
room for improving the removal of the gaseous contaminants from the
feed gas stream, ensuring that levels can be reached that are
specified for pipeline transport of the feed gas stream or the
production of liquefied natural gas. Moreover, the use of a
recompressor, interstage cooler and an expander between the two
centrifugal separators affects the energy efficiency of the
separation process, which energy efficiency is a measure of the
fuel gas consumption and the hydrocarbon loss in the liquid phase
contaminant streams during the process.
SUMMARY OF THE INVENTION
[0007] It has now been found that in an integrated process for
removing gaseous contaminants from gas streams that contain
relatively large amount of gaseous contaminants the removal of
gaseous contaminants can be improved, as well as the energy
efficiency of the overall processing when after a gas/liquid
separation the contaminants-depleted gaseous phase is introduced
into a cryogenic separation device wherein use is made of a
distillation zone in combination with a freezing zone.
[0008] Thus, the present invention concerns a process for removing
gaseous contaminants from a feed gas stream which comprises a
gaseous product and gaseous contaminants, the process
comprising:
1) providing the feed gas stream; 2) cooling the feed gas stream to
a temperature at which liquid phase contaminant is formed as well
as a gaseous phase rich in gaseous product; 3) separating the two
phases as obtained in step 2) by means of a gas/liquid separator;
and 4) introducing the gaseous phase rich in gaseous product as
obtained in step 3) into a cryogenic separation device which
comprises a freezing zone and a distillation zone which is
positioned below the freezing zone; and 5) removing from the
cryogenic separation device a bottom stream rich in liquid phase
contaminant and lean in gaseous product, and a top stream rich in
gaseous product and lean in gaseous contaminant.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a more complete understanding of the invention,
reference will be made to the accompany Figures, in which:
[0010] FIG. 1 is a schematic diagram illustrating one embodiment of
the invention;
[0011] FIG. 2 is a schematic diagram illustrating one component of
the embodiment of FIG. 1;
[0012] FIG. 3 a schematic diagram illustrating another component of
the embodiment of FIG. 1;
[0013] FIG. 4 a schematic diagram illustrating an alternative
embodiment of the component shown in FIG. 3; and
[0014] FIG. 5 a schematic diagram illustrating still another
component of the embodiment of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0015] Referring initially to FIG. 1, natural gas via a conduit 1
is passed through an expansion means 2, whereby a stream is
obtained comprising liquid phase contaminant and a methane enriched
gaseous phase. The stream flows via a conduit 3 into a gas/liquid
separator 4 wherein the two phases are separated from each other.
The liquid phase contaminant is recovered via a conduit 5, whereas
the methane enriched gaseous phase is passed via a conduit 6 into a
heat exchanger 7. In heat exchanger 7 ethane is used as an external
refrigerant whereby the ethane is cooled by means of an
ethane/propane cascade 8 as depicted in more detail in FIG. 2. The
cooling in heat exchanger 7 is such that liquid phase contaminant
and a methane enriched gaseous phase are formed. The stream which
comprises these two phases is then passed via a conduit 9 into a
cryogenic separation vessel 10, which comprises a first
distillation zone 11, a controlled freezing zone 12 and a second
distillation zone 13. From the cryogenic separation vessel 10 a
further enriched methane enriched gaseous phase is recovered via a
conduit 15 and liquid phase contaminant is recovered via a conduit
14.
[0016] In FIG. 2 a suitable heat exchanger 7 is shown which is
based on an ethane/propane cascade which comprises an ethane loop
and a propane loop. In the ethane loop an ethane stream is passed
via a conduit 16 into an expander 17 (e.g. a turbine expander or a
Joule-Thomson valve), and the cooled ethane stream so obtained is
passed via a conduit 18 into the heat exchanger 7. A stream of warm
ethane is then passed from the heat exchanger 7 to a recompressor
20 via a conduit 19 to increase the pressure of the ethane stream.
The compressed stream of ethane obtained from recompressor 20 is
then passed via a conduit 21 into the heat exchanger 22 wherein the
ethane stream is cooled and at least partly condensed. Via the
conduit 16 the ethane stream is then recycled to the expander 17.
In the propane loop a propane stream is passed via a conduit 23
into an expander 24 (e.g. a turbine expander or a Joule-Thomson
valve), and the cooled propane stream so obtained is passed via a
conduit 25 into the heat exchanger 22 of the ethane loop. A stream
of warm propane is then passed from the heat exchanger 22 via a
conduit 26 into a recompressor 27 to increase the pressure of the
propane stream. The compressed stream of propane obtained from
recompressor 27 is then passed via a conduit 28 into a heat
exchanger 29 wherein the propane stream is cooled and at least
partly condensed by means of water or air. Via the conduit 23 the
propane stream is then recycled to the expander 24.
[0017] In FIG. 3 a preferred gas/liquid separator is shown for
carrying out step 3) of the present process. The stream comprising
liquid phase contaminant and a methane enriched gaseous phase is
passed via the conduit 3 into the gas/liquid separator 4 via supply
and distribution assembly 30. Most of the liquid will flow down to
the lower end of the separator and leave the separator via the
liquid outlet 5. The gaseous stream comprising larger and smaller
droplets will flow upwards via liquid coalescer 31, centrifugal
separator 32 and a second liquid coalescer 33 to the top of the
separator vessel, and leave the separator vessel via gas outlet
6.
[0018] In FIG. 4 another preferred gas/liquid separator is shown
for carrying out step 3) of the present process. The stream
comprising liquid phase contaminant and a methane enriched gaseous
phase is passed via the conduit 3 to a gas inlet 34 in a housing 35
of the gas/liquid separator 4. The housing 35 further comprises a
separating body 36 which shows a large number of ducts 37 which are
arranged around a shaft 38, which provides an axis of rotation.
Separating body 36 has been composed of six discs 36a, 36b, 63c,
36d, 36e and 36f that have been combined by welding or gluing. In
the rotating separating body droplets of carbon dioxide and/or
hydrogen sulphide are separated from the natural gas. The separated
contaminants are discharged from the housing via a contaminants
outlet 38 which has been arranged downstream of the separating body
36, and via a discharge conduit 5. Purified natural gas leaves
housing 35 via the gas outlet 6 arranged at the opposite end of the
housing 35.
[0019] In FIG. 5 a preferred cryogenic separation device has been
shown for carrying out step 4) of the present invention. Such a
device has, for instance been described in U.S. Pat. No. 4,533,372.
The stream that has been cooled by the heat exchanger 7 is
introduced via the conduit 9 into the first distillation zone 11 of
the cryogenic separation vessel 10. The cryogenic separation vessel
10 is divided into three distinct sections, viz. the first
distillation zone 11, the controlled freezing zone 12, and the
second distillation zone 13. The stream that has been introduced
into the first distillation zone 11 will be subjected to a
distillation treatment. The internals of the first distillation
zone 11 may include suitable trays, downcomers, and weirs, which
are suitable for separating a methane enriched gaseous phase from
liquid phase contaminant. Liquid phase contaminant is withdrawn
from the bottom of the first distillation zone 11 via the conduit
14. The withdrawn liquid phase contaminant can be heated in a
reboiler 40, and a portion can be returned to the tower as reboiled
liquid phase contaminant via a conduit 41. The remainder of the
reboiled liquid phase contaminant is withdrawn as a product via a
conduit 42. In the first distillation zone 11a methane enriched
gaseous phase can leave this distillation zone and enter the
controlled freezing zone 12 via a chimney tray 43. In controlled
freezing zone 12, the methane enriched gaseous phase will contact a
spray of liquid phase contaminant emanating from nozzles or spray
jet assemblies 44. The methane enriched gaseous phase then
continues to flow up through the second distillation zone 13. A
methane enriched gaseous phase is withdrawn via the conduit 15,
partially condensed in a reflux condenser 45 and separated into
liquid phase contaminant and a methane enriched gaseous phase in a
reflux drum 46. Liquid phase contaminant from reflux drum 46 is
returned to the cryogenic separation device via a conduit 47,
whereas a methane enriched gaseous phase product can be recovered
via a conduit 48. Liquid phase contaminant is drawn from the bottom
tray 48 of the second distillation zone 13 via a conduit 49. Liquid
phase contaminant is accumulated in a vessel 50 and returned to the
controlled freezing zone via a conduit 51, a pump 52, a conduit 53
and nozzles or spray jet assemblies 44.
[0020] Suitably, the feed gas stream is a natural gas stream in
which the gaseous contaminants are carbon dioxide and/or hydrogen
sulphide, or it is a gas stream from a (partial) oxidation process
which comprises carbon dioxide as the gaseous contaminant. It has
been found that the process is especially suitable for removal of
hydrogen sulphide. The natural gas stream suitably comprises
between 0.1 and 60 vol % of hydrogen sulphide, preferably between
20 and 40 vol % of hydrogen sulphide. The natural gas stream
suitably comprises between 1 and 90 vol % of carbon dioxide,
preferably between 5 and 80 vol % of carbon dioxide.
[0021] The feed gas stream to be used in accordance with the
present invention comprises between 20 and 80 vol % of methane.
[0022] Suitably, the feed gas stream in step 1) has a temperature
between .+-.20 and 150.degree. C., preferably between -10 and
70.degree. C., and a pressure between 10 and 150 bara, preferably
between 80 and 120 bara.
[0023] The raw feed gas stream may be pre-treated to partially or
completely remove water and optionally some heavy hydrocarbons.
This can be for instance done by means of a pre-cooling cycle,
against an external cooling loop or a cold internal process stream.
Water may also be removed by means of a pre-treatment with
molecular sieves, e.g. zeolites, or silica gel or alumina oxide or
other drying agents such as glycol, MEG, DEG or TEG, or glycerol.
The amount of water in the feed gas stream is suitably less than 1
vol %, preferably less than 0.1 vol %, more preferably less than
0.0001 vol %.
[0024] The cooling in step 2) of the feed gas stream may be done by
methods known in the art. For instance, cooling may be done against
internal or an external cooling fluid. In the case that the
pressure of the feed gas is sufficiently high, cooling may be
obtained by expansion of the feed gas stream. Combinations may also
be possible. A suitable method to cool the feed gas stream is by
nearly isentropic expansion, especially by means of an expander,
preferably a turbo expander or laval nozzle. Another suitable
method is to cool the feed gas stream by isenthalpic expansion,
preferably isenthalpic expansion over an orifice or a valve,
especially over a Joule-Thomson valve.
[0025] In a preferred embodiment the feed gas stream is pre-cooled
before expansion. This may be done against an external cooling loop
or against a cold internal process stream, e.g. liquid acidic
contaminant. Preferably the gas stream is pre-cooled before
expansion to a temperature between 15 and -35.degree. C.,
preferably between 10 and -20.degree. C. Especially when the feed
gas stream has been compressed, the temperature of the feed gas
stream may be between 100 and 150.degree. C. In that case air or
water cooling may be used to decrease the temperature first,
optionally followed by further cooling.
[0026] Another suitable cooling method is heat exchange against a
cold fluidum, especially an external refrigerant, e.g. a propane
cycle, an ethane/propane cascade or a mixed refrigerant cycle,
optionally in combination with an internal process loop, suitably a
contaminants stream (liquid or slurry), a cold methane enriched
stream or washing fluid.
[0027] Suitably the feed gas stream is cooled in step 2) to a first
temperature between -30 and -80.degree. C., preferably between -40
and -65.degree. C. At these temperatures liquid phase contaminant
will be formed.
[0028] In the present invention both liquid phase contaminant and
gaseous contaminant will comprise hydrogen sulphide and carbon
dioxide, whereas solid contaminant will usually mainly contain
carbon dioxide.
[0029] In accordance with the present invention the gaseous phase
rich in gaseous product as obtained in step 3) is introduced in
step 4) into a cryogenic separation device which comprises a
freezing zone and a distillation zone which is positioned below the
freezing zone.
[0030] Preferably, use is made of a so-called controlled freezing
zone (CFZ). Such a freezing zone is designed to control the
formation and melting of solid contaminant and to prevent the
introduction of solid contaminant into the distillation zone.
[0031] Step 4) of the process according to the present invention
can suitably be carried out as follows:
a) the methane enriched gaseous phase is introduced in the
distillation zone of the cryogenic separation device for forming
liquid phase contaminant and a gaseous feed stream rich in gaseous
product for the freezing zone; b) the gaseous feed stream rich in
gaseous product so obtained is introduced into the freezing zone;
c) the gaseous phase rich in gaseous product is contacted in the
freezing zone with a cold stream for forming solid contaminant and
a gaseous phase rich in gaseous product; d) the solid contaminant
obtained in step c) is melted and a stream of melted solid
contaminant is introduced into the distillation zone; and e) at
least part of the gaseous phase rich in gaseous product obtained in
step c) is condensed to form liquid phase contaminant.
[0032] In a preferred embodiment of the present invention at least
part of the liquid phase contaminant formed in step e) is used as
the stream of liquid phase contaminant in step c).
[0033] In another preferred embodiment of the present invention at
least part of the bottom stream rich in liquid phase contaminant as
obtained in step 5) is returned to the distillation zone.
[0034] The cryogenic separation section suitably comprises a single
vertical vessel having the distillation zone in its lower section
and the freezing zone in an upper section.
[0035] Suitably, step 4) of the present invention is carried out as
follows:
(a) maintaining the distillation zone engineered to produce a
bottom stream rich in liquid phase contaminant and lean in gaseous
product and a gaseous feed stream for the freezing zone which
stream is rich in gaseous, and wherein the distillation zone is
operated at a temperature and pressure at which substantially no
solid contaminant is formed within the distillation zone; (b)
maintaining the freezing zone engineered to contact the gaseous
feed stream for the freezing zone which stream is rich in gaseous
product with a stream of a cold liquid at a temperature and
pressure whereby solid contaminant and a gaseous phase rich in
gaseous product are formed in the freezing zone; (c) introducing
the gaseous phase rich in gaseous product obtained in step 3) into
the distillation zone, (d) producing the liquid phase contaminant
and the gaseous feed stream for the freezing zone which stream is
rich in gaseous product; (e) introducing the gaseous feed stream
for the freezing zone which is rich in gaseous product into the
freezing zone; (f) contacting in the freezing zone the gaseous feed
stream for the freezing zone which is rich in gaseous product with
the stream of cold liquid; (g) forming in the freezing zone solid
contaminant and a gaseous phase further enriched in gaseous
product; (h) melting the solid contaminant and introducing the
liquid stream containing the melted solid contaminant into the
distillation zone; (i) condensing at least a portion of the gaseous
phase further enriched in gaseous product and forming the stream of
cold liquid with at least a portion of the condensed gaseous phase
further enriched in gaseous product; and (j) recovering at least a
portion of the remainder of the gaseous phase further enriched in
gaseous product as a gaseous product stream.
[0036] Suitably, the solid contaminant is melted in the freezing
zone by adding heat. Preferably, the heat is added through indirect
heat exchanger means placed within the freezing zone or it is added
through electrical heating means placed within the freezing zone or
via direct heat exchange from, e.g. a condensing vapor.
[0037] Suitably, the stream of cold liquid is introduced through
spray means placed within the freezing zone. Preferably, such spray
means comprise one or more separate spray nozzle assemblies through
which the stream of cold liquid can be pumped. The cold liquid may
have been subcooled by heat exchange prior to introducing into the
freezing zone.
[0038] The cryogenic separation section suitably comprises a single
vertical vessel having the first distillation zone in its lower
portion and the freezing zone in an upper portion.
[0039] Preferably, the condensation of at least a portion of the
methane enriched gaseous phase is carried out in a second
distillation zone which is positioned above the freezing zone.
[0040] It should be noted, however, that such a second distillation
zone is not required for carrying out the present invention.
[0041] Hence, in a suitable embodiment of the present invention the
cryogenic separation device comprises a single vertical vessel
having a first distillation zone in a lower part, a freezing zone
in an intermediate part, and a second distillation zone in an upper
part.
[0042] The cryogenic separation device to be used in accordance
with the present invention suitably comprises a first lower
distillation zone having an upper end and a lower end and
containing gas-liquid contact means, outlet means in the lower end
of the lower distillation zone suitable for allowing liquid phase
contaminant to exit the distillation zone, means for allowing
reboiled liquid phase contaminant to enter the lower end, means for
allowing liquid phase contaminant to enter the upper end of the
lower distillation zone from the freezing zone, and means for
allowing a gaseous phase rich in gaseous product to exit the first
lower distillation zone into the freezing zone while maintaining a
liquid level within a lower end of the freezing zone, whereby the
freezing zone is engineered to contact a gaseous phase rich in
gaseous product from the first lower distillation zone with a
stream of cold liquid to produce solid contaminant as well as a
gaseous phase rich in gaseous product, the freezing zone having an
upper end and a lower end and containing spray means suitable for
introducing a stream of cold liquid into the freezing zone in a
spray, and means for allowing the gaseous phase rich in gaseous
product to exit the upper end of the freezing zone.
[0043] Suitably, the means for allowing the gaseous phase rich in
gaseous product to exit the first lower distillation zone into the
freezing zone comprise a chimney tray.
[0044] Suitably, the cryogenic separation device also comprises
heating means situated in the vicinity of the means for allowing
the gaseous phase rich in gaseous product to exit the first lower
distillation zone which heating means are suitable for melting
frozen solid contaminant which may be produced in the freezing
section.
[0045] The spray means to be used in accordance with the present
invention comprise one or more levels of spray assemblies.
[0046] The cryogenic separation device suitably comprises a second
upper distillation zone having an upper end and a lower end and
containing gas-liquid contact means, inlet means in the upper end
of the second upper distillation zone for allowing reflux liquids
to contact the gas-liquid contact means in the second upper
distillation zone, means in the lower end of the second
distillation zone for collecting liquid phase contaminant and
allowing the liquid phase contaminant to exit the second
distillation zone, and means for allowing a gaseous phase rich in
gaseous product to enter the upper distillation zone from the
freezing zone.
[0047] Suitably, the means for allowing the gaseous phase rich in
gaseous product to enter the second distillation zone from the
freezing zone comprise a chimney tray.
[0048] Suitably, the gas-liquid contact means in the second
distillation zone are distillation trays.
[0049] Suitably, the cryogenic separation section also includes a
reboiler adapted to heat liquid phase contaminant exiting the first
lower distillation zone, whereby at least a fraction of the
reboiled liquid phase contaminant is returned to enter the lower
end of the first lower distillation zone.
[0050] Preferred cryogenic separation devices to be used in
accordance with the process of the present invention have, for
instance, been described in U.S. Pat. No. 4,533,372; U.S. Pat. No.
4,923,493; U.S. Pat. No. 5,062,270; U.S. Pat. No. 5,265,428; U.S.
Pat. No. 5,956,971; U.S. Pat. No. 6,053,007 and U.S. Pat. No.
5,120,338, which documents are herewith incorporated by
reference.
[0051] In the present process the gaseous phase rich in gaseous
product can suitably be recompressed in one or more compression
steps before it is introduced into the cryogenic separation device
in step 4).
[0052] Suitably, energy that is recovered in step 2) can be used
for such one or more compression steps.
[0053] In a preferred embodiment of the present invention the
gaseous phase rich in gaseous product as obtained in step 3) is
cooled in a cooling step to a temperature at which liquid phase
contaminant is formed as well as a gaseous phase rich in gaseous
product, which gaseous phase rich in gaseous product is then
introduced into the cryogenic separation device in step 4).
[0054] In another preferred embodiment of the present invention the
recompressed gaseous phase rich in gaseous product as obtained in
the one or more compression steps is cooled in a cooling step to a
temperature at which liquid phase contaminant is formed as well as
a gaseous phase rich in gaseous product, after which the gaseous
stream rich in gaseous product so obtained is introduced into the
cryogenic separation device in step 4).
[0055] The cooling of the gaseous phase rich in gaseous product
between steps 3) and 4) can be carried out by means of an internal
process stream such as a stream of liquid phase contaminant which
is separated from the gaseous phase rich on gaseous product in step
3).
[0056] In accordance with the present invention the cooling of the
gaseous phase rich in gaseous product between steps 3) and 4) can
suitably at least partly be done by means of an external
refrigerant.
[0057] Preferably, the external refrigerant to be used in step 4)
has a higher molecular weight than the gaseous phase rich in
gaseous product to be cooled. Suitable examples of such cooling
medium include ethane, propane and butane. Preferably, the cooling
medium comprises ethane and/or propane.
[0058] More preferably, the external refrigerant to be used
comprises a propane cycle, an ethane/propane mixed refrigerant or
an ethane/propane cascade. Such an ethane/propane cascade is
described in more detail hereinbelow.
[0059] The cooling between steps 3) and 4) as described herebefore
can suitably partly be done by means of an external refrigerant and
partly by means of an internal process stream.
[0060] In another embodiment of the present invention the gaseous
phase rich in gaseous product as obtained in step 3) is
recompressed in one or more compression steps before it is
introduced in the cryogenic separation device in step 4).
[0061] In yet another embodiment of the present invention, the
gaseous phase rich in gaseous product as obtained in step 3) is
firstly recompressed in one or more compression steps, than cooled
between steps 3) and 4) as described herein, and the methane
enriched gaseous phase so obtained is introduced into the cryogenic
separation device in step 4).
[0062] The cooling between steps 3) and 4) as described herebefore
is suitably carried out at a temperature between -50 and
-90.degree. C., preferably at a temperature between -30 and
-70.degree. C., and at a pressure which is between 20 and 80 bara,
preferably a pressure between 30 and 60 bara.
[0063] Suitably, such an interstage cooler will be based on a
internal process stream.
[0064] In the one or more compression steps suitably energy is used
that is recovered in step 2).
[0065] In yet another embodiment of the present invention the
gaseous phase rich in gaseous product as obtained in step 3) is
again subjected to a step 2) and subsequently to a step 3) before
it is introduced in the cryogenic separation section in step 4). In
this case the gas stream is subsequently subjected to a total
number of combinations of subsequent recompression, cooling and
separation steps. Suitably, the sequence of steps 2) and 3) can be
repeated or three times before the gaseous phase rich in gaseous
product thus obtained is introdiced into the cryogenic separation
device in step 4).
[0066] In this way a further enriched gaseous product-containing
gaseous phase can be obtained containing a low level of gaseous
contaminants.
[0067] 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 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 drying natural gas are
also possible. Other dehydration processes are also possible,
including treatment with molecular sieves or drying processes with
glycol or methanol. Suitably, water is removed until the amount of
water in the gas stream comprises at most 50 ppmw, preferably at
most 20 ppmw, more preferably at most 1 ppmw of water, based on the
total gas stream.
[0068] The hydrocarbon gas that is obtained in step 5) can be used
as product. It is also possible that it is desirable to subject the
recovered sweet hydrocarbon gas after step 5) to further treatment
and/or purification. For instance, the sweet hydrocarbon gas may be
subjected to fractionation. Further purification may be
accomplished by absorption with an alkanolamine fluid, optionally
in combination with a sulphone, such as tetramethylene sulphone
(sulpholane), with N-methyl pyrrolidone, or with methanol. Other
treatments may include a further compression, when the sweet gas is
wanted at a higher pressure.
[0069] In the process according to the present invention a variety
of gas/liquid separators can suitably be used in step 3), such as,
for instance, rotating centrifuges or cyclones.
[0070] Suitable gas/liquid separators to be used in accordance with
the present invention have, for instance, been described in WO
2008/082291, WO 2006/087332, WO 2005/118110, WO 97/44117, WO
2007/097621 and WO 94/23823, which documents are hereby
incorporated by reference.
[0071] In a preferred embodiment of the present invention, the
gas/liquid separator vessel in step 3) comprises a gas/liquid inlet
at an intermediate level, a liquid outlet arranged below the
gas/liquid inlet and a gas outlet arranged above the gas/liquid
inlet, in which vessel a normally horizontal coalescer is present
above the gas/liquid inlet and over the whole cross-section of the
vessel and in which vessel a centrifugal liquid separator is
arranged above the coalescer and over the whole cross-section of
the vessel, the liquid separator comprising one or more swirl
tubes.
[0072] When using a vertical gas/liquid separator vessel, the
process only needs a relatively small area.
[0073] According to a preferred embodiment, the gas/liquid inlet
comprises an admittance with a supply and distribution assembly
extending horizontally in the separator vessel. In its most simple
form, the inlet is a simple pipe, having a closed end and a number
of perforations evenly distributed over the length of the pipe.
Optionally, the pipe may have a tapered or conical shape. One or
more cross pipes may be present to create a grid system to
distribute the gas-liquid mixture more evenly over the
cross-section of the vessel. Preferably, the assembly includes a
chamber, e.g. a longitudinal box-like structure, connected to the
gas inlet and having at least one open vertical side with a grid of
guide vanes disposed one behind each other, seen in the direction
of the flow. By means of this supply and distribution assembly, the
gas is evenly distributed by the guide vanes over the cross-section
of the column, which brings about an additional improvement of the
liquid separation in the coalescer/centrifugal separator
combination. A further advantage is that the supply and
distribution assembly separates from the gas any waves of liquid
which may suddenly occur in the gas stream, the separation being
effected by the liquid colliding with the guide vanes and falling
down inside the column. Suitably, the box structure narrows down in
the direction of the flow. After having been distributed by the
vanes over the column cross-section, the gas flows up to the
coalescer.
[0074] In a preferred embodiment the longitudinal chamber has two
open vertical sides with a grid of guide vanes.
[0075] Suitable gas/liquid inlets are those described in e.g. GB
1,119,699, U.S. Pat. No. 6,942,720, EP 195,464, U.S. Pat. No.
6,386,520 and U.S. Pat. No. 6,537,458. A suitable, commercially
available gas/liquid inlet is a Schoepentoeter.
[0076] There are numerous horizontal coalescers available,
especially for vertical columns. A well-known example of a mist
eliminator is the demister mat. All of these are relatively tenuous
(large permeability) and have a relatively large specific
(internal) surface area. Their operation is based on drop capture
by collision of drops with internal surfaces, followed by drop
growth on these surfaces, and finally by removal of the grown drop
either by the gas or by gravity.
[0077] The horizontal coalescer can have many forms which are known
per se and may, for example, consist of a bed of layers of gauze,
especially metal or non-metal gauze, e.g. organic polymer gauze, or
a layer of vanes or a layer of structured packing. Also
unstructured packings can be used and also one or more trays may be
present. All these sorts of coalescers have the advantage of being
commercially available and operating efficiently in the column
according to the invention. See also Perry's Chemical Engineers'
Handbook, Sixth edition, especially Chapter 18. See also EP
195464.
[0078] Through the use of these three stages of coalescence and
separation, a high separation efficiency is achieved.
[0079] The centrifugal liquid separator in one of its most simple
forms may comprise a horizontal plate and one or more vertical
swirl tubes extending downwardly from the plate, each swirl tube
having one or more liquid outlets below the horizontal plate at the
upper end of the swirl tube. In another form, the centrifugal
liquid separator comprises one or more vertical swirl tubes
extending upwardly from the plate, each swirl tube having one or
more liquid outlets at the upper end. The plate is provided with a
downcomer, preferably a downcomer that extends to the lower end of
the separator vessel.
[0080] In a preferred embodiment of the invention, the centrifugal
liquid separator comprises two horizontal trays between which
vertical open-ended swirl tubes extend, each from an opening in the
lower tray to some distance below a coaxial opening in the upper
tray, means for the discharge of secondary gas and of liquid from
the space between the trays outside the swirl tubes, and means
provided in the lower part of the swirl tubes to impart to the
gas/liquid a rotary movement around the vertical axis.
[0081] The liquid separator is also preferably provided with
vertical tube pieces which project down from the coaxial openings
in the upper tray into the swirl tubes and have a smaller diameter
than these latter. This arrangement enhances the separation between
primary gas on the one hand and secondary gas and liquid on the
other hand, since these latter cannot get from the swirl tubes into
the openings in the upper tray for primary gas.
[0082] According to a preferred embodiment, the means for
discharging the secondary gas from the space between the trays
consist of vertical tubelets through the upper tray, and the means
for discharging liquid from the space between the trays consist of
one or more vertical discharge pipes which extend from this space
to the bottom of the column. This arrangement has the advantage
that the secondary gas, after having been separated from liquid in
the said space between the trays, is immediately returned to the
primary gas, and the liquid is added to the liquid at the bottom of
the column after coming from the coalescer, so that the secondary
gas and the liquid removed in the centrifugal separator do not
require separate treatment.
[0083] In order to improve even further the liquid separation in
the centrifugal separator, openings are preferably provided in
accordance with the invention at the top of the swirl tubes for
discharging liquid to the space between the trays outside the swirl
tubes. This has the advantage that less secondary gas is carried to
the space between the trays. A suitable, commercially available
centrifugal separator is a Shell Swirltube deck.
[0084] In a preferred embodiment, the separation vessel comprises a
second normally horizontal liquid coalescer above the centrifugal
liquid separator and over the whole cross-section of the vessel.
This has the advantage that any droplets still present in the gas
stream are removed. See for a further description hereinabove.
Preferably, the second coalescer is a bed of one or more layers of
gauze, especially metal or non-metal gauze, e.g. organic polymer
gauze. In another preferred embodiment, the second normally
horizontal liquid coalescer is situated above the secondary gas
outlets, for instance in the way as described in EP 83811,
especially as depicted in FIG. 4.
[0085] In another preferred embodiment of the present invention the
gas/liquid separator in step 3) comprises a centrifugal separator
which comprises a bundle of parallel channels that are arranged
within a spinning tube parallel to an axis of rotation of the
spinning tube.
[0086] Suitably, the centrifugal separator is spinned by
introducing a swirling gas stream into the spinning tube.
[0087] Preferably, such a centrifugal separator to be used in
accordance with the present invention comprises a housing with a
gas inlet for contaminated gas at one end of the vessel, a
separating body, a gas outlet for purified gas at the opposite end
of the housing and a contaminants outlet downstream of the
separating body or upstream or downstream of the separating body,
wherein the separating body comprises a plurality of ducts over a
part of the length of the axis of the housing, which ducts have
been arranged around a central axis of rotation, in which apparatus
the separating body has been composed of a plurality of perforated
discs wherein the perforations of the discs form the ducts.
[0088] It will be appreciated that the discs can be easily created
by drilling or cutting a plurality of perforations into the
relatively thin discs. By attaching several discs together these
discs form a separating body. By aligning the perforations ducts
are obtained.
[0089] It is now also very easy to attach the discs such that the
perforations are not completely aligned. By varying the number and
nature of the non-alignment of the perforations the resulting ducts
can be given any desired shape. In such cases not only ducts are
obtainable that are not completely parallel to the central axis of
rotation, but also ducts that form a helix shape around the axis of
rotation. So, in this way very easily the preferred embodiment of
having non-parallel ducts can be obtained. Hence it is preferred
that the perforations of the discs have been arranged such that the
ducts are not parallel to the central axis of rotation or form a
helix shape around the axis of rotation.
[0090] Further, it will be appreciated that it is relatively easy
to increase or decrease the diameter of the perforations. Thereby
the skilled person has an easy manner at his disposal to adapt the
(hydraulic) diameter of the ducts, and thereby the Reynolds number,
so that he can easy ascertain that the flow in the ducts is laminar
or turbulent, just as he pleases. The use of these discs also
enables the skilled person to vary the diameter of the duct along
the axis of the housing. The varying diameter can be selected such
that the separated liquid or solid contaminants that are collected
against the wall of the duct will not clog up the duct completely,
which would hamper the operation of the apparatus.
[0091] The skilled person is also now enabled to maximise the
porosity of the separating body. The easy construction of the discs
allows the skilled person to meticulously provide the disc with as
many perforations as he likes. He may also select the shape of the
perforations. These may have a circular cross-section, but also
square, pentagon, hexagon, octagon or oval cross-sections are
possible. He may therefore minimise the wall thickness of the
separating body and the wall thicknesses of the ducts. He is able
to select the wall thicknesses and the shape of the ducts such that
the surface area that is contributed to the cross-section of the
separating body by the walls is minimal. That means that the
pressure drop over the separating body can be minimised.
[0092] The apparatus can have a small or large number of ducts.
Just as explained in the prior art apparatuses the number of ducts
suitably ranges from 100 to 1,000,000, preferably from 500 to
500,000. The diameter of the cross-section of the ducts can be
varied in accordance with the amount of gas and amounts and nature,
e.g., droplet size distribution, of contaminants and the desired
contaminants removal efficiency. Suitably, the diameter is from
0.05 to 50 mm, preferably from 0.1 to 20 mm, and more preferably
from 0.1 to 5 mm. By diameter is understood twice the radius in
case of circular cross-sections or the largest diagonal in case of
any other shape.
[0093] The size of the apparatus and in particular of the
separating body may vary in accordance with the amount of gas to be
treated. In EP-B 286 160 it is indicated that separating bodies
with a peripheral diameter of 1 m and an axial length of 1.5 m are
feasible. The separating body according to the present invention
may suitably have a radial length ranging from 0.1 to 5 m,
preferably from 0.2 to 2 m. The axial length ranges conveniently
from 0.1 to 10 m, preferably, from 0.2 to 5 m.
[0094] The number of discs may also vary over a large number. It is
possible to have only two discs if a simple separation is needed
and/or when the perforations can be easily made. Other
considerations may be whether parallel ducts are desired, or
whether a uniform diameter is wanted. Suitably the number of discs
varies from 3 to 1000, preferably from 4 to 500, more preferably
from 4 to 40. When more discs, are used the skilled person will
find it easier to gradually vary the diameter of the ducts and/or
to construct non-parallel ducts. Moreover, by increasing or
decreasing the number of discs the skilled person may vary the duct
length. So, when the conditions or the composition of the gas
changes, the skilled person may adapt the duct length easily to
provide the most optimal conditions for the apparatus of the
present invention. The size of the discs is selected such that the
radial diameter suitably ranges from 0.1 to 5 m, preferably from
0.2 to 2 m. The axial length of the discs may be varied in
accordance with construction possibilities, desire for varying the
shape etc. Suitably, the axial length of each disc ranges from
0.001 to 0.5 m, preferably from 0.002 to 0.2 m, more preferably
from 0.005 to 0.1 m.
[0095] Although the discs may be manufactured from a variety of
materials, including paper, cardboard, and foil, it is preferred to
manufacture the discs from metal or ceramics. Metals discs have the
advantage that they can be easily perforated and be combined to
firm sturdy separating bodies. Dependent on the material that needs
to be purified a suitable metal can be selected. For some
applications carbon steel is suitable whereas for other
applications, in particular when corrosive materials are to be
separated, stainless steel may be preferred. Ceramics have the
advantage that they can be extruded into the desired form such as
in honeycomb structures with protruding ducts.
[0096] Typically, the ceramics precursor material is chosen to form
a dense or low-porosity ceramic. Thereby the solid or liquid
contaminants are forced to flow along the wall of the ducts and
not, or hardly, through the ceramic material of the walls. Examples
of ceramic materials are silica, alumina, zirconia, optionally with
different types and concentrations of modifiers to adapt its
physical and/or chemical properties to the gas and the
contaminants.
[0097] The discs may be combined to a separating body in a variety
of ways. The skilled person will appreciate that such may depend on
the material from which the discs have been manufactured. A
convenient manner is to attach the discs to a shaft that provides
the axis of rotation. Suitable ways of combining the discs include
clamping the discs together, but also gluing them or welding them
together can be done. Alternatively, the discs may be stacked in a
cylindrical sleeve. This sleeve may also at least partly replace
the shaft. This could be convenient for extruded discs since no
central opening for the shaft would be required. It is preferred to
have metal discs that are welded together.
[0098] The gas/liquid separator in step 3) comprises:
a) a housing comprising a first, second and third separation
section for separating liquid from the mixture, wherein the second
separation section is arranged below the first separation section
and above the third separation section, the respective separation
sections are in communication with each other, and the second
separation section comprises a rotating coalescer element; b)
tangentially arranged inlet means to introduce the mixture into the
first separation section; c) means to remove liquid from the first
separation section; d) means to remove liquid from the third
separation section; and e) means to remove a gaseous stream, lean
in liquid, from the third separation section.
[0099] In a preferred embodiment of the invention, the methane
enriched gaseous phase obtained in accordance with the present
invention is further purified, e.g. by extraction of remaining
acidic components with a chemical solvent, e.g. an aqueous amine
solution, especially aqueous ethanolamines, such as DIPA, DMA,
MDEA, etc., or with a physical solvent, e.g. cold methanol, DEPG,
NMP, etc.
[0100] The contaminated gas stream is continuously provided,
continuously cooled and continuously separated.
[0101] The present invention also relates to a device (plant) for
carrying out the process as described above, as well as the
purified gas stream obtained by the present process. In addition,
the present invention concerns a process for liquefying a feed gas
stream comprising purifying the feed gas stream by means of the
present process, followed by liquifying the feed gas stream by
methods known in the art.
* * * * *