U.S. patent application number 13/120411 was filed with the patent office on 2011-08-11 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 | 20110192192 13/120411 |
Document ID | / |
Family ID | 40351798 |
Filed Date | 2011-08-11 |
United States Patent
Application |
20110192192 |
Kind Code |
A1 |
Andrian; Diki ; et
al. |
August 11, 2011 |
PROCESS FOR REMOVING GASEOUS CONTAMINANTS FROM A FEED GAS STREAM
COMPRISING METHANE AND GASEOUS CONTAMINANTS
Abstract
The invention provides a process for removing gaseous
contaminants from a feed gas stream which comprises methane and
gaseous contaminants, the process comprising: 1) providing the feed
gas stream (1); 2) cooling the feed gas stream to a temperature at
which liquid phase contaminant is formed as well as a methane
enriched gaseous phase,--3) separating the two phases obtained in
step 2) by means of a gas/liquid separator (4); and 4) subjecting
the methane enriched gaseous phase obtained in step 3) to a
distillation treatment in a distillation section (10) thereby
obtaining a bottom stream (12) rich in liquid phase contaminant and
lean in methane, and a top stream (11) rich in methane and lean in
gaseous contaminant. The invention further concerns 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) ; Van Der Vaart; Rick; (Rijswijk, NL) |
Family ID: |
40351798 |
Appl. No.: |
13/120411 |
Filed: |
September 11, 2009 |
PCT Filed: |
September 11, 2009 |
PCT NO: |
PCT/EP2009/061798 |
371 Date: |
March 22, 2011 |
Current U.S.
Class: |
62/620 |
Current CPC
Class: |
F25J 2220/66 20130101;
F25J 2270/60 20130101; B01D 2257/304 20130101; F25J 2270/12
20130101; F25J 3/0635 20130101; F25J 3/061 20130101; B01D 53/526
20130101; Y02C 10/12 20130101; F25J 3/067 20130101; F25J 2270/66
20130101; C10L 3/10 20130101; B01D 2257/504 20130101; B01D 2258/06
20130101; Y02C 20/40 20200801; B01D 53/002 20130101; F25J 2215/04
20130101 |
Class at
Publication: |
62/620 |
International
Class: |
F25J 3/08 20060101
F25J003/08; F25J 3/02 20060101 F25J003/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 23, 2008 |
EP |
08164886.7 |
Claims
1. A process for removing gaseous contaminants from a feed gas
stream which comprises methane 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 methane enriched gaseous phase;
3) separating the two phases obtained in step 2) with a gas/liquid
separator; and 4) subjecting the methane enriched gaseous phase
obtained in step 3) to a distillation treatment in a distillation
device thereby obtaining a bottom stream rich in liquid phase
contaminant and lean in methane, and a top stream rich in methane
and lean in gaseous contaminant.
2. A process according to claim 1, in which the feed gas stream is
a natural gas stream in which the gaseous contaminants are carbon
dioxide and/or hydrogen sulphide, preferably in which the natural
gas stream comprises between 1 and 90 vol % of carbon dioxide,
preferably between 5 and 80 vol % of carbon dioxide and/or wherein
the natural gas stream comprises between 0.1 and 60 vol % of
hydrogen sulphide.
3. A process according to claim 1, in which the feed gas stream
comprises between 20 and 80 vol % of methane.
4. A process according to claim 1, in which the feed gas stream in
step 1) has a temperature between -20 and 150.degree. C.
5. A process according to claim 1, in which the cooling in step 2)
is done by isenthalpic expansion.
6. A process according to claim 5, wherein the expansion is done
using at least two expansion devices and the operating parameters
of the expansion devices are chosen such that the liquefied acidic
contaminants have a certain droplet size distribution.
7. A process according to claim 1, in which the feed gas stream is
cooled in step 2) to a temperature between -30 and -80.degree.
C.
8. A process according to claim 1, in which the methane enriched
gaseous phase obtained in step 3) is recompressed in one more
compression steps before it is subjected to the distillation
treatment in step 4.
9. A process according to claim 1, in which the methane enriched
gaseous phase obtained in step 3) or the recompressed methane
enriched gaseous phase 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 methane enriched gaseous
phase, and the methane enriched gaseous phase so obtained is
subjected to the distillation treatment in step 4).
10. A process according to claim 1, in which the bottom temperature
of the distillation section is between 0 and 30.degree. C.
11. A process according to claim 1, in which 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.
12. A process according to claim 1, in which 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.
13. A process according to claim 1, in which 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.
14. A gas/liquid separator comprising: a) a housing comprising a
first, second and third separation section for separating liquid
from a gas 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
15. (canceled)
Description
[0001] The present invention concerns a process for the removal of
gaseous contaminants from a feed gas stream which comprises methane
and gaseous contaminants, in particular the removal of gaseous
contaminants such as carbon dioxide and hydrogen sulphide from a
natural gas.
[0002] Methane comprising gas streams produced from subsurface
reservoirs, especially natural gas, associated gas and coal bed
methane, usually contain contaminants 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 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.
[0003] 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 %.
[0004] 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 which is
utilising air, water and/or an internal cold process stream, e.g.
cold gas and/or liquid obtained form step 3), 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 contaminants-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.
[0005] A disadvantage of this known method is that there is still
room for improving the removal of 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. A further disadvantage of such
a method is that there is still room to improve the hydrocarbon
efficiency of such a method, which hydrocarbon efficiency is a
measure of the fuel consumption and the hydrocarbon loss into the
liquid phase contaminant streams during the process.
[0006] It has now been found that in an integrated process for
removing gaseous contaminants improved levels of gaseous
contaminants and an improved hydrocarbon efficiency can be obtained
when after a gas/liquid separation the contaminants-depleted
gaseous phase is subjected to a distillation treatment.
[0007] Thus, the present invention concerns a process for removing
gaseous contaminants from a feed gas stream which comprises methane
and gaseous contaminants, which process comprises:
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 methane enriched gaseous phase; 3) separating the two phases
obtained in step 2) by means of a gas/liquid separator; and 4)
subjecting the methane enriched gaseous phase obtained in step 3)
to a distillation treatment in a distillation section thereby
obtaining a bottom stream rich in liquid phase contaminant and lean
in methane, and a top stream rich in methane and and lean in
gaseous contaminant.
[0008] Suitably, the feed gas stream is a natural gas stream in
which the gaseous contaminants are carbon dioxide and/or 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.
[0009] The feed gas stream to be used in accordance with the
present invention comprises between 20 and 80 vol % of methane.
[0010] 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.
[0011] The raw feed gas stream may be pre-treated to partially or
completely remove water and optionally some heavy hydrocarbons.
This can for instance be 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 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 gas feed stream is suitably less than 1 vol
%, preferably less than 0.1 vol %, more preferably less than 0.0001
vol %.
[0012] 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
an internal or 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.
[0013] 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.
[0014] Preferably, the expansion is done using at least two
expansion devices and the operating parameters of the expansion
devices are chosen such that the liquefied acidic contaminants have
a certain droplet size distribution. In this way, the droplet size
distribution can be controlled.
[0015] 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.
[0016] 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.
[0017] Suitably the feed gas stream is cooled in step 2) to a
temperature between -30 and -80.degree. C., preferably between -40
and -65.degree. C. At these temperatures liquid phase contaminant
will be formed.
[0018] In the present invention both liquid phase contaminant and
gaseous contaminant will comprise carbon dioxide and/or hydrogen
sulphide.
[0019] In step 4), the bottom temperature of the distillation
device is suitably between 0 and 30.degree. C., and preferably
between 5 and 20.degree. C., whereas the top temperature of the
distillation device is between -20 and -90.degree. C., preferably
between -30 and -70.degree. C.
[0020] In accordance with the present invention the distillation
device can suitable be a distillation column or distillation vessel
known in the art.
[0021] Suitably, the top stream in step 4) comprises between 1 and
75 mol % of the gaseous contaminants present in the feed gas
stream. Preferably, the top stream in step 4) comprises between 2
and 50 mol % of the gaseous contaminants present in the feed gas
stream.
[0022] Hence, it will be clear that with the relatively simple
process according to the invention very low levels of gaseous
contaminants can be established in the methane enriched gaseous
phase, ensuring that levels can be reached that are specified for
pipeline transport of the feed gas stream or the production of
liquefied natural gas.
[0023] In accordance with the present invention the methane
enriched gaseous phase obtained in step 3) can suitably be cooled
in a cooling step to a temperature at which liquid phase
contaminant is formed as well as a methane enriched gaseous phase,
and the methane enriched gaseous phase so obtained is subjected to
the distillation treatment in step 4).
[0024] The cooling between steps 3) and 4) can, e.g. be carried out
by means of an internal process stream, e.g. a stream of liquid
phase contaminant which is separated from the methane enriched
gaseous phase in step 3).
[0025] In accordance with the present invention the cooling of the
methane enriched gaseous phase between steps 3) and 4) can suitably
at least partly be done by means of an external refrigerant.
[0026] Preferably, the external refrigerant to be used has a higher
molecular weight than the methane enriched gaseous phase to be
cooled. Suitable examples of such cooling medium include ethane,
propane and butane. Preferably, the cooling medium comprises ethane
and/or propane.
[0027] 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.
[0028] In a preferred embodiment of the present invention the
methane enriched gaseous phase obtained in step 3) is recompressed
in one or more compression steps before the methane enriched
gaseous phase is cooled by means of the internal and/or external
refrigerant.
[0029] Suitably, the recompressed methane enriched gaseous phase
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 methane enriched gaseous phase, whereafter the
methane enriched gaseous phase so obtained is subjected to the
distillation treatment in step 4).
[0030] The cooling of the methane enriched gaseous phase between
steps 3) and 4) can suitably be partly done by means of an external
refrigerant and partly by means of an internal process stream such
as a stream of liquid phase contaminant which is separated from the
methane enriched gaseous phase in step 3).
[0031] The cooling by means of the external refrigerant may be
carried out in such a way that a stream is obtained comprising
liquid phase contaminant and a methane enriched gaseous phase. This
stream can subsequently be subjected to the distillation treatment
in step 4). In this way a further enriched methane-containing
gaseous phase can be obtained containing a low level of gaseous
contaminants, making the present process a relatively simple
process that requires relatively simple equipment only, and at the
same time improving the removal of gaseous contaminants from the
feed gas stream.
[0032] In another embodiment of the present invention the methane
enriched gaseous phase obtained in step 3) is firstly cooled by
means of an interstage cooler before the methane enriched gaseous
phase obtained in step 3) is cooled to a temperature at which
liquid phase contaminant in methane enriched phase is subsequently
subjected to the distillation treatment in step 4).
[0033] In yet another embodiment of the present invention, the
methane enriched gaseous phase obtained in step 3) is firstly
recompressed in one or more compression steps, than cooled by means
of an interstage cooler, and subsequently cooled to a temperature
at which liquid phase contaminant is formed as well as a methane
enriched gaseous phase, which methane enriched gaseous phase so
obtained is then subjected to the distillation treatment in step
4).
[0034] Suitably, such an interstage cooler can be based on a
internalprocess loop.
[0035] In the one or more compression steps suitably energy is used
that is recovered in step 2) or 4). In accordance with the present
invention the methane enriched gaseous phase obtained in step 3) is
cooled between steps 3) and 4) to a temperature between 30 and
-90.degree. C., preferably between -30 and -70.degree. C., and the
pressure is suitably between 20 and 80 bara, preferably between 30
and 60 bara.
[0036] Whereas in known separation processes for removing gaseous
contaminants from a feed gas stream the cold of the methane
enriched gaseous phase as obtained in step 3) is usually at least
partly used to pre-cool the feed gas stream. In accordance with the
present process preferably no part of the methane enriched gaseous
phase is used as an internal loop to pre-cool the feed gas stream,
but the entire cold of the methane enriched gaseous phase is
maintained in the stream to be further processed in accordance with
the present invention.
[0037] 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.
[0038] 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.
[0039] In one preferred embodiment, a gas/liquid separator is used
comprising:
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.
[0040] In another 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.
[0041] When using a vertical gas/liquid separator vessel, the
process only needs a relatively small area.
[0042] 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.
[0043] In a preferred embodiment the longitudinal chamber has two
open vertical sides with a grid of guide vanes.
[0044] 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.
[0045] 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.
[0046] 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.
[0047] 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.
[0048] 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.
[0049] 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.
[0050] 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.
[0051] 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.
[0052] 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.
[0053] 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.
[0054] Suitably, the centrifugal separator is spinned by
introducing a swirling gas stream into the spinning tube.
[0055] Preferably, the 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 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.
[0056] 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.
[0057] 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.
[0058] 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.
[0059] 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.
[0060] 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.
[0061] 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.
[0062] 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.
[0063] 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.
[0064] 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.
[0065] 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.
[0066] 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.
[0067] 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.
[0068] The contaminated gas stream is continuously provided,
continuously cooled and continuously separated.
[0069] 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 purified feed gas
stream by methods known in the art.
[0070] The invention will be further illustrated by means of the
following Figures.
[0071] Referring 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 an external
refrigerant whereby 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 a 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 distillation
column 10 from which a further enriched methane enriched gaseous
phase is recovered via a conduit 11 and liquid phase contaminant is
recovered via a conduit 12.
[0072] 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 13 into an expander (e.g. a turbine expander or a
Joule-Thomson valve), and the cooled ethane stream so obtained is
passed via a conduit 15 into the heat exchanger 7. A stream of warm
ethane is then passed from the heat exchanger 7 to a recompressor
16 via a conduit 17 to increase the pressure of the ethane stream.
The compressed stream of ethane obtained from recompressor 16 is
then passed via a conduit 18 into heat exchanger 19 wherein the
ethane stream is cooled and at least partly condensed. Via the
conduit 13 the ethane stream is then recycled to the expander 14.
In the propane loop a propane stream is passed via a conduit 20
into an expander 21 (e.g. a turbine expander or a Joule-Thomson
valve), and the cooled propane stream so obtained is passed via a
conduit 22 into the heat exchanger 19 of the ethane loop. A stream
of warm propane is then passed from the heat exchanger 19 via a
conduit 23 into a recompressor 24 to increase the pressure of the
propane stream. The compressed stream of propane obtained from
recompressor 24 is then passed via a conduit 25 into a heat
exchanger 26 wherein the propane stream is cooled and at least
partly condensed by means of water or air. Via the conduit 20 the
propane stream is then recycled to the expander 21.
[0073] 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 27. 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 28, centrifugal
separator 29 and a second liquid coalescer 30 to the top of the
separator vessel, and leave the separator vessel via gas outlet
6.
[0074] 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 31 in a housing 32
of the gas/liquid separator 4. The housing 32 further comprises a
separating body 33 which shows a large number of ducts 34 which are
arranged around a shaft 35, which provides an axis of rotation.
Separating body 33 has been composed of six discs 33a, 33b, 33c,
33d, 33e and 33f that have been combined by welding or gluing.
[0075] 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 36 which has been arranged downstream of the
separating body 33, and via the discharge conduit 5. Purified
natural gas leaves housing 32 via the gas outlet 6 arranged at the
opposite end of the housing 32.
* * * * *