U.S. patent application number 13/144322 was filed with the patent office on 2011-12-08 for method and apparatus for separating nitrogen from a mixed stream comprising nitrogen and methane.
Invention is credited to Esther Lucia Johanna Van Soest-Vercammen, Renze Wijntje.
Application Number | 20110296871 13/144322 |
Document ID | / |
Family ID | 40456496 |
Filed Date | 2011-12-08 |
United States Patent
Application |
20110296871 |
Kind Code |
A1 |
Van Soest-Vercammen; Esther Lucia
Johanna ; et al. |
December 8, 2011 |
METHOD AND APPARATUS FOR SEPARATING NITROGEN FROM A MIXED STREAM
COMPRISING NITROGEN AND METHANE
Abstract
A method and apparatus for separating nitrogen from a mixed
stream comprising nitrogen and methane employes a monolith sorption
contactor formed of a unitary construction of active carbon, the
contactor housing one or more separation flow channels, the one or
more channels having at least one inlet to, and at least one outlet
from, said contactor, the one or more channels defining one or more
first internal surfaces of the monolith sorption contactor, the
contactor further comprising one or more first external surfaces
provided with a barrier layer, the first external surfaces being
different from the first internal surfaces. The mixed stream is
passed through at least one of the separation flow channels, where
methane is sorbed. The contactor can be regenerated by contacting
the contactor with a heat exchange fluid via the barrier layer at
one or more of the external surfaces.
Inventors: |
Van Soest-Vercammen; Esther Lucia
Johanna; (Amsterdam, NL) ; Wijntje; Renze;
(Amsterdam, NL) |
Family ID: |
40456496 |
Appl. No.: |
13/144322 |
Filed: |
January 13, 2010 |
PCT Filed: |
January 13, 2010 |
PCT NO: |
PCT/EP10/50320 |
371 Date: |
August 31, 2011 |
Current U.S.
Class: |
62/636 |
Current CPC
Class: |
B01D 2256/24 20130101;
B01D 2259/416 20130101; B01D 53/0462 20130101; B01D 2253/102
20130101 |
Class at
Publication: |
62/636 |
International
Class: |
F25J 3/08 20060101
F25J003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 15, 2009 |
EP |
09150637.8 |
Claims
1. A method of separating nitrogen from a mixed stream comprising
nitrogen and methane, the method comprising at least the steps of:
(a) providing a monolith sorption contactor formed of a unitary
construction of active carbon, said contactor housing one or more
separation flow channels intersecting the monolith sorption
contactor, said one or more separation flow channels having at
least one inlet to, and at least one outlet from, said contactor,
said one or more separation flow channels defining one or more
first internal surfaces of the monolith sorption contactor, said
contactor further comprising one or more first external surfaces
provided with a barrier layer, said first external surfaces being
different from said first internal surfaces; (b) passing the mixed
stream into at least one of the one or more separation flow
channels via the at least one inlet; (c) sorbing the methane in the
sorption contactor via the one or more first internal surfaces in
the at least one of the one or more separation flow channels at a
temperature lower than or equal to -60.degree. C. to provide a
nitrogen-enriched stream at the at least one outlet; (d)
interrupting the passage of the mixed stream through the contactor;
(e) regenerating the contactor by contacting the contactor with a
heat exchange fluid stream at the one or more first external
surfaces provided with the barrier layer, to heat the contactor to
a temperature above -60.degree. C. to desorb methane and provide a
cool heat exchange fluid stream; and (f) withdrawing the desorbed
methane as a methane-enriched stream from the at least one outlet
from the contactor; wherein the barrier layer serves to provide a
fluid barrier against passage of the heat exchange fluid into the
monolith sorption contactor.
2. The method according to claim 1, wherein the sorbing step (c) is
carried out at a temperature in the range of from -160 to
-60.degree. C.
3. The method according to claim 1 wherein the regenerating in step
(e) is carried out by raising the temperature of the contactor, to
a temperature in the range of from -40 to -30.degree. C.
4. The method according to claim 1, further comprising cooling the
contactor by passing the nitrogen-enriched stream through one or
more of the one or more separation flow channels.
5. The method according to claim 1, further comprising, optionally
prior to step (c), a step of: cooling the contactor by contacting
at least one of the one or more first external surfaces having the
barrier layer with the cool heat exchange fluid stream to provide
the warm heat exchange fluid stream.
6. The method according to claim 4 wherein cooling the contactor
reduces the temperature of the contactor to less than or equal to
-60.degree. C.
7. The method according to claim 1 wherein the barrier layer
comprises an epoxy resin.
8. The method according to claim 1 wherein the mixed stream is
derived from a liquefaction unit and the heat exchange fluid is a
refrigerant from said liquefaction unit.
9. The method according to claim 1 wherein step (e) further
comprises passing a flushing fluid stream through the one or more
separation channels.
10. The method according to claim 1, further comprising the steps
of passing a purging fluid stream through the one or more
separation channels between steps (d) and (e).
11. The method according to claim 1 wherein the mixed stream is
obtained from a gas/liquid separator providing a gaseous
hydrocarbon-containing stream, and a liquid hydrocarbon-containing
stream.
12. The method according to claim 10, wherein at least a part of
the gaseous hydrocarbon-containing stream is contacted with the
activated carbon as the mixed stream.
13. The method according to claim 1 wherein the mixed stream is at
a temperature below 0.degree. C.
14. The method according to claim 1 wherein the mixed stream is at
a pressure of less than or equal to 10 bar.
15. The method according to claim 11 wherein at least a part of the
mixed stream has been liquefied upstream of the gas/liquid
separator.
16. An apparatus for separating nitrogen from a mixed stream
comprising nitrogen and methane, the apparatus comprising at least:
a source of a mixed stream comprising methane and nitrogen at a
temperature of less than or equal to -60.degree. C. in a mixed
stream line; a source of a warm heat exchange fluid stream in a
warm heat exchange fluid stream line; a source of a cool heat
exchange fluid stream in a cool heat exchange fluid stream line; a
monolith sorption contactor formed of a unitary construction of
active carbon, said contactor housing one or more separation flow
channels intersecting the monolith sorption contactor, said one or
more separation flow channels having at least one inlet in fluid
communication with the mixed stream line, and at least one outlet
in fluid communication with a nitrogen-enriched stream line, said
one or more separation flow channels defining one or more first
internal surfaces of the monolith sorption contactor, said
contactor further comprising one or more first external surfaces,
said first external surfaces being different from said first
internal surfaces and being in heat exchange communication with
said warm heat exchange fluid stream line and said cool heat
exchange fluid stream line; and a barrier layer provided on the one
or more first external surfaces to provide a fluid barrier against
passage of the warm and cool heat exchange fluids into the monolith
sorption contactor.
Description
[0001] The present invention provides a method for separating
nitrogen from a mixed stream comprising nitrogen and methane and an
apparatus therefore.
[0002] Several processes and apparatuses for the removal of
nitrogen from a mixed stream comprising nitrogen and methane, such
as a flashed LNG stream, are known. One reason for removing
nitrogen from such a stream may be in order to obtain natural gas
having a desired heating value (i.e. energy content when the gas is
burned), according to particular gas specifications or the
requirements of a consumer.
[0003] An example of a known method for removing nitrogen from a
mixed stream comprising nitrogen and methane is disclosed in US
Patent Application No. 2008/282885. US Patent Application No.
2008/282885 discloses a process for removing a first heavy gas
component such as nitrogen from a gas mixture containing the first
heavy gas component and a second lighter gas component such as
methane. The first heavy gas component is taken up by a microporous
absorbent in the form of a monolithic parallel channel
contactor.
[0004] US Patent Application No. 2008/282885 discloses the coating
of an absorbent layer onto the channels of a preformed monolith
comprised of non-absorbent material for thermal swing absorption
processes. The necessity to also apply a ceramic or metallic glaze
or sol gel coating to seal the walls of the channels is also
discussed to prevent the transmission of gas flowing through the
channels into the body of the preformed monolith. The monolithic
contactor can also be provided with paths or separate channels
which can be used to heat and cool the adsorbent.
[0005] The provision of a such a preformed monolith comprising
channels coated with absorbent, which may have to be glazed to
prevent gas entering the body of the monolith and maintain fluid
separation of the heating and cooling paths or channels requires a
complicated construction process, leading to a more expensive
contactor, and increased likelihood of operational difficulties
should the coating separating the heating and cooling channels
fail.
[0006] In a first aspect, the present invention provides a method
of separating nitrogen from a mixed stream comprising nitrogen and
methane, the method comprising at least the steps of:
(a) providing a monolith sorption contactor formed of a unitary
construction of active carbon, said contactor housing one or more
separation flow channels intersecting the monolith sorption
contactor, said one or more separation flow channels having at
least one inlet to, and at least one outlet from, said contactor,
said one or more separation flow channels defining one or more
first internal surfaces of the monolith sorption contactor, said
contactor further comprising one or more first external surfaces
provided with a barrier layer, said first external surfaces being
different from said first internal surfaces; (b) passing the mixed
stream into at least one of the one or more separation flow
channels via the at least one inlet; (c) sorbing the methane in the
sorption contactor via the one or more first internal surfaces in
the at least one of the one or more separation flow channels at a
temperature lower than or equal to -60.degree. C. to provide a
nitrogen-enriched stream at the at least one outlet; (d)
interrupting the passage of the mixed stream through the contactor;
(e) regenerating the contactor by contacting the contactor with a
heat exchange fluid stream at the one or more first external
surfaces provided with the barrier layer, to heat the contactor to
a temperature above -60.degree. C. to desorb methane and provide a
cool heat exchange fluid stream; and (f) withdrawing the desorbed
methane as a methane-enriched stream from the at least one outlet
from the contactor; wherein the barrier layer serves to provide a
fluid barrier against passage of the heat exchange fluid into the
monolith sorption contactor.
[0007] In another aspect, the present invention provides an
apparatus for separating nitrogen from a mixed stream comprising
nitrogen and methane, the apparatus comprising at least: [0008] a
source of a mixed stream comprising methane and nitrogen at a
temperature of less than or equal to -60.degree. C. in a mixed
stream line; [0009] a source of a warm heat exchange fluid stream
in a warm heat exchange fluid stream line; [0010] a source of a
cool heat exchange fluid stream in a cool heat exchange fluid
stream line; [0011] a monolith sorption contactor formed of a
unitary construction of active carbon, said contactor housing one
or more separation flow channels intersecting the monolith sorption
contactor, said one or more separation flow channels having at
least one inlet in fluid communication with the mixed stream line,
and at least one outlet in fluid communication with a
nitrogen-enriched stream line, said one or more separation flow
channels defining one or more first internal surfaces of the
monolith sorption contactor, said contactor further comprising one
or more first external surfaces, said first external surfaces being
different from said first internal surfaces and being in heat
exchange communication with said warm heat exchange fluid stream
line and said cool heat exchange fluid stream line; and [0012] a
barrier layer provided on the one or more first external surfaces
to provide a fluid barrier against passage of the warm and cool
heat exchange fluids into the monolith sorption contactor.
[0013] Embodiments of the present invention will now be described
by way of example only, and with reference to the accompanying
non-limiting drawings in which:
[0014] FIG. 1 shows a schematic view of a monolith sorbent
contactor;
[0015] FIG. 2 shows an embodiment of an exemplary application of
the monolith sorbent contactor in a method according to an
embodiment the invention;
[0016] FIG. 3 shows an embodiment of a typical process scheme
according to an embodiment the invention; and
[0017] FIG. 4 shows an embodiment of a typical process stream for
the regeneration of the monolith sorbent contactor according to an
embodiment of the invention.
[0018] For the purpose of this description, a single reference
number will be assigned to a line as well as a stream carried in
that line. The same reference numbers refer to similar components,
streams or lines.
[0019] A method of separating nitrogen from a mixed stream
comprising nitrogen and methane is proposed herein, that uses a
monolith sorption contactor formed of a unitary construction of
active carbon, which does not need an absorbent-coated preformed
monolith or require treatments to seal the channel walls.
[0020] FIG. 1 shows a typical monolith sorption contactor 2. It is
formed of a unitary construction of a sorbent material, e.g.
activated carbon, and it is provided with a barrier layer 2f. The
contactor houses one or more separation flow channels 2a which
intersect the monolith sorption contactor 2. As shown in FIG. 1,
the separation flow channels intersect end face 2g of the monolith
2. The one or more separation flow channels have at least one inlet
(2b) to allow the mixed stream to enter into the flow channels 2a.
On the other side, there is at least one outlet (not shown). The
one or more separation flow channels 2a define one or more first
internal surfaces 2d of the monolith sorption contactor 2. The
contactor 2 further comprises one or more first external surfaces
2e, different from the first internal surfaces 2d. At least part of
the one or more first external surfaces 2e is provided with barrier
layer 2f. For clarity, the barrier layer 2f in FIG. 1 is partly
shown removed so as to partly expose the external surface 2e of the
monolith sorption contactor 2 into view.
[0021] The monolith sorption contactor 2 formed of the unitary
construction of activated carbon as used herein is advantageous
because of its small coefficient of thermal expansion. This allows
the use of temperature swing adsorption over a broad temperature
range to separate the nitrogen from the mixed stream, while
minimising any problems arising from the thermal expansion and
contraction of the contactor during the heating and cooling
process.
[0022] Moreover, the amount of adsorbent required in such a thermal
swing process to achieve the separation of methane from the
nitrogen from a mixed stream having a selected % of methane at a
specified flow rate is significantly lower than would be the case
in a conventional thermal swing absorption configuration.
[0023] The contactor operates by sorbing at least a part of the
methane component of the mixed stream to provide a
nitrogen-enriched stream. The sorbed methane component can then be
subsequently desorbed from the contactor to provide a
methane-enriched stream. As used herein, the term "sorption" is
intended to denote one or both of adsorption and absorption. In a
preferred embodiment, one molecule or sorbate, such as methane, has
a preferred affinity for the active carbon sorbent over a second
molecule or sorbate, such as nitrogen.
[0024] Desorption may be facilitated by exposing the monolith
sorbent contactor to a heat exchange fluid in order to increase its
temperature. A heat exchange fluid may also be used to bring the
monolith sorbent contactor to low temperature prior to and/or while
allowing the mixed stream into the separation flow channels.
[0025] In the context of the present disclosure, the term "warm
heat exchange fluid" may refer to the heat exchange fluid admitted
to the monolith sorbent contactor to heat it, or it may refer to
the heat exchange fluid resulting from having cooled the monolith
sorbent contactor (in which case it is warmer than the original
heat exchange fluid when it was admitted to the monolith sorbent
contactor). Likewise, the term "cool heat exchange fluid" may refer
to the heat exchange fluid resulting from having warmed the
monolith sorbent contactor, or it may refer to the heat exchange
fluid admitted to the monolith sorbent contactor to cool it. Thus,
depending on whether the heat exchange fluid is being warmed or
cooled as a result of exchanging heat with the monolith sorbent
contactor, the monolith sorbent contactor could form part of a
source of a cool, respectively warm, heat exchange fluid
stream.
[0026] Hence, the method may further comprise, optionally prior to
step (c), a step of:
[0027] cooling the contactor by contacting at least one of the one
or more first external surfaces via the barrier layer with the cool
heat exchange fluid stream to provide the warm heat exchange fluid
stream.
[0028] The cold energy removed from the contactor in the
regeneration step (e) may be returned to the cool contactor in
preparation for the sorption step (c). In this way, the energy
requirements of the sorption cycle of steps (a) to (f) can be
minimised by recycling the cold energy released when the sorbent is
regenerated to subsequently cool the contactor, providing a more
efficient separation method. This can be contrasted with a thermal
swing absorption method in which the temperature of the contactor
is raised by heating elements, which could result in the loss of
the cold energy of the contactor at sorption temperature.
[0029] A unitary construction of active carbon for the monolith
contactor as described herein facilitates a more efficient energy
transfer to heat or cool the contactor. Any energy applied to heat
or cool the contactor will alter the temperature of the active
carbon sorbent with significantly less energy being lost to alter
the temperature of associated components, for instance a shell and
tube contactor or a ceramic or metallic pre-formed monolith.
[0030] The monolith sorption contactor utilised in the present
methods and apparatus has a simple construction compared to those
of the prior art. It does not require the sealing of the flow
channels to render them impermeable to the mixed stream and/or the
heat exchange fluid. Nor does it require the use of a pre-formed
monolith in which the sorbent must be applied to the separation
flow channels. Instead, the barrier layer is provided on one or
more second external surfaces of the monolith. The application of
the barrier layer to an external surface of the monolith is a
simple procedure compared to the application of a barrier layer to
an internal surface of the separation flow channels, and decreases
the possibility of failure of the barrier layer during
operation.
[0031] The unitary construction of the contactor is also beneficial
because it reduces the energy requirements of the separation method
compared to non-unitary structures of, for example, micro-flow
reactors, such as shell and tube reactors holding particulate
sorbent in the tubes, or the pre-formed monoliths having sorbent
coatings on the separation flow channels. Such prior art structures
comprise non-sorbent components, such as the metallic shell and
tube reactor or the metallic or ceramic pre-formed monolith, which
must also be heated and cooled in a temperature swing adsorption
process, requiring additional energy.
[0032] The sorbent described herein is activated carbon. Activated
or active carbon is a form of carbon which has been processed to
provide it with a large surface area which can be available for the
sorption of molecular species. The BET surface area available for
the sorption may be in excess of 500 m.sup.2/g as determined by a
BET surface area method known in the art, such as N.sub.2
adsorption at liquid nitrogen temperature using multipoint
pressures of 0.08, 0.14 and 0.20 P/P.sub.0 (relative
pressure/vapour pressure), and using adsorption analyzers such as
the TriStar 3000 apparatus of Micromeritics Instrument Corporation,
USA. BET surface area has been proposed and described by Brunauer,
S., Emmett, P. H. & Teller, E. in "Adsorption of gases in
multimolecular layers" J. Am. Chem. Soc. 60, pp. 309-319
(1938).
[0033] The unitary construction of the monolith sorbent contactor
preferably consists essentially of activated carbon and optional
further sorbents together with incidental impurities from the
manufacturing process. More preferably the unitary construction of
the monolith sorbent contactor preferably consists essentially of
activated carbon together with incidental impurities from the
manufacturing process. Thus, the monolith contactor differs
structurally from those prior art contactors formed of a preformed
monolith of metal or ceramic material having channels coated with
activated carbon.
[0034] The monolith sorbent contactor may have any desired shape,
such as a rod-, triangular prismatic- or quadrilateral
prismatic-shape etc. Rod-shaped contactors are preferred because
these can be most easily integrated into a separation unit.
[0035] The contactor comprises one or more flow separation
channels. Preferably, the flow separation channel extends through
the contactor along its longest (longitudinal) dimension. The flow
separation channel is generally linear. The flow separation channel
can have a variety of cross-sections, such as circular, triangular,
quadrilateral, pentagonal, hexagonal, heptagonal, octagonal etc.
The surface area of the wall or walls of the flow channel along its
length defines a first internal surface of the contactor.
[0036] The contactor further comprises one or more external
surfaces, such as the tubular longitudinal surface and the two
circular end surfaces of a rod-shaped contactor or the three
rectangular longitudinal surfaces and the two triangular end
surfaces of a triangular prismatic-shaped contactor. At least a
part of one or more of these external surfaces is provided,
suitably covered, with a barrier layer, such as an epoxy resin
coating, to provide one or more first external surfaces having a
barrier layer. The barrier layer is intended to provide a fluid
barrier to minimise, more preferably prevent, transfer of the heat
exchange fluid into the body of the contactor, as discussed below.
Consequently, it will be apparent that the heat exchange fluid
should only be provided to those external surfaces of the contactor
having the barrier layer.
[0037] The monolith sorption contactor formed of a unitary
construction of active carbon can be produced from, for instance, a
phenolic resin such as NOVACARB.TM. (MAST Carbon Technology,
Guildford, UK). The monoliths can be provided by controlled curing
followed by milling and classification to provide the desired
macrostructure, prior to the formation of the desired three
dimensional shape by extrusion, pressing and/or moulding, with
subsequent carbonisation and activation steps. A suitable method of
preparation is disclosed in the paper titled
"Phenolic-resin-derived activated carbons", Applied Catalysts A:
General 173 (1998), pages 289-311.
[0038] It has been found that using the surprisingly simple method
and/or apparatus discussed above, provides the highly efficient
separation of one or more hydrocarbon components, such as methane,
from the mixed stream. This provides a nitrogen-enriched stream
which can be more easily disposed of, such as by venting to the
atmosphere, without any or any significant further treatment.
[0039] The method and/or apparatus discussed above can provide, by
regenerating the monolith sorbent contactor after the sorption of
the methane and any heavier hydrocarbon components, a
methane-enriched (or nitrogen-depleted) stream for subsequent use.
The methane-enriched stream can be used more efficiently than the
original mixed stream. For example, recompression of the
methane-enriched stream, which is nitrogen-depleted and can
comprise substantially of methane and one or more other
hydrocarbons, can be carried out more efficiently with a reduction
in the nitrogen content. Any such compressed hydrocarbons can be
used as, for example a fuel or a hydrocarbon product. Alternatively
the methane-enriched stream can be liquefied to provide a liquefied
hydrocarbon stream such as liquefied natural gas (LNG).
[0040] In this way, the CAPEX and running costs of subsequently
processing the methane-enriched stream can be significantly
lowered.
[0041] Further, as a result of the simplicity and efficiency of the
method and/or apparatus disclosed herein, it or they are expected
to be very robust when compared to known line-ups.
[0042] FIG. 2 shows a first embodiment of a typical monolith
sorbent contactor 2 as depicted in FIG. 1, utilised in the method
and apparatus disclosed herein. The contactor is shown in a
longitudinal cross section. A mixed stream 40 comprising nitrogen
and methane is passed through a mixed stream pressure reducing
device 45, such as a valve as depicted and/or a hydraulic turbine,
to a monolith sorption contactor 2. The mixed stream 40 from which
the nitrogen is to be separated may be any gaseous, liquid or
partially condensed or vaporised stream, and is suitably derived
from natural gas, and is more preferably an LNG-derived stream,
suitably in the form of a stream of flash vapour.
[0043] As is known in the art, an LNG stream may have various
compositions. Usually an LNG stream to be vaporised or flashed is
comprised substantially of methane, e.g. comprising at least 60-65
mol. % methane. The flash vapour is normally enriched in components
with lower boiling temperature, and the methane content could be
between 40 and 70 mol. %, or more typically between 40 and 60 mol.
% depending on the concentration of lower boiling components such
as nitrogen.
[0044] An LNG stream may comprise varying amounts of hydrocarbons
heavier than methane, as well as other non-hydrocarbon compounds
such as nitrogen, helium and hydrogen. Any hydrocarbons heavier
than methane may be sorbed together with methane by the active
carbon sorbent.
[0045] Depending upon the source, the mixed stream 40 may also
contain varying amounts of compounds such as H.sub.2O, CO.sub.2,
H.sub.2S and other sulphur compounds, and the like. However, if the
mixed stream is a (previously) liquefied mixed stream, such as LNG,
these latter components have usually been substantially removed
because they would otherwise freeze during the liquefaction
process, causing blockages and related problems in the liquefaction
equipment. As the steps of liquefaction and removing undesired
components such as H.sub.2O, CO.sub.2, and H.sub.2S are well known
to the skilled person, they will not be further discussed here.
[0046] The active carbon of which the monolith sorption contactor 2
is made acts as the sorbent for the methane sorbate and any heavier
hydrocarbons, if present. It is preferred that the active carbon
has an affinity for the methane sorbate which is at least 5 times
that of the affinity of the active carbon for a nitrogen
sorbate.
[0047] The sorption step is carried out at a temperature of less
than or equal to -60.degree. C. Without wishing to be bound by
theory, it is believed that the sorption affinity of the active
carbon for methane and any other heavier hydrocarbon components is
optimal in a temperature range of approximately within 100.degree.
C. of the dew point of the methane component e.g. in the range of
from -165 to -60.degree. C., preferably in the range of from
-160.degree. C. to -60.degree. C.
[0048] The monolith sorption contactor 2 can be cooled to the
sorption temperature range by the mixed stream 40 itself if this is
at a suitable temperature, and/or by an external heat exchange
fluid, such as a refrigerant. As an alternative or an addition
hereto, the monolith sorption contactor 2 may be cooled by passing
the cold nitrogen-enriched stream through the separaton flow
channels 2a in the monolith. It is preferred to use as much cold
from the nitrogen-enriched stream as possible. Moreover, any
remaining methane in the nitrogen-enriched stream is also adsorbed
this way. The monoliths 2 are found to have a low pressure drop
associated with the passage of a gas flow through the separation
flow channels, such that a second pass of the nitrogen-enriched
stream through the separation flow channels is easily made. The use
of an external refrigerant as heat exchange fluid, either for the
full cooling or supplemental cooling of the monolith sorption
contactor, is discussed in greater detail in relation to FIG.
4.
[0049] In the case in which at least a part of the cooling of the
contactor 2 to the sorption temperature range is provided by the
mixed stream 40, the mixed stream may be, for instance, a partly
condensed LNG stream from a liquefaction unit, and may have at a
temperature between -165 and -140.degree. C. If the mixed stream is
used to cool the contactor 2 prior to sorption, this portion of the
stream can be recycled to the liquefaction unit for re-liquefaction
prior to being returned to the now cooled contactor 2 for
separation.
[0050] Also shown in FIG. 2 are the one or more separation flow
channels 2a, which pass through the body of the contactor 2. The
separation flow channel walls 2d are thus composed of active carbon
sorbent.
[0051] Once the contactor has been cooled to sorption temperature,
the mixed stream 40 is passed to one or more inlets 2b of the one
or more separation flow channels 2a, suitably via optional inlet
header 12. At least a part of the methane in the mixed stream will
be sorbed by the contactor 2 via the internal surfaces 2d upon
passage through the separation flow channels 2a, which internal
surfaces are formed of active carbon.
[0052] The mixed stream 40 will have a residence time in the
contactor which enables the sorption of at least a portion of the
methane component in mixed stream 40. By residence time is meant
the internal volume of the space occupied by the mixed stream
flowing through the separation flow channels divided by the average
volumetric flow rate for the mixed stream flowing through the space
at the temperature and pressure being used. The stream exiting the
one or more separation flow channels 2a, via outlets 2c and
optional outlet header 13, is a nitrogen-enriched stream 70 which
is depleted in methane and optionally heavier hydrocarbons.
[0053] In a preferred embodiment, the mixed stream 40 is provided
at a temperature at or near the sorption temperature of the
contactor 2 i.e. at a temperature less than or equal to -60.degree.
C. If the mixed stream 40 is provided at a temperature of greater
than the sorption temperature, then it will have to be pre-cooled
to the sorption temperature or the contactor 2 refrigerated to
maintain the temperature in the sorption range. The refrigeration
of the contactor 2 can be carried out by the heat exchange fluid
used to warm and cool the contactor 2, and is discussed in more
detail below.
[0054] When the contactor 2 approaches a full loading of sorbent,
such as methane and any heavier hydrocarbons present, mixed stream
pressure reducing device 45 can be closed, thereby interrupting
further flow of the mixed stream 40 into the contactor 2. Contactor
2 can then be regenerated to liberate the sorbed methane and any
heavier hydrocarbon components as methane-enriched stream 80.
[0055] After the passage of the mixed stream 40 to the contactor 2
is interrupted, but prior to regeneration, it is preferred to pass
a purging fluid stream through the one or more separation flow
channels 2a. The purging fluid stream may, for instance, be
supplied along first auxiliary line 75 to the one or more inlets 2b
of the flow channels 2a. The purging fluid can remove any residual
components of the mixed stream such as nitrogen and any unsorbed
methane from the separation flow channels 2a prior to the desorbing
of the methane and any heavier hydrocarbons. The spent purging
fluid stream can exit the flow channels 2a via outlets 2c and be
removed from the contactor 2 via a second auxiliary line (not
shown). Countercurrent purging in which the purging fluid stream is
passed from flow channel outlets 2c to flow channel inlets 2b via
the second and first auxiliary lines is also envisaged.
[0056] After the optional purging step, the contactor 2 can be
regenerated by temperature swing absorption/adsorption. The
temperature of the contactor 2 is raised above the methane sorption
range of less than or equal to -60.degree. C. to desorb methane and
any heavier hydrocarbons. The desorbed components can exit the one
or more separation flow channels 2a at outlets 2c and be removed
from the separator 2 as methane-enriched stream 80. The
methane-enriched stream may also pass through the optional outlet
header 13.
[0057] In a preferred embodiment the methane-enriched stream 80 can
be removed from the contactor 2 under reduced pressure to encourage
the desorption of the methane and any heavier hydrocarbon sorbents.
Optionally, a flushing fluid stream, such as the methane-enriched
stream itself, after optional compression, can be supplied to the
separation flow channels 2a via auxiliary incoming line 75 of the
contactor 2 to remove any residual desorbed hydrocarbons. If the
flushing fluid stream is not composed of the methane-enriched
stream, then it can be removed downstream of the outlets 2c of the
separation flow channels 2a by an auxiliary outgoing line (not
shown), in order to prevent contamination of the methane-enriched
stream 80 with the flushing fluid.
[0058] A heat exchange fluid chamber 11 may be provided surrounding
the external longitudinal surface 2e of the contactor 2, which can
be filled with a heat exchange fluid. A warm heat exchange fluid
100 may enter or leave the heat exchange fluid chamber via a warm
heat exchange fluid stream line 100, while a cool heat exchange
fluid 110 may leave respectively enter the heat exchange fluid
chamber via cool heat exchange fluid stream line 110. Preferably,
the barrier layer 2f is present everywhere on the external surface
2e that is inside the heat exchange fluid chamber 11. In one
embodiment in which the contactor 2 is rod-shaped, the heat
exchange fluid chamber 11 may be an annular chamber.
[0059] As an example, the external surface area 2e of the contactor
2 may define a tube which can be coated with the barrier layer 2f.
In this way, the external surface area 2e of the contactor 2 which
is heated by the warm heat exchange fluid stream 100 can be
maximised, while keeping the heat exchange fluid separate from the
circular ends 2g of the contactor 2 which are adjacent to the one
or more inlets 2b and one or more outlets 2c of the separation flow
channels 2a.
[0060] The temperature of the contactor 2 may be increased during
the desorbing step by contacting the contactor 2 with a heat
exchange fluid stream 100 at one or more first external surfaces 2e
having a barrier layer 2f. The heat exchange fluid is allowed
contact with the barrier layer 2f at its surface facing away from
the external surface 2e on which the barrier layer 2f is
provided.
[0061] The heat exchange fluid stream is preferably warm, i.e.
preferably having a temperature higher than that of the contactor
2. The barrier layer 2f is provided to prevent the heat exchange
fluid reaching the body of the contactor 2 and contaminating the
separation flow channels 2a. A preferred barrier layer is an epoxy
resin. Providing the warm heat exchange fluid 100 to the external
surface 2e of the contactor 2 (with the barrier layer 2f) is
advantageous because it simplifies the construction of the
contactor 2.
[0062] It is a straight-forward procedure to apply the barrier
layer 2f to an external surface 2e of the contactor 2. It is not
necessary to apply the barrier layer to all external surfaces of
the contactor 2, only those which could be in contact with the heat
exchange fluid may suffice. Thus, in the embodiment shown in FIG. 2
it is not necessary to apply a barrier layer to end external
surfaces 2g, which are adjacent to the inlet 2b and outlet 2c of
the separation flow channels 2a, because these are isolated from
the heat exchange fluid. In the embodiment shown, the barrier layer
2f need only be applied to longitudinal external surfaces 2e.
[0063] If internal heat exchange channels were to be provided
within the body of the contactor, these would have to be treated to
seal their walls against penetration of the heat exchange fluid to
prevent contamination of adjacent separation flow channels 2a. This
is a complex procedure requiring the sealing of the walls of such
heat exchange channels with a ceramic or metallic glaze.
Alternatively a pre-formed monolith having separation flow and heat
exchange channels would have to be provided in which the separation
flow channels would have to be coated with a layer of the sorbent,
again increasing the complexity of the manufacturing operation and
the cost of the completed monolith contactor. The contactor used
herein therefore provides a number of advantages in terms of
simplicity of construction and ease of use.
[0064] The warm heat exchange fluid stream 100 is provided at a
temperature above the sorption temperature of the contactor 2 i.e.
above -60.degree. C., preferably at a temperature of -50.degree. C.
or greater, even more preferably at a temperature of -40.degree. C.
or greater. For example, the warm heat exchange fluid stream 100
could be at ambient temperature or around -10.degree. C. to
0.degree. C. Under certain circumstances, temperature could also be
in the range of from -40 to -30.degree. C., e.g. in case that a
stream is used which is also used as a refrigerant stream in an LNG
production process. The temperature of the warm heat exchange fluid
stream 100 is reduced upon contact with the barrier layer of the
contactor 2 to provide a cool heat exchange fluid stream in the
form of a cooler (cooled) heat exchange fluid stream 110, while at
the same time the temperature of the contactor 2 is increased to
facilitate desorption.
[0065] It will be apparent that the cool heat exchange fluid stream
110 carries the cold energy required by the contactor 2 for the
sorption step. Thus, after regeneration of the heated contactor 2,
at least a portion, preferably all of the cool heat exchange fluid
stream 110 can be used to lower the temperature of the contactor 2
to that required for the sorption operation by reversing the flow
of the cool heat exchange fluid stream 110 to the contactor 2 (or
looping the cool heat exchange fluid back through the process via
line 100, thereby maintaining the direction of flow but using the
warm heat exchange fluid line to feed the cool heat exchange fluid
and the cool heat exchange fluid line to remove the warmed heat
exchange fluid). In this way, the cold energy required to place
contactor 2 in sorption mode can be recycled to the contactor after
each regeneration operation, increasing the efficiency of the
method and apparatus.
[0066] Thus, a sorption and regeneration cycle can be provided
utilising the heat exchange fluid to remove and return the cold
energy to the contactor. Thus, prior to the mixed stream 40 being
provided to the contactor 2 for the sorption step, the contactor 2
can be cooled by contacting at least one of the one or more first
external surfaces 2e having the barrier layer 2f with the cool heat
exchange fluid stream 110 to provide the warm heat exchange fluid
stream 100.
[0067] In a preferred embodiment, two or more contactors 2 can be
arranged in parallel, such that when one contactor 2 approaches
full loading, the mixed stream 40 can be passed to a second
unloaded contactor (not shown), so that continuous processing of
the mixed stream 40 can be achieved.
[0068] Thus, the monolith sorption contactor 2 may be part of a
contactor unit, which is any suitable device, system, or apparatus
comprising one or more monolith sorption contactors able to
selectively sorb methane and optionally any heavier hydrocarbons
from the mixed stream. The person skilled in the art will
understand that the contactor unit can have many forms, including
one or more monolith sorption contactors in series, parallel or
both.
[0069] For example, there may be at least one monolith sorption
contactor in sorbing mode and at least one monolith sorption
contactor in regeneration or desorbing mode. Depending upon the
actual requirements, there may be combinations of two, three, four
or even more monolith sorption contactors, one in sorbing mode, the
others in different stages of regenerating or desorbing mode.
[0070] Having multiple monolith sorption contactors operate in
different stages of the cycle creates possibilities of recovering
energy by transferring heat exchange fluid from one to the other.
This way, the cold vested in one of the monoliths that is brought
to regereration mode can be preserved by using it to cool down
another of the monoliths.
[0071] FIG. 3 schematically shows a process scheme for the
separation of nitrogen from a mixed stream 40 comprising nitrogen
and methane, derived from an LNG stream, whereby a methane-enriched
stream 80 having a higher heating value is obtained.
[0072] The process scheme of FIG. 3 comprises a monolith sorption
contactor 2, which can also be a contactor unit comprising one or
more monolith sorption contactors, a gas/liquid separator 3, an
expansion device 4 such as a turboexpander, a second pressure
reduction device 5 such as a Joule-Thomson valve, a liquefaction
unit 6 comprising one or more heat exchangers with associated
refrigerant circuits (not shown), a pump 7 and a liquid storage
tank 8, such as an LNG storage tank. The person skilled in the art
will understand that further elements may be present if
desired.
[0073] In operation, liquefaction unit 6 produces an at least
partially, preferably fully liquefied, hydrocarbon stream 10, such
as an LNG stream. The at least partially liquefied hydrocarbon
stream 10 is expanded in expansion device 4 to provide an expanded
hydrocarbon stream 20, and subsequently passed through pressure
reduction device 5 to provide controlled expanded hydrocarbon
stream 30, which can be a partly condensed LNG stream. Controlled
expanded hydrocarbon stream 30 is then passed to the first inlet 31
of gas/liquid separator 3, which can be an end-flash vessel.
Typically, the pressure of the controlled expanded hydrocarbon
stream 30 at inlet 31 can be between 0.5 and 10 bar, more
preferably between 1 and 5 bar, even more preferably between 1 and
2 bar. The inlet temperature to the gas/liquid separator 3 can be
between -140 and -165.degree. C. When stream 30 is a partly
condensed LNG stream, it can comprise approximately >80 mol. %
methane and >1 mol. % nitrogen.
[0074] In the gas/liquid separator 3, the controlled expanded
hydrocarbon stream 30 is separated into a gaseous overhead stream,
which is a mixed stream 40 comprising nitrogen and methane (removed
at outlet 32) and a liquid bottom stream 50 (removed at outlet
33).
[0075] The liquid bottom stream 50 is usually enriched in methane
relative to the stream 30, and comprises the majority of the
controlled expanded hydrocarbon stream 30. The liquid bottom stream
50 can be pumped as stream 60 to the liquid storage tank 8, such as
a LNG storage tank, using the pump 7. In the liquid storage tank 8
the liquid bottom stream is temporarily stored.
[0076] In the case that the process scheme of FIG. 3 is situated in
an LNG exporting terminal, the LNG stored in the tank may be
subsequently loaded into a transport vessel (not shown) before it
is transported overseas. In the case t that the process scheme of
FIG. 3 forms part of a regasification terminal (at an LNG import
location where the LNG is usually supplied by a transport vessel
rather than a liquefaction unit 6), the LNG in the tank 8 may be
subsequently passed to a vaporizer (not shown).
[0077] Due to the action of the gas/liquid separator 3, nitrogen in
the stream 30 favours passing upwardly out through the outlet 32.
Thus, the gaseous overhead stream removed at the outlet 32 of the
separator 3 is provided as a mixed stream 40 comprising nitrogen
and methane. This stream 40 is passed to the inlet 21 of the
monolith sorption contactor 2. Usually the stream 40 comprises
>15% or >25 mol. % nitrogen, such as between 30-60 mol. %
nitrogen.
[0078] During the passage of the mixed stream 40 through the
contactor 2, at least a fraction of one or more hydrocarbons, in
particular methane, present in the stream 40 is adsorbed by the
active carbon sorbent in the contactor 2, whilst at least a major
part of the nitrogen phase is passed on and removed from the
contactor 2 at outlet 22. This nitrogen enriched flow is collected
as nitrogen-enriched stream 70.
[0079] After the nitrogen-enriched stream has been collected as
stream 70 as described hereinbefore, the hydrocarbons sorbed by the
active carbon sorbent in the contactor 2 can be desorbed, thereby
regenerating the contactor 2. This is done using thermal swing
adsorption/absorption, usually involving a flushing fluid stream,
purging fluid stream, heat exchange fluid stream, etc., to remove
the desorbed hydrocarbons from the active carbon sorbent. The
desorbed hydrocarbons are removed at outlet 23 and are collected
either directly or after separation from the purging gas as a
methane-enriched stream 80, which is nitrogen depleted. Stream 80
may be used as fuel. Alternatively, stream 80 may be recombined
with the LNG stream 50, optionally after first compressing and
re-liquefying stream 80.
[0080] The person skilled in the art will understand that the
outlets 22 and 23 may be separate outlets or one and the same
outlet. Further, the person skilled in the art will understand that
instead of one contactor 2, several parallel contactors may be
used. Also, several contactors (containing different sorbent
materials, at least one of which is the monolith sorption contactor
formed of a unitary construction of active carbon described herein)
may be placed in series to enable the separation of one or more
other streams (including nitrogen).
[0081] Cold recovery from the nitrogen enriched stream 70 and/or
the nitrogen depleted stream 80 can be affected in a manner known
in the art. For instance, the nitrogen-enriched stream 70 can be
passed into a first cold recovery unit (not shown), prior to being
further treated or vented to atmosphere. Meanwhile the
methane-enriched stream 80 can be passed through a second cold
recovery unit (not shown) to provide a warmed stream, which can
then be passed through a compressor to provide a compressed
hydrocarbon stream, which could be used as fuel gas, or even
recycled into a hydrocarbon liquefaction plant (not shown).
[0082] Where the contactor 2 is placed directly after the
gas/liquid separator 3, the conditions of the mixed stream passed
to the one or more separation flow channels in the contactor 2 (for
example 1 bar and -160.degree. C.), are optimal for the thermal
swing adsorption/absorption technique for desorption. The cold
energy of the nitrogen-enriched stream 70 can be used for the
process, after which it can be vented to atmosphere. At the same
time methane will be sorbed on the active carbon.
[0083] The method and apparatus disclosed herein is further
advantageous as the re-liquefaction of the desorbed hydrocarbon(s)
such as methane requires less power than prior art processes,
because the cryogenic separation of any nitrogen therewith is no
longer needed.
[0084] In a first alternative embodiment to the arrangement shown
in FIG. 3, the contactor 2 may be located prior to the gas/liquid
separator 3 so as to separate nitrogen from the controlled expanded
hydrocarbon stream as the mixed stream, generally obtained directly
from expansion or expansions of an at least partially liquefied
hydrocarbon stream such as LNG.
[0085] FIG. 4 schematically shows a process scheme for the
separation of nitrogen from a mixed stream 40 comprising nitrogen
and methane according to a further embodiment described herein. In
a similar manner to that already discussed, a gas/liquid separator
3, such as an end-flash separator, can provide an overhead mixed
stream 40 comprising methane and nitrogen, from a suitable feed
stream, such as a partly condensed LNG stream 30.
[0086] The mixed stream 40 can be passed to monolith sorption
contactor 2 for separation of the nitrogen and methane and any
heavier hydrocarbon components into nitrogen-enriched stream 70 and
methane-enriched stream 80 as discussed in relation to FIGS. 1 and
2.
[0087] In the embodiment of FIG. 4, the heat exchange fluid which
is used to alter the temperature of the contactor 2 can be a
refrigerant provided from a refrigerant circuit, preferably the
refrigerant circuit of a cooling stage of an associated
liquefaction unit, e.g. in the case of natural gas treatment,
liquefaction unit 6 according to FIG. 3.
[0088] For instance, the heat exchange fluid can be liquid or
gaseous propane, for example from the pre-cool cycle of a
liquefaction unit or hot mixed refrigerant from a cryogenic heat
exchanger at a temperature of -30 to -40.degree. C. which used in
the liquefaction of natural gas.
[0089] FIG. 4 shows a refrigerant circuit comprising a refrigerant
compressor 9 with associated driver D1, and cooler 10, such as an
air or water cooler, which has been incorporated into the circuit
carrying the heat exchange fluid to the contactor 2.
[0090] The warm heat exchange fluid stream 100 generated by cooling
contactor 2 can be passed to the refrigerant compressor 9 or
subsequently used to heat the contactor 2 to regenerate the methane
and any heavier hydrocarbon components sorbed by the contactor
2.
[0091] If warm heat exchange fluid stream 100 is passed to
refrigerant compressor 9, it is compressed to provide compressed
heat exchange fluid stream 95. The compressed heat exchange fluid
stream 95 can then be cooled in cooler 10, to provide cool heat
exchange fluid stream 110, which can be used to cool or preferably
liquefy a natural gas stream or passed to contactor 2 to reduce the
temperature of the contactor 2 to within the sorption range,
thereby providing the warm heat exchange fluid 100.
[0092] This line-up can also be used to maintain the contactor 2 at
a temperature in the sorption range should the mixed stream be at a
higher temperature, although this embodiment is less preferred
because the cooling duty required by the contactor is then placed
on the refrigerant circuit.
[0093] Alternatively, the cool heat exchange fluid stream 110
produced by heating contactor 2 to desorption temperature can be
used in the cooling of a natural gas stream in a liquefaction
process, or stored to cool the contactor 2 to sorption temperature
upon completion of the regeneration operation.
[0094] Preferably, the desorbed methane-enriched stream is cooled
and liquefied. There are various options to achieve such
re-liquefaction, e.g. by recycling into the original feed stream to
a liquefaction system such as liquefaction unit 6 of FIG. 3.
Preferably, the methane-enriched stream from one monolith sorption
contactor that in regeneration mode is liquefied using the cool
heat exchange fluid stream originating from a the same and/or a
parallel arranged monolith sorption contactor that is being heated
by the heat exchange fluid. This cool refrigerant stream may first
be expanded prior to heat exchanging it with the methane-enriched
stream to remove heat from the methane-enriched stream at a lower
pressure level.
[0095] In a further alternative embodiment of the arrangement shown
in the accompanying Figures, the contactor 2 may be located in the
path of any gaseous mixed stream comprising hydrocarbons including
methane with a high concentration of nitrogen, including such a
stream at a high pressure (for example <70 bar).
[0096] The person skilled in the art will understand that the
present invention can be carried out in many various ways without
departing from the scope of the appended claims.
* * * * *