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.

Description

The present invention provides a method for separating nitrogen from a mixed stream comprising nitrogen and methane and an apparatus therefore.

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.

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.

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.

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.

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° 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° 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.

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:

• • a source of a mixed stream comprising methane and nitrogen at a temperature of less than or equal to −60° 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.

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:

FIG. 1 shows a schematic view of a monolith sorbent contactor;

FIG. 2 shows an embodiment of an exemplary application of the monolith sorbent contactor in a method according to an embodiment the invention;

FIG. 3 shows an embodiment of a typical process scheme according to an embodiment the invention; and

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.

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.

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.

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.

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.

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.

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.

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.

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.

Hence, the method may further comprise, optionally prior to step (c), a step of:

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.

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.

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.

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.

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.

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 m²/g as determined by a BET surface area method known in the art, such as N₂ adsorption at liquid nitrogen temperature using multipoint pressures of 0.08, 0.14 and 0.20 P/P₀ (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).

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.

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.

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.

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.

The monolith sorption contactor formed of a unitary construction of active carbon can be produced from, for instance, a phenolic resin such as NOVACARB™ (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.

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.

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).

In this way, the CAPEX and running costs of subsequently processing the methane-enriched stream can be significantly lowered.

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.

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.

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.

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.

Depending upon the source, the mixed stream 40 may also contain varying amounts of compounds such as H₂O, CO₂, H₂S 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 H₂O, CO₂, and H₂S are well known to the skilled person, they will not be further discussed here.

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.

The sorption step is carried out at a temperature of less than or equal to −60° 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° C. of the dew point of the methane component e.g. in the range of from −165 to −60° C., preferably in the range of from −160° C. to −60° C.

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.

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° 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.

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.

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.

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.

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° 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.

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.

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.

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° 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.

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.

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.

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.

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.

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.

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.

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.

The warm heat exchange fluid stream 100 is provided at a temperature above the sorption temperature of the contactor 2 i.e. above −60° C., preferably at a temperature of −50° C. or greater, even more preferably at a temperature of −40° C. or greater. For example, the warm heat exchange fluid stream 100 could be at ambient temperature or around −10° C. to 0° C. Under certain circumstances, temperature could also be in the range of from −40 to −30° 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.

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.

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.

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.

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.

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.

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.

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.

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.

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° C. When stream 30 is a partly condensed LNG stream, it can comprise approximately >80 mol. % methane and >1 mol. % nitrogen.

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).

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.

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).

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.

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.

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.

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).

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).

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° 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.

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.

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.

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.

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.

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.

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° C. which used in the liquefaction of natural gas.

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.

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.

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.

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.

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.

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.

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).

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.

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° 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° 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° 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° 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° 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° 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° 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.