EP2507567B1

EP2507567B1 - Process and apparatus for separation of nitrogen from lng

Info

Publication number: EP2507567B1
Application number: EP10787542.9A
Authority: EP
Inventors: Adrian Finn; Grant Johnson; Terence Tomlinson
Original assignee: Costain Oil Gas and Process Ltd
Current assignee: Costain Oil Gas and Process Ltd
Priority date: 2009-11-30
Filing date: 2010-11-30
Publication date: 2020-06-24
Anticipated expiration: 2030-11-30
Also published as: AU2016244250A1; AU2010322827A1; US20120285196A1; AU2018204827A1; WO2011064605A3; WO2011064605A2; GB2462555A; GB0920947D0; EP2507567A2; GB2462555B

Description

This invention relates to processes and apparatus for the separation of nitrogen from liquefied natural gas (LNG) - mixtures comprising nitrogen gas and low-boiling hydrocarbons, such as methane, ethane, propane and butane, according to the preamble of claims 1 and 13 respectively. Such processes respectively apparatus are known e.g. from US-A- 2004 0231359 .
Nitrogen is found in many natural gas reservoirs, sometimes at relatively high levels, for example greater than around 5 mol%, which can necessitate removal to meet specifications for use as fuel, but often at lower levels not requiring removal. As high quality gas fields are depleted, natural gas increasingly needs to be sourced from lower quality gas fields, containing higher levels of contaminants such as nitrogen.
Many natural gas reservoirs are not sufficiently close to gas consumers to make pipeline transportation economical and infrastructure has grown worldwide for the transportation of gas in liquefied form as LNG. The presence of greater than about 1 mol% nitrogen in LNG, can lead to auto-stratification and rollover in storage tanks, which presents a significant safety concern, and there is therefore a need for efficient techniques for the separation of nitrogen from LNG, even for relatively low nitrogen levels.
For relatively low nitrogen levels of approximately 1 to 2 mol%, nitrogen removal from LNG can be achieved by the separation of the nitrogen rich vapour, also referred to as "flash gas", which is evolved when the pressure of sub-cooled LNG is reduced to the LNG storage tank pressure - which is typically just above atmospheric pressure. For feed gas nitrogen levels of greater than about 2 mol% of nitrogen, a fractionation column is typically employed to achieve separation of nitrogen from the LNG, while avoiding excessive flash gas flow rates.
Fractionation systems used for the separation of nitrogen from liquefied natural gas typically incorporate a reboiler to produce stripping vapour, required to reduce the nitrogen level in the LNG product to less than 1 mol%.
US 2004/231359 A1 relates to a method for the rejection of nitrogen from condensed natural gas which comprises (a) introducing the condensed natural gas into a distillation column at a first location therein, withdrawing a nitrogen-enriched overhead vapor stream from the distillation column, and withdrawing a purified liquefied natural gas stream from the bottom of the column; (b) introducing a cold reflux stream into the distillation column at a second location above the first location, wherein the refrigeration to provide the cold reflux stream is obtained by compressing and work expanding a refrigerant stream comprising nitrogen; and (c) either (1) cooling the purified liquefied natural gas stream or cooling the condensed natural gas stream or (2) cooling both the purified liquefied natural gas stream and the condensed natural gas stream, wherein refrigeration for (1) or (2) is obtained by compressing and work expanding the refrigerant stream comprising nitrogen. The refrigerant stream may comprise all or a portion of the nitrogen-rich vapor stream from the distillation column.
US 2009/277217 A1 relates to an LNG facility employing an enhanced nitrogen removal system that concentrates the amount of nitrogen in the feed stream to a nitrogen removal unit (NRU) to thereby increase the separation efficiency of the NRU. In one example, the nitrogen removal system comprises a multistage separation vessel operable to separate nitrogen from a cooled natural gas stream. At least a portion of the resulting nitrogen-containing stream exiting the multistage separation vessel can be used as a refrigerant, processed to a nitrogen removal unit, and/or utilized as fuel gas for the LNG facility.
EP 0725256 A1 relates to a process in which nitrogen is removed from a natural gas feed stream by a cryogenic distillation process in which the feed stream is separated in a distillation column; a methane-rich bottoms liquid is recovered as a methane-rich product, preferably after being pumped to increase its pressure; a nitrogen-rich overhead vapour is warmed in heat exchange with intermediate vapour stream(s) to at least partially condense said stream(s) for return to the distillation column to provide reflux; and a portion of the warmed nitrogen-rich overhead vapour is utilized as a recycle nitrogen-rich heat pump stream above the critical pressure of nitrogen to provide at least part of the reboil to the distillation column and to produce a mixed vapour-liquid stream, which is returned to the distillation column to provide reflux.
An example of a conventional apparatus for separation of nitrogen from LNG is shown in Figure 1.
A nitrogen-containing LNG feed stream (101) already sub-cooled at elevated pressure is further cooled in a reboil heat exchange system (102). The resultant stream (103) is expanded in a hydraulic expansion turbine (104) to give a two-phase stream (105), which is fed to a fractionation column (106).
Liquid (140) from the bottom tray (or packed section) of the fractionation column is partially vaporised in the reboil heat exchange system (102), to produce stripping vapour (141) which is fed to the fractionation column, and thereby also providing refrigeration to further sub-cool the feed stream (101).
A LNG stream (107) having low nitrogen content is withdrawn from the bottom of the fractionation column, and is reduced in pressure across a valve (108) to give a two-phase stream (109). The two-phase stream (109) is then passed to a vapour-liquid separator (111) to separate a flash gas stream (110) and a low pressure LNG stream (112) for storage. The flash gas stream (110) is passed to a compressor (134), and a resulting compressed stream (135) is cooled by a heat exchanger (136) to give a fuel gas stream (137).
An overhead vapour stream (113) rich in nitrogen, but with significant methane content, is withdrawn from the top of the fractionation column (106).
While low temperature fractionation processes, such as that shown in Figure 1, allow a LNG product having a low nitrogen content to be obtained, the nitrogen vapour overhead from the fractionation column generally comprises significant amounts of methane as the column incorporates no rectification section. The methane-containing overhead vapour is typically used as fuel gas for power generation or to drive compression equipment. However, restrictions exist as to the nitrogen content of fuel gas which may be used in gas turbines, particularly those derived from aero engines, which can typically burn gases comprising up to 10 mol%, or up to 15 mol% nitrogen, and sometimes as high as 20 mol% nitrogen.
Alternatively, the methane-containing overhead vapour from the fractionation column is sometimes used as part of a refrigeration cycle in processes that require methane as a refrigerant. It would be preferable if the methane content of the fractionation overhead vapour could be substantially eliminated. There is therefore a need in the art for efficient separation processes that are able to separate mixtures of nitrogen and LNG to form a natural gas product that is low in nitrogen, and preferably substantially free of nitrogen, and also a nitrogen product that is low in hydrocarbons, and preferably substantially free of hydrocarbons.
One solution to the issue of high methane content of the overhead vapour from the fractionation column would be to feed the overhead vapour to a separate nitrogen rejection unit that is able to produce a nitrogen stream with low methane content suitable for venting to the atmosphere and a methane rich stream suitable for use as fuel gas. In addition to compression and heat exchange equipment, this would require additional separation equipment including one or more vapour/liquid separators and fractionation columns.
The process and apparatus of the present invention avoids the necessity for a nitrogen rejection unit by producing an overhead vapour stream from the LNG fractionation that has a suitable composition (i.e. substantially devoid of hydrocarbons) for venting to the atmosphere.
By recycling a portion of the nitrogen-containing overhead vapour stream from the fractionation column it has surprisingly been found that an improvement in separation may be obtained. More specifically, the recycled portion may be used as a nitrogen-rich reflux stream, which nitrogen-rich reflux stream may be efficiently cooled by heat exchange with one or more streams withdrawn from the fractionation, particularly against evaporating methane rich liquid streams. By avoiding a separate nitrogen rejection unit, thermodynamic losses are reduced and process efficiency is increased, leading to greater LNG production with lower power consumption, as well as improved separation in the fractionation system. The process of this invention also avoids the operating complexity of a separate nitrogen rejection unit and is robust to changes in feed composition.
In accordance with the present invention, there is provided a process for the separation of nitrogen from a liquid feed comprising liquefied natural gas and nitrogen, the process comprising the steps of:

(i) cooling the feed and passing the feed to a fractionation column;
(ii) withdrawing from the fractionation column an overhead vapour stream having an enriched nitrogen content, and a liquid stream having a reduced nitrogen content;
(iii) dividing the overhead vapour stream from step (ii) into at least first and second overhead streams;
(iv) compressing, cooling and at least partially condensing at least the first overhead stream from step (iii); and
(v) expanding the stream from step (iv) and passing the expanded stream to the fractionation column as reflux,

By recycling a portion of the overhead vapour stream to the fractionation as reflux, the process of the present invention adds a rectification section to the fractionation column, which enables an overhead stream to be obtained from the fractionation column that is substantially devoid of hydrocarbons. For example, the process of the present invention is capable of producing an overhead stream from the fractionation column that comprises less than 2 mol% methane, less than 1 mol% methane, less than 0.5 mol% methane, and potentially as low as 0.1 mol% methane.
Furthermore, by cooling and at least partially condensing the compressed first overhead stream in heat exchange with one or more streams from the fractionation column, the heat integration of the process is improved, thereby reducing energy demands.
The proportion of the overhead vapour stream from step (ii) that is recycled to the column as reflux in steps (iii) to (v) is preferably in the range of from 20 to 80 mol% of the total overhead vapour stream, more preferably 30 to 70 mol%, and most preferably 40 to 60 mol% of the total overhead vapour stream from the fractionation column. However, the exact amount of reflux depends on the nitrogen content of the feed and overhead stream purity. An advantage of the present invention is that liquid feeds comprising various quantities of nitrogen can be processed while maintaining methane content of the overhead stream simply by varying the proportion of the overhead vapour stream that is recycled to the column as reflux.
As used herein, the expression "one or more streams from the fractionation column" refers to any liquid or gas from the fractionation column that can be used as a source of refrigeration to cool a compressed overhead stream from step (iv). Thus, the expression may refer to overhead vapour withdrawn from the top of the fractionation column. The expression may also refer to the liquid stream withdrawn from the bottom of the fractionation column. The expression may further refer to a side stream obtained from an intermediate stage of the column. Still further, the expression may refer to liquid and/or vapour within the column where one or more heat exchange steps takes place within the column.
The LNG feed may consist of, or consist substantially of, methane. The feed may also comprise small amounts of other hydrocarbons such as, for example, ethane, propane, butane and/or heavier hydrocarbons. The hydrocarbons in the LNG feed usually comprise greater than 80 mol% methane, more typically greater than 85 mol% methane, and potentially up to near 100% methane. The balance of the LNG feed may comprise ethane, propane, butane and/or heavier hydrocarbons. Preferably the total content of ethane and/or propane and/or heavier hydrocarbons in the LNG feed is less than 20 mol%, more preferably less than 10 mol%, and most preferably less than 5 mol%. The total content of propane and/or heavier hydrocarbons in the LNG feed is preferably less than 10 mol%, more preferably less than 5 mol%, and most preferably less than 2 mol%. The total content of hydrocarbons heavier than propane in the LNG feed is preferably less than 5 mol%, more preferably less than 2 mol% and most preferably less than 1 mol%.
The process of the present invention may be used in particular for the separation of nitrogen from LNG feeds that comprise up to 40 mol% nitrogen. For instance, the feed may comprise up to 30 mol% nitrogen, up to 20 mol% nitrogen or up to 15 mol% nitrogen. Preferably, the feed comprises at least 1 mol% nitrogen, for example at least 2 mol% nitrogen, possibly 5 mol% nitrogen, or at least 10 mol% nitrogen.
The present invention is particularly applicable to the separation of nitrogen from feeds that comprise or consist of liquefied natural gas.
The fractionation column is typically operated in a pressure range of from 100 to 500 kPa, more preferably 100 to 300 kPa, and most preferably 120 to 200 kPa. For example, suitable operating pressures for the fractionation column include 130 kPa, 140 kPa, 150 kPa, 160 kPa, 170 kPa, 180 kPa and 190 kPa. The operating temperature of the fractionation column is dependent on the operating pressure, but the overhead temperature is generally in the range -175 °C to -190 °C and the bottom liquid temperature is generally in the range -135 °C to -160°C.
It will be appreciated that, as used herein, expressions such as "an overhead vapour stream having an enriched nitrogen content" or "a liquid stream having a reduced nitrogen content" are intended to refer to the relative nitrogen content of the respective stream when compared with the nitrogen content of the feed. Thus, an overhead vapour stream having an enriched nitrogen content is one that comprises a higher mole fraction of nitrogen than that of the feed. Similarly, a liquid stream having reduced nitrogen content is one that comprises a lower mole fraction of nitrogen than that of the feed. Generally, the feed is supplied to the fractionation column at or around the operating temperature and pressure of the column. Suitable operating temperatures and pressures for the fractionation column are discussed above. In most cases, the LNG feed will be supplied at a pressure significantly higher than the operating pressure of the fractionation column. For example, a liquefied natural gas feed would typically have a pressure in the range of from 3,000 to 10,000 kPa. In most cases it is therefore necessary to expand the cooled feed to column pressure. For example, the feed may be expanded to column pressure across an expansion valve or by way of an expansion turbine. Expansion turbines have the benefit of extracting work from the process under near-isentropic conditions and reducing the amount of upstream cooling of the feed LNG that is necessary, and thus providing a reduction in energy requirements in upstream liquefaction plant.
The cooled and expanded feed is preferably supplied to the fractionation column as a two-phase vapour-liquid mixture. More preferably, the two-phase mixture has a vapour fraction of from 1 to 40 mol%, more preferably 2 to 20 mol%, and most preferably from 3 to 10 mol%, for example 5 to 8 mol%.
In one embodiment, cooling in step (iv) is provided, at least in part, by heat exchange with at least a portion of the overhead vapour stream withdrawn from the fractionation column.
Cooling in step (iv) is provided, at least in part, by heat exchange with at least a portion of the liquid product stream withdrawn from the fractionation column. The portion of the liquid product stream passed in heat exchange with the compressed first overhead vapour stream in step (iv) is expanded first, in order to provide further cooling.
Heat to the fractionation column is preferably provided, at least in part by a reboiler. Heated vapour from the reboiler is fed back to the fractionation column to strip nitrogen from the descending LNG in the column.
Where the fractionation column comprises a reboiler, cooling in step (iv) may be provided, at least in part, by heat exchange with a stream from the fractionation column in the reboiler.
The type of reboiler that may be used is not limited, and a person of skill in the art can select suitable reboiler systems. For example, thermosyphon, forced-circulation, and kettle reboilers may be used in the process of the invention. The fractionation column may comprise an internal reboiler, located within the column, or an external reboiler located outside the fractionation column.
Where an internal reboiler is used, the reboiler is preferably immersed in boiling liquid at the bottom of the fractionation column. Where an external reboiler is used, a liquid stream is withdrawn from the column and fed to the reboiler to produce a heated vapour stream, which is returned to the column as stripping vapour.
In preferred embodiments, the bulk of the condensing duty required to cool and at least partially condense the compressed overhead stream from step (iv) is provided by heat exchange with one or more evaporating methane-rich liquids, which may be selected from the liquid product stream withdrawn from the fractionation column and/or boiling liquid at the bottom of the fractionation column and/or a liquid side-stream that is withdrawn from the column and fed to a column-side reboiler.
In another embodiment, further cooling of the first overhead stream from step (iii) may be provided by heat exchange with the LNG feed.
In another embodiment, heat exchange with a liquid stream from the fractionation column in the reboiler may be used to provide cooling of the feed in step (i).
The reflux feed is fed to the top stage of the fractionation column in step (v) to provide rectification of the column vapour, and thereby improve separation, reducing methane content of the column overhead vapour. The reflux stream is expanded before being fed to the fractionation column, for example using an expansion valve or an expansion turbine.
The liquid stream withdrawn from the fractionation column in step (ii) is expanded to form a two-phase vapour-liquid stream. As noted above, this expanded stream, or a portion thereof, is used to provide cooling in step (iv). The two-phase stream may then be passed to a vapour-liquid separator to separate a low pressure liquid hydrocarbon stream substantially free of nitrogen, and a hydrocarbon vapour stream. Preferably, the hydrocarbon vapour stream contains less than about 15 mol% nitrogen, and more preferably less than about 10 mol% nitrogen. The hydrocarbon vapour stream may advantageously be compressed and cooled to form a fuel gas product. Alternatively, this stream may be used as part of a refrigeration cycle.
In accordance with another aspect of the present invention, there is provided an apparatus for the separation of nitrogen from a feed comprising liquefied natural gas and nitrogen, the apparatus comprising:

(i) means for cooling and expanding the feed;
(ii) a fractionation column for producing an overhead vapour stream and a bottom liquid stream;
(iii) means for conveying the cooled and expanded feed from step (i) to the fractionation column;
(iv) means for dividing the overhead vapour stream from the fractionation column into at least first and second overhead streams;
(v) means for compressing at least the first overhead stream;
(vi) one or more heat exchangers for cooling and at least partially condensing the compressed stream from step (v) in heat exchange with one or more streams from the fractionation column;
(vii) means for conveying at least one stream from step (v) to the one or more heat exchangers; and
(viii) means for conveying the compressed, cooled and at least partially condensed first overhead stream from step (vi) to an expanding means and from the expanding means to the fractionation column as reflux;

In a preferred embodiment, the apparatus of the invention comprises a heat exchanger for cooling and condensing at least one stream from step (v) in heat exchange with at least a portion of the overhead vapour stream withdrawn from the column.
The fractionation column may comprise one or more reboilers, which may be internal or external to the column. The type of reboiler that may be used is not particularly limited, and thermosiphon, forced-circulation, and kettle reboilers are examples of reboiler types that are compatible with the apparatus of the invention.
Where the fractionation column comprises a reboiler, the reboiler may be a heat exchanger as specified in step (vi) which is operable to provide cooling and condensing to the at least one stream from step (v) via heat exchange with a stream from the fractionation column.
In a further embodiment, the reboiler is operable to provide cooling to the feed via heat exchange with a stream from the fractionation column.
The apparatus of the invention comprises means for expanding the bottom liquid stream from step (ii), at least a portion of which is passed to a heat exchanger in step (vi) as described above, and preferably comprises a vapour-liquid separator, and means for conveying the expanded stream to the vapour-liquid separator to separate a liquid stream and a vapour stream. The apparatus may further comprise means for compressing and cooling a vapour stream withdrawn from the vapour-liquid separator.
Suitable means for expanding the stream from step (vi) and the bottom liquid stream from step (ii) include expansion valves and expansion turbines.
Suitable operating parameters for the process of the present invention are disclosed in detail above. It is to be understood that the apparatus of the invention is operable in accordance with parameters discussed above in connection with the process of the invention. Furthermore, preferred operating parameters for the process of the invention are also preferred operating parameters for the apparatus of the invention.
The invention will now be described in greater detail with reference to preferred embodiments and with the aid of the accompanying figures, in which:

Figure 1 shows a conventional fractionation apparatus for the separation of nitrogen from a gaseous mixture comprising nitrogen gas and hydrocarbons, as described above;
Figure 2 shows a fractionation apparatus in accordance with an example; and
Figure 3 shows another embodiment of a fractionation apparatus in accordance with the present invention.

In the example shown in Figure 2, a sub-cooled liquid feed stream (201) comprising LNG and nitrogen is further cooled in reboil heat exchange system (202), to give stream (203). Stream (203) is expanded in hydraulic expansion turbine (204) to give a two-phase stream (205) which is fed to the fractionation column (206).
A liquid stream (240) is removed from the bottom tray (or packed section) of the fractionation column below the two-phase feed stream (205) and is partially vaporised to produce stripping vapour (241) in reboil heat exchange system (202), providing refrigeration to further sub-cool the feed stream (201) and condense nitrogen rich reflux stream (223).
Nitrogen rich overhead vapour (213) is removed from the fractionation column (206) and passed to a heat exchange system (214) where it is warmed to a suitable temperature (215) for atmospheric venting (217) downstream of a pressure control valve (216).
A portion (218) of the nitrogen rich overhead vapour (213) from the fractionation column is compressed in a compression system (219) to give a compressed stream (220). Compression system (219) includes inter-stage cooling which is not shown. The compressed stream (220) is then cooled (typically in heat exchange against air or water) in a cooler (221) to give a high pressure nitrogen rich stream (222). The high pressure nitrogen rich stream (222) is further cooled in heat exchange systems (214) and (202) to give a liquid stream (224) which is further sub-cooled in the heat exchange system (214) to provide a sub-cooled liquid stream (225).
The sub-cooled liquid stream (225) is let down to fractionation column pressure across a valve (226) to give a two phase stream (227) which is supplied as reflux to the fractionation column (206).
A LNG stream (207), with low nitrogen content, is removed from the fractionation column (206), and is let down to storage pressure across a valve (208) to give a two-phase stream (209). The two-phase stream (209) is then passed to a vapour-liquid separator (211) and a flash gas stream (210) and a low pressure LNG product stream (212) for storage are obtained. The flash gas stream (210) is compressed using a compressor (234), giving a stream (235), which is cooled (typically in heat exchange against air or water) in a cooler (236) to give a fuel gas stream (237).
In this example, the necessary refrigeration to cool and sub-cool the nitrogen rich reflux stream (222) to form the stream (225) is provided by heat exchange with the overhead vapour (213) from the fractionation column (206) and the liquid stream (240) that is fed to the reboil heat exchange system (202).
The embodiment of the invention shown in Figure 3 differs from Figure 2 by way of the heat exchange systems that are employed to provide the reflux stream to the fractionation column (306). Thus, a sub-cooled liquid feed stream (301) comprising LNG and nitrogen is further cooled in a reboil heat exchange system (302), to give a stream (303). Stream (303) is expanded in hydraulic expansion turbine (304) to give a two-phase stream (305) which is fed to the fractionation column (306).
A liquid stream (340) is removed from the bottom tray (or packed section) of the fractionation column (306) below the two-phase feed stream (305) and is partially vaporised to produce stripping vapour (341) in the reboil heat exchange system (302), which provides refrigeration to further sub-cool the feed stream (301).
Nitrogen rich overhead vapour (313) from the fractionation column (306) passes to a heat exchange system (314) where it is warmed to a suitable temperature (315) for atmospheric venting (317) downstream of pressure control valve (316).
A portion (318) of the nitrogen rich overhead vapour (313) from the fractionation column is compressed in a compression system (319) to give a compressed stream (320). The compression system (319) includes inter-stage cooling which is not shown. The compressed stream (320) is then cooled (typically in heat exchange against air or water) in a cooler (321) to give a high pressure nitrogen rich stream (322). The high pressure nitrogen rich stream (322) is further cooled and sub-cooled in the heat exchange system (314) to provide a sub-cooled liquid stream (325).
The sub-cooled liquid stream (325) is let down to fractionation column pressure across a valve (326) to give a two phase stream (327) which is supplied as reflux to the fractionation column (306).
A LNG stream (307), with low nitrogen content, is removed from the fractionation column (306). A portion (328) of the stream (307) is let down to just above storage pressure across a valve (330) to produce a two-phase stream (331), which is vaporised to provide refrigeration in the heat exchange system (314). A remaining portion (329) of the stream (307) is let down to storage pressure across a valve (308) to give a two-phase stream (333). The stream (332) resulting from vaporisation of the stream (331) in the heat exchange system (314) is combined with the stream (333) to give a two-phase stream (309). The two-phase stream (309) is then passed to a vapour-liquid separator (311) to separate a flash gas stream (310) and a low pressure LNG product stream (312) for storage. The flash gas stream (310) is compressed by means of a compressor (334), giving a stream (335), which is cooled (typically in heat exchange against air or water) in a cooler (336) to give a fuel gas stream (337).

Examples

Example 1 (comparative Example)

Table 1 shows operating data for the separation of nitrogen from a LNG feed comprising 10 mol% nitrogen, 85 mol% methane, 4 mol% ethane and 1 mol% propane, at a mass flow rate of 200,000 kg/h, according to the prior art separation system described in Figure 1. Reference is made to the vapour fraction, temperature, pressure, mass flow, and molar composition of specific numbered streams (numbering of streams as in Figure 1).
It will be noted that the prior art process produces an overhead vapour stream (113) from the fractionation column that comprises a significant amount of methane (37 mol%), and which requires further processing to separate the remaining nitrogen. This is in contrast with the process of the present invention in which the overhead vapour stream (113) is substantially free of methane (see below).

Example 2

Table 2 shows corresponding operating data for the separation of nitrogen from the LNG feed used in Example 1, at a mass flow rate of 200,000 kg/h, according to an example as described in Figure 2.
It will be noted that the example shown in Figure 2 gives rise to a nitrogen rich overhead vapour stream (213) which comprises 99.5 mol% of nitrogen, and only 0.5 mol% methane (in comparison with 63 mol% nitrogen and 37 mol% methane in the overhead stream (213) in Example 1). This is reflected in the amount of hydrocarbon product (212) obtained, which represents 87.1 mol% of the total feed, compared with only 81.9 mol% in Example 1.
The improved separation obtained by the process of Figure 2 can be attributed to the provision of the low temperature nitrogen rich reflux stream (227), which is obtained at - 191.6 °C and supplied to the column at a temperature 25 °C below the feed (205) to the column. The rectification of column vapour provided by this nitrogen rich reflux stream allows the temperature differential between the overhead vapour stream (213) and the liquid hydrocarbon stream (207) to be increased to 35.3 °C, and the temperature differential between the nitrogen rich overhead vapour stream (213) and the feed (205) to the column to be increased to 23.3 °C, reflective of the increased purity of the nitrogen rich overhead vapour stream (213). In Example 1, in contrast, where no nitrogen rich reflux stream is provided, the temperature differential between the column overhead vapour stream (213) and the liquid hydrocarbon stream (207) is only 10.3 °C, and the temperature differential between the column overhead vapour stream (213) and the feed (205) to the column is negligible at 0.3 °C, reflective of much poorer separation.

Example 3

Table 3 shows corresponding corresponding operating data for the separation of nitrogen from the LNG feed used in Example 1, at a mass flow rate of 200,000 kg/h, according to the process of the invention as described in Figure 3.
The process of the present invention shown in Figure 3 also gives rise to an nitrogen rich overhead vapour stream (313) which comprises 99.5 mol% of nitrogen, and only 0.5 mol% methane (in comparison with 63 mol% nitrogen and 37 mol% methane in the overhead stream (313) in Example 1).
As in Example 2, the improved separation obtained by the process is attributable to rectification of the column vapour by the low temperature nitrogen rich reflux stream (327), which is obtained at -191.6 °C and supplied to the column at a temperature 24.2 °C below the feed (205) to the column. This in turn, means that the temperature differential between the nitrogen rich overhead vapour stream (313) and the liquid hydrocarbon stream (307) is increased to 33.6 °C, and the temperature differential between the nitrogen rich overhead vapour stream (313) and the feed (205) to the column is increased to 23.5 °C, reflecting the increased purity of the nitrogen rich overhead vapour stream.
In this Example, refrigeration to cool the reflux stream (322) is provided by expanding a portion (328) of the liquid hydrocarbon stream (307). As a result, the flash gas flow is higher in this embodiment at 13265 kg/h compared with 6729 kg/h in Example 2, but as the full reboil duty in exchanger (202) is used to sub-cool the feed LNG, the feed temperature from the liquefaction process can be higher, reducing load on the upstream liquefaction refrigeration system.
It will additionally be understood that the LNG feed used in the process of the present invention may undergo additional separation and/or conditioning. Examples of such additional processes include one or more of the following:

Separation of vapour formed on expansion of the LNG feed stream. The separated vapour may, in a preferred example, then be introduced as a separate feed to the fractionation column, and more preferably to the fractionation column above the main feed;
Heating a portion of the LNG feed stream and introducing it as a separate feed to the fractionation column, preferably above the main feed. The portion of the stream is preferably heated by way of a well integrated heat exchange operation;
Cooling a portion of the LNG feed stream and introducing it as a separate feed to the fractionation column, preferably above the main feed. The portion of the stream is preferably heated by way of a well integrated heat exchange operation.

It will also be understood that the fractionation column, as described in the process of the present invention, may additionally comprise or incorporate a side condenser system. In a preferred example, a vapour side draw is taken from an intermediate point, preferably above the main LNG feed to the fractionation column. In the same way that nitrogen rich reflux is generated from the overhead vapour in the embodiments of the invention described above, the vapour side draw is warmed in a heat exchange operation; compressed and cooled; condensed primarily against an evaporating liquid stream rich in methane and sub-cooled against cold vapour in a heat exchange operation; expanded to the fractionation column pressure; and returned to the fractionation column as an intermediate two phase feed.
It is believed that the incorporation of a side condenser system reduces the required overhead reflux flow. As the vapour side draw comprises a mixture of methane and nitrogen, it can be condensed against methane rich liquid streams at a lower pressure than can the overhead stream from the fractionation column, which is purer in nitrogen. As a large proportion of the liquid required for rectification can be met by the side condenser system, compression power requirements overall can be reduced.

Claims

A process for the separation of nitrogen from a liquefied natural gas feed (201, 301), the process comprising the steps of:
(i) cooling the feed (201, 301) and passing the feed to a fractionation column (206, 306);

(ii) withdrawing from the fractionation column (206, 306) an overhead vapour stream (213, 313) having an enriched nitrogen content, and a liquid stream (207, 307) having a reduced nitrogen content;

(iii) dividing the overhead vapour stream (213, 313) from step (ii) into at least first (218, 318) and second (215, 315) overhead streams;

(iv) compressing, cooling and at least partially condensing at least the first overhead stream (218, 318) from step (iii); and

(v) expanding the stream (225, 325) from step (iv) and passing the expanded stream (227, 327) to the fractionation column (206, 306) as reflux,
wherein cooling in step (iv) is provided, at least in part, by heat exchange with at least a portion of the liquid stream (207, 307) withdrawn from the fractionation column (206, 306) and characterised in that at least a portion of the liquid stream (207, 307) withdrawn from the fractionation column (206, 306) is expanded before being passed in heat exchange with the compressed first overhead vapour stream (222, 322).
A process according to any of the preceding claims, wherein the liquefied natural gas feed (201, 301) comprises from 1 mol% to 40 mol% nitrogen.
A process according to any of the preceding claims, wherein cooling in step (iv) is provided, at least in part, by heat exchange with at least a portion of the overhead vapour stream (213, 313) withdrawn from the fractionation column (206, 306).
A process according to any of the preceding claims, wherein heat to the fractionation column (206, 306) is provided, at least in part, by a reboiler (202, 302).
A process according to Claim 4, wherein cooling in step (iv) is provided, at least in part, by heat exchange with a liquid stream (240, 340) from the fractionation column (206, 306) in the reboiler (202, 302).
A process according to Claim 4 or Claim 5, wherein the reboiler (202, 302) is an internal reboiler located within the fractionation column (206, 306).
A process according to Claim 4 or Claim 5, wherein the reboiler (202, 302) is external to the column (206, 306).
A process according to Claim 7, wherein the feed to the reboiler (202, 302) is a stream (240, 340) withdrawn from the bottom of the fractionation column (206, 306).
A process according to any of the preceding claims, wherein further cooling of the first overhead stream (213, 313) from step (iii) is provided by heat exchange in parallel with the liquefied natural gas feed (201, 301).
A process according to any of Claims 5 to 9, wherein cooling in step (i) is provided, at least in part, by heat exchange with a liquid stream (240, 340) from the fractionation column (206, 306) in the reboiler (202, 302).
A process according to any of the preceding claims, wherein the cooled feed (201, 301) in step (i) is expanded to form a two-phase feed to the fractionation column (206, 306).
A process according to anyone of the preceding claims, wherein the reflux stream (227, 327) of step (v) is at a temperature of from 5 to 50°C below the temperature of the feed to the column (206, 306) in step (i).
An apparatus for the separation of nitrogen from a feed (201, 301) comprising liquefied natural gas and nitrogen, the apparatus comprising:
(i) means for cooling (202, 302) and expanding (204, 304) the feed (201, 301);

(ii) a fractionation column (206, 306) for producing an overhead vapour stream (213, 313) and a bottom liquid stream (207, 307);

(iii) means for conveying the cooled and expanded feed from step (i) to the fractionation column (206, 306);

(iv) means for dividing the overhead vapour stream (213, 313) from the fractionation column (206, 306) into at least first (218, 318) and second (215, 315) overhead streams;

(v) means for compressing (219, 319) at least the first overhead stream (218, 318);

(vi) one or more heat exchangers (214, 202, 314) for cooling and at least partially condensing the compressed stream (222, 322) from step (v) in heat exchange with one or more streams (213, 240, 313) from the fractionation column (206, 306);

(vii) means for conveying at least one stream (213, 240, 313) from step (v) to the one or more heat exchangers (214, 202, 314); and

(viii) means for conveying the compressed, cooled and at least partially condensed first overhead stream (225, 325) from step (vi) to an expanding means (226, 326) and from the expanding means (226, 326) to the fractionation column (206, 306) as reflux;
wherein the one or more heat exchangers (214, 202, 314) for cooling and at least partially condensing the compressed stream (222, 322) from step (v) comprises a heat exchanger (314) for heat exchange with at least a portion of the bottom liquid stream (307) withdrawn from the fractionation column (206, 306); and characterised in that the apparatus further comprises an expander (330) to expand the bottom liquid stream (307) withdrawn from the column (206, 306), or a portion thereof, before heat exchange with the compressed stream (222, 322) from step (v).