Patent Application
20070245771
The present invention relates to the removal of nitrogen from liquefied natural gas (LNG) streams. It has particular, but not exclusive, application to the use of only part of the nitrogen content in fuel gas whilst venting the remaining nitrogen content to atmosphere. There is provided a method in which the nitrogen is removed in two stages at different concentrations and corresponding apparatus for natural gas liquefaction to provide a nitrogen-freed LNG product.
Gas turbines are usually used to provide the shaft work and electrical power for LNG facilities. Fuel for these gas turbines is often generated as off-gasses from the LNG process. In a conventional LNG process, nitrogen present in the feed gas is normally rejected into this fuel gas stream. However, more environmentally friendly low nitrogen oxide (NOX) burners for these turbines have a lower tolerance for nitrogen in the fuel gas than previously used burners. Accordingly, in some plant locations with high nitrogen containing feed gas, more nitrogen will be rejected from the LNG process than can be accepted by the gas turbine fuel system.
There have been a number of prior art proposals for removing nitrogen from LNG as relatively high concentration streams by fractionation in which a refrigeration or heat pump stream is used to condense overhead vapour from the fractionation column or provide reflux to the column.
EP-A-0090469 (published 5th Oct. 1983; corresponding to U.S. Pat. No. 4,415,345, issued 15th Nov. 1983) discloses a process in which nitrogen is removed from a gaseous natural gas feed by cooling and fractionating at low pressure using an open-loop nitrogen heat pump to generate liquid reflux for the fractionation. In single column embodiments, only a vapour fraction from partially condensed natural gas feed is subjected to fractionation. Reboil for the fractionation column is provided by condensing the open-loop nitrogen refrigerant and reflux for the column is provided by the condensed nitrogen refrigerant. In exemplified double column embodiments, the higher pressure column is reboiled against partially condensed natural gas feed and the open-loop nitrogen heat pump receives nitrogen from both columns and provides reboil duty to the lower pressure column and reflux to both columns. The purified LNG is warmed against natural gas feed and recovered as vapour. No LNG end product is produced in the process.
EP-A-0131 128 (published 16th Jan. 1985; corresponding to U.S. Pat. No. 4,504,295, issued 12th Mar. 1985) discloses separating a natural gas stream into a nitrogen stream and a methane stream by fractionation of a partially condensed natural gas fraction using a closed cycle heat pump loop to provide reboil and reflux heat exchange duty. No LNG end product is produced in this process.
WO-A-93/08436 (published 29th Apr. 1993; corresponding to U.S. Pat. No. 5,421,165, issued 6th Jun. 1995) discloses removal of nitrogen from an LNG stream by a process in which the LNG is cooled and expanded both dynamically and statically before fractionation. The cooling is at least partially conducted by heat exchange with a reboiling stream withdrawn from an intermediate location of the column and returned to a level below that intermediate location. The overhead vapour from the fractionation column can be compressed and used as fuel gas. Optionally a portion of the compressed overhead vapour is partially condensed against overhead vapour leaving the column, reduced in pressure and fed to the column as reflux. A portion of the condensed overhead vapour can be fractionated in an auxiliary column to provide high purity nitrogen overhead vapour and bottoms liquid, which is reduced in pressure and combined with the remaining portion prior to feeding to the fractionation column. The auxiliary column bottoms liquid can be used to provide condensation duty at the top of the auxiliary column.
EP-A-0725256 (published 7 Aug. 1996) discloses a process in which a gaseous natural gas feed is cooled and fractionated to remove nitrogen. Reboil vapour for the fractionation column is provided by cooling an open-loop nitrogen gas refrigerant in the column reboiler. Reflux for the top of the column is provided by work expanding the cooled nitrogen refrigerant gas to provide a small amount (4-5%) of liquid. At least one intermediate vapour stream from the column is partially condensed against an overhead nitrogen vapour stream and returned to the column as intermediate reflux, which is the bulk of the reflux to the column. The natural gas is pumped to a higher pressure prior to warming and is recovered as a vapour product. No LNG end product is produced in the process.
GB-A-2298034 (published 21 Aug. 1996; corresponding to U.S. Pat. No. 5,617,741, issued 8th Apr. 1997) discloses a process for removing nitrogen from a natural gas feed stream using a dual column cryogenic distillation system having a primary column and a secondary column fed from and operating at substantially the same pressure as the primary column. At least a portion of a bottoms liquid from the primary column is expanded and at least partially vaporized in heat exchange with a nitrogen-enriched vapour from the column to provide an at least partially condensed nitrogen-enriched stream that is returned to the primary column to provide higher temperature reflux. Bottoms liquid from the secondary column is at least partially vaporized in heat exchange with an overhead vapour from one of the columns to provide an at least partially condensed stream that is returned to the primary or secondary column to provide lower temperature reflux. Reboil to the columns is provided by heat exchange with natural gas feed. No LNG end product is produced in this process.
WO-A-0023164 (published 27th Apr. 2000; corresponding to U.S. Pat. No. 6,199,403, issued 13th Mar. 2001) discloses a process in which a natural gas stream is liquefied, expanded and then separated in a phase separator, which can be a nitrogen-rejection column. Reflux for the column can be provided by condensing a portion of the overhead vapour using a refrigeration system. The refrigeration system can comprise a closed-loop refrigeration system; an open-loop refrigeration system; and/or indirect heat exchange with a product stream. Some of the heat exchanger duty to condense the overhead vapour can be provided by a bottoms liquid stream withdrawn from and returned to the column. The separated LNG product liquid is pumped to a higher pressure and warmed.
U.S. Pat. No. 6,070,429 (issued 6 Jun. 2000; corresponding to WO-A-0058674, published 5 Oct. 2000) discloses a process in which a pressurized gas stream obtained from a pressurized LNG-bearing stream is separated in a cascade of 3 stripping columns at successively lower pressures to produce, from the third stripping column, a nitrogen-rich gas stream and a methane-rich liquid stream, which latter stream is suitable for recycle to an open methane cycle liquefaction process and/or use as a fuel gas. In each stripping column, a liquid bearing stream obtained by partial condensation of a first portion of a gas stream is contacted in countercurrent with a second portion of the respective gas stream to provide an overhead vapour and bottoms liquid. The overhead vapours of the first and second stripping columns provide the feed streams for the second and third stripping columns respectively. Condensation duty for the feed streams to the second and third stripping columns is provided by the overhead vapour and bottoms liquid from the third stripper. In exemplified embodiments, bottoms liquid from the second stripping column is fed to the third stripping column and the bottoms liquid from the first stripping column can be used to provide heat exchange duty to provide the partially condensed feed portion to the first stripping column.
U.S. Pat. No. 6,449,984 (issued 17 Sep. 2002; corresponding to WO-A-03004951, published 8th Jan. 2003) discloses a process in which a natural gas stream is liquefied and then fractionated to provide a nitrogen-enriched overhead vapour and LNG bottoms liquid. Reflux for the fractionation column is provided by condensing a portion of the overhead vapour. In the exemplified embodiments, the condensing duty is provided by a refrigerant stream and is integrated with a final LNG subcooling heat exchanger. Also in these embodiments, liquid is withdrawn from an intermediate location of the fractionation column, warmed against the liquefied gas stream feed to the column and returned to the column at a lower location.
WO-A-02088612 (published 7 Nov. 2002) discloses a process for removing nitrogen from a hydrocarbon-rich stream, especially natural gas, during liquefaction in which the partially condensed stream is fed to a double column nitrogen-rejection system. The higher pressure column provides a nitrogen-rich overhead vapour that is condensed against overhead vapour from the lower pressure column and fed as reflux to the lower pressure column. Bottoms liquid from the higher pressure column is cooled and fed to the lower pressure column, from which liquefied product is withdrawn as bottoms liquid. The higher pressure column is reboiled with heat duty provided by-the partially condensed feed to the higher pressure column.
US-A-2004231359 (published 25th Nov. 2004; corresponding to WO2004104143, published 2nd Dec. 2004) discloses a process in which a natural gas stream is liquefied and then fractionated in a distillation column to remove nitrogen as an overhead vapour product and purified LNG as bottoms liquid. Reflux for the column is provided by a condensed nitrogen stream. Refrigeration to provide the reflux stream and cooling the purified LNG stream and/or the liquefied natural gas feed is obtained by compressing and work expanding a refrigerant stream comprising nitrogen that may comprise all or a portion of the overhead vapour from the distillation column. In the exemplified embodiments, heat exchange duty for reboil to the fractionation column is provided by the liquefied natural gas feed to the column.
It is an object of the present invention to provide for reject of part of the nitrogen from any LNG process with minimal additional equipment and minimum impact on plant performance. This can be achieved by the invention without any changes to the configuration of the heat transfer equipment for the production of LNG and with limited additional equipment. In particular, the invention avoids the necessity of an additional heat pump compressor and permits end product LNG to be used to operate a nitrogen separation column condenser.
In a first and broadest aspect, the invention provides a method of removing nitrogen from a liquefied natural gas feed comprising subjecting the liquefied natural gas to a first fractionation to provide a first nitrogen-enriched overhead vapour stream and a nitrogen-containing bottoms liquid stream and subjecting at least a portion of said bottoms liquid stream to a second fractionation to provide a second nitrogen-enriched overhead vapour stream that is of lower purity than said first overhead vapour stream and a purified liquefied natural gas stream.
The first nitrogen-enriched-overhead vapour stream can have a nitrogen concentration in excess of 80 mol %, preferably in excess of 90 mol % and more preferably in excess of 95 mol %.
Usually, at least a portion of the first nitrogen-enriched overhead vapour stream is vented to atmosphere and the second nitrogen-enriched overhead vapour stream is used as, or added to, a fuel gas, especially for a gas turbine providing work for use in connection with liquefaction of the natural gas feed.
Preferably, the first fractionation is conducted in a distillation column refluxed with a condensed portion of the first nitrogen-enriched overhead vapour. Suitably, heat exchange duty for the condensation is provided by a sub-cooled liquefied natural gas stream comprising or derived from at least a portion of the nitrogen-containing bottoms liquid stream. The sub-cooled liquefied natural gas stream can be all or a portion of the nitrogen-containing bottoms liquid stream after sub-cooling and pressure reduction. The distillation column can be reboiled by heat exchange duty provided by the liquefied natural gas feed.
It is preferred also that the second fractionation is conducted in a flash drum. When the first fractionation is conducted in a distillation column, that column usually will be refluxed with all or a portion of the first nitrogen-enriched overhead vapour condensed in a condenser located in the flash drum. When only a portion of the nitrogen-containing bottoms liquid stream is required for condensation duty, the remainder can be fed to a second flash drum for separation into a third nitrogen-enriched overhead vapour stream that is of lower purity than said first overhead vapour stream and a second purified liquefied natural gas stream. Usually, said third nitrogen-enriched overhead vapour stream will be combined with the second nitrogen-enriched overhead vapour stream and said second purified liquefied natural gas stream will be combined with the purified liquefied natural gas stream from the second fractionation.
If the liquefied natural gas feed stream contains helium, a helium-rich stream can be separated from a stream comprising or derived from the first nitrogen-enriched overhead vapour stream by, for example, partial condensation and separation to provide a helium-enriched vapour and a nitrogen-enriched liquid. The heat exchange duty for said partial condensation can be provided by the separated helium-enriched vapour and/or nitrogen-enriched liquid.
In a second aspect, the invention provides a method of preparing a nitrogen-freed liquefied natural gas stream comprising liquefying a nitrogen-containing natural gas to provide a nitrogen-containing liquefied natural gas stream and subjecting said liquefied gas stream to nitrogen removal in accordance with the first aspect supra.
The invention also provides an apparatus for preparing a nitrogen-freed liquefied natural gas stream by a process of said second aspect, said apparatus comprising:
a refrigeration system for liquefying the nitrogen-containing natural gas feed;
a first fractionator;
a second fractionator;
a conduit for feeding nitrogen-containing liquefied natural gas from the refrigeration system to the first fractionator;
a conduit to remove the first nitrogen-enriched overhead vapour stream from the first fractionator;
a conduit for conveying the nitrogen-containing bottoms liquid stream from the first fractionator to the second fractionator;
a conduit for removing the second nitrogen-enriched overhead vapour stream from the second fractionator; and
a conduit for removing the purified liquefied natural gas stream from the second fractionator.
In accordance with a preferred embodiment of the present invention, natural gas which has been liquefied at pressure but not yet fully cooled to its storage conditions is let down to an intermediate pressure and fed into a first nitrogen-separation column. The flashing of the LNG stream into this column results in the bottoms liquid having reduced nitrogen content. The quantity of this reduction is as desired by the objective of reducing the nitrogen content of the final fuel gas. LNG withdrawn from the bottom of this column is further cooled to the temperature required by the end flash system to produce LNG of the final desired nitrogen content and fuel gas of the required heating value. This finally cooled LNG is sent to an end flash drum. The end flash drum contains a heat exchanger which is used to condense the nitrogen-separation column overhead vapour stream and provide reflux to this column. The overhead vapour of this column is a nitrogen stream which can be vented directly to atmosphere.
The overhead vapour condenser to the column may be integrated into the end flash drum of the process in which case all product LNG passes through this drum. Optionally only a portion of the LNG product may pass through this drum.
The nitrogen-separation column can have a reboiler which is reboiled by the LNG feed to the column before it is let down in pressure, optionally via a fluid expander.
The nitrogen product from the top of the column can be expanded and have refrigeration recovered from it into a stream being cooled or liquefied in the LNG process.
The invention is particularly useful for LNG plants which use spiral wound heat transfer equipment for LNG liquefaction. It requires only withdrawing the nitrogen-containing LNG after the liquefaction section and returning it at lower pressure and nitrogen depleted into the subcooling section and access end product LNG for refrigeration. For C3MR processes, this can be achieved simply by withdrawing and returning LNG between penultimate and ultimate refrigeration stages and using rundown LNG. Similarly for AP-X™, LNG can be withdrawn and returned between the Main Cryogenic Heat Exchanger and the subcooler and using rundown LNG.
Almost any portion of the nitrogen contained in the feed gas can be rejected as a pure nitrogen stream by this invention.
The following is a description by way of example only and with reference to the accompanying drawings of presently preferred embodiments of the invention.
In the drawings:
The exemplified embodiments of the invention can be applied to any LNG liquefaction process in which there is a liquefaction section followed by a subcooling section. For example, it can be applied to double or dual mixed refrigerant (DMR) and hybrid C3MR pre-cooling and liquefaction with nitrogen expander cycle LNG subcooling (AP-X™) processes as well as the illustrated C3MR process. The LNG is extracted between liquefaction and subcooling sections, fed to a nitrogen-separation column where nitrogen is rejected ‘pure’. The LNG is returned to the subcooling section after which some of the cold in the product LNG is used to operate the nitrogen-separation column condenser
Referring to
The mixed refrigerant is compressed in one or more compressors 28,30. The compressed mixed refrigerant is first cooled against a cooling medium in cooler 31 and then further cooled and partially condensed against a first level pre-cooling refrigerant in coolers 32-35. Partially condensed refrigerant is separated in separator 37 and both vapour and liquid fractions supplied to the liquefaction heat exchanger 16.
After pressure reduction, the stream 41 is separated in nitrogen-rejection column 23 to provide bottoms liquid. 19 and overhead vapour 46. The bottoms liquid 19 has reduced nitrogen content compared with the feed 41 to the column 23 and is further cooled in a second part of the heat exchanger 16 against a mixed refrigerant to a temperature at which it will remain substantially liquid when lowered to the pressure desired for the LNG product. The cold LNG stream 20 is reduced in pressure across an expansion valve 21 and the low pressure stream 42 is passed into flash drum 25 in which it is partially vaporized to provide a liquid product LNG fraction 50 and a vapour fuel fraction 36. Heat exchange duty in the flash drum 25 is provided by a heat exchanger 24 in which a portion 43 of the overhead vapour stream 46 from the nitrogen-rejection column 23 is condensed. The remainder 26 of the overhead vapour stream 46, which is relatively high purity nitrogen, is vented to atmosphere. Condensed nitrogen 44 from the heat exchanger 24 is returned to the nitrogen-rejection column 23 as reflux 45. Optionally a liquid nitrogen stream 22 can be withdrawn from the condensed stream 44 leaving condenser 24.
The embodiment of
The reboiler 47 is located at the bottom of column 23 to increase the quantity of nitrogen rejected by that column. The cooled high pressure feed gas 17 from the first section of heat exchanger 16 is used to provide heat duty in reboiler 47 and the resultant stream 48 leaving the reboiler 47 is expanded in the expansion turbine 49 prior to passing into column 23.
Refrigeration can be recovered from either or both of the overheads vapours 26 & 36 from column 23 and flash drum 25. This can be done by passing the relevant stream(s) to a heat exchanger 57 and, if required expanding the warmed overhead vapour 58 from the nitrogen-rejection column in a turboexpander 59. The stream 61 cooled by the refrigeration recovered in the heat exchanger 57 can be a sidestream of feed gas or circulating refrigerant.
The embodiment of
The embodiment of
A further embodiment of this invention relates to the recovery of an enriched crude helium stream from the overhead vapour 46 of the nitrogen-rejection column 23. The discharged portion 26 of the overhead vapour 46 in the embodiment of, for example,
This Example is based on the embodiment of
This Example is based on the embodiment of
It will be appreciated that the invention is not restricted to the details disclosed above in respect of the exemplified embodiments and that numerous modifications and variations can be made without departing from the scope of the invention as defined in the following claims.
