EP1766311A1 - Treating liquefied natural gas - Google Patents

Treating liquefied natural gas

Info

Publication number
EP1766311A1
EP1766311A1 EP05761177A EP05761177A EP1766311A1 EP 1766311 A1 EP1766311 A1 EP 1766311A1 EP 05761177 A EP05761177 A EP 05761177A EP 05761177 A EP05761177 A EP 05761177A EP 1766311 A1 EP1766311 A1 EP 1766311A1
Authority
EP
European Patent Office
Prior art keywords
stream
column
liquid
components
low boiling
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP05761177A
Other languages
German (de)
French (fr)
Inventor
Cornelis Buijs
Robert Klein Nagelvoort
Johan Jan Barend Pek
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Shell Internationale Research Maatschappij BV
Original Assignee
Shell Internationale Research Maatschappij BV
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Shell Internationale Research Maatschappij BV filed Critical Shell Internationale Research Maatschappij BV
Priority to EP05761177A priority Critical patent/EP1766311A1/en
Publication of EP1766311A1 publication Critical patent/EP1766311A1/en
Withdrawn legal-status Critical Current

Links

Classifications

    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0257—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of nitrogen
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
    • F25J3/0209—Natural gas or substitute natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
    • F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
    • F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
    • F25J3/0233—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 1 carbon atom or more
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/02—Processes or apparatus using separation by rectification in a single pressure main column system
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/40—Features relating to the provision of boil-up in the bottom of a column
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/76—Refluxing the column with condensed overhead gas being cycled in a quasi-closed loop refrigeration cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2200/00—Processes or apparatus using separation by rectification
    • F25J2200/78—Refluxing the column with a liquid stream originating from an upstream or downstream fractionator column
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2205/00—Processes or apparatus using other separation and/or other processing means
    • F25J2205/02—Processes or apparatus using other separation and/or other processing means using simple phase separation in a vessel or drum
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2215/00—Processes characterised by the type or other details of the product stream
    • F25J2215/04—Recovery of liquid products
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2230/00—Processes or apparatus involving steps for increasing the pressure of gaseous process streams
    • F25J2230/60—Processes or apparatus involving steps for increasing the pressure of gaseous process streams the fluid being hydrocarbons or a mixture of hydrocarbons
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2235/00—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams
    • F25J2235/60—Processes or apparatus involving steps for increasing the pressure or for conveying of liquid process streams the fluid being (a mixture of) hydrocarbons
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2240/00—Processes or apparatus involving steps for expanding of process streams
    • F25J2240/30—Dynamic liquid or hydraulic expansion with extraction of work, e.g. single phase or two-phase turbine
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2245/00—Processes or apparatus involving steps for recycling of process streams
    • F25J2245/90—Processes or apparatus involving steps for recycling of process streams the recycled stream being boil-off gas from storage
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/02—Internal refrigeration with liquid vaporising loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/04—Internal refrigeration with work-producing gas expansion loop
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2270/00—Refrigeration techniques used
    • F25J2270/42—Quasi-closed internal or closed external nitrogen refrigeration cycle
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/40—Vertical layout or arrangement of cold equipments within in the cold box, e.g. columns, condensers, heat exchangers etc.
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J2290/00—Other details not covered by groups F25J2200/00 - F25J2280/00
    • F25J2290/62—Details of storing a fluid in a tank

Definitions

  • the present invention relates to treating liquefied natural gas, and in particular treating liquefied natural gas that contains components having boiling points lower than methane.
  • An example of such a component is nitrogen.
  • ⁇ low boiling point components' and ⁇ components having low boiling points' will be used to refer to components having boiling points lower than methane.
  • the treatment is directed to removing low boiling point components from the liquefied natural gas in order to obtain a liquefied natural gas having a reduced content of components having low boiling points.
  • the improved method can be applied in two ways: (1) to treat the same amount of liquefied natural gas as in a conventional method, or (2) to treat a larger amount of liquefied natural gas as in a conventional method.
  • the content of low boiling point components in the liquefied gas treated with the method of the present invention is lower than that in liquefied gas treated with a conventional method.
  • the second way the content of low boiling point components is maintained and the amount of liquefied gas is increased.
  • US-A-6 199 403 discloses a method to remove a high- volatility component such as nitrogen from a feed stream rich in methane. According to US-A-6 199 403 the expanded liquefied natural gas stream enters a separation column at an intermediate level, i.e. not below a single gas- liquid contacting section.
  • US-A-5 421 165 relates to a process for denitrogenation of a feedstock of a liquefied mixture of hydrocarbons. To this end US-A-5 421 165 suggests a relatively complicated process using a denitrogenation column comprising a plurality of theoretical fractionation stages.
  • WO 02/50483 discloses several methods of removing components having low boiling points from liquefied natural gas. According to WO 02/50483 a liquid product stream having a reduced content of components having low boiling points is obtained.
  • a problem of the above processes described in WO 02/50483 is that the liquid product stream contains an undesirable high content of components having low boiling points. It is an object of the present invention to minimize the above problem.
  • a method of treating liquefied natural gas supplied at liquefaction pressure containing components having low boiling points to obtain a liquid product stream having a reduced content of components having low boiling points comprises the steps of: (a) allowing the liquefied gas to expand to separation pressure to obtain an expanded two-phase fluid; (b) introducing the expanded two-phase fluid into a column, the column comprising a single gas-liquid contacting section, wherein the expanded two-phase fluid is introduced below the single gas-liquid contacting section arranged in the column;
  • step (f) heating the gaseous stream obtained in step (c) in a heat exchanger to obtain a heated gaseous stream;
  • step (g) compressing the heated gaseous stream obtained in step (f) to fuel gas pressure to obtain fuel gas;
  • step (h) separating a recycle stream from the fuel gas obtained in step (g) ;
  • step (i) at least partly condensing the recycle stream obtained in step (h) to obtain a reflux stream;
  • step (j) introducing the reflux stream obtained in step (i) at separation pressure into the column above the single contacting section.
  • liquid product stream obtained according to the present invention contains a smaller content of components having low boiling points than one would expect.
  • the reduction of the amount of components having low boiling points in the liquid product stream may be achieved in a more cost-effective manner. Further it has been found that the process according to the present invention is especially suitable for liquefied natural gas streams (to be supplied at liquefaction pressure) comprising less than 7 mol% of components having low boiling points.
  • Figure 1 shows schematically a process flow scheme of an embodiment of the method of the present invention.
  • FIG 2 shows schematically an alternative of the process of Figure 1
  • Figure 3 shows schematically a process flow scheme of an other embodiment of the method of the present invention.
  • Figure 4 shows schematically an alternative of the process of Figure 3.
  • Figure 5 shows schematically and not to scale an alternative to part V of the process flow scheme of Figure 4.
  • Liquefied natural gas containing components having low boiling points is supplied at liquefaction pressure through conduit 1 to an expansion device in the form of expansion engine 3 and Joule-Thompson valve 5 in the discharge conduit 6 of expansion engine 3.
  • the expansion device the liquefied gas is allowed to expand to separation pressure, and an expanded two-phase fluid is obtained.
  • the liquefaction pressure is suitably in the range of from 3 to 8.5 MPa and the separation pressure is suitably in the range of from 0.1 to 0.5 MPa.
  • the expanded two-phase fluid is passed through conduit 9 to a column 10.
  • the expanded two-phase fluid is introduced into the column 10 at separation pressure via a suitable inlet device, such as vane inlet device 12.
  • the vane inlet device also known as schoepentoeter, allows efficient separation of gas and liquid.
  • the column 10 is provided with only a single gas- liquid contacting section 14.
  • the single contacting section 14 may comprise any suitable means for contacting a gas and a liquid, such as trays and packings.
  • the single contacting section 14 consists of from two to eight horizontal contacting trays 15 or a packing having a packed section with a length being equivalent to from two to eight contacting trays.
  • the expanded two-phase fluid is introduced into the column 10 below the single gas-liquid contact section 14.
  • liquid from the two-phase fluid is collected, and a liquid stream having a reduced content of components having low boiling points is removed from the bottom 16 through conduit 17 and pumped by pump 18 to a storage tank 20.
  • a liquid product stream is removed through conduit 21 and a gaseous stream through conduit 22.
  • the gaseous stream is also known as boil-off gas.
  • a gaseous stream that is enriched in components having low boiling points is removed through conduit 25.
  • the gaseous stream is heated in a heat exchanger 27 to obtain a heated gaseous stream that is passed through conduit 28 to a compressor 30.
  • compressor 30 the heated gaseous stream is compressed to fuel gas pressure to obtain fuel gas.
  • the fuel gas is removed through conduit 31 and cooled in heat exchanger 32 to remove the heat of compression.
  • the fuel gas is passed away through conduit 33.
  • the fuel gas pressure is in the range of from 1 to 3.5 MPa.
  • the recycle stream is at least partly condensed to obtain a reflux stream, which is passed to the column 10 through the conduit 34b provided with Joule-Thompson valve 37.
  • the reflux stream is introduced at separation pressure into the column 10 via inlet device, such as vane inlet device 39 above the single contacting section 14.
  • Table 1 summarizes the result of a hypothetical example, wherein the method of Figure 1 is compared to a base case.
  • the recycle stream and the feed are introduced into the column at the same level, so that the liquid phases of the two streams are mixed before introduction thereof in the column and the column has no contacting section. It was found that the liquid stream withdrawn through conduit 17 for the base case contains more nitrogen than the same stream for the present invention.
  • Table 1 shows that a lower nitrogen content in the product stream is obtained with the method of the present invention.
  • the recycle stream separated from the fuel gas is additionally compressed in an auxiliary compressor to an elevated pressure before it is at least partly condensed in heat exchanger 27.
  • the high-pressure recycle stream can be used in several ways, which will be discussed with reference to Figure 2. The parts that were already discussed with reference to Figure 1 have got the same reference numerals.
  • the auxiliary compressor included in conduit 34a is referred to with reference numeral 35.
  • the auxiliary compressor 35 can be provided with a cooler (not shown) to remove the heat of compression for the compressed recycle stream.
  • the compressed recycle stream is at least partly condensed by cooling it in heat exchanger 27. Part of the cold that is needed is provided by the gaseous stream that is enriched in components having low boiling points that is passed through conduit 25. The remainder is provided by the recycle stream.
  • Cold from the recycle stream can be obtained by expanding a part of the recycle stream to an intermediate pressure in Joule-Thompson valve 38, using the expanded fluid to cool the recycle stream in conduit 34a and supplying the expanded fluid through conduit 38a to the compressor 30.
  • the intermediate pressure to which the part of the recycle stream is expanded is in the range of from the suction pressure to the discharge pressure of the compressor 30 (ends of the range included) .
  • the stage at which the expanded recycle stream enters the compressor 30 is so selected that the pressure of the expanded recycle stream matches the pressure of the fluid in the compressor 30 in that stage.
  • the remainder of the recycle stream is expanded by the Joule-Thompson valve 37 and introduced as reflux in the column 10 as discussed with reference to Figure 1.
  • An advantage of the embodiment discussed with reference to Figure 2 is that the recycle stream is expanded from a larger pressure and thus cooled to a lower temperature. This allows a warmer feed stream, for example a feed stream at -142 0 C, compared to a feed stream temperature of -145 0 C (in the above example) . Thus the temperature of the liquefied gas from the main cryogenic heat exchanger can be higher and therefore, for the same amount of energy, more gas can be liquefied.
  • the elevated pressure of the fluid discharged from the auxiliary compressor 35 is so selected that the costs of the power required to drive the auxiliary compressor 35 are less than the value of the increased amount of gas that is liquefied.
  • the expansion is done in the expansion valves 37 and 38.
  • the expansion of the recycle stream can be done in two stages, at first in an expansion device, such as expander 36 and subsequently in the Joule-Thompson valves 37 and 38.
  • the expanded fluid can be supplied to an inlet (not shown) of the compressor 35.
  • this step comprises collecting in the bottom of the column liquid from the two-phase fluid and withdrawing from the bottom of the column a liquid stream having a reduced content of components having low boiling points; introducing the liquid stream into a flash vessel at a low pressure; removing a second gaseous stream from the top of the flash vessel; and removing from the bottom of the flash vessel a liquid stream to obtain the liquid product stream.
  • the column 10' comprises an upper part 1Ou and a lower part 101, wherein the upper part performs the function of the column 10 in Figure 1 and the lower part 101 is a flash vessel operating at a pressure that is below the pressure in the upper part 1Ou.
  • the pressure in the upper part 1Ou is in the range of from 0.2 to 0.5 MPa and the pressure in the flash vessel 101 in the range of from 0.1 to 0.2 MPa.
  • liquid from the two-phase fluid supplied through conduit 9 is collected in the bottom 16' of the upper part 1Ou of the column 10' . From that bottom 16' is withdrawn a liquid stream having a reduced content of components having low boiling points through conduit 17' . This stream is then introduced into the flash vessel 101 at a low pressure.
  • the pressure reduction is achieved by means of Joule-Thompson valve 40 in conduit 17' . Consequently a two-phase mixture is formed and that is introduced via inlet device 41 into the flash vessel 101.
  • a second gaseous stream is removed from the top 23" of the flash vessel 101 .
  • the second gaseous stream is passed through conduit 42 to heat exchanger 27, in which the second gaseous stream is heated by heat exchange with the recycle stream supplied through conduit 34a.
  • the heated stream is compressed in compressor 45, the heat of compression is removed in heat exchanger 48 and passed through conduit 49 to add the compressed second gaseous stream to the recycle stream in conduit 34a.
  • compressors 45 and 30 can be combined into one compressor (not shown) .
  • conduit 42 is connected to the suction end of that compressor, conduit 28 to an intermediate inlet and conduit 32 is connected to the discharge end of that compressor.
  • the embodiment discussed with reference to Figure 3 can as well be provided with an auxiliary compressor to compress the recycle stream separated from the fuel gas to an elevated pressure before it is at least partly condensed in heat exchanger 27.
  • the high- pressure recycle stream can be used in several ways, which will be discussed with reference to Figure 4. The parts that were already discussed with reference to Figure 3 have got the same reference numerals.
  • the auxiliary compressor included in conduit 34a is referred to with reference numeral 35.
  • the auxiliary compressor 35 can be provided with a cooler (not shown) to remove the heat of compression for the compressed recycle stream.
  • the compressed recycle stream is partially condensed by cooling it in heat exchanger 27. Part of the cold that is needed is provided by the gaseous stream that is enriched in components having low boiling points that is passed through conduit 25. The remainder is provided by the recycle stream.
  • Cold from the recycle stream can be obtained by expanding a part of the recycle stream to an intermediate pressure in Joule- Thompson valve 38, using the expanded fluid to cool the recycle stream in conduit 34a and supplying the expanded fluid through conduit 38a to the compressor 30.
  • the intermediate pressure to which the part of the recycle stream is expanded is in the range of from the suction pressure to the discharge pressure of the compressor 30 (ends of the range included) .
  • the point at which the expanded recycle stream enters the compressor 30 is so selected that the pressure of the expanded recycle stream matches the pressure of the fluid in the compressor 30 at the inlet point.
  • the remainder of the recycle stream is expanded by the Joule-Thompson valve 37 and introduced as reflux in the column 10 as discussed with reference to Figure 1.
  • An advantage of this embodiment is that the recycle stream is expanded from a larger pressure and thus cooled to a lower temperature. This allows a warmer feed stream, for example a feed stream at -142 0 C, compared to a feed stream temperature of -145 0 C (in the above example) . Thus the temperature of the liquefied gas from the main cryogenic heat exchanger can be higher and therefore, for the same amount of energy, more gas can be liquefied.
  • the elevated pressure of the fluid discharged from the auxiliary compressor 35 is so selected that the costs of the power required to drive the auxiliary compressor 35 are less than the value of the increased amount of gas that is liquefied.
  • the expansion is done in the expansion valves 37 and 38.
  • the expansion of the recycle stream can be done in two stages, at first in an expansion device, such as expander 36 and subsequently in the Joule-Thompson valves 37 and 38.
  • FIG 4 also shows that the boil-off gas from the storage tank 20 is provided via the conduit 22 to the suction end of the compressor 45.
  • compressors 45 and 30 can be combined into one compressor (not shown) .
  • conduit 42 into which conduit 22 opens
  • conduit 28 to an intermediate inlet
  • conduit 32 is connected to the discharge end of that compressor.
  • the expanded fluid can be supplied to an inlet (not shown) of the compressor 35.
  • An alternative of the embodiment shown in Figure 4 is shown in Figure 5, wherein a part of the recycle stream that is passed through conduit 34a is separated therefrom and passed through conduit 50 through the heat exchanger 27. Then the cooled recycle stream is expanded to the intermediate pressure in expander 51 and used to cool the recycle stream in conduit 34a. The expanded stream is then introduced into the compressor 30 at an intermediate stage.
  • the recycle stream passed through conduit 34a is between 10 and 90% by mass of the fuel gas that is passed through conduit 31.
  • the single contacting section 14 contains trays, however, any other contacting means such as packing can be employed as well.
  • the length of the packed section should is then preferably equivalent to from two to eight contacting trays.
  • the method of the present invention provides a simple way of reducing the amount of components having low boiling points in a liquefied natural gas stream.

Abstract

Method of treating liquefied natural gas (1) to obtain a liquid stream (21) having a reduced content of components having low boiling points comprising expanding (3) the liquefied gas to expand to obtain expanded two-phase fluid; introducing the two-phase fluid into a column (10) below a single gas-liquid contacting section (14); withdrawing from the bottom (16) a liquid stream (17) having a reduced content of components having low boiling points; withdrawing from the top (23) of the column (10) a gaseous stream (25) enriched in components having low boiling points; heating the gaseous stream in a heat exchanger (27); compressing (30) the stream to fuel gas pressure to obtain fuel gas (33); separating a recycle stream (34a) from the fuel gas; at least partly condensing (27) the recycle stream to obtain a reflux steam (34b); and introducing the reflux stream (34b) into the column (10) above the single contacting section (14).

Description

TREATING LIQUEFIED NATURAL GAS
The present invention relates to treating liquefied natural gas, and in particular treating liquefied natural gas that contains components having boiling points lower than methane. An example of such a component is nitrogen. In the specification and in the claims the expressions
Λlow boiling point components' and ^components having low boiling points' will be used to refer to components having boiling points lower than methane. The treatment is directed to removing low boiling point components from the liquefied natural gas in order to obtain a liquefied natural gas having a reduced content of components having low boiling points. The improved method can be applied in two ways: (1) to treat the same amount of liquefied natural gas as in a conventional method, or (2) to treat a larger amount of liquefied natural gas as in a conventional method. When applied in the first way, the content of low boiling point components in the liquefied gas treated with the method of the present invention is lower than that in liquefied gas treated with a conventional method. When applied in the second way, the content of low boiling point components is maintained and the amount of liquefied gas is increased.
US-A-6 199 403 discloses a method to remove a high- volatility component such as nitrogen from a feed stream rich in methane. According to US-A-6 199 403 the expanded liquefied natural gas stream enters a separation column at an intermediate level, i.e. not below a single gas- liquid contacting section.
US-A-5 421 165 relates to a process for denitrogenation of a feedstock of a liquefied mixture of hydrocarbons. To this end US-A-5 421 165 suggests a relatively complicated process using a denitrogenation column comprising a plurality of theoretical fractionation stages.
Another relatively complicated process has been described in International patent application publication No. WO 02/50483. WO 02/50483 discloses several methods of removing components having low boiling points from liquefied natural gas. According to WO 02/50483 a liquid product stream having a reduced content of components having low boiling points is obtained.
A problem of the above processes described in WO 02/50483 is that the liquid product stream contains an undesirable high content of components having low boiling points. It is an object of the present invention to minimize the above problem.
It is a further object of the present invention to provide an alternative process.
It is an even further object of the present invention to provide a simplified process to reduce the amount of components having low boiling points in a liquefied natural gas stream.
One or more of the above or other objects are achieved according to the present invention by providing a method of treating liquefied natural gas supplied at liquefaction pressure containing components having low boiling points to obtain a liquid product stream having a reduced content of components having low boiling points, which method comprises the steps of: (a) allowing the liquefied gas to expand to separation pressure to obtain an expanded two-phase fluid; (b) introducing the expanded two-phase fluid into a column, the column comprising a single gas-liquid contacting section, wherein the expanded two-phase fluid is introduced below the single gas-liquid contacting section arranged in the column;
[C) collecting in the bottom of the column liquid from the two-phase fluid and withdrawing from the bottom of the column a liquid stream having a reduced content of components having low boiling points to obtain the liquid product stream;
(d) allowing vapour from the two-phase fluid to flow through the single contacting section; (e) withdrawing from the top of the column a gaseous stream that is enriched in components having low boiling points;
(f) heating the gaseous stream obtained in step (c) in a heat exchanger to obtain a heated gaseous stream; (g) compressing the heated gaseous stream obtained in step (f) to fuel gas pressure to obtain fuel gas;
(h) separating a recycle stream from the fuel gas obtained in step (g) ;
(i) at least partly condensing the recycle stream obtained in step (h) to obtain a reflux stream; and
(j) introducing the reflux stream obtained in step (i) at separation pressure into the column above the single contacting section.
Applicants have found that the liquid product stream obtained according to the present invention contains a smaller content of components having low boiling points than one would expect.
Surprisingly, these desirable results are obtained according to the present invention with a simplified process, using a column in step (b) having only a single gas-liquid contacting section.
Using the simplified process according to the present invention, the reduction of the amount of components having low boiling points in the liquid product stream may be achieved in a more cost-effective manner. Further it has been found that the process according to the present invention is especially suitable for liquefied natural gas streams (to be supplied at liquefaction pressure) comprising less than 7 mol% of components having low boiling points.
The present invention will now be described by way of example in more detail with reference to the accompanying non-limiting drawings, wherein:
Figure 1 shows schematically a process flow scheme of an embodiment of the method of the present invention; and
Figure 2 shows schematically an alternative of the process of Figure 1;
Figure 3 shows schematically a process flow scheme of an other embodiment of the method of the present invention;
Figure 4 shows schematically an alternative of the process of Figure 3; and
Figure 5 shows schematically and not to scale an alternative to part V of the process flow scheme of Figure 4.
Reference is made to Figure 1. Liquefied natural gas containing components having low boiling points is supplied at liquefaction pressure through conduit 1 to an expansion device in the form of expansion engine 3 and Joule-Thompson valve 5 in the discharge conduit 6 of expansion engine 3. In the expansion device, the liquefied gas is allowed to expand to separation pressure, and an expanded two-phase fluid is obtained. The liquefaction pressure is suitably in the range of from 3 to 8.5 MPa and the separation pressure is suitably in the range of from 0.1 to 0.5 MPa.
The expanded two-phase fluid is passed through conduit 9 to a column 10. The expanded two-phase fluid is introduced into the column 10 at separation pressure via a suitable inlet device, such as vane inlet device 12. The vane inlet device, also known as schoepentoeter, allows efficient separation of gas and liquid.
The column 10 is provided with only a single gas- liquid contacting section 14. The single contacting section 14 may comprise any suitable means for contacting a gas and a liquid, such as trays and packings. Preferably the single contacting section 14 consists of from two to eight horizontal contacting trays 15 or a packing having a packed section with a length being equivalent to from two to eight contacting trays. The expanded two-phase fluid is introduced into the column 10 below the single gas-liquid contact section 14.
In the bottom 16 of the column 10 liquid from the two-phase fluid is collected, and a liquid stream having a reduced content of components having low boiling points is removed from the bottom 16 through conduit 17 and pumped by pump 18 to a storage tank 20. From the storage tank 20 a liquid product stream is removed through conduit 21 and a gaseous stream through conduit 22. The gaseous stream is also known as boil-off gas.
Vapour from the two-phase fluid to flow through the single contacting section 14. From the top 23 of the column 10 a gaseous stream that is enriched in components having low boiling points is removed through conduit 25. The gaseous stream is heated in a heat exchanger 27 to obtain a heated gaseous stream that is passed through conduit 28 to a compressor 30. In compressor 30 the heated gaseous stream is compressed to fuel gas pressure to obtain fuel gas. The fuel gas is removed through conduit 31 and cooled in heat exchanger 32 to remove the heat of compression. The fuel gas is passed away through conduit 33. The fuel gas pressure is in the range of from 1 to 3.5 MPa.
A recycle stream from the fuel gas and supplied to the heat exchanger 27 through conduit 34a. In the heat exchanger 27 the recycle stream is at least partly condensed to obtain a reflux stream, which is passed to the column 10 through the conduit 34b provided with Joule-Thompson valve 37. The reflux stream is introduced at separation pressure into the column 10 via inlet device, such as vane inlet device 39 above the single contacting section 14.
Table 1 summarizes the result of a hypothetical example, wherein the method of Figure 1 is compared to a base case. In the base case the recycle stream and the feed are introduced into the column at the same level, so that the liquid phases of the two streams are mixed before introduction thereof in the column and the column has no contacting section. It was found that the liquid stream withdrawn through conduit 17 for the base case contains more nitrogen than the same stream for the present invention.
Table 1. Summary of hypothetical example with the embodiment of Figure 1.
Table 1 shows that a lower nitrogen content in the product stream is obtained with the method of the present invention.
In an alternative embodiment of the present invention, the recycle stream separated from the fuel gas is additionally compressed in an auxiliary compressor to an elevated pressure before it is at least partly condensed in heat exchanger 27. The high-pressure recycle stream can be used in several ways, which will be discussed with reference to Figure 2. The parts that were already discussed with reference to Figure 1 have got the same reference numerals.
The auxiliary compressor included in conduit 34a is referred to with reference numeral 35. The auxiliary compressor 35 can be provided with a cooler (not shown) to remove the heat of compression for the compressed recycle stream. The compressed recycle stream is at least partly condensed by cooling it in heat exchanger 27. Part of the cold that is needed is provided by the gaseous stream that is enriched in components having low boiling points that is passed through conduit 25. The remainder is provided by the recycle stream. Cold from the recycle stream can be obtained by expanding a part of the recycle stream to an intermediate pressure in Joule-Thompson valve 38, using the expanded fluid to cool the recycle stream in conduit 34a and supplying the expanded fluid through conduit 38a to the compressor 30. The intermediate pressure to which the part of the recycle stream is expanded is in the range of from the suction pressure to the discharge pressure of the compressor 30 (ends of the range included) . The stage at which the expanded recycle stream enters the compressor 30 is so selected that the pressure of the expanded recycle stream matches the pressure of the fluid in the compressor 30 in that stage. The remainder of the recycle stream is expanded by the Joule-Thompson valve 37 and introduced as reflux in the column 10 as discussed with reference to Figure 1.
An advantage of the embodiment discussed with reference to Figure 2 is that the recycle stream is expanded from a larger pressure and thus cooled to a lower temperature. This allows a warmer feed stream, for example a feed stream at -142 0C, compared to a feed stream temperature of -145 0C (in the above example) . Thus the temperature of the liquefied gas from the main cryogenic heat exchanger can be higher and therefore, for the same amount of energy, more gas can be liquefied.
The elevated pressure of the fluid discharged from the auxiliary compressor 35 is so selected that the costs of the power required to drive the auxiliary compressor 35 are less than the value of the increased amount of gas that is liquefied.
In the above we discussed an embodiment in which the expansion is done in the expansion valves 37 and 38. However, it will be understood that the expansion of the recycle stream can be done in two stages, at first in an expansion device, such as expander 36 and subsequently in the Joule-Thompson valves 37 and 38.
Instead of supplying the expanded fluid through conduit 38a to the compressor 30, the expanded fluid can be supplied to an inlet (not shown) of the compressor 35.
In the embodiments discussed with reference to Figures 1 and 2, the liquid from the two-phase fluid is collected in the bottom 16 of the column 10, and from the bottom 16 a liquid stream 17 is withdrawn having a reduced content of components having low boiling points to obtain the liquid product stream. In an alternative embodiment of the invention, this step comprises collecting in the bottom of the column liquid from the two-phase fluid and withdrawing from the bottom of the column a liquid stream having a reduced content of components having low boiling points; introducing the liquid stream into a flash vessel at a low pressure; removing a second gaseous stream from the top of the flash vessel; and removing from the bottom of the flash vessel a liquid stream to obtain the liquid product stream.
This two-vessel embodiment will now be discussed with reference to Figure 3. The parts that were already discussed with reference to Figure 1 have got the same reference numerals.
The column 10' comprises an upper part 1Ou and a lower part 101, wherein the upper part performs the function of the column 10 in Figure 1 and the lower part 101 is a flash vessel operating at a pressure that is below the pressure in the upper part 1Ou. Suitably the pressure in the upper part 1Ou is in the range of from 0.2 to 0.5 MPa and the pressure in the flash vessel 101 in the range of from 0.1 to 0.2 MPa. During normal operation, liquid from the two-phase fluid supplied through conduit 9 is collected in the bottom 16' of the upper part 1Ou of the column 10' . From that bottom 16' is withdrawn a liquid stream having a reduced content of components having low boiling points through conduit 17' . This stream is then introduced into the flash vessel 101 at a low pressure. The pressure reduction is achieved by means of Joule-Thompson valve 40 in conduit 17' . Consequently a two-phase mixture is formed and that is introduced via inlet device 41 into the flash vessel 101.
Through conduit 17'' a liquid stream having a reduced content of components having low boiling points is removed, which is passed to the storage tank 20.
From the top 23" of the flash vessel 101 a second gaseous stream is removed. Suitably the second gaseous stream is passed through conduit 42 to heat exchanger 27, in which the second gaseous stream is heated by heat exchange with the recycle stream supplied through conduit 34a. The heated stream is compressed in compressor 45, the heat of compression is removed in heat exchanger 48 and passed through conduit 49 to add the compressed second gaseous stream to the recycle stream in conduit 34a.
It will be understood that compressors 45 and 30 can be combined into one compressor (not shown) . In that case, conduit 42 is connected to the suction end of that compressor, conduit 28 to an intermediate inlet and conduit 32 is connected to the discharge end of that compressor. An advantage of this method is that it can be used for large liquefaction plants.
As with the embodiment discussed with reference to Figure 1, the embodiment discussed with reference to Figure 3 can as well be provided with an auxiliary compressor to compress the recycle stream separated from the fuel gas to an elevated pressure before it is at least partly condensed in heat exchanger 27. The high- pressure recycle stream can be used in several ways, which will be discussed with reference to Figure 4. The parts that were already discussed with reference to Figure 3 have got the same reference numerals.
The auxiliary compressor included in conduit 34a is referred to with reference numeral 35. The auxiliary compressor 35 can be provided with a cooler (not shown) to remove the heat of compression for the compressed recycle stream. The compressed recycle stream is partially condensed by cooling it in heat exchanger 27. Part of the cold that is needed is provided by the gaseous stream that is enriched in components having low boiling points that is passed through conduit 25. The remainder is provided by the recycle stream. Cold from the recycle stream can be obtained by expanding a part of the recycle stream to an intermediate pressure in Joule- Thompson valve 38, using the expanded fluid to cool the recycle stream in conduit 34a and supplying the expanded fluid through conduit 38a to the compressor 30. The intermediate pressure to which the part of the recycle stream is expanded is in the range of from the suction pressure to the discharge pressure of the compressor 30 (ends of the range included) . The point at which the expanded recycle stream enters the compressor 30 is so selected that the pressure of the expanded recycle stream matches the pressure of the fluid in the compressor 30 at the inlet point. The remainder of the recycle stream is expanded by the Joule-Thompson valve 37 and introduced as reflux in the column 10 as discussed with reference to Figure 1.
An advantage of this embodiment is that the recycle stream is expanded from a larger pressure and thus cooled to a lower temperature. This allows a warmer feed stream, for example a feed stream at -142 0C, compared to a feed stream temperature of -145 0C (in the above example) . Thus the temperature of the liquefied gas from the main cryogenic heat exchanger can be higher and therefore, for the same amount of energy, more gas can be liquefied.
The elevated pressure of the fluid discharged from the auxiliary compressor 35 is so selected that the costs of the power required to drive the auxiliary compressor 35 are less than the value of the increased amount of gas that is liquefied.
In the above we discussed an embodiment in which the expansion is done in the expansion valves 37 and 38. However, it will be understood that the expansion of the recycle stream can be done in two stages, at first in an expansion device, such as expander 36 and subsequently in the Joule-Thompson valves 37 and 38.
Figure 4 also shows that the boil-off gas from the storage tank 20 is provided via the conduit 22 to the suction end of the compressor 45.
It will be understood that compressors 45 and 30 can be combined into one compressor (not shown) . In that case, conduit 42 (into which conduit 22 opens) is connected to the suction end of that compressor, conduit 28 to an intermediate inlet and conduit 32 is connected to the discharge end of that compressor.
Instead of supplying the expanded fluid through conduit 38a to the compressor 30, the expanded fluid can be supplied to an inlet (not shown) of the compressor 35. An alternative of the embodiment shown in Figure 4 is shown in Figure 5, wherein a part of the recycle stream that is passed through conduit 34a is separated therefrom and passed through conduit 50 through the heat exchanger 27. Then the cooled recycle stream is expanded to the intermediate pressure in expander 51 and used to cool the recycle stream in conduit 34a. The expanded stream is then introduced into the compressor 30 at an intermediate stage.
Suitably, the recycle stream passed through conduit 34a is between 10 and 90% by mass of the fuel gas that is passed through conduit 31.
In the embodiments discussed with reference to the Figures, the single contacting section 14 contains trays, however, any other contacting means such as packing can be employed as well. The length of the packed section should is then preferably equivalent to from two to eight contacting trays.
The method of the present invention provides a simple way of reducing the amount of components having low boiling points in a liquefied natural gas stream.

Claims

C L A I M S
1. Method of treating liquefied natural gas supplied at liquefaction pressure containing components having low boiling points to obtain a liquid product stream having a reduced content of components having low boiling points, 5 which method comprises the steps of:
(a) allowing the liquefied gas to expand to separation pressure to obtain an expanded two-phase fluid;
(b) introducing the expanded two-phase fluid into a column, the column comprising a single gas-liquid
10 contacting section, wherein the expanded two-phase fluid is introduced below the single gas-liquid contacting section arranged in the column;
(c) collecting in the bottom of the column liquid from the two-phase fluid and withdrawing from the bottom of
15 the column a liquid stream having a reduced content of components having low boiling points to obtain the liquid product stream; !•■ (d) allowing vapour from the two-phase fluid to flow through the single contacting section; 20 (e) withdrawing from the top of the column a gaseous stream that is enriched in components having low boiling points;
(f) heating the gaseous stream obtained in step (c) in a heat exchanger to obtain a heated gaseous stream; 25 (g) compressing the heated gaseous stream obtained in step (f) to fuel gas pressure to obtain fuel gas; (h) separating a recycle stream from the fuel gas obtained in step (g) ;
(i) at least partly condensing the recycle stream • 30 obtained in step (h) to obtain a reflux stream; and (j) introducing the reflux stream obtained in step (i) at separation pressure into the column above the single contacting section.
2. Method according to claim 1, wherein step (c) comprises collecting in the bottom of the column liquid from the two-phase fluid and withdrawing from the bottom of the column a liquid stream having a reduced content of components having low boiling points; introducing the liquid stream having a reduced content of components having low boiling points into a flash vessel at a low pressure; removing a second gaseous stream from the top of the flash vessel; and removing from the bottom of the flash vessel a liquid stream to obtain the liquid product stream.
3. Method according to claim 2, further comprising heating the second gaseous stream in the heat exchanger; compressing the second gaseous stream to fuel gas pressure; and adding the second gaseous stream to the recycle stream.
4. Method according to one or more of the preceding claims 1-3, wherein at least partly condensing the recycle stream in step (i) comprises indirectly heat exchanging the recycle stream with the gaseous stream(s) in the heat exchanger.
5. Method according to one or more of the preceding claims 1-4, wherein compressing the heated gaseous stream to fuel gas pressure to obtain fuel gas in step (g) further includes removing the heat of compression.
6. Method according to one or more of the preceding claims 1-5, wherein the recycle stream separated from the fuel gas in step (h) is compressed to an elevated pressure before it is at least partly condensed.
7. Method according to one or more of the preceding claims 1-6, wherein the supplied liquefied natural gas comprises less than 7 mol% of components having low boiling points.
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