Method and system for producing liquefied natural gas (LNG)

• US 20100154470A1
• Filed: 12/17/2009
• Published: 06/24/2010
• Est. Priority Date: 12/19/2008
• Status: Active Grant

• Current Assignee: Aragon AS
• Original Assignee: Kanfa Aragon AS
• Inventors: Nilsen, Inge Sverre Lund
• Granted Patent: US 9,151,537 B2
• Time in Patent Office: Days
• US Class Current: 62/613
• CPC Class Codes: F25J 1/0022 Hydrocarbons, e.g. natural gas
• CPC Class Codes: F25J 1/0037 of a return stream
• CPC Class Codes: F25J 1/005 by expansion of a gaseous refrigerant stream with extraction of work
• CPC Class Codes: F25J 1/0052 by vaporising a liquid refrigerant stream
• CPC Class Codes: F25J 1/0057 after expansion of the liquid refrigerant stream with extraction of work
• CPC Class Codes: F25J 1/0072 Nitrogen
• CPC Class Codes: F25J 1/0082 Methane
• CPC Class Codes: F25J 1/0092 Mixtures of hydrocarbons comprising possibly also minor amounts of nitrogen
• CPC Class Codes: F25J 1/0097 Others, e.g. F-, Cl-, HF-, HClF-, HCl-hydrocarbons etc. or mixtures thereof
• CPC Class Codes: F25J 1/0201 using only internal refrigeration means, i.e. without external refrigeration
• CPC Class Codes: F25J 1/0202 in a quasi-closed internal refrigeration loop F25J1/0208, F25J1/0219, F25J1/0224 take precedence
• CPC Class Codes: F25J 1/0204 as a single flow SCR cycle
• CPC Class Codes: F25J 1/0205 as a dual level SCR refrigeration cascade
• CPC Class Codes: F25J 1/0212 as a single flow MCR cycle
• CPC Class Codes: F25J 1/0215 with one SCR cycle
• CPC Class Codes: F25J 1/0216 using a C3 pre-cooling cycle
• CPC Class Codes: F25J 1/0232 integration within a pressure letdown station of a high pressure pipeline system
• CPC Class Codes: F25J 1/0238 Purification or treatment step is integrated within one refrigeration cycle only, i.e. the same or single refrigeration cycle provides feed gas cooling (if present) and overhead gas cooling
• CPC Class Codes: F25J 1/0241 wherein the overhead cooling comprises providing reflux for a fractionation step
• CPC Class Codes: F25J 1/0278 Unit being stationary, e.g. on floating barge or fixed platform
• CPC Class Codes: F25J 1/0281 : characterised by the type of prime driver, e.g. hot gas expander
• CPC Class Codes: F25J 1/0288 : using work extraction by mechanical coupling of compression and expansion of the refrigerant, so-called companders
• CPC Class Codes: F25J 1/0294 : Multiple compressor casings/strings in parallel, e.g. split arrangement
• CPC Class Codes: F25J 2210/06 : Splitting of the feed stream, e.g. for treating or cooling in different ways
• CPC Class Codes: F25J 2210/62 : Liquefied natural gas [LNG]; Natural gas liquids [NGL]; Liquefied petroleum gas [LPG]
• CPC Class Codes: F25J 2215/66 : Butane or mixed butanes
• CPC Class Codes: F25J 2220/64 : Separating heavy hydrocarbons, e.g. NGL, LPG, C4+ hydrocarbons or heavy condensates in general
• CPC Class Codes: F25J 2270/16 : with mutliple gas expansion loops of the same refrigerant
• CPC Class Codes: F25J 2270/90 : External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration

Abstract

A method and system for optimizing the efficiency of an LNG liquification system of the gas expansion type, wherein an incoming feed gas is first separated in a fractionation column by counter current contact with a cold reflux fluid, and a gaseous stream introduced into the heat exchanger system at a reduced temperature such that an intermediate pinch point is created in the warm composite curve.

Claims

1. A method for optimizing the efficiency of a LNG liquification system of the type comprising a heat exchanger system having a cooling circuit employing a gas-expansion cooling cycle comprising a gaseous cooling agent, characterized in that method comprises,

a) introducing an incoming natural gas stream into a fractionation column,

b) separating the natural gas stream, by counter current contact with a cold reflux fluid, into an overhead gas stream and bottom liquid fraction, the temperature of said overhead gas stream being thus reduced relative to the temperature of the original incoming gas stream,

c) introducing the cooled overhead gas stream into the heat exchange system whereby the overhead gas is cooled and partially condensed to form a 2-phase stream,

d) introducing the 2-phase stream into a separator where the 2-phase stream is separated into a gaseous component and a liquid component,

e) introducing the liquid component into the fractionation column, said liquid component functioning as the cold reflux,

e) introducing the gaseous component into the heat exchanger system for further cooling and condensation into a liquefied LNG product,

f) setting the parameters of the liquification system such that the cooling circuit compression work is less than the minimum compression work required for the liquefaction of the natural gas stream had it been introduced to the heat exchange system at its original temperature.

2. ) A method according to claim 1, wherein the setting of the parameters comprises determining and setting a reduced required cooling agent flow rate.

3. ) A method according to either claim 1 or 2, wherein the compression work is from 5 to 15% less than said minimum compression work.

4. ) A method according to either claim 1 or 2, wherein the cooling circuit comprises a closed gas expansion process with at least one gas expansion stage for essentially isentropically cooling the cooling agent by gas expansion.

5. ) A method according to either claim 1 or 2, wherein the cooling circuit comprises a closed gas expansion process with two or more gas expansion stages for essentially isentropically cooling the cooling agent by gas expansion, and where the cooling agent inlet temperature for the second gas expander stage is lower than the cooling agent inlet temperature for the first gas expander stage.

6. ) A method according to either of claim 1 or 2, wherein the introduction of the cooled overhead gas stream at a reduced temperature relative to the original incoming natural gas stream creates an intermediate pinch point in the warm composite curve.

7. ) A method according to either of claim 1 or 2, wherein the introduction of the cooled overhead gas stream at a reduced temperature relative to the heat exchanger warm end temperature creates a change of slope of the warm composite curve at the position of introduction of the cooled overhead gas.

8. ) A method according to claim 5, wherein the streams being cooled down from the warm end of the heat exchanger system to the intermediate pinch point consists essentially of the cooling agent streams.

9. ) A method according to either of claim 1 or 2 wherein the cooling agent is nitrogen.

10. ) A method according to either of claim 1 or 2, wherein the composition of the gaseous component from the separator, relative to the original incoming natural gas stream, consists essentially of from 87.5% to 98.2% of the propane of the feed gas, from 63.6% to 94.7% of the butanes of the feed gas, from 5.1% to 68% of the pentanes of the feed gas, and less than 4.5% of the hexane of the feed gas.

11. ) A optimized LNG liquification system comprising:

a) heat exchanger system having a cooling circuit employing a gas-expansion cycle comprising a gaseous cooling agent,

b) a fractionation column arranged for receiving an incoming natural gas stream, and cooling the natural gas stream into a cooled overhead gas stream and bottom liquid fraction, and further arranged for introducing the cooled overhead gas stream into the heat exchanger system where the overhead gas is cooled and partially condensed to form a 2-phase stream, and wherein the cooling in said fractionation column is essentially provided by a cold reflux liquid,

c) a separator arranged for receiving the 2-phase stream and separating it into a gaseous component and a liquid component, and further arranged for leading the liquid component to the fractionation column as cold reflux and for leading the gaseous component to the heat exchanger system for condensation into a LNG product.

12. ) A system according to claim 11 wherein the cooling circuit comprises a closed gas expansion process with at least one gas expansion stage for essentially isentropically cooling the cooling agent by gas expansion.

13. ) A system according to claim 11, wherein the cooling circuit comprises a closed gas expansion process with two or more gas expansion stages for essentially isentropically cooling the cooling agent by gas expansion, and where the cooling agent inlet temperature for the second gas expander stage is lower than the cooling agent inlet temperature for the first gas expander stage.

14. ) A system according to either claim 11, wherein the cooling agent is nitrogen.

15. ) A system according to any one of claims 11-14, wherein the temperatures and pressures of the system are chosen such that the composition of the gaseous component from the separator, relative to the original incoming natural gas stream, consists essentially of from 87.5% to 98.2% of the propane of the feed gas, from 63.6% to 94.7% of the butanes of the feed gas, from 5.1% to 68% of the pentanes of the feed gas, and less than 4.5% of the hexane of the feed gas.

16. ) A system according to any one of claims 11-14, wherein a reboiler is connected to the fractionation column to reduce the vapour pressure of the bottom product.

17. ) A system according to any one of claims 11-14, wherein the liquefaction circuit comprises one or more multi-stream heat exchangers configured in series or parallel, or both.

18. ) A system according to any one of claim 11-14 wherein the liquefaction circuit comprises one heat exchanger comprising a plurality of warm and cold streams in the same unit.

19. ) A system according to any one of claims 11 to 14, wherein the liquefaction circuit comprises the gaseous refrigerant at an inlet pressure of 3-10 MPa being fed to the heat exchanger or system of heat exchangers and cooled to a temperature between 0 and −

120 deg C., and further wherein the cooled gaseous refrigerant is expanded to a pressure between 5% and 40% of the inlet pressure, and then being led back to the heat exchanger or system of heat exchangers to provide cooling.

20. ) A system according any one of claims 11-14, wherein the liquefaction circuit comprises a closed gas expansion process with two or more gas expansion stages for essentially isentropically cooling the refrigerant by gas expansion, and where the refrigerant inlet temperature for the second gas expander stage is lower than the refrigerant inlet temperature for the first gas expander stage.

21. ) A system according to any one of claims 11-14, wherein the liquefaction circuit comprises two expansion stages, wherein the gaseous refrigerant at an inlet pressure of 3-10 MPa is split in two parts either before or after pre-cooling in the heat exchanger system, and where the parts are pre-cooled to different temperatures before expansion to essentially the same lower pressures and led back to the heat exchanger or system of heat exchangers to provide cooling.

22. ) A system according to claim 11, wherein the streams being cooled down from the warm end of the heat exchanger system to the position where the cooled overhead gas is introduced consist essentially of the cooling agent streams.