Method and system for producing liquefied natural gas (LNG)

1. Method for optimizing a warm end of a heat exchanger system of a process for liquefaction of a feed gas comprising hydrocarbons, the process employing a gas expansion cycle for providing cooling in a heat exchanger system, the gas expansion cycle comprising a compressor for cooling agent compression, the method comprising the steps of:

a) enriching the feed gas with butane and hydrocarbons with a lower boiling point than butane by

i) feeding the feed gas to a fractionation column where the feed gas is cooled in contact with a cold fluid rich in propane, butane and pentane and separated into a first overhead fraction with reduced content of hydrocarbons having molecular weight heavier than pentane, and a bottom fraction;

ii) the first overhead fraction being cooled in the heat exchanger system and partially condensed;

iii) separating the partially condensed first overhead fraction in a separator to generate said cold fluid rich in propane, butane and pentane, and a second overhead fraction enriched with a majority of the butane and hydrocarbons with a lower boiling point than butane that were contained in the feed gas, said second overhead fraction being further cooled down and liquefied in the heat exchanger system;

iv) operating the fractionation column and the separator at pressure and temperature such that said column and separator generate a separation of components in the feed gas at a normal boiling point range between −12° C. and 60° C.;

b) feeding a gaseous cooling agent to the warm end of the heat exchanger system for heat exchange with a cold gaseous cooling agent stream, the gaseous cooling agent and the cold gaseous cooling agent in the warm end of the heat exchanger having linear heat versus temperature relation;

c) leading said first overhead fraction to the heat exchanger system at a lower temperature than the heat exchanger system warm end temperature, wherein the introduction of the first overhead fraction causes a change of slope of a warm composite curve at the point where the first overhead fraction is introduced;

wherein the mass flow of gaseous cooling agent can be reduced such that a better temperature adaption between the gaseous and cold gaseous cooling agent streams is achieved in the warm end of the heat exchanger system, and required compression work for the cooling in the heat exchanger system can be reduced.

2. Method according to claim 1 , wherein the gaseous cooling agent is cooled at a first pressure to a temperature between 0° C. and −120° C. in heat exchange with the said cold gaseous cooling agent stream, and thereafter expanded in a gas expander to a lower pressure lower than said first pressure to generate the cold gaseous cooling agent stream.

3. Method according to claim 2 , wherein the gas expander comprises an expansion turbine, where the gaseous cooling agent stream is expanded at high isentropic efficiency from a first pressure between 3 and 10 MPa, to a second, lower pressure between 5%-40% of said first pressure.

4. Method according to claim 3 , wherein the second, lower pressure is between 10% and 30% of the first pressure.

5. Method according to claim 1 , wherein the gaseous cooling agent is nitrogen.

6. Method according to claim 1 , wherein the cold gaseous cooling agent stream is heated in the heat exchanger system and thereafter is compressed and cooled with external cooling, and thereafter reused as the gaseous cooling agent at a higher pressure.

7. Method according to claim 1 , wherein the gaseous cooling agent is split into a plurality of cooling agent parts, the cooling agent parts are cooled to different temperatures and expanded in gas expanders, thereafter returning the expanded cooling agent parts to different inlet locations on the heat exchanger system.

8. Method according to claim 7 , wherein the said cooling agent parts are cooled to the said different temperatures in separate flow channels in the heat exchanger system.

9. Method according to claim 7 , wherein the expanded cooling agent parts are heated in separate flow channels in the heat exchanger system.

10. Method according to claim 1 , wherein the said component separation at the normal boiling point range between −12° C. and 60° C. corresponds to butane (C4) with a normal boiling point between −12° C. and 0° C. being a light key component to the separation, and a C6 component with a boiling point between 50° C. and 70° C. being a heavy key component to the separation.

11. Method according to claim 1 , wherein the said second overhead fraction from the separator, relative to the feed gas, 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.

12. Method according to claim 1 , wherein before introduction of said first overhead fraction the warm streams being cooled down in the warm end of the heat exchanger system consist of gaseous cooling agent streams, and after introduction of said first overhead fraction the warm streams being cooled down consist of gaseous cooling agent streams and said first overhead fraction.