Method and system for producing LNG

Abstract

A method is described for production of LNG from an incoming feed gas ( 1 ) on an onshore or offshore installation, and it is characterised by the following steps: 1) the feed gas is led through a fractionation column ( 150 ) where it is cooled a separated in an overhead fraction with a reduced content of pentane (C5) and heavier components, and a bottom fraction enriched with heavier hydrocarbons, 2) the overhead fraction from the fractionation column is fed to a heat exchanger system ( 110 ) and is subjected to a partial condensation to form a two-phase fluid, and the two-phase fluid is separated in a suitable separator ( 160 ) a liquid ( 5 ) rich in LPG and pentane (C3-C5) which is re-circulated as cold reflux to the fractionation column ( 150 ), while the gas ( 6 ) containing lower amounts of C5 hydrocarbons and hydrocarbons heavier than C5 is exported for further processing in the heat exchanger system ( 110 ) for liquefaction to LNG with a maximum content of ethane and LPG 3) the cooling circuit for liquefaction of gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step. A system for carrying out the method is also described.

1 . Method for production of LNG from an incoming feed gas ( 1 ), characterised by the following steps:
 a) the feed gas is led through a fractionation column ( 150 ) where it is cooled and separated into an overhead fraction ( 2 ), with a reduced content of the hydrocarbons that have a molecular weight higher than pentanes (C5) and enriched with most of the butane (C4) and hydrocarbons with a lower normal boiling point than butane, and a bottom fraction ( 3 ) wherein hydrocarbons having a molecular weight from hexanes (C6) and higher of the feed gas are conserved.   b) the overhead fraction of the fractionation column is led into a heat exchanger system ( 110 ) and is subjected to a partial condensation to form a two-phase fluid, and the two-phase fluid is separated in a suitable separator ( 160 ) into a fluid ( 5 ) rich in LPG and pentane (C3-C5) which is re-circulated as cold reflux to the fractionation column ( 150 ), while the gas ( 6 ), enriched with most of the butane (C4) and hydrocarbons with a lower normal boiling point than butane, is reintroduced into the heat exchanger system ( 110 ) for liquefaction to LNG consisting essentially of methane, ethane, propane and butane, and wherein   c) the cooling circuit for liquefaction of gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step.   
   
   
       2 . Method according to  claim 1 , characterised in that the fractionation column ( 150 ) and the separator ( 160 ) are operated at pressures and temperatures which lead to the complete system (the fractionating column  150  and reflux separator  160 ) generating a component split/separation point in the normal boiling point range (NBP) between −12° C. and 60° C. 
   
   
       3 . Method according to  claims 1 - 2 , characterised in that the light key component for the separation is butane (C4) with a normal boiling point between −12° C. and 0° C., and the heavy key component is a C6 component with a boiling point between 50° C. and 70° C., whereby the overhead gas stream ( 6 ) of the system will contain the most of comprising a considerably reduced content of n-butane and hydrocarbons with a lower normal boiling point than n-butane, and the reject stream ( 3 ) of the system comprises most of C6 and components with a normal boiling point higher than C6. 
   
   
       4 . Method according to  claims 1 - 3 , characterised in that the fractionation column ( 150 ) and the separator ( 160 ) are operated so that pentane (C5, NBP=28-36° C.) is a transitional component that is distributed both in the overhead gas stream ( 6 ) of the system and the reject stream ( 3 ) of the system. 
   
   
       5 . Method according to one of the preceding claims, characterised in that the temperature of the feed gas is reduced through the fractionation column ( 150 ) so that the temperature of the gas when it is fed into the heat exchanger system ( 110 ) is lower than the temperature of the cooling gas stream at the hot end of the heat exchanger system (hot pinch point temperature). 
   
   
       6 . Method according to one of the preceding claims, characterised in that a reboiler ( 135 ) is connected to the fractionation column ( 150 ) to reduce the steam pressure of the bottom product. 
   
   
       7 . Method according to one of the preceding claims, characterised in that the heat exchanger for liquefaction (LNG production) comprises one or more multi-stream heat exchangers. 
   
   
       8 . Method according to one of the preceding claims, characterised in that it is carried out with a closed gas expansion process with at least one nitrogen expander. 
   
   
       9 . Method according to one of the preceding claims, characterised in that it is carried out with a closed hybrid cooling process with methane/nitrogen as a cooling agent, where the cooling agent is used both in the gas phase and in the liquid phase, and where the cooling circuit has at least one gas expander and at least one appliance for expansion of a liquid cooling agent. 
   
   
       10 . Method according to one of the preceding claims, characterised in that it is carried out with an open gas expansion process with at least one gas expander, in which a suitable gas at a higher pressure is used as cooling gas, and where the expanded gas at a lower pressure is not recompressed for recycling but is used for another purpose. 
   
   
       11 . System for carrying out the method according to  claims 1 - 10  comprising a fractionation column ( 150 ) for feeding in a feed gas, a heat exchanger system ( 110 ) for cooling down and partially condensing the overhead gas stream of the fractionation column, a separator ( 160 ) to separate the two-phase stream from the heat exchanger system, set up to recycle fluid from the separator to the fractionation column and import this fluid to the upper part of the column as a reflux, and appliance to lead the gas from the separator back to the heat exchanger system for further cooling down and liquefaction to LNG, characterised in that the cooling system which is used for cooling down, condensing and liquefying of gas in the heat exchanger system comprises an open or closed gas expansion process with at least one gas expansion step. 
   
   
       12 . System according to  claim 11 , characterised in that the system is designed and configured to separate the feed gas such that the overhead gas stream ( 6 ) of the system will be enriched with most of the butane (C4) and hydrocarbons with a lower normal boiling point than butane, and the bottom product in the fractionation column will be enriched with most of the C6 and components with a normal boiling point higher than C6. 
   
   
       13 . An optimized gas liquefaction system of the gas expansion type for the production of LNG from an incoming natural gas stream, characterised in that the system comprises:
 a) an open or closed gas liquefaction circuit comprising a gaseous refrigerant, at least one gas expander for cooling the refrigerant by gas expansion, and one or more heat exchangers for heat exchange between the natural gas, LNG and refrigerant streams,   b) a fractionation column arranged for receiving the incoming natural gas prior to introduction of the natural gas into the liquefaction circuit, said fractionation column being further arranged to cool and separate the incoming natural gas into an overhead gaseous fraction, with a reduced content of the hydrocarbons that have a molecular weight higher than pentanes (C5) and enriched with most of the butane (C4) and hydrocarbons with a lower normal boiling point than butane, and a bottom liquid fraction wherein hydrocarbons having a molecular weight from hexanes (C6) and heavier are conserved, and arranged to lead the overhead gaseous fraction to at least one of the heat exchangers of the liquefaction circuit, whereby the overhead feed gas fraction is cooled to a two-phase fluid, and   c) a separator arranged for receiving the two-phase fluid and separating the two-phase fluid into a gas component wherein hydrocarbons having a molecular weight from butanes and lighter are conserved, and a liquid component rich in C3-C5 hydrocarbons, and further arranged to return the liquid component back to the fractionation column as a cold reflux liquid and to return the gas component to the liquefaction circuit for further cooling and condensation to LNG.   
   
   
       14 . (canceled) 
   
   
       15 . (canceled) 
   
   
       16 . A system according to any of  claim 13 , 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. 
   
   
       17 . A system according to any of  claim 13  or  16 , 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, 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. 
   
   
       18 . A system according to any of  claim 13  or  16 - 17 , wherein cooling in the fractionation column is essentially provided by the reflux liquid from the separator. 
   
   
       19 . A system according to any of  claim 13  or  16 - 18 , wherein a reboiler ( 135 ) is connected to the fractionation column ( 150 ) to reduce the vapour pressure of the bottom product. 
   
   
       20 . A system according to any of  claim 13  or  16 - 19 , wherein the overhead gas fraction is cooled into the two-phase fluid by being in heat-exchanging, counter-current relationship with the gaseous refrigerant in one or more heat exchangers of the liquefaction circuit. 
   
   
       21 . A system according to any of  claim 13  or  16 - 20 , wherein the liquefaction circuit comprises one or more multi-stream heat exchangers configured in series or parallel, or both. 
   
   
       22 . A system according to any of  claim 13 - 22 , wherein the gas expander essentially isentropically cools the refrigerant. 
   
   
       23 . A system according any of  claim 13  or  16 - 23 , 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. 
   
   
       24 . A system according to either any of  claim 13  or  16 - 24 , wherein the gaseous refrigerant comprises nitrogen gas, or a mixture of nitrogen and hydrocarbons.