Process and plant for producing liquefied natural gas

Abstract

A process for producing liquefied natural gas, in which natural gas feed having methane and higher hydrocarbons including benzene is cooled down to a first temperature level in a first cooling step using a first mixed coolant and then subjected to a countercurrent absorption using an absorption liquid to form a methane-enriched and benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed coolant and liquefied to give the liquefied natural gas. In the plant proposed, the first and second mixed coolants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping. The present invention likewise provides a corresponding plant.

1 - 14 . (canceled) 
     
     
         15 . A process for producing liquefied natural gas, in which natural gas feed containing methane and higher hydrocarbons, including benzene, is cooled down to a first temperature level in a first cooling step using a first mixed refrigerant, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed refrigerant and liquefied to give the liquefied natural gas, wherein the first and second mixed refrigerants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping. 
     
     
         16 . The process according to  claim 15 , wherein a countercurrent absorber is used in the countercurrent absorption, which is operated with a head condenser arranged above an absorption region of the countercurrent absorber, wherein the head condenser is used for condensing the further portion of the gas fraction. 
     
     
         17 . The process according to  claim 16 , wherein the head condenser is integrated into the countercurrent absorber or is at least partially arranged within the countercurrent absorber. 
     
     
         18 . The process according to  claim 15 , wherein the first mixed refrigerant comprises in total more than 90 mole percent, preferably more than 95 mole percent ethane, isobutane and n-butane and in total less than 10 mole percent, preferably less than 5 mole percent nitrogen, methane, propane and hydrocarbons having five or more carbon atoms. 
     
     
         19 . The process according to  claim 15 , wherein the second mixed refrigerant comprises in total more than 98 mole percent nitrogen, methane and ethane and in total less than 2 mole percent propane and heavier hydrocarbons. 
     
     
         20 . The process according to  claim 15 , wherein a first heat exchanger is used in the first cooling step, wherein the first mixed refrigerant in gaseous form is subjected to, in particular, single-stage compression in a first mixed refrigerant circuit, condensed by cooling, subcooled, expanded, heated in the first heat exchanger and, in particular, completely evaporated thereby, and subsequently subjected to the compression again. 
     
     
         21 . The process according to  claim 20 , wherein a second heat exchanger is used in the second cooling step, wherein the second mixed refrigerant in gaseous from is subjected to an in particular multi-stage compression in a second mixed refrigerant circuit, condensed by cooling, subcooled, expanded, heated in the second heat exchanger and, in particular, completely evaporated thereby, and subsequently subjected to compression again. 
     
     
         22 . The process according to  claim 21 , wherein the second mixed refrigerant is used after heating in the second heat exchanger and before compression during the condensation of the further portion of the gas fraction from the countercurrent absorption and further heated thereby. 
     
     
         23 . The process according to  claim 21 , wherein the first heat exchanger (E 1 ) is used to cool down the first mixed refrigerant (WMR) and/or the first (E 1 ) and the second (E 3 ) heat exchangers are used to cool down the second mixed refrigerant (CMR). 
     
     
         24 . The process according to  claim 15 , wherein in countercurrent absorption, a rising gas phase is provided at least in part by feeding in further natural gas feed which was not subjected to the first cooling step and/or at least in part by evaporating a portion of a sump liquid formed in the countercurrent absorption. 
     
     
         25 . The process according to  claim 15 , wherein the natural gas feed contains at least 80% methane and, in the methane-free remainder, at least 50% ethane and propane. 
     
     
         26 . A method according to  claim 15 , wherein the liquefied natural gas contains at least 90% methane, wherein the methane content in the liquefied natural gas is higher than in the natural gas feed. 
     
     
         27 . A plant configured to produce liquefied natural gas, having a first heat exchanger configured to cool natural gas feed containing methane and higher hydrocarbons, including benzene, to a first temperature level in a first cooling step using a first mixed refrigerant, a countercurrent absorber configured to subject the natural gas feed to countercurrent absorption using an absorption liquid after the first cooling step by forming a benzene-depleted gas fraction, having a second heat exchanger configured to cool down a portion of the gas fraction in a second cooling step to a second temperature level using a second mixed refrigerant and is liquefied to give the liquefied natural gas, wherein the plant is configured to use low-propane or propane-free first and second mixed refrigerants, and means are provided which are configured to form the absorption liquid from a further portion of the gas fraction, wherein they condense this above the countercurrent absorption and return it to the countercurrent absorption without pumping. 
     
     
         28 . The plant according to  claim 27 , wherein the plant is configured to carry out a process for producing liquefied natural gas, in which natural gas feed containing methane and higher hydrocarbons, including benzene, is cooled down to a first temperature level in a first cooling step using a first mixed refrigerant, and then subjected to countercurrent absorption using an absorption liquid to form a benzene-depleted gas fraction, wherein a portion of the gas fraction is cooled down to a second temperature level in a second cooling step using a second mixed refrigerant and liquefied to give the liquefied natural gas, wherein the first and second mixed refrigerants are low in propane or free of propane, and the absorption liquid is formed from a further portion of the gas fraction which is condensed above the countercurrent absorption and returned to the countercurrent absorption without pumping.