Plant and Method for Liquefying Natural Gas

Abstract

Plant and method for liquefying natural gas. The plant comprises a common pre-cooling heat exchanger train ( 1 ), two natural gas liquids extraction units ( 100, 100′ ) and two main heat exchangers ( 200, 200′ ) to cool the overhead light fraction from its corresponding natural gas liquids extraction unit to liquefaction.

1 . A plant for liquefying natural gas comprising:
 a pre-cooling heat exchanger train with one pre-cooling refrigerant circuit for removing heat from the natural gas in the pre-cooling heat exchanger train, and having an inlet for natural gas and an outlet for pre-cooled natural gas;   a distributor for splitting the pre-cooled natural gas stream into at least first and second natural gas substreams;   at least two natural gas liquids extraction units each provided with an extraction unit inlet arranged to receive one of the natural gas substreams, and comprising a heavy fraction outlet, and an overhead light fraction outlet; and   at least two main cryogenic systems each comprising a main heat exchanger having a first hot side having one inlet connected to the overhead light fraction outlet of at least one of the natural gas liquids extraction units and an outlet for liquefied natural gas, and each comprising a main refrigerant circuit for removing heat from the natural gas flowing through the first hot side of the corresponding main heat exchanger.   
     
     
         2 . The plant of  claim 1 , wherein each of the natural gas liquids extraction units is provided with a reflux inlet arranged to receive a liquid reflux from a liquid reflux outlet of an overhead separator, which overhead separator is provided with an inlet in fluid communication with the overhead light fraction outlet and a vapour outlet in fluid communication with the corresponding main cryogenic heat exchanger. 
     
     
         3 . The plant of  claim 2 , wherein upstream of the overhead separator an overhead heat exchanger is provided for removing heat from the overhead light fraction, of which overhead heat exchanger the cold side is in fluid communication with at least one of the at least two main refrigerant circuits. 
     
     
         4 . The plant of  claim 1 , further comprising at least one end flash unit, connected to the outlets for liquefied natural gas of the at least two heat exchangers and comprising at least an outlet for end flash gas and an outlet for liquefied natural gas. 
     
     
         5 . The plant of  claim 1 , wherein the distributor has two outlets, and wherein the plant comprises two natural gas liquids extraction units and two main heat exchangers. 
     
     
         6 . A method of liquefying a natural gas stream, comprising:
 pre-cooling the natural gas stream in a heat exchanger train against a pre-cooling refrigerant being cycled in one pre-cooling refrigerant circuit, thereby obtaining a pre-cooled natural gas stream;   splitting the pre-cooled natural gas stream into at least first and second natural gas substreams;   simultaneously separating each of the first and second natural gas substreams in a liquid heavy fraction, and a vaporous overhead light fraction;   further cooling the vaporous overhead light fractions into full condensation against a main refrigerant in at least two main cryogenic systems, whereby in each main cryogenic system the main refrigerant is cycled in a main refrigerant circuit; and   drawing a liquefied natural gas stream therefrom.   
     
     
         7 . The method of  claim 6 , wherein further cooling comprises partially condensing each of the vaporous overhead light fractions to form light condensate and light vapour, separating the light condensate from the light vapour, feeding the light condensate as a cold reflux into the step of simultaneously separating each of the first and second partially condensed natural gas substreams and further cooling the light vapour into full condensation. 
     
     
         8 . The method of  claim 7 , wherein the partially condensing each of the vaporous overhead light fractions comprises indirect heat exchanging with the main refrigerant in at least one of the at least two main refrigerant circuits. 
     
     
         9 . The method of  claim 6 , wherein the liquefied natural gas stream is subsequently expanded whereby a mixture is formed comprising an even further cooled liquefied natural gas and a flash vapour, whereby the flash vapour is separated from the even further cooled liquefied natural gas, compressed, at least partly condensed and reinjected in the liquefied natural gas stream upstream of the separating the flash vapour. 
     
     
         10 . The plant of  claim 2 , further comprising at least one end flash unit, connected to the outlets for liquefied natural gas of the at least two heat exchangers and comprising at least an outlet for end flash gas and an outlet for liquefied natural gas. 
     
     
         11 . The plant of  claim 3 , further comprising at least one end flash unit, connected to the outlets for liquefied natural gas of the at least two heat exchangers and comprising at least an outlet for end flash gas and an outlet for liquefied natural gas. 
     
     
         12 . The plant of  claim 2 , wherein the distributor has two outlets, and wherein the plant comprises two natural gas liquids extraction units and two main heat exchangers. 
     
     
         13 . The plant of  claim 3 , wherein the distributor has two outlets, and wherein the plant comprises two natural gas liquids extraction units and two main heat exchangers. 
     
     
         14 . The plant of  claim 4 , wherein the distributor has two outlets, and wherein the plant comprises two natural gas liquids extraction units and two main heat exchangers. 
     
     
         15 . The method of  claim 7 , wherein the liquefied natural gas stream is subsequently expanded whereby a mixture is formed comprising an even further cooled liquefied natural gas and a flash vapour, whereby the flash vapour is separated from the even further cooled liquefied natural gas, compressed, at least partly condensed and reinjected in the liquefied natural gas stream upstream of the separating the flash vapour. 
     
     
         16 . The method of  claim 8 , wherein the liquefied natural gas stream is subsequently expanded whereby a mixture is formed comprising an even further cooled liquefied natural gas and a flash vapour, whereby the flash vapour is separated from the even further cooled liquefied natural gas, compressed, at least partly condensed and reinjected in the liquefied natural gas stream upstream of the separating the flash vapour.