Method And Apparatus For Producing A Liquefied Natural Gas Stream

Abstract

Method and apparatus producing a LNG stream, wherein before liquefaction heavier hydrocarbon components are removed from a NG stream to be liquefied, the method at least comprising the steps of: providing a vaporous feed stream ( 1 ) of natural gas; feeding the feed stream ( 1 ) into a distillation column ( 10 ); drawing a bottom stream ( 17 ) and an overhead stream ( 16 ) from the distillation column ( 10 ); and liquefying at least a part of the overhead stream ( 16 ) thereby obtaining a LNG stream; wherein the step of feeding the feed stream ( 1 ) into the distillation column ( 10 ) comprises the sub steps of: splitting the feed stream ( 1 ) into first ( 3 a) and second ( 3 b) substreams; feeding the first substream ( 3 a) into the distillation column ( 10 ) via a first feed point ( 7 a), at a pressure not lower than the feed stream pressure minus a pressure drop brought about by the said splitting of the feed stream ( 1 ); cooling the second substream ( 3 b); and feeding the cooled second substream ( 7 ) into the distillation column ( 10 ) at a second feed point ( 7 b) overhead of the first feed point ( 7 a).

1 . A method of producing a liquefied natural gas stream, wherein before liquefaction heavier hydrocarbon components having a molecular weight higher than that of butane are removed from a natural gas stream to be liquefied, the method at least comprising the steps of:
 providing a substantially vaporous feed stream of natural gas at a feed stream pressure and a feed stream temperature;   feeding the feed stream into a distillation column having two or more separation stages;   drawing a bottom stream from a lower part of the distillation column and an overhead stream from an upper part of the distillation column, the overhead stream containing a lower relative amount of the heavier hydrocarbon components than the bottom stream; and   liquefying at least a part of the overhead stream thereby obtaining a liquefied natural gas stream;   wherein the step of feeding the feed stream into the distillation column comprises the sub steps of:   splitting the feed stream into first and second substreams at a selected split ratio;   feeding the first substream into the distillation column via a first feed point at the bottom of the distillation column below the lowest separation stage thereof, at a pressure not lower than the feed stream pressure minus a pressure drop brought about by the said splitting of the feed stream, wherein the first substream is not warmed between splitting the feed stream and feeding of the first substream at the first feed point into the distillation column;   cooling the second substream in a heat exchanger to a lower temperature than the feed temperature;   feeding the cooled second substream into the distillation column at a second feed point overhead of the first feed point;   wherein the pressure of neither the first substream nor the second substream is lowered in a dedicated pressure lowering device.   
     
     
         2 . The method according to  claim 1 , wherein the substantially vaporous feed stream comprises >90 vol. % vapour. 
     
     
         3 . The method according to  claim 1 , wherein the pressure drop in the first substream between the splitting of the feed stream and the first feed point is <6 bar. 
     
     
         4 . The method according to  claim 1 , wherein the selected split ratio is kept smaller than 1, wherein the split ratio is defined as the mass flow rate of the first substream divided by the mass flow rate of the second substream. 
     
     
         5 . The method according to  claim 4 , wherein the selected split ratio is kept higher than 1/100. 
     
     
         6 . The method according to  claim 1 , wherein the second substream is cooled in the heat exchanger against an external refrigerant. 
     
     
         7 . The method according to  claim 1 , wherein the distillation column is provided in the form of a scrub column wherein a scrub stream is fed to the scrub column via a third feed point overhead of the second feed point, at a temperature that is lower than that of the cooled second substream. 
     
     
         8 . The method according to  claim 7 , wherein the scrub stream is substantially liquid. 
     
     
         9 . The method according to  claim 7 , wherein the scrub stream is obtained by:
 drawing a fraction from the second substream to form a third substream, wherein the residue carries on as the second substream;   cooling the third substream in a second heat exchanger to form the scrub stream.   
     
     
         10 . The method according to  claim 7 , wherein the scrub stream is obtained by:
 partly condensing the overhead stream to form a mixed phase stream of vapour and condensate, drawing the condensate from the mixed phase stream to obtain the scrub stream.   
     
     
         11 . The method according to  claim 10 , wherein the part of the overhead stream that is used as the scrub stream is not expanded between the scrub column and the third feed point. 
     
     
         12 . The method according to  claim 7 , wherein the temperature of the scrub stream is sufficiently low whereby a condensate of heavier hydrocarbons is formed. 
     
     
         13 . An apparatus for producing a liquefied natural gas stream, wherein before liquefaction heavier hydrocarbon components having a molecular weight higher than that of butane can be removed from a natural gas stream to be liquefied, the apparatus at least comprising:
 a feed stream line for carrying a substantially vaporous feed stream of natural gas at a feed pressure and a feed temperature;   a distillation column having two or more separation stages for separating the heavier hydrocarbon components from the natural gas, a bottom stream discharge opening arranged in a lower part of the distillation column for discharging a bottom stream, and an overhead stream discharge opening arranged in an upper part of the scrub column for discharging an overhead stream containing a lower relative amount of the heavier hydrocarbon components than the bottom stream; and   a cryogenic zone in which at least a part of the overhead stream can be liquefied thereby obtaining a liquefied natural gas stream;   wherein the feed stream line comprises a feed stream junction fluidly connecting a main branch with first and second branches to split the feed stream into first and second substreams at a selected split ratio, wherein the first substream connects the feed stream junction with a first feed point at the bottom of the distillation column at a pressure not lower than the feed stream pressure drop brought about by the splitting of the feed stream, and wherein the second substream connects the feed stream junction with a second feed point in the distillation column, the second feed point being overhead relative to the first feed point, wherein the second substream is provided with a heat exchanger;   wherein the first feed point is situated below the lowest separation stage of the distillation column;   wherein the first substream is not provided with a heat exchanger for heating the first substream; and   wherein no dedicated pressure lowering device is present for lowering the pressure of the first substream and the second substream.   
     
     
         14 . An apparatus according to  claim 13 , wherein the distillation column is in the form of a scrub column, the apparatus further comprising a scrub stream source connected to the scrub column via a third feed point in the scrub column overhead of the second feed point. 
     
     
         15 . An apparatus according to  claim 14 , wherein the scrub stream source comprises a second junction provided in the second substream upstream of the second feed point to feed a third substream out of the second substream, wherein the third substream is provided with a second heat exchanger. 
     
     
         16 . An apparatus according to  claim 14 , wherein the scrub stream source comprises a condenser provided to receive the overhead stream downstream the scrub column in cooperation with a separator having a condensate outlet and a vapour outlet, wherein the condensate outlet is connected to the scrub column via the third feed point. 
     
     
         17 . An apparatus according to  claim 14 , wherein between the overhead stream discharge opening and the third feed point no expander is present.