Process and apparatus for reducing the heating value of liquefied natural gas

Abstract

A process and apparatus is provided for reducing the heating value of imported LNG by removing natural gas liquid products while condensing boil-off gas. The LNG is pumped from a storage container and is then heated by cross exchange to a dew point temperature. A portion of this heated LNG is sent out to be vaporized while the remaining portion is further heated by cross exchange with demethanizer overhead vapors and is then sent as feed to the demethanizer. NGL is recovered at the bottom of the demethanizer, and the overhead vapors are mixed with boil off gas coming from the LNG storage container. These mixed vapors are condensed by cross exchanging with the LNG feed portion and then pumped to pipeline pressure and sent to the gas pipeline through the vaporizers.

1 ) A process for reducing the heating value of LNG, comprising:
 splitting an LNG stream into a separations feed stream and a vaporization feed stream;   cross heat-exchanging the separations feed stream with a combined boil-off gas and overheads stream;   separating the separations feed stream into the overheads stream and a bottoms stream; and   combining the combined boil-off gas and overheads stream with the vaporization feed stream.   
     
     
         2 . The process according to  claim 1 , wherein a flowrate ratio between the separations feed stream and the vaporization stream is in the range of about 20:80 to about 40:60. 
     
     
         3 ) The process according to  claim 1 , wherein the LNG stream pressure is in the range of about 75 to about 125 psig. 
     
     
         4 ) The process according to  claim 1 , wherein the LNG stream is at a dew point temperature. 
     
     
         5 ) The process according to  claim 1 , wherein the separations feed stream is separated into the overheads stream and the bottoms stream in a demethanizer. 
     
     
         6 ) The process according to  claim 1 , wherein the combined boil-off gas and overheads stream is formed by combining a boil-off gas stream with the separations feed stream before the separations feed stream is separated into the overheads stream and the bottoms stream. 
     
     
         7 ) The process according to  claim 1 , wherein the combined boil-off gas and overheads stream is formed by combining a boil-off gas stream with the overheads stream after the separations feed stream is separated into the overheads stream and the bottoms stream. 
     
     
         8 ) The process according to  claim 1 , wherein the overheads stream comprises at least about 98 mol % methane. 
     
     
         9 ) The process according to  claim 1 , wherein the cross heat-exchanging of the separations feed stream with the combined boil-off gas and overheads stream condenses at least a portion of the combined boil-off gas and overheads stream. 
     
     
         10 ) An apparatus for reducing the heating value of LNG, comprising:
 a pump for pumping an LNG stream;   a flow control device for splitting the LNG stream into a separations feed stream and a vaporization stream;   at least one heat exchanger for cross heat-exchanging the separations feed stream with a combined boil-off gas and overheads stream; and   a separations column for separating the separations feed stream into the overheads stream and a bottoms stream.   
     
     
         11 ) The apparatus according to  claim 10 , wherein the flow control device controls a flowrate ratio between the separations feed stream and the vaporization stream within the range of about 20:80 to about 40:60. 
     
     
         12 ) The apparatus according to  claim 10 , wherein the pump maintains the LNG stream at a pressure in the range of about 75 to about 125 psig. 
     
     
         13 ) The apparatus according to  claim 10 , wherein the LNG stream is at a dew point temperature. 
     
     
         14 ) The apparatus according to  claim 10 , wherein the separations column is a demethanizer. 
     
     
         15 ) The apparatus according to  claim 10 , further comprising a manifold disposed upstream of the separations column for combining a boil-off gas with the separations feed stream. 
     
     
         16 ) The apparatus according to  claim 10 , further comprising a manifold disposed downstream of the separations column for combining a boil-off gas with the overheads stream. 
     
     
         17 ) The apparatus of  claim 10 , further comprising a manifold disposed downstream from the heat exchanger for combining the combined boil-off gas and overheads stream with the vaporization stream. 
     
     
         18 ) The apparatus of  claim 10 , further comprising a compressor for compressing the boil-off gas stream.