Method And Apparatus for High Purity Liquefied Natural Gas

Abstract

A novel method and system for liquefying and distilling natural gas into high purity liquid methane (LNG) and NGL product streams. Heat exchangers and distillation towers are configured to produce high purity liquefied natural gas (LNG) and NGL product streams, while also rejecting excess nitrogen contained in the inlet gas stream, utilizing liquid nitrogen as the process refrigerant. A molecular sieve pretreatment system is configured to utilize the vaporized nitrogen stream for regeneration of the molecular sieve beds which are designed for removing water and carbon dioxide from the inlet gas stream

1 . A method of processing a hydrocarbon gas stream comprising an initial amount of methane, an initial amount of ethane and heavier hydrocarbon components, and an initial amount of contaminant comprising nitrogen, the method comprising:
 introducing the hydrocarbon gas stream to a first heat exchange unit;   introducing a first stream of nitrogen refrigerant to at least the first heat exchange unit to aid in cooling of the hydrocarbon gas;   flowing the cooled hydrocarbon gas from the first heat exchange unit to a separator and   separating a liquid portion from a vapor portion;   flowing the liquid portion from the separator to a demethanizer;   liquefying all of the vapor portion from the separator by heat exchange with at least a second nitrogen refrigerant stream in a second heat exchange unit to achieve total liquefaction and subcooling, thereby forming all of the vapor portion from the separator into a totally subcooled liquid reflux stream to the demethanizer, wherein the first and second nitrogen refrigerant stream may be dependent or independent streams;   flowing a vapor product from the demethanizer, wherein as the vapor product exits the demethanizer it comprises essentially all of the initial amount of methane and essentially all of the initial amount of contaminant;   flowing a liquid NGL product from the demethanizer, wherein as the liquid NGL product exits the demethanizer it comprises essentially all of the initial amount of ethane and heavier hydrocarbon components;   liquefying the vapor product of the demethanizer;   flowing the liquefied vapor product to a nitrogen recovery unit;   flowing a liquid LNG stream from the nitrogen recovery unit;   flowing a vapor contaminant stream from the nitrogen recovery unit;   combining the vapor contaminant stream from the nitrogen recovery unit into the second nitrogen refrigerant stream; and,   cooling the liquid LNG stream from the nitrogen recovery unit in the second heat exchange unit.   
     
     
         2 . The method of  claim 1 , further comprising passing the hydrocarbon gas stream through a pretreatment system prior to introducing the hydrocarbon gas to the first heat exchanger, and further comprising vaporizing at least a portion of the first or second nitrogen refrigerant streams and regenerating the pretreatment system with the vaporized nitrogen refrigerant. 
     
     
         3 . The method of  claim 2 , further comprising regenerating the pretreatment system at a temperature of 400 degrees F. or less with the vaporized nitrogen refrigerant. 
     
     
         4 . The method of  claim 1 , wherein the NGL product contains a percentage of ethane and methane. 
     
     
         5 . The method of  claim 4 , further comprising recovering greater than 99% of the ethane from the NGL product 
     
     
         6 . The method of  claim 4 , wherein the NGL product contains less than 0.5 liquid volume percent methane. 
     
     
         7 . The method of  claim 1 , wherein liquefying the vapor' product of the first pressurized distillation tower comprises forming an LNG stream containing a percentage of methane. 
     
     
         8 . The method of claim, wherein the LNG stream contains less than 0.1% ethane. 
     
     
         9 . The method of  claim 1 , further comprising forming the LNG stream containing a percentage of nitrogen. 
     
     
         10 . (canceled) 
     
     
         11 . (canceled) 
     
     
         12 . (canceled) 
     
     
         13 . The method of  claim 1 , further comprising further cooling LNG of the LNG stream with the first or second nitrogen refrigerant streams prior to the LNG being stored in a storage unit. 
     
     
         14 . The method of  claim 1 , further comprising pumping the LNG of the LNG stream to a transportation pipeline pressure, vaporizing the LNG, and transporting to sale. 
     
     
         15 . The method of  claim 1 , further comprising the ability to liquefy the inlet hydrocarbons gas into LNG at a pressure of 350 psig or less. 
     
     
         16 . The method of  claim 1 , further comprising vaporizing at least one of the nitrogen refrigerant streams through at least one of the heat exchange units, the demethanizer, the nitrogen recovery unit pressurized distillation towers, or a combination thereof. 
     
     
         17 . The method of  claim 16 , further comprising recycling at least a portion of the vaporized nitrogen refrigerant for reclamation and re-use as nitrogen refrigerant. 
     
     
         18 . The method of  claim 16 , further comprising venting at least a portion of the vaporized nitrogen refrigerant to the atmosphere. 
     
     
         19 - 21 . (canceled)