Method and system for production of liquid natural gas

Abstract

A process and system for liquefying a hydrocarbon gas is provided. The hydrocarbon feed gas is pre-treated to remove sour species and water therefrom. The pre-treated feed gas is then passed to a refrigeration zone where it is cooled and expanded to produce a hydrocarbon liquid. A closed loop single mixed refrigerant provides most of the refrigeration to the refrigeration zone together with an auxiliary refrigeration system. The auxiliary refrigeration system and closed loop single mixed refrigerant are coupled in such a manner that waste heat generated by a gas turbine drive of the compressor in the closed loop single mixed refrigerant drives the auxiliary refrigeration system and the auxiliary refrigeration system cools the inlet air of the gas turbine. In this way, substantial improvements are made in the production capacity of the system.

1 . A process for liquefying a hydrocarbon gas comprising the steps of:
 a) pre-treating a hydrocarbon feed gas to remove sour species and water therefrom;   b) providing a refrigeration zone, wherein refrigeration in the refrigeration zone is provided by circulating a mixed refrigerant from a mixed refrigerant system and an auxiliary refrigerant from an auxiliary refrigeration system through the refrigeration zone;   c) coupling the mixed refrigerant system and the auxiliary refrigeration system in a manner whereby the auxiliary refrigeration system is driven, at least in part, by waste heat generated by the mixed refrigerant; and   d) passing the pre-treated feed gas through the refrigeration zone where the pre-treated feed gas is cooled and expanding the cooled feed gas to produce a hydrocarbon liquid.   
     
     
         2 . The process according to  claim 1 , wherein the step of circulating a mixed refrigerant through the refrigeration zone comprises:
 a) compressing the mixed refrigerant in a compressor;   b) passing the compressed mixed refrigerant through a first heat exchange pathway extending through the refrigeration zone where the compressed mixed refrigerant is cooled and expanded to produce a mixed refrigerant coolant;   c) passing the mixed refrigerant coolant through a second heat exchange pathway extending through the refrigeration zone to produce a mixed refrigerant; and   d) recirculating the mixed refrigerant to the compressor.   
     
     
         3 . The process according to  claim 2 , wherein the step of passing the pre-treated feed gas through the refrigeration zone comprises passing the pre-treated feed gas through a third heat exchange pathway in the refrigeration zone. 
     
     
         4 . The process according to  claim 2 , wherein the step of circulating the auxiliary refrigerant through the refrigeration zone comprises passing the auxiliary refrigerant through a fourth heat exchange pathway extending through a portion of the refrigeration zone. 
     
     
         5 . The process according to  claim 4 , wherein the second and fourth heat exchange pathways extend in countercurrent heat exchange relation to the first and third heat exchange pathways. 
     
     
         6 . The process according to  claim 2 , wherein the waste heat is produced from the compressing step. 
     
     
         7 . The process according to  claim 2 , wherein the process further comprises cooling inlet air of a gas turbine directly coupled to the compressor with the auxiliary refrigerant. 
     
     
         8 . The process according to  claim 7 , wherein the inlet air is cooled to a temperature in a range of about 5° C.-10° C. 
     
     
         9 . The process according to  claim 2 , wherein the step of compressing the mixed refrigerant increases the pressure thereof from about 30 to 50 bar. 
     
     
         10 . The process according to  claim 2 , wherein the process comprises cooling the compressed mixed refrigerant prior to passing the compressed mixed refrigerant to the first heat exchange pathway. 
     
     
         11 . The process according to  claim 10 , wherein the compressed mixed refrigerant is cooled to a temperature less than 50° C. 
     
     
         12 . The process according to  claim 10 , wherein the compressed mixed refrigerant is cooled to about 10° C. 
     
     
         13 . The process according to  claim 10 , wherein the step of cooling the compressed mixed refrigerant comprises passing the compressed mixed refrigerant from the compressor to a heat exchanger. 
     
     
         14 . The process according to  claim 13 , wherein the heat exchanger is an air- or water-cooler. 
     
     
         15 . The process according to  claim 13 , wherein the cooling step comprises passing the compressed mixed refrigerant from the compressor to the heat exchanger and further passing the compressed mixed refrigerant cooled in the heat exchanger to a chiller. 
     
     
         16 . The process according to  claim 15 , wherein the chiller is driven at least in part by waste heat. 
     
     
         17 . The process according to  claim 16 , wherein the waste heat is produced from the compressing step. 
     
     
         18 . The process according to  claim 2 , wherein the temperature of the mixed refrigerant coolant is at or below the temperature at which the pre-treated feed gas condenses. 
     
     
         19 . The process according to  claim 18 , wherein the temperature of the mixed refrigerant coolant is less than -150° C. 
     
     
         20 . The process according to  claim 1 , wherein the mixed refrigerant contains compounds selected from a group consisting of nitrogen and hydrocarbons containing from 1 to 5 carbon atoms. 
     
     
         21 . The process according to  claim 20 , wherein the mixed refrigerant comprises nitrogen, methane, ethane or ethylene, isobutane and/or n-butane. 
     
     
         22 . The process according to  claim 20 , wherein the composition of the mixed refrigerant is in the following mole fraction percent ranges: nitrogen: about 5 to about 15; methane: about 25 to about 35; C2: about 33 to about 42; C3: 0 to about 10; C4: 0 to about 20 about; and C5: 0 to about 20. 
     
     
         23 . The process according to  claim 1 , wherein the hydrocarbon gas is natural gas or coal seam methane. 
     
     
         24 . The process according to  claim 23 , wherein the hydrocarbon gas is recovered from the refrigeration zone at a temperature at or below the liquefaction temperature of methane. 
     
     
         25 . A hydrocarbon gas liquefaction system comprising:
 a) a mixed refrigerant;   b) a compressor for compressing the mixed refrigerant;   c) a refrigeration heat exchanger for cooling a pre-treated feed gas to produce a hydrocarbon liquid, the refrigeration heat exchanger having a first heat exchange pathway in fluid communication with the compressor, a second heat exchange pathway, and a third heat exchange pathway, the first, second and third heat exchange pathways extending through the refrigeration zone, and a fourth heat exchange pathway extending through a portion of the refrigeration zone, the second and fourth heat exchange pathways being positioned in counter current heat exchange in relation to the first and third heat exchange pathways;
 an expander in fluid communication with an outlet from the first heat exchange pathway and an inlet to the second heat exchange pathway; 
   d) a recirculation mixed refrigerant line in fluid communication with an outlet from the second heat exchange pathway and an inlet to the compressor;   e) an auxiliary refrigeration system having an auxiliary refrigerant in fluid communication with the fourth heat exchange pathway;   f) a source of pre-treated feed gas in fluid communications with an inlet of the third heat exchange pathway; and   g) a hydrocarbon liquid line in fluid communication with an outlet of the third heat exchange pathway.   
     
     
         26 . The system according to  claim 25 , wherein the compressor is a single stage compressor driven by a gas turbine. 
     
     
         27 . The system according to  claim 26 , wherein the compressor is a single stage centrifugal. 
     
     
         28 . The system according to  claim 26 , the compressor is a two stage compressor driven by respective gas turbines with intercooler and interstage scrubber. 
     
     
         29 . The system according to  claim 26 , wherein the gas turbine is coupled with a steam generator in a configuration whereby, in use, waste heat from the gas turbine facilitates production of steam in the steam generator. 
     
     
         30 . The system according to  claim 29 , wherein the steam generator is coupled to a single steam turbine generator configured to produce electrical power. 
     
     
         31 . The system according to  claim 30 , wherein the amount of electrical power generated by the single steam turbine generator is sufficient to drive the auxiliary refrigeration system. 
     
     
         32 . The system according to  claim 25 , wherein the auxiliary refrigerant comprises low temperature ammonia and the auxiliary refrigeration system comprises one or more ammonia refrigeration packages. 
     
     
         33 . The system according to  claim 32 , wherein the one or more ammonia refrigeration packages are cooled by air coolers. 
     
     
         34 . The system according to  claim 26 , wherein the auxiliary refrigeration system is in heat exchange communication with the gas turbine, the heat exchange communication being configured in a manner to effect cooling of inlet air of the gas turbine by the auxiliary refrigeration system. 
     
     
         35 . The system according to  claim 25 , wherein the system comprises a cooler to cool the compressed mixed refrigerant prior to the compressed mixed refrigerant being received in the refrigeration heat exchanger. 
     
     
         36 . The system according to  claim 35 , wherein the cooler is an air-cooled heat exchanger, or a water-cooled heat exchanger. 
     
     
         37 . The system according to  claim 25 , wherein the hydrocarbon liquid in the hydrocarbon liquid line is expanded through an expander to further cool the hydrocarbon liquid.