Systems and methods for separating alkane gases with applications to raw natural gas processing and flare gas capture

Abstract

The present invention is a field-deployable system for separating methane and natural gas liquids (NGLs) from a raw gas stream comprising a compressor; a dehydrator; a refrigerator having one or more stages; and a separation subsystem adapted to separate the raw gas stream into three product streams including a methane stream that is at least 80% methane, an ethane-rich stream, and a NGLs stream having a vapor pressure of no more than 250 psia at 38° C. The methane stream is sufficiently lean to be useable in existing natural gas engines without modification. The NGLs stream has a sufficiently low vapor pressure to be transportable in standard propane containers. The ethane-rich stream may be utilized within the system itself to power its own operations. The system can be utilized to reduce flaring from liquids-rich gas production sites to an absolute minimum, produce natural gas liquids for transport, and provide dry methane gas suitable for use in portable field generators.

What is claimed is: 
     
         1 . An apparatus for separating methane and natural gas liquids from a raw natural gas stream, comprising:
 a chassis adapted to hold the system for field deployment;   one or more compressors for compressing the raw natural gas stream;   one or more dehydrators for removing water from the compressed natural gas stream;   a refrigerator having one or more stages for lowering a temperature of the dehydrated, compressed natural gas stream; and   a separation subsystem system adapted to separate the dehydrated, compressed, and refrigerated natural gas stream into three product streams consisting essentially of a methane stream that is at least 80% methane, an ethane-rich stream that is high in ethane content, and a natural gas liquids stream having a vapor pressure of no more than 250 psia at 38° C.   
     
     
         2 . The apparatus of  claim 1 , further comprising:
 a power generator tuned to run on ethane-rich gas adapted to provide electricity to run the compressor, the refrigerator, and other system components, utilizing the ethane-rich stream as its energy source.   
     
     
         3 . The apparatus of  claim 1 , wherein the methane stream is at least 85% methane. 
     
     
         4 . The apparatus of  claim 1 , wherein the ethane-rich stream is at least 30% ethane. 
     
     
         5 . The apparatus of  claim 1 , wherein the natural gas liquids stream has a vapor pressure of no more than 200 psia at 38° C. 
     
     
         6 . The apparatus of  claim 1 , wherein the natural gas liquids stream meets a Y grade standard. 
     
     
         7 . The apparatus of  claim 1 , wherein the one or more compressors compress the raw natural gas stream to a pressure range of 100 psig to 500 psig. 
     
     
         8 . The apparatus of  claim 1 , wherein the chassis is mounted on a trailer having one or more wheels. 
     
     
         9 . The apparatus of  claim 1 , wherein the refrigerator further comprises:
 a high-stage refrigeration loop having at least one heat exchanger for lowering a temperature of the dehydrated, compressed natural gas stream; and   a low-stage refrigeration loop having at least one heat exchanger for further lowering the temperature of the dehydrated, compressed natural gas stream.   
     
     
         10 . The apparatus of  claim 9 , wherein the low-stage refrigeration loop is an autocascade loop having mixed refrigerants. 
     
     
         11 . The apparatus of  claim 10 , wherein the mixed refrigerants are hydrocarbons. 
     
     
         12 . The apparatus of  claim 1 , wherein the refrigerator cools the natural gas stream to a temperature range of −40° C. to −70° C. sufficient to achieve a NGLs separation in a single separation column. 
     
     
         13 . The apparatus of  claim 1 , wherein one or more of the dehydrators employ a desiccant bed. 
     
     
         14 . The apparatus of  claim 13 , wherein two desiccant beds are employed in alternation, and wherein heat required to dry the two desiccant beds is derived from waste heat from a power generator that drives the compressors and the refrigerator. 
     
     
         15 . The apparatus of  claim 1 , wherein the separation subsystem comprises a stripping column. 
     
     
         16 . The apparatus of  claim 1 , wherein the separation subsystem comprises a distillation column. 
     
     
         17 . The apparatus of  claim 1 , wherein the separation subsystem comprises one or more flash tanks. 
     
     
         18 . The apparatus of  claim 1 , wherein the separation subsystem comprises one or more cyclones to separate liquids from gasses. 
     
     
         19 . A method for separating useable gas streams from a raw natural gas stream, comprising:
 a compression step for compressing the raw natural gas stream utilizing a compressor to a pressure range of 100 to 500 psig;   a dehydration step for removing water from the compressed natural gas stream utilizing one or more condensers and one or more desiccant beds;   a refrigeration step for reducing a temperature of the dehydrated, compressed natural gas stream utilizing a refrigerator having one or more stages to a temperature range of −40° C. to −70° C.; and   a separation step for separating the dehydrated, compressed, and refrigerated natural gas stream into three product streams consisting essentially of a methane stream of at least 80% methane, an ethane-rich stream that is high in ethane content, and a natural gas liquids stream having a vapor pressure of no more than 250 psia at 38° C.   
     
     
         20 . The method of  claim 19 , further comprising:
 a generation step for generating power utilizing a power generator tuned to run on ethane-rich gas to provide electricity to run the compressor and the refrigerator, utilizing the ethane-rich stream as its energy source.   
     
     
         21 . The method of  claim 19 , further comprising:
 a transportation step for bringing a mobile alkane gas separator system to an oil field site that is flaring associated gas; and   a deployment step for connecting said mobile alkane gas separator system to a raw associated natural gas source.   
     
     
         22 . The method of  claim 19 , wherein the methane stream is at least 85% methane. 
     
     
         23 . The method of  claim 19 , further comprising:
 compressing the methane stream to produce compressed natural gas (CNG).   
     
     
         24 . The method of  claim 19 , further comprising:
 converting the methane stream into methanol for transport;   transporting the methanol to a remote site; and   upgrading the methanol to higher alcohols at the remote site.   
     
     
         25 . The method of  claim 19 , further comprising:
 utilizing the methane stream for enhanced oil recovery.   
     
     
         26 . The method of  claim 19 , wherein the refrigeration step utilizes an autocascade refrigerator having mixed hydrocarbon refrigerants. 
     
     
         27 . The method of  claim 19 , wherein the refrigeration step cools the natural gas stream to at temperature range of −50° C. to −60° C. 
     
     
         28 . The method of  claim 19 , wherein two desiccant beds are employed in alternation, and wherein heat required to dry the two desiccant beds is derived from waste heat from a power generator. 
     
     
         29 . A system for separating a raw natural gas stream into a methane-enriched stream, an ethane-enriched stream, and a natural gas liquids stream, comprising:
 one or more compressors for compressing the raw natural gas stream;   a dehydrator subsystem for removing water from the compressed natural gas stream;   a refrigeration subsystem for lowering a temperature of the dehydrated, compressed natural gas stream to a low temperature, comprising a high-stage cascade refrigeration loop and a low-stage autocascade refrigeration loop having mixed hydrocarbon refrigerants;   a separation subsystem adapted to separate the dehydrated, compressed, and refrigerated natural gas stream into three product streams consisting essentially of a methane stream that is at least 80% methane, an ethane-rich stream that is high in ethane content, and a natural gas liquids stream having a vapor pressure of no more than 250 psia at 38° C.; and   a power generation subsystem tuned to run on ethane-rich gas adapted to provide electricity to run the compressor, the refrigerator, and other system components, utilizing the ethane-rich stream as its energy source.   
     
     
         30 . The system of  claim 29 , wherein the dehydration subsystem employs two desiccant beds in alternation, wherein heat required to dry the two desiccant beds is derived from waste heat from the power generator. 
     
     
         31 . The system of  claim 29 , wherein the natural gas liquids stream has a vapor pressure of no more than 200 psia at 38° C. 
     
     
         32 . The system of  claim 29 , further comprising a chassis for holding system components for field deployment, said chassis mountable to a trailer having one or more wheels for transportation to a field location. 
     
     
         33 . The system of  claim 29 , wherein the refrigeration subsystem comprises:
 a high-stage refrigeration loop having at least one heat exchanger for lowering a temperature of the dehydrated, compressed natural gas stream; and   a low-stage refrigeration loop having at least one heat exchanger for further lowering the temperature of the dehydrated, compressed natural gas stream,   wherein the low-stage refrigeration loop is an autocascade loop having mixed refrigerants.   
     
     
         34 . The system of  claim 29 , wherein the refrigeration subsystem cools the natural gas stream to a temperature range of −40° C. to −70° C. sufficient to achieve a NGLs separation in a single separation column. 
     
     
         35 . A natural gas liquids stream having a vapor pressure of no more than 250 psia at 38° C. produced from a raw natural gas stream at a wellhead, said natural gas liquids stream made by a process comprising the steps of:
 compressing the raw natural gas stream utilizing a compressor to a pressure range of 100 to 300 psig; 
 removing water from the compressed natural gas stream utilizing one or more condensers and one or more desiccant beds to achieve an aqueous dew point below −70° C.; 
 reducing a temperature of the dehydrated, compressed natural gas stream utilizing a refrigerator having one or more stages to a temperature range of −40° C. to −70° C.; and 
 separating the dehydrated, compressed, and refrigerated natural gas stream into said natural gas liquids stream having a vapor pressure of no more than 250 psia at 38° C. and two additional product streams consisting essentially of a methane stream of at least 80% methane and an ethane-rich stream that is high in ethane content.