Partial open-loop nitrogen refrigeration process and system for an oil or gas production operation

Abstract

A method for cooling a hydrocarbon production stream such as natural gas uses cryogenic nitrogen as a cooling medium (“refrigerant”) wherein only a portion of a nitrogen refrigerant stream is recovered, with a vapor portion of the nitrogen refrigeration stream being vented from the system. Unlike a conventional sacrificial nitrogen refrigeration process which vents all the nitrogen refrigerant after cooling a production stream, the method comprise means for recovering some of the nitrogen refrigerant thereby improving the operating efficiency of the process compared to conventional sacrificial nitrogen refrigeration processes. Also unlike conventional closed loop nitrogen refrigeration processes which recover all of the nitrogen refrigerant after cooling a production stream, the method can recover nitrogen refrigerant without the complex and costly equipment used in closed loop systems to compress nitrogen vapor.

What is claimed is: 
     
         1 . A method for cooling a production stream from an oil or gas production operation, comprising:
 (a) flowing a refrigerant feed stream comprising a non-greenhouse gas (GHG) refrigerant and a production stream comprising a hydrocarbon fluid through a production stream heat exchanger such that the production stream is cooled and the refrigerant feed stream is heated;   (b) flowing a refrigerant return stream comprising the non-GHG refrigerant out of the first production stream heat exchanger and into a pre-cooling heat exchanger wherein the refrigerant return stream is cooled;   (c) reducing pressure of the refrigerant return stream to further cool the refrigerant return stream and producing a liquid stream and a vapor stream; and   (d) recovering at least some of the liquid stream and venting at least some of the vapor stream.   
     
     
         2 . The method as claimed in  claim 1  wherein the non-GHG refrigerant is selected from a group consisting of nitrogen, ammonia, helium, neon, oxygen, air, argon and krypton. 
     
     
         3 . The method as claimed in  claim 1  wherein in step (c) the reducing pressure of the refrigerant return stream comprises flowing the refrigerant return stream into an expander and the producing a liquid stream and a vapor stream comprises flowing the refrigerant return stream into a phase separator. 
     
     
         4 . The method as claimed in  claim 3  wherein the expander is selected from a group consisting of a throttling valve and a turbo-expander. 
     
     
         5 . The method as claimed in  claim 1  further comprising flowing the vapor stream into the production stream heat exchanger such that the vapor stream is heated and the production stream is cooled. 
     
     
         6 . The method as claimed in  claim 1  further comprising flowing the liquid stream into a liquid storage tank fluidly coupled to the refrigerant feed stream, such that the refrigerant feed stream comprises at least some of the liquid stream. 
     
     
         7 . The method as claimed in  claim 1  further comprising adiabatically pressuring the non-GHG refrigerant feed stream from a selected storage pressure to a selected system pressure. 
     
     
         8 . The method as claimed in  claim 1  wherein in step (b) the refrigerant return stream is cooled by flowing the vapor stream into the pre-cooling heat exchanger such that the vapor stream is heated. 
     
     
         9 . The method as claimed in  claim 1  wherein in step (b) the refrigerant return stream is cooled by flowing the refrigerant feed stream into the pre-cooling heat exchanger before flowing into the production stream heat exchanger. 
     
     
         10 . The method as claimed in  claim 8  further comprising flowing the production stream and the vapor stream from the pre-cooling heat exchanger into a chiller heat exchanger such that the production stream is cooled and the vapor stream is heated. 
     
     
         11 . The method as claimed in  claim 8  further comprising flowing the production stream through a throttling valve such that the production stream is expanded and cooled. 
     
     
         12 . The method as claimed in  claim 11  wherein the production stream comprises natural gas which is liquefied when the production stream is cooled in the production stream heat exchanger. 
     
     
         13 . The method as claimed in  claim 1  further comprising flowing the production stream out of the production stream heat exchanger and into a phase separator to produce a lean production stream for flowing into a pipeline or downstream process, and a condensed liquid phase production stream for storage in a production liquids storage tank. 
     
     
         14 . The method as claimed in  claim 13  wherein the production stream upstream of the production stream heat exchanger comprises gaseous phase natural gas with a methane composition below a pipeline or process inlet specification, and the method further comprises cooling the production stream in the production stream heat exchanger to a temperature which condenses hydrocarbon heavy ends from the production stream to produce the lean production stream comprising gaseous phase natural gas with a methane composition at or above the pipeline or process inlet specification, and the liquid phase production stream comprising the condensed hydrocarbon heavy ends. 
     
     
         15 . The method as claimed in  claim 9  wherein the refrigerant return stream is cooled in the pre-cooling heat exchanger such that condensation occurs within the refrigerant return stream. 
     
     
         16 . The method as claimed in  claim 15  wherein after cooling in the pre-cooling the heat exchanger, flowing the refrigerant return stream into an expander wherein the refrigerant return stream is expanded and further cooled such that the refrigerant return stream leaving the expander is a saturated liquid. 
     
     
         17 . The method as claimed in  claim 1  further comprising flowing the production stream out of the production stream heat exchanger and into a phase separator to a condensed liquid phase production stream for storage in a production liquids storage tank. 
     
     
         18 . The method as claimed in  claim 1  further comprising flowing the production stream into a separator that separates an undesirable gaseous component from the production stream, then venting the undesirable gaseous component. 
     
     
         19 . The method as claimed in  claim 1  further comprising flowing the production stream into a separator to produce an undesirable liquid component stream and a condensed liquid phase production stream, then storing the liquid phase production stream in a production liquid storage tank