System and method for liquefying natural gas employing turbo expander

Abstract

An improved system and method for liquefying natural gas employing liquid nitrogen is disclosed. The improved system and method lowers the nitrogen consumption rate by using an expander, for example, a radial inflow turbo-expander, on the nitrogen side. This reduction in nitrogen consumption rate substantially reduces system operating costs.

What is claimed is: 
     
         1 . A natural gas liquefier system, comprising:
 a natural gas input coupled to a source of natural gas;   a liquid nitrogen input coupled to a source of liquid nitrogen;   a liquefier module coupled to receive the natural gas and liquid nitrogen and liquefy the natural gas by boiling the liquid nitrogen;   a turbo expander module coupled to the liquefier module to receive the boiled gaseous nitrogen, cool the gaseous nitrogen by expansion and reintroduce the colder gaseous nitrogen into the liquefier module; and   a liquefied natural gas output coupled to the liquefier module.   
     
     
         2 . A natural gas liquefier system as set out in  claim 1 , wherein the turbo expander module comprises a turbo expander coupled to the liquefier module in a closed loop. 
     
     
         3 . A natural gas liquefier system as set out in  claim 1 , wherein the liquefier module comprises at least one heat exchanger. 
     
     
         4 . A natural gas liquefier system as set out in  claim 3 , wherein the at least one heat exchanger comprises a first heat exchanger and a second heat exchanger, wherein the second heat exchanger is coupled to the liquid nitrogen source and liquefies the natural gas by boiling the liquid nitrogen and outputs liquefied natural gas to the liquefied natural gas output and outputs boiled gaseous nitrogen to the first heat exchanger, and wherein the turbo expander is coupled in a closed loop to the first heat exchanger. 
     
     
         5 . A natural gas liquefier system as set out in  claim 4 , wherein the first heat exchanger has first and second inputs for receiving gaseous nitrogen at a different temperature and different pressure from the second heat exchanger and expander, respectively. 
     
     
         6 . A natural gas liquefier system as set out in  claim 1 , further comprising a liquid nitrogen pump coupled between the liquid nitrogen input and the liquefier module. 
     
     
         7 . A natural gas liquefier system as set out in  claim 6 , further comprising a monitoring and control system coupled to the liquid nitrogen pump, turbo expander module and liquefier module. 
     
     
         8 . A natural gas liquefier system as set out in  claim 7 , wherein the turbo expander module comprises an input volume flow control mechanism. 
     
     
         9 . A natural gas liquefier system as set out in  claim 8 , wherein the monitoring and control system controls the liquid nitrogen pump and turbo expander module input volume flow control mechanism. 
     
     
         10 . A natural gas liquefier system as set out in  claim 9 , wherein the monitoring and control system receives a liquid nitrogen level signal from the liquefier module and input pressure signal from the turbo expander module. 
     
     
         11 . A natural gas liquefier system as set out in  claim 1 , further comprising a nitrogen reclaiming system coupled to receive used nitrogen gas from the liquefier module, liquefy the nitrogen gas and return the liquid nitrogen to the liquid nitrogen source. 
     
     
         12 . A natural gas liquefier system, comprising:
 a natural gas flow path including a natural gas input and liquefied natural gas outputs;   a separate nitrogen flow path including a liquid nitrogen input and a gaseous nitrogen outputs;   a liquefier module coupled to both the natural gas and nitrogen flow paths wherein the liquid nitrogen is brought into thermal contact with the natural gas to liquefy the natural gas and boil the nitrogen; and   an expander coupled to the nitrogen flow path in a closed loop to receive boiled gaseous nitrogen, cool the nitrogen gas through expansion and reintroduce the cooler nitrogen gas into the nitrogen flow path.   
     
     
         13 . A natural gas liquefier system as set out in  claim 12 , wherein the nitrogen flow path includes a nitrogen reliquefaction system coupled to receive nitrogen gas from the liquefier module, liquefy the nitrogen gas and return the liquid nitrogen to the nitrogen supply to the liquefier module. 
     
     
         14 . A natural gas liquefier system as set out in  claim 12 , wherein the liquefier module includes first and second heat exchangers, wherein the first heat exchanger is configured in the nitrogen flow path in a nitrogen superheating region having boiled nitrogen in gaseous form, and wherein the expander is coupled to the first heat exchanger. 
     
     
         15 . A natural gas liquefier system as set out in  claim 12 , wherein the second heat exchanger comprises a reboiler which receives the liquid nitrogen and the natural gas, liquefies the natural gas and boils the nitrogen and provides the boiled nitrogen gas to the first heat exchanger at a first temperature and pressure. 
     
     
         16 . A natural gas liquefier system as set out in  claim 15 , wherein the expander provides cooled nitrogen gas to the first heat exchanger at a second temperature and pressure. 
     
     
         17 . A method of reducing liquid nitrogen usage in a natural gas liquefaction process in a system having a natural gas liquefier module and a cryogenic turbo expander, comprising:
 boiling liquid nitrogen in the natural gas liquefier module creating a nitrogen superheating region in the liquefier module; and   introducing two pressure levels of nitrogen cooling media in the nitrogen superheating region by using expansion of boiled nitrogen in the turbo expander.   
     
     
         18 . A method as set out in  claim 17 , further comprising:
 monitoring nitrogen pressure input to the turbo expander; and   controlling the flow of boiled nitrogen into the turbo expander to control the pressure of boiled nitrogen.   
     
     
         19 . A method as set out in  claim 18 , further comprising:
 monitoring liquid nitrogen level in the liquefier module; and   controlling the pressure of liquid nitrogen supplied to the liquefier module.   
     
     
         20 . A method as set out in  claim 17 , further comprising reclaiming and liquefying waste nitrogen and reintroducing the reclaimed liquid nitrogen into the liquefaction system.