Robust recovery of natural gas letdown energy for small scale liquefied natural gas production

Abstract

A method for liquefaction of natural gas using refrigeration from a combination of sources including a refrigeration cycle and letdown energy of natural gas is provided. The natural gas to be liquefied (LNG) is boosted in pressure using a booster that is powered by expansion of a portion of the natural gas flow from the booster through a first natural gas turbine. A second flow of natural gas is expanded in a second natural gas turbine, and the resulting expanded stream, along with the natural gas expanded in the first natural gas turbine, are warmed against the natural gas to be liquefied. The flow rate of the natural gas in the second natural gas turbine is decoupled from the booster, thereby allowing for variation in flow rates and pressures while maintaining a constant production of LNG.

We claim: 
     
         1 . A method for the liquefaction of natural gas, the method comprising the steps of:
 a) withdrawing a pressurized natural gas stream from a natural gas pipeline;   b) boosting a first portion of the pressurized natural gas stream to a higher pressure using a first natural gas booster to produce a boosted pressurized natural gas stream;   c) expanding a first portion of the boosted pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream;   d) warming the first expanded natural gas stream in a heat exchanger against a second portion of the boosted pressurized natural gas stream to produce a first warmed natural gas stream;   e) expanding a second portion of the pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream;   f) warming the second expanded natural gas stream in the heat exchanger against the second portion of the boosted pressurized natural gas stream to produce a second warmed natural gas stream; and   g) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product.   
     
     
         2 . The method as claimed in  claim 1 , wherein the first natural gas turbine is configured to power the first natural gas booster. 
     
     
         3 . The method as claimed in  claim 1 , wherein the second natural gas turbine is configured to power a generator such that electricity is produced during step f). 
     
     
         4 . The method as claimed in  claim 1 , wherein the second natural gas turbine is configured to power a second natural gas booster. 
     
     
         5 . The method as claimed in  claim 1 , wherein the refrigeration cycle comprises a refrigerant by-pass configured to remove a portion of refrigerant coolant from an intermediate section of the heat exchanger thereby reducing the amount of cooling provided from the refrigeration cycle to the second portion of the boosted pressurized natural gas stream. 
     
     
         6 . The method as claimed in  claim 1 , wherein the first warmed natural gas stream and the second warmed natural gas stream are combined within the heat exchanger or combined before entering the heat exchanger. 
     
     
         7 . The method as claimed in  claim 1 , further comprising the steps of:
 removing an impurity from the pressurized natural gas stream using an adsorption bed, wherein the impurity is selected from the group consisting of carbon dioxide, water, and combinations thereof; and   regenerating the adsorption bed using a low pressure natural gas, wherein the low pressure natural gas is selected from the group consisting of the first warmed natural gas stream, the second warmed natural gas stream, and combinations thereof.   
     
     
         8 . The method as claimed in  claim 1 , wherein the first expanded natural gas stream and the second expanded natural gas stream are expanded to about the same pressure. 
     
     
         9 . The method as claimed in  claim 1 , further comprising the steps of:
 sending a flow of a low pressure natural gas stream to a user;   adjusting the amount of the second portion of the pressurized natural gas stream expanded in the second natural gas turbine based on the flow of the low pressure natural gas sent to the user,   wherein the low pressure natural gas is selected from the group consisting of the first warmed natural gas stream, the second warmed natural gas stream, and combinations thereof.   
     
     
         10 . The method as claimed in  claim 1 , wherein the amount of LNG product produced in step h) is unchanged by the amount of the second portion of the pressurized natural gas stream expanded in a second natural gas turbine in step f). 
     
     
         11 . The method as claimed in  claim 1 , wherein the pressure of the second portion of the boosted pressurized natural gas stream during liquefaction is unchanged by the combined flow rate of the first expanded natural gas stream and the second expanded natural gas stream. 
     
     
         12 . The method as claimed in  claim 1 , further comprising the steps of:
 modifying the amount of the second portion of the pressurized natural gas stream expanded in the second natural gas turbine in step f); and   adjusting the refrigeration provided from the refrigeration cycle to the second portion of the boosted pressurized natural gas stream in step h) in order to keep the amount of the LNG product produced within a targeted range.   
     
     
         13 . The method as claimed in  claim 1 , wherein the refrigeration cycle is selected from the group consisting of a nitrogen refrigeration cycle and a mixed refrigerant refrigeration cycle. 
     
     
         14 . The method as claimed in  claim 1 , wherein the refrigeration cycle has only one turbine-booster. 
     
     
         15 . The method as claimed in  claim 1 , wherein the nitrogen cycle comprises two turbine-boosters. 
     
     
         16 . A method for the liquefaction of natural gas, the method comprising the steps of:
 a) withdrawing a pressurized natural gas stream from a natural gas pipeline operating at a first pressure;   b) boosting a first portion of the pressurized natural gas stream in a first natural gas booster to a second pressure to produce a boosted pressurized natural gas stream;   c) expanding a first portion of the boosted pressurized natural gas stream in a first natural gas expansion turbine to a third pressure to produce a first expanded natural gas stream;   d) liquefying a second portion of the boosted pressurized natural gas stream in a natural gas liquefier using refrigeration provided by a refrigeration cycle;   e) expanding a second portion of the pressurized natural gas stream in a second expansion turbine to a fourth pressure to produce a second expanded natural gas stream;   f) warming the first expanded natural gas stream and the second expanded natural gas stream by in-direct heat exchange against the second portion of the boosted natural gas stream to produce a first and second warmed expanded natural gas stream;   g) sending the first and second warmed expanded natural gas stream to a downstream facility, wherein the downstream facility has a natural gas demand,   wherein the first natural gas expansion turbine is configured to provide compression power for the first natural gas booster.   
     
     
         17 . The method as claimed in  claim 16 , wherein the flow rate of the second portion of the pressurized natural gas stream expanded in the second expansion turbine is adjusted based on changes in the natural gas demand of the downstream facility. 
     
     
         18 . The method as claimed in  claim 17 , wherein the flow rate of the second portion of the boosted pressurized natural gas stream is independent of the natural gas demand of the downstream facility. 
     
     
         19 . The method as claimed in  claim 16 , further comprising the step of monitoring the first pressure; and adjusting the flow rates of the first portion of the boosted pressurized natural gas stream and the second portion of the pressurized natural gas stream based on the first pressure while maintaining the flow rate of the LNG stream produced in step d). 
     
     
         20 . A method for the liquefaction of natural gas, the method comprising the steps of:
 a) boosting a first pressurized natural gas stream to a higher pressure using a first natural gas booster to produce a boosted pressurized natural gas stream;   b) expanding a second pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream;   c) warming the first expanded natural gas stream in a heat exchanger against a second portion of the boosted pressurized natural gas stream to produce a first warmed natural gas stream;   d) expanding a third pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream;   e) warming the second expanded natural gas stream in the heat exchanger against the second portion of the boosted pressurized natural gas stream to produce a second warmed natural gas stream; and   f) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product, wherein the first pressurized natural gas stream, the second pressurized natural gas stream, and the third pressurized natural gas stream all originate from a common high pressure natural gas pipeline.   
     
     
         21 . A method for the liquefaction of natural gas, the method comprising the steps of:
 a) expanding a first pressurized natural gas stream in a first natural gas turbine to form a first expanded natural gas stream;   b) warming the first expanded natural gas stream in a heat exchanger against a second pressurized natural gas stream to produce a first warmed natural gas stream;   c) expanding a third pressurized natural gas stream in a second natural gas turbine to form a second expanded natural gas stream;   d) warming the second expanded natural gas stream in the heat exchanger against the second pressurized natural gas stream to produce a second warmed natural gas stream; and   e) liquefying the second portion of the boosted pressurized natural gas stream in the heat exchanger using refrigeration provided from a refrigeration cycle to form a liquefied natural gas (LNG) product,   wherein the first natural gas turbine is configured to drive a first booster, wherein the first booster is configured to compress a stream selected from the group consisting of the first pressurized natural gas stream, the first warmed natural gas stream, the second pressurized natural gas stream.