Hydraulic Turbine Between Middle and Cold Bundles of Natural Gas Liquefaction Heat Exchanger

Abstract

A system and method for liquefying a natural gas stream, including a liquefaction heat exchanger having at least three cooling bundles and arranged such that the natural gas stream passes sequentially therethrough. A first cooling bundle condenses heavy hydrocarbon components in the natural gas stream. A second cooling bundle liquefies the natural gas stream. A third cooling bundle sub-cools the LNG stream. A hydraulic turbine has an inlet operationally connected to an outlet of the second cooling bundle, and an outlet operationally connected to an inlet of the third cooling bundle. The hydraulic turbine cools the LNG stream and reduces the pressure of the LNG stream to form a reduced-pressure LNG stream.

What is claimed is: 
     
         1 . A system for liquefying a natural gas stream, comprising:
 a liquefaction heat exchanger having at least three cooling bundles and arranged such that the natural gas stream passes sequentially therethrough, including   a first cooling bundle configured to condense heavy hydrocarbon components in the natural gas stream,   a second cooling bundle configured to liquefy the natural gas stream, the second cooling bundle having an outlet for passing an LNG stream therethrough, and   a third cooling bundle having an inlet to receive the LNG, the third cooling bundle configured to sub-cool the LNG stream; and   a hydraulic turbine having an inlet operationally connected to the outlet of the second cooling bundle and an outlet operationally connected to the inlet of the third cooling bundle, the hydraulic turbine configured to cool the LNG stream and reduce a pressure of the LNG stream to form a reduced-pressure LNG stream.   
     
     
         2 . The system of  claim 1 , further comprising:
 a first set of one or more sensors situated to sense at least one of a pressure and a temperature of the LNG stream prior to entering the hydraulic turbine; and   a second set of one or more sensors situated to sense at least one of a pressure and a temperature of the LNG stream as the LNG stream exits the hydraulic turbine.   
     
     
         3 . The system of  claim 2 , wherein at least one of a) a speed of the hydraulic turbine and b) an LNG inlet flow rate to the hydraulic turbine is adjusted based on at least one of the sensed temperature of the LNG stream prior to entering the hydraulic turbine, the sensed pressure of the LNG stream prior to entering the hydraulic turbine, the sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and the sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine. 
     
     
         4 . The system of  claim 2 , further comprising a bypass valve operationally connecting the outlet of the second cooling bundle and the inlet of the third cooling bundle such that, when open, at least a portion of the LNG stream bypasses the hydraulic turbine. 
     
     
         5 . The system of  claim 4 , wherein the bypass valve is selectively controlled based on at least one of the sensed temperature of the LNG stream prior to entering the hydraulic turbine, the sensed pressure of the LNG stream prior to entering the hydraulic turbine, the sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and the sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine. 
     
     
         6 . The system of  claim 1 , further comprising a control valve disposed between the outlet of the hydraulic turbine and the inlet of the third cooling bundle, wherein the control valve is selectively controlled based at least in part on one or more of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine. 
     
     
         7 . The system of  claim 1 , further comprising a generator connected to the hydraulic turbine and configured to generate power based on the work energy generated by the hydraulic turbine. 
     
     
         8 . The system of  claim 7 , further comprising:
 a first set of one or more sensors situated to sense at least one of a pressure and a temperature of the LNG stream prior to entering the hydraulic turbine, and   a second set of one or more sensors situated to sense at least one of a pressure and a temperature of the LNG stream as the LNG stream exits the hydraulic turbine;   wherein a speed of the generator is adjusted based on at least one of the sensed temperature of the LNG stream prior to entering the hydraulic turbine, the sensed pressure of the LNG stream prior to entering the hydraulic turbine, the sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and the sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine.   
     
     
         9 . The system of  claim 7 , further comprising a variable-speed constant-frequency (VSCF) drive situated between the generator and a power system, wherein the VSCF drive is selectively controlled based at least in part on one or more of the sensed temperature of the LNG stream prior to entering the hydraulic turbine, the sensed pressure of the LNG stream prior to entering the hydraulic turbine, the sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, the sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine and the power system frequency. 
     
     
         10 . The system of  claim 1 , further comprising at least one of a mechanical brake and a compressor operationally connected to the hydraulic turbine. 
     
     
         11 . The system of  claim 10 , wherein the brake is selectively controlled based at least in part on one or more of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine. 
     
     
         12 . The system of  claim 1 , further comprising:
 a liquefied petroleum gas (LPG) stream configured to pass through the first cooling bundle and the second cooling bundle, the reduced-pressure LNG stream being at a pressure so to as to be combined with the LPG stream after the LPG stream has passed through the second cooling bundle.   
     
     
         13 . A method of liquefying a natural gas stream to produce liquefied natural gas (LNG), comprising:
 sequentially cooling the natural gas stream in first, second, and third cooling bundles of a liquefaction heat exchanger, wherein the second cooling bundle liquefies the natural gas stream to produce an LNG stream;   cooling and reducing the pressure of the LNG stream between the second cooling bundle and the third cooling bundle using a hydraulic turbine, to thereby produce a reduced-pressure LNG stream; and   producing work energy using the hydraulic turbine.   
     
     
         14 . The method of  claim 13 , further comprising:
 adjusting at least one of a) a speed of the hydraulic turbine and b) an LNG inlet rate of the hydraulic turbine based on at least one of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine.   
     
     
         15 . The method of  claim 13 , further comprising:
 selectively directing at least a portion of the LNG stream exiting the hydraulic turbine through a bypass valve that operationally connects an outlet of the second cooling bundle and an inlet of the third cooling bundle; and   selectively controlling the bypass valve based on at least one of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine.   
     
     
         16 . The method of  claim 13 , further comprising controlling a pressure of the LNG stream exiting the hydraulic turbine by disposing a control valve between an outlet of the hydraulic turbine and an inlet of the third cooling bundle, wherein the control valve is selectively controlled based at least in part on one or more of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine. 
     
     
         17 . The method of  claim 13 , further comprising:
 connecting a generator to the hydraulic turbine; and   generating power using the generator based on the work energy generated by the hydraulic turbine.   
     
     
         18 . The method of  claim 17 , further comprising:
 adjusting a speed of the generator based on at least one of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine.   
     
     
         19 . The method of  claim 17 , further comprising:
 controlling an electrical output of the generator using a variable-speed constant-frequency drive situated between the hydraulic turbine and the generator.   
     
     
         20 . The method of  claim 13 , further comprising:
 operationally connecting at least one of a mechanical brake and a compressor to the hydraulic turbine.   
     
     
         21 . The method of  claim 13 , further comprising:
 obtaining a liquefied petroleum gas (LPG) stream from a fractionation process that occurs prior to the natural gas stream being sequentially cooled in the liquefaction heat exchanger;   cooling the LPG stream in the first cooling bundle and the second cooling bundle, the reduced-pressure LNG stream being at a pressure so as to be combined with the LPG stream after the LPG stream has passed through the second cooling bundle.   
     
     
         22 . The method of  claim 21 , wherein the liquefaction heat exchanger is part of an operating LNG process, and further comprising:
 retrofitting the hydraulic turbine between the second cooling bundle and the third cooling bundle.   
     
     
         23 . A method of liquefying a natural gas stream to produce liquefied natural gas (LNG), comprising:
 sequentially cooling the natural gas stream in a liquefaction heat exchanger having first, second, and third cooling bundles, wherein the second cooling bundle liquefies the natural gas stream to produce an LNG stream;   cooling and reducing the pressure of the LNG stream between the second cooling bundle and the third cooling bundle using a hydraulic turbine;   producing work energy using the hydraulic turbine;   using the work energy, generating power using a generator connected to the hydraulic turbine;   controlling a pressure of the LNG stream exiting the hydraulic turbine using a control valve disposed between the outlet of the hydraulic turbine and an inlet of the third cooling bundle; and   adjusting at least one of
 a speed of the hydraulic turbine, 
 an LNG inlet rate of the hydraulic turbine, 
 a position of the control valve, and 
 a speed of the generator, 
   
       based on at least one of a sensed temperature of the LNG stream prior to entering the hydraulic turbine, a sensed pressure of the LNG stream prior to entering the hydraulic turbine, a sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and a sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine. 
     
     
         24 . The method of  claim 23 , further comprising:
 when the hydraulic turbine is desired to be bypassed, selectively directing at least a portion of the LNG stream exiting the middle bundle through a bypass valve that operationally connects an outlet of the second cooling bundle and an inlet of the third cooling bundle; and   adjusting a position of the bypass valve based on at least one of the sensed temperature of the LNG stream prior to entering the hydraulic turbine, the sensed pressure of the LNG stream prior to entering the hydraulic turbine, the sensed temperature of the LNG stream as the LNG stream exits the hydraulic turbine, and the sensed pressure of the LNG stream as the LNG stream exits the hydraulic turbine.