Method and system for cooling a hydrocarbon stream

Abstract

A system and method for increasing the efficiency of natural gas liquefaction processes by using a hybrid cooling system and method. More specifically, a system and method for converting a transcritical precooling refrigeration process to a subcritical process. In one embodiment, the refrigerant is cooled to sub-critical temperature using an economizer. In another embodiment, the refrigerant is cooled to a sub-critical temperature using an auxiliary heat exchanger. Optionally, the economizer or auxiliary heat exchanger can be bypassed when ambient temperatures are sufficiently low to cool the refrigerant to a sub-critical temperature. In another embodiment, the refrigerant is isentropically expanded.

1 . A method for cooling a hydrocarbon feed stream against a first refrigerant to produce a cooled hydrocarbon stream, the first refrigerant having a critical temperature, the method comprising:
 (a) compressing the first refrigerant in one or more compression stages to produce a compressed first refrigerant;   (b) cooling the compressed first refrigerant against ambient fluid in one or more ambient heat exchangers to produce a cooled first refrigerant at a first temperature;   (c) cooling a fluid stream in each of at least one cooling circuit located in downstream fluid flow communication from the one or more ambient heat exchangers, each of the at least one cooling circuit having at least one evaporation stage, each of the following steps being performed in each evaporation stage:
 (i) reducing the pressure of the first refrigerant; 
 (ii) cooling the fluid stream against the reduced pressure first refrigerant in an evaporator, resulting in vaporization of at least a portion of the reduced pressure first refrigerant; and 
 (iii) flowing at least a portion of the vaporized reduced pressure first refrigerant into one of the at least one compression stages; 
 wherein at least one fluid stream being cooled in the at least one cooling circuit comprises the hydrocarbon feed stream and step (c) produces a cooled hydrocarbon stream; 
   (d) after step (b) and before step (c), further cooling the cooled first refrigerant in at least one auxiliary heat exchanger against an auxiliary refrigerant to produce a further cooled first refrigerant at a second temperature if the first temperature is greater than or equal to the critical temperature of the first refrigerant, the second temperature being less than the critical temperature of the first refrigerant; and   (e) after step (b) and before step (c), bypassing the at least one auxiliary heat exchanger if the first temperature is less than the critical temperature of the first refrigerant.   
     
     
         2 . The method of  claim 1 , wherein the at least one auxiliary heat exchanger comprises an economizer and the auxiliary refrigerant comprises the first refrigerant. 
     
     
         3 . The method of  claim 1 , wherein the auxiliary refrigerant is at least a portion of the hydrocarbon feed stream. 
     
     
         4 . The method of  claim 1 , wherein the at least one auxiliary heat exchanger is a part of a closed loop vapor compression system. 
     
     
         5 . The method of  claim 4 , wherein the auxiliary refrigerant is a hydrofluorocarbon or propane. 
     
     
         6 . The method of  claim 1 , further comprising:
 (f) further cooling and liquefying the cooled hydrocarbon stream in at least one liquefaction heat exchanger against a second refrigerant stream to produce a liquefied natural gas stream.   
     
     
         7 . The method of  claim 6 , wherein at least one fluid stream being cooled in the at least one cooling circuit comprises the second refrigerant. 
     
     
         8 . The method of  claim 1 , wherein the first refrigerant comprises ethane, carbon-dioxide, or ethylene. 
     
     
         9 . The method of  claim 1 , wherein step (a) further comprises:
 (a) compressing the first refrigerant in a plurality of compression stages to produce a compressed first refrigerant.   
     
     
         10 . The method of  claim 9 , wherein step (c) further comprises cooling at least one fluid stream in a plurality of evaporation stages located downstream from the economizer, wherein the steps (c)(i) through (c)(iii) are performed in each of the plurality of evaporation stages. 
     
     
         11 . An apparatus for cooling a hydrocarbon feed stream, the apparatus comprising:
 at least one compression stage operationally configured to compress a first refrigerant;   at least one ambient heat exchanger in downstream fluid flow communication with the at least one compression stage, the at least one ambient heat exchanger being operationally configured to cool the first refrigerant to a first temperature by indirect heat exchange against an ambient fluid;   at least one auxiliary heat exchanger in downstream fluid flow communication with the at least one ambient heat exchanger, the auxiliary heat exchanger being operationally configured to further cool the first refrigerant to a second temperature that is below the critical temperature of the first refrigerant;   at least one cooling circuit located in downstream fluid flow communication from the at least one auxiliary heat exchanger, each of the at least one cooling circuit having at least one evaporation stage, each of the evaporation stages comprising an expansion valve in upstream fluid flow communication with an evaporator, the evaporator operationally configured to cool a fluid stream against the first refrigerant and to create a vaporized first refrigerant stream and a cooled fluid stream, each of the evaporation stages further comprising a vaporized first refrigerant circuit in fluid flow communication with one of the at least one compression stages;   a bypass system comprising a controller, at least one temperature sensor, a plurality of valves, and at least one bypass circuit in fluid flow communication with the at least one ambient heat exchanger and the at least one cooling circuit, the bypass system operationally configured to (1) prevent flow of the first refrigerant through the at least one bypass circuit and allow flow of the first refrigerant through the at least one auxiliary heat exchanger when the first temperature is greater than or equal to the critical temperature of the first refrigerant and (2) allow flow of the first refrigerant through the at least one bypass circuit and prevent flow of the first refrigerant through the at least one auxiliary heat exchanger when the first temperature is less than the critical temperature of the first refrigerant;   wherein the fluid stream of at least one of the at least one cooling circuit comprises the hydrocarbon feed stream.   
     
     
         12 . The apparatus of  claim 11 , wherein the at least one auxiliary heat exchanger comprises an economizer. 
     
     
         13 . The apparatus of  claim 11 , wherein the at least one auxiliary heat exchanger is part of a closed loop vapor compression system. 
     
     
         14 . A method for cooling a hydrocarbon feed stream against a first refrigerant to produce a cooled hydrocarbon stream, the first refrigerant having a critical temperature, wherein the method comprises:
 (a) compressing the first refrigerant in at least one compression stage to produce a compressed first refrigerant;   (b) cooling the compressed first refrigerant against an ambient fluid in at least one ambient heat exchanger to produce a cooled first refrigerant at a first temperature that is greater than or equal to the critical temperature of the first refrigerant;   (c) cooling a fluid stream in each of at least one cooling circuit located in downstream fluid flow communication from the ambient heat exchanger, each of the at least one cooling circuit having at least one evaporation stage, each of the following steps being performed in each evaporation stage:
 (i) reducing the pressure of the first refrigerant; 
 (ii) cooling the fluid stream against the reduced pressure first refrigerant in an evaporator, resulting in vaporization of at least a portion of the reduced pressure first refrigerant; and 
 (iii) flowing at least a portion of the vaporized reduced pressure first refrigerant into one of the at least one compression stages; 
 wherein the at least one evaporation stage of each of the at least one cooling circuit comprises a first evaporation stage that is located at an upstream end of the at least one cooling circuit, wherein step (c)(i) comprises the following step in each first evaporation stage: 
 (c)(i) reducing the pressure of the first portion of the first refrigerant using an isentropic expansion device to produce a first reduced pressure first refrigerant having a vapor fraction of no less than 0.2 and no more than 0.6. 
   wherein at least one fluid stream being cooled in the at least one cooling circuit is selected from the group of: the hydrocarbon stream and a second refrigerant stream.   
     
     
         15 . The method of  claim 14 , further comprising:
 (d) further cooling and liquefying the cooled hydrocarbon stream in at least one liquefaction heat exchanger against a second refrigerant stream to produce a liquefied natural gas stream.   
     
     
         16 . The method of  claim 15 , wherein at least one fluid stream being cooled in the at least one cooling circuit comprises the second refrigerant. 
     
     
         17 . The method of  claim 14 , wherein the first refrigerant is ethane, carbon-dioxide, or ethylene. 
     
     
         18 . The method of  claim 14 , wherein step (a) further comprises:
 (a) compressing the first refrigerant in a plurality of compression stages to produce a compressed first refrigerant.