Method and apparatus for cooling down a cryogenic heat exchanger and method of liquefying a hydrocarbon stream

Abstract

The present invention relates to a method and apparatus for cooling down a cryogenic heat exchanger adapted to liquefy a hydrocarbon stream, such as a natural gas stream. The method comprises: (i) receiving one or more refrigerant temperature indications, providing an indication of the temperature of the refrigerant, (ii) comparing the one or more refrigerant temperature indications with one or more associated predetermined threshold values, and (iii) based on the outcome of the comparison under (ii) selecting one of an automated warm cooling down procedure of the cryogenic heat exchanger and an automated cold cooling down procedure of the cryogenic heat exchanger.

What is claimed is: 
     
         1 . A method of cooling down a cryogenic heat exchanger adapted to liquefy a hydrocarbon stream, the method comprising the steps of
 receiving, by the cryogenic heat exchanger, the hydrocarbon stream to be liquefied and a refrigerant, to exchange heat between the hydrocarbon stream and the refrigerant, thereby at least partially liquefying the hydrocarbon stream, and to discharge the at least partially liquefied hydrocarbon stream and a spent refrigerant that has passed through the cryogenic heat exchanger,   recirculating, by a refrigerant recirculation circuit, the spent refrigerant back to the cryogenic heat exchanger, wherein the refrigerant recirculation circuit comprises at least a compressor, a compressor recycle valve, a cooler, and a first Joule Thomson (JT) valve;   performing a comparison step by a programmable controller, said comparison step comprising:   (i) receiving one or more refrigerant temperature indications, and providing an indication of a temperature of the refrigerant,   (ii) comparing the one or more refrigerant temperature indications with one or more associated predetermined threshold values, and   (iii) based on an outcome of the comparison under (ii) selecting one of an automated warm cooling down procedure of the cryogenic heat exchanger and an automated cold cooling down procedure of the cryogenic heat exchanger;   wherein the automated cold cooling down procedure is adapted to reduce a temperature of the cryogenic heat exchanger from a cold condition down to LNG production point and is allowed to start with an opened first JT valve,   wherein the automated warm cooling down procedure comprises an initial opening step, wherein the initial opening step comprises imposing an initial opening of the first JT valve, wherein the initial opening step of the first JT valve according to the automated warm cooling down procedure differs in size of the opened first JT valve according to the automated cold cooling down procedure; and   performing a TROC step by the programmable controller, the TROC step comprising adjusting the opening of the first JT valve based on a determined temperature rate of change (TROC) of the refrigerant over the first JT valve in accordance with an adjustment scheme, wherein the automated warm cooling down procedure and the automated cold cooling down procedure comprise a plurality of different adjustment schemes.   
     
     
         2 . The method of  claim 1 , further comprising the steps of:
 subsequently liquefying the hydrocarbon stream in one or more steps including at least heat exchanging the hydrocarbon stream in the cryogenic heat exchanger.   
     
     
         3 . The method according to  claim 1 , wherein the one or more refrigerant temperature indications comprise at least one of a refrigerant temperature indication of the refrigerant
 at a suction side of the first JT valve;   at a discharge side of the first JT valve;   at an entry of the cryogenic heat exchanger;   at a point inside the cryogenic heat exchanger;   at a discharge side of the cryogenic heat exchanger.   
     
     
         4 . The method according to  claim 1 , wherein the initial opening of the first JT valve of the automated warm cooling down procedure is greater than the opening of the opened first JT valve of the automated cold cooling down procedure. 
     
     
         5 . The method according to  claim 1 ,
 wherein the initial opening step of the first JT valve in the automated warm cooling down procedure comprises imposing a predetermined initial opening of the first JT valve, and   wherein the automated cold cooling down procedure comprises determining a current opening of the first JT valve and imposing the determined current opening of the first JT valve.   
     
     
         6 . The method according to  claim 1 , wherein the cold cooling down procedure comprises opening the compressor recycle valve. 
     
     
         7 . The method according to  claim 1 ,
 wherein determining the temperature rate of change (TROC) of the refrigerant over the first JT valve is done by comparing two refrigerant temperature indications obtained at a respective first and second moment in time, the first and second moment in time being separated by a predetermined time interval,   wherein the predetermined time interval according to the cold cooling down procedure is shorter than the predetermined time interval according to the warm cooling down procedure.   
     
     
         8 . The method according to  claim 5 , wherein the adjustment scheme of the cold cooling down procedure comprises waiting a predetermined time interval between imposing the determined current opening of the first JT valve and initiating adjusting the opening of the first JT valve based on the monitored temperature rate of change (TROC) of the refrigerant over the first JT valve. 
     
     
         9 . The method according to  claim 1 , wherein the automated cold cooling down procedure comprises performing an adjustment step comprising simultaneously
 (i) adjusting and closing the compressor recycle valve; and   (ii) further adjusting the first JT valve.   
     
     
         10 . The method according to  claim 1 , wherein the refrigerant recirculation circuit to recirculate the spent refrigerant back to the cryogenic heat exchanger comprises a plurality of compression stages with each compression stage comprising a compressor recycle valve and the adjustment step comprising adjusting and closing the plurality of compressor recycle valves. 
     
     
         11 . The method according to  claim 1 ,
 wherein downstream of the cooler and upstream of the first JT valve a liquid/vapor separator is provided in the refrigerant recirculation circuit, to receive a partly condensed refrigerant stream and separate the partly-condensed refrigerant stream into a liquid heavy refrigerant fraction (HMR) and a gaseous light refrigerant fraction (LMR) and to discharge the liquid heavy refrigerant fraction via a liquid outlet and the gaseous light refrigerant fraction via a gas outlet,   the method comprising the steps of the first JT valve controlling passage of the light refrigerant fraction and a second JT valve controlling passage of the heavy refrigerant fraction.   
     
     
         12 . The method according to  claim 11 , wherein the initial opening step comprises imposing the initial opening of the first JT valve and an initial opening of the second JT valve, wherein the initial opening step of the second JT valve according to the automated warm cooling down procedure differs in size from an initial opening step of the second JT valve according to the automated cold cooling down procedure. 
     
     
         13 . The method according to  claim 12 , wherein the automated cold cooling down procedure comprises performing an adjustment step comprising simultaneously
 (i) adjusting and closing the compressor recycle valve; and
 (ii) further adjusting the first and second JT valves.