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

Abstract

Method and apparatus for cooling down a cryogenic heat exchanger, employing a programmable controller that receives input signals representing sensor signals of one or more controlled variables in a selected process, and produces control signals to control one or more manipulated variables in the selected process. The programmable controller can execute a computer program that comprises a network of at least three modules. The modules in the network are interconnected such a trigger signal received by a second and a third module of the at least three modules corresponds to a communication signal that is generated when the first module of the at least three modules has reached a pre-determined target for that module.

1 . An apparatus for cooling down a cryogenic heat exchanger adapted to liquefy a hydrocarbon stream, which cryogenic heat exchanger is arranged to receive 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 spent refrigerant that has passed through the cryogenic heat exchanger, the apparatus comprising:
 a refrigerant recirculation circuit to recirculate spent refrigerant back to the cryogenic heat exchanger, the refrigerant recirculation circuit comprising at least a compressor, a compressor recycle valve, a cooler, and a first JT valve;   a programmable controller arranged to:   (i) receive input signals representing sensor signals of one or more controlled variables;   (ii) produce control signals to control one or more manipulated variables; and   (iii) execute a computer program, the computer program comprising a network of at least three modules, wherein one or more of the at least three modules receive a representations of one or more of the input signals and produce representations of one or more of the control signals;   
       and wherein the at least three modules are each arranged to:
 (a) wait until a trigger signal is received; and 
 (b) start executing a predetermined sequence of one or more computer readable instructions upon receipt of the trigger signal at least until a predetermined module target for that module is reached; 
 
       and which modules in the network are interconnected such that the trigger signal received by a second and a third module of the at least three modules corresponds to a communication signal that is generated upon the first module of the at least three modules reaching the pre-determined target for that module. 
     
     
         2 . The apparatus of  claim 1 , wherein the second and third modules are operating in parallel with each other, whereby a control action of one of these modules is constrained by a variable that is influenced by manipulating of one or more manipulated variables by at least the other one of the second and third modules. 
     
     
         3 . The apparatus of  claim 1 , wherein the one or more controlled variables comprise a rate of change in temperature over time. 
     
     
         4 . The apparatus of  claim 3 , wherein the rate of change of temperature over time comprises one or more of: temperature of the refrigerant at the suction side of the first JT valve; temperature of the refrigerant at the discharge side of the first JT valve; temperature of the hydrocarbon stream at a point inside the cryogenic heat exchanger; temperature of the hydrocarbon stream downstream of the cryogenic heat exchanger. 
     
     
         5 . The apparatus of  claim 1 , wherein the one or more controlled variables comprises a selected spatial temperature gradient in or around the cryogenic heat exchanger. 
     
     
         6 . The apparatus of  claim 5 , wherein the selected spatial temperature gradient reflects one or more of the following temperature differentials: the temperature differential between the spent refrigerant and the refrigerant at a refrigerant inlet of the cryogenic heat exchanger; the temperature differential between the refrigerant at the suction side and the refrigerant at the discharge side of the first JT valve. 
     
     
         7 . The apparatus of  claim 1 , wherein the cryogenic heat exchanger comprises a shell side for evaporating refrigerant and a tube side for auto-cooling the refrigerant. 
     
     
         8 . The apparatus of  claim 7 , wherein the selected spatial temperature gradient reflects the temperature differential between a shell side of the cryogenic heat exchanger and a refrigerant containing tube side. 
     
     
         9 . The apparatus of  claim 1 , wherein downstream of the cooler and upstream of the first JT valve a liquid/vapour separator is provided in the refrigerant recirculation circuit, to receive a partly condensed refrigerant and separate the partly-condensed refrigerant stream into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction and to discharge the liquid heavy refrigerant fraction via a liquid outlet and the gaseous light refrigerant fraction via a gas outlet, which fractions are passed to the cryogenic heat exchanger, wherein the first JT valve is arranged to control passage of one of these fractions. 
     
     
         10 . The apparatus of  claim 9 , wherein the selected spatial temperature gradient reflects one or more of: the temperature differential between the temperature differential between the spent refrigerant and the refrigerant between the gas outlet and a gaseous refrigerant inlet of the cryogenic heat exchanger; and the temperature differential between spent refrigerant and the refrigerant between the liquid outlet and a liquid refrigerant inlet of the cryogenic heat exchanger. 
     
     
         11 . The apparatus of  claim 1 , wherein the one or more controlled variables comprise one or both of: a first temperature difference in the refrigerant stream across the first JT valve; and a suction pressure in the refrigerant stream at a suction side of the compressor. 
     
     
         12 . The apparatus of  claim 1 , wherein the one or more manipulated variables at least comprise one or both of: a first JT valve setting representing a measure of amount of opening of the first JT valve; and any pressure setting that controls refrigerant pressure upstream of the first JT valve. 
     
     
         13 . The apparatus of  claim 12 , wherein the pressure setting comprises a compressor recycle valve setting representing a measure of amount of opening of the compressor recycle valve. 
     
     
         14 . The apparatus of  claim 1 , wherein the cryogenic heat exchanger comprises a hydrocarbon stream inlet and a hydrocarbon stream outlet for the hydrocarbon stream, and one or more separate refrigerant inlets and a refrigerant outlet for the refrigerant and spent refrigerant, respectively. 
     
     
         15 . A method of cooling down a cryogenic heat exchanger adapted to liquefy a hydrocarbon stream, comprising the steps of:
 providing a cryogenic heat exchanger arranged to receive 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 spent refrigerant that has passed through the cryogenic heat exchanger,   providing a refrigerant recirculation circuit to recirculate spent refrigerant back to the cryogenic heat exchanger, the refrigerant recirculation circuit comprising at least a compressor, a compressor recycle valve, a cooler, and a first JT valve;   activating a programmable controller which   (i) receives input signals representing sensor signals of one or more controlled variables;   (ii) produces control signals to control one or more manipulated variables; and   (iii) executes a computer program, the computer program comprising a network of at least three modules, wherein one or more of the at least three modules receive a representations of one or more of the input signals and produce representations of one or more of the control signals;   
       and wherein each of the at least three modules
 (a) waits until a trigger signal is received; and 
 (b) starts executing a predetermined sequence of one or more computer readable instructions upon receipt of the trigger signal at least until a predetermined module target for that module is reached; 
 
       and wherein a communication signal is generated upon the first module of the at least three modules reaching the pre-determined target for that module, which communication signal is passed on to a second and a third module of the three or more modules where the communication signal acts as the trigger signal for the second and third modules. 
     
     
         16 . A method of liquefying a hydrocarbon stream, the method comprising steps of:
 cooling down a cryogenic heat exchanger adapted to liquefy the hydrocarbon stream;   subsequently liquefying the hydrocarbon stream in one or more steps including at least heat exchanging the hydrocarbon stream in the cryogenic heat exchanger,   
       wherein said cooling down of the said cryogenic heat exchanger comprises steps of:
 providing a cryogenic heat exchanger arranged to receive 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 spent refrigerant that has passed through the cryogenic heat exchanger, 
 providing a refrigerant recirculation circuit to recirculate spent refrigerant back to the cryogenic heat exchanger, the refrigerant recirculation circuit comprising at least a compressor, a compressor recycle valve, a cooler, and a first JT valve; 
 activating a programmable controller which 
 (i) receives input signals representing sensor signals of one or more controlled variables; 
 (ii) produces control signals to control one or more manipulated variables; and 
 (iii) executes a computer program, the computer program comprising a network of at least three modules, wherein one or more of the at least three modules receive a representations of one or more of the input signals and produce representations of one or more of the control signals; 
 
       and wherein each of the at least three modules
 (a) waits until a trigger signal is received; and 
 (b) starts executing a predetermined sequence of one or more computer readable instructions upon receipt of the trigger signal at least until a predetermined module target for that module is reached; 
 
       and wherein a communication signal is generated upon the first module of the at least three modules reaching the pre-determined target for that module, which communication signal is passed on to a second and a third module of the three or more modules where the communication signal acts as the trigger signal for the second and third modules. 
     
     
         17 . A method of liquefying a hydrocarbon stream, comprising the steps of:
 cooling down a cryogenic heat exchanger adapted to liquefy the hydrocarbon stream;   subsequently liquefying the hydrocarbon stream in one or more steps including at least heat exchanging the hydrocarbon stream in the cryogenic heat exchanger,   
       wherein said cooling down of the said cryogenic heat exchanger comprises using an apparatus for cooling down a cryogenic heat exchanger adapted to liquefy a hydrocarbon stream, which cryogenic heat exchanger is arranged to receive 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 spent refrigerant that has passed through the cryogenic heat exchanger, the apparatus comprising:
 a refrigerant recirculation circuit to recirculate spent refrigerant back to the cryogenic heat exchanger, the refrigerant recirculation circuit comprising at least a compressor, a compressor recycle valve, a cooler, and a first JT valve; 
 a programmable controller arranged to: 
 (i) receive input signals representing sensor signals of one or more controlled variables; 
 (ii) produce control signals to control one or more manipulated variables; and 
 (iii) execute a computer program, the computer program comprising a network of at least three modules, wherein one or more of the at least three modules receive a representations of one or more of the input signals and produce representations of one or more of the control signals; 
 
       and wherein the at least three modules are each arranged to:
 (a) wait until a trigger signal is received; and 
 (b) start executing a predetermined sequence of one or more computer readable instructions upon receipt of the trigger signal at least until a predetermined module target for that module is reached; 
 
       and which modules in the network are interconnected such that the trigger signal received by a second and a third module of the at least three modules corresponds to a communication signal that is generated upon the first module of the at least three modules reaching the pre-determined target for that module. 
     
     
         18 . The method of  claim 17 , wherein the hydrocarbon stream is a natural gas stream. 
     
     
         19 . The method of  claim 17 , wherein the second and third modules are operating in parallel with each other, whereby a control action of one of these modules is constrained by a variable that is influenced by manipulating of one or more manipulated variables by at least the other one of the second and third modules. 
     
     
         20 . The method of  claim 17 , wherein the one or more controlled variables comprise a rate of change in temperature over time. 
     
     
         21 . The method of  claim 20 , wherein the rate of change of temperature over time comprises one or more of: temperature of the refrigerant at the suction side of the first JT valve; temperature of the refrigerant at the discharge side of the first JT valve; temperature of the hydrocarbon stream at a point inside the cryogenic heat exchanger; temperature of the hydrocarbon stream downstream of the cryogenic heat exchanger. 
     
     
         22 . The method of  claim 17 , wherein the one or more controlled variables comprises a selected spatial temperature gradient in or around the cryogenic heat exchanger. 
     
     
         23 . The method of  claim 22 , wherein the selected spatial temperature gradient reflects one or more of the following temperature differentials: the temperature differential between the spent refrigerant and the refrigerant at a refrigerant inlet of the cryogenic heat exchanger; the temperature differential between the refrigerant at the suction side and the refrigerant at the discharge side of the first JT valve. 
     
     
         24 . The method of  claim 17 , wherein the cryogenic heat exchanger comprises a shell side for evaporating refrigerant and a tube side for auto-cooling the refrigerant. 
     
     
         25 . The method of  claim 24 , wherein the selected spatial temperature gradient reflects the temperature differential between a shell side of the cryogenic heat exchanger and a refrigerant containing tube side. 
     
     
         26 . The method of  claim 17 , wherein downstream of the cooler and upstream of the first JT valve a liquid/vapour separator is provided in the refrigerant recirculation circuit, to receive a partly condensed refrigerant and separate the partly-condensed refrigerant stream into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction and to discharge the liquid heavy refrigerant fraction via a liquid outlet and the gaseous light refrigerant fraction via a gas outlet, which fractions are passed to the cryogenic heat exchanger, wherein the first JT valve is arranged to control passage of one of these fractions. 
     
     
         27 . The method of  claim 26 , wherein the one of these fractions is the light refrigerant fraction. 
     
     
         28 . The method of  claim 26 , wherein the selected spatial temperature gradient reflects one or more of: the temperature differential between the temperature differential between the spent refrigerant and the refrigerant between the gas outlet and a gaseous refrigerant inlet of the cryogenic heat exchanger; and the temperature differential between spent refrigerant and the refrigerant between the liquid outlet and a liquid refrigerant inlet of the cryogenic heat exchanger. 
     
     
         29 . The method of  claim 17 , wherein the one or more controlled variables comprise one or both of: a first temperature difference in the refrigerant stream across the first JT valve; and a suction pressure in the refrigerant stream at a suction side of the compressor. 
     
     
         30 . The method of  claim 17 , wherein the one or more manipulated variables at least comprise one or both of: a first JT valve setting representing a measure of amount of opening of the first JT valve; and any pressure setting that controls refrigerant pressure upstream of the first JT valve. 
     
     
         31 . The method of  claim 30 , wherein the pressure setting comprises a compressor recycle valve setting representing a measure of amount of opening of the compressor recycle valve. 
     
     
         32 . The method of  claim 17 , wherein the cryogenic heat exchanger comprises a hydrocarbon stream inlet and a hydrocarbon stream outlet for the hydrocarbon stream, and one or more separate refrigerant inlets and a refrigerant outlet for the refrigerant and spent refrigerant, respectively. 
     
     
         33 . The method of  claim 16 , wherein the hydrocarbon stream is a natural gas stream. 
     
     
         34 . The method of  claim 16 , wherein the second and third modules are operating in parallel with each other, whereby a control action of one of these modules is constrained by a variable that is influenced by manipulating of one or more manipulated variables by at least the other one of the second and third modules. 
     
     
         35 . The method of  claim 16 , wherein the one or more controlled variables comprise a rate of change in temperature over time. 
     
     
         36 . The method of  claim 35 , wherein the rate of change of temperature over time comprises one or more of: temperature of the refrigerant at the suction side of the first JT valve; temperature of the refrigerant at the discharge side of the first JT valve; temperature of the hydrocarbon stream at a point inside the cryogenic heat exchanger; temperature of the hydrocarbon stream downstream of the cryogenic heat exchanger. 
     
     
         37 . The method of  claim 16 , wherein the one or more controlled variables comprises a selected spatial temperature gradient in or around the cryogenic heat exchanger. 
     
     
         38 . The method of  claim 37 , wherein the selected spatial temperature gradient reflects one or more of the following temperature differentials: the temperature differential between the spent refrigerant and the refrigerant at a refrigerant inlet of the cryogenic heat exchanger; the temperature differential between the refrigerant at the suction side and the refrigerant at the discharge side of the first JT valve. 
     
     
         39 . The method of  claim 16 , wherein the cryogenic heat exchanger comprises a shell side for evaporating refrigerant and a tube side for auto-cooling the refrigerant. 
     
     
         40 . The method of  claim 39 , wherein the selected spatial temperature gradient reflects the temperature differential between a shell side of the cryogenic heat exchanger and a refrigerant containing tube side. 
     
     
         41 . The method of  claim 16 , wherein downstream of the cooler and upstream of the first JT valve a liquid/vapour separator is provided in the refrigerant recirculation circuit, to receive a partly condensed refrigerant and separate the partly-condensed refrigerant stream into a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction and to discharge the liquid heavy refrigerant fraction via a liquid outlet and the gaseous light refrigerant fraction via a gas outlet, which fractions are passed to the cryogenic heat exchanger, wherein the first JT valve is arranged to control passage of one of these fractions. 
     
     
         42 . The method of  claim 41 , wherein the one of these fractions is the light refrigerant fraction. 
     
     
         43 . The method of  claim 41 , wherein the selected spatial temperature gradient reflects one or more of: the temperature differential between the temperature differential between the spent refrigerant and the refrigerant between the gas outlet and a gaseous refrigerant inlet of the cryogenic heat exchanger; and the temperature differential between spent refrigerant and the refrigerant between the liquid outlet and a liquid refrigerant inlet of the cryogenic heat exchanger. 
     
     
         44 . The method of  claim 16 , wherein the one or more controlled variables comprise one or both of: a first temperature difference in the refrigerant stream across the first JT valve; and a suction pressure in the refrigerant stream at a suction side of the compressor. 
     
     
         45 . The method of  claim 16 , wherein the one or more manipulated variables at least comprise one or both of: a first JT valve setting representing a measure of amount of opening of the first JT valve; and any pressure setting that controls refrigerant pressure upstream of the first JT valve. 
     
     
         46 . The method of  claim 45 , wherein the pressure setting comprises a compressor recycle valve setting representing a measure of amount of opening of the compressor recycle valve. 
     
     
         47 . The method of  claim 16 , wherein the cryogenic heat exchanger comprises a hydrocarbon stream inlet and a hydrocarbon stream outlet for the hydrocarbon stream, and one or more separate refrigerant inlets and a refrigerant outlet for the refrigerant and spent refrigerant, respectively. 
     
     
         48 . The method of  claim 9 , wherein the one of these fractions is the light refrigerant fraction.