Controlling Liquefaction of Natural Gas

Abstract

A gas liquefaction process, especially for producing LNG, maintains product flow rate and temperature by controlling the refrigeration so that variation to reduce any difference between actual and required product temperatures is initiated before variation of the product flow rate to reduce any difference between actual and required flow rates.

1 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied gas (“IG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
 comparing said predetermined LG flow rate value with the actual LG flow rate;   comparing said predetermined LG temperature value with the actual LG temperature; and   varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences,   characterized in that variation of the refrigeration to reduce any LG temperature difference is initiated before variation of the LG flow rate to reduce any LG flow rate difference.   
   
   
       2 . The method of  claim 1 , wherein the gas feed is natural gas. 
   
   
       3 . The method of  claim 1 , wherein the actual LG flow rate is adjusted in response to changes in the actual LG temperature. 
   
   
       4 . The method of  claim 1 , wherein the actual LG flow rate is adjusted in response to changes in the temperature difference between a stream entering the warm end of the heat exchange means and a stream leaving that end of the heat exchanger. 
   
   
       5 . The method of  claim 1 , wherein the actual LG flow rate is adjusted in response to changes in the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”). 
   
   
       6 . The method of  claim 1  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LG outlet stream and setting the predetermined temperature value for the LG outlet stream.   
   
   
       7 . The method of  claim 6 , wherein the gas feed is natural gas. 
   
   
       8 . The method of  claim 6 , wherein the actual LG flow rate is adjusted in response to changes in the actual LG temperature. 
   
   
       9 . The method of  claim 6 , wherein the actual LG flow rate is adjusted in response to changes in the temperature difference between a stream entering the warm end of the heat exchange means and a stream leaving that end of the heat exchanger. 
   
   
       10 . The method of  claim 6 , wherein the actual LG flow rate is adjusted in response to changes in the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”). 
   
   
       11 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 comparing said predetermined LNG flow rate value with the actual LNG flow rate;   comparing said predetermined LNG temperature value with the actual LNG temperature;   comparing a predetermined value of (i) the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) or (ii) the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with the actual warm end temperature difference or actual mid-point temperature respectively;   varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates;   varying the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and   varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.   
   
   
       12 . The method of  claim 11 , wherein the MRL flow rate varies by an amount corresponding to the difference between the actual and predetermined LNG flow rates. 
   
   
       13 . The method of  claim 11 , wherein the MRL flow rate varies by an amount corresponding to the difference between the actual and predetermined warm end temperature differences. 
   
   
       14 . The method of  claim 11 , wherein the MRL flow rate varies by an amount corresponding to the difference between the actual and predetermined mid-point temperatures. 
   
   
       15 . The method of  claim 11 , wherein the MRL/MRV ratio is determined by the difference between the actual and predetermined LNG temperatures. 
   
   
       16 . The method of  claim 11 , wherein the MRL/MRV ratio is determined by the difference between the actual and predetermined warm end temperature differences. 
   
   
       17 . The method of  claim 11 , wherein the MRL/MRV ratio is determined by the difference between actual and predetermined mid-point temperatures. 
   
   
       18 . A method of  claim 11  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LNG outlet;   setting the predetermined temperature value for the LNG outlet stream; and   setting the predetermined value of (i) the warm end temperature difference value or (ii) the mid-point temperature.   
   
   
       19 . The method of  claim 18 , wherein the MRL flow rate varies by an amount corresponding to the difference between the actual and predetermined LNG flow rates. 
   
   
       20 . The method of  claim 18 , wherein the MRL flow rate varies by an amount corresponding to the difference between the actual and predetermined warm end temperature differences. 
   
   
       21 . The method of  claim 18 , wherein the MRL flow rate varies by an amount corresponding to the difference between the actual and predetermined mid-point temperatures. 
   
   
       22 . The method of  claim 19 , wherein the MRL/MRV ratio is determined by the difference between the actual and predetermined LNG temperatures. 
   
   
       23 . The method of  claim 22 , wherein the MRL/MRV ratio is determined by the difference between the actual and predetermined warm end temperature differences. 
   
   
       24 . The method of  claim 22 , wherein the MRL/MRV ratio is determined by the difference between actual and predetermined mid-point temperatures. 
   
   
       25 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 comparing said predetermined LNG flow rate value with the actual LNG flow rate;   comparing said predetermined LNG temperature value with the actual LNG temperature;   comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) with the actual warm end temperature difference;   varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;   varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by difference between the actual and predetermined warm end temperature differences; and   varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.   
   
   
       26 . The method of  claim 25  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LNG outlet stream;   setting the predetermined temperature value for the LNG outlet stream; and   setting the predetermined value of the warm end temperature difference value.   
   
   
       27 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 comparing said predetermined LNG flow rate value with the actual LNG flow rate;   comparing said predetermined LNG temperature value with the actual LNG temperature;   comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) with the actual warm end temperature difference;   varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;   varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and   varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences, the actual LNG flow rate.   
   
   
       28 . The method of  claim 27  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LNG outlet stream;   setting the predetermined temperature value for the LNG outlet stream; and   setting the predetermined value of the warm end temperature difference value.   
   
   
       29 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 comparing said predetermined LNG flow rate value with the actual LNG flow rate;   comparing said predetermined LNG temperature value with the actual LNG temperature;   comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) with the actual warm end temperature difference;   varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;   varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined LNG temperatures; and   varying, by an amount corresponding to the difference between the actual and predetermined warm end temperature differences multiplied by a value dependent on the actual MRL flow rate, the actual LNG flow rate.   
   
   
       30 . The method of  claim 29  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LNG outlet stream;   setting the predetermined temperature value for the LNG outlet stream; and   setting the predetermined value of the warm end temperature difference value.   
   
   
       31 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 comparing said predetermined LNG flow rate value with the actual LNG flow rate;   comparing said predetermined LNG temperature value with the actual LNG temperature;   comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) with the actual warm end temperature difference;   comparing a predetermined value of temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with the actual mid-point temperature;   varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates and also, when the difference between actual and predetermined warm end temperature differences exceeds a threshold value, to said difference between warm end temperature differences, the MRL flow rate;   varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and predetermined mid-point temperatures; and   varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.   
   
   
       32 . The method of  claim 31  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LNG outlet stream;   setting the predetermined temperature value for the LNG outlet stream; and   setting the predetermined warm end temperature difference value.   
   
   
       33 . A method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 comparing said predetermined LNG flow rate value with the actual LNG flow rate;   comparing said predetermined LNG temperature value with the actual LNG temperature;   comparing a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference”) with the actual warm end temperature difference;   comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined warm end temperature differences;   varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate;   varying the MRV flow rate to maintain an MRL/MRV ratio, which ratio is determined by the difference between the actual and calculated mid-point temperatures; and   varying, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.   
   
   
       34 . The method of  claim 33  wherein said predetermined values are adjustable and the method further comprises the steps of:
 setting the predetermined flow rate value for the LNG outlet stream;   setting the predetermined temperature value for the LNG outlet stream; and   setting the predetermined value of the warm end temperature difference value.   
   
   
       35 . A control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
 means for comparing the predetermined flow rate value for the LG outlet stream and comparing said value with the actual LG flow rate;   means for varying the actual LG product flow rate;   means for comparing the predetermined temperature value for the LG outlet stream and comparing said LNG temperature value with the actual LG temperature; and   means for varying the refrigeration provided by said heat exchange means in response to said LG flow rate and LG temperature comparisons to reduce any differences,   characterized in that adjustment of the means for varying the actual LG product flow rate is not initiated until the refrigeration has been adjusted to reduce any LG temperature difference.   
   
   
       36 . A control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
 means for comparing the predetermined flow rate value for the LNG outlet stream and comparing said value with the actual LNG flow rate;   means for varying the actual LG product flow rate;   means for comparing the predetermined temperature value for the LNG outlet stream and comparing said LNG temperature value with the actual LNG temperature;   means for comparing a predetermined value of (i) the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed (“warm end temperature difference value”) or (ii) the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) and comparing same with the actual warm end temperature difference or actual mid-point temperature respectively;   means for varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, one of the MRL and MRV flow rates;   means for varying the other of the MRV and MRL flow rates to maintain an MRL/MRV ratio, which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures; and   means for varying, by an amount corresponding to the other of (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures and (a) the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.