process of liquefying a gaseous methane-rich feed for obtaining liquid natural gas

Abstract

A process for liquefying a gaseous methane-rich feed ( 20 ) to obtain liquefied natural gas, wherein a gaseous methane-rich feed at elevated pressure is conducted through a heat exchanger ( 1 ), where it is cooled, liquefied and sub-cooled against evaporating refrigerant. The liquefied product ( 21 ) is discharged from the heat exchanger. The refrigerant cycles in a closed loop, wherein the evaporated refrigerant ( 22 ) is removed from the heat exchanger, and is compressed ( 30 ) to get high pressure refrigerant which is cooled ( 31,32 ) and divided into a liquid (heavy) portion ( 24 ) and a gaseous (light) portion ( 25 ), which are conducted to the heat exchanger for sub-cooling and liquefying the gaseous methane-rich feed. The sub-cooled refrigerant portions are introduced ( 35,37 ) into the shell of the heat exchanger at respective positions for being evaporated. The process is controlled by two separate process controllers, wherein the mass flow of the methane-rich feed is controlled by using a first process controller, and the loop of the refrigerant is controlled by a second controller using a model predictive control.

1 . A process for liquefying a gaseous methane-rich feed to obtain liquefied natural gas comprising the steps of:
 supplying the gaseous methane-rich feed at elevated pressure to a first path of a main heat exchanger at its warm end;   cooling, liquefying and sub-cooling the gaseous methane-rich feed against evaporating refrigerant to get a liquefied stream, which is removed from the main heat exchanger at its cold end and stored as a liquefied product; and   providing refrigerant to be evaporated in a shell of the main heat exchanger,   wherein the evaporated refrigerant is removed from the shell of the main heat exchanger at its warm end and compressed in at least one refrigerant compressor to become high-pressure refrigerant;   wherein the high-pressure refrigerant is partly condensed and separated in a refrigerant separator for obtaining, from the partly-condensed refrigerant, a liquid heavy refrigerant fraction and a gaseous light refrigerant fraction;   wherein the heavy refrigerant fraction is sub-cooled in a second path of the main heat exchanger to get a sub-cooled heavy refrigerant stream, introducing the heavy refrigerant stream at reduced pressure into the shell of the main heat exchanger at its mid-point, and allowing the heavy refrigerant stream to evaporate in the shell; and   wherein at least part of the light refrigerant fraction is cooled, liquefied and sub-cooled in a third path of the main heat exchanger to get a sub-cooled light refrigerant stream, introducing the light refrigerant stream at reduced pressure into the shell of the main heat exchanger at its cold end, and allowing the light refrigerant stream to evaporate in the shell,   characterized in that   the mass flow of the methane-rich feed is controlled by using a first process controller; and   the provision of the refrigerant is controlled by a second controller for determining control actions for a set of manipulated variables to control a set of controlled variables based on a model predictive control,   wherein the set of manipulated variables comprises the mass flow of the light refrigerant and the mass flow of the heavy refrigerant, and the set of controlled variables comprises a temperature difference between the fluid in the first path of the main heat exchanger and the fluid in the shell at its warm end, and a temperature difference between the fluid in the first path of the main heat exchanger and the fluid in the shell of the main heat exchanger at its mid point, and the set of parameters to be optimized includes the compensation of a disturbance value caused by the mass flow of the methane-rich feed.   
   
   
       2 . The process according to  claim 1 , wherein the first process controller acts based on optimization solutions provided by an external dynamic optimization tool. 
   
   
       3 . The process according to  claim 1 , wherein the set of manipulated variables of the second process controller comprises the speed of the at least one refrigerant compressor. 
   
   
       4 . The process according to any of  claims 1 ,  2 ,  3 , or  7 , wherein the high-pressure refrigerant is partly condensed, using at least one heat exchanger operating with propane evaporating at a suitable pressure. 
   
   
       5 . The process according to  claim 4 , wherein the evaporated propane is compressed by at least one propane compressor, wherein the set of manipulated variables of the second process controller further comprises the speed of the at least one propane compressor and the suction pressure of the at least one propane compressor. 
   
   
       6 . A plant including at least one process train for liquefying a gaseous methane-rich feed to obtain liquefied natural gas according to any of  claims 1 ,  2 ,  3 , or  7 ,
 wherein a dynamic optimization tool comprises a calculation means for calculating optimization solutions in order to coordinate the at least one process train for achieving maximum efficiency,   characterized in that the calculating means is configured to calculate a desired value for the mass flow of the methane-rich feed for each of the at least one process train, and for providing the desired value to the first process controller of each of the at least one process train.   
   
   
       7 . The process according to  claim 2 , wherein the set of manipulated variables of the second process controller comprises the speed of the at least one refrigerant compressor. 
   
   
       8 . A plant including at least one process train for liquefying a gaseous methane-rich feed to obtain liquefied natural gas according to  claim 4 ,
 wherein a dynamic optimization tool comprises a calculation means for calculating optimization solutions to coordinate the at least one process train for achieving maximum efficiency,   characterized in that the calculating means is configured to calculate a desired value for the mass flow of the methane-rich feed for each of the at least one process train, and for providing the desired value to the first process controller of each of the at least one process train.