A non-transitory computer readable medium embodying a computer program, the computer program comprising:
- computer readable program code for determining how to adjust manipulated variables so as to control controlled variables using a plurality of models, the manipulated variables and the controlled variables associated with a natural gas liquefaction train that comprises:
- a first heat exchanger configured to cool a natural gas stream into a liquefied natural gas stream;
- a compressor configured to compress evaporated refrigerant from a shell of the first heat exchanger;
- an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand;
- and
- a nitrogen cooling loop configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the expansion value and further cool the liquefied natural gas stream, the nitrogen cooling loop comprising a second heat exchanger having a single path;
- and
- computer readable program code for generating control signals for manipulating the manipulated variables to control the controlled variables based on the determination;
- wherein the manipulated variables comprise a nitrogen flow to the second heat exchanger of the nitrogen cooling loop, a compressor pressure of the compressor, and a flow of compressed refrigerant from the compressor;
- wherein the controlled variables comprise a calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, a rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and temperature differences within the first heat exchanger, the temperature differences associated with the evaporated refrigerant;
- wherein the models comprise a first model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and a second model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop;
- and
- wherein the computer readable program code for outputting the control signals comprises computer readable program code for outputting at least one first control signal to manipulate the nitrogen flow in order to control the calorific value and the temperature of the liquefied natural gas stream exiting the nitrogen cooling loop.