Controlling the production of a liquefied natural gas product stream

A method of controlling the production of a liquefied natural gas product stream obtained by removing heat from natural gas in a heat exchanger in which the natural gas is in indirect heat exchange with expanded heavy mixed refrigerant and expanded light mixed refrigerant, which method comprises the steps of:

• a) measuring the temperature and the flow rate of the liquefied natural gas product stream and measuring the flow rates of the heavy mixed refrigerant and of the light mixed refrigerant;
• b) selecting the flow rate of one of the refrigerants (the heavy mixed refrigerant, the light mixed refrigerant or the total mixed refrigerant) to have an operator manipulated set point, and generating a first output signal for adjusting the flow rate of the heavy mixed refrigerant and a second output signal for adjusting the flow rate of the light mixed refrigerant using (i) the operator manipulated set point for the flow rate of the one of the refrigerants, (ii) the flow rates of the heavy and light mixed refrigerants and (iii) an operator manipulated set point for the ratio of the flow rate of the heavy mixed refrigerant to the flow rate of the light mixed refrigerant;
• c) adjusting the flow rates of the heavy mixed refrigerant and the light mixed refrigerant in accordance with the first and second output signals;
• d) determining a dependent set point for the ratio of the flow rate of the liquefied natural gas product stream to the flow rate of one of the refrigerants such that the temperature of the liquefied natural gas product stream is maintained at an operator manipulated set point, and determining a dependent set point for the flow rate of the liquefied natural gas product stream using (i) the dependent set point for the ratio of the flow rate of the liquefied natural gas product stream to the flow rate of the one of the refrigerants and (ii) the flow rate of the one of the refrigerants;
• and
• e) maintaining the flow rate of the liquefied natural gas product stream at its dependent set point.