Controlling refrigerant compression power in a natural gas liquefaction process

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

wherein the method comprises circulating the heavy and light mixed refrigerant through a refrigerant cycle, the refrigerant cycle comprising a centrifugal compressor driven by an electric motor,

wherein the method comprises executing a control loop comprising:

a) determining a dependent set point for a flow rate of the liquefied natural gas product stream based on operator manipulated set points for

(i) a flow rate of one of the refrigerants (the heavy mixed refrigerant, the light mixed refrigerant or the total mixed refrigerant),

(ii) a ratio of the flow rate of the heavy mixed refrigerant to the flow rate of the light mixed refrigerant, and

(iii) a temperature of the liquefied natural gas product stream, determining the operator manipulated set point for the flow rate of the heavy mixed refrigerant and the operator manipulated set point for the flow rate of the light mixed refrigerant,

wherein determining the dependent set point for the flow rate of the liquefied natural gas product stream comprises 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 the operator manipulated set point for the temperature of the liquefied natural gas product stream,

maintaining the flow rate of the liquefied natural gas product stream at the dependent set point for the liquefied natural gas product stream and maintaining the flow rates of the heavy mixed refrigerant and the light mixed refrigerant at the operator manipulated set points for the flow rates of the heavy mixed refrigerant and the light mixed refrigerant respectively,

wherein the method comprises executing an override control loop comprising:

b) determining a residual available power value of the electric motor by determining an actual power consumption of the electric motor and compare the actual power consumption to a predetermined maximal power consumption of the electric motor;

c) if the residual available power exceeds a predetermined threshold, determining an override set point for the flow rate of the liquefied natural gas;

d) computing an override set point for the flow rate of the heavy mixed refrigerant and an override set point for the flow rate of the light mixed refrigerant associated to the override set point for the flow rate of the liquefied natural gas to maintain the operator manipulated set point for the temperature of the liquefied natural gas product stream,

e) overriding the dependent set point for the flow rate of the liquefied natural gas product stream with the override set point for the flow rate of the liquefied natural gas and overriding the operator manipulated set points for the flow rates of the heavy mixed refrigerant and the light mixed refrigerant with the override set points for the flow rates of the heavy mixed refrigerant and the light mixed refrigerant respectively.

2. The method according to claim 1 , wherein step a) further comprises

a1) measuring the temperature of the liquefied natural gas product stream.

3. The method according to claim 2 , wherein step b) further comprises

a2) 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;

a3) adjusting the flow rates of the heavy mixed refrigerant and the light mixed refrigerant in accordance with the first and second output signals;

a4) determining the 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.

4. The method according to claim 1 , wherein determining an actual power consumption of the electric motor comprises obtaining an indication of an actual electric current consumed by the electric motor.

5. The method according to claim 1 , wherein the production of the liquefied natural gas product stream comprises removing heat from natural gas in a pre-cool heat exchanger in which the natural gas is in indirect heat exchange with an expanded pre-cool refrigerant, to obtain a pre-cooled natural gas stream,

wherein the method comprises circulating the pre-cool refrigerant through a pre-cool refrigerant cycle, the pre-cool refrigerant cycle comprising a centrifugal pre-cool centrifugal compressor driven by a pre-cool electric motor,

wherein the method comprises passing the pre-cooled natural gas stream to the main heat exchanger in which the natural gas is in indirect heat exchange with the expanded heavy mixed refrigerant and the expanded light mixed refrigerant,

wherein the override control loop further comprises:

b1) determining a pre-cool residual available power value of the pre-cool electric motor by determining an actual power consumption of the pre-cool electric motor and compare the actual power consumption to a predetermined maximal power consumption of the pre-cool electric motor;

b2) select the smallest of the pre-cool residual available power value and the residual available power value,

and continue with executing steps c) and d) based on the selected one of the pre-cool residual available power value and the residual available power value.

6. The method according to claim 1 , wherein step c) comprises selecting the override dependent set point for the flow rate of the liquefied natural gas to equal the dependent set point for the flow rate of the liquefied natural gas product stream under a) plus a predetermined step size.

7. The method according to claim 1 , wherein the override control loop further comprises

f) after steps b), c), d) and e) have been performed, monitoring at least one of the following parameters:

the residual available power value of the electric motor,

the actual power consumption of the electric motor,

the flow rate of the liquefied natural gas product stream,

the flow rate of the heavy mixed refrigerant,

the flow rate of the light mixed refrigerant,

the temperature of the liquefied natural gas product stream,

and repeat steps b), c), d) and e) once the monitored parameters are stable.

8. The method according to claim 1 , wherein the control loop is executed by one or more controllers (C).

9. A system for the production of a liquefied natural gas product stream comprising a main heat exchanger arranged to remove heat from natural gas by indirect heat exchange with an expanded heavy mixed refrigerant and an expanded light mixed refrigerant,

the system comprising a refrigerant cycle arranged to circulate the heavy and light mixed refrigerant, the refrigerant cycle comprising the main heat exchanger and a centrifugal compressor driven by an electric motor,

wherein the system comprises a controller (C) arranged to perform the method according to claim 1 .