Gas liquefaction process control system

A method of operating a gas liquefaction process in which at least a portion of the refrigeration required to liquefy said gas is provided by a refrigeration system utilizing one or more gas turbine-driven compressors, said method comprising:

• (a) compressing, cooling, and partially liquefying a refrigerant stream, said compression being accomplished at least in part by a gas turbine-driven compressor;
• (b) separating the resulting partially liquefied refrigerant stream of step (a) into a vapor refrigerant stream and a liquid refrigerant stream;
• (c) cooling said vapor refrigerant stream and expanding the resulting cooled vapor refrigerant stream, thereby reducing the pressure and temperature of said stream, whereby the resulting stream is at least partially liquefied,
• (d) vaporizing the resulting refrigerant stream of step (c) by indirect heat exchange with a precooled feed gas, thereby providing a portion of the refrigeration needed to liquefy said feed gas;
• and
• (e) returning the resulting vaporized refrigerant stream of step (d) to said gas turbine-driven compressor and repeating steps (a) through (e), and during the cyclic operation of steps (a) through (e) performing the additional steps of:
• (f) controlling the pressure drop resulting from the expansion of said cooled vapor refrigerant stream in step (c) at a set value such that the compression ratio of said gas turbine-driven compressor is maintained at a selected value which optimizes the operation of said gas liquefaction process at an ambient air temperature at a given time;
• (g) predicting an ambient air temperature at a future time;
• (h) determining a new compression ratio for said gas turbine-driven compressor which would be required to optimize the operation of said gas liquefaction process at the ambient air temperature at said future time;
• (i) changing the pressure drop of said cooled vapor refrigerant stream in step (c) in anticipation of the ambient temperature at said future time from said set value to a new value which is sufficient to yield said new compression ratio;
• and
• (j) repeating steps (f) through (i) at a time interval defined by the difference between said given time and said future time;
• thereby providing a feedforward control method for the operation of said liquefaction process which anticipates the ambient temperature at said future time in order to maintain optimum operating conditions for said process.

A system for gas liquefaction comprising:

• (a) indirect heat exchange means for liquefying a precooled gas feedstream against a first and a second vaporizing refrigerant stream;
• (b) closed-loop refrigeration means for providing said first and second vaporizing refrigerant streams to said indirect heat exchange means and for removing the resulting vaporized refrigerant therefrom;
• wherein said closed-loop refrigeration means comprises:
• (1) compression means comprising at least one gas turbine-driven compressor followed by aftercooling means, first conduit means for transferring said vaporized refrigerant from said indirect heat exchange means to said compression means, and second conduit means for the flow of compressed vaporized refrigerant from said compression means;
• (2) first cooling means for cooling and partially liquefying said compressed vaporized refrigerant which is transferred from said compression means to said cooling means by said second conduit means;
• (3) separator means for separating partially liquefied refrigerant from said first cooling means, said separator means comprising a separation vessel, third conduit means for transferring partially liquefied refrigerant from said first cooling means to said separator means, and fourth and fifth conduit means for transporting a vapor refrigerant stream and a liquid refrigerant stream respectively from said separator means to said indirect heat exchange means;
• (c) second cooling means within said indirect heat exchange means for further cooling said vapor refrigerant stream, and sixth conduit means for withdrawing a further cooled vapor refrigerant stream from said second cooling means;
• (d) first expansion means for reducing the pressure of said further cooled vapor refrigerant stream thereby further cooling the strea, and seventh conduit means for transferring the stream from said first expansion means into said indirect heat exchange means to provide said first vaporizing refrigerant stream;
• (e) third cooling means within said indirect heat exchange means for further cooling said liquid refrigerant stream, and eighth conduit means for withdrawing a further cooled liquid refrigerant stream from said third cooling means;
• (f) second expansion means for reducing the pressure of said further cooled liquid refrigerant stream thereby further cooling the stream, and ninth conduit means for transferring the stream from said second expansion means into said indirect heat exchange means to provide said second vaporizing refrigerant stream;
• (g) first feedback control means for controlling the degree of pressure reduction across said first expansion means according to a first set point;
• (h) first pressure measurement and transmitter means for measuring the pressure in said first conduit means and transmitting a first signal representative of said pressure to said first feedback control means;
• (i) second pressure measurement and transmitter means for measuring the pressure in said second conduit means and transmitting a second signal representative of said pressure to said first feedback control means;
• (j) second feedback control means for controlling the degree of pressure reduction across said second expansion means according to a second set point;
• (k) first flow measurement and transmitter means for measuring the flow in said fourth conduit means and transmitting a third signal representative of said flow to said second feedback control means;
• (l) second flow measurement and transmitter means for measuring the flow in said eighth conduit means and transmitting a fourth signal representative of said flow to said second feedback control means;
• (m) conduit means for introducing said precooled gas feedstream into said indirect heat exchange means, fourth cooling means within said indirect heat exchange means for liquefying said precooled gas feedstream, and conduit means to remove a liquefied gas product from said fourth cooling means;
• (n) supervisory control computing means comprising the capability to:
• (1) calculate the amount of refrigerant in said closed-loop refrigeration means at a given time;
• (2) predict the ambient temperature at a future time;
• (3) determine new values of said first and second set points required at said future time to cause the appropriate pressure reduction across said first and second expansion means such that the specific fuel consumption to liquefy said precooled gas feedstream is minimized;
• and
• (4) generate a fifth and a sixth signal representative of said new values of said first and second set points respectively;
• (o) process parameter measurement and transmission means for measuring appropriate process parameters and transmitting signals representative of said parameters from selected locations in said system to said supervisory control computing means;
• (p) transmission means to transmit said fifth signal to said first feedback control means and first manipulative means to change said first set point from its value at a given time to said new value;
• and
• (q) transmission means to transmit said sixth signal to said second feedback control means and second manipulative means to change said second set point from its value at a given time to said new value.