Liquefier control for transient heat loads

Abstract

An apparatus and method for controlling a cryogenic liquefier during transient operation, the method comprising the steps of: providing a cryogenic liquefier, wherein the cryogenic liquefier comprises: a cryogenic liquefier configured to liquefy the cryogen gas at a temperature of approximately 20K, a cryogen feed flow conduit configured to deliver the cryogen gas to the cryogenic liquefier; a first refrigeration circuit, and a secondary refrigeration circuit, wherein the first refrigeration circuit comprises isenthalpic or near-isenthalpic expansion wherein the secondary refrigeration circuit comprises isentropic or near-isentropic expansion; switching from steady state operation to a transient operation; controlling an outlet temperature of liquid cryogen from the cryogenic liquefier by adjusting the refrigeration provided by the secondary refrigeration circuit.

We claim: 
     
         1 . A method for controlling a cryogenic liquefier during transient operation, the method comprising the steps of:
 providing a cryogenic liquefier, wherein the cryogenic liquefier comprises: a cold end heat exchanger configured to liquefy the cryogen gas at a temperature of approximately 20K, a cryogen feed flow conduit configured to deliver the cryogen gas to the cryogenic liquefier; a first refrigeration circuit, and a secondary refrigeration circuit, wherein the first refrigeration circuit comprises isenthalpic or near-isenthalpic expansion wherein the secondary refrigeration circuit comprises isentropic or near-isentropic expansion;   switching from steady state operation to a transient operation; and   controlling an outlet temperature of liquid cryogen from the cryogenic liquefier by adjusting the refrigeration provided by the secondary refrigeration circuit.   
     
     
         2 . The method as claimed in  claim 1 , wherein the step of controlling the outlet temperature further comprises manipulating a factor selected from the group consisting of: a differential pressure across a turbine system, a pressure ratio across the turbine system, rotating speed of the turbine system, a work factor across the turbine system. 
     
     
         3 . The method as claimed in  claim 1 , wherein the method switches to the transient operation based upon a change in flow rate of cryogen gas in the feed flow conduit. 
     
     
         4 . The method as claimed in  claim 1 , wherein the method switches to the transient operation based upon a determination that a cryogen storage vessel is entering a loading phase. 
     
     
         5 . The method as claimed in  claim 4 , wherein the method switches to the steady state operation based upon a determination that the cryogen storage vessel has finished its loading phase. 
     
     
         6 . A method for controlling a cryogenic liquefier having a refrigerant circuit with at least one turbine, the method comprising:
 detecting a change in a heat load on the cryogenic liquefier based on a measurement of a leading indicator parameter that is upstream of a liquefier outlet;   calculating, using a processor, a required adjustment to refrigeration production in response to the detected change in the heat load; and   proactively adjusting a refrigeration factor of the refrigerant circuit to effect the required adjustment to refrigeration production before a deviation in a liquefier outlet temperature caused by the change in the heat load occurs.   
     
     
         7 . The method of  claim 6 , wherein the leading indicator parameter comprises a flow rate of a feed gas stream entering the cryogenic liquefier. 
     
     
         8 . The method of  claim 6 , wherein the leading indicator parameter is determined from a heat balance calculation based on at least one of an inlet flow rate and an inlet temperature of a feed gas stream. 
     
     
         9 . The method of  claim 6 , wherein the refrigeration factor is a function of at least an inlet pressure and an outlet pressure of the at least one turbine. 
     
     
         10 . The method of  claim 9 , wherein the refrigeration factor is selected from the group consisting of: a differential pressure across the at least one turbine, a pressure ratio across the at least one turbine, and a work factor representative of total work produced by the at least one turbine. 
     
     
         11 . The method of  claim 6 , wherein the step of proactively adjusting the refrigeration factor comprises transmitting a control signal to a refrigerant letdown valve positioned upstream of the at least one turbine. 
     
     
         12 . The method of  claim 6 , wherein the change in the heat load is caused by introducing recovered cryogenic vapor from a transport trailer into a feed gas stream of the liquefier. 
     
     
         13 . The method of  claim 6 , further comprising using a feedback control loop based on the liquefier outlet temperature to provide a trim adjustment to the refrigeration factor. 
     
     
         14 . An apparatus for controlling a cryogenic liquefier, the apparatus comprising:
 a cryogenic liquefier comprising a refrigerant circuit with at least one turbine and an actuator configured to adjust a flow of refrigerant to the at least one turbine;   at least one sensor configured to measure a leading indicator parameter of a heat load on the cryogenic liquefier, wherein the at least one sensor is positioned to measure the parameter upstream of a liquefier outlet; and   a controller communicatively coupled to the at least one sensor and the actuator, the controller comprising a processor and a memory including computer-executable instructions that, when executed by the processor, cause the controller to:
 i) receive a signal from the at least one sensor indicating a change in the heat load; 
 ii) calculate, using a control algorithm, a required adjustment to a refrigeration factor of the refrigerant circuit; and 
 iii) transmit a control signal to the actuator to proactively implement the required adjustment to the refrigeration factor before the change in the heat load causes a deviation in a liquefier outlet temperature. 
   
     
     
         15 . The apparatus of  claim 14 , wherein the at least one sensor is a flow meter configured to measure a flow rate of a feed gas stream entering the cryogenic liquefier. 
     
     
         16 . The apparatus of  claim 14 , wherein the refrigeration factor is a function of at least an inlet pressure and an outlet pressure of the at least one turbine, and is selected from the group consisting of: a differential pressure, a pressure ratio, and a work factor. 
     
     
         17 . The apparatus of  claim 14 , wherein the apparatus does not include a gaseous cryogen buffer storage tank for buffering recovered cryogenic vapor.