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US20090025422A1 - Controlling Liquefaction of Natural Gas - Google Patents

Controlling Liquefaction of Natural Gas Download PDF

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Publication number
US20090025422A1
US20090025422A1 US11/782,990 US78299007A US2009025422A1 US 20090025422 A1 US20090025422 A1 US 20090025422A1 US 78299007 A US78299007 A US 78299007A US 2009025422 A1 US2009025422 A1 US 2009025422A1
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United States
Prior art keywords
predetermined
actual
flow rate
lng
temperature
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Abandoned
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US11/782,990
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English (en)
Inventor
Michael Andrew Sicinski
Brian Keith Johnston
Scott Robert Trautmann
Mark Julian Roberts
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Air Products and Chemicals Inc
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Air Products and Chemicals Inc
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Application filed by Air Products and Chemicals Inc filed Critical Air Products and Chemicals Inc
Priority to US11/782,990 priority Critical patent/US20090025422A1/en
Assigned to AIR PRODUCTS AND CHEMICALS, INC. reassignment AIR PRODUCTS AND CHEMICALS, INC. ASSIGNMENT OF ASSIGNORS INTEREST (SEE DOCUMENT FOR DETAILS). Assignors: ROBERTS, MARK JULIAN, SICINSKI, MICHAEL ANDREW, JOHNSTON, BRIAN KEITH, TRAUTMANN, SCOTT ROBERT
Priority to PCT/IB2008/001924 priority patent/WO2009013605A2/en
Priority to EP08776389.2A priority patent/EP2324311B1/en
Priority to JP2010517501A priority patent/JP5529735B2/ja
Priority to PE2008001249A priority patent/PE20090461A1/es
Publication of US20090025422A1 publication Critical patent/US20090025422A1/en
Priority to US13/314,882 priority patent/US9671161B2/en
Priority to JP2014022603A priority patent/JP5785282B2/ja
Abandoned legal-status Critical Current

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    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F25J1/0245—Different modes, i.e. 'runs', of operation; Process control
    • F25J1/0249—Controlling refrigerant inventory, i.e. composition or quantity
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/0002—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the fluid to be liquefied
    • F25J1/0022—Hydrocarbons, e.g. natural gas
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/003—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production
    • F25J1/0047—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle
    • F25J1/0052—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream
    • F25J1/0055—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures characterised by the kind of cold generation within the liquefaction unit for compensating heat leaks and liquid production using an "external" refrigerant stream in a closed vapor compression cycle by vaporising a liquid refrigerant stream originating from an incorporated cascade
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0244—Operation; Control and regulation; Instrumentation
    • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
    • F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
    • F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
    • F25J1/00—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures
    • F25J1/02—Processes or apparatus for liquefying or solidifying gases or gaseous mixtures requiring the use of refrigeration, e.g. of helium or hydrogen ; Details and kind of the refrigeration system used; Integration with other units or processes; Controlling aspects of the process
    • F25J1/0243—Start-up or control of the process; Details of the apparatus used; Details of the refrigerant compression system used
    • F25J1/0279—Compression of refrigerant or internal recycle fluid, e.g. kind of compressor, accumulator, suction drum etc.
    • F25J1/0292—Refrigerant compression by cold or cryogenic suction of the refrigerant gas

Definitions

  • This invention relates to the field of control systems for production of liquefied gas (LG), and more specifically, to a process and system which controls LG production and LG temperature. It has particular but not exclusive application to liquefying natural gas (NG) to produce liquefied natural gas (LNG).
  • NG liquefying natural gas
  • LNG liquefied natural gas
  • natural gas is fed to the warm end of heat exchange means, having a liquefying section in which the natural gas is liquefied and a subcooling section in which the liquefied natural gas is subcooled, and the LNG outlet stream is withdrawn from the cold end of the heat exchange means.
  • Some refrigeration duty in the liquefying section is provided by flashing a first refrigerant (“MRL”), provided by cooling in the heat exchange means the liquid portion of a phase separation of a multicomponent refrigerant (MR) and refrigeration duty in the subcooling section is provided by flashing a second refrigerant (“MRV”), provided by condensing in the heat exchange means the vapor portion of the MR phase separation.
  • the remainder of the refrigeration duty in the liquefying section is provided by spent MRV from the liquefaction section.
  • the refrigerants exiting the warm end of the heat exchanger means are combined, if not already mixed in the liquefaction section, compressed and precooled before return to the MR phase separation for recycle to the heat exchange means.
  • a process having the aforementioned features is referred to herein as “a typical NG liquefaction process”.
  • U.S. Pat. No. 5,791,160 (Mandler et at, corresponding to EP-A-0893665) describes a natural gas liquefaction control scheme where LNG product flow rate and temperature are simultaneously and independently controlled by adjusting the amount of refrigeration.
  • the control variables (the ones having a set point that can be changed by the operator) of a typical NG liquefaction process include LNG product flow rate and temperature as well as the MRL/MRV ratio.
  • Manipulated variables (the ones that are automatically controlled in response to operator setting of one or more of the control variables) include MR compressor speed and MR/LNG ratio. In this scheme the amount of refrigeration is adjusted after the actual LNG product flow rate has been changed in response to a change in the LNG product flow rate set point.
  • U.S. Patent Application Publication 2004/0255615 (Hupkes et al; corresponding to WO-A-2004/068049 & EP-A-1595101) describes the use of an advanced process controller based on model-predictive control to control a typical NG liquefaction process.
  • the controller determines simultaneous control actions for a set of manipulated variables in order to optimize at least one of a set of parameters including the production of liquefied product whilst controlling at least one of a set of controlled variables.
  • the set of manipulated variables includes MRL flow rate, MRV flow rate, MR composition, MR removal, MR compressor capacity and NG feed flow rate.
  • the set of controlled variables includes the temperature difference at the warm end of the main heat exchanger, an adjustable relating to the LNG temperature, the composition of the refrigerant entering the MR phase separator, the pressure in the shell of the main heat exchanger, and the pressure and liquid level in MR phase separator.
  • a control system for typical NG liquefaction processes has been devised in which the thermal stress on the heat exchange means is limited and the need to manipulate the MR compressor can be avoided by controlling the refrigeration so that variation to reduce any difference between actual and required LNG temperature is initiated before variation of the LNG product flow rate to reduce any difference between actual and required LNG flow rate. Accordingly, refrigeration leads LG production.
  • the invention has particular, but not exclusive, application to a typical NG liquefaction process in which the controlled variables are LNG temperature, LNG flow rate and either heat exchanger warm end temperature difference (“WETD”) or heat exchanger mid-point temperature (“MPT”) and the manipulated variables are MRL and MRV flow rates.
  • WETD heat exchanger warm end temperature difference
  • MPT heat exchanger mid-point temperature
  • the invention is not restricted to the control of NG liquefaction processes but is more generally applicable to gas liquefaction, e.g. of hydrocarbon mixtures.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
  • this aspect allows the LG flow rate and temperature to be independently set and refrigeration to be correspondingly adjusted to meet the set requirements with limited thermal stress on the heat exchange means.
  • the control system concept of the invention is applicable to LG liquefaction processes in which the LG flow rate and temperature requirements are constant but from time to time some variation is required to the actual values in order to compensate for a change in other parameters, such as NG feed temperature and composition, MR composition, ambient air temperature, cooling water temperature, atmospheric pressure etc., that has caused the actual value to deviate from the required value.
  • the invention also provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
  • FIG. 1 is a schematic flow diagram of a mixed refrigerant LNG plant process of a first exemplary embodiment of the present invention.
  • FIG. 2 is a schematic flow diagram of a mixed refrigerant LNG plant process of a second exemplary embodiment of the present invention.
  • FIG. 3 is a schematic flow diagram of a mixed refrigerant LNG plant process of a third exemplary embodiment of the present invention.
  • FIG. 4 is a schematic flow diagram of a mixed refrigerant LNG plant process of a fourth exemplary embodiment of the present invention.
  • FIG. 5 is a schematic flow diagram of a mixed refrigerant LNG plant process of a fifth exemplary embodiment of the present invention.
  • FIG. 6 is a schematic flow diagram of a modification of the mixed refrigerant LNG plant process of FIG. 3 .
  • FIG. 7 is a schematic flow diagram of a comparative mixed refrigerant LNG plant process.
  • the present invention relates to the control of liquefaction of gas, especially natural gas, in a manner that maintains the LG product at a required flow rate and temperature with limited thermal stress on the heat exchange means even when the LG flow rate and/or temperature requirements have been changed.
  • the invention resides in the manner in which refrigeration is changed by manipulated variables.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
  • the refrigeration is varied to reduce any LG temperature difference before variation of the LG flow rate to reduce any LG flow rate difference.
  • the invention also provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
  • the means for varying the actual LG product flow rate is not adjusted until the refrigeration has been adjusted to reduce any LG temperature difference.
  • the invention also provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising the steps of:
  • the invention also provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied gas (“LG”) outlet stream of a gas liquefaction in which a gas feed is liquefied by refrigeration in heat exchange means, comprising:
  • the invention has particular application to typical NG liquefaction processes and in a preferred embodiment provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • MRL/MRV ratio which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures;
  • the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • thermo difference value the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • MRL/MRV ratio which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures;
  • Another preferred embodiment of the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • thermo difference value the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • MRL/MRV ratio which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures;
  • the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • thermo difference value the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • MRL/MRV ratio which ratio is determined by one of (a) the difference between the actual and predetermined LNG temperatures and (b) the difference between the actual and predetermined warm end temperature differences or mid-point temperatures;
  • the warm end temperature difference value is predetermined; the MRL flow rate is adjusted in response to the difference between actual and predetermined LNG product flow rates and hence the LNG/MRL ratio changed; the required MRL/MRV ratio is adjusted in response to the difference between actual and predetermined warm end temperature difference value and the MRV flow rate adjusted to achieve that ratio; and the actual flow rate is adjusted in response to the difference between actual and predetermined LNG product temperatures.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the actual LNG flow rate varies, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
  • the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the actual LNG flow rate varies, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
  • the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the warm end temperature difference value is predetermined; the MRL flow rate is adjusted in response to the difference between actual and predetermined LNG product flow rates and hence the LNG/MRL ratio changed; the required MRL/MRV ratio is adjusted in response to the difference between actual and predetermined LNG product temperatures and the MRV flow rate adjusted to achieve that ratio; and the actual flow rate is adjusted in response to the difference between actual and predetermined warm end temperature difference values.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the warm end temperature difference value is predetermined; the MRL flow rate is adjusted in response to the difference between actual and predetermined LNG product flow rates and hence the LNG/MRL ratio changed; the required MRL/MRV ratio is adjusted in response to the difference between actual and predetermined LNG product temperatures and the MRV flow rate adjusted to achieve that ratio; and the actual flow rate is adjusted in response to both the difference between actual and predetermined warm end temperature difference values and the actual MRL flow rate.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • the mid-point temperature difference value is predetermined; the MRL flow rate is adjusted in response to the difference between actual and predetermined LNG product flow rates and hence the LNG/MRL ratio changed; the required MRL/MRV ratio is adjusted in response to the difference between actual and predetermined mid-point temperatures and the MRV flow rate adjusted to achieve that ratio; and the actual flow rate is adjusted in response to the difference between actual and predetermined LNG product temperatures.
  • the warm end temperature difference is predetermined and the difference between actual and predetermined warm end temperature differences used as an override control of the MRL flow rate when said difference exceeds a predetermined value.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature a predetermined value of the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • the actual LNG flow rate varies, by an amount corresponding to the difference between the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
  • the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature a predetermined value of the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature a predetermined value of temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • the actual LNG flow rate varies, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
  • the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature a predetermined value of temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means
  • the warm end temperature difference is predetermined; the MRL flow rate is adjusted in response to the difference between actual and predetermined LNG product flow rates and hence the LNG/MRL ratio changed; the required MRL/MRV ratio is adjusted in response to the difference between the actual mid-point temperature and a calculated temperature determined by the difference between the actual and predetermined warm end temperature differences and the MRV flow rate adjusted to achieve that ratio; and the actual flow rate is adjusted in response to the difference between actual and predetermined LNG product temperatures.
  • the invention provides a method of maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the actual LNG flow rate varies, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
  • the invention provides a control system for maintaining at an adjustable predetermined flow rate value and at an adjustable predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature means for comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
  • the invention provides a method of maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising the steps of:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
  • the MRL flow rate varying, by an amount corresponding to the difference between the actual and predetermined LNG flow rates, the MRL flow rate
  • the actual LNG flow rate varies, by an amount corresponding to the difference between the actual and predetermined LNG temperatures, the actual LNG flow rate.
  • the invention provides a control system for maintaining at a predetermined flow rate value and at a predetermined temperature value the liquefied natural gas (“LNG”) outlet stream of a natural gas liquefaction using heat exchange means, having a warm end to which the natural gas is fed, a liquefying section in which the natural gas is liquefied, a subcooling section in which the liquefied natural gas is subcooled and a cold end from which said LNG outlet stream is withdrawn, in which refrigeration duty is provided in the liquefying section by a first refrigerant (“MRL”) cooled in said heat exchange means and supplied for refrigeration duty at an MRL flow rate and in the subcooling section by a second refrigerant (“MRV”) cooled in said heat exchange means and supplied for refrigeration duty at an MRV flow rate, comprising:
  • warm end temperature difference value a predetermined value of the temperature difference between spent refrigerant leaving the warm end of the heat exchange means and a stream entering said warm end selected from MRL, MRV and the natural gas feed
  • mid-point temperature means for comparing the temperature of a stream at a location between the liquefying and subcooling sections of the heat exchanger means (“mid-point temperature”) with a calculated temperature value, which is determined by the difference between the actual and predetermined actual warm end temperature differences;
  • natural gas is introduced via line 100 into the warm end of a first tube side of a heat exchanger 112 in which is it liquefied and then subcooled before leaving the heat exchanger at the cold end.
  • Refrigeration duty in the heat exchange is provided by a multi component refrigerant (“MR”) circulating in a closed loop.
  • MR multi component refrigerant
  • Spent refrigerant from the heat exchanger is fed via line 144 to a compressor 102 and the compressed refrigerant is partially condensed in a cooler 104 before separation in a phase separator 106 .
  • the liquid phase (“MRL”) is fed via line 124 to a second tube side of the heat exchanger in which it is cooled before being throttled in valve 132 and introduced into the shell side of the heat exchanger 112 below the cold bundle.
  • the vapor phase (“MRV”) is fed via line 134 to a third tube side of the heat exchanger 112 in which it is cooled and then liquefied before being throttled in valve 138 and introduced into the shell side of the heat exchanger at the cold end.
  • the liquid and condensed vapor portions vaporize in the heat exchanger and combine to provide the refrigerant feed to line 144 .
  • the flow of LNG product is controlled by a valve 120 and the flows of the refrigerant portions to the heat exchanger are controlled by valves 132 and 138 respectively.
  • the temperature of the LNG product is compared in temperature indicator controller (“TIC”) 114 against the required product temperature determined by an operator set point (SP).
  • a signal proportionate to the difference in actual and required temperature is sent from the TIC 114 to a flow indicator controller (“FIC”) 116 , which in turn adjusts the position of the product valve 120 to maintain the required temperature.
  • FIC flow indicator controller
  • the product flow rate is monitored by the FIC 116 and a signal proportionate to the actual value (“PV”) of the flow is sent from the FIC 116 to a FIC 122 for comparison with a set point value determined by the operator.
  • a signal proportionate to the difference between the actual and required product flow rates is sent to FIC 126 , which compares the actual flow rate of the MRL with a required value set by that signal.
  • the MRL control valve 132 is adjusted in response to differences between the actual and required flow rates in order to adjust the refrigeration in heat exchanger 112 .
  • a signal proportionate to the difference between actual and required MRL flow rates is sent to a flow ratio indicator controller (“FRIC”) 140 where it is compared with a signal from flow indicator (“FI”) 136 measuring actual MRV flow rate in order to determine the actual MRV/MRL flow ratio.
  • the actual MRV/MRL flow rate is compared with a set point value determined by a signal received from the temperature differential indicator controller (“TDIC”) 142 .
  • TDIC temperature differential indicator controller
  • a signal proportionate to the difference between the actual and required MRV/MRL flow ratios adjusts flow valve 138 and the corresponding refrigeration provided to the heat exchanger 112 .
  • the TDIC 142 compares the actual temperature difference between the spent refrigerant in line 144 and the MRL in line 124 with a set point value determined by the operator.
  • the set point signal provided by the TDIC 142 to the FRIC 140 is proportionate to that difference in temperature.
  • the TDIC 142 could measure temperature difference between the spent refrigerant and either the MRV in line 134 or the natural gas feed in line 100 instead of the difference with the MRL as shown in FIG. 1 .
  • FI 136 could be located upstream instead of downstream of the heat exchanger 112 .
  • the FIC 126 also could be located upstream instead of downstream of the heat exchanger 112 .
  • valves 132 and 138 determined by the extent to which the flow rate, temperature and/or WETD have been changed. This will change the amount of refrigeration provided to the heat exchanger 112 and thereby change the difference between the actual and set LNG product temperature values. That change will adjust the valve 120 and hence the actual product flow rate. The change of the actual product flow rate will result in further adjustment valves 132 and 138 controlling the refrigeration supplied to the heat exchanger 112 and provide a corresponding change in the actual LNG product temperature.
  • control system will automatically change the refrigeration provided to the heat exchanger to maintain the set values if there is any change in LNG product flow rate, LNG product temperature or WETD arising from changes to any of those parameters not occasioned by changes to their required values, such as changes in NG composition, NG flow rate, partial condensation refrigeration duty for 104 , ambient air temperature, cooling water temperature, or atmospheric pressure.
  • the control system of FIG. 2 differs from that of FIG. 1 in that the LNG product valve 120 is adjusted in response to changes in the WETD and the required MRV/MRL flow ratio is determined by the difference between the actual and required LNG product temperatures.
  • the TDIC 142 sends a signal to FIC 116 instead of to FRIC 140 and TIC 114 sends a signal to FRIC 140 instead of FIC 116 .
  • the control system of FIG. 3 differs from that of FIG. 2 in that the signal from TDIC 142 to FIC 116 is dependent upon the difference between the actual and required MRL flow rates.
  • a signal proportionate to that difference is sent to a multiplier 300 to modify the signal from TDIC 142 .
  • the control system of FIG. 4 differs from that of FIG. 1 in that the required MRV/MRL flow ratio is determined by the difference between actual and required temperatures at a mid-point of the heat exchanger 112 located between the liquefying and subcooling sections of the heat exchanger, typically between the cold and warm or middle bundles of the heat exchanger.
  • a TIC 400 has a set point determined by the operator and compares that set point with the actual mid-point temperature.
  • the mid-point temperature can be that on the shell side of the heat exchanger 112 as shown in FIG. 4 or could be the temperature of the LNG or MRL or MRV at an appropriate location in the relevant tube section.
  • adjustment of the MRL flow rate by valve 132 in response to the difference between the actual and required MRV flow rates is overridden by a FIC 326 responsive to the WETD if the actual difference differs from the required difference by a predetermined amount.
  • the control system of FIG. 5 differs from that of FIG. 4 in that the required mid-point temperature is not operator set but is determined by the difference between actual and required WETDs.
  • a signal from TDIC 142 no longer provides an override to the FIC 126 control of valve 132 but provides a set point for TIC 400 .
  • the control system of FIG. 6 differs from that of FIG. 3 in that a constraint controller 146 limits the opening of the MRV valve 138 to, for example 90%, by adjusting a “Production Factor” that is multiplied by multiplier 148 to produce an LNG Master Flow Controller set point.
  • a constraint controller 146 limits the opening of the MRV valve 138 to, for example 90%, by adjusting a “Production Factor” that is multiplied by multiplier 148 to produce an LNG Master Flow Controller set point.
  • the controller 146 provides a Production Factor of 1 and does not limit production. However, if the valve position exceeds 90 % (or other predetermined maximum) amount, then the controller would begin to reduce the Production Factor until the system was in control with the required maximum valve position.
  • FIG. 7 differs from that of FIG. 1 in that the LNG product flow rate is directly adjusted to the required value and FIC 126 is controlled by the difference between actual and desired LNG product temperature difference
  • the process of FIG. 1 had some oscillations in its response to the increased rundown temperature with both the LNG flow rate and the LNG temperature oscillating somewhat as they reached the new steady state. It is believed that this was more a result of conflict between the two controllers rather than a tuning issue.
  • the process of FIG. 7 did not oscillate but did take a long time to reach the new steady state, which is believed to have been a function of tuning rather than controller interaction.
  • the process of FIG. 2 exhibited much tighter control of the temperature with little disturbance to the rest of the system.
  • the process of FIG. 3 showed similar ability to that of FIG. 2 in tracking the LNG temperature set point and slightly less disturbances to the rest of the system. Both of the processes of FIG. 2 and 3 showed the best response to this disturbance.
  • the process of FIG. 1 had difficulty in following the LNG production set point taking 2 hours to return to steady state. During that time, the LNG temperature had deviations as large as 2° F. (1.1° C.) warmer before finally reaching steady state over 2 hours after the disturbance began.
  • the process of FIG. 7 with its direct control over LNG flow rate had excellent tracking of the LNG production set point, but also saw large temperature swings before finally reaching steady state almost 3 hours after the disturbance.
  • the process of FIG. 2 lagged in the tracking of the LNG production set point taking two hours to reach steady state, but maintained tight LNG temperature control throughout the disturbance.
  • the process of FIG. 3 tracked the set point of the LNG production very well reaching steady state within an hour of the beginning of the disturbance and maintaining tight temperature control throughout.
  • the system tracked the LNG production to the new set point, however, the LNG temperature continued to rise driving the MRV valve 138 fully open.
  • the LNG temperature finally settled out approximately 4° F. (2.2° C.) warmer than the desired LNG temperature.
  • the mid point temperature between the middle and cold bundles of the heat exchanger 112 warmed up by close to 20° F. (11° C.).
  • the increased MRV flow almost doubled the cold bundle pressure drop.
  • the gas turbine reached full power and then bogged down reducing its speed by approximately 1%. The results showed that the control system had no checks to prevent it from reaching an unacceptable new operating point.
  • the process of FIG. 6 overcomes this problem by providing a check to prevent the control system from reaching an undesirable operating point.
  • the controller 146 set to limit the valve opening to 90%, the 7% production increase limited production to a 4.8% increase and maintained control of the LNG temperature throughout and did not bog down the MR turbine 102 .

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US11/782,990 US20090025422A1 (en) 2007-07-25 2007-07-25 Controlling Liquefaction of Natural Gas
PCT/IB2008/001924 WO2009013605A2 (en) 2007-07-25 2008-07-17 Controlling liquefaction of natural gas
EP08776389.2A EP2324311B1 (en) 2007-07-25 2008-07-17 Controlling liquefaction of natural gas
JP2010517501A JP5529735B2 (ja) 2007-07-25 2008-07-17 天然ガス液化の制御
PE2008001249A PE20090461A1 (es) 2007-07-25 2008-07-23 Control de la licuefaccion de gas natural
US13/314,882 US9671161B2 (en) 2007-07-25 2011-12-08 Controlling liquefaction of natural gas
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JP5529735B2 (ja) 2014-06-25
EP2324311A2 (en) 2011-05-25
US20120079850A1 (en) 2012-04-05
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JP5785282B2 (ja) 2015-09-24
JP2014134374A (ja) 2014-07-24

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