# US 8,783,061 B2 · Apparatus and method for optimizing a natural gas liquefaction train having a nitrogen cooling loop > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,783,061 B2 - Document ID: US8783061B2 - Status: Granted - Kind: B2 - Application number: 11/811,782 - Filed: 2007-06-12 - Published: 2008-12-18 - Granted: 2014-07-22 - Earliest claimed: 2007-06-12 - Assignees: Honeywell International Inc. - Applicants: Brian A. Coward - Inventors: Brian A. Coward - Web page: https://patentgrep.com/patent/US8783061B2 ## CPC - F25J 1/0055: {originating from an incorporated cascade} - F25J 1/0022: {Hydrocarbons, e.g. natural gas} - F25J 1/0072: {Nitrogen} - F25J 1/0087: {Propane; Propylene} - F25J 1/0218: {with one or more SCR cycles, e.g. with a C3 pre-cooling cycle} - F25J 1/0249: {Controlling refrigerant inventory, i.e. composition or quantity (charging or discharging refrigerants in cooling systems F25B45/00)} - F25J 1/0252: {Control strategy, e.g. advanced process control or dynamic modeling} - F25J 1/0268: {using a dedicated refrigeration means (F25J1/0296 takes precedence)} - F25J 1/0269: {Arrangement of liquefaction units or equipments fulfilling the same process step, e.g. multiple "trains" concept (F25J1/0294 takes precedence)} - F25J 1/0292: {Refrigerant compression by cold or cryogenic suction of the refrigerant gas} - G05B 13/048: {using a predictor} - G05D 7/0676: {by action on flow sources (G05D7/0688, G05D7/0694 take precedence)} - F25J 2220/62: Separating low boiling components, e.g. He, H2, N2, Air - F25J 2270/12: External refrigeration with liquid vaporising loop - F25J 2270/60: Closed external refrigeration cycle with single component refrigerant [SCR], e.g. C1-, C2- or C3-hydrocarbons - 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 (cryogenic pumps F04B37/08; gas storage vessels, gas holders F17; filing vessels with, or discharging from vessels, compressed, liquefied or solidified gases F17C; refrigeration machines, plants, or systems F25B)} - G05B: CONTROL OR REGULATING SYSTEMS IN GENERAL; FUNCTIONAL ELEMENTS OF SUCH SYSTEMS; MONITORING OR TESTING ARRANGEMENTS FOR SUCH SYSTEMS OR ELEMENTS (systems for controlling or regulating non-electric variables G05D; systems for regulating electric or magnetic variables G05F; control devices or systems insofar as characterised by mechanical features only G05G) - G05D: SYSTEMS FOR CONTROLLING OR REGULATING NON-ELECTRIC VARIABLES ## Abstract A natural gas liquefaction train includes a nitrogen cooling loop. A controller is provided for controlling one or more controlled variables by adjusting one or more manipulated variables. The one or more manipulated variables may include a nitrogen flow associated with the nitrogen cooling loop in the natural gas liquefaction train. The controller could adjust the nitrogen flow by adjusting operation of a compressor associated with the nitrogen cooling loop. The one or more controlled variables may include a rundown temperature of liquefied natural gas exiting the nitrogen loop and/or a calorific or heating value of the liquefied natural gas exiting the nitrogen loop. A second controller could control other aspects of the natural gas liquefaction train, such as by controlling a mass flow rate of a feed gas in the natural gas liquefaction train. ## Claims ### Claim 1 An apparatus comprising: at least one memory configured to store a plurality of models, the models associated with manipulated variables and controlled variables, the manipulated variables and the controlled variables associated with a natural gas liquefaction train that comprises: a first heat exchanger configured to cool a natural gas stream into a liquefied natural gas stream; a compressor configured to compress evaporated refrigerant from a shell of the first heat exchanger; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the expansion value and further cool the liquefied natural gas stream, the nitrogen cooling loop comprising a second heat exchanger having a single path; and at least one processor configured to determine how to adjust the manipulated variables using the models so as to control the controlled variables; wherein the manipulated variables comprise a nitrogen flow to the second heat exchanger of the nitrogen cooling loop, a compressor pressure of the compressor, and a flow of compressed refrigerant from the compressor; wherein the controlled variables comprise a calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, a rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and temperature differences within the first heat exchanger, the temperature differences associated with the evaporated refrigerant; wherein the models comprise a first model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and a second model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop; and wherein the at least one processor is configured to use the first and second models to determine how to manipulate the nitrogen flow in order to control the calorific value and the temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop. ### Claim 2 (depends on 1) The apparatus of claim 1, wherein the further cooling of the liquefied natural gas stream by the nitrogen cooling loop results in a lower ethane content in the liquefied natural gas stream. ### Claim 3 (depends on 1) The apparatus of claim 1, wherein the at least one processor is configured to adjust operation of a second compressor associated with the nitrogen cooling loop to adjust the nitrogen flow. ### Claim 4 (depends on 3) The apparatus of claim 3, wherein the at least one processor is configured to adjust at least one of a speed and a suction pressure of the second compressor. ### Claim 5 (depends on 1) The apparatus of claim 1, wherein the temperature differences within the first heat exchanger comprise a first temperature difference between a fluid in a first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a warm end of the first heat exchanger, the first path configured to receive a feed gas and output liquefied natural gas. ### Claim 6 (depends on 1) The apparatus of claim 1, wherein: the first heat exchanger is associated with a light refrigerant and a heavy refrigerant that are produced by separating the compressed refrigerant; and the manipulated variables further comprise a mass flow of the light refrigerant and a mass flow of the heavy refrigerant. ### Claim 7 (depends on 1) The apparatus of claim 1, wherein the at least one processor is configured to determine how to manipulate the compressor pressure of the compressor and the flow of compressed refrigerant from the compressor in order to control: a first temperature difference between a fluid in a first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a warm end of the first heat exchanger, the first path configured to receive a feed gas and output liquefied natural gas; a second temperature difference between the fluid in the first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a cold end of the first heat exchanger; and a third temperature difference between the fluid in the first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a mid-point of the first heat exchanger. ### Claim 8 A method comprising: determining how to adjust manipulated variables so as to control controlled variables using a plurality of models, the manipulated variables and the controlled variables associated with a natural gas liquefaction train that comprises: a first heat exchanger configured to cool a natural gas stream into a liquefied natural gas stream; a compressor configured to compress evaporated refrigerant from a shell of the first heat exchanger; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the expansion value and further cool the liquefied natural gas stream, the nitrogen cooling loop comprising a second heat exchanger having a single path; and outputting control signals for manipulating the manipulated variables to control the controlled variables based on the determination; wherein the manipulated variables comprise a nitrogen flow to the second heat exchanger of the nitrogen cooling loop, a compressor pressure of the compressor, and a flow of compressed refrigerant from the compressor; wherein the controlled variables comprise a calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, a rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and temperature differences within the first heat exchanger, the temperature differences associated with the evaporated refrigerant; wherein the models comprise a first model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and a second model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop; and wherein outputting the control signals comprises outputting at least one first control signal to manipulate the nitrogen flow in order to control the calorific value and the temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop. ### Claim 9 (depends on 8) The method of claim 8, wherein the further cooling of the liquefied natural gas stream by the nitrogen cooling loop results in a lower ethane content in the liquefied natural gas stream. ### Claim 10 (depends on 8) The method of claim 8, wherein outputting the at least one first control signal comprises outputting the at least one first control signal for adjusting operation of a second compressor associated with the nitrogen cooling loop to adjust the nitrogen flow. ### Claim 11 (depends on 8) The method of claim 8, wherein the temperature differences within the first heat exchanger comprise a first temperature difference between a fluid in a first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a warm end of the first heat exchanger, the first path configured to receive a feed gas and output liquefied natural gas. ### Claim 12 (depends on 8) The method of claim 8, wherein: the first heat exchanger is associated with a light refrigerant and a heavy refrigerant that are produced by separating the compressed refrigerant; and the manipulated variables further comprise a mass flow of the light refrigerant and a mass flow of the heavy refrigerant. ### Claim 13 (depends on 8) The method of claim 8, wherein outputting the control signals further comprises outputting at least one second control signal to manipulate the compressor pressure of the compressor and the flow of compressed refrigerant from the compressor in order to control: a first temperature difference between a fluid in a first path of the first heat exchanger and the evaporated refrigerant in the of the first heat exchanger at a warm end of the first heat exchanger, the first path configured to receive a feed gas and output liquefied natural gas; a second temperature difference between the fluid in the first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a cold end of the first heat exchanger; and a third temperature difference between the fluid in the first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a mid-point of the first heat exchanger. ### Claim 14 A non-transitory computer readable medium embodying a computer program, the computer program comprising: computer readable program code for determining how to adjust manipulated variables so as to control controlled variables using a plurality of models, the manipulated variables and the controlled variables associated with a natural gas liquefaction train that comprises: a first heat exchanger configured to cool a natural gas stream into a liquefied natural gas stream; a compressor configured to compress evaporated refrigerant from a shell of the first heat exchanger; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the expansion value and further cool the liquefied natural gas stream, the nitrogen cooling loop comprising a second heat exchanger having a single path; and computer readable program code for generating control signals for manipulating the manipulated variables to control the controlled variables based on the determination; wherein the manipulated variables comprise a nitrogen flow to the second heat exchanger of the nitrogen cooling loop, a compressor pressure of the compressor, and a flow of compressed refrigerant from the compressor; wherein the controlled variables comprise a calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, a rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and temperature differences within the first heat exchanger, the temperature differences associated with the evaporated refrigerant; wherein the models comprise a first model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and a second model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop; and wherein the computer readable program code for outputting the control signals comprises computer readable program code for outputting at least one first control signal to manipulate the nitrogen flow in order to control the calorific value and the temperature of the liquefied natural gas stream exiting the nitrogen cooling loop. ### Claim 15 (depends on 14) The computer readable medium of claim 14, wherein: the controlled variables further comprise a rundown temperature of the liquefied natural gas stream exiting the nitrogen loop. ### Claim 16 (depends on 15) The computer readable medium of claim 15, wherein the computer readable program code for outputting the at least one first control signal comprises computer readable program code for outputting the at least one first control signal to adjust operation of a second compressor associated with the nitrogen cooling loop in order to adjust the nitrogen flow. ### Claim 17 (depends on 14) The computer readable medium of claim 14, wherein the manipulated variables further comprise a mass flow of a light refrigerant associated with the first heat exchanger and a mass flow of a heavy refrigerant associated with the first heat exchanger. ### Claim 18 A system comprising: a plurality of controllers configured to control operation of a natural gas liquefaction train that comprises: a first heat exchanger configured to cool a natural gas stream into a liquefied natural gas stream; a compressor configured to compress evaporated refrigerant from a shell of the first heat exchanger; an expansion valve configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the first heat exchanger and cause the liquefied natural gas stream to expand; and a nitrogen cooling loop configured to receive the liquefied natural gas stream after the liquefied natural gas stream exits the expansion value and further cool the liquefied natural gas stream, the nitrogen cooling loop comprising a second heat exchanger having a single path; wherein a first of the controllers is configured to determine how to adjust manipulated variables associated with the natural gas liquefaction train so as to control controlled variables associated with the natural gas liquefaction train using a plurality of models; wherein the manipulated variables comprises a nitrogen flow to the second heat exchanger of the nitrogen cooling loop, a compressor pressure of the compressor, and a flow of compressed refrigerant from the compressor; wherein the controlled variables comprise a calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, a rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and temperature differences within the first heat exchanger, the temperature differences associated with the evaporated refrigerant; wherein the models comprise a first model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the rundown temperature of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop, and a second model associating the nitrogen flow to the second heat exchanger of the nitrogen cooling loop and the calorific value of the liquefied natural gas stream exiting the second heat exchanger of the nitrogen cooling loop; and wherein the first controller is configured to determine how to manipulate the nitrogen flow in order to control the calorific value and the temperature of the liquefied natural gas stream exiting the nitrogen cooling loop. ### Claim 19 (depends on 18) The system of claim 18, wherein the temperature differences within the first heat exchanger comprise a first temperature difference between a fluid in a first path of the first heat exchanger and the evaporated refrigerant in the shell of the first heat exchanger at a warm end of the first heat exchanger, the first path configured to receive a feed gas and output liquefied natural gas. ### Claim 20 (depends on 18) The system of claim 18, wherein the manipulated variables further comprise a mass flow of a light refrigerant associated with the first heat exchanger and a mass flow of a heavy refrigerant associated with the first heat exchanger. ### Claim 21 (depends on 18) The system of claim 18, wherein a second of the controllers is configured to control a mass flow rate of a feed gas in the natural gas liquefaction train. ### Claim 22 (depends on 7) The apparatus of claim 7, wherein the models further comprise: a third model associating the compressor pressure of the compressor and the first temperature difference; a fourth model associating the compressor pressure of the compressor and the second temperature difference; a fifth model associating the compressor pressure of the compressor and the third temperature difference; a sixth model associating the flow of compressed refrigerant from the compressor and the first temperature difference; a seventh model associating the flow of compressed refrigerant from the compressor and the second temperature difference; an eighth model associating the flow of compressed refrigerant from the compressor and the third temperature difference. ### Claim 23 (depends on 22) The apparatus of claim 22, wherein: the first model is defined in Laplace Transform form as G ⁡ ( s ) = - 0.979 ⁢ 1 1.29 ⁢ s + 1 ⁢ ⅇ - 0 ⁢ s ; the second model is defined in Laplace Transform form as G ⁡ ( s ) = - 0.687 ⁢ 1 1.78 ⁢ s + 1 ⁢ ⅇ - 0 ⁢ s ; the third model is defined in Laplace Transform form as G ⁡ ( s ) = - 0.2 ⁢ 414 ⁢ s + 1 641 ⁢ s 2 + 45 ⁢ s + 1 ⁢ ⅇ - 9 ⁢ s ; the fourth model is defined in Laplace Transform form as G ⁡ ( s ) = 0.7 ⁢ 1 5 ⁢ s + 1 ⁢ ⅇ - 0 ⁢ s ; the fifth model is defined in Laplace Transform form as G ⁡ ( s ) = 0.55 ⁢ 1 5.71 ⁢ s + 1 ⁢ ⅇ - 3 ⁢ s ; the sixth model is defined in Laplace Transform form as G ⁡ ( s ) = 0.129 ⁢ 1 35.3 ⁢ s + 1 ⁢ ⅇ - 0 ⁢ s ; the seventh model is defined in Laplace Transform form as G ⁡ ( s ) = - 0.1 ⁢ 1 15 ⁢ s + 1 ⁢ ⅇ - 0 ⁢ s ; and the eighth model is defined in Laplace Transform form as G ⁡ ( s ) = 0.0404 ⁢ 1 80.5 ⁢ s 2 + 17.9 ⁢ s + 1 ⁢ ⅇ - 6 ⁢ s .