A METHOD AND SYSTEM FOR CONTROLLING THE TEMPERATURE OF LIQUEFIED NATURAL GAS IN A LIQUEFACTION PROCESS

Technical Field

The present invention generally relates ^' to a method for controlling the temperature of a liquefied natural gas (LNG) in a liquefaction process. The present invention also relates to a systems for implementing the method and to a computer program for implementing same.

Background

Development of a Liquefied Natural Gas (LNG) liquefaction process cycle involves multivariable optimization. The variables include temperature, pressure, pressure drop, flow, mixed refrigerant flow rate, and compressor speed. Maintaining the temperature of the Liquefied Natural Gas (LNG) exiting the main cryogenic heat exchanger within a specified range is important for downstream processing and the prevention of downstream equ pment p ob1ems .

A particular problem with optimizing LNG plant operation occurs when one seeks to maximize the LNG production ^' rate for a given compressor power consumption.. Accordingly,, different pressure settings and different flow rates of the multi component refrigerant ( CR) are amongst the variables that are adjusted to give optimal operation.

In one known method, there is disclosed a method of adjusting the temperature of the LNG outlet stream via adjusting the flow rate and pressure of the refrigerant stream. The refrigeration capacity of this method was adjusted via the MCR compressors, and by adjusting the total flow rate of the MCR. However, such a method usually requires high integrity and reliable instruments, and a high level of maintenance of these instrumentations. Further, such a method requires operator personnel intervention which translates to higher operating costs of the LNG process .

In another known method, the MCR gas liquefaction process is optimized through the control of the refrigerant composition of the refrigerant stream in each heat exchange stage. Such adjustment is effected by withdrawing controlled amounts of the refrigerant components and introducing pure refrigerant components at a point before the refrigerant enters a particular heat exchanger. However, such a method typically leads to a higher consumption of the refrigerant due to draining and make-up of the refrigerant , at the same time since it needs to maintain the pressure and level of the MCR system. In addition, such a method usually requires high integrity and reliable instruments. Such a method thus requires a high level of maintenance of the instruments and operator personnel intervention.

In another known, method, an Advance Process Control (APC) controller generally manipulates the Joule-Thompson valves and the MCR compressor speed in order to set the appropriate circulation rate of MCR. However, such a method may result in substantial temperature variations within the cryogenic heat exchanger due to manual control to adjust the MCR composition. Further, when such system is operating at a reduced refrigerant speed, it may result in a lack of adequate circulation of the ref igerant which may in turn impair the efficiency and performance of the system. Accordingly, there is a need to provide a method for controlling the temperature profile of the chiller that overcomes, or at least ameliorates, the disadvantages mentioned above. Therefore, it is an object of the present invention to provide a method of MCR Control which can be automated .

Summary

According to a first aspect, there is provided a method for controlling the temperature of liquefied natural gas (LNG) produced from a chiller, in a liquefaction process, the method comprising the step of varying the composition of a refrigerant stream in thermal communication with natural gas undergoing liquefaction in said chiller to thereby control the temperature of the LNG produced from the chiller.

Advantacfeously, by varying the refrigerant composition of the refrigerant stream, the temperature profile of the chiller may be controlled. More advantageously, this method allows processing of LNG with varying compositions and heat contents while producing an "on-spec" L G, which meets the requirements as predetermined.

According to a second aspect, there is provided a method of improving the yield of LNG in an LNG production process that utilizes a chiller to liquefy natural gas, the method comprising the steps of providinj a refrigerant multi-component stream that is in fluid communication with natural gas undergoing liquefaction in said chiller; varying the composition of the multi-componen refrigerant stream based on a temperature profile of the chiller, wherein the yield of LNG produced in said method is relatively higher than the yield of LNG produced when said varying step is absent.

Advantageously, this method is capable of controlling the temperature profile of the chiller and maintaining an optimum temperature profile of the chiller. More advantageously, this method improves the overall efficiency of the liquefaction process and the overall yield of LNG produced in said process.

According to a third aspect, there is provided a chiller system for liquefaction of liquefied natural gas, the system comprising a chiller having a natural gas flow path extending therethrough in which natural gas is liquefied; a temperature profile extending along the flow path of the chiller; a refrigerant stream in thermal communication with said flow path to liquefy said natural gas to liquefied natural gas; and control means for varying the composition of the refrigerant stream to maintain the temperature profile within a pre-determined temperature profile range.

According to a fourth aspect, there is provided a computer program for controlling the temperature of a liquefied natural gas produced from a chiller in a liquefaction process, the computer program being encoded in at least one computer readable medium, the computer program comprising a first set of instructions, encoded in at least one computer readable medium, operable to implement, when executed by a processor, conversion of the temperature data representing a temperature profile between an inlet stream and an outlet stream of a chiller into a set of instructions to vary the composition of a refrigerant stream in thermal communication with the natural gas undergoing liquefaction in said chiller. Advantageously, the program provides an accurate monitoring of the temperature profile between an inlet stream and an outlet stream of a chiller, and results in a reliable and optimized performance of the chiller in the liquefaction process.

Definitions

The following words and terms used herein shall have the meaning indicated:

The term "Liquefied Natural Gas", and its acronym

"LNG", refers to Liquefied Natural Gas, that is Natural gas that has been cooled down such that it condenses and becomes liquid. LNG is primarily methane but does include other low molecular weight hydrocarbons, which has been liquefied by refrigeration below the boiling point (e.g. -161. °C (111.7K)) depending on constituents of the gas for storage and transport.

The term Natural Gas shall refer to .a gaseous mixture of hydrocarbons where an essential part is methane.

The term "temperature profile" is the context of this specification with reference to a chiller, is intended to refer, to a temperature difference or pattern between the temperature of the LNG flowing into the chiller and the temperature of the LNG flowing from the chiller. The temperatures forming the profile can therefore be measured along a LNG and MCR flow path of the chiller which extends from an inlet conduit extending into the chiller to an outlet conduit extending from the chiller. The temperature profile can be generated by measuring various temperature measurement points along the LNG profile* typically from, the inlet conduit to the outlet conduit. The term "pre-determined temperature profile range" means a selected range of temperatures for each of the measured temperature measurement points of the LNG flow path for the temperature profile of the chiller as mentioned above.

The term "plural" means two or more.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements.

As used herein the term "cryogenic temperature" means a temperature of -150°C or less.

As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value, more typically, +/- 2% of the stated value, even more typically +/- 1% of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclQsed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifically disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1, 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range. Detailed Disclosure of Embodiments

Exemplary, non-limiting embodiments of a method of controlling the temperature of a liquefied natural gas will now be disclosed.

The refrigerant which, performs · the cooling and liquefaction of the natural gas may comprise plural constituents. In one embodiment, the plural constituents of the refrigerant are selected from nitrogen and saturated hydrocarbons , having from 1 to 3 carbons. Preferably, the hydrocarbons are comprised of methane, ethane, propane and butane. Advantageously,, the use of plural constituents in the refrigerant provides refrigeration over a range of temperatures. The natural gas feed stream may be subjected to preliminary treatment processes, which comprises absorption, drying and distillation to remove moisture and higher hydrocarbons. Moisture must be removed to prevent freezing and plugging of the conduits and heat exchangers at the temperatures encountered in the process.

The method according to the present invention may also comprise splitting the natural gas into two or .more separate streams and using the separate streams to exchange heat with the refrigerant in series fashion. In this manner, heat can be exchanged between the refrigerant, and the first and second heat, exchange streams in the beat exchangers and a more effective heat transfer to the refrigerant may be achieved.

The method according to the present invention may comprise monitoring the flow rate of the natural gas inlet stream into the chiller. In one embodiment, a controller is configured to monitor the flow rate of the natural gas through the chiller.

The natural gas is passed from an inlet stream into the chiller to an LNG outlet stream from the chiller, wherein the chiller has a temperature profile between said inlet stream and said outlet stream. The method according to the present invention may comprise monitoring the temperature profile of the chiller. In one embodiment, a controller may be configured to monitor the temperature profile of the chiller. The controller may be configured to monitor the temperature profile of the exit of the chiller. The temperature profile of said chiller may be generated with the aid of temperature sensors at the outlet of the chiller, at the entrance of the chiller and at various points along the length of the chiller. In a preferred embodiment, the. temperature sensors are placed at uniform points along the chiller. In another embodiment, the temperature profile of the chiller comprises a temperature profile showing the temperature changes between the central portion of the chiller and at the entrance of the chiller. In yet another embodiment, the temperature profile of the chiller comprises a temperature profile showing the temperature changes between the central portion of the chiller and at the exit of the chiller. In a preferred embodiment, the temperature profile of the chiller comprises a temperature profile showing the temperature changes over the entire length of the chiller.

As the natural gas enters the chiller, the natural gas may be cooled to a temperature of about -28 to -30 °C to about -65 to -66 °C. More preferably, the natural gas is cooled to a temperature of about -67 °C. The natural gas may be further cooled to a temperature of about -70 to -73 °C to about -120 to -122 °C as it undergoes further heat exchange with the refrigerant stream. More preferably, the natural gas is cooled to about -123°C.

As the natural gas exits the chiller, the natural gas may be cooled to a temperature of about -120 to -122 °C to about -150 to -152 °C. More preferably, the natural gas is cooled to a temperature of about -151 °C. The method according to the present invention may comprise monitoring the temperature of the LNG outlet stream of chiller. In one embodiment, a controller may be configured to monitor the temperature of the chiller. Overcooling the natural gas stream results in a waste of energy and increases cost, while under-cooling the natural gas stream results in downstream processing and equipment problems as well as loss in LNG yield.

In one embodiment, the composition of the re frigerant stream is controlled in response to the temperature profile of the chiller. The step of varying the composition of the refrigerant stream can be achieved via control of the flow rate of at least one constituent of the refrigerant stream in the LNG process. In a preferred embodiment, the flow rate of all constituents of the refrigerant stream, may be adjusted independently of each . other. There may be at least one control valve for regulating the flow rate of each constituent of the refrigerant stream. Consequently, the composition of the refrigerant stream may be varied accordingly in response to the temperature profile of the chiller. In one exemplary embodiment, the composition of the refrigerant is controlled to thereby maintain the temperature profile of the chiller at a pre-determined temperature range. Advantageously, the method according to the present invention minimizes the inventory loss of the refrigerant due to less bleeding and/or less venting and this leads to a lower consumption of the refrigerant.

The composition of nitrogen in the refrigerant stream may be selected from the group consisting of about 1 moll to about 10 mol%, about lmol% to about 8 mol%, about 1 mol% to about 6 mol%, about 1 mol% to about 4 moll, about 1 moll to about 2 mol%, about 3 mol% to about 10 mol%, about 5 mol% to about 10 mol%, about 7 mol% to about 10 mol%, about 9 mol% to about 10 mol%. Preferably, the composition of nitrogen in the refrigerant stream is about 5.5 moll.

The composition of methane in the refrigerant stream may be selected from the group consisting of about 20 mol% to about 60 mol%, about 20 mol! to about 50 mol%, about 20 mol% to about .40 moll, about 20 moll to about 30 moll, about 30 mol% to about 60 mol%, about 40 mol% to about 60 moll, about 50 mol% to about 60 mol% . Preferably, the composition of methane in the refrigerant stream is about 40 mol%.

The composition of ethane in the refrigerant stream may be selected, from the group consisting of about 20 moll to about 60 mol%, about 20 mol% to about 50 moll, about 20 mol% to about 40 mol%, about 20 moll to about 30 moll, about 30 mol% to about 60 mol , about 40 mol% to about 60 moll, about 50 moll to about 60 moll. Preferably, the composition of ethane in the refrigerant strea is about. 46 moll .

The composition of propane in the refrigerant stream may be selected from the group consisting of about 5 moll to about 15 moll, about 5 moll to about 13 moll, about 5 moll to about 11 moll, about 5 moll to about 9 moll, about 5 mol% to about 7 mol%, about 7 mol% to about 15 mol%, about 9 moll to about 15 mol%, about 11 mo.1% to about 15 moll, about 13 mol% to about 15 moll. Preferably, the composition of nitrogen, in the refrigerant stream is about 9 mol%.

The refrigerant stream employed in the cooling and liquefaction of the natural gas may be compressed in one or more stages. Pumping or compressing the refrigerant can be achieved ^' using devices known for pumping and compressing fluids. The selection of pumps and compression equipment will be a matter of design choice and will depend on factors such as the composition of the refrigerant, its- flow rate, the desired vaporization and/or condensation temperatures, compressor speed and whether power is to be produced from the circulating refrigerant. Because it is typically more efficient to increase the pressure of a liquid than a gas, there is a preference for increasing the pressure of the refrigerant when it is primarily in a liquid phase. Suitable pumps may include centrifugal and reciprocating pumps.

In one embodiment, the step of varying the composition of the refrigerant stream comprises monitoring the pressure of the refrigerant stream. The refrigerant composition of the refrigerant stream may be controlled together with the adjustment of the compressor speed. In one embodiment, a controller may be configured to monitor the pressure of the refrigerant stream. The pressure of the refrigerant stream is dependent on the liquefaction process requirements and the capacity of the compression equipment. The pressure of the refrigerant stream may be measured with, the aid of pressure sensors prior to, after compression, and in between the compression 5ΐ:3ς}θ3. Typically, the pressure may be selected from the group consisting of about 3500 kPa to about 6000 kPa, about 3500 kPa to about 5000 kPa, about 3500 kPa to about 5600 kPa, about 3500 kPa to about 5400 kPa, about 3500 k:Pa to about 52 kPa, about 3500 kPa to about 5000 kPa, about 3700 kPa to about 6000 kPa, 3900 kPa to about 6000 kPa, 4100 kPa to about 6000 kPa, and about 4300 kPa to about 6000 kPa, 4300 kPa to about 6000kPa, 4500 kPa to about 6000 kPa. In one embodiment, the pressure of the refrigerant stream is about 4500 to about 4900 kPa .

The compressed refrigerant stream may be further cooled to partial condensation against another refrigerant loop. Thereafter, the partially condensed refrigerant stream may be phase separated in a separator, which separates the partially condensed refrigerant stream into a vapor phase and a liquid phase. In one embodiment, the step of varying the composition, of the refrigerant stream comprises monitoring the liquid level of the separator. The liquid level in the separator may be measured with the aid of level sensors or liquid flow meters. In another embodiment, the step of varying the composition of the refrigerant stream comprises monitoring the gas level of the separator. The gas level in the separator may be measured with the aid of gas flow meters or pressure sensors.

Typically, the level of the separator may be selected from the group consisting of about 20 to about 00%, about 20 to about 70%, about 20 to about 60%, about 20 to about 50%, about 20 to about 40%, about 20 to about 30%, about 30 to about 80%, about 40 to about 80%, about 50 to about 00%, about 60 to about 80%, about 70 to about 80%. The chiller can be any of the heat exchanger types well known in the art which permit indirect heat exchange between two fluid streams. Such heat exchangers can be plate and fin heat exchangers, tube and shell heat exchangers, coil wound tube heat exchangers, or any other similar devices permitting indirect heat exchange between fluids. The flow of natural gas through the heat exchanger can be upwardly, downwardly or even horizontally. The flow of natural gas through the heat exchanger is generally dependent upon the particular type of heat exchanger selected .

The method according to the present invention may further comprise controlling the liquefaction process using a distributed control system (DCS) , wherein the controller elements are distributed throughout the liquefaction process. The DCS receives information from the process variables comprising the mole composition of the refrigerant stream, , the temperature profile of the chiller, the temperature of the LNG outlet stream, and determine control actions for at least one of a set of controlled variables. The set of controlled variables comprises varying the flow rate of at least one component of the refrigerant stream. Advantageously, a method using DCS

Only require minimal operator intervention during stable operation of the process and allows greater optimization of the . refrigerant composition during stable operation of the process .

There is also provided a computer program for controlling the temperature of a liquefied natural gas produced from a chiller in a liquefaction process, the computer program being encoded in at least one computer readable medium, the computer program comprising a first set of instructions, encoded in at least one computer readable medium, operable to implement, when executed by a processor, conversion of the temperature data representing a temperature profile between an inlet stream and an outlet stream of a chiller into a set of instructions to vary the composition of a refrigerant stream in thermal communication with the natural gas undergoing liquefaction in said chiller. The refrigerant stream which is in thermal communication with the natural gas may comprise plural constituents.

The computer program may comprise a second set of instructions, encoded in the at least one computer readable medium, operable to implement, , when executed by the processor, to control the flow rate of at . least one constituent of the refrigerant stream to vary the composition of a refrigerant stream in thermal communication with the natural gas undergoing liquefaction in said chiller.

Brief Description of Drawings

The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig. 1 is a schematic flow diagram of a mixed component refrigerant liquefied natural gas (MCR LNG) system 100 which is typical of a plant controlled in accordance to one embodiment of the method disclosed herein.

Fig. 2 is a schematic outlining the logic sequence in controlling Nitrogen of the refrigerant composition in accordance to the embodiment disclosed if Fig 1. Fig. 3 is a schematic outlining the logic sequence in controlling Methane of the refrigerant composition in accordance to the embodiment disclosed if Fig 1.

Fig. 4 is a schematic outlining the logic sequence in controlling ethane of the refrigerant composition in accordance to the embodiment disclosed if Fig 1.

Fig. 5 is a schematic outlining the logic sequence in controlling Propane of the refrigerant composition in accordance to the embodiment disclosed if Fig 1.

Detailed Description of Drawings

Referring to Fig. 1, there is disclosed a schematic flow diagram of Mixed Component Refrigerant (MCR) LNG system 100 which is typical of a plant controlled in accordance to one embodiment of the method disclosed herein

A natural gas feed stream is supplied in conduit 62 to the main heat exchanger E-01. The feed stream in conduit 62 may be subjected to preliminary treatment processes, which comprises drying and distillation to remove moisture and higher hydrocarbons. Moisture must be removed to prevent freezing and plugging of the conduits and heat exchangers at the temperatures encountered in the process .

The treated natural gas is fed in conduit 62 to the main heat exchanger E-01. The natural gas undergoes heat exchange with the Mixed Component Refrigerant (MCR) as will be explained, further below. Consequently, the natural gas is cooled and condensed in the main heat exchanger E-01. The liquid phase of the cooled natural gas is removed as a bottom stream via conduit 70.

The MCR is compressed in multiple stages through a low pressure compressor stage K-01 and a high pressure compressor stage K-02. The low pressure compressor stage K-01 receives the MCR from the heat exchanger K-02, compresses the MCR and then passes the compressed MCR to high pressure compressor stage K-02. The heat of compression is removed by passing the refrigerant through heat exchanger V-03, which is cooled by an external cooling fluid. The external cooling fluid is treated sea cooling water. The compressed refrigerant in conduit 40 is further cooled to partial condensation in heat exchanger E-02 against another refrigerant loop. The partially condensed MCR in line 44 is then phase separated in separator V-01, which separates the partially condensed MCR into MCR vapor phase and MCR liquid phase. The liquid phase of the MCR is removed from the bottom of separator V-01 in conduit 54. The vapor phase of the MCR is removed from the top of separator V-01 in conduit 52 and going to the main heat exchanger E-01. It is then subjected to further cooling and expansion (using the Joule-Thompson valve) at the top of chiller E-01 to be changed from vapor to liquid form. The MCR streams 52 and 54 enter the main heat exchanger E-01 and undergo heat exchange with said treated natural gas entering the main heat exchanger E-01. The treated natural gas is reduced in temperature and liquefied before being withdrawn from the main heat exchanger E-01 as a bottom stream via condu.lt 70.

The MCR vapor 20 leaves the bottom of the main heat exchanger E-01 and is then recompressed in multiple stages through compressors K-01 and K-02, with water or other refrigerant loop cooling provided between each compression stage, before being recycled back to the main heat exchanger E-01.

A number of process measurements are utilized to derive the control signals which will be described hereinafter for controlling the liquefaction system. These process measurements are as follows.

A temperature transducer 2 in combination with a temperature measuring device such as a thermocouple (not shown), is located on the effluent side of the heat exchanger E-01. The temperature transducer 2 provides an output signal which is representative of the temperature of the liquefied natural gas stream withdrawn from the heat exchanger E-01. Control valves 12a-d, which are located in conduit lOa-d respectively, are manipulated in response to said output signal.

The flow rate of the refrigerant, components through conduits lOa-d are manipulated by the control valves 12a-d respectively. Flow controllers (not shown) would be provided, with a set point signal representative of the flow rate of refrigerant components through conduits lOa-d required to produce a desired refrigerant composition or other desired process variable. This set point signal would be compared to the actual flow rate of the refrigerant components flowing through conduit lOa-d and the flow controllers would provide an output signal required to maintain the actual flow rate of the refrigerant components substantially equal to the flow rate represented by the set point. The control valves 12a-d are thus manipulated in response to said output signal.

A pressure transducer 20 in combination with a pressure measuring device such as a pressure gauge ( not shown), is located on the discharge side of the turbine compressor KT-01. The pressure transducer 20 would be provided with a. target set point signal representative of the exiting discharge pressure of the turbine compressor KT-01. This target set point signal would be compared to J8

the actual discharge pressure of turbine compressor KT-01 and the pressure transducer PT02 would provide an output signal which is representative of the exiting discharge pressure of the turbine compressor KT-01. Control valves 12a-d, which are located in conduits lOa-d, are manipulated in response to said output signal.

In like manner, pressure transducer 4 in combination with a pressure measuring device such as a pressure gauge (not shown) , is located on the effluent side of the heat exchanger V-01. The pressure transducer 4 would be provided with a target set point signal representative of the pressure of the MCR withdrawn from heat exchanger V-01. This target set point signal would be compared to the actual pressure of the MCR from withdrawn from heat exchanger V-01 and the pressure transducer 4 would provide an output signal. Control valves 12a-d, which are located in conduits- 10a-d, are manipulated in response to said outpu t s igna1.

The concept of the disclosed method is to maintain specific temperature profile in the exchanger E-01 through proper control of the refrigerant compositions valves 12a-d opening once operational conditions are being met. The operational conditions are HP separator pressure PT01, HP separator level LT01 and discharge pressure turbine compressor PT02. Each refrigerant composition utilizes different temperature reference for its control. For control of nitrogen, the temperature reference used is the top outlet temperature TT01. For control of methane, the temperature reference used is the middle temperature TT02. For control of ethane, the temperature reference used is the bottom temperature TT03. For the control of propane, the temperature reference used is the outlet temperature TT04. Each of the parameters has its own setting value that is not to be violated when the process is running. For automation of the _. process, the valves 12a-d can be set under automatic control. The specific control of the individual components is described in more detail below with further references to Figs 2 to 5.

Referring further to Fig. 2, there is shown a schematic outlining the logic sequence in controlling nitrogen of the refrigerant composition. For the control of nitrogen of the refrigerant composition, comparison is first made with the current condition of top outlet temperature TT01 with its target setting of -152°C. If the existing TT01 is warmer than -152°C, a second step is made which is to compare the existing nitrogen composition with its target composition setting of 5.5mol%. Otherwise, if the existing TT01 is colder than -152°C, a signal will be sent to close the opening of valve 12a for nitrogen to decrease the ratio of the flow rate of nitrogen relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the second step, if the existing nitrogen is still below 5.5mol%, a third step is made which is to compare the existing discharge pressure turbine compressor PT02 with its target setting of 48.5barG. Otherwise, if nitrogen exceeds 5.5mol¾, a signal will be sent to close the opening of valve 12a for nitrogen to decrease the ratio of the flow rate of nitrogen relative _; to the total flow rate of the refrigerant flowing through the chiller to 0. At the third step, if the PT02 is below 48.5barG, a fourth step is made, which is to compare the existing HP separator pressure PT01 with its target setting of 46.5barG. Otherwise, if the PT02 is above 48.5barG, a signal is sent to close the opening of valve 12a for nitrogen to decrease the ratio of the flow rate of nitrogen relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the fourth step, if the delta difference of existing PT01 and its target is above 0.3 barG, a signal is automatically sent to Open valve 12a for nitrogen to increase the ratio of . the flow rate of nitrogen relative to the total flow rate of the refrigerant flowing through the chiller to 6:89. When the delta difference of existing PTOl and its target is below 0.3 barG and above 0.1 barG, a signal is automatically sent to reduce the opening of valve 12a for nitrogen to decrease the ratio of the flow rate of nitrogen relative to the total flow rate of the refrigerant flowing through the chiller to 3:89. When the PTOl reaches 46.5barG, a signal is sent to close the opening, of valve 12a for nitrogen to decrease the ratio of the flow rate of nitrogen relative, to the total flow rate of the refrigerant flowing through the chiller to 0. At any time when valve 12a for nitrogen is at the open position, flow of nitrogen is ensured to be allowed into the system, otherwise an alarm will appear to alert the operator. The system is also designed to hold the opening of valve 12a for nitrogen for 200 seconds before repeating at the checking steps discussed above.

Referring further to Fig. 3, there is shown a schematic outlining the logic sequence in controlling methane of the refrigerant composition. For methane control, comparison of the current condition of middle temperature TT02 with its target setting of -130°C is made. If the existing ΤΊ02 is warmer than -130°C, a second step is made, which is to compare the e isting methane with its target composition setting of 40mol%. Otherwise, if the existing T 01 is colder than -130°C, a signal is sent to close the opening of valve 12b for methane to increase the ratio of the flow rate of methane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the second step, if the existing methane is still below 40mol%, a third step is made which is to compare the existing discharge pressure turbine compressor PT02 with its target setting of 48.5barG. Otherwise, if methane exceeds 40 moll, a signal is sent to close the opening of valve 12b for methane to decrease the ratio of the flow rate of methane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the third step, if the PT02 is below of 48.5barG, a fourth . step is made, which is to compare the existing HP separator pressure PT01 with its target setting of 46.5barG. Otherwise if PT01 is above 46.5barG, a signal is sent to close the opening of valve 12b for methane to decrease the ratio of the flow rate of methane relative to the flow rate of the liquefied natural gas flowing through the chiller to 0. At the fourth step, if the delta difference of existing PT01 and its target is above 0.3 barG, a signal is automatically sent to open valve 12b for methane to increase the ratio of the flow rate of methane relative to the total flow rate of the refrigerant flowing through the chiller to 2:19. When the delta difference of existing PTOl and its target is below 0.3 barG and above 0.1 barG, the opening of valve 12b for methane will be reduced to decrease the ratio of . the flow rate of methane relative to the total flow rate of the refrigerant flowing through the chiller to 1:19. When the PT01 reaches 46.5barG, a signal is sent to close valve 12b for methane to decrease the ratio of the flow rate of methane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At any time when valve 12b for methane is at the open position, flow of methane is ensured to be allowed into the system, otherwise an alarm will appear to alert the operator. The system is also designed to hold the opening of valve 12b for methane for 200 seconds before repeating at the checking steps discussed above.

Referring further to Fig. 4, there is shown a schematic outlining the logic sequence in controlling ethane of the refrigerant composition. For the control of ethane of the refrigerant composition, comparison is first made with the current condition of bottom temperature TT03 with its target setting of -75°C. If the existing TT03 is warmer than -75°C, a second step is made which is to compare the existing HP separator level LT01 with its target setting of 48%. Otherwise, if the existing TT03 is colder than-75°C, a check is made on the existing plant capacity and LT01 simultaneously. If the plant capacity is above its minimum capacity and LT01 is below its minimum level, a signal will be sent to close the opening of valve 12c for Ethane to decrease the ratio of the flow rate of ethane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the second step, if the existing LT01 is below 48%, a third step is made which is to compare the ethane with, its target composition setting of 46mol%. Otherwise, if existing LT01 is above 48%, a signal will be sent to close the opening of valve 12c for ethane to decrease the ratio of the flow rate of ethane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the third step, if the delta difference of existing ethane and its target is above 2mol%, a signal is automatically sent to open valve 12c for Ethane to increase the ratio of the flow rate of ethane relative to the total flow rate of the refrigerant flowing through the chiller to 26:97. When the delta difference of existing ethane and its target is below 2mol% and above 0.1mol%, a signal is automatically sent to reduce the opening of valve 12c for ethane to decrease the ratio of the flow rate of ethane relative to the .flow rate of the liquefied natural gas flowing through the chiller to 13:97. When the composition of ethane reaches 46mol%, a signal is sent to close the opening of valve 12c for ethane decrease the ratio of the flow rate of ethane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At any time when valve 12c for ethane is at the open position, flow of ethane is ensured to be allowed into the system, otherwise an alarm will appear to alert the operator. The system is also designed to hold the opening of valve 12c for ethane for 200 seconds before repeating at the checking steps discussed above.

Referring further to Fig. 5, there is shown a schematic outlining the logic sequence in controlling propane of the refrigerant composition. For the control of propane of the refrigerant composition, comparison is first made with the current condition of outlet temperature TT04 with its target setting of -46°C. If the existing TT04 is warmer than -46°C, a second step is made which is to compare the existing HP separator level LT01 with its target setting of 48%. Otherwise, if the existing TT04 is colder than -46°C, a signal is sent to close the opening of valve 12d for propane to decrease the ratio of the flow rate of propane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the second step, if the existing L Ol is below 9mol%, a third step is made which is to compare the propane with its target composition setting of 9mol%. Otherwise, if existing LT01 is above 48%, a signal will be sent to close the opening of valve 12d for propane to decrease the ratio of the flow rate of propane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At the third step, if the delta difference of existing propane and its target is above 0.5mol%, a signal is automatically sent to open valve 12d for propane to increase the ratio of the flow rate of propane relative to the total flow rate of the refrigerant flowing through the chiller to 1:15. When the delta difference of existing propane and its target is below 0.5mol% and above 0.1moi%, a signal is automatically sent to reduce the opening of valve 12d for propane to decrease the ratio of the flow rate of propane relative to the total flow rate of the refrigerant flowing through the chiller to 1:30. When the composition of propane reaches 9mol%, a signal . is sent to close the opening of valve 12d for propane to decrease the ratio of the flow rate of propane relative to the total flow rate of the refrigerant flowing through the chiller to 0. At any time when valve 12d for propane is at the open position, flow of propane is ensured to be allowed into the system, otherwise an alarm will appear to alert the operator. The system is also designed to hold the opening of valve 12d for propane for 200 seconds before repeating at the checking steps discussed above .

Applications

The disclosed method of the present invention may be applied in numerous industrial applications, not least in the liquefaction of natural gas, hydrocarbons or industrial gases, and in the refrigeration of foodstuff in food processing.

By varying the refrigerant composition, the disclosed method of the present invention is capable of maintaining an optimum temperature profile of the chiller. Advantageously, the disclosed method allows for the monitoring of the temperature profile of the chiller together with the refrigerant composition. More advantageously, this method allows processing of natural gas with varying compositions and heat contents while producing a liquefied natural gas which meets the requirements as predetermined.

The disclosed method is useful for maintaining the refrigerant composition within specific ranges while taking into account the temperature profile in the chiller. Even more advantageously, the disclosed method minimizes the inventory loss of the refrigerant due to draining and makeup of the refrigerant at the same time.

It will be apparent that various other modifications and adaptations of the i2ivention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is · intended that all such modifications and adaptations come within the scope of the appended claims .