JPWO2017154181A1 - Method for determining mixed refrigerant composition of natural gas liquefier - Google Patents

Abstract

Provided is a method capable of determining a mixed refrigerant composition suitable for a new supply condition after the change even when at least one of the supply conditions of natural gas is changed. In the model creation process, when the natural gas supply conditions are determined and the liquefied natural gas cooled to a preset temperature is obtained from the natural gas liquefier, the operation data acquired from the natural gas liquefier is used. A simulation model is created based on this, and in the UA value calculation step, the simulation model is executed to calculate the UA value of the cryogenic heat exchanger. In the trial calculation process, the total consumption power is calculated by executing a simulation model for multiple mixed refrigerant cases with different refrigerant composition under the new supply conditions of natural gas, and in the composition determination process, the unit outflow of liquefied natural gas The mixed refrigerant composition of the mixed refrigerant case with the smallest total power consumption per unit amount is set as the mixed refrigerant composition under the new supply condition. [Selection] Figure 1

Description

  The present invention relates to a technology for liquefying natural gas using a mixed refrigerant in which a plurality of refrigerant raw materials are mixed.

  In a natural gas liquefaction device (hereinafter referred to as “NG liquefaction device”), the supplied natural gas (NG: Natural Gas) is cooled by a series of heat exchangers, and liquefied natural gas (LNG) is obtained. Is obtained. In major NG liquefaction equipment, mixed refrigerant (MR: Mixed Refrigerant) in which NG is pre-cooled using a pre-cooling refrigerant such as propane and then mixed with a plurality of refrigerant raw materials such as nitrogen, methane, ethane, and propane. Is used to liquefy and supercool precooled NG.

  The NG liquefaction equipment is optimally designed so that LNG can be produced efficiently after setting preconditions such as NG supply composition and supply pressure, and environmental factors (outside temperature and pressure) in the plant area of the NG liquefaction equipment. Is done. In particular, MR that mixes a plurality of refrigerant raw materials can realize a cooling curve that changes in temperature along the cooling curve of NG, and can realize an efficient liquefaction cycle with little loss.

  However, there are cases where the supply composition and supply pressure of NG deviate from the values set as the preconditions at the time of design due to changes in the natural gas produced from the gas wells over time or switching of the gas wells. The change from the value set as one of the preconditions leads to a decrease in the production amount of LNG and an increase in power consumed by the NG liquefaction device in order to maintain the target production amount. It may cause a decrease in operating efficiency (an increase in power consumption per unit output of LNG).

  Here, Patent Document 1 discloses a plurality of operation variables related to the operation of a main heat exchanger (corresponding to the “cryogenic heat exchanger” of the present application) that performs liquefaction of natural gas using known model predictive control. A predetermined set of control variables by manipulating the set of the mass (eg, the mass flow rate of the MR heavy refrigerant fraction and the light refrigerant fraction, the compensation flow rate of the refrigerant component composition (corresponding to the “mixed refrigerant” of the present application), etc.) Techniques for optimizing (for example, temperature difference between natural gas and vaporized MR on the warm end side of the main heat exchanger, temperature of liquefied natural gas, etc.) are described.

US Pat. No. 7,266,975

  However, the model predictive control described in Patent Document 1 creates a response model using an empirical response relationship of a specific control variable with respect to a change in a predetermined operation variable in each set. For example, the production amount of LNG is maximum. This is a technique for performing control using the model. For this reason, the response relationship between the manipulated variable and the control variable is restricted by the data obtained in the previous operation of the NG liquefaction apparatus.

  Usually, the operation range of the operation variable of the NG liquefier and the allowable variation range of the control variable are limited to a range in which the NG liquefier can execute efficient LNG production under the predetermined preconditions as described above. . Therefore, if the supply composition or supply pressure of NG, which is a precondition, has changed, the currently allowed fluctuation range itself is from the optimum state that can be realized by each device constituting the NG liquefaction apparatus. It may have shifted. For this reason, after the change of the supply composition and supply pressure of NG, there is a possibility that an operation state more efficient than the result of the model predictive control exists.

  The present invention has been made under such a background, and an object of the present invention is to provide a mixed refrigerant suitable for new supply conditions after the change even after at least one of the supply conditions of natural gas has changed. The object is to provide a method by which the composition can be determined.

The method for determining the mixed refrigerant composition of the natural gas liquefaction apparatus of the present invention is selected from a precooling heat exchanger for precooling natural gas with a precooling refrigerant and a refrigerant raw material group consisting of nitrogen and hydrocarbons having 1 to 3 carbon atoms Natural cryogenic heat exchanger for liquefying the precooled natural gas by a mixed refrigerant containing a plurality of refrigerant raw materials, and a plurality of compressors for compressing the precooling refrigerant and the mixed refrigerant gas A method for determining a mixed refrigerant composition of a gas liquefier,
When determining the supply composition and supply pressure of the natural gas and the composition of the mixed refrigerant and obtaining the liquefied natural gas cooled to a preset temperature, the heat transfer to the overall heat transfer coefficient of the cryogenic heat exchanger A simulation model capable of inputting operation information of the natural gas liquefier required for calculating the UA value, which is a value multiplied by the area, and the total power consumption of the plurality of compressors was acquired from the natural gas liquefier. A model creation process to be created based on operational data;
A UA value calculation step of calculating a UA value by executing a simulation model obtained as a result of the model creation using the mixed refrigerant composition at the time of the operation data acquisition and the supply composition and supply pressure of natural gas;
With respect to a plurality of mixed refrigerant cases in which the composition of the refrigerant raw material is changed from the mixed refrigerant composition at the time of the operation data acquisition under a new supply condition in which at least one of the supply composition and the supply pressure is changed. A trial calculation step of executing the simulation model adjusted so that the UA value of the heat exchanger is aligned with the UA value obtained by calculating the UA value, and calculating the total power consumption;
Among the execution results of the simulation model for each mixed refrigerant case obtained in the trial calculation step, the mixed refrigerant composition of the mixed refrigerant case in which the total consumed power per unit outflow of liquefied natural gas is minimized is supplied to the new supply. And a composition determining step for obtaining a mixed refrigerant composition under conditions.

The method for determining the mixed refrigerant composition of the natural gas liquefier may have the following characteristics.
(A) the mixed refrigerant includes four refrigerant raw materials;
The UA value of the cryogenic heat exchanger is adjusted so as to match the UA value obtained by calculating the UA value while changing the content of the first refrigerant raw material having the maximum vapor pressure contained in the mixed refrigerant. When the simulation model is executed, the temperature difference between the temperature of the liquefied natural gas at the top of the cryogenic heat exchanger and the temperature of the mixed refrigerant that cools the liquefied natural gas at the top of the tower is set in advance. A first provisional content determination step for obtaining a provisional content of the first refrigerant raw material that is equal to or less than the maximum temperature difference necessary for obtaining the liquefied natural gas cooled to the tempered temperature;
While changing the content of the second refrigerant raw material having the minimum vapor pressure contained in the mixed refrigerant, a simulation model in which the UA value obtained by the UA value calculation is adjusted is executed, and the cryogenic heat is The temperature difference between the temperature of the natural gas at the tower bottom of the exchanger and the temperature of the mixed refrigerant that cools the natural gas at the tower bottom is necessary for obtaining liquefied natural gas cooled to the preset temperature. A second provisional content determination step for obtaining a provisional content of the second refrigerant raw material that is not more than the maximum temperature difference,
The plurality of mixed refrigerant cases in the trial calculation step include the first and second refrigerant raw materials having the temporary content obtained in the first and second temporary content determination steps.
(B) In the plurality of mixed refrigerant cases in the trial calculation step, the contents of the first and second refrigerant raw materials are set in a range of ± 0.5 percentage points of the content ratio in the mixed refrigerant in each temporary content. And the contents of the remaining two refrigerant raw materials other than the first and second refrigerant raw materials are changed.

  The present invention calculates a UA value obtained by multiplying the overall heat transfer coefficient of a cryogenic heat exchanger by a heat transfer area using a simulation model of a natural gas liquefaction apparatus created based on actual operation data. The simulation model is executed under new supply conditions in which at least one of the supply composition and supply pressure of the natural gas is changed while adjusting the simulation model so that the UA value of the heat exchanger matches the calculated UA value. Based on the result of running the simulation model for a plurality of mixed refrigerant cases in which the composition of the refrigerant raw material was changed from the mixed refrigerant composition at the time of operating data acquisition, the total consumed power per unit outflow of liquefied natural gas was the highest. Since the mixed refrigerant composition of the mixed refrigerant case that becomes smaller is the mixed refrigerant composition under new supply conditions, a mixed refrigerant composition with less power consumption can be selected.

It is explanatory drawing which shows the structural example of NG liquefying apparatus. It is explanatory drawing which shows the cooling curve with respect to NG and a refrigerant | coolant in the said NG liquefying apparatus. It is explanatory drawing which shows the procedure which determines the composition of MR used for the liquefaction of NG in the said NG liquefaction apparatus.

First, an example of an NG liquefaction apparatus to which the MR composition determination method according to an embodiment of the present invention is applied will be described with reference to FIG.
As shown in FIG. 1, the NG liquefaction apparatus of this example includes precooling heat exchangers 101 to 104 that precool NG with a precooling refrigerant, a scrub column 2 that separates heavy components from NG, and precooled NG. A cryogenic heat exchanger (MCHE) 3 that liquefies, and compressors 41, 42, and 51 that compress precooled refrigerant and MR gas after heat exchange are provided.

  The NG supplied from the well source is supplied to the pre-cooling heat exchangers 101 to 104 after pre-processing for removing mercury, acid gas, and moisture contained in the NG in a pre-processing unit (not shown). The In the NG liquefaction apparatus of this example, a precooling refrigerant (hereinafter also referred to as “C3 refrigerant”) containing propane as a main component is used, and after pretreatment, for example, NG supplied at 40 to 50 ° C. is connected in series. For example, it is cooled to around −30 ° C. by four-stage precooling heat exchangers 101 to 104.

  On the upstream side of the line for supplying the C3 refrigerant to each of the precooling heat exchangers 101 to 104, an expansion valve (not shown) is provided, and the C3 refrigerant whose temperature is lowered by adiabatic expansion by the expansion valve is It supplies to the pre-cooling heat exchangers 101-104. As a result, in the precooling heat exchangers 101 to 104, the pressure level is adjusted so as to decrease sequentially from the upstream side (precooling heat exchanger 101) in the flow direction of NG to the downstream side (precooling heat exchanger 104). The NG cooling is performed using the C3 refrigerant (described as “HPC3, MPC3, LPC3, LLPC3” in FIG. 1).

  The scrub column 2 separates NG precooled in the precooling heat exchangers 101 to 104 into a gas at the top of the column containing a large amount of methane and a liquid at the bottom of the column containing a heavier hydrocarbon component than methane. Stay. The scrub column 2 of this example is provided with a reboiler 201 that heats the liquid extracted from the lower position of the scrub column 2 and returns the heated gas and liquid to the scrub column 2.

  The gas flowing out from the column top side of the scrub column 2 flows in the NG tube of the bottom bundle described later in the MCHE 3 and is cooled by MR having a relatively high temperature near the column bottom of the MCHE 3 to partially liquefy. . Thereafter, the NG gas-liquid mixed fluid extracted from the tube of the bottom bundle is supplied to the reflux drum 202 to be gas-liquid separated. The liquid after the gas-liquid separation is refluxed to the scrub column 2 by the reflux pump 203, while the gas is introduced into the NG tube of the MCHE3 middle bundle.

  Further, the liquid flowing out from the bottom side of the scrub column 2 is separated into liquid condensate and lighter gas than the condensate in a rectifying section 21 having a rectifying tower (not shown). The gas separated from the condensate is supplied to MCHE3.

  Here, the MCHE 3 of this example has a structure in which a number of tubes for NG and MR are arranged along the flow direction of the MR in a shell where the MR flows down from the tower top side toward the tower bottom side. It has become. NG and MR flow in each tube in the opposite direction to the MR flow in the shell from the tower bottom side to the tower top side of the shell.

Further, the above-mentioned multiple tubes for NG and MR are bundled to form a tube bundle. The tube bundle is divided into three regions: a top bundle arranged in the region on the tower top side of the shell, and a middle bundle and bottom bundle arranged in the region from the lower side of the top bundle to the bottom of the shell tower. Can do. Hereinafter, MCHE3 divided into three regions of top bundle / middle bundle / bottom bundle is referred to as three-bundle type MCHE3.
A part of the tube for NG is arranged so that a part of NG (NG flowing out from the top of the scrub column 2 as described above) flows out of the MCHE 3 after flowing through the bottom bundle. Further, a part of the MR tube is arranged so that a part of MR (gas MR separated by an MR separator 31 to be described later) flows through the middle bundle and the bottom bundle and then is extracted from the MCHE 3. ing. The remaining NG and MR tubes are arranged so that the NG and MR flow through the bottom bundle, middle bundle, and top bundle, and then are extracted from the top of the MCHE 3.

  In the MCHE 3, the gas after separation from the condensate supplied from the rectifying unit 21 is introduced into the NG tube of the bottom bundle and gradually cooled by the MR flowing on the shell side. Further, the gas extracted from the above-described reflux drum 202 joins the fluid. The flow of these gases (NG) flows into the middle bundle and the top bundle and is liquefied while being cooled, further subcooled, and cooled to about −150 to −155 ° C. from the top of the MCHE 3 as LNG. Extracted.

  The LNG that has flowed out of the MCHE 3 is recovered by the expander turbine 33 and then expanded by the expansion valve V5, and the end flash container 61 is flushed with nitrogen and some light end components, so that the boiling point of the LNG is approximately reduced. After adjusting to −161 ° C., it is run down to an LNG tank (not shown). The light end component flushed from the LNG in the end flush container 61 is used as a fuel gas in a factory where an NG liquefaction apparatus is installed, for example.

  Next, the MR flow (MR cycle) for liquefying and supercooling NG in the MCHE 3 will be described. The MR used for cooling the NG is extracted as a low-pressure MR (approximately -40 ° C., pressure 3.5 bara) from the bottom of the MCHE 3 shell in a gaseous state. After the droplets are separated by the suction drum 413, the low pressure MR is boosted from a low pressure to a medium pressure by the low pressure MR compressor 41, and further cooled by the after cooler 411. The medium pressure MR cooled by the after cooler 411 is increased in pressure from medium pressure to high pressure (pressure 50 to 55 bara) by the high pressure MR compressor 42 after the droplets are separated by the suction drum 423, and further the after cooler. Cooled by 421 (approx. Temperature + 30 ° C.).

  The MR compressors 41 and 42 are driven by driving units 412 and 422 such as a gas turbine using NG as fuel, a steam turbine driven by steam obtained by burning fuel gas, or an electric motor. Further, for example, the aftercoolers 411 and 421 are used to supply a tube bundle obtained by bundling a large number of tubes through which MR discharged from a corresponding one of the MR compressors 41 and 42 flows, and supply air to the tube bundle. An air-cooled heat exchanger provided with a fan or a water-cooled heat exchanger.

  The high-pressure MR is further cooled by the C3 refrigerant in the chillers 431 to 434 and supplied to the MR separator 31 as a gas-liquid mixed fluid to perform gas-liquid separation. Similarly to the pre-cooling heat exchangers 101 to 104, also in these chillers 431 to 434, the pressure level sequentially decreases from the upstream side (chiller 431) to the downstream side (chiller 434) in the flow direction of the high-pressure MR. Thus, cooling of the high-pressure MR is performed using the C3 refrigerant that has been expanded and lowered in temperature using the expansion valve (on the side of the chillers 431 to 434, for convenience of illustration, “HPC3, MPC3, LPC3, LLPC3” The description of the pressure level on the C3 refrigerant side is omitted).

  The gas MR (approximately -30 to -40 ° C) gas-liquid separated by the MR separator 31 is introduced into the MR tube from the tower bottom side of the MCHE 3 and then flows through the bottom bundle, the middle bundle and the top bundle. And cooled from the top of MCHE3 (approximately -150 to -155 ° C). The MR extracted from the MCHE 3 is expanded by the expansion valve V1, and then supplied to the shell side of the MCHE 3 through the nozzle 302 provided on the tower top side of the MCHE 3.

  On the other hand, after the liquid MR (approximately -30 to -40 ° C) that has been gas-liquid separated by the MR separator 31 is introduced from the tower bottom side of the MCHE 3 to the MR tube side, the bottom bundle and the middle bundle are separated. Flowed and cooled and withdrawn from MCHE3 (approximately -120 to -125 ° C). The liquid MR extracted from the middle bundle is expanded by the expansion valve V2 while recovering power through the expander turbine 32, and then the lower side of the nozzle 302 on the gas MR side (the top bundle). It is introduced into the shell side of MCHE 3 from the nozzle 301 arranged on the lower side.

  MR introduced to the shell side of the MCHE 3 through the nozzles 302 and 301 arranged in two upper and lower stages is liquefaction of NG flowing through the NG tube, supercooling, gas MR flowing through the MR tube, and liquid MR After being used for cooling, the low pressure MR is extracted from the bottom of the MCHE 3 and supplied to the low pressure MR compressor 41 again.

  In the MR cycle described above, from the line for extracting the gas MR from the MR separator 31 and supplying it to the MCHE 3 and the line for extracting the liquid MR from the MR separator 31 and supplying it to the MCHE 3 respectively, An extraction line for extracting the liquid MR is branched. The amount of MR supplied to the MCHE 3 can be adjusted by changing the opening degree of the valves V1 and V2. The MR component can be adjusted by changing the opening degree of the extraction valves V3 and V4.

Further, for example, nitrogen (N ₂ ), methane (C 1), ethane (C 2), and propane (C 3), which are MR refrigerant materials, are placed at the upstream position of the suction drum 413 provided in the low-pressure MR compressor 41. An MR material replenishment line that can be individually replenished is provided. Replenishment of each refrigerant material from these MR material replenishment lines can be adjusted by changing the opening degree of the replenishment amount adjustment valves V51 to V54.

  Next, the flow (C3 cycle) of the C3 refrigerant used for precooling NG and cooling the high pressure MR will be described. The C3 refrigerant gas after heat exchange with NG in the precooling heat exchangers 101 to 104 and heat exchange with the high pressure MR in the chillers 431 to 434 is separated into droplets in the suction drums 512 to 515, Depending on the pressure level of the C3 refrigerant, for example, it is supplied to the suction side of each stage of the C3 compressor 51 that performs four-stage compression.

For convenience of illustration, in the C3 cycle, the individual components of the pre-cooling heat exchangers 101 to 104, the chillers 431 to 434, and the expansion valves provided on the upstream side of these heat exchangers 101 to 104 and 431 to 434, respectively. The description is omitted, and is generally indicated as “C3 refrigerant heat exchange unit 50”.
Similarly to the MR compressors 41 and 42, the C3 compressor 51 is driven by a driving unit 511 such as a gas turbine using NG as a fuel, a steam turbine driven by steam obtained by burning fuel gas, or an electric motor. Driven.

  The C3 refrigerant compressed to a predetermined pressure by the C3 compressor 51 is reduced in temperature by the desuperheater 521 and the condenser 522, and the condensed C3 refrigerant is collected in the separator 53 and then the C3 refrigerant heat exchange unit 50. The precooling heat exchanger 101 and the expansion valve disposed on the upstream side of the chiller 431 are supplied again. Similar to the aftercoolers 411 and 421 on the MR compressors 41 and 42 side, the desuperheater 521 and the condenser 522 are configured by, for example, an air-cooled heat exchanger or a water-cooled heat exchanger.

  As described above, the configuration example of the NG liquefaction apparatus is shown using FIG. 1, but the configuration of the NG liquefaction apparatus to which the MR composition determination method according to the embodiment can be applied is not limited to this example. The present invention can be applied to various modifications that can be adopted in an actual NG liquefaction apparatus.

For example, the number of compression stages of the C3 compressor 51 may be three or five. In this case, the number of installation stages of the precooling heat exchangers 101 to 104 and the chillers 431 to 434 is also increased or decreased according to the number of compression stages of the C3 compressor 51. Further, a subcooler that performs supercooling of the C3 refrigerant may be provided between the separator 53 and the C3 refrigerant heat exchange unit 50.
The configuration of the MCHE 3 is not limited to the above-described three bundle type, and may be a two bundle type including a top bundle and a bottom bundle.

In the following description, the case where the MR composition determination method according to the embodiment is applied to the NG liquefaction apparatus shown in FIG. 1 will be described as an example representing various modifications of the NG liquefaction apparatus.
An NG liquefier that performs precooling of NG with a precooling refrigerant and liquefaction of NG with MR is designed so that NG is cooled along a cooling curve shown in FIG. The horizontal axis of FIG. 2 shows enthalpy changes of NG, C3 refrigerant, and MR, and the vertical axis shows the temperature of these fluids. In the figure, the solid line or the alternate long and short dash line indicates an NG cooling curve. A long broken line indicates a C3 refrigerant cooling curve (denoted as “pre-cooling cycle”), and a short broken line represents an MR cooling curve (denoted as “liquefaction cycle”).

For example, NG supplied to the inlet side of the pre-cooling heat exchanger 101 at a temperature of 40 ° C. is a multi-stage pre-cooling cycle using C3 refrigerant (for convenience of illustration, FIG. 2 shows a three-stage pre-cooling cycle). After precooling, NG is further liquefied and supercooled in a liquefaction cycle using MR in MCHE3.
In this liquefaction cycle, the composition of N ₂ , C 1, C ₂ , C 3 in MR (content ratio of each refrigerant raw material in MR) is determined based on the supply composition of NG supplied from the well source and the design value of supply pressure Has been.

However, the supply composition and supply pressure of NG supplied to the NG liquefaction apparatus may change due to changes in the state of production from the well source, switching of wells that produce NG, and the like.
For example, the alternate long and short dash line shown in FIG. 2 shows an example of a cooling curve of NG that is heavier than the NG of the cooling curve shown by the solid line. In this case, the temperature difference between the NG side and the MR side in the MCHE 3 becomes large, and the liquefaction efficiency of NG decreases. On the other hand, if NG is lightened and the temperature difference between MR and NG in MCHE3 becomes too small, there is a possibility that the processing capability of MCHE3 may be restricted.

In order to solve the above-described problem, in the present embodiment, the MR composition that is normally used in a fixed state is processed efficiently in response to changes in the supply composition and supply pressure of NG. To determine a new MR composition.
Hereinafter, an example of the MR composition determination method according to the embodiment will be described with reference to FIG.

  First, a simulation model of the NG liquefaction apparatus is created (model creation process: P1). The simulation model is executed in the NG liquefier for each device, such as heat exchange in the precooling heat exchangers 101 to 104 and MCHE3, NG fractionation in the scrub column 2, and compression of each refrigerant gas in the compressors 41, 42, and 51. The unit operation can be created using a known process simulator capable of expressing the unit operation.

  In the simulation model, operating conditions such as supply composition of NG, supply pressure, supply temperature, pressure and temperature of each fluid in the MCHE 3, flow rate of each refrigerant of C 3 and MR, pressure and temperature are set. These operating conditions are set based on actual operating data of the NG liquefier that determines a new MR composition. For example, in FIG. 1, a pressure gauge (PI), a thermometer (TI), a flow meter (FI), a composition analyzer (AI), and a compressor power meter (SC) from which operation data is acquired are surrounded by broken lines. It is shown by. For example, as the operation data, an average value of measured values acquired by these measuring devices during a predetermined period can be adopted.

From the created simulation model of the NG liquefier, calculate the UA value, which is a value obtained by multiplying the overall heat transfer coefficient of MCHE3 by the heat transfer area, and the power consumed by each compressor 41, 42, 51. Can do.
The UA value is UA value = q / LMTD, where q is the heat transfer amount per unit time from NG to MR in MCHE3, and LMTD is the logarithmic average temperature difference between NG temperature and MR temperature in MCHE3. It can be calculated from the relationship. The heat transfer amount q and the temperature difference LMTD are obtained from the result of executing the simulation model.

  Also, the work executed in each compressor 41, 42, 51 is calculated from the flow rate, temperature, inlet side and outlet side pressure of the MR and precooling refrigerant, and the efficiency (input) of each compressor 41, 42, 51 is calculated. The power consumption can be obtained from the ratio of work to power). And the total value of the power consumption of all the compressors 41, 42, 51 becomes total power consumption.

  Once the simulation model is created, the simulation model is executed using the MR composition at the time of the operation data acquisition, the supply composition and supply pressure of NG. As a result, if the calculated value is in good agreement with the operation data, it can be evaluated as a simulation model that appropriately represents the NG liquefaction apparatus to be studied.

  Then, based on the result of executing the simulation model, a UA value is calculated (UA value calculating step: P2). As described above, since the simulation model appropriately represents the state of the NG liquefaction apparatus at the time of operation data acquisition, the UA value calculated from the result of executing this simulation model is also the value of MCHE 3 at the time of operation data acquisition. It can be said that this is an index that appropriately represents the cooling capacity.

  After that, simulation is performed so that the UA value of MCHE3 is aligned with the calculated UA value under the new NG supply condition in which at least one of the supply composition and the supply pressure is changed from the supply condition set at the time of creating the simulation model. The MR composition is changed while adjusting the model. Note that “aligning the UA value of MCHE3 to the calculated UA value” is not limited to the case where these UA values are strictly aligned. Depending on the accuracy required for the simulation model, there may be a deviation within a range of, for example, about ± 1 to 2%.

  As described above, the UA value is represented by the ratio between the heat transfer amount per unit time in the MCHE 3 and the temperature difference between NG and MR. Therefore, when adjusting the UA value, parameters that affect these values are set. adjust. Examples of parameters include the LNG rundown amount, the opening degree of the expansion valves V1 and V2, and the like. Further, the opening degree of the extraction valves V3, V4 for extracting the gas MR and the liquid MR may be adjusted as necessary, and the replenishment amount adjustment valves V51, 52, 53, 54 in the MR component replenishment line may be adjusted. The opening degree may be adjusted.

In addition, here, even with the new MR composition, the provisional content of the refrigerant raw material with the maximum vapor pressure (N _{2 in} this example) so that the same efficient cooling as the cooling curve shown by the solid line in FIG. 2 is possible. (PAH: Preliminary Amount of the Highest Vapor Pressure Refrigerant Component) and Temporary Content (PAL: Preliminary Amount of the Lowest Vapor Pressure Refrigerant Component) (C3 in this example) 2 provisional content determination step: P3).

For PAH, under a new NG composition and supply pressure, a simulation model is executed while adjusting the content of N ₂ in MR while adjusting the UA value of MCHE3 to be equal to the calculated UA value. The temperature difference between the NG temperature at the top of the MCHE 3 and the MR temperature at the top of the MCHE 3 was cooled to a preset temperature (predetermined temperature in the range of −150 to −155 ° C. in this example). The flow rate of N ₂ when the temperature difference is equal to or less than the maximum temperature difference necessary for obtaining LNG is defined as PAH (first provisional content determination step). At this time, the content of the refrigerant raw material other than N ₂ is determined in the second provisional content determination step and the trial calculation step in the latter stage, so here the provisional values (for example, the current contents of C1, C2, and C3) ) Is set in advance.

Next, under a new NG composition and supply pressure, the PAL is adjusted so that the UA value of MCHE3 is aligned with the calculated UA value, and the simulation model is executed while adjusting the content of C3 in MR. The temperature difference between the NG temperature at the bottom of the MCHE 3 and the MR temperature for cooling the NG at the bottom is less than the maximum temperature difference necessary to obtain the LNG cooled to the preset temperature. Is set to PAL (second provisional content determination step). Herein, the amount of refrigerant materials other than C3, for N ₂ is determined in the first tentative content determination process described above, for the remaining two (C1, C2) determined by the subsequent estimation process Therefore, here, provisional values (for example, the current contents of C1, C2, and C3) are set.

Then, the UA value of MCHE3 is calculated for a plurality of MR cases in which the content of the remaining refrigerant raw materials (C1, C2) is changed under the new NG composition, supply pressure, and within the limits of PAH and PAL. The simulation model adjusted to match the adjusted UA value is executed. At this time, the contents of N ₂ and C3 in the MR are not limited to the case where the contents exactly match PAH and PAL, respectively. For example, in the range of 0.5 percentage point of the content ratio of each refrigerant raw material in PAH and PAL (if the content ratio of N ₂ in MR in PAH is 10 percent, the range is 9.5 to 10.5 percent). It may be changed.
Then, the total power consumption of the compressors 41, 42, 51 is obtained for a plurality of MR cases for which the simulation model has been executed (trial calculation step: P4).

Furthermore, PSP (Plant Specific Power, LNG unit outflow), which is a value obtained by dividing the total consumption power by the simulation value of the outflow amount of LNG from the NG liquefaction device, from the result of obtaining the total consumption power for the plurality of MR cases Calculate the total power consumption per unit). The MR composition in the MR case in which the value of the PSP is minimized is set as the MR composition suitable for the new NG composition and supply pressure (composition determination step: P5).
When the MR composition suitable for the new supply conditions is determined, the MR composition circulating in the actual NG liquefier is gradually determined in the composition determination step by adjusting the opening of the replenishment amount adjusting valves V51 to V54. Make adjustments to bring the values closer to the specified values.

  The method for determining the MR composition of the NG liquefaction apparatus according to the present embodiment has the following effects. Using the simulation model of the NG liquefaction device created based on actual operation data, calculate the UA value by multiplying the overall heat transfer coefficient of MCHE3 by the heat transfer area so that it matches the UA value at the time of operation data acquisition While adjusting the simulation model, the simulation model is executed under new supply conditions in which at least one of the supply composition and supply pressure of NG is changed. Then, based on the result of executing the simulation model for a plurality of MR cases in which the composition of the refrigerant raw material is changed from the MR composition at the time of operation data acquisition, the MR case in which the total power consumption per unit outflow of LNG is the smallest Therefore, the MR composition with less power consumption can be selected.

Here, in determining the MR composition under the new conditions of NG, after obtaining the PAH and PAL, the remaining refrigerant raw material (C1) under the conditions in which the contents of N ₂ and C3 in the MR are aligned with these PAH and PAL. It is not essential to adopt a method of changing the content of C2).
A PSP is calculated by executing a simulation model for a plurality of MR cases including the current MR composition, and if the MR case having the smallest PSP is an MR case other than the current MR composition, the operating efficiency of the NG liquefaction apparatus is increased. Can improve.

The MR _{is, N 2, C1, C2,} C3 need not include all of the refrigerant material included in the refrigerant material group consisting of. If a plurality of refrigerant raw materials selected from these refrigerant raw material groups are included, an MR case with the smallest PSP can be determined based on the result of executing a simulation model for a plurality of MR cases.

Hereinafter, the influence on the PSP by changing the MR composition when at least one of the supply conditions of NG changes will be described based on the embodiment of the present invention.
In this example, the MR composition is changed when the average molecular weight of NG supplied to the NG liquefier shown in FIG. 1 at a predetermined pressure is increased from 17.15 to 18.29 (heavy). The effect of PSP on the PSP was confirmed by a simulation model. MR uses N ₂ , C 1, C ₂ , and C 3 as refrigerant raw materials, and in the following tables, the average molecular weight is displayed as a general index instead of the content ratio of each refrigerant raw material. The process simulator used for creating the simulation model is UNISIM (registered trademark) of Honeywell.

(Reference example)
Reference examples 1 to 5 in Table 1 create a simulation model using the current operation data of the NG liquefaction apparatus, and for NG having an average molecular weight of 17.15 before increasing (heavyening) the average molecular weight of NG , PSP obtained by executing the simulation model for each MR case in which the average molecular weight of MR is gradually increased (heavy).
According to the results shown in Table 1, it can be confirmed that the PSP ratio is minimized in Reference Example 3, which is an MR case having an average molecular weight of 25.73. In MR case of Reference Example 3, the content of the respective refrigerant materials are, _{N 2} is 13 mol%, C1 is 40 mol%, C2 is 36 mol%, a C3 11 mol%.

(Example)
Average molecular weight of 25.87, is the content of the coolant material, _{N 2} is 13 mol%, C1 is 39 mol%, C2 is 37 mol%, C3 for MR cases is 11 mol%, an average molecular weight of NG The simulation model was executed under new supply conditions that were increased to 18.29 to calculate PSP, total power consumption, and LNG outflow. When the simulation model was executed, the UA value of MCHE3 was adjusted so as to match the UA value in Reference Examples 1-5.
(Comparative example)
In the MR case corresponding to Reference Example 3, a simulation model was executed under new supply conditions in which the average molecular weight of NG was increased to 18.29, and PSP, total power consumption, LNG efflux was calculated.
The results of Examples and Comparative Examples are shown in Table 2. Similar data for Reference Example 3 is also shown in Table 2.

According to the results shown in Table 2, the PSP was larger in the comparative example employing the MR case corresponding to the reference example 3 in which the PSP was the minimum before the NG became heavier. On the other hand, in the example in which MR was made heavy (corresponding to the MR case of Reference Example 5), the outflow amount of LNG increased, and as a result, the PSP became small.
According to the above results, when at least one of the supply conditions of NG changes, a UA value is obtained for a plurality of MR cases (each MR case of the example and the comparative example) using a simulation model created using operation data. It was confirmed that the MR suitable for the new supply condition can be determined by calculating and comparing the PSP by executing the simulation model while aligning.

101-104
Pre-cooling heat exchanger 3 MCHE
41 Low pressure MR compressor 42 Medium pressure MR compressor 412, 422
Drive unit 431-434
Chiller 50 C3 refrigerant heat exchange unit 51 C3 compressor 511 drive unit

Claims (3)

The precooling is performed by a precooling heat exchanger that precools natural gas using a precooling refrigerant, and a mixed refrigerant including a plurality of refrigerant raw materials selected from a refrigerant raw material group consisting of nitrogen and hydrocarbons having 1 to 3 carbon atoms. A method for determining a mixed refrigerant composition of a natural gas liquefaction apparatus comprising a cryogenic heat exchanger for liquefying natural gas and a plurality of compressors for compressing the precooling refrigerant and the mixed refrigerant gas,
When determining the supply composition and supply pressure of the natural gas and the composition of the mixed refrigerant and obtaining the liquefied natural gas cooled to a preset temperature, the heat transfer to the overall heat transfer coefficient of the cryogenic heat exchanger A simulation model capable of inputting operation information of the natural gas liquefier required for calculating the UA value, which is a value multiplied by the area, and the total power consumption of the plurality of compressors was acquired from the natural gas liquefier. A model creation process to be created based on operational data;
A UA value calculation step of calculating a UA value by executing a simulation model obtained as a result of the model creation using the mixed refrigerant composition at the time of the operation data acquisition and the supply composition and supply pressure of natural gas;
With respect to a plurality of mixed refrigerant cases in which the composition of the refrigerant raw material is changed from the mixed refrigerant composition at the time of the operation data acquisition under a new supply condition in which at least one of the supply composition and the supply pressure is changed. A trial calculation step of executing the simulation model adjusted so that the UA value of the heat exchanger is aligned with the UA value obtained by calculating the UA value, and calculating the total power consumption;
Among the execution results of the simulation model for each mixed refrigerant case obtained in the trial calculation step, the mixed refrigerant composition of the mixed refrigerant case in which the total consumed power per unit outflow of liquefied natural gas is minimized is supplied to the new supply. And a composition determination step for obtaining a mixed refrigerant composition under conditions. A method for determining a mixed refrigerant composition of a natural gas liquefaction apparatus. The mixed refrigerant includes four refrigerant raw materials,
The UA value of the cryogenic heat exchanger is adjusted so as to match the UA value obtained by calculating the UA value while changing the content of the first refrigerant raw material having the maximum vapor pressure contained in the mixed refrigerant. When the simulation model is executed, the temperature difference between the temperature of the liquefied natural gas at the top of the cryogenic heat exchanger and the temperature of the mixed refrigerant that cools the liquefied natural gas at the top of the tower is set in advance. A first provisional content determination step for obtaining a provisional content of the first refrigerant raw material that is equal to or less than the maximum temperature difference necessary for obtaining the liquefied natural gas cooled to the tempered temperature;
While changing the content of the second refrigerant raw material having the minimum vapor pressure contained in the mixed refrigerant, a simulation model in which the UA value obtained by the UA value calculation is adjusted is executed, and the cryogenic heat is The temperature difference between the temperature of the natural gas at the tower bottom of the exchanger and the temperature of the mixed refrigerant that cools the natural gas at the tower bottom is necessary for obtaining liquefied natural gas cooled to the preset temperature. A second provisional content determination step for obtaining a provisional content of the second refrigerant raw material that is not more than the maximum temperature difference,
The plurality of mixed refrigerant cases in the trial calculation step include first and second refrigerant raw materials having provisional contents obtained in the first and second provisional content determination steps. The method for determining the mixed refrigerant composition of the natural gas liquefying apparatus described. In the plurality of mixed refrigerant cases in the trial calculation step, the contents of the first and second refrigerant raw materials are set in a range of ± 0.5 percentage points of the content ratio in the mixed refrigerant in each temporary content, and 3. The method for determining a mixed refrigerant composition of a natural gas liquefying apparatus according to claim 2, wherein the contents of the remaining two refrigerant raw materials other than the first and second refrigerant raw materials are changed.

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