Cold energy control method, system and equipment for LNG (Liquefied Natural Gas) gasification process

Abstract

The invention relates to the technical field of LNG cold energy control, in particular to a cold energy control method, system and device for the LNG gasification process, and the method comprises the steps that the temperature and relative humidity of the environment where an air temperature type gasifier is located, the temperature and pressure of measuring points in all pipes and the temperature of measuring points outside all the pipes are obtained in real time; according to the temperature change characteristics of the measuring points outside the pipe at all adjacent moments in the current acquisition period, the ambient temperature and relative humidity at all moments in the current acquisition period and the pipe pass length of the single-phase liquid area at all moments in the current acquisition period, a frosting process evaluation value of the current acquisition period is obtained; and then cold energy control is carried out on the next collection period. According to the cold energy control method and device, by evaluating the frosting process of each collection period, the cold energy of the next collection period is controlled in a self-adaptive mode, and the sustainability and stability of cold energy control are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of LNG cold energy control, in particular to a cold energy control method, system and equipment for LNG gasification process. BACKGROUND

[0002] Liquefied natural gas (LNG) is stored at about -162℃ under normal pressure, and needs to be converted into normal temperature NG gas before use through LNG gasification station. A large amount of cold energy can be released in the gasification process. At present, some technologies have attempted to recycle LNG cold energy, such as for cold storage, power generation, air liquefaction or ice making. Among them, the market demand for ice making technology (especially for producing edible ice or industrial ice) is stable and huge.

[0003] LNG cold energy ice making needs to use a LNG and refrigerant heat exchanger for heat exchange. The gasification load of LNG is affected by the gas consumption of downstream users, and has dynamic change characteristics. When the LNG flow suddenly changes or the ice making load changes greatly, the refrigerant outlet temperature of the LNG-refrigerant heat exchanger fluctuates greatly. Too low temperature can cause ice blocks to be difficult to be smoothly deiced, and too high temperature can cause the ice making period to be prolonged or even overload operation of the air-cooled gasifier. In addition, if the fin of the tube side of the air-cooled gasifier is excessively frosted, the heat exchange efficiency of the fin will be reduced, thereby affecting the gasification effect of LNG, and even causing frosting shutdown, so that the stability and sustainability of cold energy ice making are low. SUMMARY

[0004] In order to solve the above technical problems, the purpose of the present application is to provide a cold energy control method, system and equipment for LNG gasification process, and the technical scheme adopted is as follows: In a first aspect, the embodiments of the present application provide a cold energy control method for LNG gasification process, which comprises the following steps: Real-time acquisition of the temperature and relative humidity of the environment in which the air-cooled gasifier is located, the temperature and pressure of each tube-in measurement point, and the temperature of each tube-out measurement point on the fin; Divide the entire data acquisition time into multiple acquisition periods, and obtain the frost thickness evaluation value at each time according to whether the temperature of each tube-out measurement point at each time meets the frosting condition; obtain the frost layer condensation rate of the current acquisition period according to the difference between the frost thickness evaluation values of all adjacent times in the current acquisition period; and obtain the frost promoting coefficient of the current acquisition period according to the proportion of data belonging to the preset temperature range and the preset humidity range among the environmental temperature and the environmental relative humidity at all times in the current acquisition period; acquire the critical pressure, the critical temperature and the mole fraction of each working medium in the LNG mixture, and acquire the bubble point temperature of each in-pipe measuring point at each time instant in combination with the pressure of each in-pipe measuring point at each time instant; acquire the heat exchange characteristic value of the current acquisition cycle according to the tube height corresponding to the in-pipe measuring point at each time instant in the current acquisition cycle, at which the measured temperature is equal to the corresponding bubble point temperature; acquire the frosting progress evaluation value of the current acquisition cycle according to the frost layer condensation rate, the frosting promotion coefficient and the heat exchange characteristic value of the current acquisition cycle, and then perform cold energy control on the next acquisition cycle.

[0005] Preferably, the method for acquiring the frosting thickness evaluation value at each time instant is as follows: acquire the air dew point temperature and the freezing point of water at each time instant; acquire the fitting curve corresponding to the temperature of all out-pipe measuring points distributed from the root to the tip of the fin at each time instant; sample the temperature of the obtained fitting curve according to a preset step length, and mark the corresponding sampling point as the fin frosting point at each time instant when the temperature of the sampling point is simultaneously lower than the air dew point temperature and the freezing point temperature of water; mark the fin height value corresponding to the fin frosting point with the largest fin height at each time instant as the frosting thickness evaluation value at each time instant.

[0006] Preferably, the frost layer condensation rate of the current acquisition cycle is determined by the average value of the differential values of the frosting thickness evaluation values at all time instants in the current acquisition cycle.

[0007] Preferably, the method for acquiring the frosting promotion coefficient of the current acquisition cycle is as follows: statistics the proportion of the environmental temperature belonging to a preset temperature range in the environmental temperature at all time instants in the current acquisition cycle, and the proportion of the environmental relative humidity belonging to a preset humidity range in the environmental relative humidity at all time instants in the current acquisition cycle; mark the average value of the obtained two proportions as the frosting promotion coefficient of the current acquisition cycle.

[0008] Preferably, the bubble point temperature of each in-pipe measuring point at each time instant satisfies the following relationship: ; wherein, ; in the formula, is the mole fraction of the xth working medium; is the phase equilibrium constant of the xth working medium; is the total number of working media in the LNG mixture; is the critical pressure of the xth working medium; is the pressure of the jth in-pipe measuring point at the ith time instant; is the critical temperature of the xth working medium; is the bubble point temperature of the jth in-pipe measuring point at the ith time instant; is an exponential function with the natural constant e as the base; by solving the above equation, the bubble point temperature of each measuring point in each tube at each time is obtained.

[0009] Preferably, the method for obtaining the heat exchange characteristic value of the current acquisition cycle is: ; in the formula, is the heat exchange characteristic value of the current acquisition cycle, is the average value of the single-phase liquid zone length at all times in the current acquisition cycle, is the total tube side value of the intermediate tube bundle of the air-cooled vaporizer; The method for obtaining the single-phase liquid zone length at each time is: The distance from each temperature measuring point in the tube to the pipeline at the LNG inlet of the intermediate tube bundle is taken as the tube side value of each temperature measuring point in the tube; Taking the tube side value as the horizontal coordinate, first taking the temperature of the temperature measuring point in the tube corresponding to each tube side value at each time as the vertical coordinate, the in-tube temperature fitting curve at each time is obtained; Then taking the bubble point temperature of the temperature measuring point in the tube corresponding to each tube side value at each time as the vertical coordinate, the bubble point temperature fitting curve at each time is obtained; The tube side value corresponding to the intersection point of the in-tube temperature fitting curve and the bubble point temperature fitting curve at each time is taken as the single-phase liquid zone length at each time.

[0010] Preferably, the method for obtaining the frosting progress evaluation value of the current acquisition cycle is: The frost layer condensation rate, the frosting promotion coefficient and the heat exchange characteristic value of the current acquisition cycle and its historical acquisition cycles are normalized respectively; The frost layer condensation rate, the frosting promotion coefficient and the heat exchange characteristic value of each acquisition cycle after normalization form the frosting index vector of each acquisition cycle; All the frosting index vectors of the current acquisition cycle and its historical acquisition cycles are taken as the input of the Topsis superior-inferior solution distance method, wherein the frost layer condensation rate and the frosting promotion coefficient are taken as the maximum type index, and the heat exchange characteristic value is taken as the minimum type index, and the output is the comprehensive score of the current acquisition cycle, which is recorded as the frosting progress evaluation value of the current acquisition cycle.

[0011] Preferably, the specific process of the cold energy control on the next acquisition cycle is: If the frosting progress evaluation value of the current acquisition cycle is less than a preset first threshold value, the PLC controller is used to adjust the opening size of the flow regulating valve until the refrigerant outlet temperature is equal to the preset standard refrigerant outlet temperature; If the frosting progress evaluation value of the current acquisition cycle is greater than or equal to the preset first threshold value and less than a preset second threshold value, the opening of the flow regulating valve is reduced by a preset proportion at the beginning of the next acquisition cycle; If the frosting progress evaluation value of the current collection cycle is greater than or equal to the preset second threshold value, the opening of the flow regulating valve is adjusted to 0 at the start of the next collection cycle, and the defrosting mechanism of the air-temperature vaporizer is started to defrost.

[0012] In a second aspect, the embodiments of the present application provide a cold energy control device for an LNG vaporization process, which comprises a data collection module, a frosting progress analysis module, and a cold energy regulation module.

[0013] The data collection module is configured to acquire, in real time, the temperature and relative humidity of the environment in which the air-temperature vaporizer is located, the temperature and pressure of each in-pipe measurement point, and the temperature of each out-pipe measurement point on the fins. The frosting progress analysis module is configured to evaluate the frosting progress of the current collection cycle according to the temperature variation characteristics of the out-pipe measurement points, the magnitude of the environmental temperature and humidity, and the length characteristics of the single-phase liquid region. The cold energy regulation module is configured to control the cold energy of the next collection cycle according to the frosting progress evaluation result of the current collection cycle.

[0014] In a third aspect, the embodiments of the present application further provide a cold energy control system for an LNG vaporization process, which comprises a memory, a processor, and a computer program stored in the memory and running on the processor, and the processor implements the steps of the cold energy control method for the LNG vaporization process according to any one of the above embodiments when executing the computer program.

[0015] As can be seen from the above embodiments, the cold energy control method, system and device for the LNG vaporization process provided by the embodiments of the present application have at least the following beneficial effects: By analyzing the temperature variation characteristics of the out-pipe measurement points over time, the present application can construct the frost layer condensation rate to evaluate the frosting rate on the fins of the tube; by analyzing the magnitude of the environmental temperature and humidity, the present application can construct the frosting promotion coefficient to evaluate the influence degree of environmental factors on the frosting progress; by analyzing the tube length at which the temperature of the in-pipe measurement point reaches the bubble point temperature, the present application can construct the heat exchange characteristic value to evaluate the heat exchange efficiency and vaporization effect of the fins; by combining the above three indexes, the present application can construct the frosting progress evaluation value to dynamically evaluate the frosting progress of the air-temperature vaporizer of each collection cycle, thereby controlling and regulating the cold energy, avoiding excessive accumulation of the frost layer and frosting shutdown, prolonging the service life of the equipment, balancing the frosting risk and ice-making demand, and improving the sustainability and stability of the LNG cold energy control. BRIEF DESCRIPTION OF DRAWINGS

[0016] In order to more clearly illustrate the technical solutions and advantages of the embodiments of the present application or the prior art, the accompanying drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the accompanying drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor.

[0017] Figure 1 The step flow chart of the cold energy control method for the LNG gasification process provided by an embodiment of the present application is shown in the figure. Figure 2 The distribution diagram of the out-tube temperature measuring points in the intermediate tube bundle of the air-cooled gasifier is shown in the figure, wherein a is the fin of the intermediate tube bundle, b is the out-tube temperature measuring point on the fin, and c is the cross section of the intermediate tube bundle. Figure 3 The structural diagram of the cold energy control equipment for the LNG gasification process provided by an embodiment of the present application is shown in the figure. DETAILED DESCRIPTION

[0018] In order to further illustrate the technical means and effects adopted by the present application to achieve the predetermined purposes, the specific embodiments, structures, features and effects of the cold energy control method, system and equipment for the LNG gasification process according to the present application are described in detail as follows by combining with the accompanying drawings and preferred embodiments. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. In addition, the specific features, structures or characteristics in one or more embodiments can be combined in any suitable form.

[0019] Unless otherwise defined and limited, such as the terms "comprise", "include" or any other variants thereof, are intended to cover non-exclusive inclusion, so that the circuit structure, article or equipment including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such article or equipment. Without more limitation, the element limited by the statement "including one" does not exclude the presence of another identical element in the article or equipment including the element. In addition, the term "and / or" used herein includes any and all combinations of one or more related listed items. All technical and scientific terms used herein have the same meaning as understood by those skilled in the art of the technology to which the present application belongs.

[0020] The specific solutions of the cold energy control method, system and equipment for the LNG gasification process provided by the present application are described in detail below by combining with the accompanying drawings.

[0021] Please refer to Figure 1which shows a step flow chart of the cold energy control method for the LNG gasification process provided by one embodiment of the present application, the method comprising the following steps: Step 1: Real-time acquisition of the temperature and relative humidity of the environment in which the air-cooled vaporizer is located, the temperature and pressure of each tube-in measuring point, and the temperature of each tube-out measuring point on the fins.

[0022] In the LNG cold and warm ice-making process, LNG from a low-temperature storage tank with a temperature of about -150℃ to -160℃ and a pressure of 0.6 MPa is divided into two paths, one of which enters the air-cooled vaporizer 1#, completes the phase change and is warmed to about 5℃ to 15℃, and is then sent to the downstream; the other path flows into the LNG-refrigerant heat exchanger through the flow regulating valve, exchanges heat with the refrigerant, and the low-temperature natural gas after heat absorption flows out of the heat exchanger, and is further warmed to 5℃ to 15℃ by the air-cooled vaporizer 2# and is sent to the downstream pipe network. The gaseous refrigerant with an initial temperature of -18℃ to +20℃ and a pressure of 0.2 MPa to 1.2 MPa is condensed into a liquid state after exchanging heat with the LNG in the LNG-refrigerant heat exchanger, and the temperature drops to -25℃ to +5℃. The liquid refrigerant then flows into the refrigerant buffer tank and the low-pressure circulating barrel in turn, and then enters the direct-cooled evaporator to participate in ice-making, and the refrigerant after heat absorption returns to the upstream heat exchanger from the evaporator, realizing the closed circulation of the refrigerant. The refrigerant specifically uses R507 refrigerant.

[0023] A sheathed embedded PT100 platinum resistance is used in the outlet pipeline of the LNG-refrigerant heat exchanger to real-time acquire the outlet temperature of the LNG-refrigerant heat exchanger.

[0024] Since the air-cooled vaporizer 1# is used for emergency backup and basically does not participate in cold energy utilization, the air-cooled vaporizer 2# is taken as the cold energy control object in this embodiment for cold energy control. The air-cooled vaporizer 2# is hereinafter referred to as the air-cooled vaporizer. The air-cooled vaporizer gradually forms frost from the LNG inlet at the bottom end, and slowly extends to the upper part of the middle tube bundle, and the middle tube bundle has low heat exchange efficiency and is more prone to frost formation. Since the middle tube bundle of the air-cooled vaporizer is more prone to frost formation, the middle tube bundle of the air-cooled vaporizer is evenly divided into N tube segments according to the tube length, and a tube-in measuring point is arranged at each tube segment boundary, and a sheathed embedded PT100 platinum resistance and a diffusion silicon pressure sensor are installed for each tube-in measuring point to real-time collect the temperature and pressure parameters in the tube segment, wherein the temperature and pressure parameters of the 1st tube-in measuring point are the inlet temperature and inlet pressure values of the LNG air-cooled vaporizer, and the temperature and pressure parameters of the N+1th tube-in measuring point are the outlet temperature and outlet pressure values of the LNG air-cooled vaporizer, and N is 10 in this embodiment.

[0025] A temperature and humidity recorder is used to real-time collect the temperature and relative humidity of the environment in which the LNG air-cooled vaporizer is located.

[0026] In addition, the air-temperature vaporizer of the multi-channel finned tube bundle has a set of heat exchange fins extending in a radial manner on the outer wall of the tube body in the height direction of the tube body and around the circumferential direction of the tube body, and an arbitrary heat exchange fin of the middle tube bundle in the air-temperature vaporizer inlet side is provided with a tube-outer temperature measuring point every preset distance along the fin height direction, Figure 2 The figure shows the distribution of the tube-outer temperature measuring points in the middle tube bundle of the air-temperature vaporizer. In this embodiment, the tube-outer temperature measuring points are arranged at a height of 1 / 4 of the middle tube bundle. The thin-film PT100 thermistor is fixed on each tube-outer temperature measuring point by using heat-conducting glue, and the temperature of each tube-outer temperature measuring point distributed along the fin height direction is obtained in real time.

[0027] The various types of data are collected in real time and synchronously, and the collection time interval is set to 30 seconds. Each preset time length is taken as a collection period, and the preset time length in this embodiment is 15 minutes.

[0028] Step two: divide the entire data collection time into multiple collection periods, obtain the frost thickness evaluation value at each time according to whether the temperature of each tube-outer measuring point at each time meets the frosting condition, obtain the frost layer condensation rate of the current collection period according to the difference between the frost thickness evaluation values of all adjacent times in the current collection period, and obtain the frost promoting coefficient of the current collection period according to the proportion of the data belonging to the preset temperature range and the preset humidity range among the environmental temperature and the environmental relative humidity at all times in the current collection period.

[0029] The air-temperature vaporizer (AAV) mainly adopts an air-temperature vaporizer mainly using natural convection or forced convection of ambient air, which has the characteristics of no additional energy consumption and convenient operation. In the LNG vaporization unit, two air-temperature vaporizers work together to support the natural gas supply demand of the downstream user. The air-temperature vaporizer is prone to frosting after a period of continuous operation, and the frost layer thermal resistance gradually increases, which seriously reduces the vaporization efficiency of the air-temperature vaporizer, and even requires shutdown for defrosting operation, thereby causing the shutdown of the LNG cold energy ice-making system and the interruption of the downstream natural gas supply.

[0030] The frosting of the air-temperature vaporizer is mainly caused by the phase change of water vapor in the air into solid crystals when it is cooled. The geometric structure and accumulation law change with the frosting process, and the frosting thickness is mainly related to the tube-outer wall temperature, the environmental temperature and the environmental relative humidity. The temperature of the air-temperature vaporizer gradually increases along the fin height direction, and the fins gradually frost in the order of root, middle and tip. When the surface temperature of the fin is lower than the dew point temperature of the air and the freezing point temperature of water at the same time, frosting occurs on the surface of the fin of the air-temperature vaporizer.

[0031] Taking the i-th moment as an example, according to the ambient temperature and relative humidity of the environment at the i-th moment, the air dew point temperature value under the current temperature and humidity can be obtained by querying the air dew point temperature table, and the freezing point temperature of water is 0℃ in this embodiment. The temperatures of all the out-of-tube temperature measuring points distributed from the fin root to the fin tip at the i-th moment are taken as the input of the least square linear fitting algorithm to obtain the fitting curve corresponding to the temperatures of all the out-of-tube temperature measuring points distributed from the fin root to the fin tip at the i-th moment. The obtained fitting curve is temperature-sampled according to a preset step length, and the preset step length is 0.1 mm in this embodiment. When the temperature of the sampling point is lower than the air dew point temperature value and the freezing point temperature of water at the same time, the corresponding sampling point is recorded as the frost point of the fin at the i-th moment. The fin height value corresponding to the frost point of the fin with the maximum fin height is recorded as the frost thickness evaluation value at the i-th moment.

[0032] The sequence composed of the frost thickness evaluation values of all the moments in the current collection period arranged in time sequence order is recorded as the frost thickness sequence of the current collection period, and the mean value of all the data in the first-order difference sequence of the frost thickness sequence is calculated as the frost layer condensation rate of the current collection period. If the frost layer condensation rate is greater than 0, it indicates that the frost layer thickness of the air-cooled vaporizer in the current collection period is in an overall upward trend, that is, the frost layer of the fin is gradually thickening. If the frost layer condensation rate is less than 0, it indicates that the frost layer thickness of the air-cooled vaporizer in the current collection period is in an overall downward trend, that is, the frost layer of the fin is gradually melting and thinning. If the frost layer condensation rate is equal to 0, it indicates that the frost layer thickness of the air-cooled vaporizer in the current collection period remains unchanged.

[0033] The ambient temperature and the ambient relative humidity are also important factors affecting the frosting process of the air-cooled vaporizer, and an absolutely low-temperature environment and a high-humidity environment are not completely conducive to the formation of the frosting process. The application will promote the suitable range of the ambient temperature and the suitable range of the ambient humidity for the frosting of the air-cooled vaporizer, which are recorded as the preset temperature range and the preset humidity range. If the air-cooled vaporizer is in the preset temperature range and the preset humidity range for a long time, the frost layer thermal resistance will increase, thereby reducing the heat exchange efficiency of the air-cooled vaporizer and affecting the vaporization effect. In the case of serious frosting, frequent shutdown and defrosting are required.

[0034] The preset temperature range is [-6℃, 0℃] and the preset humidity range is [20%, 30%] in this embodiment. When the ambient temperature is lower than the preset temperature range, the mass transfer rate of the air-cooled vaporizer is small, the frosting is slow, and the frost layer thermal resistance is small. When the ambient temperature is higher than the preset temperature range, the high temperature of the environment will promote the melting of the frost layer, thereby reducing the rate of increasing the frost layer thickness, and the frost layer thermal resistance will also become smaller. When the ambient relative humidity is lower than the preset humidity range, the air is relatively dry, which is not conducive to the formation of the frost layer. When the ambient relative humidity is higher than the preset humidity range, the moisture in the air will promote the melting of the frost layer, thereby reducing the frost layer thermal resistance.

[0035] The proportion of the environment temperature belonging to the preset temperature range in the environment temperature at all times in the current collection period and the proportion of the environment relative humidity belonging to the preset humidity range in the environment relative humidity at all times in the current collection period are counted, and the average of the two proportions is recorded as the frost-promoting coefficient of the current collection period, which is used to reflect the promoting degree of the environment temperature and humidity in the current collection period to the fin frosting process.

[0036] Step three: the critical pressure, critical temperature and mole fraction of each working medium in the LNG mixture are obtained, and the bubble point temperature of each in-pipe measuring point at each time is obtained by combining the pressure of each in-pipe measuring point at each time; the heat exchange characteristic value of the current collection period is obtained according to the in-pipe measuring point height corresponding to the in-pipe measuring point at each time in the current collection period, at which the measured temperature is equal to the corresponding bubble point temperature.

[0037] The LNG fluid entering the LNG air-cooled vaporizer is a mixture mainly composed of methane. The boiling points of the components in the LNG mixture are different, and the components in the LNG mixture vaporize in a certain order after absorbing heat, which leads to the continuous change of the gas-liquid composition in the LNG vaporization process, and further causes the air-cooled vaporizer tube bundle to have a single-phase liquid zone, a two-phase zone and a single-phase gas zone along the tube length in turn, and the bubble point marks the transition from the single-phase liquid zone to the two-phase zone.

[0038] Meanwhile, since the proportion of methane, ethane and propane in the LNG mixed fluid is extremely high, the LNG fluid is regarded as a mixture containing methane, ethane and propane in the application, and the mole fractions of the three working media are measured. In the embodiment, the mole fractions of methane, ethane and propane in the LNG fluid are 94.7%, 0.55% and 0.08% respectively. The mole fractions of methane, ethane and propane are normalized, and the normalized mole fractions are taken as the mole fractions of the working media, denoted as wherein x=1 is methane, x=2 is ethane, and x=3 is propane. As known in the art, the critical pressures of methane, ethane and propane are 4.49 MPa, 4.73 MPa and 4.26 MPa respectively, and the critical temperatures are -82.6℃, 32.2℃ and 95.7℃ respectively.

[0039] As known in the art, the bubble point temperatures of the in-pipe measuring points of the LNG air-cooled vaporizer at each time satisfy the following relationship: ; wherein, ; in the formula, is the mole fraction of the xth working medium; is the phase equilibrium constant of the xth working medium; is the total number of working media in the LNG mixture; is the critical pressure (unit: MPa) of the xth working medium; is the pressure (unit: MPa) of the jth in-pipe measuring point at the ith time. is the critical temperature (unit: K) of the xth working medium; is the bubble point temperature (unit: K) of the jth tube at the i th time; is an exponential function with the natural constant e as the base.

[0040] Through the correlation of the above two formulas, the bubble point temperature of the LNG air-cooled vaporizer at any time and any tube measurement point can be obtained by solving the equation.

[0041] The pipe distance from each tube temperature measurement point to the LNG inlet of the middle tube bundle is calculated as the tube length value of each tube temperature measurement point.

[0042] The present application constructs a first coordinate system with the tube length value as the horizontal coordinate and the temperature of the tube temperature measurement point corresponding to each tube length value at each time as the vertical coordinate, and obtains the tube temperature fitting curve at each time by linear fitting using the least squares method; wherein each time corresponds to a tube temperature fitting curve, and the tube temperature fitting curve shows an increasing trend.

[0043] The present application constructs a second coordinate system with the tube length value as the horizontal coordinate and the bubble point temperature of the tube temperature measurement point corresponding to each tube length value at each time as the vertical coordinate, and obtains the bubble point temperature fitting curve at each time by linear fitting using the least squares method, wherein the pressure of the LNG fluid in the air-cooled vaporizer decreases along the tube length, so the bubble point temperature fitting curve shows a decreasing trend. It should be noted that since the temperature value measured by the temperature and humidity sensor is in ℃, the unit of the bubble point temperature needs to be converted from K to ℃.

[0044] Since the bubble point marks the transition of the single-phase liquid zone of the LNG fluid to the two-phase zone, the intersection of the tube temperature fitting curve and the bubble point temperature fitting curve at each time is obtained, and the tube length value corresponding to the intersection is taken as the single-phase liquid zone length at each time.

[0045] Further, according to the single-phase liquid zone length at each time in the current collection period, the heat exchange characteristic value of the current collection period is obtained, which is used to represent the heat exchange efficiency of the air-cooled vaporizer in the current collection period.

[0046] In this embodiment, the heat exchange characteristic value of the current collection period is denoted as , and its specific expression is: ; in the formula, is the heat exchange characteristic value of the current collection period, is the average of the single-phase liquid zone lengths at all times in the current collection period, is the total tube length of the middle tube bundle of the air-cooled vaporizer. A large amount of heat (i.e. latent heat of vaporization) is required for the LNG to change from liquid to gas, and the shorter the single-phase liquid zone length, the greater the heat exchange characteristic value, indicating that the air-cooled vaporizer has better heat exchange efficiency and lower fin frosting progress.

[0047] Solid ice crystals gradually accumulate on the surface of the air-cooled vaporizer fins over time, developing into a dense frost crystal structure, and the frost thickness also increases. Rapid growth of the frost layer affects the gas circulation in the outer space of the vaporizer, hinders heat transfer, and if the environmental temperature and humidity are in suitable conditions, it is easy to promote fin frosting, which will reduce the heat exchange efficiency and vaporization effect, and prolong the length of the single-phase liquid zone in the air-cooled vaporizer tube.

[0048] Step four: According to the frost layer condensation rate, frost promotion coefficient and heat exchange characteristic value of the current collection period, the frost progress evaluation value of the current collection period is obtained, and then the cold energy control of the next collection period is carried out.

[0049] Further, according to the above method, the frost layer condensation rate, frost promotion coefficient and heat exchange characteristic value of the air-cooled vaporizer in the historical M collection periods are obtained. In this embodiment, M is 10. The frost layer condensation rate, frost promotion coefficient and heat exchange characteristic value of the air-cooled vaporizer in the current collection period and its historical collection periods are Max-Min normalized respectively; the normalized frost layer condensation rate, frost promotion coefficient and heat exchange characteristic value of each collection period form the frost index vector of each collection period.

[0050] All the frost index vectors of the current collection period and its historical collection periods are taken as the input of the Topsis superior-inferior solution distance method, in which the frost layer condensation rate and the frost promotion coefficient are taken as the maximum type index, and the heat exchange characteristic value is taken as the minimum type index. The output is the comprehensive score of the current collection period, which is recorded as the frost progress evaluation value of the current collection period, and is used to evaluate the fin frosting progress of the air-cooled vaporizer. The larger the value is, the higher the fin frosting progress of the current collection period is. The Topsis superior-inferior solution distance method is a known technology, and the specific process will not be described here.

[0051] As the fin frosting process of the air-cooled vaporizer advances, the frost layer thickness on the fin surface increases, the heat resistance becomes larger and larger, gradually reducing the vaporization efficiency of the air-cooled vaporizer. If the frosting process is high, it may even cause the pipe bundle of the air-cooled vaporizer to break, at which time it is necessary to stop in time. In the process of LNG cold energy ice making, in order to fully utilize the LNG cold energy, the LNG coming out of the low-temperature storage tank is mostly distributed to the pipeline where the LNG-refrigerant heat exchanger is located. Therefore, it is necessary to control the valve opening of the flow regulating valve to avoid frosting shutdown of the air-cooled vaporizer.

[0052] Specifically, a preset first threshold and a preset second threshold are set, and the cold energy control of the next collection period of the air-cooled vaporizer is carried out according to the frost progress evaluation value of the air-cooled vaporizer in the current collection period. In this embodiment, the preset first threshold is 0.5, and the preset second threshold is 0.8.

[0053] If the frosting process evaluation value of the air-cooled vaporizer in the current collection cycle is less than the preset first threshold value, it indicates that the fin frost layer thermal resistance in the current collection cycle is small, the influence degree on the heat exchange efficiency and vaporization effect of the air-cooled vaporizer is low, and the stability of the refrigerant circulation process should be paid more attention to. The standard refrigerant outlet temperature is set to -20 DEG C. At this time, the opening size of the flow regulating valve is adjusted by using the PLC controller until the refrigerant outlet temperature of the next collection cycle is equal to the preset standard refrigerant outlet temperature, so as to prevent the refrigerant outlet temperature curve of the LNG-refrigerant heat exchanger from fluctuating greatly when the LNG flow suddenly changes or the ice making load changes greatly.

[0054] If the frosting process evaluation value of the air-cooled vaporizer in the current collection cycle is greater than or equal to the preset first threshold value and less than the preset second threshold value, it indicates that the fin frost layer thermal resistance in the current collection cycle is high, the influence degree on the heat exchange efficiency and vaporization effect of the air-cooled vaporizer is high, and the opening of the flow regulating valve is reduced by the preset proportion at the beginning of the next collection cycle. In the embodiment, the preset proportion is 5%, so as to prevent the air-cooled vaporizer from frosting and stopping.

[0055] If the frosting process evaluation value of the air-cooled vaporizer in the current collection cycle is greater than or equal to the preset second threshold value, it indicates that the fin frost layer thermal resistance in the current collection cycle is very high, the influence degree on the heat exchange efficiency and vaporization effect of the air-cooled vaporizer is extremely high, and the opening of the flow regulating valve is adjusted to 0 at the beginning of the next collection cycle, and the defrosting mechanism of the air-cooled vaporizer is started to defrost.

[0056] Please refer to Figure 3 , Figure 3 is a structure diagram of the cold energy control device provided by the embodiment of the application. In the embodiment, each unit included in the terminal is used to execute each step in the embodiment corresponding to the cold energy control method in the LNG vaporization process. Please refer to Figure 3 , the cold energy control device comprises a data collection module, a frosting process analysis module and a cold energy adjusting module.

[0057] The data collection module is used to acquire the temperature and relative humidity of the environment where the air-cooled vaporizer is located, the temperature and pressure of each in-pipe measuring point, and the temperature of each out-pipe measuring point on the fin in real time. The frosting process analysis module is used to evaluate the frosting process in the current collection cycle according to the temperature variation characteristics of the out-pipe measuring point, the size of the environment temperature and humidity and the length characteristics of the single-phase liquid region. The cold energy adjusting module is used to control the cold energy in the next collection cycle according to the frosting process evaluation result in the current collection cycle.

[0058] Based on the same inventive concept as the above method, the embodiments of the present application also provide a cold energy control system for an LNG gasification process, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, and the processor implements the steps of the cold energy control method for the LNG gasification process according to any one of the above embodiments when running the computer program.

[0059] The embodiments in the present application are described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other. Each embodiment mainly explains the difference from other embodiments.

[0060] It should be noted that, unless otherwise specified and limited, terms such as "comprise", "include" or any other variants thereof are intended to cover non-exclusive inclusion, so that the circuit structure, article or device including a series of elements not only includes those elements, but also includes other elements not explicitly listed or inherent to such article or device. Without more limitation, the element defined by the phrase "including a" does not exclude the presence of another identical element in the article or device including the element. In addition, the term "and / or" used herein includes any and all combinations of one or more related listed items.

[0061] Other embodiments of the present application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. The present application is intended to cover any variations, uses, or adaptations of the application following the general principles thereof and including such departures from the present disclosure as come within known use or custom in the art to which the application pertains.

[0062] It should be understood that the present application is not limited to the precise construction that has been described and shown in the accompanying drawings, and that various modifications and changes can be made without departing from the scope thereof.

Claims

1. A method for controlling cold energy in the LNG gasification process, characterized in that, The method includes the following steps: Real-time acquisition of the ambient temperature and relative humidity of the ambient air vaporizer, the temperature and pressure at each measuring point inside the tube, and the temperature at each measuring point outside the tube on the fins; The entire data acquisition time is divided into multiple acquisition cycles. Based on whether the temperature at each external measurement point meets the frosting conditions at each moment, the frosting thickness assessment value at each moment is obtained. Based on the difference in the frosting thickness assessment values ​​of all adjacent moments in the current acquisition cycle, the frost condensation rate of the current acquisition cycle is obtained. Based on the proportion of data in the ambient temperature and relative humidity at all moments in the current acquisition cycle that belong to the preset temperature range and preset humidity range, the frosting promotion coefficient of the current acquisition cycle is obtained. The critical pressure, critical temperature, and mole fraction of each working substance in the LNG mixture are obtained. Combined with the pressure at each measurement point in the pipe at each time, the bubble point temperature at each measurement point in the pipe at each time is obtained. Based on the pipe height corresponding to the measurement point in the pipe at each time in the current acquisition cycle whose measured temperature is equal to the corresponding bubble point temperature, the heat transfer characteristic value of the current acquisition cycle is obtained. Based on the frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of the current collection cycle, the frost process evaluation value of the current collection cycle is obtained, and then cold energy control is carried out for the next collection cycle.

2. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The method for obtaining the frost thickness assessment values ​​at each time point is as follows: Obtain the air dew point temperature and the freezing point of water at various times; Obtain the fitting curves corresponding to the temperatures of all external measurement points distributed from the fin root to the tip at each time point; The obtained fitted curve is sampled for temperature according to a preset step size. When the temperature of the sampling point is lower than both the air dew point temperature and the water freezing point temperature, the corresponding sampling point is recorded as the fin frost point at each time. The wing height value corresponding to the frost point of the wing with the largest wing height at each time moment is recorded as the frost thickness evaluation value at each time moment.

3. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The frost condensation rate for the current collection period is determined by the average of the differences between the frost thickness assessment values ​​at all times within the current collection period.

4. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The method for obtaining the frost promotion coefficient of the current collection period is as follows: statistically analyze the proportion of ambient temperature within the preset temperature range at all times during the current collection period, and the proportion of ambient relative humidity within the preset humidity range at all times during the current collection period; the average of the two proportions is recorded as the frost promotion coefficient of the current collection period.

5. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The bubble point temperature at each measuring point inside the tube at each time moment satisfies the following relationship: ;in, In the formula, Let x be the mole fraction of the xth working substance; It is the phase equilibrium constant of the xth working fluid; This represents the total number of working fluids in the LNG mixture. It is the critical pressure of the xth working fluid; Let be the pressure at the j-th measurement point inside the pipe at the i-th time. It is the critical temperature of the xth working fluid; Let be the bubble point temperature at the j-th measurement point inside the tube at the i-th time. It is an exponential function with the natural constant e as the base; by solving the above equation, the bubble point temperature of each measuring point in the tube at each time point is obtained.

6. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The method for obtaining the heat transfer characteristic value of the current acquisition cycle is as follows: In the formula, This represents the heat transfer characteristic value for the current acquisition cycle. This represents the average length of the single-phase liquid region at all times during the current acquisition cycle. It is the total tube length value of the intermediate tube bundle of the ambient air vaporizer; The method for obtaining the length of the single-phase liquid region at each time point is as follows: The pipe distance from each temperature measurement point in the pipe to the LNG inlet of the middle tube bundle is taken as the pipe length value of each temperature measurement point in the pipe. Using the tube pass value as the horizontal axis, and the temperature of the tube measuring point corresponding to each tube pass value at each time point as the vertical axis, the tube temperature fitting curve at each time point is obtained. Then, using the bubble point temperature of the tube measurement point corresponding to each tube value at each time as the ordinate, the bubble point temperature fitting curve at each time is obtained. The tube length corresponding to the intersection of the tube temperature fitting curve and the bubble point temperature fitting curve at each time point is taken as the length of the single-phase liquid zone at each time point.

7. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The method for obtaining the frosting process evaluation value of the current collection period is as follows: The frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of the current collection cycle and its historical collection cycles are normalized respectively. The frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of each collection cycle are normalized to form the frost index vector for each collection cycle. The frost index vectors of the current acquisition cycle and its historical acquisition cycles are used as inputs to the Topsis superior-inferior solution distance method. Among them, the frost condensation rate and frost promotion coefficient are used as the maximum index, and the heat transfer characteristic value is used as the minimum index. The comprehensive score of the current acquisition cycle is output and recorded as the frost process evaluation value of the current acquisition cycle.

8. The cold energy control method for LNG gasification process as described in claim 1, characterized in that, The specific process for controlling the cooling energy in the next acquisition cycle is as follows: If the frost process assessment value of the current collection cycle is less than the preset first threshold, the PLC controller is used to adjust the opening of the flow regulating valve until the refrigerant outlet temperature is equal to the preset standard refrigerant outlet temperature. If the frost process evaluation value of the current collection cycle is greater than or equal to the preset first threshold and less than the preset second threshold, then the opening degree of the flow regulating valve will be reduced by the preset ratio at the beginning of the next collection cycle. If the frost process assessment value of the current collection cycle is greater than or equal to the preset second threshold, the opening of the flow regulating valve will be adjusted to 0 at the beginning of the next collection cycle, and the defrosting mechanism of the ambient air vaporizer will be activated for defrosting.

9. A cold energy control device for the LNG gasification process, characterized in that, The cold energy control method for LNG gasification process as described in any one of claims 1-8, wherein the cold energy control device comprises: The data acquisition module is used to acquire in real time the temperature and relative humidity of the ambient air vaporizer, the temperature and pressure of each measuring point inside the tube, and the temperature of each measuring point outside the tube on the fins. The frosting process analysis module is used to evaluate the frosting process of the current acquisition cycle based on the temperature change characteristics of the external measurement points, the ambient temperature and humidity, and the length characteristics of the single-phase liquid zone. The cooling energy regulation module is used to control the cooling energy for the next collection cycle based on the evaluation results of the frost process in the current collection cycle.

10. A cold energy control system for LNG gasification processes, comprising a memory, a processor, and a computer program stored in the memory and running on the processor, characterized in that, When the processor executes the computer program, it implements the cold energy control method for the LNG gasification process as described in any one of claims 1-8.

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