Control method, system and device for cold energy in LNG gasification process

Abstract

The application relates to the technical field of LNG cold energy control, in particular to a cold energy control method, system and equipment for an LNG gasification process, which comprises the following steps: acquiring the temperature and relative humidity of the environment in which an air temperature type gasifier is located, the temperature and pressure of each in-pipe measuring point and the temperature of each out-pipe measuring point in real time; acquiring a frosting progress evaluation value of a current acquisition cycle according to the temperature variation characteristics of the out-pipe measuring points at all adjacent time points in the current acquisition cycle, the environment temperature and relative humidity at all time points in the current acquisition cycle and the pipe length of the single-phase liquid zone at each time point in the current acquisition cycle, and then controlling the cold energy of a next acquisition cycle. The cold energy of the next acquisition cycle is adaptively controlled by evaluating the frosting progress of each acquisition cycle, so that the sustainability and stability of the 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 device 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 at LNG gasification station before use, and 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, and 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, and too low temperature can cause ice blocks to be difficult to be smoothly defrosted, 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 device for LNG gasification process, and the technical scheme adopted is as follows:

[0005] 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:

[0006] 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;

[0007] Divide the entire data acquisition time into multiple acquisition periods, obtain the frost thickness evaluation value of 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 of 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 the data belonging to the preset temperature range and the preset humidity range in the environmental temperature and the environmental relative humidity at all times in the current acquisition period;

[0008] 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 pipe length height of the in-pipe measuring point at each time instant in the current acquisition cycle corresponding to the equal measuring temperature and the corresponding bubble point temperature;

[0009] 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.

[0010] Preferably, the method for acquiring the frosting thickness evaluation value at each time instant is as follows:

[0011] acquire the air dew point temperature and the freezing point of water at each time instant;

[0012] acquire the fitting curve corresponding to the temperature of all out-pipe measuring points distributed from the fin root to the fin tip at each time instant;

[0013] temperature sampling is performed on the obtained fitting curve according to a preset step length, and when the temperature of the sampling point is simultaneously lower than the air dew point temperature and the freezing point temperature of water, the corresponding sampling point is recorded as the fin frosting point at each time instant;

[0014] the fin height value corresponding to the fin frosting point with the largest fin height at each time instant is recorded as the frosting thickness evaluation value at each time instant.

[0015] 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.

[0016] Preferably, the method for acquiring the frosting promotion coefficient of the current acquisition cycle is as follows: 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 is counted, 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 is counted; the average value of the obtained two proportions is recorded as the frosting promotion coefficient of the current acquisition cycle.

[0017] 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; 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.

[0018] Preferably, 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;

[0019] The method for obtaining the length of the single-phase liquid region at each time point is as follows:

[0020] 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.

[0021] 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.

[0022] Then, using the bubble point temperature of the tube measurement point corresponding to each tube pass value at each time as the ordinate, the bubble point temperature fitting curve at each time is obtained.

[0023] 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.

[0024] Preferably, the method for obtaining the frost process evaluation value of the current collection period is as follows:

[0025] 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.

[0026] 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.

[0027] 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.

[0028] Preferably, the specific process of performing cold energy control for the next acquisition cycle is as follows:

[0029] 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.

[0030] 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.

[0031] 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.

[0032] Secondly, embodiments of this application provide a cold energy control device for the LNG gasification process, the cold energy control device including: a data acquisition module, a frosting process analysis module, and a cold energy regulation module.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] Thirdly, embodiments of this application also provide a cold energy control system for the LNG gasification process. The system includes a memory, a processor, and a computer program stored in the memory and running on the processor. When the processor executes the computer program, it implements the steps of any of the above-described cold energy control methods for the LNG gasification process.

[0037] As can be seen from the above embodiments, the cold energy control method, system, and equipment for the LNG gasification process provided in this application have at least the following beneficial effects:

[0038] This application constructs a frost condensation rate by analyzing the temperature change characteristics of external measurement points over time, which can assess the rate of frost formation on the fins of the tube. By analyzing the ambient temperature and humidity, a frost promotion coefficient is constructed, which can assess the influence of environmental factors on the frost formation process. By analyzing the tube length at which the temperature at the internal measurement points reaches the bubble point, a heat transfer characteristic value is constructed, which can assess the heat transfer efficiency and vaporization effect of the fins. Combining the above three indicators, a frost formation process evaluation value is constructed to dynamically evaluate the frost formation process of the ambient air vaporizer in each sampling cycle. This allows for the control and regulation of cold energy, avoiding excessive frost accumulation and frost shutdown, extending equipment life, balancing frost risk and ice-making demand, and improving the sustainability and stability of LNG cold energy control. Attached Figure Description

[0039] To more clearly illustrate the technical solutions and advantages in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0040] Figure 1 A flowchart illustrating the steps of a cold energy control method for LNG gasification process provided in one embodiment of this application;

[0041] Figure 2 This is a schematic diagram showing the distribution of external temperature measuring points in the intermediate tube bundle of an ambient air vaporizer, where a represents the fins of the intermediate tube bundle, b represents the external temperature measuring points on the fins, and c represents the cross-section of the intermediate tube bundle.

[0042] Figure 3 This is a schematic diagram of the structure of a cold energy control device for the LNG gasification process provided in an embodiment of this application. Detailed Implementation

[0043] To further illustrate the technical means and effects adopted by this application to achieve the intended purpose of the invention, the following, in conjunction with the accompanying drawings and preferred embodiments, details the specific implementation methods, structures, features, and effects of the cold energy control method, system, and equipment for the LNG gasification process proposed in this application. In the following description, different "one embodiment" or "another embodiment" do not necessarily refer to the same embodiment. Furthermore, specific features, structures, or characteristics in one or more embodiments can be combined in any suitable form.

[0044] Unless otherwise specified and limited, terms such as “comprising,” “including,” or any other variations thereof are intended to cover a non-exclusive inclusion, such that a circuit structure, article, or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or device. Without further limitation, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the article or device that includes said element. Furthermore, the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items. All technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0045] The following description, in conjunction with the accompanying drawings, details the specific solutions provided in this application for cold energy control methods, systems, and equipment in the LNG gasification process.

[0046] Please see Figure 1 The diagram illustrates a flowchart of a cold energy control method for LNG gasification processes according to an embodiment of this application. The method includes the following steps:

[0047] Step 1: 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 fin.

[0048] In the LNG-based ice-making process, LNG from the cryogenic storage tank, at a temperature of approximately -150℃ to -160℃ and a pressure of 0.6MPa, is divided into two streams. One stream enters ambient air vaporizer #1, where it undergoes a phase change and is heated to approximately 5℃ to 15℃ before being transported downstream. The other stream flows through a flow regulating valve into the LNG-refrigerant heat exchanger, where it exchanges heat with the refrigerant. The cooled natural gas then exits the heat exchanger and is further heated to 5℃ to 15℃ by ambient air vaporizer #2 before being transported to the downstream pipeline network. Gaseous refrigerant, with an initial temperature of -18℃ to +20℃ and a pressure of 0.2MPa to 1.2MPa, condenses into a liquid state after exchanging heat with LNG in the LNG-refrigerant heat exchanger, with its temperature dropping to -25℃ to +5℃. The liquid refrigerant then flows sequentially into the refrigerant buffer tank and the low-pressure circulation tank, before entering the direct-cooling evaporator to participate in ice making. The refrigerant, after absorbing heat, returns from the evaporator to the upstream heat exchanger, completing a closed-loop refrigerant cycle. Specifically, the refrigerant used is R507.

[0049] An armored embedded PT100 platinum resistance thermometer is used in the outlet pipe of the LNG-refrigerant heat exchanger to obtain the refrigerant outlet temperature of the LNG-refrigerant heat exchanger in real time.

[0050] Since ambient air vaporizer #1 is used for emergency backup and does not primarily participate in cold energy utilization, this embodiment focuses on ambient air vaporizer #2 as the object of cold energy control. Hereinafter, ambient air vaporizer #2 will be referred to simply as the ambient air vaporizer. Frosting on the ambient air vaporizer begins at the bottom LNG inlet and gradually extends upwards towards the middle tube bundle. The middle tube bundle has low heat exchange efficiency and is more prone to frosting. Since the intermediate tube bundle of the ambient air vaporizer is more prone to frosting, this application divides the intermediate tube bundle of the ambient air vaporizer into N tube segments according to the tube length, and sets an in-tube measurement point at the boundary of each tube segment. An armored embedded PT100 platinum resistance thermometer and a diffused silicon pressure sensor are installed at each in-tube measurement point to collect the temperature and pressure parameters in the tube segment in real time. The temperature and pressure parameters of the first in-tube measurement point are the inlet temperature and inlet pressure values ​​of the LNG ambient air vaporizer, and the temperature and pressure parameters of the (N+1)th in-tube measurement point are the outlet temperature and outlet pressure values ​​of the LNG ambient air vaporizer. In this embodiment, N is taken as 10.

[0051] A temperature and humidity recorder is used to collect real-time data on the temperature and relative humidity of the environment in which the LNG ambient temperature vaporizer is located.

[0052] In addition, the multi-channel finned tube bundle air-temperature vaporizer has a set of heat exchange fins extending radially at intervals on the outer wall of the tube body in the height direction and around the circumference of the tube body. On any heat exchange fin of the middle tube bundle on the air-temperature vaporizer inlet side, an external temperature measuring point is set at preset intervals along the fin height direction. Figure 2 This is a schematic diagram showing the distribution of external temperature measuring points in the intermediate tube bundle of an ambient air vaporizer. In this embodiment, the external temperature measuring points are positioned at 1 / 4 of the height of the intermediate tube bundle. Thin-film PT100 resistance thermometers are fixed to each external temperature measuring point using thermally conductive adhesive to obtain the temperature of each external temperature measuring point distributed along the fin height direction in real time.

[0053] All types of data are collected in real time and synchronously, with a collection time interval of 30 seconds. Each preset duration is considered a collection cycle, and in this embodiment, the preset duration is 15 minutes.

[0054] Step 2: Divide the entire data acquisition time into multiple acquisition cycles. Based on whether the temperature at each external measurement point meets the frosting conditions at each time, obtain the frosting thickness assessment value at each time. Based on the difference in the frosting thickness assessment values ​​of all adjacent times in the current acquisition cycle, obtain the frost condensation rate of the current acquisition cycle. Based on the proportion of data in the ambient temperature and relative humidity at all times in the current acquisition cycle that fall within the preset temperature range and preset humidity range, obtain the frosting promotion coefficient of the current acquisition cycle.

[0055] Ambient air vaporizers (AAVs) primarily utilize natural or forced convection of ambient air, offering advantages such as no additional energy consumption and ease of operation. In an LNG vaporization unit, two AAVs work collaboratively to support the natural gas supply needs of downstream users. However, after a period of continuous operation, AAVs are prone to frosting. The increasing thermal resistance of the frost layer severely reduces the vaporization efficiency, sometimes necessitating shutdown for defrosting. This can lead to the shutdown of the LNG refrigeration system and even disruptions to downstream natural gas supply.

[0056] Frosting in ambient air vaporizers is mainly caused by water vapor in the air turning into solid crystals upon cooling. The geometric structure and accumulation pattern change as the frosting process progresses. The frosting thickness is primarily related to the temperature of the outer wall of the finned tubes, the ambient temperature, and the relative humidity. The temperature of the ambient air vaporizer gradually increases along the fin height, and frosting gradually forms on the fins in the order of root-middle-tip. Frosting occurs on the fin surface when the fin surface temperature is simultaneously lower than both the air dew point and the freezing point of water.

[0057] Taking the i-th moment as an example, based on 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 consulting the air dew point temperature table. The freezing point temperature of water is 0℃ in this embodiment. The temperatures of all external temperature measuring points distributed from the fin root to the tip at the i-th moment are used as input to the least squares linear fitting algorithm to obtain the fitting curve corresponding to the temperature of all external temperature measuring points distributed from the fin root to the tip at the i-th moment. The obtained fitting curve is sampled according to a preset step size, which is 0.1mm in this embodiment. When the temperature of the sampling point is lower than both the air dew point temperature value and the freezing point temperature of water, the corresponding sampling point is recorded as the fin frost point at the i-th moment. The fin height value corresponding to the fin frost point with the largest fin height is recorded as the frost thickness evaluation value at the i-th moment.

[0058] The frost thickness assessment values ​​at all times within the current acquisition cycle are arranged in chronological order to form a sequence, which is recorded as the frost thickness sequence for the current acquisition cycle. The mean of all data in the first-order difference sequence of the frost thickness sequence is calculated as the frost condensation rate for the current acquisition cycle. If the frost condensation rate is greater than 0, it indicates that the overall frost thickness of the ambient air vaporizer is increasing during the current acquisition cycle, meaning the frost layer on the fins is gradually thickening. If the frost condensation rate is less than 0, it indicates that the overall frost thickness of the ambient air vaporizer is decreasing during the current acquisition cycle, meaning the frost layer on the fins is gradually melting and thinning. If the frost condensation rate is equal to 0, it indicates that the frost thickness of the ambient air vaporizer remains unchanged during the current acquisition cycle.

[0059] Ambient temperature and relative humidity are also important factors affecting the frosting process of air-conditioned vaporizers. Moreover, extremely low temperature and high humidity environments are not entirely conducive to the formation of frosting. This application defines the suitable ambient temperature range and suitable ambient humidity range for promoting frosting of air-conditioned vaporizers as the preset temperature range and preset humidity range. If the air-conditioned vaporizer is in the preset temperature range and preset humidity range for a long time, it will lead to an increase in the thermal resistance of the frost layer, thereby reducing the heat exchange efficiency of the air-conditioned vaporizer and affecting the vaporization effect. In the case of severe frosting, frequent shutdowns for defrosting are required.

[0060] In this embodiment, the preset temperature range is [-6℃, 0℃], and the preset humidity range is [20%, 30%]. When the ambient temperature is below the preset temperature range, the mass transfer rate of the air-cooled vaporizer is low, frost formation is slow, and the thermal resistance of the frost layer is low. When the ambient temperature is above the preset temperature range, the higher ambient temperature promotes the melting of the frost layer, thereby reducing the rate at which the frost layer thickens, and thus the thermal resistance of the frost layer also becomes lower. When the relative humidity of the environment is below the preset humidity range, the air is relatively dry, which is not conducive to the formation of frost. When the relative humidity of the environment is above the preset humidity range, the moisture in the air promotes the melting of the frost layer, thereby reducing the thermal resistance of the frost layer.

[0061] The percentage of ambient temperature within the preset temperature range and the percentage of ambient relative humidity within the preset humidity range at all times during the current acquisition cycle are statistically analyzed. The average of these two percentages is recorded as the frost promotion coefficient for the current acquisition cycle, which reflects the degree to which the ambient temperature and humidity during the current acquisition cycle promote the frost formation process on the fins.

[0062] Step 3: Obtain the critical pressure, critical temperature, and mole fraction of each working substance in the LNG mixture, and combine this with the pressure at each measuring point in the pipe at each time point to obtain the bubble point temperature at each measuring point in the pipe at each time point; based on the pipe height corresponding to the measuring point in the pipe at each time point in the current acquisition cycle whose measured temperature is equal to the corresponding bubble point temperature, obtain the heat transfer characteristic value of the current acquisition cycle.

[0063] The LNG fluid entering the LNG ambient temperature vaporizer is a mixture mainly composed of methane. The boiling points of each component in the LNG mixture are different. After absorbing heat, these LNG mixture components vaporize in a certain order, which leads to continuous changes in the gas-liquid composition during the LNG vaporization process. Consequently, the tube bundle of the ambient temperature vaporizer has a single-phase liquid region, a two-phase region, and a single-phase gas region along the tube side. The bubble point marks the transition from the single-phase liquid region to the two-phase region.

[0064] Since methane, ethane, and propane constitute a very high proportion of the LNG mixture, this application considers the LNG fluid as a mixture containing these three working substances and measures their mole fractions. In this 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 each working substance, denoted as . Where x=1 is methane, x=2 is ethane, and x=3 is propane. As is 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 their critical temperatures are -82.6℃, 32.2℃, and 95.7℃, respectively.

[0065] As is known from the prior art, the bubble point temperature at each measuring point inside the pipe of an LNG ambient temperature vaporizer at various times 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 refers to the total number of working fluids in the LNG mixture; It is the critical pressure of the xth working fluid (in MPa). Let be the pressure (in MPa) at the j-th measurement point inside the pipe at the i-th time. It is the critical temperature (in K) of the xth working fluid. Let be the bubble point temperature (in K) of the j-th measurement point inside the tube at the i-th time. It is an exponential function with the natural constant e as its base.

[0066] By relating the two formulas above and solving the equations, we can obtain the bubble point temperature at any measurement point inside the pipe of the LNG ambient temperature vaporizer at any time.

[0067] Calculate the pipe distance from each temperature measurement point inside the pipe to the LNG inlet of the middle tube bundle, and use it as the pipe length value for each temperature measurement point inside the pipe.

[0068] This application constructs a first coordinate system with the tube length value as the horizontal axis and the temperature of the tube measurement point corresponding to each tube length value at each time as the vertical axis. The least squares method is used for linear fitting to obtain the tube temperature fitting curve at each time. Each time corresponds to a tube temperature fitting curve, and the tube temperature fitting curve shows an increasing trend.

[0069] This application constructs a second coordinate system with the tube-side value as the x-axis and the bubble point temperature at each tube-side value corresponding to that temperature measurement point at each time point as the y-axis. The least squares method is used for linear fitting to obtain the bubble point temperature fitting curve at each time point. In the ambient air vaporizer, the pressure of the LNG fluid decreases along the tube side, therefore the bubble point temperature fitting curve shows a decreasing trend. It should be noted that since the temperature values ​​measured by the temperature and humidity sensor are in °C, the unit of the bubble point temperature needs to be converted from K to °C.

[0070] Since the bubble point marks the transition of LNG fluid from a single-phase liquid region to a two-phase region, the intersection of the pipe temperature fitting curve and the bubble point temperature fitting curve at each time point is obtained, and the pipe length value corresponding to the intersection point is taken as the length of the single-phase liquid region at each time point.

[0071] Furthermore, based on the length of the single-phase liquid zone at each moment in the current acquisition cycle, the heat transfer characteristic value of the current acquisition cycle is obtained, which is used to characterize the heat transfer efficiency of the ambient air vaporizer in the current acquisition cycle.

[0072] In this embodiment, the heat transfer characteristic value of the current acquisition cycle is denoted as... Its specific expression is: 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. This is the total tube length value of the intermediate tube bundle in an ambient air vaporizer. LNG needs to absorb a large amount of heat (i.e., latent heat of vaporization) to change from a liquid to a gaseous state. The shorter the length of the single-phase liquid zone, the larger the heat exchange characteristic value, indicating that the ambient air vaporizer has better heat exchange efficiency and a lower fin frosting process.

[0073] Solid ice crystals on the surface of the air-conditioned vaporizer fins gradually accumulate over time, developing into a dense frost crystal structure, and the frost thickness also increases accordingly. The rapid growth of the frost layer affects the gas flow in the space surrounding the vaporizer and hinders heat transfer. If the ambient temperature and humidity are suitable, it is easy to promote frost formation on the fins. Frost formation on the fins will reduce heat exchange efficiency and vaporization effect, and prolong the length of the single-phase liquid zone inside the air-conditioned vaporizer tube.

[0074] Step 4: Based on the frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of the current collection cycle, obtain the frost process evaluation value of the current collection cycle, and then perform cold energy control for the next collection cycle.

[0075] Furthermore, following the above method, the frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of the ambient air vaporizer for the historical M sampling cycles are obtained. In this embodiment, M is set to 10. The frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of the ambient air vaporizer in the current sampling cycle and its historical sampling cycles are respectively Max-Min normalized; the frost condensation rate, frost promotion coefficient, and heat transfer characteristic value of each sampling cycle after normalization are used to form the frost index vector for each sampling cycle.

[0076] The Topsis superiority / inferiority distance method uses all frosting index vectors from the current and historical data collection cycles as inputs. Frost condensation rate and frosting promotion coefficient are considered as large-scale indices, while heat transfer characteristic values ​​are considered as small-scale indices. The output is a comprehensive score for the current data collection cycle, denoted as the frosting progress evaluation value, used to assess the frosting progress of the ambient air vaporizer fins. A higher value indicates a higher level of fin frosting progress in the current data collection cycle. The Topsis superiority / inferiority distance method is a well-known technique, and its specific process will not be elaborated further.

[0077] As the frosting process of the ambient air vaporizer fins progresses, the thickness of the frost layer on the fin surface increases, leading to greater thermal resistance and gradually reducing the vaporization efficiency of the ambient air vaporizer. If the frosting process is too advanced, it may even cause damage and rupture of the ambient air vaporizer tube bundle, requiring immediate shutdown. In the LNG cold energy ice-making process, to fully utilize the LNG cold energy, most of the LNG from the cryogenic storage tank is distributed to the pipeline where the LNG-refrigerant heat exchanger is located. Therefore, it is necessary to control the valve opening of the flow control valve to prevent the ambient air vaporizer from frosting and shutting down.

[0078] Specifically, a first preset threshold and a second preset threshold are set. Based on the frost formation assessment value of the ambient air vaporizer in the current sampling cycle, the cooling energy of the ambient air vaporizer is controlled for the next sampling cycle. In this embodiment, the first preset threshold is set to 0.5, and the second preset threshold is set to 0.8.

[0079] If the frosting progress assessment value of the ambient air vaporizer in the current sampling cycle is less than the preset first threshold, it indicates that the thermal resistance of the fin frost layer in the current sampling cycle is small, and the impact on the heat exchange efficiency and vaporization effect of the ambient air vaporizer is low. More attention should be paid to the stability of the refrigerant circulation process. Set the standard refrigerant outlet temperature to -20℃. At this time, use the PLC controller to adjust the opening of the flow regulating valve until the refrigerant outlet temperature in the next sampling cycle is equal to the preset standard refrigerant outlet temperature. This will prevent large fluctuations in the refrigerant outlet temperature curve of the LNG-refrigerant heat exchanger when the LNG flow rate changes suddenly or the ice-making load changes significantly.

[0080] If the frost progress assessment value of the ambient air vaporizer in the current collection cycle is greater than or equal to the preset first threshold and less than the preset second threshold, it indicates that the thermal resistance of the fin frost layer in the current collection cycle is high, which has a high impact on the heat exchange efficiency and vaporization effect of the ambient air vaporizer. Therefore, at the beginning of the next collection cycle, the opening degree of the flow regulating valve is reduced by a preset ratio. In this embodiment, the preset ratio is 5% to prevent the ambient air vaporizer from frost-induced shutdown.

[0081] If the frost formation assessment value of the ambient air vaporizer in the current collection cycle is greater than or equal to the preset second threshold, it indicates that the thermal resistance of the frost layer on the fins is very high in the current collection cycle, which has a very high impact on the heat exchange efficiency and vaporization effect of the ambient air vaporizer. If the ambient air vaporizer continues to work at this time, the frost layer on each fin will stick together solidly, increasing the difficulty of defrosting. Therefore, at the beginning of the next collection cycle, the opening of the flow regulating valve will be adjusted to 0, and the defrosting mechanism of the ambient air vaporizer will be activated for defrosting.

[0082] Please see Figure 3 , Figure 3 This is a schematic diagram of the structure of a cold energy control device for the LNG gasification process provided in an embodiment of this application. In this embodiment, the terminal includes units used to execute the steps in the corresponding embodiment of the cold energy control method for the LNG gasification process. See also... Figure 3 The cold energy control equipment includes: a data acquisition module, a frosting process analysis module, and a cold energy regulation module.

[0083] 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.

[0084] 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.

[0085] 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.

[0086] Based on the same inventive concept as the above method, this application also provides a cold energy control system for the LNG gasification process, including a memory, a processor, and a computer program stored in the memory and running on the processor. When the processor executes the computer program, it implements the steps of any of the above-described cold energy control methods for the LNG gasification process.

[0087] The various embodiments in this application are described in a progressive manner. The same or similar parts between the various embodiments can be referred to each other. Each embodiment focuses on describing the differences from other embodiments.

[0088] It should be noted that, unless otherwise specified and limited, terms such as “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a circuit structure, article, or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such article or device. Without further limitations, an element defined by the phrase “comprising one…” does not exclude the presence of other identical elements in the article or device that includes said element. Furthermore, the term “and / or” as used herein includes any and all combinations of one or more of the associated listed items.

[0089] Other embodiments of this application will readily occur to those skilled in the art upon consideration of the specification and practice of the invention herein. This application is intended to cover any variations, uses, or adaptations of this application that follow the general principles of this application and include common knowledge or customary techniques in the art not invented in this application.

[0090] It should be understood that this application is not limited to the precise structure described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope.

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 ambient temperature and relative humidity of the environment where the air-cooled vaporizer is located, temperature and pressure at each measuring point inside the tube, and 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. 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. The frost condensation rate of 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. The method for obtaining the frost promotion coefficient of the current collection period is as follows: Calculate the proportion of ambient temperature within the preset temperature range at all times in the current collection period, and the proportion of ambient relative humidity within the preset humidity range at all times in the current collection period; record the average of the two proportions as the frost promotion coefficient of the current collection period. 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 pass 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.

2. 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 refers to 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.

3. 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.

4. 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.

5. 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 1-4, 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.

6. 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 1-4.

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