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CN-122015003-A - BOG recovery system and method based on LNG-ANG coupling condition

CN122015003ACN 122015003 ACN122015003 ACN 122015003ACN-122015003-A

Abstract

The invention belongs to the technical field of clean energy storage and efficient utilization, and particularly relates to a BOG recovery system and method based on LNG-ANG coupling conditions. According to the LNG-ANG deep coupling technology, the heat and mass transfer coupling model comprising LNG variable property characteristics and adsorption bed dynamic boundary characteristics is constructed, and based on dynamic regulation and control of the model, accurate control of LNG refrigerant from rough cooling to 'cold supply on demand' is realized, cold waste or heat removal deficiency is avoided, and system energy consumption is remarkably reduced. The invention introduces Yong efficiency, takes energy grade matching as an optimization target, realizes theoretical crossing from 'energy conservation' to 'Yong balance', enables the system to operate under the thermodynamic optimal working condition, and realizes efficient cascade utilization of energy.

Inventors

  • CHEN SHUJUN
  • Xiong xuan
  • FU YUE
  • CHEN LINGLI
  • FU YIMING
  • ZHENG LONG

Assignees

  • 中国石油大学(华东)

Dates

Publication Date
20260512
Application Date
20260413

Claims (10)

  1. 1. BOG recovery system based on LNG-ANG coupling condition, characterized by comprising: The modified activated carbon adsorption storage tank is a pressure vessel with a vacuum heat insulation layer, is filled with a high-heat-conductivity modified activated carbon adsorbent, and is provided with a built-in tube bundle type heat exchange structure; The LNG cold energy supply unit is used for conveying liquefied natural gas of the LNG receiving station into the heat exchange structure in the adsorption storage tank and used as a dynamic cold source; The BOG air inlet unit is used for introducing BOG generated by the receiving station into the adsorption storage tank to perform low-temperature adsorption; The desorption regeneration unit is used for providing a heat source and regenerating the adsorbent after adsorption saturation; the system is provided with a plurality of temperature sensors and pressure sensors inside and around the adsorption storage tank according to the requirement, and each temperature sensor and each pressure sensor are connected with the control unit; The control unit is internally provided with a thermal mass transfer coupling model based on variable properties and dynamic boundary conditions, and is used for adjusting the flow of the LNG cold energy supply unit and the air inlet rate of the BOG air inlet unit according to the temperature field and pressure field data monitored in real time; The heat and mass transfer coupling model comprises an adsorption storage tank internal thermal fluid energy sub-model and an adsorption storage tank internal cold fluid energy sub-model, wherein the adsorption storage tank internal thermal fluid energy sub-model is used for calculating total generated heat energy Q Heat of the body based on adsorption dynamics and porous medium heat transfer, and the adsorption storage tank internal cold fluid energy sub-model is used for calculating cold energy Q Cold water provided by LNG in a heat exchange tube; The control unit takes an energy matching principle Q Heat of the body ≈Q Cold water as a target, adjusts the flow of LNG in the heat exchange tube in real time, and based on a reverse heat conduction algorithm, inverts the size and the position of an adsorption heat source item in the adsorption bed layer in real time according to the temperature of the outer wall surface of the heat exchange tube, so as to realize the dynamic regulation and control of the temperature field of the adsorption bed layer.
  2. 2. The BOG recovery system based on the LNG-ANG coupling condition of claim 1, wherein the modified activated carbon adsorbent is characterized in that coconut shell activated carbon with specific surface area not lower than 2000m < 2 >/g is taken as a matrix, 2-5wt% of heat conducting filler is compounded, the heat conducting filler is selected from one or more of graphite nano-sheets, expanded graphite, carbon fibers or metal powder, the adsorbent is molded and extruded into spherical activated carbon through a binder, and is subjected to secondary activation treatment to dredge pore channels, and the volume adsorption capacity of methane is not lower than 140V/V within a temperature range of 160K-298K.
  3. 3. The BOG recovery system based on the LNG-ANG coupling condition of claim 1, wherein the built-in tube bundle heat exchange structure is arranged in a staggered mode, the outer surface of the heat exchange tube is welded with enhanced heat transfer fins, the reverse heat transfer algorithm is used for inverting the size of the adsorption heat inside the adsorption bed layer and the position of the adsorption heat inside the bed layer in real time based on the temperature of the outer wall surface of the heat exchange tube, and the LNG mass flow required for counteracting the adsorption heat generated inside the adsorption bed layer is calculated by combining an LNG variable property model And output to the regulating valve; The formula of (2) is: ; Wherein, the The system needs to regulate and control the LNG instantaneous mass flow at the time t; the heat generation rate of the adsorption bed layer at the t moment is obtained through a dynamic boundary and a heat conduction inversion model (or an air inlet flow meter); According to the LNG inlet temperature and pressure measured by a sensor in the storage tank, the inlet specific enthalpy is read in real time by calling a built-in physical property database (such as an NIST database); and reading the outlet specific enthalpy in real time according to the LNG outlet temperature and pressure measured by the heat exchange tube outlet sensor.
  4. 4. A BOG recovery system based on LNG-ANG coupling conditions according to claim 3, wherein the adsorption tank thermal fluid energy submodel, i.e. the total generated thermal model, is: (1); Wherein Q Heat of the body is the total generated heat in the process of storing natural gas, U 1 is the total internal energy in the adsorption storage tank, E k1 is the total kinetic energy in the adsorption storage tank, Q 1 is the sum of the convective heat transfer between natural gas and adsorbent in the adsorption storage tank, the heat transfer between adsorbent and the tank wall of the adsorption storage tank, Q 2 is the heat absorbed from LNG heat exchange tubes, ε is the porosity of adsorbent, Is the density of the natural gas, and is the density of the natural gas, For the density of the adsorbent, c pg is the specific heat of natural gas, c Ps is the specific heat of the adsorbent, Is the Darcy speed of the natural gas, T is the temperature in an adsorption storage tank, P is the pressure in the adsorption storage tank, lambda e is the effective heat conductivity of the adsorbent, h i is the specific enthalpy of each component of the natural gas, The diffusion flux for each component of the natural gas, H w is the heat exchange coefficient of the LNG and the wall of the heat exchange tube, A is the area of the LNG heat exchange tube, T w is the temperature of the wall of the heat exchange tube, and T LNG is the temperature of natural gas; The cold fluid energy quantum model in the adsorption storage tank, namely the variable physical property model is as follows: (2); Wherein Q Cold water is cold energy generated by convective heat transfer in the LNG flowing process, U 2 is internal energy of LNG in the heat exchange tube, E k2 is kinetic energy of LNG in the heat exchange tube, and Q 3 is energy generated by heat conduction in the LNG flowing process; for the density of LNG, c pl is the specific heat of LNG, For LNG flow rate in the heat exchange tube, lambda eff,f is the thermal conductivity containing turbulent motion; The dynamic boundary model is as follows: Firstly, a one-dimensional transient heat conduction equation of the heat exchange tube wall along the radial direction is established as a physical basis for solving reverse heat conduction, and the transient heat conduction equation of the heat exchange tube wall is as follows: (3); Wherein, the The volume specific heat capacity of the wall material of the heat exchange tube; The transient temperature of the heat exchange tube wall at the radial position r and the moment t; the heat conductivity coefficient of the heat exchange tube wall material; The initial conditions of equation (3) are: ; Wherein, the The initial temperature distribution of the heat exchange tube wall at the end of precooling or at the last control period; The inner wall boundary condition of equation (3) is: ; Wherein, the The instantaneous heat flux density of the inner wall of the heat exchange tube at the time t is the unknown boundary quantity to be calculated; Calculating an inversion target functional by adopting a least square method or a regularization method: (4); Wherein, the Objective functions requiring minimization; T mea （t i ) the actual temperature acquired at the time T i by a temperature sensor arranged on the outer wall of the heat exchange tube; t calc （t i,q ) under the premise of assuming that the heat flux density of the inner wall is q, calculating the theoretical temperature of the outer wall by a formula (3); tikhonov regularization term; setting initial iteration value of inner wall heat flux density The control unit continuously and iteratively modifies the unknown by a conjugate gradient method or a sequential function rule Up to Reaching a value of less than or equal to 10 -4 , i.e. when the calculated theoretical outer wall temperature is extremely close to the actual acquired outer wall temperature, the temperature is obtained The real instantaneous heat flux density of the inner side of the tube is obtained; Will invert the heat flux density of the inner wall As the boundary condition of the inner wall of the formula (3), combining the temperature measurement boundary of the outer wall and the initial temperature condition, solving the transient heat conduction equation of the heat exchange tube wall to obtain the transient temperature field of the heat exchange tube wall And then at Extracting the temperature of the inner wall : ; Wherein, the The transient temperature of the inner wall of the heat exchange tube at the moment t; Further, the side stream temperature is combined with LNG Calculating real-time convection heat transfer coefficient in the outlet pipe ; According to newton's law of cooling: ; The real-time convection heat exchange coefficient in the heat exchange tube can be obtained : ; Wherein, the Inversion to obtain the instantaneous heat flux density of the inner wall; inversion is carried out to obtain the transient temperature of the inner wall of the pipe; The real-time main body temperature of the LNG fluid in the tube is obtained through coupling calculation of the inlet and outlet temperature, pressure, mass flow and physical property database of the heat exchange tube; The convective heat transfer coefficient in the tube which dynamically changes along with the change of flow velocity, fluid physical property and temperature difference; Calculating a third class of boundaries that dynamically change over time: the adsorption bed side edge coupling equation is: ; the effective heat conductivity coefficient of the adsorption bed layer; The temperature gradient of the adsorption bed layer in the normal direction of the pipe wall; The local temperature at the interface between the adsorption bed layer and the outer wall of the heat exchange tube; Equivalent convection heat exchange coefficient is obtained based on inversion And the heat resistance of the pipe wall is calculated to be converted into the comprehensive heat transfer coefficient of the outer wall of the pipe; Dynamic temperature of LNG cold fluid in the pipe; And taking the calculation result as a dynamic boundary condition of an adsorption bed energy equation to realize accurate coupling of the adsorption side and the heat exchange side model.
  5. 5. The BOG recovery system based on LNG-ANG coupling conditions according to claim 1, wherein the control unit has built-in a thermodynamic matching optimization module targeting Yong efficiency maximization, the Yong efficiency is defined as: ; Wherein: ; Wherein, the For the actual utilization of the adsorbent bed cooling Yong, Total cooling Yong provided for the LNG fluid, h and s are specific enthalpy and specific entropy of the fluid under real-time temperature and pressure conditions respectively, h 0 、s 0 is specific enthalpy and specific entropy of the fluid under environmental reference temperature conditions respectively, and T 0 is environmental reference temperature; The control unit enables the system to operate at the peak point of the Yong efficiency curve by adjusting the LNG flow so as to realize the homogenization of the temperature difference field of the cold and hot fluid.
  6. 6. The BOG recovery system based on the LNG-ANG coupling condition of claim 1, wherein the built-in tube bundle heat exchange structure comprises at least three heat exchange straight tubes, reinforced heat transfer fins are welded outside the tubes, the tube diameter, the tube spacing and the fin size of the heat exchange tubes are optimally designed through CFD, the heat exchange tubes are made of low-temperature resistant stainless steel, and LNG to be gasified is introduced into the tubes as a refrigerant.
  7. 7. A BOG recovery method based on LNG-ANG coupling conditions, characterized in that a BOG recovery system based on LNG-ANG coupling conditions as defined in any one of claims 1-6 is used, comprising the steps of: S1, a precooling stage, namely controlling an LNG cold energy supply unit to feed LNG into a heat exchange structure of an adsorption storage tank, and reducing the temperature of an adsorption bed layer to a preset working temperature zone by utilizing sensible heat and latent heat of the LNG; The S2 collaborative adsorption stage comprises the steps of starting a BOG air inlet unit, introducing the BOG pressurized by a compressor into an ANG adsorption tank and capturing by an active carbon bed layer, simultaneously starting an LNG refrigerant supply pipeline, enabling low-pressure LNG in a storage tank to flow into an enhanced heat exchange tube bundle in the adsorption tank after being pressurized by a immersed pump, absorbing latent heat and adsorption heat of the BOG, transferring the absorbed LNG to a downstream gasifier to be converted into gaseous natural gas and then collecting the gaseous natural gas into an output tube network, and in the process, collecting recorded data of a flowmeter, a temperature sensor and a pressure sensor arranged at each node in real time, and calculating instantaneous total generated heat Q Heat of the body and instantaneous heat transfer capacity Q Cold water of LNG cold fluid by utilizing a built-in thermal mass transfer coupling model; S3, dynamically regulating and controlling, namely regulating the LNG mass flow in the heat exchange tube by the control unit according to the calculation result of the S2 stage based on the energy matching principle Q Heat of the body ≈Q Cold water , and regulating a flow set value through a correction algorithm when the temperature gradient inside the adsorption bed layer is monitored to exceed a set threshold or the physical properties of the LNG in the heat exchange tube are subjected to severe fluctuation, so as to maintain the low-temperature environment of the adsorption front; s4, saturation judgment and switching, namely judging that the adsorption is saturated when the methane concentration at the outlet of the adsorption storage tank reaches a set threshold value or the pressure in the tank reaches a design upper limit, stopping air inlet and cooling, and switching to a desorption regeneration unit; S5, desorption regeneration, namely firstly opening a valve of a heating medium inlet and outlet and an external heater, introducing a heating medium into a heat exchange tube bundle in an ANG adsorption tank, enabling the temperature of an adsorbent to rise from a low temperature state to a desorption temperature, namely more than or equal to 298K, methane desorption leads to pressure rebound in the tank, when the pressure reaches an output threshold value, opening a top exhaust valve, sending high-pressure methane gas generated by desorption into an output tube network or a BOG compressor to the downstream, when the pressure is reduced to the vicinity of normal pressure, closing the top exhaust valve and opening a vacuum pump, removing residual methane under the dual action of high temperature and negative pressure, sending the extracted gas into a collecting tube network, finally closing the vacuum pump, the heater and related valves, introducing a small amount of normal-temperature natural gas or inert gas for micro-positive pressure replacement purging, removing residual impurities in a bed layer, completing deep regeneration and resetting to a standby state.
  8. 8. The BOG recovery method based on the LNG-ANG coupling condition according to claim 7, wherein the preset working temperature range in step S1 is 160K-200K, the temperature gradient setting threshold in step S3 is 10K, and the desorption setting temperature in step S5 is 353k±5K.
  9. 9. The BOG recovery method based on LNG-ANG coupling condition according to claim 7, wherein the adjustment of LNG flow in step S3 considers physical distortion of supercritical LNG near a quasi-critical point, and calculates an in-tube local heat convection coefficient using a modified turbulence model selected from RNG k-epsilon model or SST k- ω model.
  10. 10. The BOG recovery method based on LNG-ANG coupling conditions according to claim 7, wherein in step S3, the energy matching principle Q Heat of the body ≈Q Cold water is implemented by a PID algorithm built in the control unit, and the PID algorithm corrects the flow rate set value when it is monitored that the internal temperature gradient of the adsorption bed layer exceeds a set threshold or that the physical properties of LNG in the heat exchange tube undergo severe fluctuation, so as to maintain a low-temperature environment of the adsorption front.

Description

BOG recovery system and method based on LNG-ANG coupling condition Technical Field The invention belongs to the technical field of clean energy storage and efficient utilization, and particularly relates to a BOG recovery system and method based on LNG-ANG coupling conditions. Background Liquefied Natural Gas (LNG) is increasingly important in global energy structures as a clean, efficient fossil energy source. In the LNG receiving station, the storage and distribution warehouse and related transferring links, due to unavoidable factors such as environmental heat leakage, atmospheric pressure fluctuation, pumping operation and the like, boil-Off Gas (BOG) can be continuously generated in the LNG storage tank, if the BOG cannot be timely and effectively treated, the pressure of the storage tank is increased, a safety valve is triggered to start to jump and discharge, serious greenhouse Gas pollution and energy waste are caused, and huge economic loss and potential safety hazard are brought. Therefore, developing a BOG processing technology with high efficiency, economy and low carbon has become a key problem to be solved in the LNG industry. Currently, the mainstream BOG processing technology mainly includes a recondensing process and a direct compression export process. Patent CN104033727a discloses a typical process and apparatus for recovering cold energy from LNG receiving stations for BOG treatment, and the core is to use the cold energy of LNG to condense and recover compressed BOG in a recondenser. The technology reduces the equipment load and the size of the recondenser by introducing cold recovery, and saves the energy consumption to a certain extent. However, this type of process is still highly dependent on the continuous export of LNG. When the receiving station is under a low output load (such as a limit minimum output working condition), the LNG cold quantity available for condensing the BOG is insufficient, so that the BOG cannot be completely processed, and the stability and the adaptability of the process face serious challenges. Patent CN117537262a discloses a device for storing BOG by low-temperature adsorption by using LNG cold energy, and by setting a plurality of adsorption tanks, the low-temperature adsorption storage of BOG is realized by using LNG cold energy. Patent CN119665136a proposes a system for directly liquefying BOG by using a refrigerator and maintaining the supercooling degree of a storage tank. These technical attempts provide new ideas for flexible handling of BOGs, but still have the following core problems to be solved: First, the mechanism of thermal mass transfer and energy matching is cognitively deficient. Cryogenic adsorption is a strongly exothermic process, particularly during the BOG high density adsorption phase, where the instantaneous release of significant amounts of heat of adsorption, if not removed effectively, can lead to a sharp rise in the adsorbent bed temperature, possibly leading to ignition risk or destruction of the adsorbent structure. In the prior art, a simple natural convection or heat exchange structure which is not optimized for low-temperature working conditions is mostly adopted, and the high-efficiency heat management requirement is difficult to meet. The LNG flowing in the heat exchange tube is near a critical point, physical parameters such as density, specific heat capacity, viscosity and the like of the LNG are subjected to severe nonlinear changes (namely 'variable physical' characteristics) along with temperature and pressure, and meanwhile, adsorption waves and thermal waves in the adsorption bed layer are continuously changed along with time, so that the boundary condition of the heat exchange tube presents a highly dynamic 'dynamic boundary' characteristic. The traditional heat exchange design method based on the assumption of normal physical properties and boundary setting cannot accurately describe the complex process, so that the calculation deviation of the heat exchange area is large, the supply and demand of cold and heat energy are unbalanced, and the system operation stability is poor. Second, there are short plates for engineering applications of high performance adsorbent materials. The heat conductivity coefficient of the conventional activated carbon at low temperature is extremely low (< 0.5W/m.K), the intra-crystal diffusion resistance is increased, the adsorption/desorption rate is severely restricted, and the requirement of the BOG on the quick response in an industrial scene is difficult to meet. Although some metal organic framework Materials (MOFs) exhibit extremely high theoretical adsorption capacities in the laboratory, they have problems of poor mechanical strength, difficulty in molding, high cost, etc., making it difficult to land in large-scale industrial applications. In summary, the existing BOG processing technology lacks a set of systematic solutions capable of deeply understanding and utilizing flu