A self-flowing wall cooling method for LNG membrane tanks

Abstract

The present invention discloses a self-flowing wall cooling method applied to LNG membrane tanks, belonging to the field of natural gas technology, and the following steps: Calculate LNG physical property parameters and establish LNG membrane tank model: First, use HYSYS simulator to obtain the initial properties of LNG, and couple the gas-liquid phase change saturation temperature curve of LNG, and fit the relationship between BOG saturation temperature and saturation pressure as the input material properties for subsequent CFD simulation. Next, by establishing a tank enclosure structure model in Designbuilder software, the corresponding wall properties are obtained. The present invention simulates the BOG generation inside the LNG membrane storage tank through CFD, thereby introducing a design and calculation method for a self-flowing wall cooling system specifically for the LNG membrane tank wall, and applies the wall cooling system to the LNG membrane storage tank, effectively avoiding system overpressure, filling the gap in this aspect in the relevant field, and providing a scientific and reliable basis for determining the membrane tank construction method and operation countermeasures for controlling the BOG generation under similar circumstances.

Description

Technical Field

[0001] The invention relates to the technical field of natural gas, and in particular to a self-flowing wall cooling method applied to an LNG membrane tank. Background Art

[0002] Natural gas is the cleanest fossil fuel, and its storage capacity is huge and its supply is abundant, which has driven its growing demand worldwide. Due to its density advantage, most users import it as liquefied natural gas (LNG). With the continuous increase in my country's LNG imports, the gradual expansion of the scale of natural gas supply and the increasing complexity of user types, not only large coastal LNG receiving stations have developed rapidly, but also small LNG receiving stations have developed rapidly, and more and more well-insulated LNG storage tanks have been built as key facilities for regasification terminals. However, despite the thick insulation layer of the storage tank, it is still impossible to completely prevent heat from leaking from the surrounding environment into the storage tank, and part of the mechanical energy of the submersible pump in the storage tank is converted into thermal energy when it is running, which will cause the LNG in the tank to gasify and produce flash gas, which is BOG (Boil Off Gas).

[0003] Due to the change of LNG volume in the storage tank, as well as the input of external energy such as ambient temperature and atmospheric pressure changes, a large amount of BOG is generated in the storage tank, which increases the system pressure. If the BOG is not handled and recovered in time, the system will have the risk of overpressure operation. At the same time, the increase in pressure in the tank will also change the boiling point temperature of LNG, resulting in a more intense phase change reaction, which has a greater impact on the safe operation of the storage tank or receiving station. At present, the methods for recovering BOG include recondensation method, direct output method, reliquefaction method, etc. The recondensation method is the main method for recovering BOG at the receiving station. These common methods all require that BOG be collected outside the tank before a unified recovery process is carried out, which consumes a lot of energy, has a low loss, and is less economical. Therefore, it is necessary to study a new design and calculation method for the self-flowing wall cooling system, combine the refrigeration system with the wall of the LNG membrane storage tank, directly recondense BOG inside the storage tank, recover BOG, reduce the temperature of the inner wall of the tank, reduce the evaporation loss of LNG, prevent the system from overpressure, and have good economic benefits. Summary of the invention

[0004] The purpose of the present invention is to provide a self-flowing wall cooling method for LNG membrane tanks, to conduct heat conduction insulation on membrane tanks that are different from traditional 9% nickel steel full-containment tanks in terms of function and structure, to control the amount of BOG generated, and to further provide a scientific basis and reference for BOG recovery schemes during the operation of membrane storage tanks, so as to solve the problems raised in the above-mentioned background technology.

[0005] To achieve the above object, the present invention provides the following technical solution: a self-flowing wall cooling method applied to an LNG membrane tank, comprising the following steps:

[0006] Step 1. Use HYSYS to input the molar percentage of the initial components of LNG as methane 99.86%, ethane 0.04%, and nitrogen 0.1%, and calculate the LNG physical properties, namely, density, specific heat, and gas phase saturation temperature versus pressure curves; then establish an LNG membrane tank model in Designbuilder, and obtain the overall heat transfer coefficient of the circumferential and bottom walls based on the double-layer wall heat transfer enclosure structure model of the insulation layer combined with concrete, and then calculate the maximum total cooling and heating loads of the tank in summer and winter based on the outdoor air temperature, thereby obtaining the heat transfer per unit area of ​​the circumferential and bottom walls of the tank, providing thermal boundary conditions for CFD numerical simulation;

[0007] Step 2: Fluent is used to perform numerical simulation of the flow field in the LNG storage tank under six different storage conditions, including summer, winter, low, medium and high initial liquid levels; the relationship between saturated temperature and pressure is obtained by fitting the curve of gas phase saturation temperature versus pressure in step 1, which is used as the input material properties of the simulation, and the heat transfer coefficient at different positions on the wall and the maximum cold and hot load are used as the thermal boundary conditions of the simulation. The VOF model and Lee model are used to simulate the LNG state change process in the membrane tank and the BOG generation process in the tank. The mass flow rate of BOG generated at the gas-liquid interface is monitored according to the evolution law under different working conditions of the six combinations to obtain the BOG generation amount under the most unfavorable working condition;

[0008] Among all the working conditions, the temperature field changes most dramatically when the LNG initial liquid level is high in summer, which will lead to obvious liquid level changes and evaporation, increasing the amount of BOG generated. Therefore, this working condition is the most unfavorable working condition during design.

[0009] Step 3: According to the BOG production under the most unfavorable working conditions, considering that 99% of the BOG produced by evaporation is methane gas, the process of its phase change through the capillary grid is a pure condensation process. The load that the capillary grid needs to provide is equal to the cold load Q required for condensing BOG, that is, the load of its phase change process, which can be obtained by multiplying the BOG production mass flow rate under the most unfavorable working conditions by the latent heat of evaporation of LNG, that is, determined by the following formula:

[0010] Q=m BOG r

[0011] Where Q is the cooling load required for condensing BOG, kW; m BOG is the BOG mass flow rate generated at the gas-liquid interface in the tank during simulation stability, kg / s; r is the latent heat of evaporation of LNG, obtained according to the LNG physical property parameters corresponding to the tank pressure during simulation stability, kJ / kg;

[0012] Step 4: Consider that the heat exchange cold fluid in the capillary grid uses LNG liquid with an inlet temperature of -162°C and an outlet temperature of -161°C, that is, the inlet and outlet temperature difference Δt of the cold fluid is 1K; the LNG liquid density ρ and the LNG liquid specific heat c are selected according to the corresponding temperature of the LNG physical property parameters, considering the commonly used capillary network parameters:

[0013] director Capillary Capillary spacing flow 20×2.0mm 4.3×0.8 mm 10 mm <![CDATA[0.16L / (s.m 2 )]]>

[0014] Therefore, the cooling capacity per unit area Q1 of the capillary grid under this size specification can be calculated by the following formula:

[0015] Q1=V·ρ·c·Δt

[0016] Where Q1 is the cooling capacity per unit area, W / m 2 ; V is the cold fluid flow rate, m 3 / (sm 2 );ρ is the density of LNG liquid, kg / m 3 ; c is the specific heat of LNG liquid, J / (kg.K); Δt is the inlet and outlet temperature difference of the cold fluid, K. After determining the cold load Q required for condensing BOG and the refrigeration capacity per unit area of ​​the capillary grid Q1, the required capillary grid laying position can be determined. The capillary grid is combined with the insulation wall of the membrane tank to form an integrated structure. The required capillary grid area S can be determined using the following formula:

[0017] S=Q / Q1

[0018] Where S is the required capillary grid area, m 2 ; Q is the cooling load required for condensing BOG, kW; Q1 is the cooling capacity per unit area, W / m 2 Taking a cylindrical membrane tank with an inner diameter of d meters as an example, the laying height can be determined using the following formula:

[0019]

[0020] Where, h is the capillary grid laying height, m; S is the required capillary grid area, m 2 ; d is the inner diameter of the cylindrical membrane tank, m, that is, it is only necessary to lay the capillary grid along the inner wall surface in the space h meters above where the flash gas is concentrated, and combine the capillary grid with the insulation inner wall surface of the upper space of the membrane tank to form an integrated structure, which can provide the cooling load required for condensing BOG.

[0021] The capillary refrigeration system is suitable for LNG membrane tanks, that is, compared with 9% nickel steel full-containment tanks, the inner tank only bears the role of liquid tightness and gas tightness, and the liquid weight and other loads are transferred to the concrete outer tank through the cold insulation material and borne by the outer tank. In addition, the cold insulation structure of the membrane tank adopts a large amount of off-site prefabrication, and the on-site construction volume is small, which is conducive to the laying of the capillary grid, which can be combined with the insulation wall of the membrane tank to form an integrated structure; the insulation layer between the inner tank and the outer tank of the membrane tank usually includes the main shielding film, plywood, reinforced polyurethane foam, stainless steel anchor strips, and moisture-proof layer. The capillary grid can be combined with it as an integrated refrigeration core layer, embedded between the main shielding film layer and the reinforced polyurethane foam layer, and fixed by plywood and separated from other surface layers by the secondary shielding film, so that it can isolate outdoor heat conduction, reduce the temperature of the inner wall of the tank, and prevent the transmission of radiant heat inside; it can also effectively control the BOG produced at the wall, condense and recover the BOG, prevent the system from overpressure, and ensure safe operation. At the same time, since the temperature difference between the gas phase and the liquid phase inside the storage tank is only 1°C, the condensation problem common in capillary refrigeration systems is effectively avoided.

[0022] The method for determining the amount of BOG generated under the most unfavorable working condition is: use HYSYS to input the molar percentage of the initial components of LNG, calculate the curves of LNG density, specific heat and gas phase saturation temperature changing with pressure, and then use Fluent software to establish a numerical model of the LNG membrane tank under the storage conditions of the closed tank body in different seasons and different initial liquid levels, and then perform numerical simulation of the flow field to simulate the state change process of LNG in the tank. According to the evolution law under different working conditions, the corresponding generation process of BOG in the membrane tank is obtained, the mass flow rate of BOG is monitored, and the BOG generation under the most unfavorable working condition is obtained.

[0023] The outdoor temperature in step 1 can be obtained by retrieving the meteorological data released by the local meteorological department, and based on the local temperature extremes in different summer and winter seasons in the past.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The present invention simulates the BOG generation inside the LNG membrane storage tank through CFD, thereby introducing a design and calculation method for a gravity wall cooling system specifically for the LNG membrane tank wall. The wall cooling system is applied to the LNG membrane storage tank, which effectively avoids system overpressure, fills the gap in this aspect in the relevant field, and provides a scientific and reliable basis for determining the membrane tank construction method and operation countermeasures for controlling the BOG generation under similar circumstances. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is the saturation temperature curve of LNG;

[0027] Figure 2This is a graph showing the change in BOG mass flow rate at the gas-liquid interface under three different initial liquid level conditions: low, medium, and high in summer and winter.

[0028] Figure 3 This is a schematic diagram of the wall cooling pipe installation;

[0029] Figure 4 It is a schematic diagram of the cross-sectional structure of the wall cooling channel.

[0030] Numbers in the figure: 1. Tank bottom; 2. Cooling pipe; 3. Outer tank; 4. Inner tank; 5. Dome; 6. Cooling pipe outlet; 7. Cooling pipe inlet; 8. Insulation layer between inner and outer tanks. DETAILED DESCRIPTION

[0031] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0032] The present invention provides Figures 1 to 4 A self-flowing wall cooling method for LNG membrane tanks is shown, comprising the following steps:

[0033] Step 1. Use HYSYS to input the molar percentage of the initial components of LNG as methane 99.86%, ethane 0.04%, and nitrogen 0.1%, and calculate the LNG physical properties, namely, density, specific heat, and gas phase saturation temperature versus pressure curves; then establish an LNG membrane tank model in Designbuilder, and obtain the overall heat transfer coefficient of the circumferential and bottom walls based on the double-layer wall heat transfer enclosure structure model of the insulation layer combined with concrete, and then calculate the maximum total cooling and heating loads of the tank in summer and winter based on the outdoor air temperature, thereby obtaining the heat transfer per unit area of ​​the circumferential and bottom walls of the tank, providing thermal boundary conditions for CFD numerical simulation;

[0034] Step 2: Fluent is used to perform numerical simulation of the flow field in the LNG storage tank under six different storage conditions, including summer, winter, low, medium and high initial liquid levels; the relationship between saturated temperature and pressure is obtained by fitting the curve of gas phase saturation temperature versus pressure in step 1, which is used as the input material properties of the simulation, and the heat transfer coefficient at different positions on the wall and the maximum cold and hot load are used as the thermal boundary conditions of the simulation. The VOF model and Lee model are used to simulate the LNG state change process in the membrane tank and the BOG generation process in the tank. The mass flow rate of BOG generated at the gas-liquid interface is monitored according to the evolution law under different working conditions of the six combinations to obtain the BOG generation amount under the most unfavorable working condition;

[0035] Step 3: According to the BOG production under the most unfavorable working conditions, considering that 99% of the BOG produced by evaporation is methane gas, the process of its phase change through the capillary grid is a pure condensation process. The load that the capillary grid needs to provide is equal to the cold load Q required for condensing BOG, that is, the load of its phase change process, which can be obtained by multiplying the BOG production mass flow rate under the most unfavorable working conditions by the latent heat of evaporation of LNG, that is, determined by the following formula:

[0036] Q=m BOG r

[0037] Where Q is the cooling load required for condensing BOG, kW; m BOG is the BOG mass flow rate generated at the gas-liquid interface in the tank during simulation stability, kg / s; r is the latent heat of evaporation of LNG, obtained according to the LNG physical property parameters corresponding to the tank pressure during simulation stability, kJ / kg;

[0038] Step 4: Consider that the heat exchange cold fluid in the capillary grid uses LNG liquid with an inlet temperature of -162°C and an outlet temperature of -161°C, that is, the inlet and outlet temperature difference Δt of the cold fluid is 1K; the LNG liquid density ρ and the LNG liquid specific heat c are selected according to the corresponding temperature of the LNG physical property parameters, considering the commonly used capillary network parameters:

[0039] director Capillary Capillary spacing flow 20×2.0mm 4.3×0.8 mm 10 mm <![CDATA[0.16L / (s.m 2 )]]>

[0040] Therefore, the cooling capacity per unit area Q1 of the capillary grid under this size specification can be calculated by the following formula:

[0041] Q1=V·ρ·c·Δt

[0042] Where Q1 is the cooling capacity per unit area, W / m 2 ; V is the cold fluid flow rate, m 3 / (sm 2 );ρ is the density of LNG liquid, kg / m 3 ; c is the specific heat of LNG liquid, J / (kg.K); Δt is the temperature difference between the inlet and outlet of the cold fluid, K

[0043] After determining the cooling load Q required for condensing BOG and the cooling capacity per unit area of ​​the capillary grid Q1, the required capillary grid laying position can be determined. The capillary grid is combined with the insulation wall of the film tank to form an integrated structure. The required capillary grid area S can be determined using the following formula:

[0044] S=Q / Q1

[0045] Where S is the required capillary grid area, m 2 ; Q is the cooling load required for condensing BOG, kW; Q1 is the cooling capacity per unit area, W / m2 Taking a cylindrical membrane tank with an inner diameter of d meters as an example, the laying height can be determined using the following formula:

[0046]

[0047] Where, h is the capillary grid laying height, m; S is the required capillary grid area, m 2 ; d is the inner diameter of the cylindrical membrane tank, m, that is, it is only necessary to lay the capillary grid along the inner wall surface in the space h meters above where the flash gas is concentrated, and combine the capillary grid with the insulation inner wall surface of the upper space of the membrane tank to form an integrated structure, which can provide the cooling load required for condensing BOG.

[0048] The capillary refrigeration system is suitable for LNG membrane tanks. Compared with 9% nickel steel full-containment tanks, the inner tank only bears the role of liquid tightness and gas tightness. The liquid weight and other loads are transferred to the concrete outer tank through the cold insulation material and borne by the outer tank. In addition, the cold insulation structure of the membrane tank adopts a large amount of off-site prefabrication, and the on-site construction volume is small, which is conducive to the laying of the capillary grid and can be combined with the insulation wall of the membrane tank to form an integrated structure; the insulation layer between the inner tank and the outer tank of the membrane tank usually includes the main shielding film, plywood, reinforced polyurethane foam, stainless steel anchor strips, and moisture-proof layer. The pipe grid can be combined with the integrated refrigeration core layer, embedded between the main shielding film layer and the reinforced polyurethane foam layer, fixed by plywood, and separated from other surface layers by the secondary shielding film. This can not only isolate outdoor heat conduction, reduce the temperature of the inner wall of the tank, and prevent the transfer of radiant heat inside; it can also effectively control the BOG produced at the wall, condense and recover BOG, prevent system overpressure, and ensure safe operation. At the same time, since the temperature difference between the gas phase and the liquid phase inside the storage tank is only 1°C, it also effectively avoids the condensation problem common in capillary refrigeration systems.

[0049] The method for determining the amount of BOG generated under the most unfavorable working condition is: use HYSYS to input the molar percentage of the initial components of LNG, calculate the curves of LNG density, specific heat and gas phase saturation temperature changing with pressure, and then use Fluent software to establish a numerical model of the LNG membrane tank under the storage conditions of the closed tank body in different seasons and different initial liquid levels, and then perform numerical simulation of the flow field to simulate the state change process of LNG in the tank. According to the evolution law under different working conditions, the corresponding generation process of BOG in the membrane tank is obtained, the mass flow rate of BOG is monitored, and the BOG generation under the most unfavorable working condition is obtained.

[0050] The outdoor temperature in step 1 can be obtained by retrieving the meteorological data released by the local meteorological department, and based on the local temperature extremes in different summer and winter seasons in the past.

[0051] (1) Calculate the physical properties of LNG and establish a LNG membrane tank model: The phase change process of LNG to BOG is related to the saturation temperature and saturation pressure of BOG. ​​Since the phase change point is related to the material properties of LNG, in order to establish a realistic liquefied natural gas phase change model as much as possible, the initial properties of LNG are first obtained using the HYSYS simulator. The molar percentages of the initial components of LNG are 99.86% methane, 0.04% ethane, and 0.1% nitrogen. The corresponding molar fractions of each component are entered into the HYSYS simulator, and the PR equation is used as the state equation. After input, the gas phase saturation temperature versus pressure curve is obtained, and the relationship between saturation temperature and pressure is fitted as the input material property for subsequent CFD simulation. After unit correction, the piecewise polynomial input into the FLUENT evaporation and condensation model is obtained, and its R2 = 0.999:

[0052] T sat =1.10e -18 p 3 -1.12e -11 p 2 +4.52e -5 p+112.52In the formula: Tsat is the saturation temperature, K; p is the saturation pressure, kPa.

[0053] Next, by establishing a tank enclosure model in the Designbuilder software, the tank model is simplified into a cylindrical model. The size of the calculation domain is 86 meters in diameter and 43 meters in height. The heat transfer of the enclosure structure is one-dimensional steady-state heat transfer. For multi-layer wall heat transfer, in order to calculate the overall heat transfer coefficient U, it is necessary to consider the convection heat transfer from the surrounding air to the outer wall, the conduction through the multi-layer insulation wall and the internal convection to the inside of the tank, and obtain the corresponding wall properties, that is, the heat transfer coefficients of the circumferential wall and the bottom wall are 0.084 and 0.088W / m2.K respectively. The outer tank is set in an atmospheric environment, and the ambient pressure is atmospheric pressure. Considering solar radiation heat, corrections are made for different orientations. The outdoor air temperature and humidity and solar irradiance select the hourly parameters of a typical year in the local area. The initial temperature under the condition of LNG injection into the tank is -161℃. Through calculation, the maximum total cooling and heating loads of the tank in summer and winter are 688.55kW and 532.74kW respectively, which are used as thermal boundary conditions for subsequent CFD simulation.

[0054] (2) Numerical simulation of the flow field in the LNG storage tank under different storage conditions was performed to calculate the BOG generation under the most unfavorable conditions: static conditions were considered, that is, a closed tank. The heat inlet into the tank includes heat transfer through the side walls, top plate and bottom plate, as well as heat leakage between the dome and the top plate. The heat Q entering through the tank side walls, top plate and bottom plate depends on the temperature of the surrounding air, the wall insulation properties and the liquid level in the tank. Since the external temperature and the initial liquid level will have a great influence on the phase change process, the conditions of different seasons and different initial liquid levels were considered. The initial liquid levels were set to high liquid level, middle liquid level and low liquid level, respectively. The high liquid level is 9 / 10 liquid level, the middle liquid level is 1 / 2 liquid level, and the low liquid level is 1 / 3 liquid level. Fluent software was used to simulate the state change process of LNG in the tank. According to the evolution law under different working conditions, the corresponding generation process of BOG in the membrane tank was obtained, and the BOG generation under the most unfavorable conditions was obtained. In Fluent software, the VOF model is used to study the gas-liquid two-phase flow and mass transfer process of the fluid in the tank and track the gas-liquid interface. For the evaporation and condensation model, the Lee model is used to consider the phase change including the influence of static pressure. The modeling and meshing use a structured grid composed of quadrilateral units to provide higher resolution and better convergence. A two-dimensional axisymmetric model is used to sweep the flow domain 360° around the rotation axis to obtain simulation results with three-dimensional geometric characteristics. The initial conditions of the temperature field are -160℃ in the gas phase and -161℃ in the liquid phase, and the boundary values ​​are determined according to the working conditions. The Realizable k-ε model and standard wall function are used. The parameters of each field quantity in the control equation use the second-order upwind format to improve the accuracy. For the spatial discretization scheme, the PRESTO scheme is used for pressure, the PISO is selected for the pressure-velocity coupling scheme, and the Geo-reconstruct scheme is used for the volume fraction. For the residuals of the continuity equation, momentum equation, volume fraction equation and turbulence equation, the convergence criterion is set to 10-3. The energy equation adopts a convergence criterion of 10-6.

[0055] The cross-sectional temperatures at 10s, 1h, 2h, 3h, 6h, 8h, 12h, 18h, and 24h were selected for observation under different working conditions. By analyzing the temperature field in the liquid at different times, the evaporation phenomenon in LNG can be seen. When the initial liquid level is high, the temperature and liquid level change more drastically, which will increase the evaporation rate of BOG. ​​There are significant differences in the amount of BOG generated at each moment under different working conditions, but the common point is that the initial changes are drastic, and after 3 to 4 hours of simulation, it gradually tends to be stable, with a small fluctuation amplitude. When stable, the temperature difference between the gas phase and the liquid phase is about 1°C. The temperature of the gas phase in winter is slightly lower than that in summer. The overall values ​​of different working conditions in winter are less than the corresponding values ​​under summer conditions. Among all working conditions, the temperature field changes most drastically when the initial LNG liquid level is high in summer, which will lead to more obvious liquid level changes and evaporation, resulting in an increase in the amount of BOG generated, which can be used as the most unfavorable working condition during design. The mass flow rate of BOG generated under this working condition is about 0.5kg / s when stable.

[0056] (3) According to the BOG production under the most unfavorable working conditions, determine the cooling load required for condensing BOG: Under high liquid level conditions in summer, the BOG production is about 0.5kg / s when stable, and the pressure in the tank is about 0.5MPa. 99% of the BOG produced by evaporation is methane gas, and the process of phase change through the capillary grid is a pure condensation process. The heat load calculation formula is:

[0057] Q=m BOG r

[0058] Where Q is the cooling load required for condensing BOG, kW; m BOG is the BOG mass flow rate generated at the gas-liquid interface in the tank during simulation stability, which is about 0.5kg / s; r is the latent heat of evaporation of LNG, which is obtained according to the LNG physical properties corresponding to the tank pressure during simulation stability, and is taken as 503.126kJ / kg.

[0059] Q=(0.5×503.126)kW=251.563kW

[0060] (4) According to the required cooling load, the cold fluid flow rate Q1, the number of cooling channels N and the pipe radius R are determined, thereby obtaining the fluid flow rate v. The cooling channel pipe and the membrane tank insulation wall are combined to form an integrated structure: the heat exchange cold fluid in the cooling channel uses LNG liquid, with an inlet temperature of -162°C and an outlet temperature of -161°C. The required cold fluid flow rate is calculated as follows:

[0061]

[0062] Wherein, Q1 is the required cold fluid flow rate, m3 / s; Q is the cold load required for condensing BOG, W; ρ is the density of LNG liquid, which is 422.05kg / m3 at this temperature; c is the specific heat of LNG liquid, which is 3481J / (kg.K) at this temperature; Δt is the temperature difference between the inlet and outlet of the cold fluid, i.e. 1K.

[0063]

[0064] In order to ensure the uniformity of heat exchange in the tank, N cooling channels are evenly laid along the inner wall of the tank. The radius of each cooling channel is R, and the flow rate v of the cold fluid in each channel can be calculated as follows:

[0065]

[0066] Where v is the velocity of the cold fluid in each pipe, m / s; Q1 is the required cold fluid flow, m 3 / s; N is the number of cooling channels; R is the radius of each cooling channel pipe, m.

[0067] Taking a cylindrical membrane tank with an inner diameter of 43 meters and a height of 40 meters as an example, assuming that 8 cooling channels are evenly arranged along the inner wall, and the radius of each cooling channel is 0.1m, the flow rate of the cold fluid in each channel can be calculated

[0068] The insulation layer between the inner tank and the outer tank of the film tank usually includes the main shielding film, plywood, reinforced polyurethane foam, stainless steel anchor strips, and moisture-proof layer. The cooling pipe can be fixed to the inner wall, which can not only isolate the outdoor heat conduction, reduce the temperature of the inner wall of the tank, and prevent the transfer of radiant heat inside; it can also effectively control the BOG produced at the wall, condense and recover BOG, prevent system overpressure, and ensure safe operation. At the same time, due to the temperature difference of 1°C between the gas phase and the liquid phase in the storage tank, it also effectively avoids the condensation problem common in the wall cooling system.

[0069] The present invention simulates the BOG generation inside the LNG membrane storage tank through CFD, thereby introducing a design and calculation method for a gravity wall cooling system specifically for the LNG membrane tank wall. The wall cooling system is applied to the LNG membrane storage tank, which effectively avoids system overpressure, fills the gap in this aspect in the relevant field, and provides a scientific and reliable basis for determining the membrane tank construction method and operation countermeasures for controlling the BOG generation under similar circumstances.

[0070] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A self-flowing wall cooling method for LNG membrane tanks, characterized in that: The steps include: Step 1. Use HYSYS to input the molar percentage of the initial components of LNG as methane 99.86%, ethane 0.04%, and nitrogen 0.1%, and calculate the LNG physical properties, namely, density, specific heat, and gas phase saturation temperature versus pressure curves; then establish an LNG membrane tank model in Designbuilder, and obtain the overall heat transfer coefficient of the circumferential and bottom walls based on the double-layer wall heat transfer enclosure structure model of the insulation layer combined with concrete, and then calculate the maximum total cooling and heating loads of the tank in summer and winter based on the outdoor air temperature, thereby obtaining the heat transfer per unit area of ​​the circumferential and bottom walls of the tank, providing thermal boundary conditions for CFD numerical simulation; Step 2: Fluent is used to perform numerical simulation of the flow field in the LNG storage tank under six different storage conditions, including summer, winter, low, medium and high initial liquid levels; the relationship between saturated temperature and pressure is obtained by fitting the curve of gas phase saturation temperature versus pressure in step 1, which is used as the input material properties of the simulation, and the heat transfer coefficient at different positions on the wall and the maximum cold and hot load are used as the thermal boundary conditions of the simulation. The VOF model and Lee model are used to simulate the LNG state change process in the membrane tank and the BOG generation process in the tank. The mass flow rate of BOG generated at the gas-liquid interface is monitored according to the evolution law under different working conditions of the six combinations to obtain the BOG generation amount under the most unfavorable working condition; Among all the working conditions, the temperature field changes most dramatically when the LNG initial liquid level is high in summer, which will lead to obvious liquid level changes and evaporation, increasing the amount of BOG generated. Therefore, this working condition is the most unfavorable working condition during design. Step 3: According to the BOG production under the most unfavorable working conditions, considering that 99% of the BOG produced by evaporation is methane gas, the process of its phase change through the capillary grid is a pure condensation process. The load that the capillary grid needs to provide is equal to the cold load Q required for condensing BOG, that is, the load of its phase change process, which can be obtained by multiplying the BOG production mass flow rate under the most unfavorable working conditions by the latent heat of evaporation of LNG, that is, determined by the following formula: Q=m BOG r Where Q is the cooling load required for condensing BOG, kW; m BOG is the BOG mass flow rate generated at the gas-liquid interface in the tank during simulation stability, kg / s; r is the latent heat of evaporation of LNG, obtained according to the LNG physical property parameters corresponding to the tank pressure during simulation stability, kJ / kg; Step 4: Consider that the heat exchange cold fluid in the capillary grid uses LNG liquid with an inlet temperature of -162°C and an outlet temperature of -161°C, that is, the inlet and outlet temperature difference Δt of the cold fluid is 1K; the LNG liquid density ρ and the LNG liquid specific heat c are selected according to the corresponding temperature of the LNG physical property parameters, and the capillary network parameters are: main tube 20×2.0mm, capillary 4.3×0.8mm, capillary spacing 10mm, flow rate 0.16L / (sm 2 ); Therefore, the cooling capacity per unit area of ​​the capillary grid Q1 can be calculated by the following formula: Q1=V·ρ·c·Δt Where Q1 is the cooling capacity per unit area, W / m 2 ; V is the cold fluid flow rate, m 3 / (sm 2 );ρ is the density of LNG liquid, kg / m 3 ; c is the specific heat of LNG liquid, J / (kg.K); Δt is the inlet and outlet temperature difference of the cold fluid, K. After determining the cold load Q required for condensing BOG and the refrigeration capacity per unit area of ​​the capillary grid Q1, the required capillary grid laying position can be determined. The capillary grid is combined with the insulation wall of the membrane tank to form an integrated structure. The required capillary grid area S can be determined using the following formula: S=Q / Q1 Where S is the required capillary grid area, m 2 ; Q is the cooling load required for condensing BOG, kW; Q1 is the cooling capacity per unit area, W / m 2 For a cylindrical membrane tank with an inner diameter of d meters, the laying height can be determined using the following formula: Where, h is the capillary grid laying height, m; S is the required capillary grid area, m 2 ; d is the inner diameter of the cylindrical membrane tank, m, that is, it is only necessary to lay the capillary grid along the inner wall surface in the space h meters above where the flash gas is concentrated, and combine the capillary grid with the insulation inner wall surface of the upper space of the membrane tank to form an integrated structure, which can provide the cooling load required for condensing BOG.

2. The method for self-flowing wall cooling applied to LNG membrane tanks according to claim 1 is characterized in that: The method for determining the amount of BOG generated under the most unfavorable working condition is: use HYSYS to input the molar percentage of the initial components of LNG, calculate the curves of LNG density, specific heat and gas phase saturation temperature changing with pressure, and then use Fluent software to establish a numerical model of the LNG membrane tank under the storage conditions of the closed tank body in different seasons and different initial liquid levels, and then perform numerical simulation of the flow field to simulate the state change process of LNG in the tank. According to the evolution law under different working conditions, the corresponding generation process of BOG in the membrane tank is obtained, the mass flow rate of BOG is monitored, and the BOG generation under the most unfavorable working condition is obtained.

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