Marine lng flash vaporization liquefaction system and method

Abstract

The application discloses a liquefied natural gas flash gas liquefaction system and method for a ship and relates to the technical field of liquefied natural gas storage. The following scheme is provided, which comprises a nitrogen expansion refrigeration unit and further comprises a liquefaction heat exchange mechanism, wherein the liquefaction heat exchange mechanism comprises a plate heat exchange assembly and a circulating flow channel adjusting mechanism; the plate heat exchange assembly comprises a heat exchange plate group composed of alternately superimposed first heat exchange plates and second heat exchange plates; the circulating flow channel adjusting mechanism comprises a synchronous frame capable of moving along a direction parallel to plate superimposition, a valve pipe fixed to the synchronous frame and arranged in correspondence with the angular hole channel, and a driving unit for driving the synchronous frame to move. By means of mechanical adjustment, dynamic changes in BOG load are matched in real time, so that the system can be operated close to an optimal energy efficiency point in the whole working condition range, and operation energy consumption is reduced.

Description

Technical Field

[0001] This invention relates to the field of liquefied natural gas (LNG) storage technology, and more particularly to a marine LNG flash liquefaction system and method. Background Technology

[0002] On ships that use LNG as fuel or for transport, LNG in storage tanks will inevitably generate BOG (Blank Gas) due to external heat intrusion. To ensure the safety of the tank pressure, especially in port operations where the main engine is shut down and during loading and unloading, BOG needs to be condensed and recovered through a reliquefaction system. Existing marine reliquefaction systems typically use independent cycles such as nitrogen expansion refrigeration to provide cooling capacity, and achieve heat exchange between BOG and refrigerant through a main cryogenic heat exchanger with a fixed structure (such as a brazed plate heat exchanger).

[0003] However, in actual ship operation, the rate of BOG generation is affected by various factors such as ambient temperature, sea state, and operational status (e.g., loading and unloading), exhibiting drastic and frequent fluctuations. The heat exchange area and flow channel characteristics of the fixed-structure main cryogenic heat exchanger cannot be changed after design and manufacturing, resulting in inherent defects in the system when facing a wide range of variable load conditions. When the BOG is at low load, the heat exchanger has excess capacity, forcing the large refrigeration unit to operate in the inefficient zone or to start and stop frequently, resulting in energy waste. When the BOG is under high load or the load increases suddenly, the fixed heat exchange capacity becomes a bottleneck, the system response is slow, and the tank pressure may approach the safety limit, which may bring risks. Existing technologies typically rely on adjusting the power of the refrigeration unit to passively adapt to the fixed downstream heat exchange capacity. This is a solution with low overall energy efficiency and slow response. Therefore, there is an urgent need for a technology that enables the heat exchanger itself to have the ability to adaptively adjust its operating conditions, so as to fundamentally improve the operating efficiency and safety of the reliquefaction system. Summary of the Invention

[0004] In view of this, the purpose of the present invention is to solve the above-mentioned problems.

[0005] To achieve the above-mentioned technical objectives, the present invention provides a marine liquefied natural gas flash liquefaction system: It includes nitrogen expansion refrigeration units, and also includes: A liquefaction heat exchange mechanism, comprising a plate heat exchange assembly and a circulation channel adjustment mechanism; The plate heat exchange assembly includes a heat exchange plate group composed of alternating stacked first heat exchange plates and second heat exchange plates, and a first side sealing plate and a second side sealing plate respectively pressed against both sides of the heat exchange plate group. The first side sealing plate and the second side sealing plate are provided with openings at the corner hole channels corresponding to the heat exchange plate group. The circulating flow channel adjustment mechanism includes a timing frame that can move parallel to the plate stacking direction, a valve pipe fixed on the timing frame and corresponding to the corner hole channel, and a drive unit that drives the timing frame to move.

[0006] Preferably, the drive unit includes a support frame fixed to the first side sealing plate, a motor mounted on the support frame, a lead screw driven by the motor, and a threaded sleeve threaded to the lead screw and fixedly connected to the synchronization frame; the support frame is provided with a guide rod, and the synchronization frame is slidably connected to the guide rod.

[0007] Preferably, one end of the valve tube is provided with a valve head, and the valve head is fitted with an axially sliding sealing seat, and the sealing seat is provided with a sealing ring; a pull rod is provided inside the valve tube, and the pull rod is connected to the sealing seat through a transmission mechanism to drive the sealing seat to move axially, so that the sealing ring is pressed against or disengaged from the valve head.

[0008] Preferably, the valve head is provided with a tapered sealing surface; the transmission mechanism includes a connecting rod hinged to the pull rod and a pressure rod hinged to the sealing seat, the middle part of the pressure rod being hinged to the valve head.

[0009] Preferably, the nitrogen expansion refrigeration unit includes a compressor, a cooler, a heat exchanger, and a turbine expander distributed along the nitrogen cooling process path.

[0010] Preferably, it also includes a controller and a monitoring unit; The monitoring unit includes a pressure sensor for monitoring the pressure of the liquefied natural gas storage tank, a flow meter for monitoring the mass flow rate of flash vapor, and a sensor for monitoring the operating current and speed of the compressor. The controller is communicatively connected to the monitoring unit and the drive unit of the circulating flow channel adjustment mechanism. The controller calculates a dynamic safety margin value based on the data collected by the monitoring unit, and controls the drive unit to adjust the insertion depth of the valve tube according to the different threshold ranges in which the dynamic safety margin value is located.

[0011] A method for flash liquefaction of marine liquefied natural gas based on the above, the method comprising: Real-time pressure of liquefied natural gas storage tanks, real-time flow rate of flash vapor, and real-time operating parameters of nitrogen compressors are collected. Based on the real-time pressure, real-time flow, and real-time operating parameters, a dynamic safety margin value characterizing the current safety buffer capacity of the system is calculated. Based on the comparison result between the dynamic safety margin value and the preset safety margin threshold, the insertion depth of the valve tube in the circulating flow channel adjustment mechanism is controlled to adjust the effective heat exchange area of ​​the plate heat exchange assembly.

[0012] Preferably, the method for calculating the dynamic safety margin value includes: The pressure change trend is calculated based on real-time pressure, the relative operating efficiency of the compressor is evaluated based on real-time operating parameters, and the pressure change trend and relative operating efficiency are used as penalty terms and deducted from the basic margin based on the current pressure and the upper limit of the safe pressure to obtain the dynamic safety margin value.

[0013] Preferably, the preset safety margin threshold is an interval threshold, including a first threshold and a second threshold, and the first threshold > the second threshold ≥ 0.

[0014] Preferably, the step of controlling based on the comparison result of the dynamic safety margin value and the preset threshold includes: When the dynamic safety margin value is higher than the first threshold, it is determined to be in economic mode, and a first control command is generated; When the dynamic safety margin value is lower than the first threshold but higher than the second threshold, it is determined to be in enhanced mode, and a second control command is generated; When the dynamic safety margin value is lower than the second threshold, it is determined to be in a safe mode, and a third control command is generated.

[0015] As can be seen from the above technical solutions, this application has the following beneficial effects: 1. By mechanically adjusting and matching the dynamically changing BOG load in real time, the number of working plates is reduced at low loads to lower system flow resistance and cooling power consumption; at high loads, the number of working plates is increased to fully tap the heat exchange potential and avoid inefficient operation of the refrigeration unit under partial loads, so that the system can operate close to the optimal energy efficiency point in the entire operating range, thus reducing operating energy consumption. 2. By calculating and comprehensively reflecting the dynamic safety margin of pressure status, changing trends, and equipment efficiency in real time, risks can be predicted in advance. Based on this decision, the effective working area of ​​the heat exchanger can be proactively adjusted, changing the traditional "passive response after pressure exceeds the limit" to "proactive intervention before risk accumulation," effectively suppressing abnormal increases in tank pressure and reducing the risk of overpressure. 3. Not only can it adapt to fluctuations in external BOG loads, but it can also sense and compensate for the impact on the overall system capability when the performance of key built-in equipment, such as compressors, deteriorates. This adaptive capability significantly improves the stability and reliability of the system under different sea conditions and equipment statuses. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in the embodiments of the present invention 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 embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0017] Figure 1 A schematic diagram of the overall structure of a marine liquefied natural gas flash liquefaction system provided by the present invention; Figure 2 A top view of a marine liquefied natural gas flash liquefaction system provided by the present invention; Figure 3 A schematic diagram of the overall structure of the liquefaction heat exchange mechanism of a marine liquefied natural gas flash liquefaction system provided by the present invention; Figure 4 A schematic diagram of the overall structure of the first and second heat exchange plates of a marine liquefied natural gas flash liquefaction system provided by the present invention. Figure 5 A schematic diagram of the overall structure of the circulation channel regulating mechanism of a marine liquefied natural gas flash liquefaction system provided by the present invention; Figure 6 A schematic diagram of the overall structure of the valve pipe of a marine liquefied natural gas flash liquefaction system provided by the present invention; Figure 7 This invention provides a partial cross-sectional view of the valve head of a marine liquefied natural gas flash liquefaction system. Figure 8 This is a cross-sectional view of the valve head of a marine liquefied natural gas flash liquefaction system provided by the present invention.

[0018] Explanation of reference numerals in the attached figures: 10. Nitrogen expansion refrigeration unit; 11. Compressor; 12. Cooler; 13. Heat exchanger; 14. Turbine expander; 20. Liquefaction heat exchange mechanism; 21. Plate heat exchange assembly; 211. First side sealing plate; 212. Second side sealing plate; 213. First heat exchange plate; 2131. Through hole; 2132. Flow diversion zone; 2133. Heat exchange zone; 2134. Flow channel seal; 214. Second heat exchange plate; 215. Fixing rod; 22. Circulation flow channel adjustment mechanism; 221. Valve pipe; 2211. Valve head; 2212. Sealing seat; 2213. Sealing ring; 2214. Pressure rod; 2215. Connecting rod; 2216. Pull rod; 222. Synchronization frame; 223. Support frame; 224. Screw; 225. Motor; 226. Screw sleeve; 227. Guide rod; 228. Synchronization bar. Detailed Implementation

[0019] The following description is exemplary in nature and is not intended to limit the scope, application, or use of this disclosure. It should be understood that in all these figures, the same or similar reference numerals indicate the same or similar parts and features. The figures are merely schematic representations of the concept and principles of embodiments of this disclosure and do not necessarily show the specific dimensions and scale of the various embodiments of this disclosure. Certain details or structures of embodiments of this disclosure may be exaggerated in particular portions of certain figures.

[0020] Example 1, see Figures 1-8 As shown, a marine liquefied natural gas flash vapor liquefaction system includes a nitrogen expansion refrigeration unit 10 and a liquefaction heat exchange mechanism 20. Nitrogen gas enters the nitrogen expansion refrigeration unit 10 and is cooled, serving as the cooling medium for liquefied natural gas flash vapor heat exchange. The liquefied natural gas flash vapor is BOG. ​​The cooled liquid nitrogen and the liquefied natural gas flash vapor exchange heat in the liquefaction heat exchange mechanism 20, causing the liquefied natural gas flash vapor to liquefy. The liquefied natural gas is LNG, and the LNG is returned to the LNG storage tank for storage. Specifically, the nitrogen expansion refrigeration unit 10 includes a compressor 11, a cooler 12, a heat exchanger 13, and a turboexpander 14 distributed along the nitrogen cooling process path. For example, nitrogen gas flows sequentially through the compressor 11, is compressed and heated, is cooled to room temperature by the cooler 12, and then pre-cooled by the regenerating heat exchanger 13 before entering the turboexpander 14 for adiabatic expansion, thereby obtaining the required cryogenic nitrogen gas. This cryogenic nitrogen gas is transported to the liquefaction heat exchange mechanism 20 as a cooling medium. After completing the cooling of the BOG (Boiled Oxide Gas), it returns to the regenerating heat exchanger 13 for reheating and finally returns to the compressor 11 inlet, forming a complete closed-loop refrigeration cycle. This is known and publicly available technology and will not be elaborated upon here.

[0021] Furthermore, the liquefaction heat exchange mechanism 20 includes a plate heat exchange assembly 21 and a circulation channel adjustment mechanism 22 for adjusting the total length of the internal circulation channel of the plate heat exchange assembly 21. The plate heat exchange assembly 21 includes a heat exchange plate group and a first side sealing plate 211 and a second side sealing plate 212 sealed and fixed on both sides of the heat exchange plate group. The first side sealing plate 211 and the second side sealing plate 212 are fixed together by a fixing rod 215. The heat exchange plate group is formed by alternating stacking of a plurality of first heat exchange plates 213 and second heat exchange plates 214. Both ends are pressed and sealed by the first side sealing plate 211 and the second side sealing plate 212 through the fixing rod 215. Each heat exchange plate has through holes 2131 at its four corners. When all the plates are stacked, they form four through-hole channels. The key difference between the first heat exchange plate 213 and the second heat exchange plate 214 is the corrugated flow channel on its surface. The corrugated flow channel includes a flow distribution area 2132 and a heat exchange area 2133. The corner holes connected by the two are complementary. For example, the flow channel of the first heat exchange plate 213 connects the upper left and lower right corner holes, while the flow channel of the second heat exchange plate 214 connects the lower left and upper right corner holes. The plates are sealed with flow channel seals 2134. The purpose is to naturally form two completely independent fluid channel networks inside the stacked heat exchange plate group, which are only heat-conducted through the metal plate walls. One network is used for the flow of BOG, and the other network is used for the flow of cryogenic nitrogen.

[0022] It is worth mentioning that, in order to realize the flow channel adjustment function, the first side sealing plate 211 and the second side sealing plate 212 in this embodiment are both provided with connection ports through the four corner holes; Specifically, the circulating flow channel adjustment mechanism 22 is used to dynamically change the effective flow channel length of the heat exchange plate group. It includes a synchronous frame 222 that moves parallel to the side of the heat exchange plate group. The synchronous frame 222 is driven by a drive unit fixed to the outside of the first side sealing plate 211. The drive unit includes a support frame 223, a motor 225, a lead screw 224 driven by the motor 225, and a threaded sleeve 226 that meshes with the lead screw 224 and is fixed on the synchronous frame 222. At the four corners of the synchronous frame 222, four internally penetrating valve pipes 221 are fixed, and their positions are aligned with the four corner holes of the heat exchange plate group.

[0023] For example, when the motor 225 starts, the lead screw 224 rotates, driving the lead sleeve 226 and the synchronous frame 222 fixed thereto to move linearly along the guide rod 227 on the support frame 223; the movement of the synchronous frame 222 will drive all valve tubes 221 to adjust their positions synchronously in the corresponding corner holes. The insertion depth of the valve tube 221 directly determines how many flow channels of the first heat exchange plate 213 and the second heat exchange plate 214 are connected to the BOG flow and nitrogen flow channel loop from the corner hole inlet, thereby realizing the adjustment of the effective heat exchange area; that is, the corner hole channel closer to the first side sealing plate 211 after the insertion point is isolated, and the corresponding heat exchange plate flow channel stops working because there is no fluid inflow. Therefore, by controlling the insertion depth of the valve tube 221, the number of plates actually participating in the heat exchange process can be continuously adjusted.

[0024] To ensure the sealing reliability of valve pipe 221 during long-term operation, a valve head 2211 is fixed at the front end of valve pipe 221. A slidable sealing seat 2212 is provided on the valve head 2211. A conical sealing surface is provided on the side of valve head 2211 facing the sealing seat 2212. A sealing ring 2213 is fitted and fixed on the sealing seat 2212 facing the conical sealing surface. Furthermore, a pull rod 2216 is slidably connected inside the valve pipe 221. A transmission mechanism is provided at the end of the pull rod 2216. The transmission mechanism includes a connecting rod 2215 and a pressure rod 2214, and is connected to the sealing seat 2212 through the connecting rod 2215 and the pressure rod 2214. Specifically, a barrel groove is opened through the outer surface of the valve head 2211. The middle part of the pressure rod 2214 is hinged to the barrel groove of the valve head 2211. The two ends of the pressure rod 2214 are respectively hinged to the sealing seat 2212 and the connecting rod 2215. The other end of the connecting rod 2215 is hinged to the end of the pull rod 2216. For example, when the valve pipe 221 needs to be moved to adjust its position, the pull rod 2216 moves towards the sealing seat 2212. Through the transmission of the connecting rod 2215 and the pressure rod 2214, the sealing seat 2212 is moved backward away from the conical sealing surface. At this time, the sealing ring 2213 is in a relaxed state, its inner diameter shrinks, and it is no longer in contact with the inner wall of the corner hole, thus avoiding wear during the movement. When the valve pipe 221 reaches the target position, the pull rod 2216 is pulled in the opposite direction, causing the sealing seat 2212 to move forward. The sealing ring 2213 is radially squeezed and expanded by the conical sealing surface of the valve head 2211, thus tightly fitting the inner wall of the corner hole and achieving a reliable static seal for high-pressure fluid.

[0025] It is worth mentioning that the ends of two adjacent pull rods 2216 are fixed with a timing bar 228, which is driven by a small hydraulic cylinder, electric push rod or pneumatic cylinder to drive the pull rod 2216 to move. The specific driving method is not specifically limited here.

[0026] Example 2: Based on the above examples, a marine liquefied natural gas flash liquefaction system further includes a controller and a monitoring unit. The monitoring unit is used to collect dynamic data. The monitoring unit includes a first acquisition module, a second acquisition module, and a third acquisition module, wherein each module and the controller are connected by wired and / or wireless means. The first acquisition module is a pressure sensor installed in the vapor space of the LNG storage tank, used to monitor the LNG storage tank pressure value in real time; the second acquisition module is a thermal mass flow meter installed on the BOG outlet pipeline of the LNG storage tank, used to monitor the BOG mass flow value in real time; the third acquisition module is a current sensor and a speed sensor installed on the compressor 11, used to acquire the operating current and speed of the compressor 11, respectively; the real-time pressure value is marked as P, the mass flow value is marked as G, and the current and speed are marked as I and N, respectively.

[0027] The controller continuously receives dynamic data collected by the monitoring unit and executes a preset control algorithm based on the dynamic data to generate corresponding control commands. The controller sends the control commands to the drive unit of the circulating flow channel regulating mechanism 22, namely the motor 225, and the nitrogen expansion refrigeration unit 10 through the control bus or drive circuit, thereby coordinating the insertion position of the control valve pipe 221 and the operating power of the refrigeration unit 10 to achieve intelligent regulation of the liquefaction process.

[0028] Example 3: Based on the above examples, a method for flash liquefaction of marine liquefied natural gas includes: S1, real-time acquisition of dynamic data, including real-time pressure value P, mass flow rate value G, real-time current value I, and real-time rotational speed value N; S1, based on dynamic data, calculates the pressure change rate and the compressor relative efficiency coefficient using formulas, and is denoted as follows: and ; S3, based on and The dynamic safety margin is assessed and denoted as M; S4, calculate the dynamic safety margin The system compares the data with a preset safety margin threshold, determines and generates a corresponding control strategy, and then executes the corresponding control strategy.

[0029] Specifically, the pressure change rate characterizes the instantaneous trend of pressure change in the storage tank, and is obtained by numerically differentiating the real-time pressure sequence, i.e. In the discrete sampling system of the controller, the differential method is used for implementation, specifically as follows: ,in and The instantaneous pressure value of adjacent sampling periods , For a fixed sampling period; relative efficiency coefficient To characterize the real-time efficiency of the nitrogen-generating expansion refrigeration unit 10, firstly, based on the real-time current... Real-time rotation speed And the pre-stored compressor performance mapping relationship, to estimate the compressor's current isentropic efficiency, which is denoted as... Subsequently, the ratio of this ratio to the rated efficiency is calculated, with the rated efficiency denoted as... ,Right now ; The value represents the current safety buffer capacity of the liquefaction system; the lower the value, the higher the risk. (Dynamic safety margin) The calculation formula is: In the formula, The maximum safe operating pressure is preset according to the design specifications of the storage tank; The maximum permissible rate of pressure change is preset according to the system safety response requirements; and These are preset weighting coefficients used to adjust the contribution of pressure change trends and equipment performance degradation to safety assessments, respectively. Specific values ​​can be determined through simulation and debugging based on the actual ship system.

[0030] The specific method for determining the control strategy is as follows: Dynamic safety margin Compared with a preset safety margin threshold, which is an interval threshold, the two thresholds are labeled as follows: and , > ≥0, the dynamic safety margin threshold shall be determined by those skilled in the art based on the actual situation, and no specific limitation shall be made here; like If the system is determined to be in economic mode, a first control command is generated. The first control command includes controlling the circulating flow channel regulating mechanism 22 to drive the valve pipe 221 to move to a first preset position so that the minimum number of heat exchange plates are put into operation; at the same time, controlling the nitrogen expansion refrigeration unit 10 to operate at a first preset power. like If the system is determined to be in enhanced mode, a second control command is generated. The second control command includes controlling the circulating flow channel regulating mechanism 22 to drive the valve pipe 221 to move to a second preset position, so that the significantly increased heat exchange plates are put into operation to improve the system's heat exchange capacity; at the same time, the nitrogen expansion refrigeration unit 10 can be controlled to operate at a second preset power. like If the system is determined to be in safe mode, a third control command is generated. The third control command includes controlling the circulating flow channel regulating mechanism 22 to drive the valve pipe 221 to move to the third preset position, so that all heat exchange plates are put into operation; at the same time, controlling the nitrogen expansion refrigeration unit 10 to operate at the third preset power to provide maximum liquefaction capacity and ensure the safety of the storage tank.

[0031] It should be noted that the first preset position < the second preset position < the third preset position, the first preset power < the second preset power < the third preset power, and the first preset position, the second preset position, the third preset position, the first preset power, the second preset power and the third preset power are all determined by those skilled in the art based on the actual situation, and are not specifically limited here.

[0032] For example, in scenario one, if the machine is parked normally and the environment is mild: at this time, BOG generation is stable, the pressure is stable or slowly rising, the Vp value is small, the compressor is operating normally, and the calculated M value is higher than the preset safety margin threshold. The system determines it to be in "economic mode". The controller instructs the motor 225 to keep the valve pipe 221 in a shallow insertion depth, for example, only 30% of the heat exchange plates at the front end are used. At this time, the total length of the flow channel is short, the flow resistance is small, and the BOG processing load and the power of the refrigeration unit 10 can be maintained at a low level, so as to achieve energy-saving operation. Scenario 2: If a sunny day suddenly turns into intense sunlight, or if the ship encounters strong winds and waves that intensify hull rolling, the external heat load increases sharply, or liquid sloshing exacerbates evaporation, leading to a surge in BOG formation and a rapid increase in pressure (Vp). This causes the M value to drop rapidly to the preset safety margin threshold, i.e., the warning range. The system immediately enters "enhanced mode". The controller instructs the motor 225 to drive the valve tube 221 to push deeper into the plate bundle, and activate more heat exchange plates, such as increasing to 70%. The expansion of the effective heat exchange area significantly improves the instantaneous liquefaction capacity of the system, thereby actively and timely curbing the rapid rise of tank pressure and mitigating the safety risks caused by sudden changes in operating conditions. Scenario 3: If the equipment performance degrades after prolonged high-temperature operation, or if extreme and harsh operating conditions accumulate, and if the compressor 11 experiences a performance decline due to prolonged operation, or if the load is excessively high under extreme conditions, causing the M value to fall below the preset safety margin threshold, i.e. The system immediately switches to "safe mode," and the controller instructs valve pipe 221 to be fully inserted to activate all heat exchange plates. At the same time, the nitrogen expansion refrigeration unit 10 is increased to its maximum power to prioritize the safety of the storage tank pressure with the maximum capacity of the entire system.

[0033] It is worth mentioning that, in addition, the controller records the dominant factors that cause the M value to decrease. If long-term data analysis shows that most warnings are triggered by an excessively low κ value, the system will generate a predictive maintenance prompt that suggests checking the performance of compressor 11, thus extending control to health management.

[0034] The exemplary implementation of the solution proposed in this disclosure has been described in detail above with reference to preferred embodiments. However, those skilled in the art will understand that various modifications and alterations can be made to the above specific embodiments without departing from the spirit of this disclosure, and various combinations can be made to the various technical features and structures proposed in this disclosure without exceeding the protection scope of this disclosure, which is determined by the appended claims.

Claims

1. A marine liquefied natural gas flash liquefaction system, comprising a nitrogen expansion refrigeration unit (10), characterized in that, Also includes: The liquefaction heat exchange mechanism (20) includes a plate heat exchange assembly (21) and a circulation channel adjustment mechanism (22). The plate heat exchange assembly (21) includes a heat exchange plate group consisting of alternating stacked first heat exchange plates (213) and second heat exchange plates (214), and a first side sealing plate (211) and a second side sealing plate (212) respectively pressed on both sides of the heat exchange plate group. The first side sealing plate (211) and the second side sealing plate (212) are provided with openings at the corner hole channels of the heat exchange plate group. The circulating flow channel adjustment mechanism (22) includes a timing frame (222) that can move along a direction parallel to the plate stacking direction, a valve pipe (221) fixed on the timing frame (222) and corresponding to the corner hole channel, and a drive unit that drives the timing frame (222) to move.

2. The marine liquefied natural gas flash liquefaction system according to claim 1, characterized in that, The drive unit includes a support frame (223) fixed on the first side sealing plate (211), a motor (225) mounted on the support frame (223), a lead screw (224) driven by the motor (225), and a threaded sleeve (226) threadedly connected to the lead screw (224) and fixedly connected to the timing frame (222); the support frame (223) is provided with a guide rod (227), and the timing frame (222) is slidably connected to the guide rod (227).

3. A marine liquefied natural gas flash liquefaction system according to claim 1, characterized in that, One end of the valve tube (221) is provided with a valve head (2211), and the valve head (2211) is fitted with an axially sliding sealing seat (2212), and the sealing seat (2212) is provided with a sealing ring (2213); a pull rod (2216) is provided inside the valve tube (221), and the pull rod (2216) is connected to the sealing seat (2212) through a transmission mechanism to drive the sealing seat (2212) to move axially, so that the sealing ring (2213) presses against or disengages from the valve head (2211).

4. A marine liquefied natural gas flash liquefaction system according to claim 3, characterized in that, The valve head (2211) is provided with a conical sealing surface; the transmission mechanism includes a connecting rod (2215) hinged to the pull rod (2216) and a pressure rod (2214) hinged to the sealing seat (2212), the middle part of the pressure rod (2214) being hinged to the valve head (2211).

5. A marine liquefied natural gas flash liquefaction system according to claim 1, characterized in that, The nitrogen expansion refrigeration unit (10) includes a compressor (11), a cooler (12), a heat exchanger (13), and a turbine expander (14) distributed along the nitrogen cooling process path.

6. A marine liquefied natural gas flash liquefaction system according to claim 5, characterized in that, It also includes a controller and a monitoring unit; The monitoring unit includes a pressure sensor for monitoring the pressure of the liquefied natural gas storage tank, a flow meter for monitoring the mass flow rate of flash vapor, and a sensor for monitoring the operating current and speed of the compressor (11). The controller is communicatively connected to the monitoring unit and the drive unit of the circulating flow channel adjustment mechanism (22). The controller calculates a dynamic safety margin value based on the data collected by the monitoring unit, and controls the drive unit to adjust the insertion depth of the valve tube (221) according to the different threshold ranges of the dynamic safety margin value.

7. A method for flash liquefaction of marine liquefied natural gas based on the system described in any one of claims 1-6, characterized in that, The methods include: Real-time pressure of liquefied natural gas storage tank, real-time flow rate of flash vapor, and real-time operating parameters of nitrogen compressor (11) are collected; Based on the real-time pressure, real-time flow, and real-time operating parameters, a dynamic safety margin value characterizing the current safety buffer capacity of the system is calculated. Based on the comparison result between the dynamic safety margin value and the preset safety margin threshold, the insertion depth of the valve tube (221) in the circulating flow channel adjustment mechanism (22) is controlled to adjust the effective heat exchange area of ​​the plate heat exchange assembly (21).

8. A method for flash liquefaction of marine liquefied natural gas according to claim 7, characterized in that, The method for calculating the dynamic safety margin includes: The pressure change trend is calculated based on real-time pressure, the relative operating efficiency of the compressor (11) is evaluated based on real-time operating parameters, and the pressure change trend and relative operating efficiency are used as penalty terms and deducted from the basic margin based on the current pressure and the upper limit of the safe pressure to obtain the dynamic safety margin value.

9. A method for flash liquefaction of marine liquefied natural gas according to claim 7, characterized in that, The preset safety margin threshold is an interval threshold, including a first threshold and a second threshold, and the first threshold > the second threshold ≥ 0.

10. A method for flash liquefaction of marine liquefied natural gas according to claim 9, characterized in that, The control steps based on the comparison between the dynamic safety margin value and the preset threshold include: When the dynamic safety margin value is higher than the first threshold, it is determined to be in economic mode, and a first control command is generated; When the dynamic safety margin value is lower than the first threshold but higher than the second threshold, it is determined to be in enhanced mode, and a second control command is generated; When the dynamic safety margin value is lower than the second threshold, it is determined to be in a safe mode, and a third control command is generated.

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