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CN-122014991-A - Self-coupling constant-pressure boiling and energy level cascade utilization system

CN122014991ACN 122014991 ACN122014991 ACN 122014991ACN-122014991-A

Abstract

The invention relates to the technical field of deep-cooling fluid thermodynamic process control and energy recovery. In particular to a self-coupling constant-pressure boiling and energy level cascade utilization system which solves the problems of low energy efficiency and complex structure caused by the fact that the existing cryogenic antifreezing technology relies on external heat source heating and open-loop emptying. The system comprises a liquid nitrogen supply unit, a single-tube bundle integrated immersed heat and mass exchange unit and a pressure energy cascade recovery network. The heat exchange unit has the core characteristics that an auxiliary heating element is not arranged on the shell side of the heat exchange unit, sensible heat released by fluid in a pipe is only utilized to drive the shell side liquid nitrogen to boil, self-coupling back pressure is established in a closed space through a back pressure regulating component, and the boiling point of the liquid nitrogen is physically clamped in a safe temperature zone. Meanwhile, the system utilizes expansion work generated by liquid nitrogen gasification to carry out lossless pressurization on the upstream tank car through a recovery network. The invention abandons the double heat exchanger structure, realizes thermodynamic self-balance and energy closed loop, and obviously reduces the entropy production and operation cost of the system.

Inventors

  • WANG FANYU
  • WANG YAOWU
  • DU LIXIA

Assignees

  • 陕西融科低温设备有限公司

Dates

Publication Date
20260512
Application Date
20260313

Claims (10)

  1. 1. The self-coupling constant-pressure boiling and energy level cascade utilization system is characterized by comprising a second liquefied gas storage unit (1), a first cryogenic working medium supply and self-coupling phase change unit (2), a single-tube bundle integrated immersed heat and mass exchange unit (3) and a pressure energy and substance cascade recovery network (5); the second liquefied gas storage unit (1) is used for storing second liquefied gas and is used as a heat load source of a system and comprises a liquid phase outlet (12) and a gas phase space interface (11); the self-coupling phase change unit (2) is a container for accommodating the first cryogenic working medium and providing a phase change cold source; The single-tube bundle integrated immersed heat and mass exchange unit (3) comprises a shell side (301), a tube side (302) and a back pressure regulating component (31); the shell side (301) comprises a shell side inlet (33) and a shell side outlet (34), wherein the back pressure regulating component (31) is provided with a back pressure regulating component inlet (35) and a back pressure regulating component outlet (36), and the tube side (302) comprises a tube side inlet (37) and a tube side outlet (38); the shell side inlet (33) is communicated with the first cryogenic working medium supply and self-coupling phase change unit (2), and the shell side outlet (34) is communicated with the back pressure regulating assembly inlet (35); the tube side (302) is connected in series in the flow path of the second liquefied gas and is completely immersed below the liquid level of the shell side (301); The pressure energy is communicated with a material step recovery network (5), and the input end of the pressure energy is communicated with the outlet (36) of the back pressure regulating assembly; It is also characterized in that: The shell side (301) of the single-tube bundle integrated immersed heat and mass exchange unit (3) is configured as a self-coupling constant-pressure boiling cavity, and the single-tube bundle integrated immersed heat and mass exchange unit (3) is only provided with the tube side (302) as a unique heat source input and does not comprise an auxiliary heating element or a second heat exchange tube bundle for introducing an external heat source; The single-tube-bundle integrated immersed heat and mass exchange unit (3) is configured to drive phase change boiling of a first cryogenic working medium in a shell side (301) by utilizing sensible heat released by a second liquefied gas in the tube side (302), and establish self-coupling back pressure in the closed shell side (301) by utilizing gas phase components generated by boiling, so that the boiling temperature of the first cryogenic working medium is physically clamped in a safe interval higher than the three-phase point of the second liquefied gas; the pressure energy and substance cascade recovery network (5) is configured to collect high-pressure low-temperature gas generated after the phase change of the first cryogenic working medium discharged by the shell side (301), and convey pressure potential energy and substance flow of the gas to a downstream pressure utilization terminal, so that closed-loop cascade utilization of cold energy, pressure energy and substances is realized.
  2. 2. The system of claim 1, wherein the thermodynamic constraint relationship between the self-coupling back pressure and the safety interval is that the self-coupling back pressure is maintained by the back pressure regulating component (31) in an antifreezing pressure interval of 0.6 MPa to 0.8 MPa (absolute), the saturated boiling temperature T sat of the first cryogenic working fluid (liquid nitrogen) is physically locked between-179 ℃ and-172 ℃ in the pressure interval, the temperature interval satisfies the thermodynamic inequality of T freeze <T sat <T target , wherein T freeze is the triple point temperature (-182.5 ℃) of the easily frozen component (methane) in the second liquefied gas, and T target is the supercooled target temperature required by the process.
  3. 3. The system according to claim 1, wherein the pressure energy and material cascade recovery network (5) comprises a surge tank (52) and a tank truck unloading pressurization circuit (55), wherein one end of the surge tank (52) is connected to a back pressure regulating assembly outlet (36), the other end is connected to one end of the tank truck unloading pressurization circuit (55), the other end of the tank truck unloading pressurization circuit (55) is configured to be detachably connected to a tank truck gas phase space interface (81) of a transportation tank truck (8) for transporting the second liquefied gas, the tank truck unloading pressurization circuit (55) is configured with a precision pressure reducing valve (56) for regulating the high pressure gas phase working medium from the shell side (301) to 0.35 MPa to 0.5MPa (gauge pressure) and then injecting the high pressure gas phase working medium into the tank truck gas phase space, and the system is configured to directly utilize the volume expansion work generated by the first cryogenic working medium endothermic gasification to maintain the pressure of the transportation tank truck (8) under unloading conditions, to replace the operation of the transportation tank truck (8) with a vaporizer and to raise the positive pressure pump suction head (npa) to prevent net cavitation.
  4. 4. The system according to claim 1, further comprising a thermodynamic active intervention circulation unit (4), wherein the thermodynamic active intervention circulation unit (4) comprises a circulation pump (41) connected to the bottom of the second liquefied gas storage unit (1) and an atomizing spray assembly (42) positioned inside the gas phase space of the second liquefied gas storage unit (1), wherein the system is configured to pump the cryogenic liquid in the second liquefied gas storage unit (1) from a liquid phase outlet (12) at the bottom thereof under static storage conditions, to enter a tube side inlet (37) through the circulation pump (41) to be supercooled below the bubble point temperature in the single tube bundle integrated submerged thermal mass exchange unit (3) and to enter a gas phase space interface (11) of the second liquefied gas storage unit (1) through a tube side outlet (38), to be ejected by the atomizing spray assembly (42), to absorb gas phase space heat by using supercooled liquid droplets and to induce a pressure collapse effect, so as to reduce the internal pressure of the second liquefied gas storage unit (1).
  5. 5. The system according to claim 1, wherein the pressure energy and substance cascade recovery network (5) further comprises a common gas supply circuit (54), wherein the common gas supply circuit (54) comprises an air temperature reheater (51) and an interface (53) connected to an in-station instrument air pipe network which are sequentially connected, and the system is configured to reheat the recovered gas phase working medium to normal temperature and then to serve as a power gas source of a pneumatic actuator to replace an air compressor system.
  6. 6. The system according to claim 1, wherein the single-tube bundle integrated submerged heat and mass exchange unit (3) adopts a stress self-eliminating structure, the tube side (302) comprises a heat exchange core body, the heat exchange core body is fixed inside a shell side (301) through a top single-point suspension mechanism, the bottom of the heat exchange core body is in a free suspension state, a thermal compensation gap larger than the axial cold shrinkage amount under the maximum design temperature difference is reserved between the bottom of the heat exchange core body and the bottom of the shell side (301), and a fluid dynamic dead zone and an inverted cone-shaped sedimentation tank (32) are arranged at the bottom of the shell side (301) and are used for collecting and discharging solid impurities precipitated in the first cryogenic working medium.
  7. 7. The system according to claim 1, characterized in that the system is configured with an all-condition intelligent control center (6), wherein the all-condition intelligent control center (6) is internally provided with a thermodynamic equation of state (EOS) model of a first cryogenic working medium, wherein the all-condition intelligent control center (6) is configured to collect the internal pressure of a shell side (301) in real time, calculate the real-time saturation temperature back by using the model and calculate the freezing margin, and when the freezing margin is lower than a preset safety threshold, trigger a hard constraint control logic with the highest priority to forcedly adjust the back pressure to raise the boiling temperature.
  8. 8. The system of claim 1, wherein the first cryogenic medium is liquid nitrogen and the second liquefied gas is liquefied natural gas, and wherein the system is configured to heat and gasify liquid nitrogen using only sensible heat of LNG without introducing an electric heating or auxiliary heat source and subcool the LNG to less than-165 ℃.
  9. 9. The system of claim 1, further comprising an anoxic safety interlock subsystem, wherein the subsystem includes an ambient oxygen content detector disposed in the plant area, and wherein the system is configured to automatically shut off the supply of the first cryogenic medium and switch the gas phase vent path to the high altitude safety vent when the ambient oxygen content is detected to be below a preset safety value (e.g., 19.5%).
  10. 10. The system of claim 6, wherein the heat exchange core is a multi-layer spiral wound tube bundle structure, and the spiral angle is configured to meet the hydrodynamic condition that the dean number is greater than a critical value, so as to induce secondary circulation in the tube, strengthen convective heat exchange inside the tube and inhibit adhesion of frozen nuclei.

Description

Self-coupling constant-pressure boiling and energy level cascade utilization system Technical Field The invention relates to the technical field of deep cold fluid thermodynamic process control and energy recovery, in particular to a self-coupling constant-pressure boiling and energy level cascade utilization system applied to a Liquefied Natural Gas (LNG) storage peak shaving station, a receiving station and a filling terminal. Background In the end application of LNG industry, in order to suppress flash gas (BOG) generation caused by pipe heat invasion and prevent cavitation during pumping, it is generally necessary to use liquid nitrogen (LIN, atmospheric boiling point 77.3K) as a cold trap to perform deep supercooling treatment on LNG (main component methane, triple point 90.69K). However, there is a central thermodynamic contradiction in this heat transfer process that the boiling temperature on the liquid nitrogen side tends to be well below the freezing point of methane in order to obtain a highly efficient heat transfer driving force (Δt). According to the Fourier heat conduction law and the phase change dynamics principle, once the temperature of the heat exchange tube wall is lower than 90.69K, heterogeneous nucleation crystallization occurs on the methane side, so that a runner is frozen and blocked and even equipment is damaged. 1. To solve the above freezing problem, early techniques attempted to raise its saturated boiling temperature by raising the liquid nitrogen side pressure using the clausius-clappelone equation. Representative is, for example, U.S. patent application No. 2010/036097A 1 (publication date 2010, 12 months, 30). This patent discloses a method for densifying liquid methane using a liquid nitrogen bath. The core logic is to utilize natural evaporation after absorbing heat of liquid nitrogen to carry out self-pressurization, and control the pressure in the container by adjusting a pressure relief valve connected to an emptying torch, so as to maintain the boiling point of the liquid nitrogen to be higher than the triple point of methane. However, from the perspective of modern energy engineering and low carbon economy, this technical route has serious endogenous drawbacks: (1) Yong loss of the open loop system, which is essentially a thermodynamic process of the open loop. The high pressure gas (typically >0.3 MPa) generated after gasification of liquid nitrogen contains high grade pressure potential energy. However, this patent explicitly states that the gas is vented "VENT STACK" to be vented directly [52, fig.1]. This process uses only the latent heat of liquid nitrogen and wastes its huge expansion work and pressure Yong thoroughly. For commercially operated LNG peaking stations, such a huge continuous energy rejection is not acceptable. (2) The limitation of passive control is that the technology relies mainly on mechanical pressure relief valves for passive regulation. Under the transient working condition that the peak shaver station load fluctuates severely (such as the start and stop of a unloading pump), the response lag and dead zone characteristic of the mechanical valve are extremely easy to cause pressure overshoot or undershoot, and the accurate anti-freezing safety margin cannot be maintained on a dynamic boundary. 2. In order to cope with the freezing risk under the dynamic working condition, another technical route selects a complex feedback control and post-hoc remedy mechanism. Representative are for example chinese patent application CN112228769a (publication day 2021, month 01, 15). This patent proposes a filling system based on freeze-proof control. The method is characterized by adopting complex PID regulation, and definitely designing a freezing emergency treatment step, namely switching pipelines immediately once a freezing signal is detected, and introducing high-pressure nitrogen to perform forced blowing and rewarming melting. The technical route has obvious logic paradox in continuous industrial production: (1) The underlying logic of its control philosophy is "allow freezing to occur and then remedy". This violates intrinsic safety principles in chemical process safety management. In the continuous output process of the LNG peak shaving station, once freezing occurs and a melting process is triggered, the process flow is interrupted, and the supply-keeping task is seriously influenced. (2) The system redundancy is too high, in order to realize the melting function, the system has to be provided with a special rewarming pipeline and a complex switching valve group (such as a fourth valve, a seventh valve, an eighth valve and the like) [6, fig.1], and the construction cost and the fault probability point of the system are obviously increased. 3. Recent techniques attempt to introduce an external heat source to actively interfere with the refrigerant state in order to achieve more precise temperature control. Representative are for example chinese pa