LNG concentration detection method and device based on phase change self-cleaning and mechanical pressure limiting

Abstract

The application discloses a LNG concentration detection method and device based on phase change self-cleaning and mechanical pressure limiting, and belongs to the technical field of gas concentration detection. The application realizes the quantitative introduction of liquefied natural gas into a sealed chamber and heating through the collaborative work of "phase change self-cleaning" and "pure mechanical physical trigger pressure limiting", uses the volume expansion generated by the self-gasification as a driving force, and fully replaces the original air in the sealed chamber through repeated "breathing type" exhaust, so that the methane concentration detection deviation caused by the residual original gas is limited to within 0.5%; at the same time, the pressure generated by the gasification forces the elastic expansion body to deform, triggers the sensor in a pure mechanical touch mode, and controls the opening and closing of the exhaust valve, so that self-adaptive pressure limiting is realized. The application eliminates the electronic pressure measurement and closed-loop control components from the root, significantly reduces the device cost, and greatly improves the deployment feasibility and operation reliability in distributed scenarios such as tank car loading and unloading, gas filling stations and the like.

Description

Technical Field

[0001] This invention relates to the field of gas concentration detection technology, and in particular to a method and apparatus for LNG concentration detection based on phase change self-cleaning and mechanical pressure limiting. Background Technology

[0002] In the liquefied natural gas (LNG) storage and transportation sector, the methane concentration in the evaporated gas directly determines the calorific value and quality of the natural gas, serving as a core basis for trade transactions. Taking long-distance LNG tanker transportation as an example, the preferential evaporation of lighter components leads to continuous changes in the composition within the tank. Furthermore, the industry's practice of adulterating LNG with low-cost substances such as liquid nitrogen has made online methane concentration detection in the evaporated gas during unloading and refueling a rigid requirement. However, since the evaporated gas being tested is typically under low temperature and high pressure, how to perform sample pretreatment and concentration analysis in a low-cost and highly reliable manner in a distributed mobile environment has remained a technical bottleneck restricting the widespread adoption of online detection technology in this field.

[0003] Currently, the industry uses gas chromatography to detect the components of LNG vaporized gas. This method can quantitatively detect multiple components and its accuracy meets the requirements for trade transfer. However, gas chromatography systems are complex, costly, and require specialized maintenance. They are mainly suitable for fixed centralized scenarios such as receiving stations and liquefaction plants, and are difficult to deploy in batches and monitor online in distributed mobile scenarios such as tank truck loading and unloading and gas stations.

[0004] Tunable semiconductor laser absorption spectroscopy is considered a promising technology for on-site online detection due to its low cost, fast response, and continuous operation. However, when the analyte vaporized gas is under low temperature and high pressure, the molecular absorption spectral lines are severely broadened and saturated, resulting in a loss of linearity between absorbance and concentration, rendering conventional inversion algorithms ineffective. Therefore, the sample must first be converted to a gas at room temperature and pressure. To address this, existing solutions typically employ a complete low-temperature precision pressure control system. This system consists of a low-temperature pressure sensor, a low-temperature precision pressure relief valve, signal conditioning circuitry, and a closed-loop control algorithm. It processes the low-temperature, high-pressure vaporized gas to ambient pressure through a step-by-step decompression process before sending the gas to the laser detection unit for analysis.

[0005] The aforementioned solutions face significant technical limitations in practical deployment. The fundamental reason lies in the fact that the entire system relies on a basic architecture of electronic pressure measurement and closed-loop electronic control for pressure regulation. On one hand, the numerous and complex components of cryogenic pressure sensors, precision pressure relief valves, and associated signal conditioning circuits result in high system costs, making mass deployment in distributed scenarios such as LNG tank trucks and refueling stations difficult. On the other hand, the reliability of electronic sensors and precision valves operating in cryogenic environments for extended periods is uncertain. Drift or failure directly impacts the accuracy of pressure control, leading to sample pretreatment failures or inaccurate test results. There is an irreconcilable contradiction between the system complexity and cryogenic operational reliability in existing technical solutions. Summary of the Invention

[0006] To address the aforementioned issues, this paper provides a method and apparatus for LNG concentration detection based on phase change self-cleaning and mechanical pressure limiting, thereby solving the problems that low-temperature and high-pressure gases cannot be directly used for laser absorption spectroscopy detection in existing technologies, while existing pretreatment systems suffer from high costs and low reliability.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is: an LNG concentration detection method, comprising the following steps: S1: introducing a quantitative amount of liquefied natural gas into a sealed chamber; S2: The sealed chamber is heated, causing the liquefied natural gas to absorb heat and vaporize. The volume expansion caused by the vaporization of the liquefied natural gas increases the pressure inside the sealed chamber and forces the elastic expansion body connected to the inside of the sealed chamber to deform. When the elastic expansion body deforms to the point of triggering a trigger sensor, the gas outlet valve is opened to discharge the mixed gas in the sealed chamber. After the gas is discharged, the pressure in the sealed chamber drops, the elastic expansion body recovers its deformation, the trigger sensor is reset, and the gas outlet valve is closed. The process of gas discharge triggered by the deformation of the elastic expansion body and stopping the discharge after the deformation recovers is repeated until the measured amount of liquefied natural gas is completely vaporized, so that the original gas in the sealed chamber is fully replaced, thereby limiting the methane concentration detection deviation introduced by the residual original gas to within 0.5%, and obtaining natural gas vapor gas at normal pressure. S3: The methane concentration in the natural gas vapor inside the sealed chamber is detected by a laser detection unit.

[0008] Preferably, the volume of the metered liquefied natural gas is not less than one-half the volume of the sealed chamber.

[0009] Preferably, the elastic expansion body is connected to the sealing chamber through a slender neck tube, the length and wall thickness of which are configured to ensure that the temperature at the end of the elastic expansion body is above -20°C during operation.

[0010] Preferably, the trigger sensor is installed in such a position that the elastic expansion body triggers the trigger sensor when the pressure on the inner surface of the sealed chamber is between 5 mbar and 20 mbar.

[0011] Preferably, the target heating temperature is between 15°C and 35°C.

[0012] Preferably, step S3 further includes: before detecting the methane concentration of the natural gas vapor in the sealed chamber using the laser detection unit, activating the gas homogenization unit to make the gas temperature and concentration in the sealed chamber uniform.

[0013] Preferably, the quantitative liquefied natural gas is introduced into the storage tank in the sealed chamber, and the storage tank forms a limited thermal contact with the heating wall of the sealed chamber, so that the liquefied natural gas receives heat through the tank wall.

[0014] Preferably, the elastic expander is a hollow sac-like body.

[0015] An LNG concentration detection device, employing the above-mentioned LNG concentration detection method, includes: a sealed chamber, equipped with a liquid inlet, a gas outlet, and a connection interface; A heating unit is used to heat the sealed chamber; An elastic expansion body is connected to the interior of the sealing chamber through the communication interface. The elastic expansion body deforms under the pressure in the sealing chamber and recovers its deformation when the pressure drops. A trigger sensor is installed at a predetermined position on the deformation path of the elastic expansion body, generates a trigger signal when the elastic expansion body deforms to the trigger position, and resets when the deformation recovers. An exhaust valve is connected to the exhaust port. A laser detection unit is used to detect the methane concentration in the gas inside the sealed chamber; The control unit is used to control the introduction of a fixed amount of liquefied natural gas into the sealed chamber, control the start of the heating unit, receive the trigger signal and reset signal from the trigger sensor and control the opening of the gas outlet valve according to the trigger signal, control the closing of the gas outlet valve according to the reset signal, and start the laser detection unit after all the liquefied natural gas has been vaporized.

[0016] Preferably, the above-mentioned LNG concentration detection device further includes: a neck tube, wherein the elastic expansion body is connected to the communication interface through the neck tube, and the length and wall thickness of the neck tube are configured such that the temperature at the end of the elastic expansion body is higher than -20°C in the working state; A liquid storage tank is disposed in the sealed chamber, located below the liquid inlet, and a limited thermal contact is formed between the liquid storage tank and the heating wall of the sealed chamber.

[0017] By adopting the above technical solution, the present invention has the following beneficial effects.

[0018] (1) This invention solves the fundamental pain point of existing technologies in processing liquefied natural gas vaporized gas by introducing a combination of "phase change self-cleaning" and "pure mechanical physical trigger pressure limiting", which is due to the complexity, high cost and inability to be deployed in batches in distributed mobile scenarios caused by the reliance on low temperature precision electronic pressure control system. Unlike the traditional method of using low temperature pressure sensors, precision pressure relief valves and closed-loop control circuits for step-by-step pressure reduction, this invention introduces a quantitative amount of liquefied natural gas into the sealed chamber and heats it, causing it to vaporize and generate a volume expansion of about 600 times. This expansion acts as a driving force to push the original gas in the sealed chamber out, realizing the self-replacement of the sample. At the same time, the pressure generated by the expansion forces the elastic expansion body connected to the sealed chamber to deform. This deformation triggers the sensor by physical contact, directly controlling the opening and closing of the gas outlet valve. Vaporization expansion serves as both the power source for displacement and the trigger for pressure limiting. Both share the same physical process and work in tandem. With a minimalist, purely mechanical structure, it simultaneously completes sample purification and pressure adaptive balancing, eliminating the need for a complete electronic pressure measurement and control closed loop. This significantly reduces the cost of the device and greatly improves its deployment feasibility and operational reliability in scenarios such as tank truck loading and unloading and gas stations.

[0019] (2) This invention continuously enhances the synergistic effect of phase change self-cleaning and mechanical pressure limiting through repeated cycles of the "breathing" exhaust process. Each complete cycle of heating and vaporization leading to pressure increase, elastic expansion body deformation triggering exhaust, pressure drop after exhaust, and elastic expansion body deformation recovery stopping exhaust will expel a portion of the original air in the sealed chamber, while the newly added gas comes entirely from the vaporization of liquefied natural gas itself. As liquefied natural gas continues to vaporize, this cycle automatically repeats until all liquid is exhausted and the original air in the sealed chamber is gradually and completely replaced. This process utilizes the phase change energy inherent in liquefied natural gas itself, requiring no external purging gas source or human intervention to obtain pure vaporized gas containing only methane components, ensuring the accuracy of subsequent concentration detection from the sample preparation stage.

[0020] (3) This invention solves the problem of instantaneous and violent boiling caused by the Leidenfrost effect when cryogenic liquids come into contact with high-temperature walls by setting up a liquid storage tank in a sealed chamber, thus creating limited thermal contact between the introduced liquefied natural gas liquid and the heated wall surface. After the liquid enters the storage tank, it receives heat through the tank wall, achieving stable and controllable evaporation, providing a continuous and stable gas source for the above-mentioned "breathing" exhaust cycle. The limited thermal contact structure of the storage tank is compatible with the phase change self-cleaning scheme, avoiding the interference of pressure shock caused by boiling on the triggering accuracy of the elastic expansion body, and ensuring that the triggering and resetting of each exhaust cycle are accurate and reliable.

[0021] (4) This invention connects the elastic expansion body and the sealing chamber with a slender neck tube. By utilizing the thermal resistance structure of the neck tube itself, a temperature gradient is established between the sealing chamber and the elastic expansion body at low temperature liquid inlet, ensuring that the working temperature of the elastic expansion body end is always higher than its embrittlement temperature. This solves the problem of embrittlement failure of elastic materials caused by the low temperature environment of liquefied natural gas. The design of this neck tube enables the core actuator of the purely mechanical pressure limiting unit to maintain stable elastic response characteristics throughout the entire detection process, ensuring the reliability of the "deformation triggering-deformation recovery" pressure limiting mechanism under long-term repeated use. This makes the low-cost solution possess the durability required for industrial field applications. Attached Figure Description

[0022] The following provides a detailed discussion of the manufacture and application of preferred embodiments of the present invention. However, it should be understood that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are merely illustrative of specific ways of manufacturing and using the present invention and do not limit the scope of the invention. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0023] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0024] The following provides a detailed discussion of the manufacture and application of preferred embodiments of the present invention. However, it should be understood that the present invention provides many applicable inventive concepts that can be embodied in various specific environments. The specific embodiments discussed are merely illustrative of specific ways of manufacturing and using the invention and do not limit the scope of the invention.

[0025] In one specific embodiment of the present invention, an LNG concentration detection method based on phase change self-cleaning and mechanical pressure limiting is provided. This method introduces a quantitative amount of liquefied natural gas into a sealed chamber and heats it. The volume expansion generated by the vaporization of the liquefied natural gas completes sample replacement and adaptive pressure control, ultimately achieving online detection of the methane concentration in the evaporated gas.

[0026] To facilitate a thorough understanding of the core concept of this invention, the two core mechanisms that form the cornerstone of the technical solution of this invention, namely "phase change self-cleaning" and "mechanical pressure limiting", will be explained in detail first.

[0027] "Phase change self-cleaning" is the sample purification mechanism of this invention. Its core lies in utilizing the enormous volume expansion generated by the "phase change" process of liquefied natural gas (LNG) from a liquid to a gaseous state as a driving force to gradually displace the original air in the sealed chamber, achieving "self-cleaning" of the chamber. Unlike traditional methods that require an external purging gas source or a complex vacuum system to prepare pure sample gas, this invention transforms the LNG itself into the executor of sample purification. When a quantitative amount of LNG is heated and vaporized in the sealed chamber, its volume expands dramatically by approximately 600 times. The expansion work generated by this physical process is sufficient to completely displace and expel the original air in the sealed chamber. Through repeated "breathing" exhaust circulation, the original air is gradually diluted and expelled step by step. Finally, after all the LNG has vaporized, only pure methane gas derived from the LNG itself remains in the sealed chamber. This mechanism combines the "analyte" and the "purification method" into one, fundamentally eliminating the need for the essential external purging or vacuum-assisted systems required in traditional methods.

[0028] "Mechanical pressure limiting" is the adaptive pressure control mechanism of this invention. Its core lies in employing a purely mechanical physical linkage structure of "elastic expansion body + trigger sensor," replacing the complex electronic pressure control system of traditional solutions, which consists of a low-temperature pressure sensor, a precision pressure relief valve, and a closed-loop control circuit. In this invention, no electronic pressure threshold is preset. Whether the elastic expansion body triggers venting depends solely on whether its deformation reaches the installation position of the trigger sensor. When the gas pressure inside the sealed chamber increases, the high-pressure gas drives the elastic expansion body to expand outward; when the pressure decreases, the elastic expansion body contracts and recovers under its own elastic force. The trigger sensor acts only as a "detector" of the spatial position of the elastic expansion body, not a "measuring instrument" of the pressure value. This purely physical mapping relationship of "pressure-deformation-displacement-signal" allows the pressure limiting action to be entirely adaptively driven by the physical process, requiring no readings from electronic pressure sensors, no calculations from closed-loop control algorithms, and no cooperation from precision pressure relief valves. This is an adaptive, self-balancing pressure control closed loop achieved purely through physical shape changes.

[0029] The "phase change self-cleaning" and "mechanical pressure limiting" work in synergy. The volume expansion generated by the vaporization of liquefied natural gas serves both as the displacement power source driving the original air out and as the pressure limiting trigger source driving the deformation of the elastic expansion body to trigger the exhaust. Each "breathing" exhaust cycle is both a purification process that brings us one step closer to the goal of complete replacement and a pressure regulation action that releases the pressure in the sealed chamber to a safe range. This invention integrates sample purification, pressure regulation, and sample state pretreatment in a purely mechanical, self-driven, and adaptive manner. The implementation of the above mechanism is explained in detail below with specific steps.

[0030] As attached Figure 1This paper presents an LNG concentration detection method based on phase change self-cleaning and mechanical pressure limiting. The method specifically includes the following steps: Step S1: Introduce a quantitative amount of liquefied natural gas into the sealed chamber.

[0031] In this embodiment, the sealing chamber is a sealed container made of pressure-resistant stainless steel, and its volume is determined according to the sample volume required for a single test. The sealing chamber is equipped with a liquid inlet, a gas outlet, and a communication interface. The liquid inlet is located at the bottom or lower side of the sealing chamber and is used to introduce liquefied natural gas (LNG) liquid into the chamber. The liquid inlet is connected to a liquid inlet unit, which is used to quantitatively inject trace amounts of cryogenic liquid. Its start / stop and injection volume are controlled by a control unit. The single injection volume is approximately half the volume of the sealing chamber. This injection volume is determined based on the fact that the vaporization volume ratio of LNG can be estimated using the ideal gas law. The density of liquid methane is taken as an example. r L ≈422 kg / m³, molar mass of methane M =0.016 kg / mol, vaporization temperature 296°C K Pressure 101325 Pa, gas constant R =8.314 J / (mol·K), from PV g = nRT and n =( r L × V L ) / M The volume expansion factor after gasification can be calculated. K = V g / V L =( r L × R × T ) / ( M × P The volume of gas produced after the liquid injected into the sealed chamber, which is half the volume of the chamber, is approximately 634 times the volume of the sealed chamber. This 300-fold increase in gas volume is not retained in the sealed chamber all at once, but is gradually discharged through repeated cycles of "vaporization and pressurization – opening the outlet valve – discharging the mixed gas – pressure drop – closing the outlet valve." Each cycle dilutes and partially discharges the original air in the sealed chamber. Under a conservative model of thorough mixing, the proportion of the original residual air can be determined using a continuous dilution model. C / C 0 = exp(- V g / V c Estimate the value of the data, where... C 0 represents the initial air concentration;C The residual air concentration after replacement; V g This refers to the total volume of LNG after vaporization. V c For the volume of the sealed chamber, when V g / V c At approximately 300°C, the impact of residual air on methane concentration is far less than 0.1%, meaning the methane concentration detection deviation introduced by residual gas can be limited to within 0.5%, meeting the accuracy requirements for methane concentration detection in LNG trade transactions. Even if localized mixing inhomogeneity or dead zones exist in the actual unit, the total volume of the nascent gas is hundreds of times larger than the sealed chamber volume, providing a sufficiently large displacement margin to ensure this deviation limit is met. This displacement margin is the technical basis for limiting the LNG inlet flow rate to no less than half the sealed chamber volume.

[0032] In a preferred embodiment, a liquid storage tank is provided inside the sealed chamber, located directly below the liquid inlet. The storage tank is a recessed structure that forms limited thermal contact with the bottom of the sealed chamber. When a trace amount of liquefied natural gas (LNG) enters through the liquid inlet, the liquid accumulates in the storage tank rather than spreading directly on the high-temperature wall surface of the sealed chamber. The storage tank and the heated wall surface of the sealed chamber receive heat indirectly through the tank wall, rather than in direct contact. This "limited thermal contact" design effectively avoids the instantaneous and violent boiling of the liquid caused by the Leidenfrost effect, allowing the LNG to absorb heat and evaporate in a stable and controllable manner, ensuring the stable operation of the "breathing" exhaust process in the subsequent step S2.

[0033] Step S2: The sealed chamber is heated, causing the liquefied natural gas to absorb heat and vaporize. The volume expansion caused by the vaporization of the liquefied natural gas increases the pressure inside the sealed chamber and forces the elastic expansion body connected to the inside of the sealed chamber to deform. When the elastic expansion body deforms to the point of triggering a trigger sensor, the gas outlet valve is opened to discharge the mixed gas in the sealed chamber. After the gas is discharged, the pressure in the sealed chamber drops, the elastic expansion body recovers its deformation, the trigger sensor is reset, and the gas outlet valve is closed. The process of expelling gas triggered by the deformation of the elastic expansion body and stopping the expelling after the deformation recovers is repeated until a fixed amount of liquefied natural gas is completely vaporized, so that the original gas in the sealed chamber is completely replaced, and natural gas vapor gas at normal pressure is obtained.

[0034] Specifically, in this step, the control unit activates the heating unit. The heating unit is an electric heating device wrapped around the outer wall of the sealed chamber, with a target heating temperature of 15°C to 35°C. This temperature range ensures that the final gas sample is at a suitable ambient temperature for tunable semiconductor laser absorption spectroscopy detection. The outer wall of the sealed chamber may be covered with a heat insulation layer to reduce the impact of external frost on the optical window during low-temperature liquid ingress.

[0035] As the heating unit heats the sealed chamber, the liquefied natural gas (LNG) in the storage tank absorbs heat indirectly through the tank walls and vaporizes—the core of the "phase change" process. When LNG changes from a liquid to a gaseous state, its volume expands dramatically, approximately 600 times. Because the sealed chamber is a closed space, the large amount of methane gas produced during vaporization causes a sharp increase in the total gas volume within the chamber, leading to a rise in gas pressure. This volume expansion is the driving force behind the coordinated operation of "phase change self-cleaning" and "mechanical pressure limiting" in this invention.

[0036] The aforementioned "breathing" exhaust process can be intuitively understood as follows: Initially, a certain amount of air exists in the sealed chamber. The newly generated methane from LNG vaporization increases the pressure in the sealed chamber. This pressure increase triggers the deformation of the elastic expansion body, opening the exhaust valve. After the exhaust valve opens, a mixture of air and newly generated methane is discharged, with each exhaust carrying away a portion of the original air. After exhaust, the pressure drops, the elastic expansion body returns to its original shape, and the exhaust valve closes. Heating continues, the liquid continues to vaporize, and the pressure in the sealed chamber rises again, entering the next exhaust cycle. This cycle of "vaporization pressurization—triggered exhaust—discharge of mixed gas—pressure drop—stop exhaust" repeats until the measured amount of LNG is completely vaporized. The core of this process is the combination of the large-scale generation of new gas and the repeated discharge of the mixed gas, which gradually dilutes and discharges the original air in the sealed chamber. Because the total volume of methane produced by vaporization is much larger than the volume of the sealed chamber, even if a single exhaust cannot completely remove all the air, after hundreds of times of dilution and replacement, the proportion of residual air is reduced to a level that has a negligible impact on the concentration detection results. Therefore, the "breathing" exhaust process has a clear physical cause and effect relationship, and those skilled in the art can understand and implement it based on the structure and steps disclosed in the specification.

[0037] The sealing chamber's connecting interface is connected to an elastic expansion body via a slender neck tube, creating gas communication between the interior of the elastic expansion body and the interior of the sealing chamber. The elastic expansion body is a hollow, sac-like structure made of elastic material. When the pressure inside the sealing chamber increases, high-pressure gas enters the elastic expansion body through the connecting interface and the neck tube, forcing the elastic expansion body to expand outward, resulting in a perceptible deformation displacement. This is the physical realization of the "pressure-deformation" mapping link in the "mechanical pressure limiting" mechanism.

[0038] A trigger sensor is installed at a predetermined position along the deformation path of the elastic expansion body. The trigger sensor can be any of a micro switch, photoelectric proximity switch, or capacitive proximity switch. As the pressure inside the sealed chamber continuously increases, the deformation displacement of the elastic expansion body gradually increases until its outer wall touches the trigger sensor. The trigger sensor then generates a trigger signal and sends it to the control unit. Upon receiving the trigger signal, the control unit controls the opening of the exhaust valve. The exhaust valve is a normally closed solenoid valve; its inlet is connected to the exhaust port, and its outlet can be connected to a gas recovery pipeline to guide the discharged methane gas back to the process system or for harmless treatment. After the exhaust valve opens, the mixed gas inside the sealed chamber is discharged outwards through the exhaust port and the exhaust valve under the action of the internal and external pressure difference. This is the execution action of "exhaust displacement" in "phase change self-cleaning," and also the execution action of "pressure relief" in "mechanical pressure limiting." That is, one operation serves two mechanisms simultaneously.

[0039] As gas is released, the pressure inside the sealed chamber drops. With the decrease in pressure, the elastic expander gradually recovers its original shape under its own elastic force, and its outer wall detaches from the trigger sensor. The trigger sensor resets, and the trigger signal disappears. Upon detecting the disappearance of the trigger signal, the control unit closes the exhaust valve, stopping the exhaust. This completes one full exhaust cycle.

[0040] Even after the vent valve is closed, unvaporized liquefied natural gas remains in the sealed chamber. The heating unit continues heating, causing the liquid to vaporize further, and the pressure inside the sealed chamber rises again, repeating the entire process of venting triggered by the deformation of the elastic expansion body and stopping venting upon deformation recovery. This process is referred to as the "breathing" venting process in this invention. Each cycle of "breathing" venting represents a step forward in the "phase change self-cleaning" mechanism towards complete replacement, and also a cycle in which the "mechanical pressure limiting" mechanism adaptively adjusts the pressure inside the sealed chamber.

[0041] In each breathing-style exhaust cycle, the gas discharged from the outlet valve is a mixture of the original air in the sealed chamber and methane gas produced by the vaporization of liquefied natural gas. As this cycle repeats, the proportion of the original air in the sealed chamber is gradually diluted and discharged with each exhaust operation. The gas remaining in the sealed chamber after each exhaust, as well as the gas newly added for subsequent vaporization, all originate from the liquefied natural gas itself. When the measured amount of liquefied natural gas has been completely vaporized and no new methane gas is generated, the original air in the sealed chamber has been completely replaced by the breathing-style exhaust process described above, leaving only pure vaporized gas containing only methane components. The "phase change self-cleaning" process is thus completed, and the analyte itself has completed the entire sample purification process.

[0042] At this point, since no more gas is continuously generated, the pressure inside and outside the sealed chamber tends to balance after the last venting. The remaining tiny pressure difference is only the elastic force required to maintain the shape of the elastic expansion body, and this pressure difference is negligible. The gas sample is at atmospheric pressure. The "mechanical pressure limiting" mechanism ensures that the pressure inside the sealed chamber remains within a safe and controllable range throughout the entire vaporization and replacement process, eventually automatically returning to atmospheric pressure, providing an ideal gas state for subsequent laser detection.

[0043] In a preferred embodiment, the neck tube connecting the elastic expansion body and the sealing chamber is a slender, thin-walled stainless steel tube made of a material with low thermal conductivity. The length and wall thickness of the neck tube are designed to utilize its own thermal resistance structure to establish a temperature gradient between the cryogenically inlet sealing chamber and the elastic expansion body. This ensures that, during operation, although the sealing chamber end may be at a low temperature, the temperature of the elastic expansion body end remains above -20°C. This design ensures that the elastic expansion body is always at room temperature or near-room temperature, preventing the elastic material from becoming brittle and failing due to low temperatures, and guaranteeing the response reliability and service life of the purely mechanical pressure limiting unit under long-term repeated use.

[0044] In another preferred embodiment, the installation position of the trigger sensor can be adjusted according to the desired pressure limiting effect. The closer the installation position of the trigger sensor is to the initial shape of the elastic expander, the smaller the deformation displacement required for triggering, and the lower the corresponding trigger exhaust pressure in the sealed chamber. In this embodiment, the installation position of the trigger sensor is set such that when the gas gauge pressure in the sealed chamber is in the range of 5 mbar to 20 mbar, the elastic expander expands to the trigger position and triggers the trigger sensor. This pressure range is extremely low, ensuring that the pressure in the sealed chamber remains within a safe range close to atmospheric pressure throughout the entire vaporization and replacement process. Furthermore, after all the liquefied natural gas has been vaporized, the residual pressure difference has a negligible impact on the final gas sample state.

[0045] Regarding the repeatability and consistency of sensor triggering during the repeated deformation of the elastic expander, it should be noted that the elastic expander and trigger sensor in this application constitute a low-pressure triggering and depressurization mechanism, rather than a precision pressure measurement mechanism. That is, the function of this structure is to open the vent valve to depressurize when the pressure inside the sealed chamber rises to near a low threshold relative to the external pressure, and to close the vent valve after the pressure returns to normal, rather than outputting a precise pressure value. As long as this trigger-depressurization-reset-closure cycle can continue, two effects can be achieved: first, the gas generated by LNG vaporization is continuously discharged, gradually diluting and expelling the original air; second, the pressure difference between the inside and outside of the sealed chamber is always kept at a low level, avoiding high pressure accumulation. Therefore, this application does not require the elastic expander to have repeatability accuracy at the level of a pressure sensor, nor does it require the trigger pressure to be completely consistent in each cycle. Even if the trigger pressure fluctuates within the range of 5 mbar to 20 mbar, as long as the vent valve can open when the pressure inside the sealed chamber is slightly higher than the external pressure and close after depressurization, a "breathing" exhaust cycle can be maintained. During the final detection, since the LNG has been completely vaporized and no new gas is produced, the pressure inside and outside the sealed chamber tends to be balanced. The residual slight pressure difference will not change the mole fraction relationship between methane and residual air, nor will it substantially affect the concentration calculation results. Therefore, the repeatability requirements of the elastic expansion body and the trigger sensor are engineering switch-level requirements. Being able to trigger the opening of the gas outlet valve within the set low pressure range and reset it to close after the pressure drops will satisfy the technical objective of this application.

[0046] It is understood that the aforementioned elastic expansion body, trigger sensor, neck tube, and their connections and control relationships together constitute the "mechanical pressure limiting unit" of this invention. This mechanical pressure limiting unit uses the physical deformation of the elastic expansion body to directly trigger pressure relief, replacing the low-temperature precision pressure sensor, low-temperature precision pressure relief valve, and corresponding closed-loop control circuit in the traditional solution with a purely mechanical "pressure-deformation-displacement-signal" mapping chain. During the vaporization process, it adaptively limits the pressure in the sealed chamber within a safe range, without the need for preset electronic thresholds or complex closed-loop control algorithms.

[0047] Step S3: The methane concentration in the natural gas vapor inside the sealed chamber is detected by the laser detection unit.

[0048] Once all the liquefied natural gas has vaporized in step S2, and the original gas in the sealed chamber has been completely replaced, resulting in pure natural gas vapor at normal pressure and temperature, the control unit first activates the gas homogenization unit. The gas homogenization unit is a miniature fan or magnetic stirrer installed inside the sealed chamber to accelerate the homogenization of the gas temperature and concentration within the sealed chamber. After the temperature sensor indicates that the gas temperature inside the sealed chamber has stabilized, the control unit activates the laser detection unit.

[0049] The laser detection unit is a tunable semiconductor laser absorption spectroscopy methane detection module. Its laser emitter and receiver are respectively positioned outside two opposing optical windows on the sealed chamber. The optical windows are made of light-transmitting material, forming the incident and exit channels for the laser beam. The inner surface of the windows can be coated with an anti-fog coating to suppress condensation. After the laser detection unit is activated, the laser beam passes through the optical windows and the gas sample inside the sealed chamber. The receiver analyzes the absorption spectral intensity of methane molecules at characteristic wavelengths to deduce the methane concentration in the gas inside the sealed chamber and outputs the detection result.

[0050] It should be clarified that the "methane concentration detection deviation introduced by residual original gas" mentioned in this application refers to the deviation component contributed solely by the original gas remaining from the exhaust gas replacement process in step S2 throughout the entire detection method. This deviation component, together with the errors of subsequent steps such as the laser detection unit and the inversion algorithm, constitutes the overall error of the detection method. Through the aforementioned thorough replacement process, this application has reliably limited this deviation component to within 0.5%, thereby ensuring that the subsequently measured methane concentration results can be directly used for trade transfer measurement.

[0051] In this embodiment, the "phase change self-cleaning" process works in conjunction with the "mechanical pressure limiting" mechanism. The volume expansion generated by the vaporization of liquefied natural gas plays two roles simultaneously: firstly, it acts as a driving force to expel the original air in the sealed chamber, completing the self-replacement of the sample; secondly, it acts as a pressure source to force the elastic expander to deform, thereby triggering the venting action. That is, vaporization expansion is both the driving force for replacement and the trigger for pressure limiting. Each cycle of breathing-style venting not only expels a portion of the original air, bringing us one step closer to the goal of complete replacement, but also completes a pressure release, keeping the pressure in the sealed chamber within a safe range. The replacement process requires repeated venting, and repeated venting is a natural result of the continuous operation of the pressure limiting mechanism. This synergistic working method allows this solution to integrate sample purification, pressure regulation, and state pretreatment with a minimalist purely mechanical structure, a system transformation that cannot be achieved by simply superimposing individual features.

[0052] In addition, stable and controllable evaporation provides a continuous and stable gas source for the breathing exhaust cycle, avoiding the interference of pressure shock caused by boiling on the triggering accuracy of the elastic expansion body, ensuring that the triggering and resetting of each exhaust cycle are accurate and reliable, and guaranteeing the smooth operation of the entire phase change self-cleaning and mechanical pressure limiting synergy process.

[0053] In this embodiment, the volume of the sealed chamber is 1 liter. In step S1, the control unit controls the liquid inlet unit to inject more than 0.5 liters of liquefied natural gas (LNG) liquid into the sealed chamber. In this embodiment, the injected LNG liquid is 0.5 liters, and the liquid enters the storage tank. In step S2, the heating unit is started, with a target heating temperature of 25°C. The LNG is heated indirectly through the storage tank wall and vaporizes smoothly. As gas is generated, the pressure in the sealed chamber increases, and the gas enters the elastic expansion body through the neck tube connected to the sealed chamber, causing the elastic expansion body to expand. When the pressure on the inner surface of the sealed chamber reaches 10 millibars, the deformation displacement of the elastic expansion body is just enough to make it touch the trigger sensor located at that position. The trigger sensor sends a signal, the control unit opens the exhaust valve, and the mixed gas is discharged outward. The pressure then drops, the elastic expansion body contracts and disengages from the trigger sensor, and the exhaust valve closes. The above-mentioned breathing-type exhaust cycle is automatically repeated. Assuming that the total amount of gas produced by the complete vaporization of LNG is approximately 300 liters, in the repeated exhaust cycle, the original 1 liter of air in the sealed chamber is gradually diluted and discharged. After all 0.5 liters of liquefied natural gas (LNG) has vaporized, the sealed chamber is left with pure methane gas, whose pressure is essentially balanced with the external atmospheric pressure, and whose temperature stabilizes at around 25°C. In step S3, the control unit activates the gas homogenization unit. Once the gas temperature and concentration are uniform, the laser detection unit is activated to invert the methane concentration in the sealed chamber and directly output the detection result. At this point, since the original air has been completely replaced, the measurement error introduced by the residual air is less than 0.1%, fully meeting the accuracy requirements for trade transactions. The entire detection process requires no cryogenic pressure sensor, precision pressure relief valve, or closed-loop control circuit, making the device structure extremely simplified.

[0054] Corresponding to the above method, a specific embodiment of the present invention also provides an LNG concentration detection device based on phase change self-cleaning and mechanical pressure limiting. This device includes a sealed chamber, a heating unit, an elastic expansion body, a trigger sensor, an outlet valve, a laser detection unit, and a control unit.

[0055] The sealing chamber is equipped with a liquid inlet, a gas outlet, and a communication port. A heating unit is used to heat the sealing chamber. An elastic expansion body is connected to the interior of the sealing chamber via the communication port. The elastic expansion body deforms under pressure within the sealing chamber and recovers its shape when the pressure drops. A trigger sensor is installed at a predetermined position along the deformation path of the elastic expansion body. It generates a trigger signal when the elastic expansion body deforms to the trigger position and resets when the deformation recovers. A gas outlet valve is connected to the gas outlet port. A laser detection unit is used to detect the methane concentration in the gas within the sealing chamber.

[0056] The control unit is used to control the introduction of a fixed amount of liquefied natural gas into the sealed chamber; control the start of the heating unit; receive trigger signals and reset signals from the trigger sensor, and control the opening of the gas outlet valve according to the trigger signal and the closing of the gas outlet valve according to the reset signal; and start the laser detection unit after all the liquefied natural gas has been vaporized.

[0057] In a preferred embodiment, the device further includes a neck tube and a reservoir. The neck tube connects the elastic expansion body to the communication interface, and its length and wall thickness are configured to ensure that the end temperature of the elastic expansion body is above -20°C during operation. The reservoir is disposed within a sealed chamber, below the inlet interface, and forms limited thermal contact between the reservoir and the heated wall surface of the sealed chamber.

[0058] It is understood that the above embodiments are merely illustrative of the present invention and are not intended to limit the invention. In other embodiments of the present invention, the heating unit may employ other types of heating devices; the trigger sensor may employ other types of proximity, contact, or non-contact sensors; the elastic expansion body may employ a capsule-like or membrane-like structure made of other elastic materials; and the gas equalization unit may employ other forms of gas stirring devices. Furthermore, the specific embodiments described are not intended to limit the scope of the present invention. Those skilled in the art will readily understand based on the present invention that existing or future-developed processes, machines, manufactures, material compositions, means, methods, or steps may perform substantially the same functions or obtain substantially the same results as the embodiments of the present invention. Therefore, the appended claims are intended to include such processes, machines, manufactures, material compositions, means, methods, or steps within their scope.

Claims

1. A method for detecting LNG concentration, characterized in that, Includes the following steps: S1: Introduce a fixed quantity of liquefied natural gas into the sealed chamber; S2: The sealed chamber is heated, causing the liquefied natural gas to absorb heat and vaporize. The volume expansion caused by the vaporization of the liquefied natural gas increases the pressure inside the sealed chamber and forces the elastic expansion body connected to the inside of the sealed chamber to deform. When the elastic expansion body deforms to the point of triggering a trigger sensor, the gas outlet valve is opened to discharge the mixed gas in the sealed chamber. After the gas is discharged, the pressure in the sealed chamber drops, the elastic expansion body recovers its deformation, the trigger sensor is reset, and the gas outlet valve is closed. The process of gas discharge triggered by the deformation of the elastic expansion body and stopping the discharge after the deformation recovers is repeated until the measured amount of liquefied natural gas is completely vaporized, so that the original gas in the sealed chamber is fully replaced, thereby limiting the methane concentration detection deviation introduced by the residual original gas to within 0.5%, and obtaining natural gas vapor gas at normal pressure. S3: The methane concentration in the natural gas vapor inside the sealed chamber is detected by a laser detection unit.

2. The LNG concentration detection method according to claim 1, characterized in that, The volume of the quantitative liquefied natural gas is not less than one-half the volume of the sealed chamber.

3. The LNG concentration detection method according to claim 1, characterized in that, The elastic expansion body is connected to the sealed chamber through a slender neck tube, the length and wall thickness of which are configured to ensure that the temperature at the end of the elastic expansion body is above -20°C during operation.

4. The LNG concentration detection method according to claim 1, characterized in that, The trigger sensor is installed in such a position that the elastic expansion body triggers the trigger sensor when the pressure on the inner surface of the sealed chamber is between 5 mbar and 20 mbar.

5. The LNG concentration detection method according to claim 1, characterized in that, The target temperature for heating is 15°C to 35°C.

6. The LNG concentration detection method according to claim 1, characterized in that, Step S3 further includes: before detecting the methane concentration of the natural gas vapor in the sealed chamber using the laser detection unit, activating the gas homogenization unit to make the gas temperature and concentration in the sealed chamber uniform.

7. The LNG concentration detection method according to claim 1, characterized in that, The metered liquefied natural gas is introduced into the storage tank inside the sealed chamber, and the storage tank forms a limited thermal contact with the heating wall of the sealed chamber, allowing the liquefied natural gas to receive heat indirectly through the tank wall.

8. The LNG concentration detection method according to claim 1, characterized in that, The elastic expander is a hollow sac-like body.

9. An LNG concentration detection device, employing the LNG concentration detection method according to any one of claims 1 to 8, characterized in that, include: The sealed chamber is equipped with a liquid inlet, an air outlet, and a communication interface. A heating unit is used to heat the sealed chamber; An elastic expansion body is connected to the interior of the sealing chamber through the communication interface. The elastic expansion body deforms under the pressure in the sealing chamber and recovers its deformation when the pressure drops. A trigger sensor is installed at a predetermined position on the deformation path of the elastic expansion body, generates a trigger signal when the elastic expansion body deforms to the trigger position, and resets when the deformation recovers. An exhaust valve is connected to the exhaust port. A laser detection unit is used to detect the methane concentration in the gas inside the sealed chamber; The control unit is used to control the introduction of a fixed amount of liquefied natural gas into the sealed chamber, control the start of the heating unit, receive the trigger signal and reset signal from the trigger sensor and control the opening of the gas outlet valve according to the trigger signal, control the closing of the gas outlet valve according to the reset signal, and start the laser detection unit after all the liquefied natural gas has been vaporized.

10. The LNG concentration detection device according to claim 9, characterized in that, The LNG concentration detection device also includes: The neck tube connects the elastic expansion body to the communication interface through the neck tube. The length and wall thickness of the neck tube are configured such that the end temperature of the elastic expansion body is higher than -20°C in the working state. A liquid storage tank is disposed in the sealed chamber, located below the liquid inlet, and a limited thermal contact is formed between the liquid storage tank and the heating wall of the sealed chamber.

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