Natural gas hydrocarbon dew point measurement system and method based on two-region reference interferometric distance measurement

Abstract

The present application belongs to the technical field of natural gas hydrocarbon dew point measurement, and particularly relates to a natural gas hydrocarbon dew point measurement system and method based on double-region reference interferometric distance measurement. The system and method pre-process the sample to be detected, divide the sample into two paths, pass the measurement gas and the remaining reference gas into the measurement mirror surface and the reference mirror surface of the dew point measurement device respectively, and synchronously collect the interference signals of the reflected light of the two regions. The system and method extract the direct current component through low-pass filtering of the cutoff frequency, calculate the phases of the measurement region and the reference region respectively, introduce a temperature-pressure cross compensation model and a quadratic baseline drift compensation model, calculate the dynamic differential phase, calculate the mean value, standard deviation and change rate of the thickness, and determine whether the hydrocarbon dew point is reached by using a multi-condition judgment threshold. The system and method realize the anti-mirror surface pollution and aging in the actual process of hydrocarbon dew point measurement, accurately determine the condensation dew point, and improve the detection sensitivity and precision.

Description

Technical Field

[0001] This invention belongs to the field of natural gas hydrocarbon dew point measurement technology, and particularly relates to a natural gas hydrocarbon dew point measurement system and method based on dual-region reference interferometric ranging. Background Technology

[0002] Hydrocarbon dew point is a crucial quality indicator for commercial natural gas. Standards such as GB17820 and ISO13686 require that the hydrocarbon dew point of natural gas, at its highest transmission pressure, must be lower than the lowest ambient temperature encountered along the pipeline, thus ensuring that no liquid precipitation occurs under any conditions. Accurate real-time monitoring of hydrocarbon dew point is a vital step in the production, processing, and long-distance transportation of natural gas. Accurate monitoring and analysis of hydrocarbon dew point can prevent liquid precipitation, avoid pipeline blockages, and prevent damage to equipment such as compressors and valves.

[0003] The hydrocarbon dew point is the temperature at which the heavy hydrocarbon components in a gas begin to condense into the first drop of liquid hydrocarbon under a given pressure. However, natural gas itself is a multi-component mixture, and its condensation is not a single, sharp temperature point, but a gradual precipitation process within a temperature range. Therefore, measuring the appearance of the first drop of hydrocarbon dew carries a great deal of uncertainty. Currently, methods for measuring hydrocarbon dew point are mainly divided into two categories: direct measurement and indirect calculation. The former relies on the cooling mirror method. Standards SY / T7484-2020 and GB / T27895-2011 use instruments based on this principle, requiring the difference between two measurements to be within 2.0℃. However, the rapid cooling rate of the instrument, its slow response to hydrocarbon condensation, and the combined effects of external factors such as mirror wear and aging during long-term use can result in measurement errors that are much greater than 2℃. The latter relies on gas chromatography and equation of state calculations to obtain theoretical calculations of hydrocarbon dew point. Its accuracy heavily depends on the completeness and accuracy of the analysis. If the analysis of heavy hydrocarbons above C6+ is incomplete or inaccurate, the calculation results will have a large deviation. In addition, the selection of different calculation models and equations of state themselves also have certain errors, leading to a certain deviation between the predicted and actual hydrocarbon dew point values.

[0004] Therefore, this invention aims to provide a natural gas hydrocarbon dew point measurement system and method based on the dual-region reference interferometry ranging principle, in order to combat mirror contamination and aging, accurately determine the condensation critical point, and improve detection sensitivity and accuracy during the actual process of hydrocarbon dew point measurement. Specifically, the method employs a dual-region condensation mirror with physically separated measurement and reference regions; an independent gas path control system that can separately introduce sample gas and dry gas into the two regions; and a differential interferometry detection module that can simultaneously acquire laser interference signals from the two regions and calculate the phase difference. Summary of the Invention

[0005] The purpose of this invention is to provide a natural gas hydrocarbon dew point measurement system and method based on dual-region reference interferometric ranging, so as to combat mirror contamination and aging, accurately determine the condensation critical point, and improve detection sensitivity and accuracy in the actual process of hydrocarbon dew point measurement.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows:

[0007] Firstly, a method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging is provided, including the following steps:

[0008] S1: The sample gas to be tested is sequentially adjusted to the set test pressure by the pressure regulating valve, and then pre-treated by the dehydration module for deep dehydration and the particulate matter removal module for particulate matter removal.

[0009] S2: The pretreated sample gas to be tested is split into two streams through a three-way valve. One stream is directly delivered to the measuring mirror of the dew point measuring device as the measuring gas; the other stream is processed sequentially by the low-temperature adsorption module and the inorganic adsorption module to obtain the reference gas.

[0010] S3: After partially purified reference gas is adjusted to the preset pressure through the pressure limiting valve, it is introduced into the gas chromatograph. The impurity composition is analyzed by the detector and dedicated chromatographic column of the gas chromatograph to ensure the purity of the reference gas.

[0011] S4: The measuring gas and the remaining reference gas are respectively introduced into the measuring mirror position and the reference mirror position of the dew point measuring device. The helium-neon laser is split into two paths by a polarization beam splitter with a specified beam splitting ratio. The laser beams are perpendicularly incident on the two regions through a beam splitter. The interference signal of the reflected light from the two regions is synchronously collected by an analog-to-digital converter.

[0012] S5: Preprocess the acquired interference signal, multiply the preprocessed signal with the orthogonal reference signal, extract the DC component by low-pass filtering at the cutoff frequency, and calculate the phase of the measurement area and the reference area respectively;

[0013] S6: Introduce a temperature-pressure cross-compensation model to compensate and correct the phase values ​​of the measurement area and the reference area; calculate the dynamic differential phase based on the secondary baseline drift compensation model;

[0014] S7: Continuously read the liquid film thickness data for a specified number of times, calculate the mean thickness, standard deviation and rate of change, and use a multi-condition judgment threshold to determine whether the hydrocarbon dew point has been reached.

[0015] Preferably, the specific process of step S1 is as follows:

[0016] S11: The sample gas to be tested is delivered to the automatic control pneumatic pressure regulating valve and adjusted to the set test pressure;

[0017] S12: The sample gas to be tested, after being pressure regulated by the pressure regulating valve, is transported to the dehydration module for deep dehydration treatment to remove water vapor from the sample gas. The dehydration material is 3A / 4A molecular sieve composite packing.

[0018] S13: The dehydrated sample gas to be tested is delivered to the particulate matter removal module to remove solid particulate matter from the sample gas. The particulate matter removal module is made of agglomerated fine filter element.

[0019] Preferably, the specific process of step S2 is as follows:

[0020] S21: The sample gas to be tested after being processed by the particulate matter removal module is split into two streams by a three-way valve according to a preset ratio.

[0021] S22: The sample gas to be tested in branch 1 is directly delivered to the measuring mirror position of the dew point measuring device for subsequent condensation and dew detection;

[0022] The sample gas to be tested in branch 2 undergoes two-stage purification treatment through a low-temperature adsorption module and an inorganic adsorption module to finally obtain high-purity methane reference gas, which is then delivered to the reference mirror position of the dew point measuring device.

[0023] Preferably, the specific process of step S3 is as follows:

[0024] S31: The high-purity methane reference gas obtained from branch 2 is split into two paths again through a three-way valve, including a main path and a secondary path: Main path: directly delivered to the reference mirror position of the dew point measuring device as a stable benchmark for interferometric measurement; Secondary path: used for purity verification.

[0025] S32: Adjust the pressure of the secondary reference gas to the optimal analytical pressure of the gas chromatograph; the pressure limiting valve controls the pressure of the secondary reference gas at the preset pressure;

[0026] S33: The pressure-adjusted secondary reference gas is delivered to the gas chromatograph inlet. After precise quantification via the injection valve, it is fed into the column system. The carrier gas carries the sample gas and flows within the column. Different impurity components elute from the column in a specific order due to their varying interaction strengths with the stationary phase. The eluent impurity components sequentially enter the detector, which converts the component concentration into an electrical signal. This signal is then recorded as a chromatographic peak by the data acquisition system. Based on the calibration curve, the specific content of each impurity component is calculated.

[0027] S34: Determine the purity of the reference gas.

[0028] Reference gas purity determination criteria: The total content of all impurities detected by gas chromatograph is less than the specified value, that is, the methane purity is greater than the specified value, and the content of single C2-C6 hydrocarbon components is less than the specified value, and the total content of inorganic components is less than the specified value.

[0029] Application of the judgment result: If the purity meets the standard: the main reference gas is normally introduced into the reference mirror position of the dew point measuring device, and the measurement process continues; if the purity does not meet the standard: the system triggers an alarm, indicating that the adsorption module has failed, and the purification and verification are carried out again after replacing or regenerating the adsorption packing.

[0030] Preferably, the specific process of step S4 is as follows:

[0031] S41: Perform optical system calibration.

[0032] The polarization beam splitter has a 1:1 splitting ratio, ensuring that the laser is uniformly divided into two paths with the same intensity.

[0033] Beam splitter and optical path adjustment: Adjust the angle of the beam splitter to ensure that the two laser beams, after reflection or transmission, are perpendicularly incident on the center of the measurement area and the reference area, respectively;

[0034] S42: The measuring gas is introduced into the measuring mirror position of the dew point measuring device through a sealed pipeline. The gas pressure is kept consistent with the pretreatment pressure, and the flow rate is stably controlled at the set value by the flow meter.

[0035] Reference gas: High-purity methane reference gas is introduced into the reference mirror position through an independent sealed pipeline, and the reference gas flow rate is matched with the measurement gas flow rate;

[0036] Real-time monitoring of the pressure difference between the two gas paths ensures that there is no leakage or cross-contamination between the measuring gas and the reference gas;

[0037] S43: Dual-zone temperature control:

[0038] Cooling start-up: Start the semiconductor cooling stack to cool the single-crystal silicon cold mirror as a whole, and gradually reduce the mirror surface temperature to close to the hydrocarbon dew point of the sample gas;

[0039] Temperature monitoring: The mirror temperature of the measurement area and the reference area are collected in real time to ensure the time correspondence between temperature data and optical signals;

[0040] Temperature difference closed-loop control: Based on the collected temperature data, the refrigeration power distribution of the refrigeration stack is dynamically adjusted through the PID regulation algorithm to strictly control the temperature difference between the measurement area and the reference area within the preset range;

[0041] S44: Laser beam splitting and dual-region incidence:

[0042] Laser source activation: Turn on the 632.8nm helium-neon laser source. After the laser beam is collimated into parallel light by the collimating lens, it is incident on the polarization beam splitter.

[0043] Uniform beam splitting: A polarization beam splitter splits the incident laser into two beams with perpendicular polarization directions at a 1:1 splitting ratio, which are defined as the measurement optical path laser and the reference optical path laser, respectively.

[0044] Vertical incidence positioning: After the two laser beams are reflected / transmitted by their respective beam splitters, the optical path angle is adjusted to ensure that both laser beams are vertically incident on their respective mirror areas, and the diameter of the incident spot matches the area of ​​the region.

[0045] S45: Synchronous acquisition of reflected light interference signals:

[0046] Signal generation: After the laser is incident on the reference optical path in the reference region, it undergoes specular reflection, forming a reference interference signal;

[0047] Measurement area: As the mirror temperature drops to the hydrocarbon dew point, the heavy hydrocarbon components in the measurement gas begin to condense and form a liquid film. The presence of the liquid film causes a change in the reflected optical path of the laser in the measurement optical path, forming a changing interference signal with the original light from the laser source.

[0048] Synchronous signal acquisition:

[0049] The reflected light from the two regions is refracted back through the optical path and then transmitted together to the optical detector. The detector converts the optical signal into an electrical signal, i.e., an interference signal.

[0050] Two electrical signals, the interference signal in the measurement area and the interference signal in the reference area, are simultaneously input to the analog-to-digital converter to convert the analog signal into a digital interference signal.

[0051] Preferably, the specific process of step S5 is as follows:

[0052] S51: Perform signal preprocessing on the digital interference signal;

[0053] S52: The preprocessed digital interference signal is demodulated by multiplying it with an orthogonal reference signal, and converted into a modulated signal with an extractable DC component. Phase information is stripped away through multiplication demodulation.

[0054] S53: DC component extraction: Filter out the high-frequency components of the multiplication signal and retain the low-frequency DC components carrying phase information, in-phase components and quadrature components.

[0055] S54: Phase Calculation: The phase value of the original interference signal is deduced by using in-phase and quadrature components.

[0056] Preferably, the specific process of signal preprocessing in step S51 is as follows:

[0057] S511: Linearly amplifies the digital interference signal through a preamplifier;

[0058] S512: An active bandpass filter is used to filter the frequency of the amplified signal, suppressing power frequency interference, low-frequency drift, and high-frequency noise, while retaining the effective frequency components of the interference signal.

[0059] S513: Nonlinear denoising based on wavelet transform, preserving signal mutation characteristics, eliminating random noise, and preserving signal mutation characteristics during liquid film formation;

[0060] S514: Spatial domain filtering is performed on the wavelet-denoised signal to eliminate impulse noise. The final preprocessed signal is... , .

[0061] Preferably, the specific process of step S6 is as follows:

[0062] S61: Obtain compensation parameters, including two types: one is pre-calibrated fixed coefficients: temperature-phase coupling coefficient. k t Pressure-phase coupling coefficient k p Secondary drift coefficient a Linear drift coefficient b Initial drift offset c Another type is dynamic parameters collected in real time during the measurement process: the actual temperature difference ∆T between the two regions, and the actual pressure difference ∆T between the two regions. P System uptime t ;

[0063] S62: Perform temperature-pressure cross-compensation: original phase , The interference signals caused by temperature and pressure differences need to be accurately removed by cross-compensation formula to obtain the compensated phase that is only related to the mirror state.

[0064] S63: Baseline drift compensation: For baseline phase drift caused by optical component aging and circuit noise drift during system operation that is unrelated to the liquid film, the drift amount is quantified by a quadratic fitting model and subtracted in subsequent difference calculations.

[0065] Preferably, the specific process of step S7 is as follows:

[0066] S71: Liquid film thickness conversion: converting the abstract dynamic differential phase Δ ϕ raw Converted into quantifiable physical thickness of the liquid film d ,unit: nm Establish a direct correlation between phase change and liquid film thickness;

[0067] S72: By continuously sampling and statistical analysis, the random errors of a single sampling are avoided from affecting the judgment results, and three key indicators of the liquid film thickness—mean, standard deviation, and rate of change—are extracted.

[0068] S73: Perform multi-condition threshold determination: Identify real liquid film precipitation and false signals by combining the main threshold to determine the trend and the auxiliary criteria to eliminate interference through a combination of logic.

[0069] S74: When the multi-condition threshold determination is passed, confirm that the mirror temperature of the dew point measuring device at this time is the hydrocarbon dew point of the natural gas.

[0070] Secondly, a natural gas hydrocarbon dew point measurement system based on dual-region reference interferometric ranging is provided, including:

[0071] The system includes a sample gas pretreatment module, a gas path splitting and purification module, a reference gas purity verification module, a dual-region interference detection module, a signal processing and phase calculation module, a multi-parameter compensation and differential phase calculation module, and a liquid film thickness conversion and dew point determination module.

[0072] The sample gas pretreatment module is equipped with a pressure regulating valve, a dehydration module and a particulate matter removal module connected in series.

[0073] The gas path diversion and purification module includes a three-way valve, a low-temperature adsorption module, and an inorganic adsorption module. The input end of the three-way valve is connected to the output end of the particulate matter removal module. The first output end of the three-way valve is directly connected to the measuring mirror position of the dual-region interference detection module. The second output end of the three-way valve is connected in series with the low-temperature adsorption module and the inorganic adsorption module and then connected to the reference mirror position of the dual-region interference detection module.

[0074] The reference gas purity verification module includes a second three-way valve, a pressure limiting valve, and a gas chromatograph. The input end of the second three-way valve is connected to the output end of the inorganic adsorption module. The first output end of the second three-way valve is connected to the reference mirror position of the dual-region interference detection module. The second output end of the second three-way valve is connected to the gas chromatograph via the pressure limiting valve.

[0075] The dual-region interferometric detection module includes a dew point measurement device, a laser source, a polarization beam splitter, a beam splitter, and an analog-to-digital converter. The dew point measurement device is divided into a measurement region and a reference region with equal area. The dew point measurement device is equipped with a semiconductor cooling stack and two independent platinum resistance thermometers. The laser source is a 632.8nm helium-neon laser source. The polarization beam splitter is used to split the laser into two paths, which are then perpendicularly incident on the two regions by the beam splitter. The analog-to-digital converter is used to synchronously acquire the interference signals of the reflected light from the two regions.

[0076] The signal processing and phase calculation module is used to perform pre-gain amplification, bandpass filtering, threshold denoising and median filtering on the acquired interference signal in sequence. The pre-processed signal is multiplied with the orthogonal reference signal to extract the DC components I and Q. The phase of the measurement area and the reference area are calculated by arctan2(Q,I) respectively.

[0077] The multi-parameter compensation and differential phase calculation module is used to introduce a temperature-pressure cross-compensation model to compensate and correct the phase value, and to calculate the dynamic differential phase based on the secondary baseline drift compensation model.

[0078] The liquid film thickness conversion and dew point determination module is used to convert the differential phase into liquid film thickness and determine whether the hydrocarbon dew point has been reached.

[0079] The beneficial effects of this invention include:

[0080] 1. Significantly Improved Measurement Accuracy: Through dual-region reference design and multi-parameter compensation mechanism, the influence of interference factors such as temperature fluctuations, pressure changes, and system baseline drift on the measurement results is effectively eliminated. It also solves the error exceeding tolerance problems caused by excessively rapid cooling rates and mirror aging in traditional cooling mirror methods, as well as the calculation deviation problems caused by incomplete heavy hydrocarbon analysis and model selection bias in gas chromatography. A multi-condition threshold judgment logic of mean liquid film thickness + standard deviation + rate of change is adopted. The main threshold and auxiliary criteria work synergistically to effectively avoid false signal misjudgments caused by transient interference. Stability verification over three consecutive windows and a uniformity requirement within 1.5 nm ensure the accuracy of hydrocarbon dew point critical point capture, solving the judgment problem caused by the condensation process of natural gas multi-component mixtures being a temperature range rather than a sharp point.

[0081] 2. Strong anti-interference ability and adaptable to long-term operation under complex working conditions: Comprehensive suppression of gas path interference: The sample gas undergoes deep dehydration and particulate matter removal pretreatment to avoid the risk of water vapor condensation interference and particulate matter scratching the mirror surface; the reference gas undergoes two-stage purification and real-time purity verification by gas chromatograph to ensure the stability of the reference signal and block the influence of gas path impurities on interference detection from the source.

[0082] 3. Balancing measurement efficiency and reliability to meet actual industrial needs: Real-time dynamic response: The AD760616-bit analog-to-digital converter is used to synchronously acquire interference signals. Combined with 50ms step size liquid film thickness data acquisition and fast threshold determination algorithm, real-time monitoring of hydrocarbon dew point is achieved. The response speed is far superior to the traditional cooling mirror method, meeting the needs of real-time monitoring in long-distance natural gas transportation and production processes. It can provide timely warning of liquid precipitation risks and avoid pipeline blockage and equipment damage.

[0083] 4. The temperature difference between the two regions is controlled within ±0.002℃ through PID regulation. The temperature-pressure cross-compensation model and the secondary baseline drift compensation model dynamically correct the measurement deviation, ensuring the measurement stability under different operating conditions. The gas path adopts a sealed design and pressure difference monitoring to prevent cross-contamination between the measuring gas and the reference gas. Key parameters (such as temperature-phase coupling coefficient and drift coefficient) are standardized and updated in real time, further ensuring the reliability and repeatability of the system in long-term operation. Attached Figure Description

[0084] Figure 1 This is a schematic flowchart of the natural gas hydrocarbon dew point measurement method based on dual-region reference interferometric ranging according to the present invention.

[0085] Figure 2 is a schematic diagram of the flow of the sample gas to be tested according to the present invention. Detailed Implementation

[0086] The following is in conjunction with the appendix Figure 1 Figure 2 provides a further detailed description of the invention:

[0087] Example 1

[0088] See appendix Figure 1 As shown in Figure 2, the method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging is characterized by the following steps:

[0089] S1: Sample gas pretreatment: The sample gas to be tested is pretreated by sequentially adjusting the pressure to the set test pressure through the pressure regulating valve, performing deep dehydration in the dehydration module, and removing particulate matter through the particulate matter removal module.

[0090] The pressure regulating valve is an automatic control pneumatic pressure reducing valve with parameters meeting the following requirements: inlet pressure 6000psi, outlet pressure 0-1500psi, flow coefficient 0.06, maximum driving gas pressure 110psi, and accuracy 0.5%. It also adopts an explosion-proof driving gas interface. The dehydration module uses 3A / 4A molecular sieve composite packing, with a working pressure of 0.1-10.0MPa and an outlet atmospheric pressure dew point <-80℃. The particulate matter removal module uses a coalescing fine filter element with a filtration accuracy of 0.5μm.

[0091] S2: Gas Path Splitting and Purification: The pretreated sample gas is split into two paths through a three-way valve. One path is used as the measuring gas and is directly delivered to the measuring mirror position of the dew point measuring device. The other path is processed sequentially through the low-temperature adsorption module and the inorganic adsorption module to obtain the reference gas.

[0092] The low-temperature adsorption module uses a composite packing material of silica gel, alumina and molecular sieve, and is cooled to -60℃ by Peltier cooling to remove C2-C6 hydrocarbon components to <1ppm; the inorganic adsorption module uses HPC-Q9-30-NX alloy adsorption material to remove inorganic components such as N2, CO, and CO2, so that the purity of methane gas is >7N.

[0093] S3: Reference gas purity verification: After partially purifying the reference gas, the pressure is adjusted to 0.15 MPa through a pressure limiting valve and then introduced into the gas chromatograph. The impurity composition is analyzed by the PDHID / PED / DID detector and three dedicated chromatographic columns of the gas chromatograph to ensure the purity of the reference gas.

[0094] The three chromatographic columns are: a fused silica capillary column with a length of 10m, an inner diameter of 320μm, and an inner wall coated with 10μm thick 5A molecular sieve material; a fused silica capillary column with a length of 10m, an inner diameter of 320μm, and an inner wall coated with 10μm thick polymer packing material; and a fused silica capillary column with a length of 20m, an inner diameter of 150μm, and an inner wall coated with 20μm thick nonpolar methyl polysiloxane.

[0095] S4: Dual-region interferometric detection: The measuring gas and the remaining reference gas are respectively introduced into the measuring mirror position and the reference mirror position of the dew point measuring device. The mirror of the dew point measuring device is a 38mm diameter single-crystal silicon cold mirror, which is gold-plated and divided into a measuring area and a reference area by femtosecond laser etching technology. A 50μm deep isolation groove is set between the areas and the surface roughness Ra≤0.5nm. A sealed structure is adopted to prevent airflow from interfering with each other. Temperature is measured by a semiconductor cooling stack (maximum cooling power 150W) and an independent PT1000 platinum resistance thermometer with an accuracy of ±0.001℃. The temperature difference between the two areas is controlled within ±0.002℃ by PID regulation. A 632.8nm helium-neon laser is split into two paths by a polarization beam splitter with a beam splitter ratio of 1:1. The laser beams are perpendicularly incident on the two areas by a beam splitter. The interference signal of the reflected light from the two areas is synchronously acquired by an AD760 616-bit analog-to-digital converter.

[0096] S5: Signal Processing and Phase Calculation: The acquired interferometric signal is preprocessed sequentially, including 100x pre-gain amplification, 1kHz~1MHz bandpass filtering, 5-level decomposition threshold denoising using a db4 wavelet basis, and 3×3 median filtering; the preprocessed signal is then compared with an orthogonal reference signal. V ref1 =cos(ωt), V ref2 Multiplying by sin(ωt), and then low-pass filtering at a 10Hz cutoff frequency, the DC components I and Q are extracted. The original phases of the measurement and reference regions are then calculated using arctan2(Q,I). ϕ m , ϕ r .

[0097] S6: Multi-parameter compensation and differential phase calculation: A temperature-pressure cross-compensation model is introduced to compensate and correct the phase value. The compensation formula is as follows:

[0098] ;

[0099] ;

[0100] in, The phase value after compensation for the measurement area. The phase value after compensation for the reference region. k tThis is the temperature-phase coupling coefficient, in rad / ℃. k p ΔT is the pressure-phase coupling coefficient, in rad / MPa, ΔT is the actual temperature difference between the two regions, and ΔP is the actual pressure difference between the two regions.

[0101] Based on the secondary baseline drift compensation model ϕ drift (t)= a · t²+b · t+c ,in, ϕ drift (t) represents the baseline drift. t For system uptime, a For the second drift coefficient, b For linear drift coefficients, c Calculate the dynamic differential phase Δ as the initial drift offset. ϕ raw = ϕ m' - ϕ r' - ϕ drift (t).

[0102] S7: Liquid Film Thickness Conversion and Dew Point Determination: Based on the formula d=λ ·Δ ϕ raw / (4 π · n Convert the differential phase to liquid film thickness. d ,in l =632.8nm is the laser wavelength. n The refractive index of hydrocarbon liquids ranges from 1.32 to 1.45, dynamically corrected based on natural gas composition. Liquid film thickness data is continuously read 50 times with a 50ms acquisition step, and the average thickness is calculated. d Standard deviation s and rate of change c A multi-condition threshold is used: main threshold d >5nm, corresponding to Δ ϕ >0.02πrad), the auxiliary criterion is that three consecutive windows satisfy d >5nm and s <1.5nm. When the above conditions are met simultaneously, the hydrocarbon dew point is determined to have been reached.

[0103] In this embodiment, the specific process of step S1 is as follows:

[0104] S11: The sample gas to be tested is delivered to the automatic control gas pressure regulating valve and adjusted to the set test pressure. The inlet pressure of the pressure regulating valve can withstand 6000psi, which is suitable for high-pressure conditions of natural gas in the field. The outlet pressure range is 0-1500psi, the flow coefficient is 0.06, and the maximum driving gas pressure is 110psi.

[0105] S12: The sample gas to be tested, after being pressure regulated by the pressure regulating valve, is delivered to the dehydration module for deep dehydration treatment to remove water vapor from the sample gas, avoid water vapor condensation interfering with the formation and detection of hydrocarbon liquid films, and protect subsequent instrument components. The dehydration material uses 3A / 4A molecular sieve composite packing.

[0106] S13: The dehydrated sample gas to be tested is delivered to the particulate removal module to remove solid particles from the sample gas, preventing particles from scratching the cold mirror surface of the dew point measuring device and avoiding interference signal distortion caused by mirror damage. The particulate removal module is made of coalescing fine filter element.

[0107] The specific process of step S2 is as follows:

[0108] S21: The sample gas to be tested after being processed by the particulate matter removal module is split into two streams by a three-way valve according to a preset ratio.

[0109] S22: The sample gas to be tested in branch 1 is directly delivered to the measuring mirror position of the dew point measuring device, retaining the original hydrocarbon components of the sample gas for subsequent condensation and dew detection.

[0110] The sample gas to be tested in branch 2 undergoes two-stage purification treatment through a low-temperature adsorption module and an inorganic adsorption module to finally obtain high-purity methane reference gas, which is then delivered to the reference mirror position of the dew point measuring device.

[0111] The low-temperature adsorption module removes C2-C6 hydrocarbons, ensuring that the reference gas retains only methane as the main component. The low-temperature adsorption module uses a composite packing material of silica gel, alumina and molecular sieve. The three packing materials work together to improve the adsorption capacity and selectivity for different hydrocarbons.

[0112] The inorganic adsorption module removes inorganic components, including N2, CO, and CO2, and further purifies methane to achieve a reference gas purity of 7N, or 99.99999%, ensuring a dew point as low as -163℃, far lower than the sample gas dew point, thus preventing condensation. The material used is HPC-Q9-30-NX alloy adsorption material, which exhibits strong adsorption selectivity for inorganic components such as N2, CO, and CO2, and does not adsorb methane.

[0113] The 7N high-purity methane reference gas, after two-stage purification, is transported to the "reference mirror position" of the dew point measuring device through a sealed gas path. Due to its extremely low dew point and absence of hydrocarbon impurities, the mirror surface can always be kept dry and clean, providing a stable reference for the interference signal in the measurement area.

[0114] Example 2

[0115] Based on Example 1, the specific process of step S3 is as follows:

[0116] S31: The high-purity methane reference gas obtained from branch 2 is split into two paths again through a three-way valve, including a main path and a secondary path:

[0117] Main path: Directly supplies the reference mirror position to the dew point measuring device, serving as a stable benchmark for interferometric measurements;

[0118] Secondary path: Used for purity verification. The split ratio must meet the analytical flow requirements of the gas chromatograph, be compatible with subsequent detection conditions at 0.15 MPa pressure, and not affect the flow stability of the reference gas in the main path.

[0119] S32: Adjusts the pressure of the secondary reference gas to the optimal analytical pressure of the gas chromatograph to ensure detection accuracy and repeatability; the pressure limiting valve precisely controls the pressure of the secondary reference gas at 0.15MPa, which is the optimal working pressure for the PDHID / PED / DID detector and column of the gas chromatograph, balancing separation effect and detection sensitivity.

[0120] S33: The pressure-adjusted reference gas is delivered to the gas chromatograph inlet. After precise quantification via the injection valve, it is fed into the column system. The carrier gas (high-purity helium, purity ≥99.9999%) carries the sample gas through the column. Different impurity components elute from the column in a specific order due to their varying interaction strengths with the stationary phase. The eluent impurity components sequentially enter the PDHID / PED / DID detectors, which convert the component concentrations into electrical signals. These signals are recorded as chromatographic peaks by the data acquisition system (peak area is positively correlated with impurity content). Based on the calibration curve (calibrated using standard gases), the specific content of each impurity component is calculated.

[0121] S34: Determine the purity of the reference gas.

[0122] Reference gas purity determination criteria: The total content of all impurities (N2, CO, CO2, C2-C6 hydrocarbons) detected by gas chromatography is <1 ppm (10⁻). 6 That is, the methane purity is >99.99999% (7N), and the content of single C2-C6 hydrocarbon components is <1ppm, and the total content of inorganic components (N2, CO, CO2) is <1ppm.

[0123] Application of the judgment results:

[0124] If the purity meets the standard: the main reference gas is normally introduced into the reference mirror position of the dew point measuring device, and the measurement process continues.

[0125] If the purity does not meet the standard: the system will trigger an alarm, indicating that the adsorption module has failed (e.g., the adsorption material is saturated). The adsorption packing material needs to be replaced or regenerated before purification and verification are carried out again to avoid the unqualified reference gas affecting the measurement results.

[0126] Gas chromatographs need to be calibrated regularly with standard gases (including high-purity methane standard gas with known concentrations of N2, CO, CO2, and C2-C6 hydrocarbons) to ensure the accuracy of impurity quantification.

[0127] The detection gas path (from the pressure limiting valve to the gas chromatograph inlet) must be made of inert material, such as PTFE or quartz, to avoid the adsorption or reaction of impurities and ensure that the detection results truly reflect the original state of the reference gas.

[0128] In this embodiment, the specific process of step S4 is as follows:

[0129] S41: Perform optical system calibration.

[0130] The polarization beam splitter has a 1:1 splitting ratio, ensuring that the laser is uniformly divided into two paths with the same intensity.

[0131] Beam splitter and optical path adjustment: Adjust the angle of the beam splitter to ensure that the two laser beams, after reflection / transmission, are perpendicularly incident on the center of the measurement area and the reference area, respectively. Perpendicular incident can maximize the intensity of reflected light and reduce refraction loss.

[0132] S42: The measurement gas from step S2 (pretreated sample gas retaining the original hydrocarbon components) is precisely introduced into the measurement mirror position of the dew point measuring device through a sealed pipeline. The gas pressure in the corresponding measurement area of ​​the mirror is kept consistent with the pretreatment pressure in the previous step, and the flow rate is stably controlled at the set value by a BROOKSSLA5850S1BAB1C2A1 flow meter to ensure that the airflow in the measurement area is stable and the liquid film condenses evenly.

[0133] Reference gas: High-purity methane reference gas is introduced into the reference mirror position through an independent sealed pipeline. The reference gas flow rate is matched with the measurement gas flow rate to avoid mirror temperature fluctuations caused by differences in airflow rates. Its dew point is -163℃ to ensure that no condensation occurs during the entire measurement process and to keep the mirror dry and clean.

[0134] The system uses a sealed structure between the zones to monitor the pressure difference between the gas paths in real time, ensuring that there is no leakage or cross-contamination between the measuring gas and the reference gas. If an abnormal pressure difference is detected, the system triggers airflow calibration to avoid cross-contamination affecting the signal.

[0135] S43: Dual-zone temperature control (synchronous cooling + temperature difference control):

[0136] Cooling Start-up: Start the semiconductor cooling stack to cool the single-crystal silicon cold mirror as a whole. The cooling stack is in close contact with the back of the mirror to ensure cooling efficiency. The cooling goal is to gradually reduce the mirror temperature to close to the hydrocarbon dew point of the sample gas. The cooling rate can be adjusted by PID to avoid uneven liquid film condensation caused by excessively rapid cooling.

[0137] Precise temperature monitoring: The mirror temperature of the measurement area and the reference area are collected in real time through two independent PT1000 platinum resistance thermometers. The acquisition frequency is synchronized with the ADC sampling frequency to ensure the time correspondence between temperature data and optical signals.

[0138] Temperature difference closed-loop control: Based on the temperature data collected by PT1000, the cooling power distribution of the cooler stack is dynamically adjusted through the PID regulation algorithm to strictly control the temperature difference between the measurement area and the reference area within ±0.002℃. The core purpose is to eliminate the influence of temperature difference on the laser phase and ensure that the difference between the two signals is caused only by the formation of liquid film, rather than temperature interference.

[0139] S44: Laser beam splitting and dual-region incidence:

[0140] Laser source activation: Turn on the 632.8nm helium-neon laser source. The output wavelength is stable and the monochromaticity is good, ensuring the consistency of the interference signal. After the laser beam is collimated into parallel light by the collimating lens, it is incident on the polarization beam splitter (PBS).

[0141] Uniform beam splitting: A polarization beam splitter splits the incident laser into two beams with perpendicular polarization directions at a 1:1 splitting ratio, which are defined as the measurement optical path laser and the reference optical path laser, respectively.

[0142] Vertical incidence positioning: After the two lasers are reflected / transmitted by their respective beam splitters, the optical path angles are adjusted to ensure that both lasers are vertically incident on their respective mirror areas. The laser in the measurement optical path is incident on the center of the measurement area, and the laser in the reference optical path is incident on the center of the reference area. The diameter of the incident spot is matched with the area to avoid spot overflow and signal mixing.

[0143] S45: Synchronous acquisition of reflected light interference signals:

[0144] Signal generation:

[0145] Reference region: Since the reference gas is dry and does not condense, the mirror surface remains clean. After the laser is incident on the reference optical path, it undergoes mirror reflection. The stability of the optical path of the reflected light forms a reference interference signal.

[0146] Measurement area: As the mirror temperature drops to the hydrocarbon dew point, the heavy hydrocarbon components in the measurement gas begin to condense and form a liquid film. The presence of the liquid film causes a change in the reflected optical path of the laser in the measurement optical path, which forms a change interference signal with the original light from the laser source. The amount of change in optical path is positively correlated with the thickness of the liquid film.

[0147] Synchronous signal acquisition:

[0148] The reflected light from the two regions is refracted back through the optical path and then transmitted together to an optical detector, such as an interferometer detector. The detector converts the optical signal into an electrical signal, i.e., an interference signal.

[0149] Two electrical signals, interference signal in the measurement area V m (t), Interference signal in the reference region V r (t), synchronously input to AD760616-bit analog-to-digital converter, converting analog signals into digital interference signals at a conversion rate of 200kSPS, while recording the acquisition timestamp to ensure that the time of each set of measurement signals corresponds perfectly with that of the reference signal;

[0150] Interference signal in the measurement area: V m (t)=V0 cos ( ϕ m (t)+ ϕ noise );

[0151] Reference region interference signal: V r (t)=V0 cos ( ϕ r (t)+ ϕ noise );

[0152] in, ϕ (t) represents the target phase. ϕ noise V0 represents the noise phase and the amplitude of the interference signal. ϕ m (t) represents the phase of the target in the measurement area at time t. ϕ r (t) represents the reference phase of the reference region.

[0153] Preliminary signal processing: The acquired digital interference signal is temporarily stored in the buffer module to retain the original signal characteristics, awaiting signal conditioning in the subsequent S5 stage.

[0154] Example 3

[0155] Based on Example 1 or Example 2, the specific process of step S5 is as follows:

[0156] S51: Perform signal preprocessing on the digital interference signal;

[0157] S52: The preprocessed digital interferometric signal is demodulated by multiplying it with an orthogonal reference signal, converting it into a modulated signal with an extractable DC component. Phase information is stripped away through "multiplication demodulation".

[0158] Orthogonal reference signal generation: The system's built-in signal generator produces two orthogonal reference signals with a 90° phase difference, as shown in the following formula:

[0159] ;

[0160] ;

[0161] in, The reference signal angular frequency is matched with the interference signal carrier frequency to ensure demodulation effectiveness. t It is a time variable;

[0162] Signal multiplication: The preprocessed measurement area / reference area signals are multiplied by the two orthogonal reference signals respectively to obtain four modulated signals (two from the measurement area and two from the reference area), as shown in the following formula:

[0163] Multiply the measurement area by the reference signal:

[0164] ;

[0165] ;

[0166] Multiply the reference region by the reference signal:

[0167] ;

[0168] ;

[0169] in, This is the demodulated signal (in-phase branch) of the measurement area. The interference signal after preprocessing in the measurement area, For in-phase orthogonal reference signals, This is the demodulated signal (orthogonal branch) of the measurement area. For orthogonal reference signals, The demodulated signal (in-phase branch) is the reference area. The interference signal after preprocessing in the reference region. The demodulated signal (orthogonal branch) is the reference area.

[0170] By using trigonometric functions to product and sum, the high-frequency interference signal is decomposed into low-frequency components (containing phase information) and harmonic components (which are subsequently filtered out).

[0171] S53: DC component extraction: Using a 10Hz low-pass filter, the high-frequency components of the multiplication signal are filtered out, and the low-frequency DC components I (in-phase component) and Q (quadrature component) carrying phase information are retained.

[0172] Filtering parameters: A passive low-pass filter is used, with a cutoff frequency of... f c =10Hz (ensure only the DC component is retained and all high-frequency interference is filtered out).

[0173] DC component extraction formula: I and Q are extracted through integration (low-pass filtering is essentially time averaging of the signal), the formula is as follows:

[0174] DC component of the measurement area ( I m , Q m ):

[0175] ;

[0176] ;

[0177] DC components in the reference region (Iᵣ, Qᵣ):

[0178] ;

[0179] ;

[0180] S54: Phase Calculation: Solved using the arctan2 algorithm: The core objective is to inversely deduce the phase value of the original interference signal using the I (in-phase) and Q (quadrature) components. The phase range is [-π, π], covering the complete phase period.

[0181] The phase calculation formula uses the four-quadrant arctan2(Q, I) arctan2(Q, I) to avoid the quadrant ambiguity problem of the single-variable arctan2(Q, I) arctan2(Q, I), and the formula is as follows:

[0182] Phase of measurement area:

[0183] ;

[0184] Reference phase:

[0185] .

[0186] principle:

[0187] The essence of an interference signal is amplitude modulation plus phase modulation, and its general form is:

[0188] V(t) = V0·cos( ϕ(t)+ ϕ noise );

[0189] in, ϕ (t) represents the target phase. ϕ noise This is the noise phase. (By comparing with...) V ref1 、V ref2 After multiplication and low-pass filtering, the resulting I and Q satisfy:

[0190] ;

[0191] ;

[0192] Therefore, arctan2(Q,I) can be directly derived from it. ϕ (t), which is the true phase of the measurement region and the reference region.

[0193] The specific process of signal preprocessing in step S51 is as follows:

[0194] S511: The digital interference signal is linearly amplified by 100 times through a preamplifier to enhance weak interference signals (the signal change amplitude caused by the liquid film is small) and improve the signal-to-noise ratio of subsequent processing;

[0195] Amplified signal:

[0196] V m-amp (t) = 100 × V m (t);

[0197] V r-amp (t) = 100 × V r (t);

[0198] Among them, V m (t) represents the original interference signal in the measurement area, V r (t) represents the original interference signal in the reference region;

[0199] S512: An active bandpass filter is used to filter the frequency of the amplified signal, suppressing 50Hz power frequency interference, low-frequency drift, and high-frequency noise, and retaining only the effective frequency components of the interference signal.

[0200] Passband 1kHz~1MHz, stopband attenuation ≥40dB, filtered signal: , ;

[0201] S513: Nonlinear denoising based on wavelet transform, preserving signal abrupt change characteristics (phase abrupt change during liquid film formation), eliminating random noise, and preserving signal abrupt change characteristics during liquid film formation (avoiding excessive smoothing and loss of key information).

[0202] Wavelet decomposition: for , Five levels of decomposition were performed using the db4 wavelet basis to obtain five detail coefficients. d 1 ~d 5 and 1 approximation coefficient a 5;

[0203] Thresholding: A heuristic thresholding function is used to denoise the detail coefficients. The formula is:

[0204] λ= s 1(2lnN) 1 / 2 ;

[0205] N is the signal length. s 1 represents the noise standard deviation.

[0206] Wavelet reconstruction: using the denoised detail coefficients d1'~d5' and approximation coefficients a 5. Reconstruct the signal to obtain V m-vv (t), V r-vv (t).

[0207] S514: Performs spatial domain filtering on the wavelet-denoised signal to eliminate impulse noise (such as transient circuit interference), making the signal smoother and preventing single-point noise from affecting phase calculation.

[0208] Window size: 3×3, with 3 consecutive sampling points in the time dimension as a group, for a total of 50 groups of sliding windows;

[0209] Filtering formula: For signal sequence x 1 ,x 2 ,...,x n The first one in the window k The output value is:

[0210] y k = median ( x k-1 ,x k ,x k+1 );

[0211] median To take the median;

[0212] Final preprocessed signal:V m-pre (t), V r-pre (t).

[0213] The specific process of step S6 is as follows:

[0214] S61: Obtain compensation parameters, including two types: one is pre-calibrated fixed coefficients: temperature-phase coupling coefficient. k t Pressure-phase coupling coefficient k p Secondary drift coefficient a Linear drift coefficient b Initial drift offset c Another type is dynamic parameters collected in real time during the measurement process: the actual temperature difference ∆ between the two regions. T ∆ Actual pressure difference between the two regions P System uptime t ;

[0215] S62: Perform temperature-pressure cross-compensation to eliminate environmental interference: original phase , The interference signals caused by temperature and pressure differences (not caused by liquid film) need to be accurately removed by cross-compensation formula to obtain the compensated phase that is only related to the mirror state (liquid film / dry).

[0216] For the original phase of the measurement area After subtracting the temperature-pressure disturbance term, the compensated phase is obtained. :

[0217] ;

[0218] For the original phase of the reference region Using the same interference subtraction logic, and because both regions are affected by the same temperature-pressure environment, the interference terms are consistent, resulting in a compensated phase. .

[0219] .

[0220] S63: Perform baseline drift compensation to eliminate system-specific interference.

[0221] During system operation, aging of optical components and circuit noise drift can cause baseline phase drift that is unrelated to the liquid film. This drift needs to be quantified by a quadratic fitting model and subtracted in subsequent difference calculations.

[0222] Based on coefficients calibrated before measurement a , b、c and real-time runtimet The baseline drift at the current moment is calculated using a quadratic function model. :

[0223] ;

[0224] For example: if the calibration is obtained a =10 -8 rad / s 2 b = 5 × 10 -6 Given rad / s and c=0.001rad, the drift amount at t=100s is:

[0225] ϕ drift (100) = 10 -8 ×100 2 +5×10 -6 ×100+0.001=0.00151rad, which is the system's own drift interference that needs to be deducted at the current moment.

[0226] In this embodiment, the specific process of step S7 is as follows:

[0227] S71: Liquid film thickness conversion: converting the abstract dynamic differential phase Δ ϕ raw Converted into quantifiable physical thickness of the liquid film d ,unit: nm A direct relationship between phase change and liquid film thickness is established, as shown in the following formula:

[0228] ;

[0229] Where λ is the inherent laser wavelength of the helium-neon laser source, with a fixed value of 632.8 nm, Δ ϕ raw It is a dynamic differential phase, with a value range of [-π, π] rad. n The refractive index of hydrocarbon liquids ranges from 1.32 to 1.45. It is adjusted in real time according to the actual composition of natural gas (such as the proportion of C2-C6 hydrocarbons). For example, when the content of heavy hydrocarbons is high, the upper limit of 1.45 is used, and when the proportion of methane is extremely high, the lower limit of 1.32 is used.

[0230] Calculation logic description:

[0231] When a laser is incident perpendicularly on a liquid film, the reflected light will produce an "optical path difference between the upper and lower surfaces of the liquid film," which is related to the phase difference Δ. ϕ raw Satisfy Δ ϕ raw =4 πnd / λ is generated by reflections from the upper and lower surfaces of the liquid film. πA phase abrupt change, after superposition, results in a total optical path difference corresponding to a phase difference of 4. πnd / λ, therefore the thickness formula is derived by reverse derivation.

[0232] For example: if S6 outputs Δ ϕ raw = 0.02 prad The laser wavelength λ = 632.8 nm After correction based on natural gas composition n =1.35, then the liquid film thickness is: d = 632.8 × 0.02 π / 4 × 1.35 = 2.34 nm ;

[0233] When d=5 nm When, corresponding to Δ ϕ raw =0.02 prad This is consistent with the definition of the main threshold.

[0234] S72: By using continuous sampling and statistical analysis, the random errors of a single sampling are avoided from affecting the judgment results. Three key indicators are extracted: the mean (stability), standard deviation (uniformity), and rate of change (growth trend) of the liquid film thickness.

[0235] Sampling parameters: The acquisition step size is 50ms, that is, 20 samples per second, and the liquid film thickness data is read continuously for 50 times to obtain the thickness sequence. d 1, d 2,..., d 50 The corresponding time window is 50ms × 50 = 2.5s;

[0236] 3D index calculation: mean thickness : ;

[0237] Standard deviation (Reflects the degree of fluctuation in liquid film thickness, determining uniformity):

[0238] ;

[0239] rate of change :

[0240] ;

[0241] Mean thickness: Eliminates random noise from a single sampling and reflects the true thickness level of the liquid film;

[0242] Standard deviation: If the standard deviation is too large, it indicates that the liquid film precipitation is uneven and needs to be eliminated;

[0243] Rate of change: If the rate of change is positive and stable, it indicates that the liquid film is continuously precipitating, which is consistent with the physical characteristic of "heavy hydrocarbons begin to condense" at the hydrocarbon dew point.

[0244] S73: Perform multi-condition threshold determination: Through a combination of logic that uses the main threshold to determine the trend and auxiliary criteria to eliminate interference, the system accurately identifies real liquid film precipitation and false signals, avoiding misjudgment of hydrocarbon dew point. See Table 1 for the specific determination logic:

[0245] Table 1. Logic Table for Determining Real Liquid Film Deposition and False Signals

[0246]

[0247] The logical order of the decision is as follows:

[0248] First, calculate the mean thickness, standard deviation, and rate of change of the current window; verify whether the current window meets the condition of "mean thickness > 5nm and standard deviation < 1.5nm"; if it meets the condition, record it as a "valid window" and accumulate the number of valid windows; when the number of valid windows reaches 3 (consecutive), proceed to the next step of dew point confirmation; if it does not reach the condition, continue to collect the next set of 50 data points and repeat the determination.

[0249] S74: When the multi-condition threshold determination is passed, confirm that the mirror temperature of the dew point measuring device at this time is the hydrocarbon dew point of the natural gas.

[0250] When all the judgment conditions are met for three consecutive windows, the system automatically records the mirror temperature of the current measurement area, which is acquired by a PT1000 platinum resistance thermometer.

[0251] This temperature is the hydrocarbon dew point of natural gas at the current test pressure. Since the hydrocarbon dew point is the temperature at which heavy hydrocarbons begin to condense under a specific pressure, the test pressure is precisely set in S1.

[0252] The system outputs the final result, including the hydrocarbon dew point temperature, the corresponding test pressure, and auxiliary data such as the mean / standard deviation / rate of change of the liquid film thickness, thus completing one measurement.

[0253] A natural gas hydrocarbon dew point measurement system based on dual-region reference interferometric ranging is provided to implement any one of the natural gas hydrocarbon dew point measurement methods based on dual-region reference interferometric ranging, including:

[0254] The system includes a sample gas pretreatment module, a gas path splitting and purification module, a reference gas purity verification module, a dual-region interference detection module, a signal processing and phase calculation module, a multi-parameter compensation and differential phase calculation module, and a liquid film thickness conversion and dew point determination module.

[0255] The sample gas pretreatment module is equipped with a pressure regulating valve, a dehydration module and a particulate matter removal module connected in series.

[0256] The gas path diversion and purification module includes a three-way valve, a low-temperature adsorption module, and an inorganic adsorption module. The input end of the three-way valve is connected to the output end of the particulate matter removal module. The first output end of the three-way valve is directly connected to the measuring mirror position of the dual-region interference detection module. The second output end of the three-way valve is connected in series with the low-temperature adsorption module and the inorganic adsorption module and then connected to the reference mirror position of the dual-region interference detection module.

[0257] The reference gas purity verification module includes a second three-way valve, a pressure limiting valve, and a gas chromatograph. The input end of the second three-way valve is connected to the output end of the inorganic adsorption module. The first output end of the second three-way valve is connected to the reference mirror position of the dual-region interference detection module. The second output end of the second three-way valve is connected to the gas chromatograph via the pressure limiting valve.

[0258] The dual-region interferometric detection module includes a dew point measurement device, a laser source, a polarization beam splitter, a beam splitter, and an analog-to-digital converter. The dew point measurement device is divided into a measurement region and a reference region with equal area. The dew point measurement device is equipped with a semiconductor cooling stack and two independent platinum resistance thermometers. The laser source is a 632.8nm helium-neon laser source. The polarization beam splitter is used to split the laser into two paths, which are then perpendicularly incident on the two regions by the beam splitter. The analog-to-digital converter is used to synchronously acquire the interference signals of the reflected light from the two regions.

[0259] The signal processing and phase calculation module is used to perform pre-gain amplification, bandpass filtering, threshold denoising and median filtering on the acquired interference signal in sequence. The pre-processed signal is multiplied with the orthogonal reference signal to extract the DC components I and Q. The phase of the measurement area and the reference area are calculated by arctan2(Q,I) respectively.

[0260] The multi-parameter compensation and differential phase calculation module is used to introduce a temperature-pressure cross-compensation model to compensate and correct the phase value, and to calculate the dynamic differential phase based on the secondary baseline drift compensation model.

[0261] The liquid film thickness conversion and dew point determination module is used to convert the differential phase into liquid film thickness and determine whether the hydrocarbon dew point has been reached.

Claims

1. A method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging, characterized in that, Includes the following steps: S1: The sample gas to be tested is pre-treated by sequentially passing through the pressure regulating valve to adjust to the set test pressure, the dehydration module for deep dehydration, and the particulate matter removal module for particulate matter removal. S2: The pretreated sample gas to be tested is split into two streams through a three-way valve. One stream is directly delivered to the measuring mirror of the dew point measuring device as the measuring gas; the other stream is processed sequentially by the low-temperature adsorption module and the inorganic adsorption module to obtain the reference gas. S3: After partially purified reference gas is adjusted to the preset pressure through the pressure limiting valve, it is introduced into the gas chromatograph. The impurity composition is analyzed by the detector and dedicated chromatographic column of the gas chromatograph to ensure the purity of the reference gas. S4: The measuring gas and the remaining reference gas are respectively introduced into the measuring mirror position and the reference mirror position of the dew point measuring device. The helium-neon laser is split into two paths by a polarization beam splitter with a specified beam splitting ratio. The laser beams are perpendicularly incident on the two regions through a beam splitter. The interference signal of the reflected light from the two regions is synchronously collected by an analog-to-digital converter. S5: Preprocess the acquired interference signal, multiply the preprocessed signal with the orthogonal reference signal, extract the DC component by low-pass filtering at the cutoff frequency, and calculate the phase of the measurement area and the reference area respectively; S6: The phase values ​​of the measurement area and the reference area are compensated and corrected by the temperature-pressure cross-compensation model; the dynamic differential phase is calculated based on the secondary baseline drift compensation model. S7: Continuously read the liquid film thickness data for a specified number of times, calculate the mean thickness, standard deviation and rate of change, and use a multi-condition judgment threshold to determine whether the hydrocarbon dew point has been reached; The specific process of step S7 is as follows: S71: Liquid film thickness conversion: converting the abstract dynamic differential phase Δ ϕ raw Converted into quantifiable physical thickness of the liquid film d ,unit: nm Establish a direct correlation between phase change and liquid film thickness; S72: By continuously sampling and statistical analysis, the random errors of a single sampling are avoided from affecting the judgment results, and three key indicators of the liquid film thickness—mean, standard deviation, and rate of change—are extracted. S73: Perform multi-condition threshold determination: Identify real liquid film precipitation and false signals by combining the main threshold to determine the trend and the auxiliary criteria to eliminate interference through a combination of logic. S74: When the multi-condition threshold determination is passed, confirm that the mirror temperature of the dew point measuring device at this time is the hydrocarbon dew point of the natural gas.

2. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 1, characterized in that, The specific process of step S1 is as follows: S11: The sample gas to be tested is delivered to the automatic control pneumatic pressure regulating valve and adjusted to the set test pressure; S12: The sample gas to be tested, after being pressure regulated by the pressure regulating valve, is transported to the dehydration module for deep dehydration treatment to remove water vapor from the sample gas. The dehydration material is 3A / 4A molecular sieve composite packing. S13: The dehydrated sample gas to be tested is delivered to the particulate matter removal module to remove solid particulate matter from the sample gas. The particulate matter removal module is made of agglomerated fine filter element.

3. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 2, characterized in that, The specific process of step S2 is as follows: S21: The sample gas to be tested after being processed by the particulate matter removal module is split into two streams by a three-way valve according to a preset ratio. S22: The sample gas to be tested in branch 1 is directly delivered to the measuring mirror position of the dew point measuring device for subsequent condensation and dew detection; The sample gas to be tested in branch 2 undergoes two-stage purification treatment through a low-temperature adsorption module and an inorganic adsorption module to finally obtain high-purity methane reference gas, which is then delivered to the reference mirror position of the dew point measuring device.

4. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 3, characterized in that, The specific process of step S3 is as follows: S31: The high-purity methane reference gas obtained from branch 2 is split into two paths again through a three-way valve, including a main path and a secondary path: Main path: directly delivered to the reference mirror position of the dew point measuring device as a stable benchmark for interferometric measurement; Secondary path: used for purity verification. S32: Adjust the pressure of the secondary reference gas to the optimal analytical pressure of the gas chromatograph; the pressure limiting valve controls the pressure of the secondary reference gas at the preset pressure; S33: The pressure-adjusted secondary reference gas is delivered to the gas chromatograph inlet. After precise quantification via the injection valve, it is fed into the column system. The carrier gas carries the sample gas and flows within the column. Different impurity components elute from the column in a specific order due to their varying interaction strengths with the stationary phase. The eluent impurity components sequentially enter the detector, which converts the component concentration into an electrical signal. This signal is then recorded as a chromatographic peak by the data acquisition system. Based on the calibration curve, the specific content of each impurity component is calculated. S34: Determine the purity of the reference gas. Reference gas purity determination criteria: The total content of all impurities detected by gas chromatograph is less than the specified value, that is, the methane purity is greater than the specified value, and the content of single C2-C6 hydrocarbon components is less than the specified value, and the total content of inorganic components is less than the specified value. Application of the judgment result: If the purity meets the standard: the main reference gas is normally introduced into the reference mirror position of the dew point measuring device, and the measurement process continues; If the purity does not meet the standard: the system will trigger an alarm, indicating that the adsorption module has failed. After replacing or regenerating the adsorption packing, purification and verification will be carried out again.

5. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 1, characterized in that, The specific process of step S4 is as follows: S41: Perform optical system calibration. The polarization beam splitter has a 1:1 splitting ratio, ensuring that the laser is uniformly divided into two paths with the same intensity. Beam splitter and optical path adjustment: Adjust the angle of the beam splitter to ensure that the two laser beams, after reflection or transmission, are perpendicularly incident on the center of the measurement area and the reference area, respectively; S42: The measuring gas is introduced into the measuring mirror position of the dew point measuring device through a sealed pipeline. The gas pressure is kept consistent with the pretreatment pressure, and the flow rate is stably controlled at the set value by the flow meter. Reference gas: High-purity methane reference gas is introduced into the reference mirror position through an independent sealed pipeline, and the reference gas flow rate is matched with the measurement gas flow rate; Real-time monitoring of the pressure difference between the two gas paths ensures that there is no leakage or cross-contamination between the measuring gas and the reference gas; S43: Dual-zone temperature control: Cooling start-up: Start the semiconductor cooling stack to cool the single-crystal silicon cold mirror as a whole, and gradually reduce the mirror surface temperature to close to the hydrocarbon dew point of the sample gas; Temperature monitoring: The mirror temperature of the measurement area and the reference area are collected in real time to ensure the time correspondence between temperature data and optical signals; Temperature difference closed-loop control: Based on the collected temperature data, the refrigeration power distribution of the refrigeration stack is dynamically adjusted through the PID regulation algorithm to strictly control the temperature difference between the measurement area and the reference area within the preset range; S44: Laser beam splitting and dual-region incidence: Laser source activation: Turn on the 632.8nm helium-neon laser source. After the laser beam is collimated into parallel light by the collimating lens, it is incident on the polarization beam splitter. Uniform beam splitting: A polarization beam splitter splits the incident laser into two beams with perpendicular polarization directions at a 1:1 splitting ratio, which are defined as the measurement optical path laser and the reference optical path laser, respectively. Vertical incidence positioning: After the two laser beams are reflected / transmitted by their respective beam splitters, the optical path angle is adjusted to ensure that both laser beams are vertically incident on their respective mirror areas, and the diameter of the incident spot matches the area of ​​the region. S45: Synchronous acquisition of reflected light interference signals: Signal generation: After the laser is incident on the reference optical path in the reference region, it undergoes specular reflection, forming a reference interference signal; Measurement area: As the mirror temperature drops to the hydrocarbon dew point, the heavy hydrocarbon components in the measurement gas begin to condense and form a liquid film. The presence of the liquid film causes a change in the reflected optical path of the laser in the measurement optical path, forming a changing interference signal with the original light from the laser source. Synchronous signal acquisition: The reflected light from the two regions is refracted back through the optical path and then transmitted together to the optical detector. The detector converts the optical signal into an electrical signal, i.e., an interference signal. Two electrical signals, the interference signal in the measurement area and the interference signal in the reference area, are simultaneously input to the analog-to-digital converter to convert the analog signal into a digital interference signal.

6. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 5, characterized in that, The specific process of step S5 is as follows: S51: Perform signal preprocessing on the digital interference signal; S52: The preprocessed digital interference signal is demodulated by multiplying it with an orthogonal reference signal, and converted into a modulated signal with an extractable DC component. Phase information is stripped away through multiplication demodulation. S53: DC component extraction: Filter out the high-frequency components of the multiplication signal and retain the low-frequency DC components carrying phase information, in-phase components and quadrature components. S54: Phase Calculation: The phase value of the original interference signal is deduced by using in-phase and quadrature components.

7. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 6, characterized in that, The specific process of signal preprocessing in step S51 is as follows: S511: Linearly amplifies the digital interference signal through a preamplifier; S512: An active bandpass filter is used to filter the frequency of the amplified signal, suppressing power frequency interference, low-frequency drift, and high-frequency noise, while retaining the effective frequency components of the interference signal. S513: Nonlinear denoising based on wavelet transform, preserving signal mutation characteristics, eliminating random noise, and preserving signal mutation characteristics during liquid film formation; S514: Spatial domain filtering is performed on the wavelet-denoised signal to eliminate impulse noise. The final preprocessed signal is... , .

8. The method for measuring the dew point of natural gas hydrocarbons based on dual-region reference interferometric ranging according to claim 1, characterized in that, The specific process of step S6 is as follows: S61: Obtain compensation parameters, including two types: one is pre-calibrated fixed coefficients: temperature-phase coupling coefficient. k t Pressure-phase coupling coefficient k p Secondary drift coefficient a Linear drift coefficient b Initial drift offset c Another type is dynamic parameters collected in real time during the measurement process: the actual temperature difference ∆ between the two regions. T ∆ Actual pressure difference between the two regions P System uptime t ; S62: Perform temperature-pressure cross-compensation: original phase , The interference signals caused by temperature and pressure differences need to be accurately removed by cross-compensation formula to obtain the compensated phase that is only related to the mirror state. S63: Baseline drift compensation: For baseline phase drift caused by optical component aging and circuit noise drift during system operation that is unrelated to the liquid film, the drift amount is quantified by a quadratic fitting model and subtracted in subsequent difference calculations.

9. A natural gas hydrocarbon dew point measurement system based on dual-region reference interferometric ranging, used to implement the natural gas hydrocarbon dew point measurement method based on dual-region reference interferometric ranging as described in any one of claims 1-8, characterized in that, include: The system includes a sample gas pretreatment module, a gas path splitting and purification module, a reference gas purity verification module, a dual-region interference detection module, a signal processing and phase calculation module, a multi-parameter compensation and differential phase calculation module, and a liquid film thickness conversion and dew point determination module. The sample gas pretreatment module is equipped with a pressure regulating valve, a dehydration module and a particulate matter removal module connected in series. The gas path diversion and purification module includes a three-way valve, a low-temperature adsorption module, and an inorganic adsorption module. The input end of the three-way valve is connected to the output end of the particulate matter removal module. The first output end of the three-way valve is directly connected to the measuring mirror position of the dual-region interference detection module. The second output end of the three-way valve is connected in series with the low-temperature adsorption module and the inorganic adsorption module and then connected to the reference mirror position of the dual-region interference detection module. The reference gas purity verification module includes a second three-way valve, a pressure limiting valve, and a gas chromatograph. The input end of the second three-way valve is connected to the output end of the inorganic adsorption module. The first output end of the second three-way valve is connected to the reference mirror position of the dual-region interference detection module. The second output end of the second three-way valve is connected to the gas chromatograph via the pressure limiting valve. The dual-region interferometric detection module includes a dew point measurement device, a laser source, a polarization beam splitter, a beam splitter, and an analog-to-digital converter. The dew point measurement device is divided into a measurement region and a reference region with equal area. The dew point measurement device is equipped with a semiconductor cooling stack and two independent platinum resistance thermometers. The laser source is a 632.8nm helium-neon laser source. The polarization beam splitter is used to split the laser into two paths, which are then perpendicularly incident on the two regions by the beam splitter. The analog-to-digital converter is used to synchronously acquire the interference signals of the reflected light from the two regions. The signal processing and phase calculation module is used to perform pre-gain amplification, bandpass filtering, threshold denoising and median filtering on the acquired interference signal in sequence. The pre-processed signal is multiplied with the orthogonal reference signal to extract the DC components I and Q. The phase of the measurement area and the reference area are calculated by arctan2(Q,I) respectively. The multi-parameter compensation and differential phase calculation module is used to introduce a temperature-pressure cross-compensation model to compensate and correct the phase value, and to calculate the dynamic differential phase based on the secondary baseline drift compensation model. The liquid film thickness conversion and dew point determination module is used to convert the differential phase into liquid film thickness and determine whether the hydrocarbon dew point has been reached.

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