Research On Detection Method Of VOCs Concentration In Oil And Gas Recovery System Of Oil Storage Depot Based On TDLAS Technology

With the continuous consumption of fossil energy,environmental pollution problems have threatened people’s life and health.Therefore,the state has introduced many policies and regulations to control pollution.One of them is to install oil and gas recovery systems in oil storage depots to recover volatile organic compounds（VOCs）,which are the main source of hydrocarbon pollution.The state also requires regular testing of the concentration of non-methane hydrocarbons（NMHC）at the discharge port of the oil and gas recovery device,the molar mass fraction concentration of VOCs emitted from any leakage point of the oil and gas collection system,and other parameters to ensure the device’s operation.The oil and gas recovery system has significantly improved the environment around the storage depot in recent years,but some problems still exist,such as old and dilapidated system,equipment failure and loose seals.These problems are not found and repaired in time during the testing window.Moreover,the traditional detection methods,such as gas chromatography and hydrogen flame ionization detector（FID）,cannot achieve real-time online detection due to their complex process and low efficiency.Therefore,this paper investigates the online detection method for oil and gas recovery systems in oil storage depots.Using the Lambert-Beer law as the basic principle,the Tunable Diode Laser Absorption Spectroscopy（TDLAS）technique measures gas concentrations by exploiting the selective absorption of light at specific wavelengths by the target gas molecules.This technique offers advantages such as high automation,rapid detection,non-invasiveness,and explosion-proof suitability.This paper discusses the online detection of oil and gas recovery system based on TDLAS technology.Firstly,the basic theory of TDLAS technology and the selection of molecular absorption spectral lines are introduced in detail.Secondly,the working principle and structure of oil and gas recovery system of oil storage reservoir are systematically analyzed,and the points that need to be paid attention to in the process of system detection are analyzed.Thirdly,the principle and detection process of gas chromatography and FID field detection equipment are introduced in this paper,and by using the above two detection methods to detect the analyzed discharge port and sealing point,four key detection points that need to be focused on are finally determined:the discharge port of the oil and gas recovery device,the rotating seal of the vacuum pump,the rotating seal of the supply pump,and the rotating seal of the return pump.Fourthly,the theoretical equation of detection is further derived and the concentration inversion equation of different points is obtained by combining the proportion of methane in VOCs of oil and gas recovery system in different locations.Fifthly,throughput analysis,the design scheme of the online detection device is proposed,and the characteristics of each part and the advantages that the scheme has are introduced in detail.The scheme was simulated and analyzed by MATLAB to verify the correctness of the derived equations,and it was studied that when the accuracy of the temperature sensor is±0.5K and the accuracy of the air pressure sensor is±0.0025atm,it will theoretically bring no more than 0.4%error for the system Finally,by establishing a laboratory system,CH₄ and oil gas concentrations were measured in an environment with a temperature of288.15K and a pressure of 1.006atm.The feasibility of the detection was analyzed from three perspectives:system accuracy,linearity,and precision.This has laid the foundation for the miniaturization and industrial application of subsequent systems.