The Automatic Detection Method of Total Hydrocarbons in Air

What are Total Petroleum Hydrocarbons?

Total petroleum hydrocarbons are a chemical mixture, all constituent chemicals are mainly composed of carbon and hydrogen. Scientists classify the total petroleum hydrocarbons according to their reaction in soil or water, which can be divided into many petroleum hydrocarbons groups. Each group is made up of many individual chemicals.

• Total petroleum hydrocarbons may enter the environment unintentionally through industrial emissions, or by-products from commercial or personal use.
• Total petroleum hydrocarbons may enter the water directly through spills or leaks.
• Some total petroleum hydrocarbons float in water and form a surface film.
• Other total petroleum hydrocarbons sink into the sediment.
• Bacteria and microorganisms in water may decompose some of the total petroleum hydrocarbons.
• Some total petroleum hydrocarbons may migrate to the soil and remain there for an extended time.

The Automatic Detection Method of Total Hydrocarbons:

This detection method is an online flame ionization detection method. The sample directly enters the flame ionization detector (FID) through the system flow path to measure the content of Total hydrocarbon (THC) in the air. In addition, the sample is introduced into a selective combustion system (such as a catalytic converter) that decomposes non-methane total hydrocarbons and enters the FID detector to measure the content of Methane in the air. After deducting methane from THC, Total nonmethane hydrocarbon (TNMHC) content, the measured concentrations are expressed relative to methane.

• Scope of application: It is suitable for the detection of methane, total hydrocarbons, and non-methane total hydrocarbons in the air. The measurement range depends on the design of the instrument. Generally, it is recommended to select an instrument with a measurement range of not more than 100 ppm of total hydrocarbons in the air.
• Interference: High moisture content can affect sample concentration measurements.
• Sampling and saving: Unless otherwise stipulated by other environmental protection laws and regulations, the location of the sampling port is generally within the height range of 3 to 15 meters above the ground, and the placement position of other air sampling ports is in principle far away from the ground. Within the height range of 1.2 to 1.5 meters above the ground.

What Related Equipment and Materials are Required?

1. Automatic analyzer for methane, total hydrocarbons, and non-methane total hydrocarbons: The air sample is directly introduced into the FID detector, and the measured result is the concentration of total hydrocarbons. In addition, the sample is introduced into a selective combustion system, which is used to burn the non-methane hydrocarbons in the sample to remove the methane concentration after introduction into the FID detector. The non-methane hydrocarbon concentration is obtained by subtracting the methane concentration from the total hydrocarbon concentration.
2. Filter: Filter particulate pollutants in the air, such as Teflon and other materials that are not easy to absorb total hydrocarbons.
3. Recorder: A recorder or data logging system compatible with the analyzer must be able to display and record the concentration of each measurement in an appropriate range.
4. Sampling equipment:
   • Sampling port: The shape of the sampling port should avoid turbulent flow, such as using a geometrically symmetrical circular opening.
   • Air extraction motor: The air extraction volume must meet the total flow required by the instrument.
   • Gas delivery pipeline: The material of the pipeline in contact with the sampling gas must not adsorb hydrocarbons, and will not release substances that affect the hydrocarbon analysis of the detector, generally Teflon or other inert substances. The other pipelines used for fuel gas and standard gas can be made of Teflon or metal.
5. Calibration equipment:
   • Flow control valve: It can adjust and control the flow. If it is used for dilution (such as the mass flow controller used in the dynamic dilution system), the accuracy must reach ±2% within the range of the flow.
   • Flowmeter: calibrated flowmeter with an accuracy of ±2%.
   • Cylinder pressure control valve: pressure controller with inert material inner membrane and inner components.
   • Mixing tank: a container for fully mixing A, propane standard gas, and zero-point standard gas.
   • Output manifold: an air distribution pipe made of Teflon or other inert materials, connected to the analyzer with a pressure-resistant pipeline of appropriate diameter, and equipped with a valve to prevent the entry of the atmosphere.

What Related Detection Reagents are There?

• Methane (CH4) standard gas: methane standard gas containing the concentration required for calibration or high-concentration methane standard gas for dilution.
• Propane (C3H8) standard gas: the standard gas of propane containing the concentration required for calibration or the standard gas of high concentration for dilution.
• Zero-point standard gas: high-purity air with a total hydrocarbon concentration of less than 0.1 ppm (calculated as methane concentration).
• Hydrogen: fuel gas, used for FID analysis, and its total hydrocarbon concentration should be less than 0.1 ppm (calculated as methane concentration).
• Combustion-supporting gas: air or other suitable gas used for FID analysis, and its total hydrocarbon concentration should be less than 0.1 ppm (calculated as methane concentration).

What are the Steps?

1. General operation steps: After the sampling equipment, methane, total hydrocarbons, and non-methane total hydrocarbons automatic analyzers and recorders are properly installed, first check the piping system and other equipment, and then carry out the inspection work after confirmation.
   • Set the operating conditions.
   • Zero point/full-scale inspection, if the zero offset or full-scale offset of the respective measurement systems for methane, total hydrocarbons, and non-methane total hydrocarbons does not meet the specifications, perform multi-point calibration again.
   • Perform a sample gas sampling analysis.
2. Calibration steps:
   • Dynamic dilution method—Use high-concentration methane or propane standard gas to dilute to various desired concentrations by dynamic dilution.
     • Combined calibration system.
     • All flow meters used should be calibrated, and the volume has been converted to the conditions of 25 ℃, 760 mmHg.
     • Set the calibration range of the methane, total hydrocarbons, or non-methane total hydrocarbons analyzer to be the same as the general operating range.
     • Connect the analyzer and recorder.
     • Adjust the zero-point standard gas flow control valve to set the total gas flow so that enough flow can be input into the analyzer. Pass the gas until the analyzer shows a stable value, then adjust the zero-control knob of the analyzer to obtain a signal of 5% compensation of the recorder, and record the reading as the zero-point background value.
     • Adjust the flow rate of zero-point standard gas and standard gas from methane or propane to generate standard gas with full-scale concentration.
   • Multiple cylinder method - use methane or propane standard gases with confirmed concentrations for calibration.
     • Multi-cylinder calibration system device.
     • Input the zero-point standard gas and the standard gas of full-scale concentration respectively. After adjusting the zero-point and full-scale of the analyzer, input each standard concentration gas. Draw a calibration curve according to the input standard concentration of methane or propane and the response of the analyzer.

How to Carry Out Quality Control?

1. Correction frequency:
   • Newly installed or relocated instruments.
   • After the main equipment of the instrument has been maintained.
   • The routine zero-point offset or full-scale offset every working day does not meet the specifications.
   • Periodic calibration every 6 months.
2. Multi-point calibration: When performing multi-point calibration with five different concentrations of methane or propane standard gas such as 0%, 20%, 40%, 60%, and 80% of the measurement range, the linear correlation coefficient must be greater than 0.995.
3. Medium concentration inspection: After the system has been calibrated at zero point and full scale. The second source methane or propane standard gas with a concentration of about 50% full scale must be imported every working day. It should be less than ±2% of the full scale. If the system displays non-methane total hydrocarbons, the value error of the measurement system should be less than ±4% of the full scale.
4. The flow control valve shall be calibrated every 6 months.