Gas detection sampling preprocessing system

1、 System Overview

Gas detection sampling preprocessing systemIt is a key link connecting the gas sampling point and the analytical instrument. Its main function is to physically or chemically process the collected raw gas, remove interfering components, adjust the gas state, ensure that the gas entering the analytical instrument meets the instrument detection requirements, and thus improve the accuracy, reliability, and stability of the analysis results. This system is widely used in various fields such as environmental monitoring, industrial process control, scientific research experiments, and medical health, and is an important component in ensuring the quality of gas analysis data.

2、 System composition and functions of each component

（1） Sampling unit

The sampling unit is the part of the system that comes into direct contact with the gas environment to be tested, responsible for extracting or collecting the target gas from the sampling point into the system.

1. Sampling probe/sampling tubeSelect appropriate materials (such as stainless steel, polytetrafluoroethylene, glass, etc.) and structures based on different sampling environments and gas characteristics (such as temperature, humidity, dust content, corrosiveness, etc.). For example, in high-temperature flue gas sampling, metal sampling probes with cooling jackets are often used; In clean gas sampling, simple glass or quartz sampling tubes can be used. The design of the sampling probe should minimize interference with the airflow and ensure the collection of representative gas samples.

2. sampling pumpProvide power to transport gas samples from the sampling point to the subsequent processing unit. Select the appropriate sampling pump type based on the required flow rate, pressure, and gas properties of the system, such as diaphragm pump, peristaltic pump, rotary vane pump, etc. The flow stability and negative pressure capability of the sampling pump are key performance indicators that directly affect the sampling efficiency and sample representativeness.

3. Flow control and metering deviceLike a mass flow controller（MFC）Rotary flow meters, soap film flow meters, etc. are used for precise control and measurement of the flow rate of sampled gases. A stable sampling flow rate is an important factor in ensuring the repeatability of analysis results and also facilitates quantitative analysis.

（2） Preprocessing unit

The preprocessing unit is the core of the system, which uses various processing techniques to remove interference factors based on the characteristics and analysis requirements of different gas samples.

1. filtration deviceUsed to remove particulate matter (dust, aerosols, etc.) from gases. Common filters include metal sintered filters, glass fiber filter membranes, polytetrafluoroethylene filter membranes, etc. The selection of filters should consider filtration efficiency, pore size, temperature resistance, and corrosion resistance. They should be replaced or cleaned regularly to prevent blockages from affecting flow and filtration efficiency.

2. dehumidify/Drying deviceWhen the moisture content in the gas is too high, it may interfere with the analytical instrument or detection results (such as damaging the sensor, causing condensation, affecting the adsorption or reaction of certain gas components), dehumidification treatment is required. Common methods include:

ocooling dehumidificationBy using semiconductor refrigeration or compressor refrigeration to cool the gas below the dew point, moisture is condensed and precipitated. This is suitable for high flow, high humidity gases, but may result in the loss of some gas components that are easily soluble in water.

oAdsorption dryingUse desiccants (such as silica gel, molecular sieve, calcium chloride, phosphorus pentoxide, etc.) to remove moisture through adsorption. Adsorbents have a certain adsorption capacity and need to be regenerated or replaced regularly. For low dew point requirements, dual tower switching adsorption drying is often used to achieve continuous operation.

3. Dehydration (chemical method)/脱水In certain specific situations, chemical reagents such as concentrated sulfuric acid may be used to absorb moisture (but attention should be paid to its strong corrosiveness and absorption of certain gases).

4. Device for removing interfering gas componentsIf the sample contains other gas components that interfere with the target analyte, selective removal is required. For example:

oSelective adsorbentActivated carbon can adsorb organic vapors, Hoggart agents can remove oxygen (under specific conditions), alkaline absorbents (such as...)NaOH）Can remove acidic gases（CO₂TheSO₂TheHClEtc.), acidic absorbents (such asH₂SO₄）Can remove alkaline gases（NH₃Etc.).

oCatalytic conversion deviceUsing catalysts to convert interfering gases into harmless or non interfering substances. For example, using a conversion furnace toNO₂convert toNO, or under the action of specific catalysts, oxidize certain organic interferents toCO₂andH₂O.

5. Heating and insulation deviceDuring the sampling and transmission process, in order to prevent high boiling point components in the gas from condensing or some easily reactive components from changing due to temperature reduction, it is necessary to heat and insulate the sampling pipeline and some pre-treatment components. For example, collecting smokeVOCsUsually, full heating is required (such as120℃or180℃）To prevent its condensation loss.

6. Enrichment and concentration deviceWhen the concentration of the gas component to be tested is lower than that detected by the analytical instrument, it is necessary to enrich and concentrate the target component. Common methods include solid adsorbent adsorption-Hot desorption, low-temperature cold trap concentration, etc. For example, trace amounts in the atmosphereVOCsThe analysis often usesTenaxWait for the adsorbent to enrich.

（3） Transmission unit

Responsible for delivering the pre treated clean gas to the analytical instrument in a stable and non-destructive manner.

1. Transmission pipelineMaterials with good chemical inertness, low adsorption, and good sealing properties, such as polytetrafluoroethylene, should be selected（PTFE）Pipes, stainless steel pipes, etc. The pipeline should be as short as possible to reduce dead volume and avoid gas adsorption, desorption, or reaction during transmission. For gases that require heating and transmission, a heat tracing pipeline should be used.

2. Voltage and current stabilizing deviceBefore entering the analytical instrument, it is sometimes necessary to further stabilize the gas pressure and flow rate to ensure that the instrument operates under the specified conditions.

（4） Control and Monitoring Unit

With the improvement of automation, modernGas sampling preprocessing systemUsually equipped with control and monitoring units.

1. sensorReal time monitoring of operating parameters of key parts of the system, such as temperature sensors, humidity sensors, pressure sensors, liquid level sensors (used to detect condensed water), etc.

2. controllerAs follows:PLCProgrammable logic controllers (PLCs), microcontrollers, industrial control computers, etc. automatically control various executing components (such as the start and stop of sampling pumps, adjustment of flow controllers, switching of heating devices, triggering of alarm devices, etc.) based on feedback from sensors, achieving automated operation and fault diagnosis of the system.

3. Display and alarm deviceUsed to display system operating status parameters (flow rate, temperature, pressure, humidity, etc.) and fault information, and to emit sound and light alarm signals in case of abnormal situations (such as flow rate exceeding limit, filter blockage, temperature abnormality, insufficient gas source pressure, etc.), reminding operators to handle them in a timely manner.

3、 System design principles

1. Sample representativenessThe system design should ensure that the collected gas samples can truly reflect the gas composition and concentration at the sampling point, avoiding contamination, loss, or component changes during the sampling process.

2. targetedBased on the properties of the gas to be tested (composition, concentration, temperature, humidity, pressure, dust content, corrosiveness, etc.) and the requirements of the analytical instrument, select appropriate pretreatment techniques and devices to achieve targeted analysis.

3. efficiencyThe preprocessing process should be able to quickly and effectively remove interfering factors, shorten sample processing time as much as possible, and reduce sample retention in the system.

4. Stability and reliabilityEach component of the system should be selected with reliable quality products, designed with reasonable process flow to ensure long-term stable operation and reduce maintenance.

5. Automation and IntelligenceIf conditions permit, try to increase the automation level of the system as much as possible, achieve unmanned operation, automatic sampling, automatic processing, automatic data recording and transmission, and improve work efficiency and data reliability.

6. safetyFor the sampling of toxic, harmful, flammable and explosive gases, the system should have good sealing and safety protection measures to prevent gas leakage from causing personal injury or environmental pollution.

7. MaintainabilityThe system structure should be easy to operate, maintain, and overhaul, and key components should be easy to replace.

4、 Application Fields

1. Environmental air quality monitoring: Regarding the atmosphereSO₂TheNOxTheCOTheO₃TheVOCsThePM2.5ThePM10Sampling and pre-treatment of pollutants.

2. Fixed pollution source exhaust gas monitoringMonitoring of pollutants in flue gas emissions from power plants, chemical plants, steel plants, etc.

3. Industrial process gas analysisIn industrial production processes such as petrochemicals, metallurgy, semiconductors, and food fermentation, online monitoring of process gas components is carried out to optimize production processes, ensure product quality, and ensure safe production.

4. Indoor air quality monitoringMonitor indoor air for formaldehyde, benzene derivativesTVOCTheCO₂Wait.

5. Scientific research experimentProvide sample gases that meet the requirements for laboratory gas analysis instruments.

6. medical fieldSuch as anesthesia gas monitoring, exhaled gas analysis, etc.

7. Emergency monitoringRapid sampling and pre-treatment analysis of toxic and harmful gases at the site of sudden environmental pollution incidents or accidents.

Gas detection sampling preprocessing systemThe performance of the system is directly related to the quality of gas analysis data. Therefore, in the process of design, selection, installation, and operation maintenance, various influencing factors must be fully considered, and optimized configuration must be carried out according to specific application scenarios and analysis requirements to ensure that the system can work stably and reliably in the long term.