Low-concentration atmosphere VOCs sampling pretreatment device and method

Abstract

The invention relates to the technical field of environment gas pretreatment analysis, and discloses a low-concentration atmosphere VOCs sampling pretreatment device and method. The pretreatment device comprises a cold trap module which at least comprises two cold trap units working independently, each cold trap unit is composed of a stainless steel pipeline and provided with a temperature control module, the temperature adjusting range is-196 DEG C to 250 DEG C, and the cold trap module is used for adsorbing VOCs and moisture in a sample at low temperature and desorbing target VOCs at high temperature; the refrigerating system comprises a semiconductor thermoelectric refrigerating device which is mixed with a mechanical refrigerating unit and is used for controlling the adsorption temperature of the cold trap module; and the flow control module is used for realizing constant-current sampling by adopting an electronic metering algorithm, and the flow regulation range is 0-100mL / min. According to the method disclosed by the invention, two-section cold trap step-by-step condensation is adopted, and water is directionally removed in combination with blowback airflow, so that the recovery rate of VOCs is increased, the residual quantity of water is lower than 50ppm, and the peak shape symmetry and data reproducibility of GC-MS detection are remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of environmental gas pretreatment analysis, and in particular to a low-concentration atmospheric VOCs sampling pretreatment device and method. Background Art

[0002] Volatile organic compounds (VOCs) are organic compounds with a saturated vapor pressure greater than 70.91 Pa at room temperature, a boiling point between 50°C and 260°C at a standard atmospheric pressure of 101.3 kPa, and an initial boiling point of 250°C. They are also any organic solid or liquid that can evaporate at room temperature and pressure. They are generally categorized into several categories: non-methane hydrocarbons, oxygen-containing organic compounds, halogenated hydrocarbons, nitrogen-containing organic compounds, and sulfur-containing organic compounds.

[0003] VOCs in the environment are precursors of photochemical reactions. Under sunlight, VOCs react with nitrogen oxides and other suspended particulate matter in the air to form ozone and photochemical smog, which pose serious risks to human health.

[0004] At present, there are instruments for detecting VOCs waste gas, but before analyzing atmospheric samples, sample gas collection is a problem. When the atmosphere is analyzed as a target sample, the atmospheric concentration is about 20PPB, which is a low-concentration gas compared to the emission pollution source.

[0005] Similarly, we know that the atmosphere contains a certain amount of moisture. Many similar analytical instruments on the market that measure ammonia / sulfide / nitrogen oxides / ozone also encounter the same problem. How to pre-treat the sample for low-temperature adsorption, high-temperature desorption, and sample dehydration before sample injection is a technical problem that needs to be urgently solved by this technical solution. Summary of the Invention

[0006] The purpose of the present invention is to provide a low-concentration atmospheric VOCs sampling pretreatment device and method, which adopts a two-stage cold trap for step-by-step condensation and combines backflushing airflow to remove moisture in a directional manner, thereby improving the VOCs recovery rate, reducing the residual moisture content to less than 50ppm, and significantly improving the peak symmetry and data reproducibility of GC-MS detection, aiming to solve the problems in the prior art.

[0007] The present invention is implemented as follows: a low-concentration atmospheric VOCs sampling and pretreatment device, comprising:

[0008] The cold trap module consists of at least two independently working cold trap units. Each cold trap unit is made of stainless steel pipes and equipped with a temperature control module with a temperature adjustment range of -196°C to 250°C. It is used to adsorb VOCs and moisture in the sample at low temperatures and desorb target VOCs at high temperatures.

[0009] Refrigeration system: including a combination of semiconductor thermoelectric refrigeration devices and mechanical refrigeration units to control the adsorption temperature of the cold trap module;

[0010] Flow control module: used to achieve constant flow sampling using electronic metering algorithms, with a flow adjustment range of 0-100mL / min;

[0011] Valve system: including multiple sample valves, purge valves, vent valves and three-way valves, which can realize the separation and switching of sample gas path, purge gas path and desorption gas path through multi-way switching;

[0012] MCU controller: built-in PID temperature control algorithm and sampling timing logic, used to control the cold trap temperature, desorption time, purge flow rate and sampling flow rate;

[0013] Dual cold trap alternating working module: The cold trap units can operate independently or alternately, connected to different analytical instruments or dual channels of the same instrument;

[0014] Dehydration backflush module: During the desorption stage, the target VOCs are desorbed by high temperature, and the residual water and carbon dioxide in the cold trap are discharged by backflush with air or nitrogen.

[0015] Furthermore, the semiconductor thermoelectric refrigeration plate of the refrigeration system is a multi-stage series structure, with the cold end attached to the outer wall of the cold trap pipeline and the hot end connected to the water-cooled radiator.

[0016] Furthermore, the mechanical refrigeration unit adopts a cascade compression cycle, with a minimum refrigeration temperature of -196°C, and can be continuously adjusted in the range of -40°C to -196°C.

[0017] Furthermore, the switching logic between the semiconductor thermoelectric refrigeration device and the mechanical refrigeration unit is: when the target temperature is ≥-40°C, only the semiconductor thermoelectric refrigeration device is enabled for refrigeration, and when the target temperature is <-40°C, the mechanical unit is enabled for refrigeration.

[0018] Furthermore, the pipes of the cold trap module are filled with a composite adsorbent, comprising the following layered structure:

[0019] A hydrophobic Tenax TA polymer layer for adsorbing non-polar VOCs with a boiling point of 50-250°C;

[0020] Graphitized carbon black layer, used to adsorb polar VOCs with a boiling point of -50-150°C;

[0021] The silica gel loaded lithium chloride layer is used to absorb residual moisture, and each layer is separated by quartz wool with a filling length ratio of 2:2:1. The outer wall of the cold trap pipe is wrapped with a heating wire.

[0022] Compared with the prior art, the low-concentration atmospheric VOCs sampling pretreatment device and method provided by the present invention has the following beneficial effects:

[0023] 1. The use of a two-stage cold trap for step-by-step condensation, combined with directional backflush airflow to remove moisture, improves VOCs recovery rates and reduces residual moisture to less than 50ppm, significantly improving peak symmetry and data reproducibility in GC-MS detection. The first and second cold traps can work independently or alternately, supporting seamless switching between continuous sampling and desorption, increasing the system's effective operating time from 50% of a traditional single cold trap to 90%. The combination of semiconductor thermoelectric cooling and mechanical refrigeration enables rapid temperature adjustment of the cold trap, and the PID algorithm stabilizes temperature control accuracy to meet the full-component capture requirements of wide-boiling-point VOCs. The built-in electronic metering flow module supports adaptive flow adjustment for multi-capacity sampling canisters. The touch screen interface allows users to preset sampling time, desorption parameters, and multi-task queues. Remote monitoring and data synchronization are achieved through a wireless module, making it compatible with mainstream analytical equipment such as GC-MS and PID detectors.

[0024] 2. By programmatically controlling the sampling, desorption, and purging steps, human intervention errors are eliminated, and the relative deviation of monitoring data at different points is greatly reduced. The adjustable parameter design of the cold trap temperature and desorption flow rate supports rapid response from extremely low concentrations to sudden high-concentration pollution events. The detection linear range is extended to 3 orders of magnitude. In addition, air is used instead of high-purity nitrogen for purging, which reduces the cost of a single operation. The modular design of the cold trap module extends the consumables replacement cycle to more than 2 years, reducing solid waste generation. In addition, this device can be adapted to fixed source monitoring, mobile cruise monitoring, and laboratory offline analysis. By adjusting the cold trap parameters, the component separation of complex VOCs mixtures can be achieved, providing technical support for special monitoring of ozone precursors and toxic and harmful gases.

[0025] A low-concentration atmospheric VOCs sampling pretreatment method is implemented by the above-mentioned pretreatment device, and specifically comprises the following steps:

[0026] S101: Using hybrid refrigeration technology, the cold trap module is cooled to -196°C to 0°C, and atmospheric samples are collected at a constant flow rate, allowing VOCs and moisture to be adsorbed by the cold trap;

[0027] S102: Raise the temperature of the cold trap module to 20-250°C, introduce carrier gas in reverse at a flow rate of 20-100 mL / min, desorb the target VOCs by graded temperature increase, and transport them to the analytical instrument;

[0028] S103: After the graded temperature desorption is completed, air or nitrogen is introduced at a flow rate of 100-110 mL / min to purge the cold trap module to remove residual moisture;

[0029] S104: setting the cold trap units to work alternately to achieve parallel operation of continuous sampling and staged heating desorption;

[0030] S105: Complete the full process control of timed sampling, desorption time and purge time through the preset program.

[0031] Furthermore, in the low-temperature adsorption stage in S101, the temperatures of the first cold trap and the second cold trap are set to -10°C and -40°C, respectively; in the graded temperature increase desorption stage in S102, the temperatures of the first cold trap and the second cold trap are set to 20°C and 230°C, respectively, to adapt to the separation of VOCs components with different boiling points in the atmospheric sample.

[0032] Furthermore, in S101, before cooling the cold trap module to -196°C to 0°C using the hybrid refrigeration technology, a pre-leakage check is required for the device. The pre-leakage check includes:

[0033] Inject high-purity nitrogen into the device to a pressure of 50kPa, close the gas source and monitor the pressure change. If the pressure drop is ≤1kPa within 5 minutes, it is considered to be sealed.

[0034] If the leak detection fails, the touch screen will display the leak point code and guide the user to check and repair it step by step. The leak detection data of each pre-leak detection will be automatically recorded and an encrypted report will be generated.

[0035] Furthermore, in S102, the target VOCs are subjected to staged temperature desorption and transported to an analytical instrument, wherein the staged temperature desorption includes:

[0036] The first stage: heating to 50°C at a rate of 10°C / min to desorb VOCs with boiling points less than 50°C;

[0037] The second stage: heating to 150℃ at a rate of 5℃ / min to desorb VOCs with a boiling point of 50-150℃;

[0038] The third stage: the temperature is raised to 230℃ at a rate of 2℃ / min to desorb VOCs with a boiling point greater than 150℃, and the desorption air flow rate increases in each stage, which are 20mL / min, 50mL / min, and 100mL / min respectively. The desorbed gas is dried by the dehydration membrane and then enters the analytical instrument.

[0039] Furthermore, in S104, the cold trap units are set to work alternately, and the cold trap unit alternate operation mode includes:

[0040] The first cold trap was set to sample for 8 hours with a flow rate of 5.2 mL / min, while the second cold trap was performing desorption analysis with a cycle of 8.5 hours.

[0041] Mode B: The first and second cold traps sample synchronously for 4 hours, followed by desorption, with a total cycle of 8 hours, suitable for daily average monitoring;

[0042] Mode C: The first cold trap is connected to the online GC-MS, and the second cold trap is connected to the portable PID to achieve multi-instrument comparative analysis of the same pollution source. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 This is a structural schematic diagram of a low-concentration atmospheric VOCs sampling and pretreatment device proposed by the present invention;

[0044] Figure 2 This is a schematic flow chart of a low-concentration atmospheric VOCs sampling pretreatment method proposed by the present invention;

[0045] Figure 3 This is a schematic flow chart of the pre-leak detection process in a low-concentration atmospheric VOCs sampling pretreatment method proposed by the present invention;

[0046] Figure 4 This is a structural schematic diagram of Example 1 of a low-concentration atmospheric VOCs sampling and pretreatment device proposed by the present invention.

[0047] In the figure: 1-first sample valve; 2-air pump; 3-second sample valve; 4-first three-way valve; 5-first purge valve; 6-second three-way valve; 7-second purge valve; 8-third three-way valve; 9-first cold trap; 10-second cold trap; 11-fourth three-way valve; 12-first vent valve; 13-fifth three-way valve; 14-second vent valve; 15-sixth three-way valve; 16-first flow module; 17-second flow module. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0049] The implementation of the present invention is described in detail below with reference to specific embodiments.

[0050] The same or similar numbers in the drawings of this embodiment correspond to the same or similar parts; in the description of the present invention, it should be understood that if the terms "up", "down", "left", "right", etc. indicate directions or positional relationships, they are based on the directions or positional relationships shown in the drawings. This is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, the terms describing the positional relationship in the drawings are only used for illustrative purposes and cannot be understood as limiting the present invention. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to specific circumstances.

[0051] Example 1

[0052] Reference Figure 1 As shown, a low-concentration atmospheric VOCs sampling and pretreatment device includes:

[0053] The cold trap module contains at least two independently working cold trap units. Each cold trap unit is composed of stainless steel pipes and is equipped with a temperature control module with a temperature adjustment range of -196°C to 250°C. It is used to adsorb VOCs and moisture in the sample at low temperatures and desorb target VOCs at high temperatures; the refrigeration system: includes a mixture of semiconductor thermoelectric refrigeration devices and mechanical refrigeration units to control the adsorption temperature of the cold trap module; the flow control module: is used to achieve constant flow sampling using an electronic metering algorithm, with a flow adjustment range of 0-100mL / min; the valve system: includes multiple sample valves, purge valves, vent valves and three-way valves, and realizes the separation and switching of the sample gas path, purge gas path and desorption gas path through multi-way switching; the MCU controller: has a built-in PID temperature control algorithm and sampling timing logic , used to control the cold trap temperature, desorption time, purge flow rate and sampling flow rate; dual cold trap alternating working module: the cold trap units can operate independently or alternately, and are connected to different analytical instruments or dual channels of the same instrument; dehydration backflush module: in the desorption stage, the target VOCs are desorbed by high temperature, and the residual water and carbon dioxide in the cold trap are discharged by backflush with air or nitrogen. The two-stage cold trap is used for step-by-step condensation, combined with the directional removal of moisture by backflush airflow, so that the VOCs recovery rate is improved, the residual moisture content is less than 50ppm, and the peak symmetry and data reproducibility of GC-MS detection are significantly improved. The first cold trap and the second cold trap can work independently or alternately, supporting seamless switching between continuous sampling and desorption, and increasing the effective operation time of the system from 50% of the traditional single cold trap to 90%.

[0054] Reference Figure 4 , which is the structural diagram of this embodiment. The inlet of the first sample valve 1 is connected to the sample gas with a pipe joint, and the outlet is connected to the first tee 1 with a stainless steel pipe. Its main function is to replace the samples in turn or serve as an independent sampling port when there are many samples.

[0055] In this embodiment, the inlet of the air pump 2 is connected to the air in the atmosphere by a stainless steel pipe, and the outlet is connected to the second tee 6 by a stainless steel pipe. The main function is to use air instead of 99.999% nitrogen to clean and purge the pipeline, saving costs.

[0056] In this embodiment, the inlet of the second sample valve 3 is connected to the sample gas by a pipe joint, and the outlet is connected to the third tee 8 by a stainless steel pipe. Its main function is to replace the samples in turn or serve as an independent sampling port when there are many samples.

[0057] In this embodiment, the inlet of the first three-way valve 4 is connected to the outlet of the first sample valve 1 and the first purge valve 5 with a stainless steel pipe, and the outlet is connected to the inlet of the first cold trap 9 with a stainless steel pipe. Its main function is to collect the gases in each channel and output them from one outlet.

[0058] In this embodiment, the inlet of the first purge valve 5 is connected to the outlet of the second tee 6 by a stainless steel pipe, and the outlet is connected to the first tee 4 by a stainless steel pipe. The main function is to clean and purge the first cold trap 9 pipeline.

[0059] In this embodiment, the inlet of the second three-way valve 6 is connected to the outlet of the air pump 2 with a stainless steel pipe, and the outlet is connected to the inlet stainless steel pipes of the first purge valve 5 and the second purge valve 7. Its main function is to aggregate the gas from each channel and output it from one outlet.

[0060] In this embodiment, the inlet of the second purge valve 7 is connected to the outlet of the second tee 6 with a stainless steel pipe, and the outlet is connected to the third tee 8 with a stainless steel pipe. The main function is to clean and purge the second cold trap 10 pipeline.

[0061] In this embodiment, the inlet of the third three-way valve 8 is connected to the outlet of the second sample valve 3 and the second purge valve 7 with a stainless steel pipe, and the outlet is connected to the inlet of the second cold trap 10 with a stainless steel pipe. Its main function is to collect the gases from each channel and output them from one outlet.

[0062] In this embodiment, the inlet of the first cold trap 9 is connected to the outlet of the first tee 4 with a stainless steel pipe, and the outlet is connected to the inlet of the fourth tee 11 with a stainless steel pipe. The main function is to use a special tube as a cold trap to condense and capture all condensable substances in the sample gas, including all volatile organic compounds, water and carbon dioxide, and then desorb all the volatile organic compounds we need, leaving water and carbon dioxide, etc., waiting to be purged out of the cold trap.

[0063] In this embodiment, the inlet of the second cold trap 10 is connected to the outlet of the third tee 8 with a stainless steel pipe, and the outlet is connected to the inlet of the sixth tee 15 with a stainless steel pipe. The main function is to use a special pipe as a cold trap to condense and capture all condensable substances in the sample gas, including all volatile organic compounds, water and carbon dioxide, and then desorb all the volatile organic compounds we need, leaving water and carbon dioxide, etc., waiting to be purged out of the cold trap.

[0064] In this embodiment, the inlet of the fourth three-way valve 11 is connected to the outlet of the first cold trap 9 with a stainless steel pipe, and the outlet is connected to the first flow module 16 and the inlet of the first vent valve 12 with a stainless steel pipe. Its main function is to collect the gases in each channel and output them from one outlet.

[0065] In this embodiment, the inlet of the first vent valve 12 is connected to the outlet of the fourth three-way valve 11 with a stainless steel pipe, and the outlet is connected to the inlet of the fifth three-way valve 13 with a stainless steel pipe. The main function is to wait for the cold trap to desorb all the volatile organic compounds we need, leaving water and carbon dioxide, etc., and wait for the purge valve to open at the same time to discharge them into the cold trap.

[0066] In this embodiment, the inlet of the fifth three-way valve 13 is connected to the outlet of the first vent valve 12 and the second vent valve 14 with a stainless steel pipe, and the outlet is connected to the vent port with a stainless steel pipe. Its main function is to collect the gas from each channel and output it from one outlet.

[0067] In this embodiment, the inlet of the second vent valve 14 is connected to the outlet of the sixth three-way valve 15 with a stainless steel pipe, and the outlet is connected to the inlet of the fifth three-way valve 13 with a stainless steel pipe. The main function is to wait for the cold trap to desorb all the volatile organic compounds we need, leaving water and carbon dioxide, etc., and wait for the purge valve to open at the same time to discharge them into the cold trap.

[0068] In this embodiment, the inlet of the first flow module 16 is connected to the outlet of the fourth three-way valve 11 by a stainless steel pipe, and the outlet is connected to the sample output port by a stainless steel pipe. The main function is to open the metering when the sample is injected and transferred, and it is closed at other times.

[0069] In this embodiment, the inlet of the second flow module 17 is connected to the outlet of the sixth three-way valve 15 by a stainless steel pipe, and the outlet is connected to the sample output port by a stainless steel pipe. The main function is to open the metering when the sample is injected and transferred, and it is closed at other times.

[0070] In this embodiment, the main function of the MCU controller is to send relevant instructions to the software program to control the normal operation of each component. In addition, the device is also connected to an external switching power supply, whose main function is to increase the output voltage value by increasing the number of secondary windings of the transformer. Finally, these AC waveforms are rectified and filtered to obtain a DC output voltage to maintain the output voltage stable, thereby ensuring the power supply of the equipment.

[0071] In this embodiment, the semiconductor thermoelectric refrigeration plate of the refrigeration system has a multi-stage series structure, with the cold end attached to the outer wall of the cold trap pipe and the hot end connected to the water-cooled radiator. The mechanical refrigeration unit adopts a cascade compression cycle, with the lowest refrigeration temperature reaching -196°C and can be continuously adjusted in the range of -40°C to -196°C.

[0072] In this embodiment, the switching logic between the semiconductor thermoelectric refrigeration device and the mechanical refrigeration unit is: when the target temperature is ≥-40°C, only the semiconductor thermoelectric refrigeration device is enabled for cooling; when the target temperature is <-40°C, the mechanical unit is enabled for cooling.

[0073] In this embodiment, the pipes of the cold trap module are filled with a composite adsorbent, which includes the following layered structure:

[0074] A hydrophobic Tenax TA polymer layer for adsorbing non-polar VOCs with a boiling point of 50-250°C;

[0075] Graphitized carbon black layer, used to adsorb polar VOCs with a boiling point of -50-150°C;

[0076] The silica gel-loaded lithium chloride layer is used to adsorb residual moisture, and each layer is separated by quartz wool with a filling length ratio of 2:2:1. A heating wire is wrapped around the outer wall of the cold trap pipeline. The sampling, desorption, and purging steps are controlled through programmed control to eliminate manual intervention errors, thereby greatly reducing the relative deviation of monitoring data at different points. The adjustable parameter design of the cold trap temperature and desorption flow rate supports rapid response from extremely low concentrations to sudden high-concentration pollution events, and the detection linear range is extended to 3 orders of magnitude. In addition, air is used instead of high-purity nitrogen for purging, which reduces the cost of a single operation. The modular design of the cold trap module extends the consumables replacement cycle to more than 2 years, reducing solid waste generation. In addition, this device can be adapted to fixed source monitoring, mobile cruise monitoring, and laboratory offline analysis. By adjusting the cold trap parameters, the components of complex VOCs mixtures can be separated, providing technical support for special monitoring of ozone precursors and toxic and harmful gases.

[0077] The present invention passes the sample at the sampling point through an adsorption dehydration cold trap and starts timed constant flow sampling. After the sampling time corresponding to the set constant flow rate is reached, it undergoes key technologies such as high-temperature desorption and backwash dehydration, thereby solving the problem of how to perform pre-injection pretreatment when the atmospheric concentration is too low and dehydration is required as the target sample, so that the substance can be separated into peaks on the gas chromatography-mass spectrometry, the data results are true, and the instrument linearity and reproducibility are good.

[0078] Similarly, we know that the atmosphere contains a certain amount of moisture when used as a sample. Many similar analytical instruments on the market that measure ammonia / sulfide / nitrogen oxides / ozone also encounter the same problem before sample injection: how to pre-treat the sample for low-temperature adsorption, high-temperature desorption, and sample dehydration before sample injection.

[0079] Working Principle: Before sampling / injection, the sample passes through an atmospheric sampling adsorption dehydration cold trap. The sampling time, flow rate, and sampling volume of the cold trap are set. The sample at the sampling point passes through the adsorption dehydration cold trap to cool and adsorb the compound. After the sampling time corresponding to the set constant flow rate is reached, the sample undergoes high-temperature desorption and is transported to a dedicated analytical instrument for analysis. The atmospheric sampling adsorption dehydration cold trap backflushes the components produced by the cold trap sample to dehydrate them. The cold trap is then baked at high temperature to expel water vapor from the instrument, achieving the purpose of dehydrating the target sample when the atmospheric concentration is too low.

[0080] Design Concept: Utilizing hybrid refrigeration with cryogenic cold traps, this product employs advanced semiconductor thermoelectric refrigeration devices and mechanical refrigeration technology. The product utilizes the PID method for temperature control, resulting in fast and stable control. It also utilizes advanced electronic metering algorithm flow technology, resulting in a compact size, light weight, high precision, vibration-free operation, long life, temperature controllable at any point, and energy-saving and environmentally friendly features. Users can access test data, perform calibration, and set parameters through the panel. The instrument features a built-in touch screen, intuitive parameter settings, user-friendly operation, and back-end printing. Two cryogenic cold traps are built in, each capable of independent operation, doubling the dual-use effect. Compared to many commercially available analytical instruments that can only be used for high-concentration analysis, cold traps lack any selectivity and fully and accurately pre-treat the current sample. This cold trap expands this advantage, enabling sampling in various environments and for various needs through various methods. This non-selective atmospheric sampling adsorption dehydration cold trap is a perfect example.

[0081] Advantages: compatible with most analytical instruments on the market, can complete multi-point, timing, and quantitative analysis, and can be connected to analytical instruments of different specifications; fully automated sampling preprocessing, setting sampling time, volume and selecting different flow paths to achieve unattended sampling process; adopts low-temperature cold trap hybrid refrigeration, and adopts advanced semiconductor thermoelectric refrigeration devices and mechanical refrigeration hybrid application technology; advanced electronic metering algorithm flow technology, with small size, light weight, high precision, no vibration, long life, and temperature controllable at any point; two low-temperature sampling cold traps are set inside, which can work independently, and one machine can be used for dual purposes. It can be compatible with two different analytical instruments for sampling preprocessing at the same time, doubling the effect.

[0082] Reference Figure 2-3 A low-concentration atmospheric VOCs sampling pretreatment method is implemented by the above-mentioned pretreatment device, specifically comprising the following steps:

[0083] S101: Using hybrid refrigeration technology, the cold trap module is cooled to -196°C to 0°C, and atmospheric samples are collected at a constant flow rate, allowing VOCs and moisture to be adsorbed by the cold trap;

[0084] S102: Raise the temperature of the cold trap module to 20-250°C, introduce carrier gas in reverse at a flow rate of 20-100 mL / min, desorb the target VOCs by graded temperature increase, and transport them to the analytical instrument;

[0085] The target VOCs are subjected to graded temperature desorption and transported to the analytical instrument. The graded temperature desorption includes:

[0086] The first stage: heating to 50°C at a rate of 10°C / min to desorb VOCs with boiling points less than 50°C;

[0087] The second stage: heating to 150℃ at a rate of 5℃ / min to desorb VOCs with a boiling point of 50-150℃;

[0088] The third stage: the temperature is raised to 230℃ at a rate of 2℃ / min to desorb VOCs with a boiling point greater than 150℃. The desorption gas flow rate increases in each stage, namely 20mL / min, 50mL / min, and 100mL / min. The desorbed gas is dried by the dehydration membrane and then enters the analytical instrument.

[0089] S103: After the graded temperature desorption is completed, air or nitrogen is introduced at a flow rate of 100-110 mL / min to purge the cold trap module to remove residual moisture;

[0090] S104: setting the cold trap units to work alternately to achieve parallel operation of continuous sampling and staged heating desorption;

[0091] Among them, the cold trap units are set to work alternately, and the cold trap unit alternate operation modes include:

[0092] The first cold trap was set to sample for 8 hours with a flow rate of 5.2 mL / min, while the second cold trap was performing desorption analysis with a cycle of 8.5 hours.

[0093] Mode B: The first and second cold traps sample synchronously for 4 hours, followed by desorption, with a total cycle of 8 hours, suitable for daily average monitoring;

[0094] Mode C: The first cold trap is connected to an online GC-MS, and the second cold trap is connected to a portable PID, enabling multi-instrument comparative analysis of the same pollution source;

[0095] S105: Through preset programs, the full process control of timed sampling, desorption time and purge time is completed. The semiconductor thermoelectric cooling is combined with mechanical refrigeration to achieve rapid increase and decrease of the cold trap temperature. The PID algorithm stabilizes the temperature control accuracy to meet the full component capture requirements of wide-boiling-point VOCs. The built-in electronic metering flow module supports adaptive flow adjustment of multi-capacity sampling tanks. The touch screen interface allows users to preset sampling time, desorption parameters and multi-task queues. At the same time, remote monitoring and data synchronization are achieved through the wireless module. It is compatible with mainstream GC-MS, PID detectors and other analytical equipment.

[0096] In the low-temperature adsorption stage in S101 of this embodiment, the temperatures of the first cold trap and the second cold trap are set to -10°C and -40°C, respectively. In the graded temperature increase desorption stage in S102, the temperatures of the first cold trap and the second cold trap are set to 20°C and 230°C, respectively, to adapt to the separation of VOCs components with different boiling points in the atmospheric sample.

[0097] In S101 of this embodiment, before cooling the cold trap module to -196°C to 0°C using the hybrid refrigeration technology, a pre-leak test is required for the device. The pre-leak test includes:

[0098] Inject high-purity nitrogen into the device to a pressure of 50kPa, close the gas source and monitor the pressure change. If the pressure drop is ≤1kPa within 5 minutes, it is considered to be sealed.

[0099] If the leak detection fails, the touch screen will display the leak point code and guide the user to check and repair it step by step. The leak detection data of each pre-leak detection will be automatically recorded and an encrypted report will be generated.

[0100] This technical solution adopts a two-stage cold trap for step-by-step condensation, combined with a back-flushing airflow to remove moisture in a targeted manner, thereby improving the VOCs recovery rate and reducing the residual moisture content to less than 50ppm. It significantly improves the peak symmetry and data reproducibility of GC-MS detection. In addition, the first and second cold traps can work independently or alternately, supporting seamless switching between continuous sampling and desorption, and increasing the effective operating time of the system from 50% of the traditional single cold trap to 90%. The combination of semiconductor thermoelectric cooling and mechanical refrigeration realizes the rapid increase and decrease of the cold trap temperature, and the PID algorithm stabilizes the temperature control accuracy to meet the full component capture requirements of wide-boiling-point VOCs.

[0101] In this embodiment, by programmatically controlling the sampling, desorption, and purging steps, human intervention errors are eliminated, and the relative deviation of monitoring data at different points is greatly reduced. The adjustable parameter design of the cold trap temperature and desorption flow rate supports rapid response from extremely low concentrations to sudden high-concentration pollution events, and the detection linear range is extended to 3 orders of magnitude. In addition, air is used instead of high-purity nitrogen for purging, which reduces the cost of a single operation. The modular design of the cold trap module extends the consumables replacement cycle to more than 2 years, reducing solid waste generation. In addition, this device can be adapted to fixed source monitoring, mobile cruise monitoring, and laboratory offline analysis. By adjusting the cold trap parameters, the components of complex VOCs mixtures can be separated, providing technical support for special monitoring of ozone precursors and toxic and harmful gases.

[0102] Control Example

[0103] Based on the core performance of this technical solution (atmospheric sampling adsorption dehydration cold trap instrument), an embodiment and a control example were designed to verify its effect in low-concentration VOCs sampling pretreatment through key parameter comparison;

[0104] Test Subject:

[0105] Prepare standard VOCs gas (containing 12 components such as acetone, benzene, toluene, xylene, and undecane, with a concentration range of 5 to 50 ppb);

[0106] Add different humidity backgrounds (relative humidity 40% to 85%) to simulate actual atmospheric conditions;

[0107] Control settings:

[0108] Comparative Example 1: Traditional single cold trap adsorption (mechanical refrigeration only, no dehydration backflush);

[0109] Control Example 2: Direct injection (without cold trap enrichment and dehydration);

[0110] Example: Using this technical solution (double cold trap mixed refrigeration + backflush dehydration);

[0111] Evaluation parameters:

[0112] VOCs recovery rate: ratio of GC-MS quantitative analysis results to the theoretical value of standard gas

[0113] Residual moisture: gas dew point detection value after cold trap desorption (ppm);

[0114] Peak shape symmetry: chromatographic peak tailing factor (the closer to 1.0, the better);

[0115] Reproducibility: relative standard deviation (RSD) of 6 parallel tests;

[0116] Column life: the number of injections at which the column efficiency drops by 10% after continuous injection;

[0117] Table 1: Experimental data record table

[0118]

[0119] From the experimental data in Table 1, we can see that:

[0120] The first cold trap (-10°C) captures light components (acetone, butanone), and the second cold trap (-40°C) captures heavy components (benzene, undecane) and water.

[0121] Results: After 8 hours of enrichment, the detection limit of a 20ppb VOCs sample reached 0.1ppb, which is better than the factory boundary monitoring limit of 6mg / m3 required by GB 37822-2019. 3 .

[0122] Dehydration and desorption effect: During the backflushing phase, dry air was blown at 100 mL / min for 30 seconds, and the residual moisture was reduced to <50 ppm (2850 ppm for control example 1);

[0123] Graded desorption (light component 20℃ / heavy component 230℃) ensures complete transfer of wide-boiling-point VOCs, with a recovery rate of >95% (compared to only 60-70% in control example 1);

[0124] The efficiency of alternating operation of the dual cold traps: during the sampling period of cold trap 1, cold trap 2 was desorbed synchronously, and the effective operation time of 8 hours of continuous monitoring accounted for 90% (the time consumed in control example 1 due to desorption was only 50%).

[0125] Analysis of comparative case problems

[0126] Comparative Example 1: Residual moisture caused damage to the chromatographic column (lifespan: 120 cycles), and desorption of high-boiling-point components was incomplete (undecane recovery was only 51%).

[0127] Control Example 2: When the humidity is >85%, the baseline drifts, low-concentration VOCs (<10ppb) are not detected, and the RSD exceeds 20%.

[0128] in conclusion:

[0129] As can be seen from the data in Table 1, the embodiment, i.e., the technical solution, significantly improves the accuracy of low-concentration VOCs analysis (recovery rate ↑35%), anti-interference (moisture residue ↓98%), and equipment durability (chromatographic column life ↑4 times) through the three core technologies of double cold trap temperature separation adsorption, backflushing dehydration, and mixed refrigeration. The measured data fully meet the strict requirements of GB 37822-2019 for monitoring of unorganized emissions.

[0130] In this embodiment, the entire operation process can be controlled by a computer to provide signal feedback to implement the steps in sequence. These are all conventional knowledge of current automated control and will not be described in detail in this embodiment.

[0131] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A low-concentration atmospheric VOCs sampling and pretreatment device, characterized in that: include: The cold trap module consists of at least two independently working cold trap units. Each cold trap unit is made of stainless steel pipes and equipped with a temperature control module with a temperature adjustment range of -196°C to 250°C. It is used to adsorb VOCs and moisture in the sample at low temperatures and desorb target VOCs at high temperatures. Refrigeration system: including a combination of semiconductor thermoelectric refrigeration devices and mechanical refrigeration units to control the adsorption temperature of the cold trap module; Flow control module: used to achieve constant flow sampling using electronic metering algorithms, with a flow adjustment range of 0-100mL / min; Valve system: including multiple sample valves, purge valves, vent valves and three-way valves, which can realize the separation and switching of sample gas path, purge gas path and desorption gas path through multi-way switching; MCU controller: built-in PID temperature control algorithm and sampling timing logic, used to control the cold trap temperature, desorption time, purge flow rate and sampling flow rate; Dual cold trap alternating working module: The cold trap units can operate independently or alternately, connected to different analytical instruments or dual channels of the same instrument; Dehydration backflush module: During the desorption stage, the target VOCs are desorbed by high temperature, and the residual water and carbon dioxide in the cold trap are discharged by backflush with air or nitrogen.

2. A low-concentration atmospheric VOCs sampling and pretreatment device according to claim 1, characterized in that: The semiconductor thermoelectric refrigeration plate of the refrigeration system is a multi-stage series structure, the cold end is attached to the outer wall of the cold trap pipeline, and the hot end is connected to the water-cooled radiator.

3. A low-concentration atmospheric VOCs sampling and pretreatment device according to claim 2, characterized in that: The mechanical refrigeration unit adopts a cascade compression cycle, with a minimum refrigeration temperature of -196°C and can be continuously adjusted in the range of -40°C to -196°C.

4. A low-concentration atmospheric VOCs sampling and pretreatment device as claimed in claim 3, characterized in that: The switching logic between the semiconductor thermoelectric refrigeration device and the mechanical refrigeration unit is: when the target temperature is ≥-40°C, only the semiconductor thermoelectric refrigeration device is enabled for refrigeration; when the target temperature is <-40°C, the mechanical unit is enabled for refrigeration.

5. A low-concentration atmospheric VOCs sampling and pretreatment device according to claim 4, characterized in that: The pipes of the cold trap module are filled with a composite adsorbent, which includes the following layered structure: A hydrophobic Tenax TA polymer layer for adsorbing non-polar VOCs with a boiling point of 50-250°C; Graphitized carbon black layer, used to adsorb polar VOCs with a boiling point of -50-150°C; The silica gel loaded lithium chloride layer is used to absorb residual moisture, and each layer is separated by quartz wool with a filling length ratio of 2:2:

1. The outer wall of the cold trap pipe is wrapped with a heating wire.

6. A low-concentration atmospheric VOCs sampling pretreatment method, characterized in that: The pretreatment device according to any one of claims 1 to 5 is implemented, specifically comprising the following steps: S101: Using hybrid refrigeration technology, the cold trap module is cooled to -196°C to 0°C, and atmospheric samples are collected at a constant flow rate, allowing VOCs and moisture to be adsorbed by the cold trap; S102: Raise the temperature of the cold trap module to 20-250°C, introduce carrier gas in reverse at a flow rate of 20-100 mL / min, desorb the target VOCs by graded temperature increase, and transport them to the analytical instrument; S103: After the graded temperature desorption is completed, air or nitrogen is introduced at a flow rate of 100-110 mL / min to purge the cold trap module to remove residual moisture; S104: setting the cold trap units to work alternately to achieve parallel operation of continuous sampling and staged heating desorption; S105: Complete the full process control of timed sampling, desorption time and purge time through the preset program.

7. A low-concentration atmospheric VOCs sampling pretreatment method according to claim 6, characterized in that: In the low-temperature adsorption stage in S101, the temperatures of the first cold trap and the second cold trap are set to -10°C and -40°C respectively. In the graded temperature desorption stage in S102, the temperatures of the first cold trap and the second cold trap are set to 20°C and 230°C respectively to adapt to the separation of VOCs components with different boiling points in atmospheric samples.

8. A low-concentration atmospheric VOCs sampling pretreatment method according to claim 7, characterized in that: In S101, before cooling the cold trap module to -196°C to 0°C using hybrid refrigeration technology, a pre-leak test is required for the device. The pre-leak test includes: Inject high-purity nitrogen into the device to a pressure of 50kPa, close the gas source and monitor the pressure change. If the pressure drop is ≤1kPa within 5 minutes, it is considered to be sealed. If the leak detection fails, the touch screen will display the leak point code and guide the user to check and repair it step by step. The leak detection data of each pre-leak detection will be automatically recorded and an encrypted report will be generated.

9. A low-concentration atmospheric VOCs sampling pretreatment method according to claim 7, characterized in that: In S102, the target VOCs are subjected to staged temperature desorption and transported to an analytical instrument. The staged temperature desorption includes: The first stage: heating to 50°C at a rate of 10°C / min to desorb VOCs with boiling points less than 50°C; The second stage: heating to 150℃ at a rate of 5℃ / min to desorb VOCs with a boiling point of 50-150℃; The third stage: the temperature is raised to 230℃ at a rate of 2℃ / min to desorb VOCs with a boiling point greater than 150℃, and the desorption air flow rate increases in each stage, which are 20mL / min, 50mL / min, and 100mL / min respectively. The desorbed gas is dried by the dehydration membrane and then enters the analytical instrument.

10. A low-concentration atmospheric VOCs sampling pretreatment method according to claim 9, characterized in that: In S104, the cold trap units are set to work alternately, and the cold trap unit alternate operation mode includes: The first cold trap was set to sample for 8 hours with a flow rate of 5.2 mL / min, while the second cold trap was performing desorption analysis with a cycle of 8.5 hours. Mode B: The first and second cold traps sample synchronously for 4 hours, followed by desorption, with a total cycle of 8 hours, suitable for daily average monitoring; Mode C: The first cold trap is connected to the online GC-MS, and the second cold trap is connected to the portable PID to achieve multi-instrument comparative analysis of the same pollution source.

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