A constant-current automatic sampling system for ambient air and a sampling method thereof

Abstract

The present invention discloses an ambient air constant-current automatic sampling system and a sampling method thereof. The system includes a vacuum sampling tank, a buffer container and a switching valve. The buffer container is respectively connected to the vacuum sampling tank and the atmospheric environment through the switching valve. Through the switching control of the switching valve, the buffer container is alternately connected to the atmospheric environment and the vacuum sampling tank to achieve the pressure balance of the sampled gas. The present invention also discloses a constant-current ambient air sampling method. The present invention breaks the foreign technical monopoly on high-precision atmospheric constant-current sampling devices, ensures the accuracy of atmospheric sampling flow control, ensures the inertness of the overall gas flow path, has flexible flow control and adjustment methods, uses commonly used industrial mass-produced devices and materials, realizes consumable-free and easy maintenance, and constructs an economical and efficient fully automatic ambient air constant-current automatic sampling system, contributing to the country's atmospheric environment monitoring and governance.

Description

Technical Field

[0001] The invention belongs to air sampling technology, and in particular relates to an ambient air constant-current automatic sampling system and a sampling method thereof. Background Art

[0002] Volatile organic compounds (VOCs) are important precursors to ozone pollution. In order to actively promote the construction of the ambient air VOCs monitoring system and capacity, find out the key types of VOCs that generate ozone, grasp the concentration level and change pattern, and carry out ozone pollution prevention and control work in a targeted manner, the state has formulated an ambient air VOCs monitoring work plan, which clearly stipulates the sampling mode of ambient air: using pre-vacuumed Suma tanks, continuously collect atmospheric samples at a constant flow rate within a set period of time 8 times a day (3 hours each time), and seal one tank at a negative pressure each time. In non-key areas and time periods, atmospheric samples are continuously collected at a constant flow rate every 24 hours and sealed at a negative pressure. For specific technical details and background, please refer to the 2018 Key Area Ambient Air Volatile Organic Compound Monitoring Plan in the "Annex to the Environmental Office Monitoring Letter

[2017] 2024".

[0003] To meet the needs of timed continuous long-term atmospheric sampling, the following mainstream solutions are currently available:

[0004] 1. Mechanical micropore flow limiting air resistance combined with mechanical constant flow valve, the specific real-time scheme includes manual and automatic methods:

[0005] 1. Manually open and close the mechanical valve at a fixed time, and use a microporous gas flow damping device and a mechanical constant flow valve to control the sampling flow. This method is extremely labor-intensive, has a large sampling time control error, and is difficult to implement for multi-point and multi-flow sampling. It has now been gradually replaced by automated sampling devices.

[0006] 2. Automatic sampling device with automatic valve switching control. The core of its sampling constant flow control is still the microporous air resistance with a mechanical constant flow valve, which switches the multi-channel Suma tank through the automatic system to achieve the purpose of timed sampling. This is the most widely used sampling method at present.

[0007] In order to achieve the core goal of not changing the composition of the sampled gas, all parts in contact with the sample gas are required to be made of inert materials. Therefore, the microporous air resistance of this solution is generally made of gem materials, and all metal surfaces of the constant flow valve must be silanized. The microporous air resistance itself is easily blocked by particulate matter in the sampled air. It is necessary to add a multi-stage inert gas filter device at the front end to ensure that the particles in the final gas are less than 2 microns. This also increases the cost of additional consumables and maintenance expenses. Since the constant flow core is a mechanical device, each combination of air resistance and constant flow valve can only meet the needs of one sampling flow rate and cannot be adjusted dynamically. The mechanical device itself also needs to calibrate the constant flow rate regularly. The calibration process is cumbersome and manual operation is prone to introduce additional errors. Excessive physical vibration must be avoided during transportation and use to avoid affecting the accuracy of flow control.

[0008] 2. A solution that uses an electronic gas mass flow controller as the core of constant flow control and combines it with automatic multi-way Suma tank valve switching.

[0009] This scheme attempts to use an electronic gas mass flow controller to replace the mechanical constant flow device, and has the advantage of flexible adjustment of the sampling flow rate. However, it has not become a mainstream sampling scheme at present. The main disadvantage is that the electronic mass flow sensor itself cannot be made of inert materials, and the silanization coating process is not suitable for the surface treatment of the sensor. Therefore, the overall inertness of this scheme cannot be compared with the mechanical constant flow device, and there is a potential possibility of damage to the VOCs analysis targets with high activity such as sulfides. At the same time, the proportional valve used in the flow controller inevitably uses elastic sealing materials, generally fluororubber, and it is difficult to ensure that it does not release additional organic gas in a high vacuum environment, causing secondary pollution to the sampled gas. High-precision electronic mass flow controllers are also relatively expensive. In order to ensure reliability, an inert filter must be used to pre-filter the sampled gas, and the consumables and maintenance costs are high. In addition, the electronic mass flowmeter is sensitive to changes in temperature and humidity of the use environment and the measured gas, so the sampling flow accuracy control will be reduced under complex climatic conditions. The sampling plan released by the country also requires that all sampling pipelines can be disassembled and cleaned to avoid cross contamination between different sampling points. On this point, the electronic mass flow controller as part of the gas flow path obviously cannot meet this requirement. Summary of the invention

[0010] The technical problem to be solved by the present invention is to provide an ambient air constant-current automatic sampling system and a sampling method thereof, which can ensure the accuracy of atmospheric sampling flow control, ensure the inertness of the overall gas flow path, have flexible flow control and adjustment methods, use commonly used industrial mass-produced devices and materials, achieve no consumables and easy maintenance, and build an economical and efficient fully automatic ambient air constant-current automatic sampling system.

[0011] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0012] On the one hand, an ambient air constant-flow automatic sampling system includes a vacuum sampling tank, a buffer container and a switching valve. The buffer container is connected to the vacuum sampling tank and the atmospheric environment respectively through the switching valve. Through the switching control of the switching valve, the buffer container is alternately connected to the atmospheric environment and the vacuum sampling tank to achieve air pressure balance of the input gas.

[0013] The switching valve includes a first two-way valve and a second two-way valve respectively arranged at two ends of the buffer container, the first two-way valve controls the connection between the buffer container and the atmospheric environment, and the second two-way valve controls the connection between the buffer container and the vacuum sampling tank.

[0014] The switching valve comprises a two-position three-way valve, and three ends of the two-position three-way valve respectively control the communication with the atmospheric environment, the buffer container, and the vacuum sampling tank.

[0015] The buffer container is made of a polytetrafluoroethylene pipe or a silanized metal pipe.

[0016] On the other hand, the sampling method of the sampling system comprises the following steps:

[0017] a. Control the buffer container to be connected to the atmosphere and fill the buffer container to one atmospheric pressure in the initial state;

[0018] b. Control the buffer container to be sealed from the atmosphere and switch it to be connected to the vacuum sampling tank so that the gas in the buffer container and the vacuum environment in the vacuum sampling tank reach pressure balance;

[0019] c. Keep the buffer container and vacuum sampling tank closed and repeat steps a, b, and c.

[0020] The switching control interval is calculated using the following formula:

[0021]

[0022]

[0023]

[0024]

[0025] Where:

[0026] T(i) is the interval time of the i-th switching control;

[0027] N(i) is the number of gas molecules in the vacuum sampling tank after the i-th valve cut;

[0028] n0 is the number of gas molecules in the vacuum sampling tank at the initial pressure state;

[0029] nd is the number of gas molecules in the buffer container at atmospheric pressure;

[0030] V1 is the capacity of the vacuum sampling tank;

[0031] V2 is the buffer container capacity;

[0032] P0 is the initial pressure of the vacuum sampling tank;

[0033] Pa is the atmospheric pressure;

[0034] Ta is the ambient temperature;

[0035] R is the ideal gas constant.

[0036] R = 8.314 J / (mol*K).

[0037] The ambient air constant current automatic sampling system and method of the present invention have the following advantages:

[0038] 1. The system is mainly composed of mature devices that have been mass-produced in China, avoiding the use of expensive imported devices and greatly reducing the cost of the environmental protection detection system.

[0039] 2. The entire gas flow path can be made of inert materials, which will not have any effect on the sampled gas components, and there will be no residual adsorption and no cross contamination.

[0040] 3. The sampling flow rate can be dynamically adjusted through the software program, which is flexible and convenient for users to remotely control and program sequence control.

[0041] 4. The sampling flow time resolution can be flexibly adjusted through the buffer container to meet various types of sampling requirements.

[0042] 5. The flow control has good repeatability and the reliability of negative pressure sealing is guaranteed in principle.

[0043] 6. The system components have excellent environmental tolerance. The system performance is not affected by the temperature and humidity of the operating environment and the sampling gas. The physical structure of the whole machine can also withstand high-intensity vibration and impact.

[0044] 7. There are no components in the system that are easily clogged or contaminated by particulate matter, and no filter consumables are required.

[0045] 8. It can automatically perform flow QC inspection and calibration, and realize automatic and continuous operation without human intervention.

[0046] 9. The system has high reliability, all components can be disassembled and cleaned according to national standards, and the cost of use and maintenance is low.

[0047] 10. The system has good scalability and the number of sampling channels can be flexibly increased according to demand. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments:

[0049] Figure 1 It is a principle diagram of the ambient air constant-current automatic sampling system of the present invention.

[0050] Figure 2 It is another principle diagram of the ambient air constant-current automatic sampling system of the present invention.

[0051] Figure 3 4 is a sampling interval time curve diagram of an embodiment of the present invention. DETAILED DESCRIPTION

[0052] The ambient air constant current automatic sampling system of the present invention is as follows Figure 1 , 2 As shown, it mainly includes a vacuum sampling tank 1, a buffer container 2 and a switching valve. The buffer container 2 is connected to the vacuum sampling tank 1 and the atmospheric environment respectively through the switching valve. Through the switching control of the switching valve, the buffer container 2 is alternately connected to the atmospheric environment and the vacuum sampling tank 1 to achieve air pressure balance of the input gas.

[0053] As an embodiment, the switching valve is as follows Figure 1 As shown, it includes a first two-way valve 3a and a second two-way valve 3b respectively arranged at both ends of the buffer container 2, the first two-way valve 3a controls the connection between the buffer container 2 and the atmospheric environment, and the second two-way valve 3b controls the connection between the buffer container 2 and the vacuum sampling tank 1. By alternately opening and closing the first two-way valve 3a and the second two-way valve 3b, valve-cut sampling can be achieved: first close the second two-way valve 3b and open the first two-way valve 3a to fill the buffer container 2 to atmospheric pressure, and then close the first two-way valve 3a and open the second two-way valve 3b to balance the pressure in the vacuum sampling tank 1 and the buffer container 2, and complete one sampling.

[0054] As an embodiment, the switching valve is as follows Figure 2 As shown, it includes a two-position three-way valve 4, and the three ends of the two-position three-way valve 4 respectively control the connection with the atmosphere, the buffer container 2, and the vacuum sampling tank 1. Using the two-position three-way valve 4, the common end is connected to the buffer container 2 with one end closed, and the other two ports are connected to the atmosphere, and the other is connected to the vacuum sampling tank 1. Each opening and closing of the two-position three-way valve 4 completes a sampling action.

[0055] The vacuum sampling tank 1 can be a Suma tank, and the buffer container 2 uses a closed volume as the input gas, and can be made of a polytetrafluoroethylene pipe or a silanized metal pipe.

[0056] Through the above-mentioned but not limited to the above-mentioned two switching valves, the buffer container can be alternately connected to the atmospheric environment and the vacuum sampling tank to achieve the pressure balance of the sampled gas. The specific flow environment air sampling method includes the following steps:

[0057] a. Control the buffer container to be connected to the atmosphere and fill the buffer container to one atmospheric pressure in the initial state;

[0058] b. Control the buffer container to be sealed from the atmosphere and switch it to be connected to the vacuum sampling tank so that the gas in the buffer container and the vacuum environment in the vacuum sampling tank reach pressure balance;

[0059] c. Keep the buffer container and vacuum sampling tank closed and repeat steps a, b, and c.

[0060] With each injection step, the vacuum pressure in the vacuum sampling tank continues to rise, and the initial pressure of the buffer container before each connection is atmospheric pressure. Therefore, the amount of gas entering the vacuum sampling tank from the buffer container during the balancing process is gradually decreasing. In order to achieve the purpose of constant flow injection, it is necessary to cooperate with the timing algorithm of valve cutting, so that the interval time between each two valve cuttings is also decreasing in sequence, and finally the injection flow rate (the amount of gas entering the vacuum sampling tank each time the valve cuts / the interval time of injection steps) is kept constant. Constant flow injection control of any flow rate can be achieved by combining the initial injection interval time and the buffer container. The smaller the buffer capacity, the faster the valve cutting frequency is required, and the higher the time resolution of the injection flow control. Conversely, the larger the buffer container, the slower the valve cutting frequency, and the lower the corresponding sampling time resolution. The valve cutting frequency and flow control time resolution can be balanced according to actual needs.

[0061] The following is the algorithm derivation process of constant flow injection:

[0062] Assume: The capacity of the vacuum sampling tank is: V1

[0063] Atmospheric pressure: Pa

[0064] The initial pressure of the vacuum sampling tank is: P0

[0065] The number of valve cuts is: i

[0066] Buffer volume: V2

[0067] Ambient temperature: Ta

[0068] According to the ideal gas formula: PV = nRT, it can be deduced: and

[0069] Number of gas molecules in the buffer volume at atmospheric pressure:

[0070] The number of gas molecules under the initial pressure state in the vacuum sampling tank:

[0071] Assume that the number of gas molecules in the vacuum sampling tank after the i-th valve cut is: N(i),

[0072] Assume that the pressure after the i-th valve is connected and the buffer volume and the vacuum sampling tank pressure are balanced is:

[0073] P(i),

[0074] Formula 1:

[0075] Formula 2:

[0076] Substituting formula 2 into formula 1, we get the recursive formula 3 of the ni series:

[0077]

[0078] The general formula 4 for solving the sequence is:

[0079]

[0080] The pressure inside the vacuum sampling tank after i injections can be derived from formula 4:

[0081] Formula 5:

[0082]

[0083] Assume the first valve cutting interval time is: t1

[0084] The sampling flow rate of each valve cut is:

[0085] Keeping the flow constant, the i-th valve cutting interval can be obtained:

[0086]

[0087] The R in the formula is the ideal gas constant, R = 8.314 J / (mol*K).

[0088] Therefore, the control program can dynamically adjust the interval time after each valve cut to achieve constant current sampling control.

[0089] Example

[0090] The design uses a 6L vacuum sampling tank, the initial tank vacuum pressure is set to 10Pa according to the national standard, the buffer capacity is selected to be 5mL, and the final sealing pressure of the vacuum sampling tank is <= 90% of atmospheric pressure. The sampling period is set to 24 hours.

[0091] It is known that V1=6L, Pa=101325Pa, P0=10Pa, V2=0.005L, Ta=298.15K

[0092] Gas constant R = 8.314 J / mol*K,

[0093] Using Formula 5, it can be calculated that when the sampling valve is cut for i=2760 times, the final pressure P(2760)=0.9 atmospheres.

[0094] Use formula 4 and substitute the recursive formula for the interval time:

[0095] The calculation shows that when t1 = 80 seconds,

[0096] The sampling flow rate under the second condition combination r=2.599x10 -6 mol / s

[0097] T(i) can be adjusted dynamically with the number of sampling times, gradually shortening the trend graph. Figure 3 As shown by Figure 3 You can see that the initial interval is 80 seconds, the final interval is 8 seconds, and the whole process takes 24 hours.

[0098] However, those skilled in the art should recognize that the above embodiments are only used to illustrate the present invention, and are not intended to limit the present invention. As long as they are within the spirit of the present invention, any changes or modifications to the above embodiments will fall within the scope of the claims of the present invention.

Claims

1. A sampling method for an ambient air constant current automatic sampling system, the sampling system comprising a vacuum sampling tank, characterized in that: It also includes a buffer container and a switching valve. The buffer container is connected to the vacuum sampling tank and the atmospheric environment respectively through the switching valve. Through the switching control of the switching valve, the buffer container is alternately connected to the atmospheric environment and the vacuum sampling tank to achieve the pressure balance of the sampled gas. The sampling method includes the following steps: a. Control the buffer container to be connected to the atmosphere and fill the buffer container to one atmospheric pressure in the initial state; b. Control the buffer container to be sealed from the atmosphere and switch it to be connected to the vacuum sampling tank so that the gas in the buffer container and the vacuum environment in the vacuum sampling tank reach pressure balance; c. Keep the buffer container and the vacuum sampling tank closed and repeat steps a, b, and c; In step b, the interval time of the switching control is calculated using the following formula: Where: T(i) is the interval time of the i-th switching control; N(i) is the number of gas molecules in the vacuum sampling tank after the i-th valve cut; n0 is the number of gas molecules in the vacuum sampling tank at the initial pressure state; nd is the number of gas molecules in the buffer container at atmospheric pressure; V1 is the capacity of the vacuum sampling tank; V2 is the buffer container capacity; P0 is the initial pressure of the vacuum sampling tank; Pa is the atmospheric pressure; Ta is the ambient temperature; R is the ideal gas constant; r is the sampling flow rate each time the valve is switched.

2. The sampling method of the ambient air constant current automatic sampling system according to claim 1 is characterized in that: The switching valve includes a first two-way valve and a second two-way valve respectively arranged at two ends of the buffer container, the first two-way valve controls the connection between the buffer container and the atmospheric environment, and the second two-way valve controls the connection between the buffer container and the vacuum sampling tank.

3. The sampling method of the ambient air constant current automatic sampling system according to claim 1 is characterized in that: The switching valve comprises a two-position three-way valve, and three ends of the two-position three-way valve respectively control the communication with the atmospheric environment, the buffer container, and the vacuum sampling tank.

4. The sampling method of the ambient air constant current automatic sampling system according to claim 1 is characterized in that: The buffer container is made of a polytetrafluoroethylene pipe or a silanized metal pipe.

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