CN102359895B - Normal temperature enrichment and sampling method for xenon in air - Google Patents

Abstract

A normal-temperature enrichment sampling method and device for xenon in the atmosphere, using a hollow fiber semi-permeable membrane group as an air pretreatment device, using four-stage adsorption columns to enrich and concentrate xenon in the air step by step, wherein two sets of first-stage adsorption columns are used Parallel structure, alternate work, continuous uninterrupted sampling, using activated carbon as an adsorbent, the invention has reasonable design, simple structure, easy operation, reduces the difficulty of removing impurities in the subsequent concentration process, and is suitable for long-term monitoring of the atmospheric environment. The representativeness of sampling is strong, and there is no time window for sampling the rare gas xenon in the ambient air.

Description

A normal temperature enrichment sampling method for xenon in the atmosphere

technical field

The invention belongs to a nuclear environment monitoring method, in particular to the enrichment and separation of xenon, a rare gas in the atmosphere.

Background technique

Radionuclide monitoring technology is one of the four key verification technologies stipulated in the Comprehensive Nuclear-Test-Ban Treaty (CTBT). The characteristic radionuclides produced and released by nuclear activities play a decisive role in the qualitative identification of suspicious events in CTBT verification. The sampling and analysis technology for these nuclides has always been the focus of radionuclide monitoring technology research. The radioactive noble gases of interest in the international radionuclide monitoring system are radioactive xenon isotopes (mainly including ^131m Xe, ^133m Xe, ¹³³ Xe, ¹³⁵ Xe).

The volume fraction of xenon, a rare gas in the air, is 8.7×10 ^-8 (v/v), and the content of radioactive xenon isotopes is even lower, so it is difficult to analyze directly. Radioactive gas xenon sampling is to separate and enrich xenon isotopes from the ambient atmosphere, and make it meet the quantitative detection sensitivity of total xenon analysis instruments and the requirements of radioactive xenon isotope content of radioactive measuring instruments, so it must be obtained from a large amount of air (greater than 10m ³ ) concentrates xenon to a very small volume (several to hundreds of ml).

Literature search information: Invention by Zhou Chongyang et al. of the 63653 Unit of the Chinese People's Liberation Army, the patent No. CN201728039U "A Separation Device for Enriching Gas Xenon" uses a first-level 5A molecular sieve impurity removal column, a first-level activated carbon adsorption column, and a second-level 5A Molecular sieve plus activated carbon impurity removal column, secondary activated carbon adsorption column, third-stage 5A molecular sieve plus activated carbon impurity removal column, and third-stage activated carbon adsorption column to enrich and separate xenon. A total of 6 adsorption columns are used, involving 2 types and 2 specifications each Adsorbent, and continuous sampling cannot be achieved. Another invention by Zhou Chongyang and others, the patent No. CN 101985080A "A Method for Enrichment and Separation of Xenon Using Activated Carbon", can only process xenon standard gas with a concentration of 10-1000ppm, and cannot directly sample from the air. The article entitled "SAUNA-a system for automatic sampling, processing, and analysis of radioactive xenon" published in the periodical "Nuclear Instruments and Methods in Physics Research A" No. 508 (2003) by A.Ringbom of Sweden, etc., adopts Two sets of parallel 4A molecular sieve adsorption columns and activated carbon adsorption columns are used for enrichment, followed by serial connection of 4A molecular sieve adsorption columns, activated carbon adsorption columns, 3A molecular sieve adsorption columns, 5A molecular sieve adsorption columns, 5A molecular sieve chromatography columns, and carbon molecular sieve adsorption columns to separate and purify xenon , can achieve continuous sampling, but a total of 8 adsorption columns are used, involving 5 kinds of adsorbents such as 3A, 4A, 5A molecular sieves, carbon molecular sieves, and activated carbon.

After the air is pretreated by the hollow fiber semi-permeable membrane group, the invention realizes the enrichment and concentration of xenon in the air and removes other impurities at the same time only by using four-stage adsorption columns. The adsorbent also uses only one kind of activated carbon, which can continuously sample , simple structure and convenient method.

Contents of the invention

The present invention proposes a continuous sampling method and device for enriching xenon in the atmosphere at normal temperature. The method uses a hollow fiber semi-permeable membrane group as an air pretreatment device, and uses 4-stage adsorption columns to enrich and concentrate xenon in the air step by step. , The first-stage adsorption column adopts two sets of parallel structures, which work alternately to realize continuous and uninterrupted sampling. Activated carbon is used as an adsorbent. This method has reasonable design, simple structure, and easy operation. It can be used for sampling the rare gas xenon in the ambient air.

Solution of the present invention is:

A normal temperature enrichment sampling method for xenon in the atmosphere, comprising the following steps:

1] Air pretreatment:

The hollow fiber semi-permeable membrane group is used to initially concentrate the xenon in the air;

2] Four-stage activated carbon adsorption and desorption step by step:

Four cascaded activated carbon adsorption and desorption units are used to gradually concentrate xenon in the air and remove other impurities; the adsorption temperature is 10-30°C, and the desorption temperature is 200-300°C; the adsorption-desorption unit is activated carbon adsorption column;

3) Sample collection:

The xenon desorbed from the last stage adsorption column is pressurized into the sample collection bottle through the diaphragm to complete the collection.

The activated carbon adsorbents packed in the adsorption columns at all levels are heated and activated before the adsorption and desorption sampling.

The above-mentioned first-stage adsorption-desorption unit is two sets of parallel-connected activated carbon adsorption columns, and the two sets of parallel-connected activated carbon adsorption columns realize alternate adsorption and desorption through valves.

The steps for the preliminary concentration of xenon in the air by using the hollow fiber semi-permeable membrane group are as follows: pass the compressed air through the hollow fiber semi-permeable membrane group, and utilize the different diffusion rates of each component in the hollow fiber membrane to realize the xenon in the air. Separation of components and preliminary concentration of xenon in air.

A normal-temperature enrichment sampling device for xenon in the atmosphere, comprising a compressed air source, a four-stage cascaded adsorption-desorption unit, a vacuum pump, a carrier gas cylinder, and a sample collection bottle, and each stage of the adsorption-desorption unit includes activated carbon adsorption column and the adsorption column heating device arranged on the periphery of the activated carbon adsorption column, the compressed air source is connected to the inlet pipe of the first-stage adsorption-desorption unit, and the gas outlet of the last-stage adsorption-desorption unit passes through the diaphragm booster pump Connected to the sample collection bottle pipeline, the carrier gas cylinder is connected to the inlet end of the last three-stage adsorption-desorption unit through pipelines and valves, and the vacuum pump is respectively connected to the gas outlet end of the fourth-stage adsorption-desorption unit through vacuum pipelines and valves , which is special in that: a hollow fiber semi-permeable membrane group is set between the compressed air source and the first-stage adsorption-desorption unit; the first-stage adsorption-desorption unit includes two parallel and alternate adsorption-desorption Activated carbon adsorption column.

The above-mentioned activated carbon is 14-25 mesh coconut shell activated carbon; the carrier gas of the carrier gas cylinder is nitrogen or helium.

The advantages of the present invention are:

1. Use the hollow fiber semi-permeable membrane group as the air pretreatment device to remove O ₂ , CO ₂ and H ₂ O in the air, and initially concentrate the xenon in the air, which reduces the difficulty of removing impurities in the subsequent concentration process.

2. Use a total of 4 stages of activated carbon adsorption columns to concentrate xenon in the air step by step and remove other impurities. The adsorption of xenon by activated carbon columns at all levels is carried out at room temperature (10-30°C), and only one adsorbent is used and adsorbed at room temperature , which reduces the complexity and implementation difficulty of the method.

3. Use two sets of parallel first-stage adsorption columns Ca1 and Cb1 to alternately sample, so that the sampling process is continuous and uninterrupted. It is suitable for long-term monitoring of the atmospheric environment. The samples taken are highly representative and have no time window.

4. The enrichment factor of xenon in the present invention is greater than 2.5×10 ⁵ , the decontamination factor of radon is greater than 1×10 ⁵ , and the xenon (volume fraction 8.7×10 ^-8 ) in the air with an effective sampling volume greater than 50m ³ can be separated from The enrichment is a sample with a volume of 150ml (the xenon concentration is greater than 2.5%), which far exceeds the requirements on the sample for total xenon measurement and radioactive xenon isotope measurement.

5. The present invention is not only applicable to atmospheric nuclear environment monitoring, but also applicable to small-scale production of xenon.

6. The idea of continuous uninterrupted sampling and four-stage adsorption column concentration adopted in the present invention can be extended to the design of other types of gas samplers.

Description of drawings

Fig. 1 is the structural schematic diagram of the continuous sampling device of enriching xenon in the atmosphere at normal temperature;

Fig. 2 is the working principle diagram of the hollow fiber semi-permeable membrane group;

Among them: 1-air compressor, 2-dryer, 3-hollow fiber semi-permeable membrane group, 4-diaphragm pump, 5-vacuum pump, 6-carrier gas cylinder, 7-sample collection bottle, MFM-gas mass flow meter, RH-gas dew point meter, O ₂ -oxygen content detector, MFC-gas mass flow controller, V-valve, P-pressure transmitter, C-adsorption column, PC-pressure controller.

Detailed ways

Device of the present invention:

Air pretreatment unit: Air dryer 2, gas mass flow meter MFM, hollow fiber semi-permeable membrane group 3, gas dew point meter RH, oxygen content detector O ₂ , gas The mass flow controller MFC0 and the valves V0 and V0 are connected to the valves V1, Va11 and Vb11 through two three-way or one four-way joints;

Primary adsorption unit: Va11 connects pressure transmitter Pa1, valve Va12 and primary adsorption column Ca1 through two three-way or one four-way joints, and the outlet of Ca1 is connected through two three-way or one four-way joints after passing through the filter Valves Va13, Va14, Va15; Vb11 are connected to pressure transmitter Pb1, valve Vb12 and primary adsorption column Cb1 through two three-way or one four-way joints, and the outlet of Cb1 passes through two three-way or one four-way after passing through the filter The joints are connected to valves Vb13, Vb14, Vb15; Va13, Vb13 are connected to gas mass flow controller MFC1 and pressure controller PC through a four-way joint, and the outlet of MFC1 is connected to valves Va12 and Vb12, between Va14 and Vb14 through a three-way joint. The valves V2 and V3 are connected through two three-way joints or one four-way joint, V3 is connected with valve V21, and the vacuum pipeline is connected between Va15 and Vb15 through a three-way joint;

Secondary adsorption unit: V21 is connected to pressure transmitter P2, valve V22 and secondary adsorption column C2 through a four-way joint. The outlet of C2 is connected to valves V23, V24, V25 through a four-way joint after passing through the filter, and V22 is connected to gas The outlet of the mass flow controller MFC2, the inlet of MFC2 is connected to the carrier gas pipeline, V24 is connected to the valve V31, and V25 is connected to the vacuum pipeline;

Tertiary adsorption unit: V31 is connected to pressure transmitter P3, valve V32 and tertiary adsorption column C3 through a four-way joint. The outlet of C3 is connected to valves V33, V34, V35 through a four-way joint after passing through the filter, and V32 is connected to gas The outlet of the mass flow controller MFC3, the inlet of MFC3 is connected to the carrier gas pipeline, V34 is connected to the valve V41, and V35 is connected to the vacuum pipeline;

Four-stage adsorption unit: V41 is connected to pressure transmitter P4, valve V42 and four-stage adsorption column C4 through a four-way joint. The outlet of C4 is connected to valves V43, V44, and V45 through a four-way joint after passing through a filter. V42 is connected to gas The outlet of the mass flow controller MFC4, the inlet of MFC4 is connected to the carrier gas pipeline, V44 is connected to the micro-diaphragm booster pump 4, and V45 is connected to the vacuum pipeline;

Source unit: the outlet of the diaphragm pump is connected to valve V46, and V46 is connected to pressure transmitter P5, valve V48 and sample collection bottle 7 with valve V47 through a four-way joint, and V48 is connected to vacuum pipeline; the vacuum pipeline is connected to vacuum pump 5 , the carrier gas pipelines are all connected to the carrier gas cylinder 6.

The method of the invention: the hollow fiber semi-permeable membrane group is used as an air pretreatment device, and the xenon is preliminarily concentrated. Under pressure, various gases have different adsorption, diffusion, and permeation rates in the hollow fiber membrane. Those with high permeation rates such as O ₂ , CO ₂ , H ₂ O, etc. are enriched outside the low pressure, while those with low permeation rates such as N _2. Xe, etc. are enriched inside the high pressure, so as to realize the separation of mixed gas. The air pretreatment device can provide a gas source with a pressure of 700kPa, a flow rate of not less than 40L/min, a dew point lower than -50°C Td, an _O2 content of less than 0.1%, a _CO2 content of less than 40ppm, and an Xe content of more than 0.14ppm. Select 14-25 mesh coconut shell activated carbon as the adsorbent of the adsorption columns at all levels, adsorb xenon at room temperature (10-30°C), desorb xenon at high temperature (200-300°C), and use nitrogen or helium as the carrier gas . A micro-diaphragm booster pump is used in the source preparation process, and the xenon gas desorbed from the four-stage adsorption column is transferred to the sample collection bottle through the diaphragm pump. The enrichment factor for xenon is greater than 2.5×10 ⁵ , and the decontamination factor for radon is greater than 1×10 ⁵ , which can separate and enrich xenon (volume fraction 8.7×10 ^-8 ) in the air with an effective sampling volume greater than 50m ³ into volume 150 ml sample (xenon concentration greater than 2.5%).

Principle of the present invention:

The air is treated by the hollow fiber semi-permeable membrane group, and the xenon in it is adsorbed by the activated carbon column at room temperature (10-30°C); a total of 4 levels of activated carbon adsorption columns are used to gradually concentrate the xenon in the air and remove other impurities. Xenon is adsorbed at room temperature and desorbed at high temperature (200-300°C); two sets of parallel primary adsorption columns Ca1 and Cb1 are used to alternately sample, and after Ca1 is saturated, switch to Cb1 for adsorption, while Ca1 is desorbed and desorbed. For regeneration, Cb1 is saturated and then Ca1 is switched for adsorption, while Cb1 is desorbed and regenerated repeatedly to ensure the continuous and uninterrupted primary adsorption process.

Working process of the present invention:

According to the connection relationship shown in Figure 1, a continuous sampling device for enriching xenon in the atmosphere at room temperature is established.

Among them: the first-level adsorption column is 3m long and the inner diameter is 59mm, and three 1m-long heating rods can be connected in series, each with an interpolation power of 1.2kW; the second-level adsorption column is 1m long, the inner diameter is 28mm, and the interpolation power is 800W heating rod; the third-stage adsorption column is 300mm long, the inner diameter is 22mm, and a heating rod with an internal power of 500W is inserted; the fourth-stage adsorption column is 150mm long, and the inner diameter is 10mm, and a heater with an external power of 200W is installed. Load 14-25 mesh coconut shell activated carbon. The carrier gas is nitrogen or helium.

Preparation:

Before sampling, it is necessary to heat and activate the activated carbon adsorbents packed in the adsorption columns at all levels to facilitate the enrichment of xenon in the air by activated carbon. The specific method is:

Turn on the vacuum pump and valves Va15, Vb15, V25, V35, and V45, and vacuum the adsorption columns at all levels until the pressures of Pa1, Pb1, P2, P3, and P4 are all <1kPa. Turn on the heaters on the adsorption columns at all levels, raise the temperature to 200°C and keep it for 30 minutes, then stop heating, stop vacuuming when it drops to <30°C, and turn off the vacuum pump and all valves. Set the flow rates of mass flow controllers MFC2, MFC3, and MFC4 to 200, 35, and 20 mL/min respectively, open MFC2, MFC3, and MFC4 and valves V22, V32, V42, V21, V3, Va14, and Vb14, and add carrier gas protection. When the pressures Pa1, Pb1, P2, P3, and P4 are all > 110kPa, close all mass flow controllers and valves.

Steps:

(1) Air pretreatment

Turn on the air pretreatment unit, and control the gas production and pressure of the membrane group by adjusting the opening size of the needle valve at the high-pressure inner outlet of the hollow fiber semi-permeable membrane group, thereby adjusting the air pretreatment unit’s response to impurity gases O ₂ , CO ₂ , H ₂ O The degree of removal and the degree of pre-concentration of the target gas Xe to achieve pressure ≥ 700kPa, flow rate ≥ 40L/min, dew point ≤ -50 ° C Td, _O2 content ≤ 0.1%, _CO2 content ≤ 40ppm, Xe content ≥ 0.14ppm .

(2) Primary adsorption column Ca1 enrichment

Set the flow rate of the mass flow controller MFC0 to 40L/min, and the exhaust pressure of the pressure controller PC to 700kPa, open MFC0, PC and valves V0, Va11, Va13, and take samples from the primary adsorption column Ca1, and stop sampling after 3 hours. Close valves Va11, Va13. Open the valves Vb11 and Vb13, and switch to the primary adsorption column Cb1 for sampling.

(3) Primary adsorption column Ca1 desorption purification

Set the flow rate of mass flow controller MFC1 to 900mL/min, open MFC1 and valves Va12, Va14, V2, and turn on the heater on the primary adsorption column Ca1, and close valve V2 after the temperature rises to 130°C. Open the valves V3, V21, V23, heat the primary adsorption column Ca1 to 300°C and keep it for 30min, close the MFC1 and valves Va12, Va14, V3, V21. Stop heating the primary adsorption column Ca1 and cool to <30°C. This process transfers the desorbed xenon in the primary adsorption column Ca1 to the secondary adsorption column C2.

(4) Secondary adsorption column C2 desorption purification

Open MFC2 and valve V22, and simultaneously open the heater on the secondary adsorption column C2, and close valve V23 after the temperature rises to 130 °C. Open valves V24, V31, V33, heat the second adsorption column C2 to 300°C and keep it for 30min, close MFC2 and valves V22, V24, V31, V33. Stop heating the secondary adsorption column C2 and cool down to <30°C. This process transfers the desorbed xenon in the secondary adsorption column C2 to the tertiary adsorption column C3.

(5) Primary adsorption column Cb1 enrichment

Stop sampling after 3 hours of sampling in the primary adsorption column Cb1, and close the valves Vb11 and Vb13. Open the valves Va11 and Va13, and switch to the primary adsorption column Ca1 for sampling.

(6) Primary adsorption column Cb1 desorption purification

Open MFC1 and valves Vb12, Vb14, V2, and turn on the heater on the primary adsorption column Cb1, and close valve V2 after the temperature rises to 130°C. Open the valves V3, V21, V23, heat the primary adsorption column Cb1 to 300°C and keep it for 30min, close the MFC1 and the valves Vb12, Vb14, V3, V21. Stop heating the primary adsorption column Cb1 and cool to <30°C. This process transfers the desorbed xenon in the primary adsorption column Cb1 to the secondary adsorption column C2.

Step (4) is repeated to transfer the desorbed xenon in the secondary adsorption column C2 to the tertiary adsorption column C3.

Repeat all the above-mentioned processes once, and carry out steps (2), (3), (5), and (6) twice in total, and step (4) for a total of 4 times, that is, the primary adsorption column Ca1 and Cb1 are sampled twice each , to the secondary adsorption column C2 for 2 transfers each, and C2 to the tertiary adsorption column C3 for 4 transfers.

(7) Three-stage adsorption column C3 desorption purification

Turn on MFC3 and valves V32 and V33, and at the same time turn on the heater of the third-stage adsorption column C3, and close valve V33 after the temperature rises to 130°C. Open valves V34, V41, V43, heat the tertiary adsorption column C3 from 130°C to 300°C and keep it for 15min, close MFC3 and valves V32, V34, V41, V43. Stop heating the third-stage adsorption column C3, and cool to <30°C. This process transfers the desorbed xenon in the third-stage adsorption column C3 to the fourth-stage adsorption column C4.

Repeat all the above-mentioned processes once, and perform steps (2), (3), (5), and (6) 4 times in total, step (4) 8 times in total, and step (7) 2 times in total, that is, perform primary adsorption The columns Ca1 and Cb1 were sampled 4 times each, transferred to the secondary adsorption column C2 4 times each, C2 was transferred to the tertiary adsorption column C3 8 times, and C3 was transferred to the fourth adsorption column C4 2 times.

(8) Four-stage adsorption column C4 desorption and source preparation

Turn on the vacuum pump and valves V48 and V47, evacuate the sample collection bottle until the pressure P5<1kPa, and close the valves V47, V48 and the vacuum pump. Turn on the heater of the fourth-stage adsorption column C4, raise the temperature to 200°C and keep it for 5 minutes, open the valves V44, V46, V47 and the diaphragm pump, and turn off the diaphragm pump when the value of P5 no longer rises. Open MFC4 and valve V42, when the pressure P4 is about 100kPa, close the valve V42, turn on the diaphragm pump, turn off the diaphragm pump when the P5 value does not rise, repeat this process, and close the diaphragm when the P5 value is about 100kPa Pump, heater for MFC4, C4 and valves V44, V46, V47. Open the valve V45, and keep the temperature of the fourth-stage adsorption column C4 at 200° C. for 30 minutes. Close valve V45, stop heating the fourth-stage adsorption column C4, and cool to <30°C. This process is the source process of sample collection bottle.

The concentrated xenon gas sample in the sample collection bottle is sent for radioactive xenon isotope measurement and total xenon concentration measurement to obtain the radioactive activity and total xenon amount of xenon isotope in the sample, and then calculate the activity concentration of radioactive xenon isotope in the ambient air taken.

The sample collection bottle is a steel cylinder with a volume of 150ml, the concentration of xenon in the sample is about 3.3% (v/v), the total amount of xenon obtained is about 5ml (standard condition), the effective air sampling volume is about 57m ³ , and the enrichment factor for xenon is greater than 3.5 ×10 ⁵ .

The aforementioned process is a 24-hour sampling process plus subsequent enrichment, source preparation and measurement processes. According to this method, long-term and continuous sampling and monitoring of the radioactive rare gas xenon in the ambient air can be realized.

The hollow fiber semi-permeable membrane group used in the device of the present invention can be Generon; it can also be realized by using the hollow fiber membrane nitrogen generator disclosed in Patent No. 200720094935.6.

This method has been used to develop a fixed station atmospheric xenon sampler, serving the monitoring research of the atmospheric nuclear environment and the nuclear test nuclide verification research.

Claims (4)

1. a normal temperature enrichment sampling method of xenon in the atmosphere is characterized in that: comprise the following steps: 1] air pretreatment: The hollow fiber semi-permeable membrane group is used to pretreat the air to remove O ₂ , CO ₂ and H ₂ O in the air, and initially concentrate xenon; 2] Four-stage activated carbon adsorption and desorption step by step: Four cascaded activated carbon adsorption and desorption units are used to gradually concentrate xenon in the air and remove other impurities; the adsorption temperature is 10-30°C, and the desorption temperature is 200-300°C; the temperature of the first-stage adsorption column is raised to 130 °C, the desorbed gas transfers to the second-stage adsorption column for adsorption, heats up to 300°C and maintains it for 30 minutes, and terminates; after the second-stage adsorption column heats up to 130°C, the desorbed gas transfers to the third-stage adsorption column for adsorption, Heating to 300°C and keeping it for 30 minutes and then terminating; after the temperature of the third-stage adsorption column is raised to 130°C, the desorbed gas transfers to the fourth-stage adsorption column for adsorption, and it is heated to 300°C and kept for 15 minutes before terminating; the adsorption-desorption unit It is an activated carbon adsorption column; 3) Sample collection: The xenon desorbed by the last stage adsorption column is transferred to the sample collection bottle through the diaphragm pressurization to complete the collection. 2. The normal-temperature enrichment and sampling method for xenon in the atmosphere according to claim 1, characterized in that: before the adsorption and desorption sampling, the activated carbon adsorbents packed in the adsorption columns at all levels are heated and activated. 3. The normal temperature enrichment sampling method of xenon in the atmosphere according to claim 1 or 2 is characterized in that: the first-stage adsorption column is two sets of parallel activated carbon adsorption columns, and the two sets of parallel activated carbon adsorption columns pass through the valve Realize alternate adsorption and desorption. 4. The normal-temperature enrichment sampling method for xenon in the atmosphere according to claim 3, characterized in that: the step of preliminary concentrating the xenon in the air by using a hollow fiber semi-permeable membrane group is specifically: passing compressed air through the hollow fiber semi-permeable membrane group. The permeable membrane group uses the different diffusion rates of each component in the hollow fiber semi-permeable membrane group to realize the separation of various components in the air and the preliminary concentration of xenon in the air.