An experimental device and method for measuring gas diffusion coefficient of porous media under stepless humidity control

Abstract

The application discloses a kind of porous medium gas diffusion coefficient experimental device and method of stepless humidity, including external constant temperature and humidity box, gas diffusion tester, standard salt solution, gas concentration sensor etc..The test device can determine the influence of any saturation soil as horizontal barrier layer on gas diffusion under specific temperature and humidity.At the same time, the application also provides a kind of test method of model device for testing gas diffusion coefficient of horizontal barrier layer, by measuring test environment temperature, humidity, oxygen concentration, soil moisture content and soil compaction degree, the saturation of soil, gas diffusion coefficient, air humidity and temperature and the influence of different barrier materials, compaction degree on the barrier performance of horizontal barrier layer are obtained.

Description

Technical Field

[0001] This invention relates to the fields of environmental and geotechnical engineering, specifically to an experimental apparatus and its method for testing the gas diffusion coefficient of porous media under a stepless humidity-controlled environment. Background Technology

[0002] Barrier engineering is an in-situ isolation technology that controls the migration of pollutants in the soil of contaminated sites. Site closure and barrier engineering is a crucial component of barrier engineering, serving two main functions: preventing the leakage of polluting gases and reducing rainfall infiltration. Existing standards and testing methods only specify requirements for the liquid permeability coefficient of the barrier layer in cover barrier engineering, neglecting the requirement to prevent the leakage of polluting gases. The leakage of polluting gases mainly occurs through convection and diffusion, with gas diffusion being widespread in various contaminated sites such as industrial organic pollution sites, landfills, and solid waste landfills. However, domestic research on testing methods for the gas diffusion coefficient of porous media such as soil is relatively weak, and existing testing methods do not comprehensively consider variables. For the gas diffusion coefficient, the saturation of the porous medium plays a decisive role; changes in saturation during testing can lead to distortion in the calculation of the gas diffusion coefficient, resulting in an underestimation of the gas barrier performance of the porous medium. The saturation of porous media is positively correlated with ambient humidity; controlling ambient humidity can determine whether the porous medium will lose water, absorb moisture, or remain unchanged in the environment.

[0003] Currently, there is a lack of research that considers the influence of different environmental humidity and temperature on the gas diffusion coefficient of porous media such as soil, and there is a lack of corresponding model devices and testing methods. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a testing model device and its method for testing the gas diffusion coefficient of soil under stepless humidity control. This device can simulate the gas diffusion process of porous media such as soil under different temperature and humidity conditions based on factors such as ambient humidity, ambient temperature, liquid saturation of the test medium, wet-dry cycle of the test medium, and the test medium (including sand, silt, clay, modified natural soil, cement-soil covering and isolation materials, or composite structural materials of geomembrane and soil). It can also measure the values ​​of ambient humidity and oxygen concentration to obtain the gas diffusion coefficient.

[0005] To achieve the above objectives, the present invention adopts the following technical solution:

[0006] An experimental apparatus for gas diffusion coefficient in porous media under stepless humidity regulation includes a chamber, a solution box, a porous plate, a diffusion chamber, an inlet pipe, an outlet pipe, and a data acquisition instrument.

[0007] The cabin is sealed;

[0008] The perforated plate is installed inside the cabin and fixed in place.

[0009] The solution box is located below the well plate and contains a humidity-controlled solution for humidity adjustment.

[0010] The pre-adjusted humidity of the constant humidity solution should be as close as possible to the target humidity. The solute selected for the solution is not unique. For example, LiBr solution (6.6% relative humidity at 20℃) can be used under low humidity, while K2SO4 solution (97.6% relative humidity at 20℃) can be used under high humidity.

[0011] The diffusion chamber is placed on a porous plate. The upper part of the diffusion chamber has an opening, and a ring cutter is provided at the opening. The soil sample to be tested is laid in the ring cutter. One end of the air inlet pipe and the air outlet pipe are connected to the diffusion chamber, and the other end penetrates the chamber and is located outside. The penetration is sealed. The air inlet pipe is connected to a nitrogen cylinder. The air inlet pipe has a left air valve, and the air outlet pipe has a right air valve.

[0012] The data acquisition unit is connected to the inside of the diffusion chamber via a pipe. An oxygen sensor is installed on the pipe inside the diffusion chamber. The data acquisition unit is connected to an external data processing terminal.

[0013] As a further preferred option, the chamber is also equipped with a constant temperature device, a high-precision thermometer and hygrometer, a sealed box, an evaporator, an electronic analytical balance, and a soil sample storage box. The sealed box, evaporator, electronic analytical balance, and soil sample storage box are placed on a porous plate. The soil sample storage box contains soil samples, the sealed box contains ultrapure water, and the evaporator is used to add ultrapure water from the sealed box to adjust the humidity.

[0014] As a further preferred option, the sidewall of the cabin is also equipped with a left hatch, a forward-tilting hatch, and an operating opening, with gloves sealed on the operating opening.

[0015] As a further preferred option, the chamber is equipped with four pipettes with volume ranges of 0-10μl, 0-100μl, 0-1ml, and 0-10ml, respectively.

[0016] As a further preferred option, the perforated plate is 2cm thick with an opening rate of 80%, and the overall area of ​​the solution box accounts for more than 90% of the bottom area of ​​the cabin.

[0017] As a further preferred option, an oxygen sensor with a testing accuracy of ±1%.

[0018] As a further preferred option, the constant temperature device has a temperature accuracy of ±0.5℃ and is placed 10cm above the soil sample to be tested. The high-precision thermometer and hygrometer has a temperature accuracy of ±0.5℃ and a humidity accuracy of ±2%, and is placed vertically on the left and right walls of the box, with the center line aligned with the soil sample to be tested. The electronic analytical balance has an accuracy of ±0.001 grams.

[0019] An experimental method for an experimental apparatus for the gas diffusion coefficient of porous media under stepless humidity regulation includes the following steps:

[0020] Step 1: Place the soil sample to be tested in the soil sample storage box, select salt and water with a humidity close to the target humidity to prepare a constant humidity solution in the solution box and place it under the porous plate, weigh ultrapure water and place it in the sealed box, and close the left hatch and the front up-turning hatch.

[0021] Step 2: Keep the solution box lid open. After 7 days, read and record the readings of the high-precision temperature and humidity meter every day. After the readings of the temperature and humidity meters on both sides are within 1% for 3 consecutive days, close the solution box to complete the pre-humidification step.

[0022] Step 3: Based on the chamber volume, calculate the difference between the target humidity at the test temperature and the pre-conditioned humidity inside the chamber, referring to the Antoine equation and the ideal gas equation. Calculate the mass of evaporated water required for secondary humidification inside the chamber to reach the target humidity using formulas 1-3. Weigh the required mass of ultrapure water (Δm) from the sealed container using gloves and place it in the evaporator. Increase the set temperature of the thermostat until the water inside the evaporator has completely evaporated, then lower the set temperature to the test temperature. After 7 days, read the high-precision temperature and humidity meter reading. If the difference from the target humidity exceeds 1%, repeat step 3 until the difference between the high-precision temperature and humidity meter reading and the target humidity is less than 1%, then proceed to step 4.

[0023]

[0024]

[0025]

[0026] In the formula P e RH is the saturated vapor pressure of water at temperature T, in mmHg; T is the temperature, in °C; RH is the relative humidity; A, B, C, and R are constants that need to be obtained from tables; P1 is the partial pressure of water vapor after pre-conditioning; P2 is the partial pressure of water vapor at the target humidity; V is the net volume of the device.

[0027] Step 4: Open the soil sample storage box with gloves, take out the soil sample to be tested, place it on the diffusion chamber, and carefully seal the gap between the white petroleum jelly sealing ring and the diffusion chamber.

[0028] Step 5: Open the left and right gas valves and control the nitrogen cylinder output flow rate to below 0.5 liters / minute. The gas enters the diffusion chamber from the inlet pipe and the gas inside the diffusion chamber is discharged from the outside of the chamber through the outlet pipe. Close the left and right gas valves after the data processing terminal displays that the oxygen concentration in each diffusion chamber has reached below 3%.

[0029] Step 6: Read the data from the data processing terminal (19), select the data of the middle linear segment, and calculate the effective gas diffusion coefficient of the soil sample to be tested according to formulas (4)-(5);

[0030]

[0031]

[0032] In the formula, C0 is the oxygen concentration in the atmosphere; D p It is the effective gas diffusion coefficient of the medium being measured; α1 is hL = α n L n tan(α n L n The first positive root of ); ε is the gas porosity of the test medium; t is the diffusion time; C(L,t) is the oxygen concentration inside the chamber at time t; C i The oxygen concentration inside the chamber at t=0 is L; the height of the sample medium is h; the height of the diffusion chamber is C. r It is a relative concentration.

[0033] As a further preferred option, during steps 2 to 6, the left hatch and the forward-tilting hatch remain closed, and all operations are performed using gloves.

[0034] Beneficial effects:

[0035] Compared with existing technologies, the present invention provides a test model device for testing the gas diffusion coefficient of soil under stepless humidity control and its usage method, which has the following advantages:

[0036] 1) Testing of porous media such as soil under different humidity conditions was achieved. By considering factors such as ambient humidity, ambient temperature, compaction degree of the test medium, liquid saturation of the test medium, and the test medium itself (including sand, silt, clay, modified natural soil, cement-soil covering materials, or composite materials of geomembrane and soil), the gas diffusion process of porous media such as soil was simulated under different temperature and humidity conditions. The values ​​of ambient humidity and oxygen concentration were measured to obtain the gas diffusion coefficient D. p This provides a scientific basis for evaluating the changes in the air barrier performance of the cover barrier during operation;

[0037] 2) It achieves arbitrary humidity control inside the cabin. Compared with existing instruments with the same function, this device utilizes the high precision characteristics of saturated salt solution and electronic analytical balance, which improves the accuracy of humidity regulation compared with the traditional humidity meter plus spray device.

[0038] 3) The testing cost is low, the operation is simple, and multiple samples can be tested simultaneously. The internal diffusion chamber and the entire environmental chamber are two independent parts; for large-scale testing, only the number of internal diffusion chambers needs to be increased. If an internal diffusion chamber is damaged, it can be replaced individually. Attached Figure Description

[0039] Figure 1 This is an example diagram of the device structure of the present invention;

[0040] The components include: 1. Left air valve; 2. Left hatch; 3. Porous plate; 4. Solution box; 5. Humidity-controlled solution; 6. Nitrogen cylinder; 7. Ring cutter; 8. Soil sample to be tested; 9. Gloves; 10. Temperature control device; 11. Diffusion chamber; 12. High-precision thermometer and hygrometer; 13. Right air valve; 14. Data acquisition instrument; 15. Sealed box; 16. Evaporator; 17. Electronic analytical balance; 18. Front-opening hatch; 19. Data processing terminal; 20. Soil sample storage box. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0042] This invention discloses a test model device for testing the gas diffusion coefficient of soil under stepless humidity control, which has a precise humidity control module and is divided into two levels of humidity control. The constant humidity solution 5 is made of supersaturated salt solution and, together with the closed constant humidity box 4, constitutes the pre-humidification module. The constant temperature device 10, the high-precision thermometer and hygrometer 12, the sealed box 15, the evaporator 16, and the electronic analytical balance 17 constitute the two-level humidity control system.

[0043] The cabin material of this invention is plexiglass, and the sealing material consists of buckles, rubber sealing rings, and epoxy resin adhesive.

[0044] The solution box 4, sealing box 15, evaporator 16, and soil sample storage box 20 of the present invention meet the sealing requirements and can ensure that the quality of the sample remains unchanged for 14 days after it is sealed in them.

[0045] This invention discloses a testing model device and its method for testing the gas diffusion coefficient of soil under a stepless humidity-controlled environment, using clay as the test medium. Specific embodiments are described below. This example uses a pre-designed cover barrier layer to determine whether the proposed in-situ natural soil meets the gas barrier requirements.

[0046] As part of the sample preparation, two equal portions of in-situ natural soil were weighed and prepared into two ring cutter samples with the same compaction degree. The ring cutters were wrapped in plastic wrap and cured in a curing room at 25°C for 48 hours, and labeled as Sample A and Sample B, respectively. After Sample B was cut, the soil water and soil characteristic curves of the clay at this compaction degree were tested using a dew point meter or traditional filter paper method or salt solution saturation cylinder method, so as to obtain the corresponding relationship between the saturation degree of the in-situ natural soil and the ambient humidity at this temperature. For example, Sample A, made from this natural in-situ soil, maintained a saturation degree of 80% under the conditions of 90% compaction degree, temperature of 20°C, and humidity of 98%.

[0047] Following step 1, place sample A in soil sample storage box 20. Pour the evaporator into solution box 4 until the page height is above 0.5 cm, then sprinkle in dry fine particles of potassium sulfate (K2SO4) until about 0.3 cm of undissolved particles accumulate at the bottom of solution box 4. Place this supersaturated salt solution 5 under the porous plate. Weigh 50 g of the evaporator and place it in the sealed box 15, then close the left hatch 2 and the front upward-opening hatch 18.

[0048] Step 2: Keep the lid of solution box 4 open, turn on the thermostat 10 and set the temperature to 20℃. After 7 days, read and record the readings of the high-precision thermo-hygrometer 12 for three consecutive days. After the readings of the thermo-hygrometer 12 on both sides are maintained at 97.6% ± 1% for 3 consecutive days, close solution box 4 to complete the pre-humidification step.

[0049]

[0050] Solve for the saturated vapor pressure P of water at 20℃ e =462.456Pa.

[0051]

[0052] According to equation (2), since the humidity of the supersaturated salt solution is 97.6%, the current water vapor pressure P1 = 451.357 kPa, and the water vapor pressure at the target humidity of 98% is P2 = 453.207 kPa.

[0053]

[0054] According to equation (3), Δm = 0.014g is calculated.

[0055] Step 3: Calculate the volume of the box, assuming the internal volume of the box is 1m³. 3The difference between the target humidity at the test temperature and the humidity inside the chamber after pre-conditioning is calculated by referring to the Antoine equation and the ideal gas equation. The amount of evaporator that still needs to be evaporated by Δm = 0.014g is weighed from the sealed box 15 and placed in the evaporator 16. The temperature of the constant temperature device is increased from 10 to 30°C. After the water inside the evaporator 16 has completely evaporated, the temperature of the constant temperature device 10 is reduced to 20°C. After 2 days, the reading of the high-precision thermo-hygrometer 12 is read. If the difference between the reading and the target humidity exceeds 1%, step 3 is repeated until the difference between the reading of the high-precision thermo-hygrometer 12 and the target humidity is less than 1%, then step 4 is performed.

[0056] Step 4: Open the soil sample storage box 20, take out the soil sample 8 to be tested, and place it on the diffusion chamber 11. Carefully use the white petroleum jelly to seal the gap between the ring cutter 7 and the diffusion chamber 11.

[0057] Step 5: Open the left gas valve 1 and the right gas valve 13, and control the output flow rate of the nitrogen cylinder 6 to below 0.5 liters / minute until the data processing terminal 19 displays that the oxygen concentration in each diffusion chamber 11 has reached below 3%, then close the left gas valve 1 and the right gas valve 13.

[0058] Step 6: Read the data from the data processing terminal 19, select the data of the middle linear segment, and calculate the gas diffusion coefficient of the soil sample to be tested according to equations (4)-(5);

[0059]

[0060]

[0061] In the formula, C0 is the oxygen concentration in the atmosphere; D p It is the effective gas diffusion coefficient of the medium being measured; α1 is hL = α n L n tan(α n L n The first positive root of ); ε is the gas porosity of the test medium; t is the diffusion time; C(L,t) is the oxygen concentration inside the chamber at time t; C i L is the oxygen concentration inside the chamber at t=0; L is the height of the sample medium being tested; L a It is the height of the diffusion chamber; C r It is a relative concentration.

[0062] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An experimental apparatus for the gas diffusion coefficient of porous media under stepless humidity regulation, characterized in that: Includes a cabin, a solution box (4), a perforated plate (3), a diffusion chamber (11), an air inlet pipe, an air outlet pipe, and a data acquisition instrument (14); The cabin is sealed; The perforated plate (3) is installed in the cabin and fixed in place; The solution box (4) is located below the porous plate (3), and the solution box (4) contains a constant humidity solution (5) for humidity control; The diffusion chamber (11) is placed on the porous plate (3). The upper part of the diffusion chamber (11) has an opening, and a ring cutter (7) is provided at the opening. The soil sample (8) to be tested is laid in the ring cutter (7). One end of the air inlet pipe and the air outlet pipe are connected to the diffusion chamber (11), and the other end penetrates the chamber and is located outside. The penetration is sealed. The air inlet pipe is connected to a nitrogen cylinder (6). The air inlet pipe has a left air valve (1), and the air outlet pipe has a right air valve (13). The data acquisition instrument (14) is connected to the inside of the diffusion chamber (11) through a pipe. The pipe inside the diffusion chamber (11) has an oxygen sensor. The data acquisition instrument (14) is connected to an external data processing terminal (19). The chamber is also equipped with a constant temperature device (10), a high-precision thermometer and hygrometer (12), a sealed box (15), an evaporator (16), an electronic analytical balance (17), and a soil sample storage box (20). The evaporator (16), the electronic analytical balance (17), and the soil sample storage box (20) are placed on a porous plate (3). The soil sample storage box (20) contains soil samples, the sealed box (15) contains ultrapure water, and the evaporator (16) is used to add ultrapure water from the sealed box (15) to adjust the humidity.

2. The experimental apparatus for gas diffusion coefficient in porous media under stepless humidity regulation according to claim 1, characterized in that: The side wall of the cabin is also provided with a left cabin door (2), a front-flipping cabin door (18) and an operating hole, and a glove (9) is sealed on the operating hole.

3. The experimental apparatus for gas diffusion coefficient in porous media under stepless humidity conditioning according to claim 1, characterized in that: The porous plate (3) is 2cm thick and has an opening rate of 80%. The solution box (4) has an overall area of ​​more than 90% of the bottom area inside the cabin.

4. The experimental apparatus for gas diffusion coefficient in porous media under stepless humidity regulation according to claim 1, characterized in that: The oxygen sensor has a testing accuracy of ±1%.

5. The experimental apparatus for gas diffusion coefficient in porous media under stepless humidity regulation according to claim 1, characterized in that: The constant temperature device (10) has a temperature accuracy of ±0.5℃ and is placed 10cm above the soil sample (8) to be tested. The high-precision thermometer and hygrometer (12) has a temperature accuracy of ±0.5℃ and a humidity accuracy of ±2% and is placed on four vertically on each of the left and right walls of the box, with the center line at the same height as the soil sample (8) to be tested. The electronic analytical balance (17) has an accuracy of ±0.001 grams.

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