WO2017085348A1

WO2017085348A1 - Device for measuring the diffusion coefficient of gases in a porous material under real conditions

Info

Publication number: WO2017085348A1
Application number: PCT/ES2016/070822
Authority: WO
Inventors: David BENAVENTE GARCÍA; Enrique Gadea Ramos; Concepcion PLA BRU
Original assignee: Universidad De Alicante
Priority date: 2015-11-19
Filing date: 2016-11-18
Publication date: 2017-05-26
Also published as: ES2558851A1; ES2558851B2

Abstract

The present invention relates to a device and method for determining the diffusion coefficient of at least one gas present in a mixture of gases in a sample of a permeable material, with different relative humidity values over a wide range of temperatures, comprising a first gas mixture chamber (1) and a second gas mixture chamber (2) arranged in a continuous manner, but separated from one another by a third chamber (3) designed to house a sample.

Description

Equipment for measuring the gas diffusion coefficient through a porous material in real conditions

DESCRIPTION

FIELD OF THE INVENTION

The present invention falls within the general field of materials science and in particular refers to a device and a method for determining the diffusion coefficient of at least one gas present in a gas mixture, through a porous material , in varying conditions of humidity, temperature and composition.

STATE OF THE PREVIOUS TECHNIQUE

Gaseous diffusion causes the movement of the molecules of the components of a gas mixture as a result of the concentration gradient in a system. It is the main mechanism for transporting gases through porous materials (soils, rocks, concrete, etc.). Gas diffusion processes are defined by the diffusion coefficient of the gas under study, which varies depending on the type of material through which diffusion occurs and which relates the flow of matter with this existing concentration gradient.

The application of this gaseous diffusion coefficient to the field of materials science, mainly the study of natural materials such as soils and rocks, is of interest if it is considered that the materials from which it is intended to determine the diffusion coefficient, are usually subjected at varying hygrometric conditions and which are determinants in the variation of the calculated coefficient.

Currently, the determination and quantification of the diffusion coefficient in environments subject to changes in their hygrometric conditions is achieved through methodological-experimental approaches, studying the factors that control this transport of gases through the porous system of the material in question. This calculation of the diffusion coefficient has been addressed by different research groups for different purposes, given the scientific and technical interest associated with the calculation of this parameter. As regards the study of gas diffusion associated with porous materials, there is no strictly defined protocol. As an example, one could consult works such as Turcu, et al, 2005. Continuous soil carbon dioxide and oxygen measurements and estimation of gradient-based gaseous flux. Vadose Zone Journal 4 (4): 1161-1169), where the study of gas diffusion in a soil column is approached through which a concentration gradient is created. However, the experimental system used in the laboratory does not allow the variation of the test conditions, that is, a diffusion coefficient is calculated without considering temperature values or volumetric water content in the soil. In addition, the injection of the necessary gas used to perform the experiment is performed directly on the sample without the existence of a homogenization chamber. This experimental design, therefore, would have to be discarded if the material tested was a compact solid, since the adequacy of the sample to the device would not be achieved.

On the other hand, Ganot et al. , 2014. Impact of thermal convection on C0 ₂ flux across the earth-atmosphere boundary in high-permeability soils. Farming! and Forest Meteorology 184 (0): 12-24) reproduce an experimental design related to the previous one in which they do study the thermal variations caused in the system and its impact on the gases studied, although focusing on the convective process of the movement of a mass of gas and not deepening the diffusive process.

Similarly, O'Brien et al., 2014. Using field analogue soil column experiments to quantify radon-222 gas migration and transport through soils and bedrock of Halifax, Nova Scotia, Canada. Environmental Earth Sciences 72 (7): 2607-2620), propose an interesting experiment in which they study the diffusion of radon in a soil column, but, again, considering the test conditions constant. Sanci et al; 2009. Assessment of soil moisture influence on C0 ₂ flux: a laboratory experiment. Environmental Geology 58 (3): 491-497) develop an experimental in which they conclude that the variation of the water content in the soil is decisive when calculating the value of the diffusion coefficient of a gas, however its research remains aimed at developing a calibration system for dynamic gas flow measurement chambers and not to determine a final result of the gas diffusion coefficient. The device they use is much more complex than the one presented in this proposal. Albanito et al., 2009. Automated diffusion chambers to diurnal monitor and seasonal dynamics of the soil C0 ₂ concentration profile. European Journal of Soil Science 60 (4): 507-514) or Risk et al., (2012) (Risk, D, Nickerson, N, Creelman, C, McArthur, G, Owens, J, 2011. Forced Diffusion soil flux : A new technique for continuous monitoring of soil gas efflux Agriculture! And Forest Meteorology 151 (12): 1622-1631) also propose different methods for calculating the diffusion coefficient and estimation of gas flow. While the first presents an experimental to develop in the laboratory (valid only for plastic membrane type materials) the second proposes the determination of a gas flow value through a forced diffusion chamber, which in any case would not be valid to determine coefficients of diffusion in robust solid materials.

Jabro, 2009. Water Vapor Diffusion Through Soil as Affected by Temperature and Aggregate Size. Transport in Porous Media 77 (3): 417-428) proposes a method to calculate the diffusion coefficient of water vapor in the soil. Consider that temperature is a determining factor in the process. However, the methodology used is less rigorous than that presented with this new equipment, since the test conditions are achieved with saline solutions. This same author also presents a paper (Jabro et al. 2012. Estimation of C0 ₂ diffusion coefficient at 0-10 cm depth in undisturbed and tilled soils. Archives of Agronomy and Soil Science 58 (1): 1-9) in which compiles the different theoretical formulas existing in the literature to obtain, in this case, a value of the diffusion coefficient of C0 ₂ in which the influence of temperature conditions and water content on the material under study is identified when determining the value of the diffusion coefficient.

Extrapolation of experimental laboratory systems to real conditions is also carried out. As an example, see the works Pingintha et al., 2010. Assessment of the soil C0 ₂ gradient method for soil C0 ₂ efflux measurements: comparison of six models in the calculation of the relative gas diffusion coefficient. Tellus Series B-Chemical and Physical Meteorology 62 (1): 47-58) where they reproduce an operating procedure similar to that proposed in this new proposal but by testing the materials (soils in this case) in their real environment. As a consequence, these works assess the changes in humidity and temperature in nature, relating them to the variation in the diffusion coefficient of the gas studied. These changes are those that, in the equipment presented, are They reproduce in the form of a laboratory test. This extrapolation to the laboratory makes it possible to test samples that would otherwise be impossible to test given their difficult access.

There is therefore a need to provide a device and a method to determine the implication of each of the factors (temperature, humidity, etc.) in the gas diffusion processes, depending on the microstructural characteristics of the porous system, which is capable of improve the results obtained with the current experimental systems, and with which the variations of the diffusion coefficient of a gas through a certain material can be determined, when there are several compounds in the gas mixture (presence of more than one gas or even presence of water vapor). And, that also, allows to study the process together with the variations of the thermohygrometric test conditions, as well as the possibility of studying different types of gases and testing a wide variety of porous materials, in such a way that a rigorous calculation of the coefficient is developed of diffusion and a detailed vision of what are the determining factors in the gas diffusion process.

EXPLANATION OF THE INVENTION

The present invention solves the problems described in the state of the art since it provides a device and a method that allows determining the diffusion coefficient of at least one gas present in a homogeneous or heterogeneous gas mixture, with different degrees of humidity on a determined permeable material (soils and / or rocks, synthetic materials, etc.), being able to reproduce on a laboratory scale situations existing in reality, where it is interesting to calculate the gas diffusion coefficient.

Thus, in a first aspect, the present invention relates to a device for determining the diffusion coefficient (hereinafter, device of the present invention) of at least one gas present in a gas mixture, through a sample of porous material, comprising a first gas mixing chamber and a second gas mixing chamber, arranged continuously and separated from each other by a third chamber, configured to house a sample, where the first chamber comprises a sensor system for monitoring the gas conditions, to measure the humidity and concentration conditions of the compounds present in the gas mixture of said chamber, which is connected to a control system, which keeps said conditions constant, and from which a gas inlet and outlet duct is connected to a system of inlet and outlet valves; and where the second chamber is sealed by all its walls except for the part thereof that is in contact with the third chamber and comprises a monitoring sensor that measures the gas conditions of the chamber and a control system that records the difference gas concentration of the chamber during the measurement process; From the chamber part an inlet conduit connected to an inlet valve system and an outlet conduit connected to an outlet valve system.

In a particular aspect of the present invention, the first chamber and / or the second chamber comprise a temperature control chamber, more particularly, the temperature control chambers are connected to a temperature control system.

In a particular aspect of the present invention the sensors of the first chamber and the second chamber are temperature, gas concentration and humidity sensors.

In a particular aspect of the present invention, the valve system of the first chamber is constituted by a valve for the inlet of the gas to be studied, an inlet valve for water vapor and an outlet valve.

In a particular aspect of the present invention, the first chamber and the second chamber operate with different thermohygrometric conditions.

In another particular aspect of the present invention, the first chamber and the second chamber operate with the same pressure (atmospheric pressure).

In a second aspect, the present invention relates to a method for determining the diffusion coefficient of at least one gas present in a gas mixture, through a sample of characterized porous material. The determination is carried out in the device of the present invention and comprises the following steps: a) Insert the sample into the third chamber, b) Introduce the mixture of gases to be studied in the first chamber and keep the humidity and concentration of each of the gases that make up the mixture constant,

c) Maintain the first chamber and the second chamber with the same pressure (atmospheric pressure) throughout the test,

d) Determine the difference in concentration and humidity in the compounds of the gas mixture in the second chamber between the start of the process and after it,

e) Determine the diffusion coefficient of the gas by means of the numerical treatment from the data obtained in step d).

In a particular aspect of the present invention, in the first chamber the concentration of each of the gases that make up the mixture is maintained constant, as well as the degree of humidity, either by the continuous circulation of a mixture of gases with a composition determined or by means of the valve system that allows each of the components of the mixture to be replaced independently depending on their diffusion to the other chamber.

The device and method of the present invention provides the following advantages:

• It allows the calculation of the diffusion coefficient of one or more compounds through a given sample under variable hygrometric conditions, simulating the real conditions in which diffusion processes occur in nature. Since the diffusion coefficient is closely related to parameters such as temperature or degree of humidity, this flexibility in changing conditions, allows to determine the effect that these parameters have in the calculation of the final value. There are two different atmospheres, separated by a membrane constituted by the material under study. This allows to reproduce differentiated conditions in each camera independently.

• Samples of different size and composition can be studied, using different sample holders and depending on the objective of the study.

• The equipment can operate in different modes depending on the needs and objectives of the analysis, performing short and repetitive cycles or long test cycles.

• The sample is not altered during the test, since it is housed in a separate chamber. • You can work with several gases at the same time: on the one hand the water vapor, which in addition to guaranteeing the humidity conditions required for each experiment allows the calculation of its gas diffusion coefficient and other gas or non-condensable gases in the experiment conditions (C0 ₂ , 0 ₃ , CH ₄ , etc.)

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 shows the device of the present invention

Figure 2 shows the device of the invention incorporating the temperature control system and a particular embodiment referring to the valve system.

References:

(1) first camera

(2) second camera

(3) third chamber

(5) first chamber inlet and outlet valve system

(6) second chamber inlet valve system

(7) second chamber outlet valve system

(8) sensors for monitoring the gas conditions of the first chamber

(9) sensors for monitoring the gas conditions of the second chamber.

(10) temperature control chamber of the first chamber

(1 1) temperature control chamber of the second chamber

(12) control system

(13) temperature control system

(15) gas inlet valve in the first chamber

(16) water vapor inlet valve in the first chamber

(17) first chamber outlet valve.

DETAILED EXHIBITION OF REALIZATION MODES

The device and method of the present invention allowed the calculation of the diffusion coefficient of one or more compounds present in a mixture of gases with different relative humidity in a wide range of temperatures.

The device comprises a system of valves that control the entrance of the gas mixture, wet or not, to be studied (5) to the first chamber (1), a system of valves (6) that control the entrance of gases to the second camera (2) for establish the initial conditions of the same, a gas outlet valve of the second chamber (7) that allows gas purge and closes to ensure the tightness of the second chamber during measurement, of a first chamber (1 ) where the concentration of the different compounds that form the mixture of gases and humidity are kept constant and that is in contact with the third chamber (3) that contains the sample to be studied which, in turn, is in contact with a second chamber (2) that is conditioned with certain initial conditions of concentration of gases and humidity and remains watertight during the measurement, recording, by one or more sensors (9) the evolution of the concentrations of the gaseous compounds to be studied and / or of temperature and / or humidity. The equipment is automatically controlled by a control system (12).

Both gas chambers are at the same pressure (atmospheric pressure), so that the passage of gases through the sample is produced solely by the diffusion phenomenon.

In a particular embodiment, as shown in Figure 1, the device of the present invention has a system of gas inlet and outlet valves (5) of the first chamber (1) consisting solely of an inlet connection and another outlet where a mixture of gases with the composition and humidity required for the study is circulated through the first chamber (1).

In another particular embodiment, (figure 2) the device of the present invention comprises a system of gas inlet and outlet valves (5) of the first chamber (1) consisting of at least one gas inlet valve to be studied. (15), at least one water vapor inlet valve (16) and one outlet valve (17), through which the different compounds that form the gas mixture are introduced, keeping the gas concentrations to be studied constant and of the humidity depending on the decrease of the same due to the diffusion through the sample and, of a system of sensors (8) of concentration of gas and / or of temperature and / or humidity that allow the measurement of said values so that they can be kept constant by the valve system (5).

In another particular embodiment, shown in Figure 2, the first chamber (1) and / or the second chamber (2), comprise a temperature control chamber (10) and (11) respectively, which may be connected to a system of temperature control (13). This temperature chamber system can operate interchangeably with both the valve system of Figure 1 and that of Figure 2.

An embodiment of the invention is described below.

The procedure began by introducing a sample of porous rock into the chamber (3) and then proceeding by introducing the mixture of gases (C0 ₂ and water vapor) into the first chamber (1) with humidity conditions (65%) and about concentrations (4000 ppm of concentration of C0 ₂ and water vapor, until reaching 65% relative humidity, equivalent to a vapor pressure 624-10 ³ micromol / m ³ at 20 ° C temperature) of the components of the mixture.

In the second chamber (2), the initial concentration and humidity conditions (40 ppm of C0 ₂ and water vapor equivalent to 35% relative humidity and vapor pressure of 329-10 ³ micromol / m ³ at 20 °) were started C temperature) and, keeping this chamber watertight except for the part of it in contact with the third chamber (3) where the sample is housed.

The evolution of the concentration of the study gas or gases (C0 ₂ and water vapor), present in the first gas chamber (1), was recorded. The evolution of the concentration of the compounds to be studied (C0 ₂ and water vapor), in the second chamber (2) are the result of their diffusion through the sample, being able to obtain the diffusion coefficient through numerical treatment of the data collected from each experiment, according to existing methodologies published in the bibliography.

The test was terminated when 4000 ppm of C0 ₂ concentration was reached in the upper chamber (2). In this specific example after 5 hours of testing. From the data recorded by the control system (12), the concentration evolution data with respect to time were obtained, from which the diffusion coefficient was calculated according to the following methodology:

Obtained from: Zhang et al. 2005 {Zhang, ZH, Ouriadov, AV, Willson, C, Balcom, BJ, 2005. Membrane gas diffusion measurements with MRI. "Journal of Magnetic Resonance 176 (2): 215-222) and Albanito et al., (2009) {Albanito, F, Saunders, M, Jones, MB, 2009. Automated diffusion chambers to daytime monitor! And seasonal dynamics of the soil C0 ₂ concentration profile European Journal of Soil Science 60 (4): 507-514). Two chambers joined by a membrane are considered, from which we want to calculate the diffusion coefficient of C0 ₂ . In one of the chambers there is an initial (C1) and constant concentration for the entire experiment. In the other chamber, concentration 0 (C2) ppm is started and the test is terminated when the concentration here has reached the initial concentration established for chamber 1 (C1).

For a mass balance:

r ^{C1 ~ C2} ¿(Ci - c ₂ ) r ^t0

^J C1 ° -C2 ° _(c - Á ^L 2J = ^{B Ddt}

AH 1 1

Where B = camera shape parameter = - (- I—); where A = chamber passage surface; L = thickness of the membrane (in this case, thickness of the specimen; V0 = volume of the chamber that is filled with gas; V1 = infinity, since the concentration in chamber 1 always remains constant). H = partition coefficient. For C0 _{2 it} is considered 1, since C0 ₂ is insoluble.

Integrating:

-ln (Cl - C2)] ¾ ₀ ^" _ ^c _c ² ₂ o = BDt] [°

ln (Cl - C2) - ln (Cl ₀ - C2 ₀ ) =

2 "". C2

7 ^ - = -l + e- ^BDt ; - = l

Cl ₀

BDt; and from here you get D.

For the case study, the calculated diffusion coefficient of C0 ₂ was 4.18 10 ^"7 m ² / s.

At any time, when deemed necessary, the second chamber (2) can be purged and return to other initial conditions, being able to repeat the diffusion study and being able to check if the diffusion coefficients of the compounds of interest change, and in what way, as a function of time, with what can be known if the behavior of the sample changes over time. The fact that the device is equipped with two gas mixing chambers is essential to be able to establish, separately, different thermo-hygrometric conditions in each of the chambers independently since, in addition, each of the chambers can Control at a certain temperature.

Claims

1. Device for determining the diffusion coefficient of at least one gas present in a gas mixture, through a sample of permeable material characterized in that it comprises a first gas mixing chamber (1) and a second gas mixing chamber (2) arranged continuously and separated from each other by a third chamber (3) configured to house a sample,

where the first chamber (1) comprises a system of sensors for monitoring the conditions of the gas (8) that measures the humidity conditions and the concentration of the compounds present in the gas mixture present in the first chamber (1), which it is connected to a control system (12) that maintains said constant conditions, and from which a gas inlet and outlet duct is connected to a system of inlet and outlet valves (5);

where the second chamber (2) is sealed on all its walls except for the part thereof that is in contact with the third chamber (3) and comprises a monitoring sensor that measures the gas conditions of the second chamber (2) and a control system (12) that records the difference in gas concentration of the second chamber (2) during the measurement process, from the chamber (2) starts an inlet conduit connected to an inlet valve system (6 ) and an outlet conduit connected to an outlet valve system (7).

2. Device according to claim 1, wherein the first chamber (1) and / or the second chamber (2) comprises a temperature control chamber (10), (1 1) respectively.

3. Device according to claim 2, wherein the temperature control chambers are connected to a temperature control system (13).

Device according to any one of the preceding claims, wherein the first chamber (1) comprises a valve system (5), constituted by a gas inlet valve to be studied (15), a water vapor inlet valve (16) and an outlet valve (17).

5. Device according to any one of the preceding claims, wherein the first chamber (1) and the second chamber (2) operate under different conditions te rm o- h i g ro m étri cas.

6. Device according to any of the preceding claims, wherein the first chamber (1) and the second chamber (2) operate with the same pressure (atmospheric pressure).

7. Method for determining the diffusion coefficient of at least one gas present in a gas mixture, through a sample of permeable material characterized in that it is carried out in a device according to claim 1-4, and by which it comprises following steps: a) Insert the sample into the chamber (3),

b) Enter in the first chamber (1), the gas mixture to be studied and keep the humidity and concentration of each of the gases that make up the mixture constant,

c) Maintain the first chamber (1) and the second chamber (2) with the same pressure (atmospheric pressure) throughout the test,

d) Determine the difference in concentration and humidity in the compounds of the gas mixture in the second chamber (2) during the start of the process and after it,

8. The method according to claim 1, wherein the gas mixture in the first chamber (1) is kept constant by continuous recirculation of said mixture or by replenishing each component of the mixture.