Bruno Cousin

Phase Change of Water in a Hygroscopic Porous Medium. Phenomenological Relation and Experimental Analysis for Water in Soil

Abstract The phenomenological relation of non-equilibrium liquid–gas phase change in a porous medium is described at the macroscopic level using the difference in chemical potentials between the liquid and its vapor. The experiments conducted consisted in lowering the partial pressure of water vapor in the pores of a hygroscopic soil and analyzing the return to equilibrium by two measurements: the macroscopic temperature and the partial pressure of vapor. The central hypothesis of the study is that the characteristic time associated with thermal equilibrium is much lower than the characteristic time associated with mass transfers. From these measurements, it is possible to determine the relation that links phase change rate to the logarithm of the ratio of partial vapor pressure divided by the equilibrium pressure (RH). The representation of this relation according to RH reveals two regimes in the return to equilibrium. The characteristics of these regimes are analyzed according to water content, temperature, and total gas phase pressure.

Interpreting the Drying Kinetics of a Soil Using a Macroscopic Thermodynamic Nonequilibrium of Water Between the Liquid and Vapor Phase

Two preliminary experiments show that a nonequilibrium situation can be easily encountered during a natural drying process. This leads us to reconsider the thermodynamic local equilibrium assumption and propose a macroscopic two-equation model that takes into account mass exchange kinetics between the liquid and vapor phase. Numerical simulation of this theoretical model is then compared to experimental drying kinetics of soil columns. The discrepancies observed between the theoretical prediction and the experimental results are discussed. This contribution emphasizes the importance of such nonequilibrium phenomenon when modeling water transport in hygroscopic porous media.

A New Experimental Method to Determine the Sorption Isotherm of a Liquid in a Porous Medium

Sorption from the vapor phase is an important factor controlling the transport of volatile organic compounds (VOCs) in the vadose zone. Therefore, an accurate description of sorption behavior is essential to predict the ultimate fate of contaminants. Several measurement techniques are available in the case of water, however, when dealing with VOCs, the determination of sorption characteristics generally relies on gas chromatography. To avoid some drawbacks associated with this technology, we propose a new method to determine the sorption isotherm of any liquid compounds adsorbed in a soil. This method is based on standard and costless transducers (gas pressure, temperature) leading to a simple and transportable experimental device. A numerical estimation underlines the good accuracy and this technique is validated on two examples. Finally, this method is applied to determine the sorption isotherm of three liquid compounds (water, heptane, and trichloroethylene) in a clayey soil.

Evaporation of a volatile organic compound in a hygroscopic soil - influence of the airflow and its VOC vapour saturation

This article presents an experimental and theoretical study of volatile organic compounds volatilisation in soil during a decontamination process by vapour extraction or venting. A phase change law is proposed in the case of a sandy-silty soil when the convective gaseous phase is vapour charged. A simple experimental method for analysing the phase change is presented. Finally, an efficiency coefficient is introduced to quantify the contribution of airflow velocity on venting. Cet article présente une étude expérimentale et théorique de la volatilisation d’un COV dans un sol soumis à une décontamination par ventilation. On propose une relation de changement de phase dans le cas convectif pour un limon sableux hygroscopique lorsque l’air en mouvement est chargé de vapeur de COV. Une méthode expérimentale simple d’analyse du changement de phase est proposée. Finalement un coefficient d’efficacité est introduit pour quantifier l’influence de la vitesse de l’air sur l’efficacité de la ventilation.

Influence of Ambient Pressure on the Evaporation of Volatile Organic Pollutants (VOP) during Soil Decontamination: Case of the Trichloroethylene (TCE)

This article presents the results on the volatilization of the volatile organic pollutants (VOP) during decontamination process from the soil. The choice of TCE as a volatile organic pollutant is explained by the fact that it is relatively highly soluble in water, compared to other compounds, which excludes any possibility of adsorption of its vapors on the walls of the experimental device during testing. Its saturation vapor pressure very high (≈7700 Pa at 20oC and nearly 12,000 Pa at 30oC) facilitates its monitoring using a pressure transducer relatively less accurate and less expensive. The results obtained on the evaporation of TCE show a linear dependence with the pressure. The coefficient of volatilization is 3.2 times greater for an atmospheric pressure of 90 kPa than for a pressure of 100 kPa. This coefficient would be multiplied by 20 when the pressure passes from 100 kPa to 10 kPa.

Phase Change of Volatile Organic Compounds in Soil Remediation Processes

Liquid-vapor phase change is a central phenomenon in remediation of soils contaminated by volatile organic compounds. An original experimental study leads to validate a law of phase change of heptane in a soil. The dependence of the phenomenological coefficient to liquid content and temperature is established. A simulation of self-removal of heptane in a soil, involving phase change of pollutant and diffusion of the vapor, is compared with experimental results. Finally, treatment by venting of a column of contaminated soil is studied experimentally in order to improve the understanding of the phenomena involved. The results obtained are likely to direct future modelisation of the processes.

Highlighting of a local non-equilibrium thermodynamics during the transfer of trichlorethylene (TCE) in the superficial layers of arid soils

In this paper, we analyse and model the mass transfer of trichloroethylene in the surface layer of soil. Our study essentially focuses on arid soils taking into account the phase change liquid–vapour. We have then examined the validity of the assumption of local equilibrium by comparing the values of instantaneous pressure of the trichloroethylene during transfer process and the equilibrium vapour pressure. It appears that the assumption of local equilibrium during the transfer of trichloroethylene cannot be admitted.