Drying Of Porous Media Research Articles

Pore-Network Modeling of Isothermal Drying in Porous Media

Pore-Network Modeling of Isothermal Drying in Porous Media

Effect of liquid films on the drying of porous media

AbstractThe effect is studied of capillarity‐driven viscous flow through macroscopic liquid films during the isothermal drying of porous materials. A mathematical model that accounts for viscous flow in a 2‐D pore network, through both the liquid films and the bulk liquid phase, is presented. The results are compared with previous works that do not account for the effect of films and with experimental findings by other authors. It is shown that film flow is a major transport mechanism in the drying of porous materials, its effect being dominant when capillarity controls the process, which is the case in typical applications. By contrast, viscous flow in the bulk contributes negligibly. The results are then generalized to drying under an applied temperature gradient. © 2004 American Institute of Chemical Engineers AIChE J, 50: 2721–2737, 2004

Characterization of the First Falling Rate Period During Drying of a Porous Material

This study deals with the evolution of the surface state and its influence on the drying of porous media. Surface temperature and saturation values are obtained using optical metrology. Analysis of the experimental results allows discussion of the apparition of constant drying rate period and to characterize the transition to the falling rate period. A mathematical model is developed to account for these observations and experimental results for some physical properties of a model material. It allows determination of the internal profile of moisture and the penetration of the drying front during the falling rate period.

Adsorption and diffusion in nanoporous materials from stochastic and process-based reconstruction techniques

Digitized binary domains representing the porous structure of an alumina membrane are produced using a process-based technique following an algorithm of ballistic deposition of non-overlapping monosize spheres. The diffusivity in the Knudsen regime of inert, point-like tracers, is determined using a random walk simulation procedure. Computed and experimental data show very good agreement. The porous structure of Vycor glass is generated by a stochastic method respecting statistical properties of the original structure such as the porosity and the two point correlation function. The Knudsen diffusivity of inert, point-like tracers is again determined as above and the agreement with the experiment is satisfactory. In a further step, both adsorption and desorption of CH 2 Br 2 in porous Vycor glass are simulated and the relative permeability curve of the system is computed and agrees well with measured data. It is shown that the adsorption process in Vycor can be simulated by the very same stochastic procedure used for the reconstruction of the dry porous medium. The same approach leads also to the simulation of the desorption branch, in the absence of thermodynamical hysteresis, by taking under consideration the accessibility of certain sites of the porous structure to the gaseous phase.

Theoretical Study of Stress Reversal Phenomena in Drying of Porous Media

AbstractDrying of capillary porous materials may generate internal stresses, the magnitude of which depends on both the drying techniques and the mechanical properties of dried materials. Acoustic emission (EA), a method to monitor for damage control during drying, shows an enhanced emission of acoustic impulses and energy from the material in some stages of drying. A particularly interesting phenomenon is the occurrence of enhanced emission of the acoustic impulses at the final stage of drying, that is, at the time when the drying induced stresses are expected to disappear. This phenomenon is explained by the stress reversal, i.e. the change of stress signs from negative (compressive) inside the material at the beginning to the positive (tensional) at the final stage of drying. A theoretical analysis is presented in this paper using the viscoelastic model. Stress reversal was predicted and analyzed from drying cylinders of ceramics‐like material and wood.

Enhancement of an Insufficient Dye-Formation in the Ninhydrin Reaction by a Suitable Post Treatment Process

A study of the reaction between ninhydrin and alanine has been carried out taking into account that, adhered on the surface of a dry porous medium such as paper, a quasi-heterogeneous reaction has to take place. Instead of amino acids released from human sweat glands, aqueous solutions of alanine were taken to deliver a given amount of amino acid to the sample. The dye density, obtained after using a standard development process, could noticeably be increased by setting a drop of water on the dye dot, thus indicating that not all the alanine had been used for dye formation during the usually applied process. The incomplete reaction can be explained by the problem of bringing the reactants into contact with each other when both are in the solid phase in the porous surrounding. The temporary presence of water allows a reorientation of the insoluble reactants. With fingerprints an increase in both the rate of development and the final dye density could be obtained when the sample was post treated after the developing process with the blank solvent, thus also the background coloration could be decreased. The ideas presented in this paper may form the basis for a modification of developing processes with ninhydrin in order to increase the proportion of amino acids present (in the sample) used in dye formation without data loss.

Experimental Investigations on Drying of Porous Media Using Infrared Radiation

Increased interest is being shown in infrared drying today because of the environmental and technological advantages offered by this method. In order to assess the advantages of this drying process, extensive trials have been carried out. The objective of this investigation was to study the drying rate of infrared drying. This was achieved with the use of scanning pyrometer and image analysis.

Resonant behavior of saturated porous media

The resonant behavior of a porous medium composed of periodic channel structures was studied. The natural frequencies of the dry pore system were obtained by theoretical analysis. The resonant frequencies of the saturated porous medium were calculated by numerical simulation, which considered the two‐phase coupling effects of fluid and solid. The numerical results showed that most of the resonant frequencies of the saturated porous medium match with the natural frequencies of the corresponding dry porous medium but that some of the resonant frequencies are different from those of the solid matrix depending on the porosity of the pore system. Both the theoretical study and the numerical study suggest that the resonant frequencies of a porous medium are functions of the porosity or the fracture density.

Evaporation of a Stagnant Liquid

We model the evaporation of a stagnant liquid from an initially filled block to a flowing gas stream. The motivation for this problem arises from applications in the drying of porous media, when the pressure is low, and in the recovery of oil from fractured reservoirs by gas injection, when the pressure is high. A similarity solution is developed which accounts for diffusion in both phases. Diffusion in the liquid phase can be important in high-pressure applications, where the gas may dissolve in the liquid phase. The motion of the interface and the evaporation rates are calculated as a function of the various thermodynamic parameters for systems of interest, including n-alkanes, methane, nitrogen, or carbon dioxide. The effect of counter-diffusion is shown to slow the evaporation process, although not by an order of magnitude, in typical cases.

TRANSPORE : A GENERIC HEAT AND MASS TRANSFER COMPUTATIONAL MODEL FOR UNDERSTANDING AND VISUALISING THE DRYING OF POROUS MEDIA

ABSTRACT In this work a sophisticated numerical model is presented that describes the drying of porous media. This model, which is known as TransPore, has evolved over the years through the direct inputs of both authors. Nowadays, TransPore can be used to analyse the drying of media that are of completely arbitrary shape and size, under a variety of drying conditions. The engine of the computational model uses a number of state-of-the-art numerical methods that ensure the simulation results describe the particular drying process accurately, whilst guaranteeing the most efficient and effective usage of computer resources. For example, the numerical discretisation method is based on a completely conservative hybrid finite element control volume technique that uses a finite element mesh for its background gradient interpolation. Furthermore, flux limiting is used to reduce numerical dispersion in the drying kinetics and the generated non-linear system is resolved using the full Newton method for the outer iteration coupled together with a preconditioned conjugate gradient technique for the inner iteration. A graphical interface has been linked to the model to enable online visualisation of the drying process. The mathematical model allows both homogeneous and heterogeneous porous media to be simulated. The resultant software is an extremely powerful and effective tool for investigating existing dryer designs and for proposing new and innovative drying schedules that provide optimal drying quality in minimal drying time.

Effect of dynamic competitive sorption on the transport of volatile organic chemicals through dry porous media

Dynamic (time‐varying) competitive sorption, modulated through fluctuating relative humidity, is shown to significantly affect the diffusive flux of 1,2,4‐trichlorobenzene through soil columns. The flux varied by a factor of up to 2 during a 24 hour cycle in the experimental system when subjected to humidity fluctuations of ±40%. This effect was observed despite conditions in which sufficient water coverage would have been present to significantly depress volatile organic chemical (VOC sorption). Many vadose zone chemical transport models do not incorporate vapor‐solid sorption, which is known to be important in low‐moisture soils. The models that do incorporate vapor sorption do not account for dynamic variations of the partitioning behavior associated with variable moisture content. We present and validate a mathematical model for VOC transport through soils that accounts for dynamic nonlinear competitive sorption on volatile species transport. Simulations are presented which show that the effect of a thin dry zone at the soil‐air interface can result in large variations in the instantaneous chemical release rate. This implies that for some conditions in the field, VOC sampling should be conducted over longer periods to avoid bias due to short‐term spikes in the chemical release rate.

An experimental and theoretical study of the nonlinear heat conduction in dry porous media

Heat transfer in porous media is important in various engineering fields, including contaminated soil incineration. Most heat transfer models are theoretical in nature. Consequently, this study was undertaken to perform both theoretical and experimental studies of heat transfer in two different sand matrices. A mathematical model based on Fourier's law of heat conduction for a one-dimensional system with the variable thermal conductivity was developed. The experimental part included heating sand samples placed in a small reactor within an infrared furnace. The transient temperature profiles of the sand layers were monitored by thermocouples. The bulk thermal conductivity was estimated to be linearly proportional to the temperature. The temperature profiles predicted by the model of heat conduction with a variable bulk thermal conductivity was compared by the observed temperatures in Quartz and Sea sands matrices up to 1300 K. Copyright © 1999 John Wiley & Sons, Ltd.

An experimental and theoretical study of the nonlinear heat conduction in dry porous media

Heat transfer in porous media is important in various engineering fields, including contaminated soil incineration. Most heat transfer models are theoretical in nature. Consequently, this study was undertaken to perform both theoretical and experimental studies of heat transfer in two different sand matrices. A mathematical model based on Fourier's law of heat conduction for a one-dimensional system with the variable thermal conductivity was developed. The experimental part included heating sand samples placed in a small reactor within an infrared furnace. The transient temperature profiles of the sand layers were monitored by thermocouples. The bulk thermal conductivity was estimated to be linearly proportional to the temperature. The temperature profiles predicted by the model of heat conduction with a variable bulk thermal conductivity was compared by the observed temperatures in Quartz and Sea sands matrices up to 1300 K. Copyright © 1999 John Wiley & Sons, Ltd.

CONJUGATE HEAT AND MASS TRANSFER IN CONVECTIVE DRYING OF POROUS MEDIA

A methodology for the analysis of conjugate problems in the convective drying of porous media is presented. In this study, the interface between porous medium and external convective flow is treated as an internal boundary within a two-phase system rather than a geometric limit. The problems of solid drying and convection boundary layer are connected by expressing the continuity of the state variables and their respective fluxes through the interface. The performance of the proposed methodology is evaluated by applying it to wood-drying problems. The analysis of the drying of porous media as a conjugate problem allows the assessment of the effect of the heat and mass transfer within the solid on the transfer in the adjacent fluid, providing good insight on the complexity of the transfer mechanisms.

Comparison of Iterative Methods for Improved Solutions of the Fluid Flow Equation in Partially Saturated Porous Media

Abstract. The Picard and modified Picard iteration schemes are often used to numerically solve the nonlinear Richards equation governing water flow in variably saturated porous media. While these methods are easy to implement, they are only linearly convergent. Another approach to solve the Richards equation is to use Newton's iterative method. This method, also known as Newton–Raphson iteration, is quadratically convergent and requires the computation of first derivatives. We implemented Newton's scheme into the mixed form of the Richards equation. As compared to the modified Picard scheme, Newton's scheme requires two additional matrices when the mixed form of the Richards equation is used and requires three additional matrices, when the pressure head-based form is used. The modified Picard scheme may actually be viewed as a simplified Newton scheme. Two examples are used to investigate the numerical performance of different forms of the 1D vertical Richards equation and the different iterative solution schemes. In the first example, we simulate infiltration in a homogeneous dry porous medium by solving both, the h based and mixed forms of Richards equation using the modified Picard and Newton schemes. Results shows that, very small time steps are required to obtain an accurate mass balance. These small times steps make the Newton method less attractive. In a second test problem, we simulate variable inflows and outflows in a heterogeneous dry porous medium by solving the mixed form of the Richards equation, using the modified Picard and Newton schemes. Analytical computation of the Jacobian required less CPU time than its computation by perturbation. A combination of the modified Picard and Newton scheme was found to be more efficient than the modified Picard or Newton scheme.

Dispersion and transport of gas-phase contaminants in dry porous media: effect of heterogeneity and gas velocity

The purpose of these experiments was to study the effects of physical heterogeneity and velocity on transport of gas-phase contaminants in dry porous media. Experiments were conducted at gas velocities ranging from 6 to 200 cm min −1 to examine the contributions of longitudinal molecular diffusion, hydrodynamic dispersion, and rate-limited diffusive mass transfer to solute spreading. Methane was used as a nonreactive tracer, and trichloroethene, benzene and toluene were used as reactive tracers. Total dispersion of methane during transport through a homogeneous porous medium was the sum of longitudinal molecular diffusion and hydrodynamic dispersion. The latter was the major process contributing to total dispersion at gas velocities greater than 40 cm min −1. The contribution of longitudinal molecular diffusion was negligible at gas velocities greater than 150 cm min −1. Transport of tracers in the heterogeneous (macroporous) medium exhibited preferential flow and tailing at gas velocities greater than about 100 cm min −1 as a result of rate-limited mass transfer between macropore and micropore domains. The spreading associated with rate-limited mass transfer between macropore and micropore domains was the main contributor to total dispersion at gas velocities greater than 120 cm min −1. The transport of the reactive tracers was successfully predicted using data obtained for the nonreactive tracer.

Experimental assessment of gas transport mechanisms in natural porous media: Parameter evaluation

An understanding of vapor transport in natural porous media is critical to the assessment of a wide range of environmental problems. In this work a comprehensive experimental program was undertaken to evaluate the relative importance of different gaseous transport mechanisms in natural porous media. The dusty gas model was used as a framework for this evaluation. The experimental program was divided into two parts: the first emphasizes the measurement of porous media transport parameters, and the second explores flux mechanisms in organic vapor transport. Results of the first part are presented in this paper. Single and binary gas experiments were conducted to obtain governing transport parameters (coefficients of permeability, Knudsen diffusion, and diffusibility) in dry porous media. To conduct these experiments, a special experimental apparatus was built that incorporates a diffusion cell that represents an open system where the pressure gradient and absolute pressures can be regulated by controlling the flow rates of the component gases at both ends of the soil sample. Soils tested included three uniform materials, a sea sand, an Ottawa sand, and kaolinite clay, and five graded mixtures of these uniform soils. Results of the single‐gas experiments illustrate the importance of Knudsen diffusion in permeability measurements. Two new transport parameter correlations are developed from the presented data and compared with available literature correlations. The correlation for the Knudsen diffusion radius has a functional dependence upon the square root of intrinsic permeability, selected as a characteristic length of the porous medium. Binary diffusion experiments, using an equimolar pair of gases, are employed to develop a correlation for a composition independent diffusibility.

An Experimental Approach for the Characterisation of Rigid Porous Media and Unsaturated Conductivity Relations

A laboratory procedure for the characterisation of rigid, porous media is developed and tested. The overall objective of the characterisation is to provide a basis for the choice of a relevant unsaturated conductivity relation. With the proposed procedure, the porous materials are characterised through hydraulic conductivity measurements, saturation-pressure measurements, porosity measurements and air entry pressure measurements. The measurements were conducted using water and a selected non-aqueous phase liquid (n-decane), and two types of homogeneous, rigid porous media of different origin (fritted glass samples with guaranteed homogeneity by the manufacturer, and chalk). The laboratory method successfully characterised the fritted glass samples as homogeneous on basis of calculated tortuosity values, measured bubble pressures and measured pore-size density curves. The contact angle between the liquid and the solid had negligible impact when comparing the drainage of water with the drainage of n-decane, and also when comparingn -decane drainage in dry porous media withn -decane drainage in media where a film of water separated then -decane from the pore wall. Also, the differences in saturated conductivities between n-decane and water in both samples of chalk and fritted glass were well predicted on basis of differences in density and viscosity of the liquids. The laboratory procedure was furthermore used for characterisation of the pore space available for non-aqueous liquid flow in partially water-saturated porous media. Then -decane conductivities in the fritted glass samples partially saturated with water were considerably lower than the n-decane conductivities in the dry fritted glass samples.

Propagation of condensation front in steam injection into dry porous media

The front speed and the liquid saturation distribution in the condensate flow region have been examined for one-dimensional injection of dry steam into a lower temperature, dry porous medium with a constant inlet pressure. Existing models for the volumetric viscous and inertial forces are used along with an upstream region with an immobile liquid, which is followed by a two-phase region, a condensation zone, a liquid region, and a downstream noncondensable gas flow region. The down- and upflow (along and against gravity) are examined assuming a quasi-steady behavior. For downflow, the liquid region grows with the condensation front location. The asymptotic front speed is obtained in closed form for both the near-field regime, where the steam flow rate is high (i.e. deviation from a Darcy behavior is significant), and the far-field regime. For upflow, the liquid region gradually disappears, the liquid saturation distribution in the upstream immobile region undergoes two transitions, and correlation expressions are found for the location of these transitions. An experiment is performed, and the experimental results confirm the predictions for both down- and upflows.

Two Dimensional Heat and Mass Transfer During Convective Drying of Porous Media

ABSTRACT In this paper, we study numerically two-dimensional heat and mass transfer during convective drying in porous media. The set of macroscopic equations is very comprehensive and takes into account the effect of gaseous pressure. A numerical code has been established. This code has allowed us to determine t.he space-time evolution of the temperature, the total pressure and the moisture content.