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Fickian Diffusion (fickian + diffusion)

Distribution by Scientific Domains
Chemistry60%

Terms modified by Fickian Diffusion

  • fickian diffusion model
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    Selected Abstracts

    Computer programs for estimating substrate flux into steady-state biofilms from pseudoanalytical solutions

    COMPUTER APPLICATIONS IN ENGINEERING EDUCATION, Issue 1 2002
    Chetan T. Goudar
    Abstract Fixed-film processes employing microorganisms attached to an inert surface (biofilms) are widely used for biological treatment of municipal and industrial wastewater. For optimal design and analysis of these processes, mathematical models are necessary that describe the dynamics of contaminant transport within these biofilms and the associated contaminant utilization by the microorganisms. However, these governing equations that typically involve Fickian diffusion for contaminant transport and Monod kinetics for contaminant utilization are inherently nonlinear and have no closed form solutions except under special conditions. This can restrict their use in the classroom as cumbersome numerical techniques must be used for their solution. This problem is well documented in the literature and several authors have presented pseudoanalytical solutions that replace numerical solutions with algebraic equations. In the present study, we present pseudoanalytical solution-based computer programs for estimating substrate flux and biofilm thickness for a steady-state biofilm. Depending upon the intended end use, these programs can either partially or totally automate the solution process. In the partial automation mode, they can serve to enhance student understanding of important concepts related to steady-state biofilms, while complete automation can help bring more challenging and realistic problems associated with steady-state biofilms into the classroom. The programs have been tested on MATLAB version 5.0 and are available as freeware for educational purposes. © 2002 Wiley Periodicals, Inc. Comput Appl Eng Educ 10: 26,32, 2002; Published online in Wiley InterScience (www.interscience.wiley.com.); DOI 10.1002/cae.10017 [source]

    Modelling of colloid leaching from unsaturated, aggregated soil

    EUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2007
    M. Laegdsmand
    Summary The migration of colloids in soils can enhance the leaching of strongly sorbing contaminants. We present a model for the simulation of colloid leaching from unsaturated, aggregated soil media under stationary flow. Transport in the intra-aggregate pores is simulated by convection,dispersion, and transport in the interaggregate pores, and a stagnant layer of water surrounding the aggregates, is simulated by diffusion. The model describes the release of colloids from soil aggregates, sorption and desorption processes at the air,water interfaces, and flocculation and subsequent straining from the flowing water. All three processes were simulated as functions of ionic strength. Transport of ions in intra-aggregate pores was simulated by Fickian diffusion. The model was calibrated against experimental results of colloid leaching from columns packed with natural soil aggregates. The aggregates were of two soils differing in organic matter content. On each soil a single calibrated parameter set could describe the experiments with the three ionic strengths. The parameters for release of colloids from the aggregate surface and the sorption properties of the air,water interface were different for the two soils. The key parameters for leaching were the thickness of the stagnant layer of water surrounding the aggregates, the mechanical dispersion, the maximum concentration of colloids at the surface of the aggregates, the sorption capacity and rate coefficient of the colloids at the air,water interface, and the colloid diffusion coefficient. Simulations were also done with two additional irrigation intensities at one ionic strength. Simulated leaching was greater than measured leaching at both irrigation intensities, but the diffusion-controlled release of colloids from the aggregates was simulated correctly. [source]

    Modelling Approach for Planar Self-Breathing PEMFC and Comparison with Experimental Results,

    FUEL CELLS, Issue 4 2004
    A. Schmitz
    Abstract This paper presents a model-based analysis of a proton exchange membrane fuel cell,(PEMFC) with a planar design as the power supply for portable applications. The cell is operated with hydrogen and consists of an open cathode side allowing for passive, self-breathing, operation. This planar fuel cell is fabricated using printed circuit board,(PCB) technology. Long-term stability of this type of fuel cell has been demonstrated. A stationary, two-dimensional, isothermal, mathematical model of the planar fuel cell is developed. Fickian diffusion of the gaseous components,(O2, H2, H2O) in the gas diffusion layers and the catalyst layers is accounted for. The transport of water is considered in the gaseous phase only. The electrochemical reactions are described by the Tafel equation. The potential and current balance equations are solved separately for protons and electrons. The resulting system of partial differential equations is solved by a finite element method using FEMLAB,(COMSOL Inc.) software. Three different cathode opening ratios are realized and the corresponding polarization curves are measured. The measurements are compared to numerical simulation results. The model reproduces the shape of the measured polarization curves and comparable limiting current density values, due to mass transport limitation, are obtained. The simulated distribution of gaseous water shows that an increase of the water concentration under the rib occurs. It is concluded that liquid water may condense under the rib leading to a reduction of the open pore space accessible for gas transport. Thus, a broad rib not only hinders the oxygen supply itself, but may also cause additional mass transport problems due to the condensation of water. [source]

    The Influence of Mass Transfer on a Porous Fuel Cell Electrode

    FUEL CELLS, Issue 1-2 2004
    Y.-P. Sun
    Abstract A one-dimensional model for a porous fuel cell electrode using a liquid electrolyte with dissolved reactant is presented. The model consists of a Poisson, second-order ordinary differential equation, describing the effect of the electric field and a one-dimensional; Fickian diffusion, second-order ordinary differential equation describing the concentration variation associated with diffusion. The model accounts for mass transport and heterogeneous electrochemical reaction. The solution of this model is by the approximate Adomian polynomial method and is used to determine lateral distributions of concentration, overpotential and current density and overall cell polarisation. The model is used to simulate the effects of important system and operating parameters, i.e. local diffusion rates, and mass transport coefficients and electrode polarisation behaviour. [source]

    Design and In vitro evaluation of a film-controlled dosage form self-converted from monolithic tablet in gastrointestinal environment

    JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 11 2010
    Tingting Zhang
    Abstract The purpose of this study is to design an easily manufactured sustained drug delivery system, which can be converted to a film coated system during the dissolution process and then control the drug release according to near zero-order kinetics. Two kinds of pH-sensitive and oppositely charged hydrophilic polymers, chitosan and alginate, were physically mixed as the matrix. Slightly water-soluble drugs such as theophylline, aspirin, and acetaminophen were utilized as model drugs. In vitro drug release and swelling tests were undertaken in simulated gastrointestinal environments. The formation and properties of the film formed during the dissolution process were identified using different techniques. It was demonstrated that formation of the film was based on the interaction of the polymers on tablet surface with the change of system pH. In 0,4,h drug release depended on the intrinsic properties of the polymers, however, characteristics of the film played a leading role in controlling drug release after 4,h. By studying the ratio of relaxation over Fickian diffusion and relationship between tablets swelling and drug release, it was revealed that the film probably modified drug release behavior by limiting polymer erosion. The in vivo behavior of this hydrophilic matrix system will be investigated. © 2010 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:4678,4690, 2010 [source]