General Transport Model Research Articles

Separation of aqueous organic multi-component solutions by reverse osmosis — development of a mass transfer model

In this paper the separation of aqueous organic multi-component solutions through a polyamide reverse osmosis membrane was systematically investigated. To analyze coupling effects and competing reactions between substances at or inside the membrane and to determine the influence of the solution components on the separation behaviour, experiments with model solutions were carried out. For the model solutions one of 34 different organic substances was added to an aqueous 1-propanol solution. The substances were chosen due to their environmental relevance and regarding their physico-chemical properties. The observed changes in separation and mass transfer of 1-propanol were used to develop a general mass transport model for reverse osmosis. This model was based on the Kimura-Sourirajan Analysis (KSA) developed by Sourirajan in 1963. It was modified to include the interactions among different substances in multi-component solutions by means of their physico-chemical properties. The model describes the mass transport through a reverse osmosis membrane and enables the prediction of the retention behaviour of organic multi-component solutions.

Assimilation of satellite observations of long‐lived chemical species in global chemistry transport models

Use of data assimilation techniques such as optimal interpolation or the Kaiman filter in global chemistry transport models (CTM) is becoming more common. However, owing to high computational requirements, it is often difficult to apply these techniques to multidimensional models containing extensive photochemical schemes. We present a sequential assimilation approach developed for use with general global chemistry transport models. It allows fast assimilation and mapping of satellite observations and provides estimates of analysis errors. The suggested data assimilation scheme evolved from the one described by Levelt et al. [1998]. It is a variant of the suboptimal Kaiman filter and is based on ideas described by Menard et al. [2000] and Menard and Chang [200O]. One of the most important features of the developed scheme is its ability to routinely estimate variance of the analysis and to predict variance evolution in the model. The developed technique (or its variants) has been successfully interfaced with a number of different global models and used for assimilation of several types of measurements, including aerosol extinction ratios. Some of these experiments are described by Lamarque et al. [1999] and W. D. Collins et al. (Forecasting aerosols using a chemical transport model with assimilation of satellite aerosol retrievals: Methodology for INDOEX, submitted to Journal of Geophysical Research, 2000, hereinafter referred to as Collins et al., submitted manuscript, 2000). We illustrate the method using assimilation of ozone observations made by the Upper Atmosphere Research Satellite/Microwave Limb Sounder in the three‐dimensional chemistry transport model ROSE [Research for Ozone in the Stratosphere and its Evolution; Rose and Brasseur, 1989].

Numerical solution of hyperbolic models of transport in bidisperse solids

This article considers the numerical solution of generalized transport models for adsorption that include the presence of finite local relaxation times. Such models arise in adsorption problems where the small diffusivity in micropores can lead to local nonequilibrium even at the macroscopic process time scale. We consider the important case of a bidisperse-structured adsorbent, the transport in which is a problem of long-standing interest to chemical engineers. In the formulation considered, the microparticle model involves a non-linear hyperbolic differential equation with source terms, and in the macroparticle model the differential equation involves a singularly perturbed parabolic diffusion problem. There are some difficulties in solving these equations due to the hyperbolic nature of the microparticle transport, conventionally assumed to obey the Fickian model leading to a parabolic diffusion equation. In this paper, an efficient numerical technique is presented to simulate these processes. A combination of the upwind method and fitted mesh collocation method is applied to solve the problem. Numerical solutions were found to match the analytical solution of the traditional model when the adsorption isotherm is linear, for macropore diffusion control or micropore diffusion control. Simulations of the adsorption and desorption dynamics are also presented for a Langmuir isotherm. The numerical scheme offers a more generalized alternative that can be used for both the model forms, with and without consideration of finite local relaxation times.

Non-isothermal Moisture Transport calculations with DIM 3.1 / Nicht-isotherme Berechnungen des Feuchtigkeitstransportes mit Hilfe des Porgrammes DIM 3.1

Abstract The application of a general thermodynamical mass and energy transport model to the coupled heat and moisture transfer in porous materials results in a balance equation system and the related constitutive equations of the considered quantities. The constitutive equations describe moisture transport in a phase-separated manner leading into phase-divided hygric transport coefficients (liquid water permeability, water vapour diffusivity). A conceptual model is presented in the paper in order to circumvent the difficulties resulting from non-isothermal overlaying moisture transport processes. Since phase-divided hygric transport coefficients are not directly measurable, but moisture transport coefficients in distinct hygric ranges, moisture conductivities and a phase dividing function are introduced. The moisture conductivities include liquid water and water vapour transport. For a known phase dividing function, the phase-divided hygric transport coefficients of the balance equation system can be calculated from the measurable moisture conductivities. The influence of a variation of the introduced phase-dividing function on non-isothermal moisture transport processes is investigated by means of computer simulations.

Diffusion of lysozyme in gels and liquids: A general approach for the determination of diffusion coefficients using holographic laser interferometry

A study on diffusion measurements of the protein lysozyme in liquids and agarose gels, at different pH and ionic strengths, has been performed using holographic laser interferometry. The measurements showed that the diffusive flux was very dependent on pH and ionic strength when the protein was not at its isoelectric point or when the charge of the lysozyme molecules was not screened by ions in the solution. Evaluation of the experimental data with Fick’s law, resulted in diffusion coefficients for lysozyme that are strongly dependent on pH and ionic strength. Evaluation of the experimental data using a more general transport model, based on chemical potential gradients instead of concentration gradients resulted in lysozyme diffusion coefficients that are independent of pH and ionic strength. The chemical potential was estimated by using the Poisson–Boltzmann equation.

Modulated photoconductivity study of electron drift mobility in amorphous silicon

The modulated photoconductivity technique, a convenient means of measuring the drift mobility of photocarriers, has been applied to investigate carrier transport in hydrogenated amorphous silicon. The frequency resolved spectra of drift mobility that can be obtained from the measurements were analyzed in accordance with a generalized transport model that included possible carrier interactions between localized states through tunneling transitions. Theory suggests that a tunneling-assisted thermalization of nonequilibrium carriers appreciably affects the transport process. The experimental results are reasonably accounted for by the introduced model, leading to quantitative assessments for transport mechanisms.

A generalized transport model for biased cell migration in an anisotropic environment.

A generalized transport model is derived for cell migration in an anisotropic environment and is applied to the specific cases of biased cell migration in a gradient of a stimulus (taxis; e.g., chemotaxis or haptotaxis) or along an axis of anisotropy (e.g., contact guidance). The model accounts for spatial or directional dependence of cell speed and cell turning behavior to predict a constitutive cell flux equation with drift velocity and diffusivity tensor (termed random motility tensor) that are explicit functions of the parameters of the underlying random walk model. This model provides the connection between cell locomotion and the resulting persistent random walk behavior to the observed cell migration on longer time scales, thus it provides a framework for interpreting cell migration data in terms of underlying motility mechanisms.

Modeling of pyrite oxidation in saturated and unsaturated subsurface flow systems

Pyrite oxidation (FeS2) causes acidification and mobilization of metals. Mathematical modeling of pyrite oxidation in variably saturated reactive flow systems is challenging because (1) it occurs through a complex interplay of multi-phase flow and transport processes, and (2) aqueous concentrations of key species vary over tens orders of magnitude in different redox conditions. Here, we present a general multi-phase reactive transport model for redox processes. Two alternative implementations were made in the TOUGHREACT and TOUGH2-CHEM simulation codes which use sequential iteration and simultaneous solution, respectively. Both codes are used to simulate a fully and a variably saturated pyrite oxidation problem with simple 1-D flow and reaction conditions. Results from both codes indicate that the effects of oxygen partial pressure reduction due to reactions on the fluid flow is not significant under ambient conditions. However, it must be noted that when fluid flow and chemical reactions are strongly coupled, such as when boiling takes place in geothermal reservoirs, this could be essential. The fully simultaneous approach has a complete process description. The sequential iteration approach is found to be more efficient computationally. The oxygen gas diffusion process plays a dominant role in the chemical evolution for pyrite oxidation in unsaturated conditions. An example in 2-D fractured rock is presented to demonstrate pyrite oxidation under complex flow and geochemical conditions. This example shows that pyrite oxidation exerts strong influence on hydrogeochemical evolution in variably saturated flow systems. The alteration of primary rock minerals and the development of secondary mineral assemblages predicted are consistent with field observations. This example serves as a prototype for oxidative weathering processes with broad significance for geoscientific, engineering, and environmental applications.

A new statistical method for transfer coefficient calculations in the framework of the general multiple-compartment model of transport for radionuclides in biological systems

A new and simple statistical procedure (STATFLUX) for the calculation of transfer coefficients of radionuclide transport to animals and plants is proposed. The method is based on the general multiple-compartment model, which uses a system of linear equations involving geometrical volume considerations. By using experimentally available curves of radionuclide concentrations versus time, for each animal compartment (organs), flow parameters were estimated by employing a least-squares procedure, whose consistency is tested. Some numerical results are presented in order to compare the STATFLUX transfer coefficients with those from other works and experimental data.

The structure and mechanism of the family of retinal proteins from halophilic archaea

Retinal proteins from halophilic archaea provide a unique opportunity to analyze vectorial ion translocation. Studies on its structure, conformational changes, proton conduction and electrogenic steps have helped to elucidate the catalytic cycle of bacteriorhodopsin in increasing detail. Experimental modulation of the vectoriality and ion specificity by altering the substrate availability, point mutations and light conditions for the different retinal proteins allows the proposal of a general model of ion transport for this protein family.

Multidimensional Transport Effects on Forced-Flow Chemical Vapor Infiltration

A general multidimensional transport and reaction model that was formulated for the chemical vapor infiltration process in a previous study is used to carry out a comprehensive study of multidimensional transport effects in forced-flow chemical vapor infiltration. Results are obtained on the effects of the anisotropy and geometry of the preform, thermal gradients, and the distribution of flow at the face of the preform. For a simple, slab-shaped preform geometry, all these parameters are found to influence strongly the distance from the entrance of the preform up to which significant multidimensional transport effects are encountered in the partial pressure and density profiles. The key factor controlling the appearance and extent of multidimensional transport effects is the distribution of flow over the entrance face of the preform.

The components of fractional transport rate

Fractional transport rates are defined as the product of spatial grain entrainment, displacement length, and displacement frequency. Grain entrainment is defined relative to the population of grains on the bed surface at an initial time in order to account for partial transport, a condition in which only a portion of the exposed grains of a single size Di are mobilized over the duration of a transport event. A relation for grain displacement frequency is calculated from an assumed relation for displacement length and observed values of fractional transport rate, bed surface size distribution, and spatial entrainment in flume experiments. Fractional transport rates are independent of Di for fully mobilized fractions and decrease rapidly with Di for partially mobile fractions; the latter results from a decrease in both the displacement frequency and the mobilized proportion of each fraction. These two terms, which together determine the fractional entrainment rate, have different relations with flow strength and Di and should be independently included in a general fractional transport model. Grain‐size similarity in fractional transport rates does not hold for conditions of partial transport. Partial transport is a grain‐scale representation of incipient motion and can be directly related to the reference shear stress τri a surrogate for the critical shear stress for incipient motion based on transport rate rather than grain mobility. Fractional mobilization at different flows may be predicted from τri, and partial transport appears to be the general condition at the reference transport rate. Using τri as input, the transport component relations are used to predict the bed mobilization and fractional transport rates for a field case.

Transport properties of a quantum dot with superconducting leads.

We report a numerical study of transport properties of a quantum dot with superconducting leads. We introduce a general phenomenological model of quantum dot transport, in which electron tunnel rates are computed within the Fermi's Golden Rule approach. The low temperature current-voltage (I-V) characteristics are in qualitative agreement with experimental observations of Ralph et al. [Phys. Rev. Lett., 74, 3241 (1995)]. At higher temperatures, our results reveal new effects due to the thermal excitation of quasiparticles in the leads as well as the thermal population of excited quantum levels in the dot. We also study the photon-assisted tunneling phenomena in our system and point out its potential for millimeter wave applications.

Structural Model Effects on the Predictions of Chemical Vapor Infiltration Models

The effects of pore structure evolution on the predictions of chemical vapor infiltration models are investigated in this study. A general multicomponent reaction and transport model is used to describe transport and reaction in the pore space, and structure evolution is modeled by representing the void space by a population of cylindrical capillaries (capillary models) or of the solid phase by a population of solid cylinders (fiber models). The capillaries are assumed to be randomly arranged in space without preferred orientation, whereas the fibers are taken to be parallel to a line, parallel to a plane, or without preferred orientation (one‐, two‐, or three‐directional structures, respectively). The obtained results show that the way in which the pore structure evolves during densification plays a dramatic role in determining the overall behavior of the deposition system. In the case of the fiber structures, the results are influenced significantly not only by the directionality but by the direction of diffusion relative to the fiber axes as well.

Prediction of locations with sediment deposits in sewers

The present paper presents an application of MOUSE ST, a general deterministic sediment transport model for sewer systems. MOUSE ST is used to predict the locations subjected to sedimentation in the sewer system of Ljubljana, Slovenia. The prediction is made by means of a sediment transport model with a movable bed. This model is run in parallel with the hydrodynamic MOUSE model. The results, in terms of locations with sediment deposits, are compared with field data from the sewer system in Ljubljana. Further, the model is used to predict the effect of the removal of the sediment deposits on the combined sewer overflows.

Transport Equations for Homogeneous and Variable-Composition Semiconductor Devices at Low Temperatures

Transport equations for use in the analysis of homogeneous and variable composition semiconductor devices at low temperatures are reviewed. In these equations, Fermi-Dirac statistics and position-dependent band structure are taken into account. The general transport models developed previously by other workers are recast into a simple form so as to be convenient for use in numerical device simulation of semiconductors at low temperatures.

Climatic effects of biomass burning

We have made a series of general circulation model (GCM) and chemical transport model (CTM) simulations to examine further the consequences of biomass burning. We take a three-pronged approach to this study; (1) examine only the direct short-wave forcing of the smoke clouds by imposing smoke ‘clouds’ in the model, (2) examine the indirect effect of biomass smoke by allowing the biomass smoke to modify the optical properties of existing clouds, reducing the effective (mass) droplet radius, and (3) examine the downstream effect by advecting biomass smoke using semi-Lagrangian transport in the GCM. Preliminary results from the GCM simulations suggest strong local coolings (2–4°C or more) where biomass burning occurs with somewhat reduced regional coolings. Initial results of the transport of smoke by the Australian National University CTM (ANU-CTM) show the biomass smoke to remain fairly localized with the advection of smoke extending the effects downwind.

Prediction of locations with sediment deposits in sewers

The present paper presents an application of MOUSE ST, a general deterministic sediment transport model for sewer systems. MOUSE ST is used to predict the locations subjected to sedimentation in the sewer system of Ljubljana, Slovenia. The prediction is made by means of a sediment transport model with a movable bed. This model is run in parallel with the hydrodynamic MOUSE model. The results, in terms of locations with sediment deposits, are compared with field data from the sewer system in Ljubljana. Further, the model is used to predict the effect of the removal of the sediment deposits on the combined sewer overflows.

Resolution limits from charge transport in optically addressed spatial light modulators

Spatial resolution of optically addressed spatial light modulators (OASLMs) is degraded by several different transfer processes in these devices. We have developed a general transient charge transport model to calculate and simulate the resolution limits of OASLMs due to the following charge spreading mechanisms during the transfer process in which the input image is converted into a particular charge distribution in the photosensor layer. (i) The effect of charge drift in the photosensor bulk on resolution increases with the thickness of the photosensor and the light-modulating layers. It also increases with the total amount of photogenerated charge collected at the interface. (ii) The effect of charge diffusion in the photosensor bulk on resolution is largely independent of the carrier mobility in the semiconductor photosensor. In most cases the corresponding spatial frequency f50% is proportional to √Vsc/dsc, where dsc is the photosensor thickness and Vsc is the voltage drop in that layer. To have high-sensitivity OASLMs the transit time of charge carriers from the photosensor bulk to the interface must be much shorter than the recombination lifetime. (iii) The effects that charge drift, diffusion, and trapping at the photosensor-light-modulating layer interface have on resolution depend strongly on the interface properties. Decreasing the mobility or the trapping time of charge carriers at the interface can dramatically improve the resolution of OASLMs. The resolution ranges from 3 to 875 line pairs/mm for respective diffusion lengths of 10 to 0.1 μm at the interface. The combined effect on resolution from each of the charge spreading and other resolution-degrading mechanisms is also discussed.

Electron and ion thermal structures above 1000 km: A multi‐ion generalized transport model for subsonic flows

A multispecies (electrons, H+, He+, O+) generalized transport model has been developed that is based on 16‐moment transport theory. This model provides a simultaneous and self‐consistent solution for the densities, drift velocities, parallel and perpendicular temperatures, and parallel and perpendicular heat flows of the electrons and each of the three ion species. Solutions have been obtained for a wide range of ionospheric boundary conditions, but this study concentrates on the relations between temperature gradients applied at a lower boundary (1000 km) and the total temperatures and temperature anisotropies that exist at higher altitudes, for electrons, H+ ions, and He+ ions. It is seen that applying even a modest temperature gradient for a particular species has a major effect on the thermal structure of that species at higher altitudes. Further, while the thermal structures of the various species are coupled due to Coulomb collisions, this coupling is far more significant for certain pairs of species than for others. For a particular species, the presence of a large temperature gradient at the lower boundary may or may not result in highly non‐Maxwellian distribution functions at higher altitudes, depending on the relative magnitudes of the species temperatures and heat flows at those altitudes. Also, in comparing the total electron heat flows predicted by 16‐moment transport theory with those predicted by the standard, collision‐dominated transport theory, close agreement is found. For the ions, however, the differences between the 16‐moment and standard models can be very large, depending on the boundary conditions.