Estimation Of Transport Parameters Research Articles

Improved Method for Determining Partition and Diffusion Coefficients in Human Dermis

Accurate values of partition and diffusion coefficients within the lower skin layers are among the information required to estimate skin concentrations of permeants following topical application. For highly lipophilic compounds these parameters also play a significant role in determining systemic absorption rates. In this study methodology is described for making more accurate in vitro measurements of partition and diffusion coefficients in dermis. Isolated human dermis obtained from surgical reduction was mounted in side-by-side diffusion cells in the presence and absence of a dialysis membrane (5000 MW cut-off) placed between the dermis and the donor solution. Permeation of N,N-diethyltoluamide (DEET) and diclofenac across the composite membrane system, as well as steady-state skin concentrations, were measured by radiochemical techniques. For the highly protein bound compound, diclofenac, dermis permeability, and partition coefficient determined in the presence of the dialysis membrane were significantly higher than those determined in its absence. No significant differences were observed for the moderately protein bound DEET. The results show that, in the absence of a stratum corneum barrier, attention must be given to the diffusion of soluble proteins in order to obtain accurate estimates of transport and partitioning parameters for highly protein bound solutes in dermis.

Influence of distributed flow losses and gains on the estimation of transient storage parameters from stream tracer experiments

Interactions between mobile stream water and transient storage zones have been the subject of careful attention for decades. However, few studies have considered explicitly the influence of water exchange between the channel and neighbouring hydrological units when modelling transient storage processes, especially the lateral inflow coming from hillslope contributions and outflow to a deep aquifer or to hyporheic flow paths extending beyond the study reach. The objective of this study was to explore the influence of different conceptualizations of these hydrologic exchanges on the estimation of transient storage parameters. Slug injections of sodium chloride (NaCl) were carried out in eight contiguous reaches in the Cotton Creek Experimental Watershed (CCEW), located in south-east British Columbia. Resulting breakthrough curves were subsequently analysed using a Transient Storage Model (TSM) in an inverse modelling framework. We estimated solute transport parameters using three distinct, hypothetical spatial patterns of lateral inflow and outflow, all based on variations of the same five-parameter model structure. We compared optimized parameter values to those resulting from a distinct four-parameter model structure meant to represent the standard application of the TSM, in which only lateral inflow was implemented for net gaining reaches or only lateral outflow for net losing reaches. In the five-parameter model, solute mass was stored predominantly in the transient storage zone and slowly released back to the stream. Conversely, solute mass was predominantly removed from the stream via flow losses in the four-parameter model structure. This led to contrasting estimates of solute transport parameters and subsequent interpretation of solute transport dynamics. Differences in parameter estimates across variations of the five-parameter model structure were small yet statistically significant, except for the transient storage exchange rate coefficient α, for which unique determination was problematic. We also based our analysis on F med 200 , the fraction of median transport time due to transient storage. Differences across configurations in F med 200 estimates were consistent but small when compared to the variability of F med 200 among reaches. Optimized parameter values were influenced dominantly by the model structure (four versus five parameters) and then by the conceptualization of spatial arrangement of lateral fluxes along the reach for a set model structure. When boundary conditions are poorly defined, the information contained in the stream tracer breakthrough curve is insufficient to identify a single, unambiguous model structure representing solute transport simulations. Investigating lateral fluxes prior to conducting a study on transient storage processes is necessary, as assuming a certain spatial organization of these fluxes might set ill-defined bases for inter-reach comparisons. Given the difficulty in quantifying the spatial patterns and magnitudes of lateral inputs and outputs, we recommend small-scale laboratory tracer experiments with well-defined and variable boundary conditions as a complement to field studies to provide new insights into stream solute dynamics.

Periodic homogenization and consistent estimates of transport parameters through sphere and polyhedron packings in the whole porosity range

This paper presents a study of transport parameters (diffusion, dynamic permeability, thermal permeability, trapping constant) of porous media by combining the homogenization of periodic media (HPM) and the self-consistent scheme (SCM) based on a bicomposite spherical pattern. The link between the HPM and SCM approaches is first established by using a systematic argument independent of the problem under consideration. It is shown that the periodicity condition can be replaced by zero flux and energy through the whole surface of the representative elementary volume. Consequently the SCM solution can be considered as a geometrical approximation of the local problem derived through HPM for materials such that the morphology of the period is "close" to the SCM pattern. These results are then applied to derive the estimates of the effective diffusion, the dynamic permeability, the thermal permeability and the trapping constant of porous media. These SCM estimates are compared with numerical HPM results obtained on periodic arrays of spheres and polyhedrons. It is shown that SCM estimates provide good analytical approximations of the effective parameters for periodic packings of spheres at porosities larger than 0.6, while the agreement is excellent for periodic packings of polyhedrons in the whole range of porosity.

CXTFIT/Excel–A modular adaptable code for parameter estimation, sensitivity analysis and uncertainty analysis for laboratory or field tracer experiments

We implemented the widely used CXTFIT code in Excel to provide flexibility and added sensitivity and uncertainty analysis functions to improve transport parameter estimation and to facilitate model discrimination for multi-tracer experiments on structured soils. Analytical solutions for one-dimensional equilibrium and nonequilibrium convection dispersion equations were coded as VBA functions so that they could be used as ordinary math functions in Excel for forward predictions. Macros with user-friendly interfaces were developed for optimization, sensitivity analysis, uncertainty analysis, error propagation, response surface calculation, and Monte Carlo analysis. As a result, any parameter with transformations (e.g., dimensionless, log-transformed, species-dependent reactions, etc.) could be estimated with uncertainty and sensitivity quantification for multiple tracer data at multiple locations and times. Prior information and observation errors could be incorporated into the weighted nonlinear least squares method with a penalty function. Users are able to change selected parameter values and view the results via embedded graphics, resulting in a flexible tool applicable to modeling transport processes and to teaching students about parameter estimation. The code was verified by comparing to a number of benchmarks with CXTFIT 2.0. It was applied to improve parameter estimation for four typical tracer experiment data sets in the literature using multi-model evaluation and comparison. Additional examples were included to illustrate the flexibilities and advantages of CXTFIT/Excel. The VBA macros were designed for general purpose and could be used for any parameter estimation/model calibration when the forward solution is implemented in Excel. A step-by-step tutorial, example Excel files and the code are provided as supplemental material.

Measurement of chemical diffusion coefficient and surface exchange on mixed ionic electronic conductors using periodical pO 2 oscillations

A new method for estimation of transport parameters in mixed ionic electronic conductors (MIEC) is proposed. Microscopic diffusion and surface exchange coefficients have been obtained with a dynamic conductivity analysis. The estimation of diffusion coefficient of interstitial oxygen and the surface exchange coefficient for well known compositions La 2NiO 4 and La 2CuO 4 shows very good agreement with SIMS data reported in the literature. The novel characterization turns out to be a time saving method which allows the precise estimation of diffusion and surface exchange coefficients in mixed conductors.

Inverse modeling of variable-density groundwater flow in a semi-arid area in Iran using a genetic algorithm

Flow and mass transport parameter estimation was done by creating an inverse model of a seawater intrusion system using a genetic algorithm (GA) method as the optimization procedure. Firstly, the SEAWAT code was used for the forward solution part and then a program was written in MATLAB for coupling the forward and inverse processes. The auto-calibration objective function was defined with the root mean square errors (RMSE) between the observed and the simulated values. A simple GA was used to minimize the RMSE criterion. The methodology was applied to a coastal aquifer with heterogeneous formations in a semi-arid area near salty Tashk Lake (electrical conductivity 61,420 µS/cm), Fars province, Iran. In the last two decades, the overexploitation of groundwater has caused a major water level drawdown and, consequently, salt-water intrusion. Firstly, flow and transport parameters (hydraulic conductivity, porosity, specific storage coefficient and longitudinal dispersivity) were estimated simultaneously in steady-state and, secondly, in the developed code, these results were used as initial values of the parameters in transient-state. Results show a good match for observed and simulated data. It can be concluded that GA is a helpful tool for automatic calibration of variable density fluid systems such as seawater intrusion cases.

Time lag parametric identification of a hydrogen permeability model

An inverse problem with dynamic boundary conditions for determining the parameters of a hydrogen permeability nonlinear model is considered. Algorithms are proposed for estimating transport parameters of adsorption, desorption, dissolution, and diffusion depending on the body of experimental information.

Novel thermo‐responsive semi‐interpenetrating network microspheres of gellan gum‐poly(N‐isopropylacrylamide) for controlled release of atenolol

AbstractThermoresponsive microspheres of gellan gum‐poly(N‐isopropylacrylamide), i.e., GG‐P(NIPAAm) semi‐interpenetrating polymer networks (semi‐IPNs) have been prepared by ionic crosslinking and used to study the controlled release (CR) of atenolol (ATL), an antihypertensive drug. Interaction of the drug with polymers was studied by Fourier transform infrared (FTIR) spectroscopy. Differential scanning calorimetry (DSC) was used to confirm the polymorphism and molecular level dispersion of ATL. Scanning electron microscopy (SEM) indicated spherical nature and smooth surfaces of the microspheres with some debris attached on their surfaces. Mean particle size measured by laser light diffraction ranged between 34 and 76 μm. Equilibrium swelling performed at 25°C and 37°C in pH 7.4 phosphate buffer exhibited thermoresponsive nature of the polymers. In vitro drug release performed at 25°C and 37°C indicated temperature‐dependency of ATL release, which was extended up to 12 h. In vitro release profiles at both the temperatures confirmed thermoresponsive nature of the polymers giving pulsatile trends. The % cumulative release data have been fitted to an empirical equation to estimate transport parameters and to understand the nature of drug release. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Estimation of Transport Parameters during Ultrafiltration of Pickling Effluent from a Tannery

Pickling effluent, being an industrial one, contains a large number of components with unknown transport coefficients. A combined model based on osmotic pressure and the Kedem-Katchalsky mechanism is used to predict these coefficients relevant to this process, e.g., effective osmotic coefficient, solute diffusivity, solute permeability, and reflection coefficient. These coefficients are evaluated utilizing the model and a large number of experimental data generated during ultrafiltration of pretreated pickling effluent. The performance criteria are evaluated in terms of the chemical oxygen demand (COD), the total dissolved solids (TDS), the total solids (TS), pH, and the conductivity of the permeate. The experiments also reveal that the proposed scheme is successfully reducing the COD to well below the prescribed limits and underscore the importance of transmembrane pressure drop and cross flow velocity on the permeate flux and quality.

Estimation of kinetic and transport parameters by quantitative evaluation of EIS and XPS data

The relatively large number of adjustable parameters often precludes the unambiguous interpretation of electrochemical impedance spectra in terms of a unique kinetic model. In the present paper, the possibilities offered by a combination between in situ electrochemical impedance spectroscopic data and ex situ surface analytical information to improve the credibility of the estimates of the kinetic and transport parameters are discussed. Two electrode systems in which passive oxide films are formed—stainless steel in simulated pressurised water reactor coolant and tungsten in sulphate–fluoride solutions—are used as representative examples to demonstrate the different approaches taken to analyse the experimental data in terms of the Mixed-Conduction Model. Ways to extract information on the rate-limiting steps of the process of passive film formation, growth and restructuring by quantitative comparison of the model equations to electrochemical impedance and X-ray photoelectron spectroscopic data are described and the significance of the obtained parameters for the kinetics of the overall process of metal and alloy dissolution in the passive state is discussed.

Predictive Modeling of Transient Storage and Nutrient Uptake: Implications for Stream Restoration

This study examined two key aspects of reactive transport modeling for stream restoration purposes: the accuracy of the nutrient spiraling and transient storage models for quantifying reach-scale nutrient uptake, and the ability to quantify transport parameters using measurements and scaling techniques in order to improve upon traditional conservative tracer fitting methods. Nitrate (NO3−) uptake rates inferred using the nutrient spiraling model underestimated the total NO3− mass loss by 82%, which was attributed to the exclusion of dispersion and transient storage. The transient storage model was more accurate with respect to the NO3− mass loss (±20%) and also demonstrated that uptake in the main channel was more significant than in storage zones. Conservative tracer fitting was unable to produce transport parameter estimates for a riffle-pool transition of the study reach, while forward modeling of solute transport using measured/scaled transport parameters matched conservative tracer breakthrough curves for all reaches. Additionally, solute exchange between the main channel and embayment surface storage zones was quantified using first-order theory. These results demonstrate that it is vital to account for transient storage in quantifying nutrient uptake, and the continued development of measurement/scaling techniques is needed for reactive transport modeling of streams with complex hydraulic and geomorphic conditions.

Analysis of Virus Transport in Groundwater and Identification of Transport Parameters

The present study deals with the analysis of virus transport in groundwater and estimation of transport parameters. The parameter estimation is formulated as a least-squares minimization problem in which the parameters are estimated by minimizing the deviations between the model predicted and experimentally observed virus concentrations. A parameter estimation procedure is developed by coupling a hybrid finite volume numerical model simulating one-dimensional virus transport with the Levenberg-Marquadart algorithm. The numerical model employs a globally second order accurate explicit finite volume method for the advective transport and an implicit finite difference method for the dispersive transport. The comparison of model prediction with analytical solution shows that the numerical model accurately predicts virus movement in both advective and dispersive dominated transport. The parameter estimation indicates that for the case of estimating more than three unknown parameters, the inverse procedure resu...

Comparison of Nonaqueous Phase Liquids' Conductivity and Air Permeability of Different Soils

It was recently shown that estimated transport parameters for nonaqueous phase liquids (NAPLs) could differ significantly from measured parameters because of poor estimates for fluid conductivity. Most of the estimation procedures of the different multiphase flow simulators assume that porous media are rigid and noninteracting (ideal porous medium). This assumption is not valid in aggregated soil systems consisting in part of clay minerals. Inadequate conductivity estimates may be a direct result of desaggregation or clay volume changes (pore‐size distribution changes) when contact with different fluids occurs. To expand the experimental data set of models describing the movement of organic liquids polluting the soils and possibly to discover new relationships between the NAPL conductivity and the other soil parameters, a series of experiments was set up in which the air permeability was measured with a PL 300 permeameter (UGT) and the fluid (NAPL and water) conductivity was measured using the falling head method. The measurements were carried out on artificial soil columns. The soils originated from different horizons of characteristic Hungarian soils. The relationships between the NAPL conductivity and the various other soil parameters were examined statistically. To give a better prediction for the NAPL conductivity, a new estimation method that is based on the easily measurable soil parameters was suggested.

Solid–liquid mass transfer analysis in a multi-phase tank reactor containing submerged coated inclined-plates: A computational fluid dynamics approach

Although there is a voluminous literature on the estimation of interphase transport parameters in conventional slurry bubble column reactors, these correlations are inadequate in photoreactors equipped with specialized internals to facilitate light harvesting efficiency of the photocatalyst. This is particularly germane to the present externally illuminated bubble column reactor containing titania-coated plates immersed in the liquid column at different angles of inclination. Thus, a computational fluid dynamics (CFD) procedure utilizing the Eulerian–Eulerian approach has been used to solve the governing differential equations for the solid liquid mass transport problem based on the standard k – ε model incorporating additional terms that take account of the interfacial turbulent momentum transfer. Mass transfer from the surface of the coated-quartz plates to the liquid phase was modeled using the Launder–Spalding wall functions. The plates were coated with benzoic acid as solid substrate with water and air as the liquid and gas phases, respectively. The increase in mass transfer due to reduction of the boundary layer thickness during air-induced liquid recirculation on either side of the submerged inclined plates was correlated with difference between turbulent and molecular Schmidt numbers via an adjustable parameter, A. CFD simulation using the Launder–Spalding wall function (with A = 1.08 ) gave better agreement with experimental transient concentration profiles than calculations based on FLUENT's enhanced wall function for the plate orientations ( θ = 0 ∘ , 22 . 5 ∘ , and 45 ∘ ) studied. The solid-to-liquid mass transfer was higher on the lower-side of the plate than the upper-side. In particular, mass transfer coefficient was higher with the inclined plate than with the vertical or horizontal orientation suggesting an added advantage for the application of the system as a solar photoreactor.

Perfluorinated nanocomposite membranes modified by polyaniline: Electrotransport phenomena and morphology

This work summarizes results on the modification of perfluorinated sulfocationic membranes MF-4SC by in situ chemical polymerization of aniline. The investigation of transport properties of polyaniline/MF-4SC composite membranes after bulk modification – conductivity, diffusion and electroosmotic permeability, proton permselectivity – as well as porosimetry and polarization behavior is carried out as functions of aniline polymerization parameters and acid concentration. The fibrous-cluster model of a composite membrane is proposed for the estimation of transport and structural parameters, taking into account different mechanism of charge transfer in structural fragments of the composite. The atomic force microscopy images and curves of water distribution on the effective pore radii in the composite membranes testify to a morphological transition from the nano- to the microsize of polyaniline inclusions with increasing the aniline polymerization time. This effect is confirmed by the analysis of two-phase model transport and structural parameters. High values of the “true” proton transport numbers of composites are obtained and discussed. The dynamic hydration numbers of protons and chloride co-ions are estimated using the “true” transport numbers of protons and the electroosmotic coefficients of composites. The current–voltage curves of composite membranes in the “free standing” state after bulk and surface modification by polyaniline are investigated. The effect of stabilization of limiting current density is observed for MF-4SC membrane after bulk modification. The effect of current–voltage curves asymmetry is observed for different orientation of the polyaniline layer towards the current direction for an anisotropic composite membrane after surface modification.

A Spreadsheet-Based Method for Estimating the Skin Disposition of Volatile Compounds: Application to N,N-Diethyl-m-Toluamide (DEET)

The disposition of N,N-diethyl-3-methylbenzamide (DEET) applied to split-thickness human cadaver skin was measured in modified Franz cells maintained at 32°C and fitted with a vapor trap. Ethanolic solutions of DEET (1% w/w) spiked with 14 C radiolabel were applied to skin at a dose of 10 μL per cell, corresponding to a DEET dose of 127 μ g/cm 2 . Room air was drawn over the skin at velocities ranging from 10–100 mL/min. Evaporation of radiolabel from the skin surface and absorption into the receptor solution were monitored for 24 hr post-dose. The percentage of radioactivity collected in the vapor trap after 24 hr increased with airflow, ranging from 16 ± 4% at 10 mL/min to 59 ± 7% at 70 mL/min. The percentage of radioactivity absorbed through the skin after 24 hours decreased with increasing airflow, ranging from 69 ± 7% at 10 mL/min to 20 ± 1% at 80 mL/min. Tissue retention after 24 hr was 6–14% of the radioactive dose with no clear correlation to airflow. This data as well as DEET absorption data from two previous in vitro studies in which dose and location (fume hood or bench top) was varied were analyzed in terms of a recently developed diffusion/evaporation model for skin implemented on an Excel spreadsheet. A priori model calculations based on independently estimated transport parameters (Model 1) were compared with calculations based on fitted parameters (Models 2 and 3). The analysis of the combined dataset (n = 272 observations) showed that the Model 1 estimates matched the cumulative disposition profiles to within a root mean square error of 12.4% of the applied dose (r 2 = 0.65), whereas the Model 2 and Model 3 fits matched to within 9.4% (r 2 = 0.80) and 6.5% (r 2 = 0.91), respectively. The Model 3 fits were obtained using a concentration-dependent diffusivity of DEET in the stratum corneum, the value of which increased 3.4-fold between low concentrations and saturation. This result was consistent with the mild skin penetration enhancement effect for DEET reported elsewhere. [Supplementary materials are available for this article. Go to the publisher's online edition of Journal of Occupational and Environmental Hygiene for the following free supplemental resource: a word document containing tables and figures including more information on the spreadsheet skin absorption model.]

Application of nanofiltration for the rejection of nickel ions from aqueous solutions and estimation of membrane transport parameters

The present work deals with the application of a thin-film composite polyamide nanofiltration membrane for the rejection of nickel ions from aqueous wastewater. The operating variables studied are feed concentration (5–250 ppm), applied pressure (4–20 atm), feed flowrate (5–15 L/min) and pH (2–8). It is observed that the observed rejection of nickel ions increases with increase in feed pressure and decreases with increase in feed concentration at constant feed flowrate. The maximum observed rejection of the metal is found to be 98% and 92% for an initial feed concentration of 5 and 250 ppm, respectively. The effect of pH on the rejection of nickel ions and permeate flux are studied, and found that the variation in pH is having more effect on the latter than the former. The experimental data are analyzed using membrane transport models; combined-film theory-solution-diffusion (CFSD), combined-film theory-Spiegler–Kedem (CFSK) and combined-film theory-finely porous (CFFP) models; to estimate membrane transport parameters and mass transfer coefficient, k. Also, enrichment factor, concentration polarization modulus and Peclet number are found from various parameters. From CFFP model the effective membrane thickness and active skin layer thickness are found.

Formulation and in-vitro evaluation of novel starch-based tableted microspheres for controlled release of ampicillin

This study reports the preparation of starch-based tableted microspheres that are crosslinked with epichlorohydrin (EPI) using a modified water-in-oil (w/o) emulsification technique. Ampicillin (AMP), a broad spectrum antibiotic was encapsulated up to the extent of 70% into the microspheres. The microspheres were characterized by Fourier transform infrared spectroscopy (FT-IR) to confirm the crosslinking reaction and chemical stability of AMP. Differential scanning calorimetry (DSC) was studied on the placebo and drug-loaded microspheres to confirm the polymorphism of AMP. Results of this study indicated a molecular level dispersion of AMP in the developed microspheres. Scanning electron microscopy (SEM) confirmed the spherical nature and smooth surfaces of the microspheres produced. Mean particle size of the microspheres as measured by laser light scattering ranged between 96 and 158 μm. Diffusion coefficients ( D) of water transport through the microspheres were determined using an empirical equation. Values of D decrease with increasing crosslinking as well as increasing content of starch in the microspheres. In-vitro release studies were performed in 1.2 and 7.4 pH media to simulate the gastric and intestinal conditions. The results indicated a dependence on the amount of polymer and extent of crosslinking. Release data were fitted to an empirical relation to estimate transport parameters and to understand the transport mechanism. Statistical analyses of release data was performed using analysis of variance (ANOVA) method. Suitable microspheres were selected and compressed into tablets using the directly compressible excipients. SEM photographs of the fractured part of the tablet revealed the presence of discrete microspheres in the tablets, suggesting that the system chosen is ideal for tableting. Tablets significantly lowered the initial burst effect when compared to microsphere formulations. The tablets were effective in releasing the AMP over an extended period of about 24 h.

Estimation–identification problem for diffusive transport in porous materials based on single reservoir test: Results for silica hydrogel

In this paper the problems of selection of an appropriate model of diffusive transport in silica hydrogel and estimation of model parameters are considered. The analytical solutions and simulations of diffusive transport in a single reservoir test with equilibrium model of sorption are developed. The mathematical models refer to transport of solute from a porous material to water or in reverse direction assuming arbitrary initial concentrations and Dirichlet or mixed type of boundary conditions. In order to quantify the relative importance of the model parameters, logarithmic sensitivity analysis of solute concentration with respect to estimated parameters describing diffusion, sorption, and interfacial effects is performed. The application of the discussed models for estimation of transport parameters is discussed using experimental results for diffusion of sodium chloride in silica hydrogel.

Oxygen diffusion and consumption in active aerobic granules of heterogeneous structure

The interior structure of aerobic granules is highly heterogeneous, hence, affecting the transport and reaction processes in the granules. The granule structure and the dissolved oxygen profiles were probed at the same granule in the current work for possible estimation of transport and kinetic parameters in the granule. With the tested granules fed by phenol or acetate as carbon source, most inflow oxygen was consumed by an active layer thickness of less than 125 microm on the granule surface. The confocal laser scanning microscopy scans also revealed a surface layer thickness of approximately 100 microm consisting of cells. The diffusivities of oxygen transport and the kinetic constant of oxygen consumption in the active layers only were evaluated. The theoretical models adopted in literature that ignored the contributions of the layered structure of aerobic granule could have overlooked the possible limitations on oxygen transport.