Mass Transport Parameters Research Articles

Inverse gas chromatography study on the effect of humidity on the mass transport of alcohols in an ethylene-vinyl alcohol copolymer near the glass transition temperature

Inverse gas chromatography (IGC) was used to study the effect of moisture on transport properties of three low molecular weight alcohols (methanol, ethanol, and 1-butanol) through high barrier copolymers of ethylene-vinyl alcohol with an ethylene content of 38% mol (EVOH38) at 40 °C. The value of the partition coefficient ( K) was obtained by using two approaches: (a) the fit of the slope of sorption isotherms obtained through the method of Kiselev and Yashin; and (b) the solution to the model of Romdhane and Danner obtained by using the law of moments. The second method also allowed the estimation of the diffusion coefficient ( D p) at the different humidity conditions. None of these two methods were applicable at low values of relative humidity. With the first method, the diffusion of the permeants through the copolymer was not fast enough to allow them to reach the core of the EVOH particles used as stationary phase resulting in sorption values unrealistically low. The fit of the chromatograms obtained by using the second method also suggested questionable values of the mass transport parameters. Although the theoretical curve perfectly described the chromatogram, the low extent of the interaction and the slow diffusion resulted in interdependent values of the coefficients K and D p, with infinite pairs of values providing the same curve profile. As the relative humidity of the carrier gas increased, the diffusivity and the sorption of the alcohols also increased, making both methods applicable. In the case of the partition coefficient, the sorption of the biggest molecules (ethanol and 1-butanol) was the most affected, the increment of K for methanol being moderate. As regards the D p value, methanol was the most influenced compound and 1-butanol the least. Finally, a sharp increment of the D p of the three alcohols was observed between 35 and 47% RH and attributed to the plasticization of the copolymer.

On the role of the surfactant aliquat ® 336 on the kinetics of oxygen reduction reaction and on the rate of hydrogen peroxide electrosynthesis

This paper reports an investigation on the effect of the surfactant aliquat ® 336 (A336) on the mass transport parameters of the oxygen reduction reaction and on H 2O 2 electro-generation rate. Results have shown that most of the positive effects of adding A336 to the electrolyte are observed at concentrations around the critical micelle concentration (CMC). Results analyzed in this paper have shown that adding A336 at CMC shifted half-wave potential for O 2 reduction by 80 mV more negative and, furthermore, extended limiting current plateaus for the reaction O 2 → H 2O 2 over extra 100 mV more negative. Limiting currents for O 2 reduction are higher in the presence of A336, and the O 2 diffusion coefficient increased from 2 × 10 −6 cm 2 s −1 to 9 × 10 −6 cm 2 s −1. All these findings were verified by controlled potential electrolysis in a flow electrochemical reactor, where it was also found that the H 2O 2 electro-generation rate reached a maximum for A336 concentrations close to the CMC.

Propylene Hydrogenation in a Continuous Polymeric Catalytic Membrane Reactor

A continuous polymeric catalytic membrane reactor (pCMR), using a poly-(dimethylsiloxane) (PDMS) composite membrane that contained palladium nanoclusters, was studied and modeled for the propylene hydrogenation. The PDMS catalytic membrane has an average thickness of 4.4 μm, and was loaded with 5 wt % of nanosized palladium clusters ∼9 nm in diameter. The reaction was performed at 308 K, and several feed compositions and flow rates were tested. The pCMR model proposed represents the experimental data (i.e., the flow rates and mixture compositions in the permeate and retentate chambers, and, therefore, the overall advancement) fairly well. In the hydrogenation rate equation, the propylene adsorption parameter and the kinetic constant were obtained by fitting the model to the experimental data. On the other hand, the hydrogen kinetic parameter and the mass-transport parameters were obtained from independent experiments.

INFLUENCE OF SAMPLE SIZE AND SHAPE ON TRANSPORT PARAMETERS DURING DRYING OF SHRINKING BODIES

ABSTRACT An experimental investigation on the influence of sample size and shape on heat and mass transport parameters under natural convection air‐drying is presented. Potato cylinders with length of 0.05 m and thicknesses of 0.005, 0.008, 0.010 and 0.016 m, and circular slices with diameter of 0.05 m and thickness of 0.01 m were dried in a laboratory scale hot‐air cabinet dryer. Results indicate that each transport parameter exhibits a linear relationship with sample thickness. Convective heat and mass transfer coefficients (hcand hm) decreased whereas moisture diffusion coefficient (Deff) increased with increasing thickness. Considering no sample shrinkage effect in the parameter analysis, for the thickness range considered, the values of hcare found to be underestimated in the range of 29.0–30.6%, whereas those of hmand Deff are overestimated in the range of 33.7–38.0% and 75.9–128.1%, respectively. Using Levenberg–Marquardt algorithm for optimization, a correlation for Biot number for mass transfer (Bim) as a function of drying time and sample thickness is proposed. A close agreement was observed between dimensionless moisture contents predicted by this relation and those obtained from experiments for different sample thicknesses at drying air temperature of 60C. For the same thickness and drying conditions, circular slices caused an increase in each transport parameter significantly.

Leaching of Heavy Metals Stabilized in Asphalt Matrix

Long-term impact of stabilized/solidified hazardous waste on the environment was evaluated using a combination of long-term leaching tests and mathematical modeling. The waste used in this research is a dust collected from filters used in the production of nickel-cadmium accumulators. It contains nickel hydroxide, cobalt hydroxide, and cadmium oxide, and based on toxicity characteristic leaching procedure test it is a hazardous waste. This waste was stabilized using two different aqueous anionic asphalt emulsions. Several extraction tests were used to evaluate the long-term leaching of stabilized waste. Then using a mathematical model, the long-term leaching test data was analyzed to obtain mass transport parameters for the contaminants in the stabilized wastes. From the leaching test results mathematical model parameters needed to estimate the transport and release of contaminants to the real environment under alkaline pH from the stabilized waste were obtained. The model simulation of the performance of the stabilized waste under the actual environmental conditions of the disposal site showed that there is insignificant release of the contaminants from the stabilized waste. Hence it was concluded based on the experimental and modeling data that if the environment is able to maintain an alkaline pH condition, then the stabilized waste will not damage the environment.

Mass transport in groundwater near hanging-wall interceptors

This study investigated mass transport near trenches designed to capture contaminated groundwater. Numerical models simulated migration of contaminant plumes toward trenches oriented perpendicular to regional groundwater flow, partially penetrating a hypothetical unconfined aquifer. Plumes originated at the top of the simulated groundwater flow system. The smallest trench necessary to capture a contaminant plume was identified for various sets of mass transport parameters. Results suggest that, in predominantly horizontal flow systems such as those simulated here: (i) vertical hydraulic conductivity has relatively little effect on downward propagation of contaminant plumes and required trench size, (ii) transverse vertical dispersivity exerts significant control on plume and trench depth, and (iii) recharge dilutes and thus reduces plume and trench width, but may induce downward vertical hydraulic gradients that deepen plumes and trenches. For all cases considered, trenches oriented perpendicular to regional groundwater flow, placed close to the leading edge of a contaminant plume, and sized slightly larger than the plume (in transverse cross-section) facilitate plume capture.

Characterization of mass transport processes in IMAC chromatography by dynamics methods

The characterization of mass transport processes in IMAC chromatography and the pH effect on the process have been done. To do that the adsorption equilibrium constant and adsorption rate constant for Cu, Zn-superoxide dismutase (Cu, Zn-SOD) on Cu-IDA-agarose matrix have been determined in batch and in a continuous operating systems (chromatographic column) by dynamic methods. The experiments were performed at pH of 7.00, 7.40 and 7.80 and, in case of chromatographic columns, flow rates of 1.56–2.66 ml/min. From the analysis of the experimental results it can be concluded that the controlling steps for the Cu, Zn-SOD adsorption on Cu(II)-IDA-agarose beads are both external and internal mass transfer processes, which was experimentally confirmed with the fact that the adsorption kinetic process is too fast. The medium pH affects equilibrium and mass transport parameters in the following way: a decrease in the medium pH towards the enzyme isoelectric point increases the adsorption equilibrium constant and both molecular and effective intraparticle diffusion coefficients; and decreases internal, external and global mass transfer resistances, and effective axial dispersion.

A finite element model for protein transport in vivo

BackgroundBiological mass transport processes determine the behavior and function of cells, regulate interactions between synthetic agents and recipient targets, and are key elements in the design and use of biosensors. Accurately predicting the outcomes of such processes is crucial to both enhancing our understanding of how these systems function, enabling the design of effective strategies to control their function, and verifying that engineered solutions perform according to plan.MethodsA Galerkin-based finite element model was developed and implemented to solve a system of two coupled partial differential equations governing biomolecule transport and reaction in live cells. The simulator was coupled, in the framework of an inverse modeling strategy, with an optimization algorithm and an experimental time series, obtained by the Fluorescence Recovery after Photobleaching (FRAP) technique, to estimate biomolecule mass transport and reaction rate parameters. In the inverse algorithm, an adaptive method was implemented to calculate sensitivity matrix. A multi-criteria termination rule was developed to stop the inverse code at the solution. The applicability of the model was illustrated by simulating the mobility and binding of GFP-tagged glucocorticoid receptor in the nucleoplasm of mouse adenocarcinoma.ResultsThe numerical simulator shows excellent agreement with the analytic solutions and experimental FRAP data. Detailed residual analysis indicates that residuals have zero mean and constant variance and are normally distributed and uncorrelated. Therefore, the necessary and sufficient criteria for least square parameter optimization, which was used in this study, were met.ConclusionThe developed strategy is an efficient approach to extract as much physiochemical information from the FRAP protocol as possible. Well-posedness analysis of the inverse problem, however, indicates that the FRAP protocol provides insufficient information for unique simultaneous estimation of diffusion coefficient and binding rate parameters. Care should be exercised in drawing inferences, from FRAP data, regarding concentrations of free and bound proteins, average binding and diffusion times, and protein mobility unless they are confirmed by long-range Markov Chain-Monte Carlo (MCMC) methods and experimental observations.

Identification of biomolecule mass transport and binding rate parameters in living cells by inverse modeling

BackgroundQuantification of in-vivo biomolecule mass transport and reaction rate parameters from experimental data obtained by Fluorescence Recovery after Photobleaching (FRAP) is becoming more important.Methods and resultsThe Osborne-Moré extended version of the Levenberg-Marquardt optimization algorithm was coupled with the experimental data obtained by the Fluorescence Recovery after Photobleaching (FRAP) protocol, and the numerical solution of a set of two partial differential equations governing macromolecule mass transport and reaction in living cells, to inversely estimate optimized values of the molecular diffusion coefficient and binding rate parameters of GFP-tagged glucocorticoid receptor. The results indicate that the FRAP protocol provides enough information to estimate one parameter uniquely using a nonlinear optimization technique. Coupling FRAP experimental data with the inverse modeling strategy, one can also uniquely estimate the individual values of the binding rate coefficients if the molecular diffusion coefficient is known. One can also simultaneously estimate the dissociation rate parameter and molecular diffusion coefficient given the pseudo-association rate parameter is known. However, the protocol provides insufficient information for unique simultaneous estimation of three parameters (diffusion coefficient and binding rate parameters) owing to the high intercorrelation between the molecular diffusion coefficient and pseudo-association rate parameter. Attempts to estimate macromolecule mass transport and binding rate parameters simultaneously from FRAP data result in misleading conclusions regarding concentrations of free macromolecule and bound complex inside the cell, average binding time per vacant site, average time for diffusion of macromolecules from one site to the next, and slow or rapid mobility of biomolecules in cells.ConclusionTo obtain unique values for molecular diffusion coefficient and binding rate parameters from FRAP data, we propose conducting two FRAP experiments on the same class of macromolecule and cell. One experiment should be used to measure the molecular diffusion coefficient independently of binding in an effective diffusion regime and the other should be conducted in a reaction dominant or reaction-diffusion regime to quantify binding rate parameters. The method described in this paper is likely to be widely used to estimate in-vivo biomolecule mass transport and binding rate parameters.

New mechanism of gas transport at the interface solid/liquid

It was recently shown that an abnormally fast transport of CO molecules takes place at the electrode/electrolyte interface of Pt and PtRu electrodes in H 2SO 4 and HClO 4 solutions. In the present paper, this phenomenon is tested for other gases, such as hydrogen and oxygen. The fast transport is also observed at the solid/electrolyte solution interface of other electrode materials and at the glass/electrolyte interface. Several experiments are shown, demonstrating that mass transfer takes place at a velocity, which is more than one order of magnitude higher than expected for usual diffusion conditions. Assuming radial mass transfer at the interface of a Pt disc, the activation energy, E a = 23 kJ mol −1, was calculated from Arrhenius plots. The same value was measured in H 2SO 4 and HClO 4 as supporting electrolytes. The mass transport parameter, Y, at 298 K was 4.8 × 10 −3 cm 2 s −1 and 2.9 × 10 −3 cm 2 s −1 in 0.5 M H 2SO 4 and 1 M HClO 4 respectively.

Experimental Method to Determine the Mass Transport Resistance of a Polymer Electrolyte Fuel Cell

A technique to measure an average mass-transport resistance of a polymer electrolyte fuel cell in situ is described. Experimental polarization curve data are extrapolated to the limiting current, at which the driving force for diffusion can be accurately estimated. The technique gives very reproducible results and is sensitive to changes in the mass-transport characteristics of the cell. Using a resistance-in-series approach, the mass-transport characteristics of the electrode and gas diffusion media (GDM) can be separated. For the particular system used in this study, it is estimated that the GDM accounts for about 25% of the total mass-transport resistance of the fuel cell assembly. Bulk gas diffusion accounts for less than half of the measured mass-transport resistance. The measured mass-transport parameter may also be utilized for comparison of fuel cell systems, analysis of the effect of operating conditions, or for tracking changes during durability testing.

Interfacial and Bulk Thermal and Mass Transport Parameters in Fuel Cell Media

not Available.

Characterization of ac Voltammetric Reaction−Diffusion Dynamics: From Patterns to Physical Parameters

Despite the widespread use of electrochemical sensing techniques, the determination of the physical parameters from the current response of rapid voltammetric measurements has been difficult for two reasons: large capacitance contributions overwhelm the current response of transient measurements and the reaction dynamics are inherently nonlinear and nonstationary. In this work, we present a signal processing methodology that in combination with a large-amplitude/high-frequency voltage waveform method, ac voltammetry, is able to determine the underlying physical parameters in heterogeneous electrochemical reaction-diffusion processes. Through a large number of numerical calculations, we explore the effect of kinetic, thermodynamic, and mass transport parameters on two components of the current response, the even and the odd. We study the even component directly whereas for the odd component, which is considerably influenced by capacitance, we use the Hilbert transform, which is suitable for the analysis of nonstationary and nonlinear data sets, to minimize the capacitance contribution. The theoretical analysis is applied to measurements of well-characterized electrochemical reactions, Ru(NH3)6(2+/3+) and Fe(CN)6(4-/3-), using two different electrode materials, glassy carbon and platinum, and the physical parameters deduced are in excellent agreement with published results.

Theoretical Analysis of Non-Catalytic Growth of Nanorods on a Substrate

A theoretical analysis explaining the whole process of the growth of nanorods on a substrate without a catalyst is presented. Prior to the growth of the nanorods, the reaction precursors form nuclei on the substrate. The nuclei undergo cluster migration caused by the surface diffusion of adatoms on the substrate, and this migration continues until the mean free time of the adatoms is larger than surface diffusion time. The most probable mechanism by which cluster migration takes place is the one that leads to the minimization of the cluster free-energy, namely the migration of six adatoms into one fixed adatom. This cluster migration continues during several (typically smaller than 6) consecutive nuclei growth steps. After the process of cluster migration comes to an end, the nuclei grow in an isotropic manner by collection of the adatoms, until the nucleus reaches the thermodynamic size limit. The one-dimensional growth of nanorods on the nuclei, which is associated with the critical radius, begins when the reactant dose is smaller than a certain value, which is determined by the thermodynamic size limit and the mass transport parameter. The mass transport of the reaction precursors leads to the expansion of the radius and elongation of the height of the nanorods, and the growth rate of the height is greater than that of the radius. This difference in the growth rate causes the aspect ratio to increase with increasing growth time. By comparing the experimental data in the literature (ZnO nanorods), the presented analysis explains well the noncatalytic growth of nanorods on a substrate.

Modelling of the concentration–time relationship based on global diffusion-charge transfer parameters in a flow-by reactor with a 3D electrode

A concentration versus time relationship model based on the isothermal diffusion-charge transfer mechanism was developed for a flow-by reactor with a three-dimensional (3D) reticulated vitreous carbon (RVC) electrode. The relationship was based on the effectiveness factor ( η) which lead to the simulation of the concentration decay at different electrode polarisation conditions, i.e. −0.1, −0.3 and −0.59 V versus SCE; the charge transfer process was used for the former and mix and a mass transport control was used for the latter. Charge transfer and mass transport parameters were estimated from experimental data using Electrochemical Impedance Spectroscopy (EIS) and Linear Voltammetry (LV) techniques, respectively.

Study of the oxygen reduction reaction (ORR) at Pt interfaced with phosphoric acid doped polybenzimidazole at elevated temperature and low relative humidity

The oxygen reduction reaction on platinum interfaced with phosphoric acid doped PBI at elevated temperature and low relative humidities has been investigated by using a micro band electrode technique. Both the kinetic and the mass transport parameters in the Pt/PBI-H 3PO 4 system are comparable to those of the Pt/H 3PO 4 system under similar conditions. The study suggests that it is the amorphous H 3PO 4 phase that functions as the electrolyte. The oxygen reduction reaction is first order with respect to both proton concentration and oxygen saturation concentration in the electrolyte, which indicates that the proton transfer is the rate-determining step in oxygen reduction. The H 3PO 4 doping level and the water content of the electrolyte affect the ORR exchange current density, oxygen diffusion, and the oxygen solubility in PBI-H 3PO 4 membranes. The dissolved O 2 molecules permeate mainly through the amorphous H 3PO 4. However, the oxygen solubility in PBI-H 3PO 4 is higher than its solubility in H 3PO 4, which is explained by the presence of the crystalline PBI region formed during electrolyte preparation.

Estimation of Mass Transport Parameters of Organic Compounds through High Density Polyethylene Geomembranes Using a Modifield Double-Compartment Apparatus

A modified double-compartment apparatus (MDCA) is used to estimate mass transport parameters of organic compounds through high density polyethylene (HDPE) geomembranes and to investigate the effects of aging and external tension of HDPE geomembranes on the mass transport of organic compounds. A developed one-dimensional partition–diffusion mass transport model successfully explains the mass transport of the organic compounds through the HDPE geomembranes in a dilute aqueous solution–geomembrane system. Similar to batch immersion tests, the HDPE–water partition coefficient ( KHDPE–W ) values of organic compounds are found to have close relationships with the octanol–water partition coefficient and the aqueous solubility; furthermore, the diffusion coefficient (D) values decrease with the increase of their molecular diameter. For HDPE geomembranes served in the landfill liner for 5 years and stretched by 8% of their initial length, KHDPE–W values for organic compounds increase by 5–58% , D values for organi...

Air gap membrane distillation: 1. Modelling and mass transport properties for hollow fibre membranes

A predictive model for air gap membrane distillation in a counter current flow configuration using fibre membranes is presented. The water vapour transport across the membrane is described by the dusty-gas model that uses constant membrane mass transport parameters to describe simultaneous Knudsen diffusion, molecular diffusion and viscous flow. This makes the model suitable to describe the membrane distillation process for a wide range of pressures and temperatures. The membrane mass transport properties had to be determined experimentally in separate experiments to obtain a predictive model. The Knudsen diffusion and viscous flow membrane parameters ( K 0 and B 0, respectively) were determined with single gas permeation experiments. The molecular diffusion membrane parameter ( K 1) was determined with binary gas diffusion experiments. High membrane permeability in combination with small membrane fibre radius, a combination that is advantageous for membrane distillation, made it necessary to pay special attention to effects as pressure drop along the fibre and boundary layer resistances in order to obtain accurate membrane parameters. The gas permeation data show that calculation of K 1 from the K 0 and B 0 values assuming parallel cylindrical pores is accurate within 30% for some membranes but can be wrong by a factor of two for other membranes. This means that (relatively simple) single gas permeation experiments in combination with a cylindrical pore membrane model are, unfortunately, not sufficient to obtain reliable membrane mass transfer properties for model calculations.

Deactivation-influenced propane steam reforming: reactor analysis and parameter estimation

Industrial heterogeneous catalytic reactors generally operate under conditions in which both mass transfer and catalyst activity decay subvert true reaction rates. Consequently, model-based improvements in reactor design are reliant on analysis of data from ideal conditions with separate runs to obtain kinetic, deactivation and mass transport parameters. Recent work by Levenspiel has, however, shown that an integrated model for a first-order reaction in a plug flow reactor containing a catalyst undergoing exponential activity decay may be effectively employed to analyze data from an industrial hydrocarbon processing unit. Thus, the objective of this study was to develop useful relations for a more general class of reactions with power-law kinetics, which may be accompanied by either exponential, hyperbolic or reciprocal catalyst decay forms for both PFR and CSTR systems in the presence of pore-diffusional limitations. As a result, it is now possible to evaluate time-dependent conversion data from practical catalytic reactors against these coupled reaction–deactivation models in order to secure kinetic and deactivation constants. Data from a deactivation-disguised propane steam reforming reaction were used to demonstrate validity and effect reactor analysis in this study.

Electrochemical sensors for detection of hydrogen in air: model of the non-Nernstian potentiometric response of platinum gas diffusion electrodes

The potentiometric response of three different platinum gas diffusion electrodes deposited on H 3PO 4 doped polybenzimidazole (PBI) was investigated under humidified atmospheres that contained H 2 or mixtures of H 2 and O 2. Continuum modelling was used to analyse the response. It is shown that the non-Nernstian response under H 2 H 2O N 2 mixtures can be explained by a difference of water activity on both sides of the membrane. Under H 2 O 2 N 2 mixtures, the oxygen mass transport parameters have a strong effect on the electrode sensitivity.