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Effective Diffusivity (effective + diffusivity)
Selected AbstractsDetermination of Drying Characteristics and Effective Diffusivity for Sugar CubesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 11 2004C. Akosman Abstract In this study, the drying characteristics of sugar cubes have been investigated. Drying experiments with sugar cubes were conducted in a laboratory-type tray dryer at temperatures ranging from 45 to 95,°C and different air velocities (0.43, 0.56, and 0.7,m/s). Drying rates increased with the increase in temperature and air velocity. At high temperatures, the moisture content of the solid reached equilibrium moisture value in short time periods. Experimental data were analyzed by using the analytical solution of the unsteady-state diffusion equation to determine the effective diffusivity in sugar cubes. [source] Contaminant Transport in Fractured Chalk: Laboratory and Field ExperimentsGROUND WATER, Issue 6 2003K. Witthüser Laboratory experiments were performed on chalk samples from Denmark and Israel to determine diffusion and distribution coefficients. Batch tests were used to define sorption isotherms for naphthalene and o-xylene. Linear sorption isotherms were observed and described with Henry-isotherms. Because of the high purity and low contents of clay minerals and organic carbon, Danish and white Israeli chalk generally have low retardation capacities. Con-trarily, gray Israeli chalk, with organic carbon fractions as high as 1.092%, remarkably retards organic contaminants. The Koc concept is not applicable to predicting distribution coefficients based on the organic carbon content in the chalk samples. Effective diffusivities of o -xylene, naphthalene, and several artificial tracers were determined using through-diffusion experiments. Based on measured diffusion coefficients and available literature values, a chalk specific exponent of 2.36 for Archie's law was derived, allowing a satisfactory estimate of relative diffusivities in chalk. A field-scale tracer test with uranine and lithium was performed in the Negev desert (Israel) to examine the transfer-ability of diffusivities determined on small rock samples in the laboratory. Due to low recovery rates of the tracer, a modified single fissure dispersion model was used for inverse modeling of the breakthrough curves. Resulting diffusivities deviate insignificantly from the laboratory values, which are considered to be representative for the investigated part of the aquifer and applicable in transport models. [source] THIN-LAYER DRYING KINETICS OF SESAME HULLS UNDER FORCED CONVECTION AND OPEN SUN DRYINGJOURNAL OF FOOD PROCESS ENGINEERING, Issue 3 2007MAJDI A. AL-MAHASNEH ABSTRACT Sesame hulls are a useful by-product of the sesame processing industry. The sesame hulls are produced at a high moisture content (68% wet basis) and need further drying to prevent deterioration. In this study, both open sun drying (OSD) and forced convection drying (FCD) at 42, 55, and 76C and 1.2 m/s air velocity were investigated. Six common thin-layer drying models were fitted to the experimental data. Several statistical parameters were used to evaluate the performance of thin-layer drying models, including r2, x2, root mean square error (RMSE) and residuals. Sesame hull drying was found to take place completely in the falling rate region. The modified Page model was found to describe OSD data well, while the Wang and Singh model was the best model for describing FCD. Effective diffusivity was found to be 1.89 × 10 - 8 m2/s and 7.36 × 10 - 10 to 1.20 × 10 - 9 m2/s for OSD and FCD, respectively. Activation energy was also found to be 12.95 kJ/mol for FCD. [source] Effective Diffusivities and Convective Coefficients for CaO-CaSO4 and CaO-CaCl2 PelletsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 1 2004C. Akosman Abstract Diffusion and convective flow in the pores of pellets formed by compressing mixtures of calcined limestone and CaSO4/CaCl2 powders have been studied experimentally by using the single pellet moment technique. The experiments were conducted in a diffusion cell by flowing nitrogen gas (carrier) through both faces of the pellet. Limestone powder was calcined in an atmosphere of N2 at 800,°C and mixed with CaSO4/CaCl2 for diffusion experiments. Effective diffusivity of helium has been estimated by exposing the upper face of the pellet to a pulse of and matching the response peak on the lower face of the pellet with theoretical expressions. The values of the effective diffusivities increased with temperature, but decreased with increasing CaSO4/CaCl2 content in the pellet. The convective flow contribution to the diffusion flux was found to increase with increasing pressure drop across the pellet. [source] Effect of solvent concentration on the extraction kinetics and diffusivity of Cyclosporin A in the fungus Tolypocladium inflatumBIOTECHNOLOGY & BIOENGINEERING, Issue 1 2007May Ly Abstract The kinetics of solid-liquid extraction and extraction yields of the immunosuppressant drug Cyclosporin A (CyA) from the mycelia of Tolypocladium inflatum were examined in this study. A 2 L stirred, baffled vessel was used to extract CyA from wet mycelia mass. Three different organic solvents were used, namely, methanol, acetone, and isopropanol at different concentrations in aqueous mixtures at room temperature. It was found that the best solvent was acetone at 50% v/v concentration achieving 100% extraction of CyA from the mycelia of T. inflatum. Although acetone proved to be the better solvent for CyA extraction, further studies were performed using methanol. A linear relationship was found between extraction yield of CyA and methanol concentration with 100% CyA extraction at 90% v/v methanol. The partition coefficients of CyA between the solid mycelia phase and the aqueous solvent phase were found to decrease exponentially with increasing methanol concentration. A liquid extraction model was developed based on the diffusion equation to correlate the kinetic data of CyA extraction from the solid mycelia of T. inflatum. Non-linear regression analysis of experimental data was used with the diffusion equation in order to calculate the effective diffusivities of CyA in the mycelia of T. inflatum. For all three organic solvents used, the effective diffusivities of CyA were found to be between 4.41,×,10,15 and 6.18,×,10,14 m2/s. This is the first time CyA effective diffusivities in T. inflatum are reported in the literature. Biotechnol. Bioeng. 2007;96: 67,79. © 2006 Wiley Periodicals, Inc. [source] Effects of ionic strength on lysozyme uptake rates in cation exchangers.BIOTECHNOLOGY & BIOENGINEERING, Issue 2 2005I: Uptake in SP Sepharose FF Fluorescence scanning confocal microscopy was used in parallel with batch uptake and breakthrough measurements of transport rates to study the effect of ionic strength on the uptake of lysozyme into SP Sepharose FF. In all cases the adsorption isotherms were near-rectangular. As described previously, the intraparticle profiles changed from slow-moving self-sharpening fronts at low salt concentration, to fast-moving diffuse profiles at high salt concentration, and batch uptake rates correspondingly increased with increasing salt concentration. Shrinking core and homogeneous diffusion frameworks were used successfully to obtain effective diffusivities for the low salt and high salt conditions, respectively. The prediction of column breakthrough was generally good using these frameworks, except for low-salt uptake results. In those cases, the compressibility of the stationary phase coupled with the shrinking core behavior appears to reduce the mass transfer rates at particle-particle contacts, leading to shallower breakthrough curves. In contrast, the fast uptake rates at high ionic strength appear to reduce the importance of mass transfer limitations at the particle contacts, but the confocal results do show a flow rate dependence on the uptake profiles, suggesting that external mass transfer becomes more limiting at high ionic strength. These results show that the complexity of behavior observable at the microscopic scale is directly manifested at the column scale and provides a phenomenological basis to interpret and predict column breakthrough. In addition, the results provide heuristics for the optimization of chromatographic conditions. © 2005 Wiley Periodicals, Inc. [source] Effective Diffusivities and Convective Coefficients for CaO-CaSO4 and CaO-CaCl2 PelletsCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 1 2004C. Akosman Abstract Diffusion and convective flow in the pores of pellets formed by compressing mixtures of calcined limestone and CaSO4/CaCl2 powders have been studied experimentally by using the single pellet moment technique. The experiments were conducted in a diffusion cell by flowing nitrogen gas (carrier) through both faces of the pellet. Limestone powder was calcined in an atmosphere of N2 at 800,°C and mixed with CaSO4/CaCl2 for diffusion experiments. Effective diffusivity of helium has been estimated by exposing the upper face of the pellet to a pulse of and matching the response peak on the lower face of the pellet with theoretical expressions. The values of the effective diffusivities increased with temperature, but decreased with increasing CaSO4/CaCl2 content in the pellet. The convective flow contribution to the diffusion flux was found to increase with increasing pressure drop across the pellet. [source] Role of geometrical dimensions in electrophoresis applications with orthogonal fieldsELECTROPHORESIS, Issue 15 2005Mario A. Oyanader Abstract The role of geometrical dimensions in electrophoresis applications with axial and orthogonal (secondary) electric fields is investigated using a rectangular capillary channel. In particular, the role of the applied orthogonal electrical field in controlling key parameters involved in the effective diffusivity and effective (axial) velocity of the solute is identified. Such mathematically friendly relationships are obtained by applying the method of spatial averaging to the solute species continuity equation; this is accomplished after the role of the capillary geometrical dimensions on the applied electrical field equations has been studied. Moreover, explicit analytical expressions are derived for the effective parameters, i.e., diffusivity and convective velocity as functions of the applied (orthogonal) electric field. Previous attempts (see Sauer et al., 1995) have only led to equations for these parameters that require numerical solution and, therefore, limited the use of such results to practical applications. These may include, for example, the design of separation processes as well as environmental applications such as soil reclamation and wastewater treatment. An illustration of how a secondary electrical field can aid in reducing the optimal separation time is included. [source] Moisture barrier and physical properties of acetylated derivatives with increasing acetylation degreeEUROPEAN JOURNAL OF LIPID SCIENCE AND TECHNOLOGY, Issue 5 2009Claire Bourlieu Abstract Four acetostearin products with increasing acetylation degree were synthesized by chemical interesterification followed by fractionation/blending stages. Their physical properties and functional barrier properties were studied and compared to the properties of technical tristearin. Increasing acetylation degree (AD) modified the triacylglycerols crystal habits and probably led to an increase in acyl chain fluidity, which induced, at macroscopic levels, a decrease in solid fat content (SFC), in melting point, in surface and bulk material hydrophobicity, and an increased moisture effective diffusivity. Water vapor permeability (WVP) coefficients of the materials were partially influenced by the AD factor, but also by the development of macroscopic cracks in lipids presenting high SFC. Acetylated stearin up to 47% (acetyl mol/mol of esterified chain) presented the lowest WVP at 20,°C resulting from an adequate balance between hydrophobicity and mechanical properties of the material. [source] Vertical partitioning of CO2 production within a temperate forest soilGLOBAL CHANGE BIOLOGY, Issue 6 2006ERIC A. DAVIDSON Abstract The major driving factors of soil CO2 production , substrate supply, temperature, and water content , vary vertically within the soil profile, with the greatest temporal variations of these factors usually near the soil surface. Several studies have demonstrated that wetting and drying of the organic horizon contributes to temporal variation in summertime soil CO2 efflux in forests, but this contribution is difficult to quantify. The objectives of this study were to partition CO2 production vertically in a mixed hardwood stand of the Harvard Forest, Massachusetts, USA, and then to use that partitioning to evaluate how the relative contributions of CO2 production by genetic soil horizon vary seasonally and interannually. We measured surface CO2 efflux and vertical soil profiles of CO2 concentration, temperature, water content, and soil physical characteristics. These data were applied to a model of effective diffusivity to estimate CO2 flux at the top of each genetic soil horizon and the production within each horizon. A sensitivity analysis revealed sources of uncertainty when applying a diffusivity model to a rocky soil with large spatial heterogeneity, especially estimates of bulk density and volumetric water content and matching measurements of profiles and surface fluxes. We conservatively estimate that the O horizon contributed 40,48% of the total annual soil CO2 efflux. Although the temperature sensitivity of CO2 production varied across soil horizons, the partitioning of CO2 production by horizon did not improve the overall prediction of surface CO2 effluxes based on temperature functions. However, vertical partitioning revealed that water content covaried with CO2 production only in the O horizon. Large interannual variations in estimates of O horizon CO2 production indicate that this layer could be an important transient interannual source or sink of ecosystem C. [source] Confocal imaging of chromatographic fouling under flow conditionsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 10 2007Sun Chau Siu Abstract BACKGROUND: The fouling impact of selected fouling species was assessed by utilising confocal scanning laser microscopy (CSLM) to image a packed chromatographic bed during operation. A custom-made flow cell was packed with Q Sepharose FF and loaded with partially clarified E. coli homogenate. Selective, multicoloured fluorescent dyes were used to label a bovine serum albumin (BSA) test protein (Cy5.5), dsDNA (PicoGreen) and host cell proteins (HCPs) (Cy3). The fouling caused by the various fluorescently labelled components was visualised as a result of the fluorescence emitted by the PicoGreen-labelled dsDNA and the Cy3-labelled protein in the foulant stream, and by testing the adsorptive capacity of a test protein (BSA) onto the resin prior to and post-fouling as well as following the application of a common CIP procedure. RESULTS: Values for the effective diffusivity of BSA (De) were derived from the confocal images and the fouling impact was assessed by comparing De values obtained from different fouling scenarios. Under the most extreme conditions examined, fouling caused a 20% reduction in capacity compared to a fresh bed. BSA diffusivity did not appear to be affected by the fouling conditions studied. Sequential CIP using 15 CVs of 1 mol L,1 NaCl then 15 CVs of 1 mol L,1 NaOH was shown to be effective in removing nucleic acids and HCPs. Subsequent BSA adsorption showed that the CIP regime successfully restored the column capacity to its original value. In contrast, 15 CVs of 1 mol L,1 NaCl were ineffective in removing dsDNA but substantially removed HCPs. CONCLUSION: CSLM was demonstrated to be a useful tool for visualising fouling mechanisms. Comparing the results obtained by this technique using different modes of chromatographic operation provided insights into the fouling characteristics of finite baths versus packed beds. Copyright © 2007 Society of Chemical Industry [source] Development and simulation studies of an unsteady state biofilter model for the treatment of cyclic air emissions of an ,-pinene gas streamJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2005Christina Dirk-Faitakis Abstract This paper describes the development and simulation of an unsteady state biofilter model used to predict dynamic behaviour of cyclically-operated biofilters and compares it with experimental results obtained from three, parallel, bench-scale biofilters treating both periodically fluctuating concentrations and constant concentrations of an ,-pinene-laden gas stream. The dynamic model, using kinetic parameters estimated from the constant concentration biofilter, was able to predict the performance of cyclic biofilters operating at short cycle periods (ie, in the order of minutes and hours). Steady state kinetic data from a constant concentration biofilter can be used to predict unsteady state biofilter operation. At a 24 h cycle period, the dynamic model compared well with experimental results. For long cycle periods (ie, hours and days), removal efficiency decreased after periods of non-loading: the longer the period of non-loading, the poorer the biofilter's performance at the re-commencement of pollutant loading. At longer time scales the model did not effectively predict transient behaviour, as adsorption and changes in kinetic parameters were not accounted for. Modelling results showed that similar biofiltration performance for the cyclic and constant concentration biofiltration of ,-pinene is expected for biofilters operating solely in the first order kinetics regime. Poorer performance for cyclic biofilters following Monod kinetics spanning the entire kinetics range is expected as the cycle amplitude increases. The most important parameters affecting the performance of a cyclically-operated biofilter with short cycle periods are: amplitude of cyclic fluctuations, Cg, max/Cg, relative value of the half-saturation constant in the Monod expression, Ks, and effective diffusivity of ,-pinene in the biofilm, De. Copyright © 2005 Society of Chemical Industry [source] DEHYDRATION CHARACTERISTICS OF PAPAYA (CARICA PUBENSCENS): DETERMINATION OF EQUILIBRIUM MOISTURE CONTENT AND DIFFUSION COEFFICIENTJOURNAL OF FOOD PROCESS ENGINEERING, Issue 5 2009R. LEMUS-MONDACA ABSTRACT This study determined the drying kinetics behavior of papaya at different temperatures (40, 50, 60, 70 and 80C). Desorption isotherms were determined at 5, 20 and 50C over a relative humidity range of 10,95%. The Guggenheim, Anderson and de Boer models were found to be suitable for description of the sorption data. The samples reached equilibrium moisture of 0.10 ± 0.01 g water/g dry matter. Fick's second law model was used to calculate the effective diffusivity (6.25,24.32 × 10 - 10 m2/s). In addition, experimental data were fitted by means of seven mathematical models. The kinetic parameters and the diffusion coefficient were temperature-dependent and were evaluated by an Arrhenius-type equation. The modified Page model obtained the best-fit quality on experimental data according to statistical tests applied. PRACTICAL APPLICATIONS The main utility of this study is the application of both different empirical models and the diffusional model in tropical fruits' dehydration, which can be considered a basis for a very accurate estimation of drying time and the optimization of the same process. Two newly mathematical models are proposed in this study, through which a good fit on the data of experimental moisture content was achieved. The study of drying of papaya cultivated in Chile engages a great technological interest, because this product is widely used in the development of different products such as candying, canning, juice, syrup and marmalade. In addition, papayas cultivated in Chile, along with other tropical fruits cultivated in Brazil, Colombia and the Caribbean, have become especially important in the exportations of these countries. The major markets for these products include the U.S.A., the European Community and Asia. [source] Nitrobenzene adsorption in activated carbon as observed by NMRAICHE JOURNAL, Issue 1 2003L. Pel The adsorption of nitrobenzene in activated carbon was investigated by nuclear magnetic resonance (NMR). Because of the short relaxation times and carbon is an electrically conducting material, a specially adapted NMR setup was used. It was found that during the adsorption process the nitrobenzene profiles can be scaled using the Boltzmann transformation and the overall effective diffusivity can be approximated by an exponential function of the nitrobenzene content. [source] Release of potassium sorbate from active films of sodium alginate crosslinked with calcium chloridePACKAGING TECHNOLOGY AND SCIENCE, Issue 6 2009E. M. Zactiti Abstract The release of potassium sorbate from alginate films was evaluated considering different active agent concentrations in the film and three levels of alginate crosslinking. The mechanism involved in the diffusional process was investigated using the Power Law Model. The results indicated that potassium sorbate diffusion in alginate films has characteristics of Fickian and non-Fickian behaviour. Effective diffusion coefficients obtained using the solution in series derived from Fick's Second Law are close to values obtained by the short-time solution, indicating that the influence of swelling on effective diffusivity, although perceptible, is small, allowing the use of an average effective diffusivity. The values of effective diffusivity found indicate that alginate films have a potential use as systems for release of active substances. Copyright © 2009 John Wiley & Sons, Ltd. [source] In situ effective diffusion coefficient profiles in live biofilms using pulsed-field gradient nuclear magnetic resonanceBIOTECHNOLOGY & BIOENGINEERING, Issue 6 2010Ryan S. Renslow Abstract Diffusive mass transfer in biofilms is characterized by the effective diffusion coefficient. It is well documented that the effective diffusion coefficient can vary by location in a biofilm. The current literature is dominated by effective diffusion coefficient measurements for distinct cell clusters and stratified biofilms showing this spatial variation. Regardless of whether distinct cell clusters or surface-averaging methods are used, position-dependent measurements of the effective diffusion coefficient are currently: (1) invasive to the biofilm, (2) performed under unnatural conditions, (3) lethal to cells, and/or (4) spatially restricted to only certain regions of the biofilm. Invasive measurements can lead to inaccurate results and prohibit further (time-dependent) measurements which are important for the mathematical modeling of biofilms. In this study our goals were to: (1) measure the effective diffusion coefficient for water in live biofilms, (2) monitor how the effective diffusion coefficient changes over time under growth conditions, and (3) correlate the effective diffusion coefficient with depth in the biofilm. We measured in situ two-dimensional effective diffusion coefficient maps within Shewanella oneidensis MR-1 biofilms using pulsed-field gradient nuclear magnetic resonance methods, and used them to calculate surface-averaged relative effective diffusion coefficient (Drs) profiles. We found that (1) Drs decreased from the top of the biofilm to the bottom, (2) Drs profiles differed for biofilms of different ages, (3) Drs profiles changed over time and generally decreased with time, (4) all the biofilms showed very similar Drs profiles near the top of the biofilm, and (5) the Drs profile near the bottom of the biofilm was different for each biofilm. Practically, our results demonstrate that advanced biofilm models should use a variable effective diffusivity which changes with time and location in the biofilm. Biotechnol. Bioeng. 2010;106: 928,937. © 2010 Wiley Periodicals, Inc. [source] Windows of operation for bioreactor design for the controlled formation of tissue-engineered arteriesBIOTECHNOLOGY PROGRESS, Issue 3 2009Spyridon Gerontas Abstract The availability of large numbers of units of artificial arteries would offer significant benefits to the clinical management of bypass surgery. Tissue engineering offers the potential of providing vessels that can mimic the morphology, function, and physiological environment of native vessels. Ideally this would involve culturing stem cells in vitro within a biodegradable tubular scaffold so as to construct tissue for implantation. Essential to establishing a robust process for the production of tissue-engineered arteries is the understanding of the impact of changes in the operating conditions and bioreactor design on the construct formation. In this article, models of transport phenomena were developed to predict the critical flow rates and mass transfer requirements of a prototype bioreactor for the formation of tissue-engineered arteries. The impact of the cell concentration, tube geometry, oxygen effective diffusivity in alginate, substrate and metabolite concentration levels, feed rate, and recycle rate on the design of the bioreactor was visualized using windows of operation and contour plots. The result of this analysis determined the best configuration of the bioreactor that meets the cellular transport requirements as well as being reliable in performance while seeking to reduce the amount of nutrients to be used. © 2009 American Institute of Chemical Engineers Biotechnol. Prog., 2009 [source] Determination of Drying Characteristics and Effective Diffusivity for Sugar CubesCHEMICAL ENGINEERING & TECHNOLOGY (CET), Issue 11 2004C. Akosman Abstract In this study, the drying characteristics of sugar cubes have been investigated. Drying experiments with sugar cubes were conducted in a laboratory-type tray dryer at temperatures ranging from 45 to 95,°C and different air velocities (0.43, 0.56, and 0.7,m/s). Drying rates increased with the increase in temperature and air velocity. At high temperatures, the moisture content of the solid reached equilibrium moisture value in short time periods. Experimental data were analyzed by using the analytical solution of the unsteady-state diffusion equation to determine the effective diffusivity in sugar cubes. [source] | ||||||||||||||||