Showing papers on "Mass transfer coefficient published in 2005"

Two‐scale continuum model for simulation of wormholes in carbonate acidization

Abstract: A two-scale continuum model is developed to describe transport and reaction mechanisms in reactive dissolution of a porous medium, and used to study wormhole formation during acid stimulation of carbonate cores. The model accounts for pore level physics by coupling local pore-scale phenomena to macroscopic variables (Darcy velocity, pressure and reactant cup-mixing concentration) through structure-property relationships (permeability-porosity, average pore size-porosity, and so on), and the dependence of mass transfer and dispersion coefficients on evolving pore scale variables (average pore size and local Reynolds and Schmidt numbers). The gradients in concentration at the pore level caused by flow, species diffusion and chemical reaction are described using two concentration variables and a local mass-transfer coefficient. Numerical simulations of the model on a two-dimensional (2-D) domain show that the model captures the different types of dissolution patterns observed in the experiments. A qualitative criterion for wormhole formation is developed and it is given by Λ ∼ O(1), where Λ = . Here, keff is the effective volumetric dissolution rate constant, DeT is the transverse dispersion coefficient, and uo is the injection velocity. The model is used to examine the influence of the level of dispersion, the heterogeneities present in the core, reaction kinetics and mass transfer on wormhole formation. The model predictions are favorably compared to laboratory data. © 2005 American Institute of Chemical Engineers AIChE J, 2005

Mass transfer modeling of apricot kernel oil extraction with supercritical carbon dioxide

Abstract: Effects of process parameters on extraction of apricot (Prunus armeniaca L.) kernel oil with supercritical carbon dioxide (SC-CO2) were investigated. The parameters included particle size (mean particle diameter < 0.425–1.5 mm), solvent flow rate (1–5 g/min), pressure (300–600 bar), temperature (40–70 °C) and co-solvent concentration (up to 3.0 wt.% ethanol). The model of broken and intact cells represented the apricot kernel oil extraction well. Grinding was necessary to release the oil from intact oil cells of kernel structure. About 99% apricot kernel oil recovery was possible if particle diameter decreased below 0.425 mm. Two extraction periods were distinguished. The released oil on the surface of particles was extracted in the fast extraction period while the unreleased oil in the intact cells was extracted in the slow extraction period. The amount oil recovered in the slow extraction period was negligible compared to the amount recovered in the fast extraction period. Extraction rate in the fast extraction period increased with increase in solvent flow rate, pressure, temperature, and ethanol addition. Mass transfer coefficients in the fluid phase and in the solid phase changed between 0.7 and 3.7 min−1, and between 0.00009 and 0.0005 min−1, respectively. Mass transfer coefficient in the fluid phase increased with decrease in particle size and pressure, and with increase in solvent flow rate, temperature and ethanol concentration.

Oxygen transfer phenomena in 48-well microtiter plates: determination by optical monitoring of sulfite oxidation and verification by real-time measurement during microbial growth.

Abstract: Oxygen limitation is one of the most frequent problems associated with the application of shaking bioreactors. The gas-liquid oxygen transfer properties of shaken 48-well microtiter plates (MTPs) were analyzed at different filling volumes, shaking diameters, and shaking frequencies. On the one hand, an optical method based on sulfite oxidation was used as a chemical model system to determine the maximum oxygen transfer capacity (OTR(max)). On the other hand, the Respiration Activity Monitoring System (RAMOS) was applied for online measurement of the oxygen transfer rate (OTR) during growth of the methylotropic yeast Hansenula polymorpha. A proportionality constant between the OTR(max) of the biological system and the OTR(max) of the chemical system were indicated from these data, offering the possibility to transform the whole set of chemical data to biologically relevant conditions. The results exposed "out of phase" shaking conditions at a shaking diameter of 1 mm, which were confirmed by theoretical consideration with the phase number (Ph). At larger shaking diameters (2-50 mm) the oxygen transfer rate in MTPs shaken at high frequencies reached values of up to 0.28 mol/L/h, corresponding to a volumetric mass transfer coefficient (k(L)a) of 1,600 1/h. The specific mass transfer area (a) increases exponentially with the shaking frequency up to values of 2,400 1/m. On the contrary, the mass transfer coefficient (k(L)) is constant at a level of about 0.15 m/h over a wide range of shaking frequencies and shaking diameters. However, at high shaking frequencies, when the complete liquid volume forms a thin film on the cylindric wall of the well, the mass transfer coefficient (k(L)) increases linearly to values of up to 0.76 m/h. Essentially, the present investigation demonstrates that the 48-well plate outperforms the 96-well MTP and shake flasks at widely used operating conditions with respect to oxygen supply. The 48-well plates emerge, therefore, as an excellent alternative for microbial cultivation and expression studies combining the advantages of both the high-throughput 96-well MTP and the classical shaken Erlenmeyer flask.

Prediction of gas-liquid mass transfer coefficient in sparged stirred tank bioreactors.

Abstract: Oxygen mass transfer in sparged stirred tank bioreactors has been studied. The rate of oxygen mass transfer into a culture in a bioreactor is affected by operational conditions and geometrical parameters as well as the physicochemical properties of the medium (nutrients, substances excreted by the micro-organism, and surface active agents that are often added to the medium) and the presence of the micro-organism. Thus, oxygen mass transfer coefficient values in fermentation broths often differ substantially from values estimated for simple aqueous solutions. The influence of liquid phase physicochemical properties on kLa must be divided into the influence on k(L) and a, because they are affected in different ways. The presence of micro-organisms (cells, bacteria, or yeasts) can affect the mass transfer rate, and thus kLa values, due to the consumption of oxygen for both cell growth and metabolite production. In this work, theoretical equations for kLa prediction, developed for sparged and stirred tanks, taking into account the possible oxygen mass transfer enhancement due to the consumption by biochemical reactions, are proposed. The estimation of kLa is carried out taking into account a strong increase of viscosity broth, changes in surface tension and different oxygen uptake rates (OURs), and the biological enhancement factor, E, is also estimated. These different operational conditions and changes in several variables are performed using different systems and cultures (xanthan aqueous solutions, xanthan production cultures by Xanthomonas campestris, sophorolipids production by Candida bombicola, etc.). Experimental and theoretical results are presented and compared, with very good results.

Mass Transfer in a Rotating Packed Bed with Viscous Newtonian and Non-Newtonian Fluids

Abstract: A theoretical analysis was developed to predict the apparent viscosity of a non-Newtonian fluid in a rotating packed bed. It is based on laminar liquid film flow on a rotating disk with the assumption of the randomly inclined surfaces in the rotating packed bed. In addition, experiments of deoxygenation were performed in glycerol solutions and CMC solutions, which are Newtonian and shear-thinning fluids, respectively. It is shown that mass transfer coefficients decreased with increasing viscosity, while the centrifugal force still revealed effective in enhancing mass transfer in viscous media. A correlation for mass transfer coefficient was proposed and valid for both the Newtonian and non-Newtonian fluids. Compared with a packed column, the influence of mass transfer coefficients by liquid viscosity was less in a rotating packed bed.

Drying Kinetics of Curcuma longa Rhizomes

Abstract: identified as due mainly to the peridermis layer (peridermial resistance). The accuracy of the model assuming identified as due mainly to the peridermis layer (peridermial resistance). The accuracy of the model assuming identified as due mainly to the peridermis layer (peridermial resistance). The accuracy of the model assuming identified as due mainly to the peridermis layer (peridermial resistance). The accuracy of the model assuming peridermial resistance only in the radial direction and solved using the finite differences method was illustrated, peridermial resistance only in the radial direction and solved using the finite differences method was illustrated, peridermial resistance only in the radial direction and solved using the finite differences method was illustrated, peridermial resistance only in the radial direction and solved using the finite differences method was illustrated, peridermial resistance only in the radial direction and solved using the finite differences method was illustrated, and the mass transfer coefficient was identified (k = 9.7 × 10 and the mass transfer coefficient was identified (k = 9.7 × 10 and the mass transfer coefficient was identified (k = 9.7 × 10 and the mass transfer coefficient was identified (k = 9.7 × 10 and the mass transfer coefficient was identified (k = 9.7 × 10 -5 kg water/m kg water/m kg water/m kg water/m kg water/m 2 /s).

Drying Kinetics of Curcuma longa Rhizomes

Abstract: Modeling drying kinetics is very useful for optimization purposes. Hot air drying kinetics were carried out in monolayer at different temperatures (60°C, 70°C, 80°C, 90°C, and 100°C) and for different sample types (peeled and unpeeled rhizomes of different sizes). Mathematical models based on Fick's law were used to describe water removal, considering different boundary conditions and geometries. Effective moisture diffusivities identified from modeling presented an Arrhenius-type relationship. An additional mass transfer resistance was identified as due mainly to the peridermis layer (peridermial resistance). The accuracy of the model assuming peridermial resistance only in the radial direction and solved using the finite differences method was illustrated, and the mass transfer coefficient was identified (k = 9.7 × 10-5 kg water/m2/s). Keywords: Curcuma longa, drying, modeling, peridermial resistance

Mechanism of mass transfer from bubbles in dispersions: Part II: Mass transfer coefficients in stirred gas–liquid reactor and bubble column

Abstract: Experimental data on the average mass transfer liquid film coefficient ( k L ) in an aerated tank stirred by Rushton turbine and in bubble column are presented. Liquid media were used as 0.8 M sodium sulphite solution, pure or with the addition of Sokrat 44 (copolymer of acrylonitrile and acrylic acid) or short-fiber carboxymethylcellulose (CMC) for the Newtonian and long-fiber CMC for the non-Newtonian viscosity enhancement and ocenol ( cis -9-octadecen-1-ol) or polyethylenglycol (PEG) 1000 for surface tension change. Volumetric mass transfer coefficient ( k L a ) and specific interfacial area ( a ) were measured by the Danckwerts’ plot method. Coefficients k L measured by pure oxygen absorption in pure sulphite solution and Newtonian viscous liquids are well fitted by the “eddy” model in the form of k L = 0.448 ( e v / ρ ) 0.25 ( D / v ) 0.5 with a mean deviation of 20%. Surface-active agents (ocenol and PEG) and non-Newtonian additive (long-fiber CMC) reduced k L value significantly but their effect was not described satisfactorily neither by surface tension nor by surface pressure. It is shown that the decisive quantity to correlate k L in the stirred tank and bubble column is power dissipated in the liquid phase rather than the bubble diameter and the slip velocity. Absorption of air did not yield correct k L data, which did not depend on or slightly decreased with increasing power. This is due to the application of an improper gas phase mixing model for absorption data evaluation.

Stripping of ammonia from aqueous solutions in the presence of carbon dioxide. Effect of negative enhancement of mass transfer.

Abstract: The stripping of ammonia from aqueous solutions in the presence of carbon dioxide in the low concentration region is studied experimentally and theoretically. The phase equilibria method of Edwards et al . (1978) is selected by comparison of different methods with newer experimental data. The differential two adjacent films model enabling component fluxes calculations is used in a stripper model. In the experimental part the systems NH 3 –water–air and CO 2 –water–air are used in order to select the best physical mass transfer coefficients correlations. The system NH 3 –CO 2 –water-air modified with strong acids or bases is used for experimental verification of the process model in a wide range of process variables. Model predictions in conditions of negative enhancement of mass transfer are presented. The influence of this effect on ammonia stripping efficiency in the presence of carbon dioxide is analysed.