Showing papers on "Mass transfer coefficient published in 1992"

Design parameters estimation for scale-up of high-pressure bubble columns

Abstract: Experiments were carried out in bubble columns for a number of liquids at pressures between 0.1 and 2.0 MPa for two column sizes. Based on the experimental results as well as extensive literature data, the extent of the effect column dimensions have on gas holdup were determined, both at low and high pressures (which is of importance to scale-up). It was also demonstrated that none of the published empirical gas holdup equations incorporate the influence of gas density accurately. Therefore, a new improved gas hold-up equation is developed that incorporates the influence of gas and liquid properties with an average error of approximately 10%. Finally, it is also discussed to what extent the influence of pressure on other important design parameters such as the interfacial area, the liquid volumetric mass transfer coefficient, and gas and liquid mixing, can be estimated on the basis of empirical equations.

Liquid-phase mass transfer coefficients in bioreactors.

Abstract: Liquid-phase mass transfer coefficient in bioreactors have been examined. A theoretical model based on the surface renewal concept has been devloped. The predicted liquid-phase mass transfer coefficients are compared with the experimental data for a mycelial fermentation broth (Chaetomium cellulolyticum) and model media (carboxymethyl cellulose) in a bench-scale bubble column reactor. The liquid-phase mass transfer coefficient is evaluated by dividing the volumetric mass transfer coefficient obtained experimentally by the specific surface area estimated using the available correlations. The available literature data in bubble column and stirred tank bioreactors is also used to test the validity of the proposed model. A reasonable agreement between the model and the experimental data is found.

Membrane-Based Solvent Extraction

Abstract: Solvent extraction is a common industrially used equilibrium-based separation process. In such a process, a solute (or solutes) in a solution, aqueous or organic, is extracted into an immiscible solvent, organic or aqueous, by dispersing one of the immiscible phases as drops in the other phase. This creates a large interfacial area and increases the extraction rate considerably. After the extraction is over, the phases are separated and the dispersed phase coalesced. There are two general categories of equipment for solvent extraction. A mixer-settler arrangement provides a single equilibrium stage; a number of them connected together provide multistage extraction. Continuous countercurrent contacting equipment whether in the form of columns or centrifugal devices can generate the equivalent of many stages in one device (Treybal 1963).

Xanthan production in stirred tank fermenters: Oxygen transfer and scale‐up

Abstract: The microbial polysaccharide xanthan was produced by cultivation of Xanthomonas campestris in four stirred tanks with volumes ranging from 0.072 to 3 m3. Many of the previously suggested correlations for the volumetric mass transfer coefficient described the present data very unsatisfactorily. When Rushton turbines were employed as stirrers, the experimental results agreed well with a correlation suggested by Kawase and Moo-Young; for the more energy-efficient INTERMIG impellers, this correlation was modified by a factor of three. Constant specific oxygen transfer rate was found to be the appropriate scale-up criterion for constant space-time yield and constant product quality, specifically, the molar mass which determines the viscosity yield.

Heat transfer in the flow of a viscoelastic fluid over a stretching sheet

Abstract: The problem of heat transfer in the viscoelastic fluid flow over a stretching sheet is examined. The important physical quantities such as the skin-friction coefficient and the heat transfer coefficient, are determined. It is found that the heat transfer coefficient decreases with the non-Newtonian parameter.

Determination of Heat and Mass Transfer Coefficients in Thin Layer Drying of Grain

Abstract: Five models simulating the process of simultaneous heat and mass transfer in the drying of a layer of barley are formulated By using the inverse method, the transfer coefficients for all five models are estimated from measured values of instantaneous surface temperature and average moisture content A finite element method is used to solve the nonlinear coupled system of two partial differential equations modeling the drying process It is concluded that the mass transfer coefficient is 108 ¥ 10–6 ms–1 for all five models, and that this number is much smaller than that calculated from the Lewis relation The heat transfer coefficient is found to vary from 43 to 59 Wm–2 K–1, depending on the form of the drying model

Model selection in air drying of foods

Abstract: The purpose of this investigation is to compare various drying models with respect to (a) the accuracy in calculating the material moisture content and temperature versus time and (b) the computation time required. Mechanistic as well as phenomenological heat and mass transfer models are considered. The mechanistic models are formulated by considering different combinations of mechanisms between (1) moisture diffusion in the solid towards its external surface (2) vaporization and convective transfer of the vapor into the air stream (3) convective heat transfer from the air to the solid's surface (4) conductive heat transfer within the solid mass. The phenomenological model incorporates the drying constant while the mechanistic models incorporate the mass diffusivity, the mass transfer coefficient in the air boundary layer, the thermal conductivity, and the heat transfer coefficient in the air boundary layer. The proposed methodology is applied to experimental data of four vegetables, namely, pota...

Hydraulic Permeability, Mass Transfer, and Retention of PEGs in Cross-flow Ultrafiltration through a Symmetric Microporous Membrane

Abstract: The flow and retention of 0.1% w/w aqueous solutions of several polyethylene glycols with molecular weights ranging from 3,000 to 35,000 dalton are studied when they are tangentially filtered through a nuclear track-etched symmetric microporous membrane made from polycarbonate with transmembrane pressure differences going up to 200 kPa. The work was done within the framework of the film layer theory for the concentration polarization phenomenon which allows one to obtain the mass transfer coefficient for the cell used as a function of the feed circulation speed and the molecular weight of the solute. The retention curves obtained lead to a sieve radius smaller than the nominal one.

Effect of Slag Foaming on the Reduction of Iron Oxide in Molten Slag by Graphite

Abstract: A kinetic study has been made on the reduction of iron oxide in molten slag, held in an alumina crucible under argon gas atmosphere, with graphite, cylindrical in shape. The composition of the primary slag was 20.5%Li2O-38.4%CaO-41.1%SiO2 (molar ratio=1:1:1). The initial concentration of iron oxide was changed between 3% and 12.5%. The experimental temperature was 1300°C. The rotation speed of the graphite cylinder was varied from zero to 1900 rpm. The reaction rate was largely affected by slag foaming. At higher FeO concentrations, the apparent rate constant was almost independent of the mechanical stirring. At lower concentrations, the effect of the rotation on the apparent rate constant was found to be small at lower speeds but tended to become larger at higher speeds. This was an indication that the melt was strongly agitated by CO gas bubble evolution. The effect of foaming is taken into account by introducing gas holdup factor into ordinary (foam-free) kinetic equations. Values of the mass transfer coefficient of oxygen in the molten slag, calculated by applying the penetration theory, are close to those of the apparent rate constant, measured experimentally. This implies that the rate of the reduction is controlled predominantly by the mass transfer in the slag phase.

Multiphase hydrodynamics and solid-liquid mass transport in an external-loop airlift reactor : a comparative study

Abstract: The solid-solid mass transfer performance of an external-loop airlift reactor was measured by dissolution of benzoic acid coated on nylon-6 particles, and the hydrodynamics of the gas-liquid-solid multiphase system in the airlift reactor were investigated. The solid-liquid system was designed to simulate the micro-carrier culture of animal cells, and some typical suspensions of immobilized enzyme particles. The solid-liquid mass transfer coefficient remained constant below a superficial air velocity of 0.04 ms−1 for the particles examined, but increased rapidly with further increase in gas velocity. Solids loading (0.3-3.5% w/w) did not affect the mass transfer coefficient in turbulent flow. The mass transfer coefficient was correlated with energy dissipation rate in the airlift reactor. The mass transfer coefficient in stirred vessels, bubble columns, fluidized beds, and airlift reactors was compared. Over an energy dissipation Reynolds number of 4-400, the solid-liquid mass transfer coefficient...

The role of mass transfer in the electrolytic reduction of hexavalent chromium at gas evolving rotating cylinder electrodes

Abstract: The rate of electrolytic reduction of hexavalent chromium from acidic solution at a hydrogen-evolving rotating cylinder lead cathode was studied under conditions of different current densities, Cr6+ concentrations and rotation speeds. The rate of the reaction was found to follow a first order rate equation. The specific reaction rate constant was found to increase with increasing rotation speed until a limiting value was reached with further increase in rotation speed. Mechanistic study of the reaction has shown that at relatively low rotation speeds the reduction of Cr6+ is partially diffusion controlled, at higher speeds the reaction becomes chemically controlled. The limiting specific reaction rate constant was related to the operating current density by the equationK=0.044i 1.385. The current efficiency of Cr6+-reduction was measured as a function of current density, initial Cr6+ concentration and rotation speed. Possible practical applications are discussed.

Effect of slag composition on the kinetics of the reduction of iron oxide in molten slag by graphite

Abstract: A kinetic study has been made on the reduction of iron oxide in molten slag with graphite. The composition of the primary slag was changed so that SiO2 concentration varied between 0.33 and 0.50 mole fraction, and slag basicity between 1 and 2. The experimental temperature was 1300°C. The reaction rate is significantly affected by the slag composition. From values of the mass transfer coefficient, calculated by using penetration theory, it is presumed that for the slags with basicity of 2, the reaction rate is controlled by mass transfer in the slag phase, but chemical reaction resistance is predominant in slags with lower basicities. By applying a mixed-control model to the latter case, the apparent chemical reaction rate constant was calculated. The rate constant decreases largely with increasing the silica activity. The reaction rate also decreases with the presence of phosphorus in the slag. These behaviors indicate that the reaction rate is very sensitive to the interfacial chemisorption of the surface active agents.