Showing papers on "Mass transfer coefficient published in 1989"

Modeling the transport of volatile organics in variably saturated media

Abstract: The understanding of the processes of dissolution, volatilization, and gas-liquid partitioning in porous media is very limited. The few models which attempt to characterize the transport of volatile organics such as petroleum products and halogenated hydrocarbon solvents in variably saturated media all assume that mass transfer processes are at equilibrium. In addition, gas phase advection is neglected by assuming that gas phase pressures are uniformly atmospheric and that density gradients are negligible. In this study a model was developed to solve for water phase flow and transport and density dependent gas phase flow and transport. Simple expressions for dissolution, volatilization, and gas-liquid partitioning, employing the concept of an overall mass transfer coefficient, were incorporated into the model. The transport of trichloroethylene in a variably saturated vertical cross section, under a variety of conditions, was simulated. Results of the simulations appeared qualitatively correct. The importance of gas phase processes in increasing subsurface contamination from volatile organics, and in dissipating residual amounts of these substances, was demonstrated. The lack of similar analytical and/or numerical models, or suitable experimental studies, excluded the possibility of validating, or verifying, the model.

Drying of carrots. i. drying models.

Abstract: Three models of different complexity are proposed to describe the falling rate period of the carrot drying process with shrinkage. A moving or fixed boundary problem as well as a constant or local moisture and temperature dependent effective diffusivity are considered. The moving boundary problem is solved by an explicit finite difference method. Heat transfer coefficient and effective diffusivity identification were carried out. The results of the heat transfer coefficient show a good agreement with other sources. Using experimental data and the models. describing the heat and mass transfer three different expressions for the effective diffusivity are established. Two of them are only temperature dependent considering or not particle shrinkage. The third one takes into account temperature and local moisture as well as shrinkage. Drying of foods is a complicated process involving simultaneous coupled heat and mass transfer phenomena which occur inside the material being dried (Chiang and Petersen...

Study of gaseous substrate fermentations: Carbon monoxide conversion to acetate. 2. Continuous culture

Abstract: The fermentation of gaseous substrates such as CO, H2, and CO2 may be performed in a continuous stirred tank reactor, as well as the traditional batch reactor. In this article, the conversion of carbon monoxide by Peptostreptococcus productus is demonstrated in a stirred tank reactor under both mass transfer-controlled and nonmass transfer-controlled conditions. Utilizing a non-steady-state procedure, intrinsic rates are evaluated under non-mass transfer-controlled conditions in a time period of only 5–6 hours. A steady-state procedure was used to evaluate CSTR performance under mass transfer-controlled conditions. The mass transfer coefficient was calculated, followed by the development of a model to predict CSTR behavior for this gas phase substrate.

Suspension-to-Surface heat transfer in a circulating-fluidized-bed combustor

Abstract: Heat transfer coefficients were measured for circulating beds of sand particles of mean size 222 to 229 {mu}m at temperatures of 340-880{degrees}C. Transfer coefficients were obtained for both a 1.22-m-long, 12.7-mm-OD vertical tube and a 1.59-m-long, 148-mm-wide membrane wall near the top of a 152-mm-square by 7.32-m-tall combustion column. For both surfaces and all temperatures, average heat transfer coefficients increased almost linearly with local suspension density which ranged from 0 to 70 kg/m{sup 3}. Radiation played a significant role, especially at high temperatures and low suspension densities. Heat transfer coefficients also varied significantly with the lateral position of the tube. The vertical length of heat transfer surface is shown to be an important parameter allowing seemingly discrepant published results to be reconciled.

Flow-through and flow-by porous electrodes of nickel foam. II. Diffusion-convective mass transfer between the electrolyte and the foam

Abstract: The work described here concerns the diffusion-convective mass transfer to flow-through and flow-by porous electrodes of nickel foam. Empirical correlations giving the product\(\bar k_d a_e\) of the mass transfer coefficient\(\bar k_d\) and the specific surface areaae of the material as a function of the pressure drop per unit electrode height and as a function of the grade characterizing the foam are proposed. The performance of various materials are compared in terms of\(\bar k_d a_e\) vs the mean linear electrolyte flow velocity.

Particle Size and Shape Effects on Adsorption Rate Parameters

Abstract: Individual carbon particles were examined using a Quantimet‐720‐23A programmable and computerized image analyzer to determine their size and shape characteristics. Particles in a given sieve size fraction are well represented by the log‐normal distribution. The use of the Sauter mean of particle diameters from projected area measurements is recommended to account for the effects due to the variations in particle size and shape. The solid phase diffusion coefficient estimated from batch reactor analysis, is found to be invariant with this particle diameter. Power‐law correlations are developed to indicate the effect of batch adsorber parameters and particle size on the external mass transfer coefficient.

Pressure Drop and Mass Transfer in Two-Pass Ribbed Channels

Abstract: The combined effects of the sharp 180-deg turn and of the rib configuration on the pressure drop and mass transfer characteristics in a two-pass square channel with a pair of opposite rib-roughened walls (to simulate turbine airfoil cooling passages) were determined for a Reynolds number range of 10,000-60,000. Heat transfer enhancements were compared for the first pass and for the two-pass channel with the sharp 180-deg turn. Correlations for the fully-developed friction factors and loss coefficients were obtained.

Effect of mass transfer coefficient on the elution profile in nonlinear chromatography

Abstract: The kinetic model of chromatography includes a mass balance equation and a mass transfer rate equation. A numerical analysis of the system of partial differential equations obtained is presented, and the characteristics of the finite difference method used to solve it are described. The results obtained are discussed, and the influence of the value of the mass transfer coefficient on the chromatographic elution profile is analyzed. In the limit case of linear chromatography (linear equilibrium isotherm), the result is very similar to the result obtained from the solution of the ideal nonequilibrium equation. In the other limit case, at large values of the rate constant, the result is very similar to the one obtained by solving numerically the equations of nonlinear equilibrium chromatography.

The Concept of Convective Mass Transfer for Prediction of Surface-Runoff Pollution by Soil Surface Applied Chemicals

Abstract: Amodel describing chemical transfer from soil to surface runoff is presented as a convective mass transfer process through a boundary layer connecting a soil mixing cell to the runoff mixing cell. The boundary layer is assumed to be laminar above the soil surface, restricting chemical transport through this zone to molecular diffusion. The determined mass transfer coefficient is proportional to the chemical diffusion coefficient and inversely proportional to the laminar boundary layer depth. It is increasing with the increase of soil surface roughness, runoff hydraulic gradient and runoff hydraulic radius. A model for chemical runoff effluent concentration is proposed in which rainfall induced surface runoff is represented as a well-mixed reactor. Transfer of soil solutes to the reactor is assumed to occur by a rate limited process proportional to the soil concentration. This approach differs from others in the literature which assume instantaneous equilibrium between runoff and soil solution concentrations. Predicted outflow concentrations were calculated using both the equilibrium and the rate-limited models and the differences were analyzed. The main difference between the two models occurs during the early stages of runoff when the equilibrium model predicts greater loss of chemical to the outflow. The mass transfer model is characterized by two time scales that control the cumulative mass loss to the field outlet. One represents the runoff volume residence time and the other the diffusive soil mixing time.

Mechanisms of mass transfer of dissolved gas from a gas-evolving electrode and their effect on mass transfer coefficient and concentration overpotential*

Abstract: Dissolved gases in the boundary layer of gas-evolving electrodes, in constrast to all other substances generated or consumed at electrodes, undergo two superimposed mass transfer mechanisms. Their interaction results in an increase of the effective mass transfer coefficient of disolved gas. A correction factor to the coefficient calculated from available mass transfer equations is proposed. The effect on the concentration overpotential is discussed.

Numerical analysis of solute transport during transient irrigation: 2. The effect of immobile water

Abstract: In this study, the two-component mobile-immobile water model is modified to describe solute transport under transient water flow induced by periodic cycles of irrigation, evaporation, and redistribution. To characterize transient flow, the soil water content is divided into a time-invariant immobile water content Θi, and a time-dependent mobile water content Θm which obeys the Richards equation. Solute transport is described by the two-component mobile-immobile water model, modified to take into account the time dependence of Θm. Results of the calculations show important differences between the behavior of the flux and residence concentrations under steady state water flow, particularly when the mass transfer coefficient α is relatively small. In this case, both resident and flux concentrations show a dispersion scale effect in their rate of spreading about the center of mass position in the soil. There were significant differences in solute transport predicted by the transient two-component model compared to the steady state model. The apparent velocity derived from the time variations of the position of the center of mass of the resident concentration decreased with time in the transient model, but was relatively constant in the steady state one. Apparent dispersion about the center of mass increased with time in both models, but was much higher in the transient model, caused principally by a buildup of solute at the surface during evaporation cycles.

Increase in viscosity and its influence on polymerization processes

Abstract: In the course of polymerization in homogeneous systems, the viscosity of the reaction mass increases by several orders of magnitude. The viscosity increase is affected by reaction conditions, concentration and properties of the formed polymer. Empirical correlations for homogeneous and heterogeneous polymerizations are given. Viscosity significantly affects the polymerization kinetics as well as heat, mass and momentum balances of the polymerization reactor. The influence of viscosity and its increase on conductive heat transfer, stirrer power input and cooling capacity, molecular diffusion and mass transfer coefficients, mixing time and residence time distribution in homogeneous and heterogeneous polymerizations in stirred tank and tubular reactors is reviewed.

Mass transfer phenomena and hydrodynamics in agitated gas—liquid reactors and bubble columns at elevated pressures: State of the art = Stoffübergangserscheinungen und hydrodynamik in gerührten gas-flüssigkeitsreaktoren und blasensäulen bei hohen drucken: Stand der technik

Abstract: All important studies on the influence of pressure on mass transfer phenomena in gas—liquid systems and reactors are reviewed critically. Points of agreement and conflict are indicated and discussed. It is concluded that: (1) the initial bubble size at a single orifice decreases with increasing pressure; (2) the gas-phase mass transfer coefficient k G is inversely proportional to the pressure to the power n , where n depends on the mass transfer mechanism; (3) the liquid-phase mass transfer coefficient k L is not influenced by pressure; (4) the gas hold-up e G in bubble columns increases with increasing pressure. However, insufficient data on the influence of the operating pressure on the interfacial areas in gas—liquid contactors are available.

Combined effects of rib angle-of-attack and pitch-to-height ratio on mass transfer from a surface with transverse ribs

Abstract: An experimental study is performed to investigate the effects of rib angle-of-attack ($ ) and pitch-to-height ratio (pit) on the mass transfer from a rib-roughened surface. For a given Reynolds number, Re - 4 x 104 distributions of local mass transfer coefficient with p/ e = 5 and 10 and θ = 60° and 90° are obtained in great detail. By integrating the measured local data, the surface with p/ e = 5 and 6 90° is found to have the highest pitch-averaged mass transfer coefficient among the four cases studied. This is in sharp contrast to the conventional notion that a rib surface with p/ e - 10 always yields a higher heat transfer enhancement than with p/ e - 5. A change of 9 from 90° to 60° results in opposite effects on the average mass transfer for pie = 5 and 10. The $ variation also strongly affects the local mass transfer characteristics, as expected. Very high mass transfer coefficients exist immediately ahead of a rib corner. This is speculated to be caused by vortex helical motion and periodic bounda...

Vacuum freeze-drying kinetics and modelling of a liquid in a vial

Abstract: Freeze-drying experiments were carried out for milk and water in a vial, and the influence of the heating plate temperature was examined. A simple simulation based on a 'uniformly retreating ice front' type model was performed. A numerical model enables the changes in both water content and temperature of these products to be described satisfactorily during the experiment. Three transfer parameters (water vapour diffusivity in the dry layer, the external mass transfer coefficient, and the contact resistance to the heat transfer) were determined. The contact resistance between the product and the heating plate is the most important resistance to heat transfer, even controlling the dehydration kinetics.

A Study of Contacting Systems in Water and Wastewater Disinfection by Ozone. 1. Mechanism of Ozone Transfer and Inactivation Related to the Contacting Method Selection

Abstract: The paper proposes that the study of ozone disinfection contacting systems should be associated with the mechanism of the process. The mass transfer rate should be the main controlling factor affecting ozone disinfection efficiency, because the disinfection process is controlled by it. Its function to be played should be closely associated with the performance of contactor selected. It appears that there is a quantitative relationship between the mass transfer coefficient and the inactivation effect. The typical experimental results identify these concepts and some important conclusions are drawn.