Mass transfer surface

M ass Transfer. Mass transfer in a fluidized bed can occur in several ways. Bed-to-surface mass transfer is important in plating appHcations. Transfer from the soHd surface to the gas phase is important in drying, sublimation, and desorption processes. Mass transfer can be the limiting step in a chemical reaction system. In most instances, gas from bubbles, gas voids, or the conveying gas reacts with a soHd reactant or catalyst. In catalytic systems, the surface area of a catalyst can be enormous. Eor Group A particles, surface areas of 5 to over 1000 m /g are possible. 

If a concentration gradient exists within a fluid flowing over a surface, mass transfer will take place, and the whole of the resistance to transfer can be regarded as lying within a diffusion boundary layer in the vicinity of the surface. If the concentration gradients, and hence the mass transfer rates, are small, variations in physical properties may be neglected and it can be shown that the velocity and thermal boundary layers are unaffected 55. For low concentrations of the diffusing component, the effects of bulk flow will be small and the mass balance equation for component A is ... 

MiUy, P. J., Toledo, R. T., Chen, J., and Kazem, B. (2007). Hydrodynamic cavitation to improve bulk fluid to surface mass transfer in a nonimmersed ultraviolet system for minimal processing of opaque and transparent fluid foods. /. Food Sci. 72, M407-M413. 

Again, care has to be taken for the non-ideal (or real) behavior of the measurement system. Applications are limited by non-specific absorption of molecules on the surface, mass transfer effects (under conditions of laminar flow a 1-5-pm layer between sensor surface and volume flow is not whirled and has to be passed by passive diffusion) or limited access for the immobilized molecules [158-160]. 

Roughness values, 15 203 Rough surfaces, mass transfer at,... 

Figure 9.10. Characterization of the surface mass transfer coefficients for the tank tests. The 95% confidence interval is included (Shierholz et al., 2006). CB, coarse bubble FB, fine bubble LACSD, Los Angeles County Sanitary District WES, U.S. Army Engineer Waterways Experiment Station.

Based on the Sherwood number definition and the solutions for a range of inlet velocities, evaluate the surface mass transfer (kg/m2 s) of gallium to the surface. Assume that only gallium remains on the surface and that methyl groups desorb back to the gas,... 

For gas-solid fluidized beds, Wen and Fane (1982) suggested that the determination of the bed-to-surface mass transfer coefficient can be conducted by using the corresponding heat transfer correlations, replacing the Nusselt number with the Sherwood number, and replacing the Prandtl number by Sc(cpp)/(cpp)/(l — a). Few experimental results on bed-to-surface mass transfer are available, especially for gas-solid fluidized beds operated at relatively high gas velocities. 

For unit total surface, mass transfer (mol/area time) = / kt 1 2(sQ st)dtACA... 

Sengupta, T.K. and Sinha, A.P. (1995). Surface mass transfer A receptivity mechanism for boundary-layers. In Proc. 6 Asian Cong. Fluid Mech., (Eds. Y.T. Chew and C.P. Tso), 1242-1245. 

A voltammetric sensor is characterized by the current and potential relationship of an electrochemical cell. Voltammetric sensor utilizes the concentration effect on the current-potential relationship. This relationship depends on the rate by which the reactant (commonly the sensing species) is brought to the electrode surface (mass transfer) and the kinetics of the faradaic or charge transfer reaction at the electrode surface. In an electrochemical reaction, the interdependence between the reaction kinetics and the mass transfer processes establishes the concentration of the sensing species at the electrode surface relative to its bulk concentration and, hence, the rate of the faradaic process. This provides a basis for the operation of the voltammetric sensor. 

High frozen food quality due to high freezing rate fine-grain crystal structure sharp reduction of the surface mass transfer easy incorporation of antioxidants, flavors, aromas, and micronutrients and improved nutritional value and sensory properties. 

There is a significant contrast here with Section 5.4.2(e), where we found that the results for reversible systems observed at spherical electrodes could be extended generally to electrodes of other shapes. This is true for a reversible system because the potential controls the surface concentration of the electroactive species directly and keeps it uniform across the surface. Mass transfer to each point, and hence the current, is consequently driven in a uniform way over the electrode surface. For quasireversible and irreversible systems, the potential controls rate constants, rather than surface concentrations, uniformly across the surface. The concentrations become defined indirectly by the local balance of interfacial electron-transfer rates and mass-transfer rates. When the electrode surface is not uniformly accessible, this balance varies over the surface in a way that is idiosyncratic to the geometry. This is a complicated situation that can be handled in a general way (i.e., for an arbitrary shape) by simulation. For UME disks, however, the geometric problem can be simplified by symmetry, and results exist in the literature to facilitate the quantitative analysis of voltammograms (12). 

FIGURE 6.15 Reduction of the local mass transfer coefficient by surface mass transfer of a foreign gas [31]. (Reprinted by permission of Pergamon Press.)... 

Along the stagnation line of a swept cylinder, the recovery enthalpy is less than the stagnation enthalpy. Thus, the effect of surface mass transfer on the recovery factor, as shown in Fig. 6.25, should be considered in establishing the proper driving potential for the... 

Sometimes it is more convenient to scale the transpiration rate to c/0, the skin friction coefficient in the absence of surface mass transfer. With... 

The aforementioned theories show very small effects of Mach number on the normalized skin friction and heat transfer data cflcfa and St/Sto, respectively. Some experiments [118] support these results, while others [119,120,121] show surface mass transfer to be less effective in reducing skin friction and heat transfer as Mach number increases. In view of these differences, it is necessary to rely on the aggregate of data presented in Ref. 109 to assess the uncertainties involved in a particular approach. 

FIGURE 6.44 Compressibility effects on the reduction of the Stanton number by surface mass transfer on bodies with zero axial pressure gradient and including effects of foreign gas injection. 

The character of the bubbles (i.e. shape and size) is also likely to be affected by the presence of the deposit. For instance the so-called wick boiling mechanism mentioned earlier, is likely to play an important role in the heat transfer process. The evaporation may be considered to take place at the bottom of the steam chimney or on the walls of that channel. If the steam chimneys are absent as might be the case with small pore size or without interconnecting chaimels, heat transfer is only possible by conduction. It could also be possible to consider that the liquid film was directly on the heating surface. Mass transfer rather than heat transfer might also form the basis of a mathematical model. 

Figure 3 shows that beyond 750 rev/min for the hydrolysis (curve A) and beyond S(X)rev/min for oxidation the speed (curve B) had no effect on conversion and hence on the rates of reaction, thereby indicating absence of liquid-to-membrane surface mass transfer resistance both inside and outside the capsules. The reaction could be taken as kinetically controlled and governed by eq.(5) beyond the said speeds in each case. This was further confirmed by studying the effect of temperature and the values of activation energies, which will be discussed later. Since the capsules were well dispersed in the agitated outer phase the bulk concentration of benzyl chloride within a capsule would be uniform. Further experiments were conducted beyond these speeds which were safe to maintain the fidelity of the capsules. 

Cag ) comes into contact with the surface of concrete(take x = 0 at the surface). (2) Diffusive CO2 (molar concentration, Ca2 diffusion coefficient, Da2 in concrete) feels the mass transfer resistance at the surface of concrete (surface mass transfer coefficient, Kg g), unless the equilibrium Ca2 = al satisfied ( k-ag + ). The retardation effects of finishes on the diffusion of CO2 (thickness,... 

Retardation Effects of Polymeric Finishes on Carbonation. The retardation effects of polymeric finishes can be well explained, using equations 9. and 12., where I is the constant term which includes total surface mass transfer constant, and describes the retardation of carbonation or induction period before the carbonation begins to proceed. From equation 9. it can be seen that the effects depend on the thickness and diffusion coefficients of finishes (Figure 13). 

Figure 13. Influence of the surface mass transfer resistance on the progress of carhonation under accelerated condition.

Slurry reactors are commonly used in situations where it is necessary to contact a liquid reactant or a solution containing the reactant with a solid catalyst. To facilitate mass transfer and effective utilization of the catalyst, one usually suspends a powdered or granular form of the catalyst in the liquid phase. This type of reactor is useful when one of the reactants is normally a gas at the reaction conditions and the second reactant is a liquid (e.g., in the hydrogenation of various oils). The reactant gas is bubbled through the liquid, dissolves, and then diffuses to the catalyst surface. Mass transfer limitations on reaction rates can be quite significant in those instances where three phases (the solid catalyst and the liquid and gaseous reactants) are present and necessary to proceed rapidly from reactants to products. 

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