Mass transfer solid-liquid mixing

The recommended correlations for gas-liquid and solid-liquid mass transfer for the different regimes are summarized in Table 17.6. The effect of liquid back-mixing is usually unimportant. 

Sicardi S, Conti R, Baldi G, Cresta R. (1979) Solid-liquid mass transfer in stirred vessels. In Proceedings of Third European Conference on Mixing, England, p 217-228. 

As depicted in Figure 2.2, the dissolved gaseous reactant has to overcome the resistance offered by the liquid-solid film before it reaches the solid catalyst surface where the reaction between the adsorbed reactive species takes place. The intrinsic capacity of a catalyst is realized when all mass transfer processes are at equilibrium. Therefore, it is required to know the rate of the solid-liquid mass transfer step. Such an estimate should reveal the relative importance of this step and also establish the controlling step in an overall process. In a multiphase system, the mass transfer between the liquid and particulate phases is considered to be good when there is an intimate mixing between the two phases. In the case of solid-liquid mass transfer, the minimum desirable condition is suspension of the solid in the liquid or in the gas-liquid dispersion as the case may be. 

For type B experiments. Criteria I-VIII should be checked. If one of these criterion are not met, then consider decreasing the mixing time 9m or increasing the mass-transfer rates between phases, that is i g-i, and i s i by some means. For example, if criteria VII and VIII are not met, the primary options are (a) use more finely divided material or (b) add baffles to the vessel, since a dramatic increase in the solid-liquid mass-transfer coefficient will result. 

Another class of applications is the high shear mixers used to break up agglomerates of particles as well as to cause rapid dissolving of solids into solvents. A further type includes the catalytic processes such as hydrogenation, in which there is a basic gas-liquid mass transfer to be satisfied, but in addition, effective mixing and shear rate on the catalyst particle fluid film as well as degradation must be considered. 

The hydrodynamic parameters that are required for stirred tank design and analysis include phase holdups (gas, liquid, and solid) volumetric gas-liquid mass-transfer coefficient liquid-solid mass-transfer coefficient liquid, gas, and solid mixing and heat-transfer coefficients. The hydrodynamics are driven primarily by the stirrer power input and the stirrer geometry/type, and not by the gas flow. Hence, additional parameters include the power input of the stirrer and the pumping flow rate of the stirrer. 

Often, foods must be treated as engineering materials. Heat has to be transported so that the components become cooked or harmful microorganisms and toxins become inactivated. Even in the kitchen, mixing involves the mass transfer of liquids and solids to form metastable structures, which are fixed by subsequent treatment by heat or cooling. Heat transfer properties are crucial in the formation of ice crystals in ice cream and fat crystals in chocolate products. Food materials have been used as a source of industrial components soybean proteins to manufacture auto parts in the 1940s, casein to make buttons and knitting pins, and starches in adhesives and thickeners. 

In general, mass transfer is not limited by diffusion rates in the supercritical phase. In fact, significant buoyancy effects in supercritical fluids enhance mass transfer rates with convective mixing (58). Usually, mass transfer limitations will occur in either a liquid or solid phase. 

In the case of the mixed kineticsthe diffusion mass transfer in the solid and liquid phases occur with comparable rates) on an ideally flat electrode with = 0 in the presence of a solid phase adsorption effect the diffusion problem (2)-(12) is reduced to the onedimensional task. Its solution is obtained using the method of the integral Laplace-Carson transformation in the form of concentration profiles and voltammogram ... 

Gas—solids fluidization is the levitation of a bed of solid particles by a gas. Intense soflds mixing and good gas—soflds contact create an isothermal system having good mass transfer (qv). The gas-fluidized bed is ideal for many chemical reactions, drying (qv), mixing, and heat-transfer appHcations. Soflds can also be fluidized by a Hquid or by gas and Hquid combined. Liquid and gas—Hquid fluidization appHcations are growing in number, but gas—soHds fluidization appHcations dominate the fluidization field. This article discusses gas—soHds fluidization. 

An important mixing operation involves bringing different molecular species together to obtain a chemical reaction. The components may be miscible liquids, immiscible liquids, solid particles and a liquid, a gas and a liquid, a gas and solid particles, or two gases. In some cases, temperature differences exist between an equipment surface and the bulk fluid, or between the suspended particles and the continuous phase fluid. The same mechanisms that enhance mass transfer by reducing the film thickness are used to promote heat transfer by increasing the temperature gradient in the film. These mechanisms are bulk flow, eddy diffusion, and molecular diffusion. The performance of equipment in which heat transfer occurs is expressed in terms of forced convective heat transfer coefficients. 

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