Mass transfer solid-liquid, impeller speed

Sykes and Gomezplata13 determined the liquid-solid mass-transfer coefficient for 0.32-cm-diameter spherical particles suspended in stirred aqueous iodine solutions. The particle density was within 5 percent of the liquid density. The effects of impeller speed (200 through 600 rev min- ), Schmidt number (770 through 11,300), and impeller type (fan-disk turbine, propeller, and 45° paddle and turbine) on the mass-transfer coefficient were examined. The data were correlated with an average deviation of 8 percent by the following expression ... 

Maximum stable drop diameter, m Impeller diameter, m Diffusivity of dissolved component or reactant in liquid, m /s Gravitational acceleration, m/s Height of liquid in vessel, m Mass transfer coefficient, m/s Mass transfer coefficient for a single spherical droplet immersed in a liquid flowing at constant velocity past the droplet, m/s Mass of liquid, kg Rate of mass transfer of solute or reactant, kg/s Impeller speed, rotations/s Minimum speed to just suspend solid particles in vessel, rotations/s Minimum impeller speed to completely incorporate dispersed phase into continuous phase in liquid-liquid systems, rotations/s Power dissipation, W Time, s... 

Henry s constant for component x (kmol/m )/(N/m ) reaction rate constant for CO2 hydrolysis (m /kmol/s) gas-liquid mass transfer coefficient (1/s) solid-liquid mass transfer coefficient (m/s) term defined by Equation (CS10.13) (-) molecular weight of component i (kg/kmol) speed of rotation of impeller (rps) critical speed for complete dispersion (rps)... 

At impeller speeds well below the reaction may be gas-liquid and/or solid-liquid mass transfer controlled. If the solid loading (s ) is varied nnder these conditions N N, then only... 

Njs is defined as the minimum impeller speed at which all the solids in the vessel are suspended. This is the speed at which the surface area of all the solids in the vessel are in complete contact with the liquid and hence is an optimal operating point for mass transfer rate in the vessel (Figure 4-19). Solid suspension measurements are very sensitive to the precise shape of the vessel base. Measurements should not be made in a vessel with a drain in the middle... 

Effect of Impeller Speed on Solid-Liquid Mass Transfer... 

Experiments indicate that solid-liquid mass transfer rate increases relatively rapidly with increasing impeller speeds up to the just suspended state, Njs. This is a result of increases in both the interfacial area per volume, Up, and the mass transfer coefficient, kst- Beyond Njs, ap is independent of agitation because all the solid surface available for mass transfer is now exposed, but the mass transfer coefficient, ksL, continues to increase, although at a much lower rate. The overall effect is illustrated in Figure 10-6. 

Impeller diameter (ft, m) diffusivity (ft /h, mVs) mass-mean diameter (ft, m) mean particle diameter of the ith size (ft, m) particle size or diameter (ft, or m) gravitational constant (32.17 ft/sec or 9.81 m/sec ) diffusional mass transfer coefficient rate of diffusional mass transfer impeller speed (rps) number of particles in the ith size class impeller speed for just suspended state of particles (rps) impeller power (hp, W) vessel diameter (ft, m) particle-free settling velocity (ft/s, or m/s) particle-hindered settling velocity (ft/s, or m/s) mass ratio of suspended solids to liquid time 100 (kg solid/kg liquid) X100 liquid depth in vessel (ft, m)... 

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