Effect of Impeller Speed on Solid-Liquid Mass Transfer

2 Effect of Impeller Speed on Solid-Liquid Mass Transfer 

Many authors (Nienow, 1975 Nienow and Miles, 1978 Chaudhari, 1980 Conti and Sicardi, 1982) have reported the effect of agitation on the diffusional mass transfer coefficient, ksLUp. It is sufficient to say that the diffusional mass transfer rate is affected primarily by the impact of agitation on the hydrodynamic environment near the surface of the particle, in particular the thickness of the diffusional boundary layer surrounding the solid. The hydrodynamic environment near the particle surface depends on the properties of the fluid properties as well as those of the particles. The specific variables were introduced in Section 10-2.1.1. In addition to these, the diffusivity. Da, also influences the diffusional mass transfer. 

The important hydrodynamic variables are the relative velocity. Vs, between the solids and the liquid (also know as slip velocity) and the rate of renewal of the liquid layer near the solid surface. The relative velocity, Vg, obviously varies from point to point within the vessel, and the average value is difficult to estimate. So, in practice, the relative velocity. Vs, is assumed equal to the free settling velocity, Vt. The renewal of the boundary layer depends on the intensity of turbulence around the solid particle as well as the convective velocity distribution in the vessel. 

In general, the specific impact of agitation must be determined experimentally for each system. The correlations discussed below are presented to provide a guide and insight into the expected effects of various variables on solid-liquid mass transfer. 

Relative mass transfer as function of impeller power 

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