Transfer Resistance of Adsorbent Particles

The condition of no slip at a solid boundary means that each particle in the bed is surrotinded by a laminar sublayer, through which mass transfer occurs by molecumdiffusion. The thickness of the laminar sublayer, and hence the mass transfer Coefficient, is determined by the hydrodynamic conditions. It is convenient to correlate mass transfer rates in terms of an effective mass transfer coefficient (kj), defined according to a linear driving force equation  

The appropriate dimensionless group characterizing film mass transfer is the Sherwood [number, defined by Sh = IR kjfD which is the analog of the Nusselt number for heat transfer. A simple analysis of heat conduction from 

Experimental mass transfer data at low Reynolds numbers show a great deal of scatter, but most of the reported values fall somewhat bi ow the limit of 2.0 predicted from the Ranz-Marshall correlation. For examii le, expressed in terms of Sherwood number, the correlation of Petrovic and iTiodos for gases is equivalent to 

FIGURE 7A Correlation of Sherwood number with for gas and liquid systems. 

Equation (7.16) is similar to the Ranz-Marshall correlation [ q. (7.13)] and shows the sairi limiting behavior at low Reynolds number, but both the coefficient and power of the Reynolds number are somewhat larger. In the application of this expression it is important to note that axial dispersion coefficients mu t be properly estimated [e.g., according to Eq. (7.11) if the isotherm is hi ly favorable] otherwise the overall dispersion arising from external mass transfer resistance and axial mixing may be underestimated. 

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