4.11: Untitled Page 66

volume

(4‐64)

with the intention of evaluating

at entrances and exits. The area integral

of the molar flux,

, can be represented as

c v n dA 

c v n dA 

c v n dA

(4‐65)

A

A

A

A

A

A

A

A e

A i

here

w

A e represents the entrances and exits at which convection dominates and A i represents an interfacial area over which diffusive fluxes may dominate. In this text, our primary interest is the study of control volumes having entrances and exits at which convective transport is much more important than diffusive transport, and we have illustrated this type of control volume in Figure 4‐6.

Multicomponent systems

114

There the entrances and exits for both the water and the air are at the top and bottom of the column, while the surface of the control volume that coincides with Figure 4‐6. Entrances and exits at which convection dominates e

th liquid‐solid interface represents an impermeable boundary at which

. For systems of this type, we express Eq. 4‐65 as

(4‐66)

In order to simplify our discussion about the flux at entrances and exits, we direct our attention to an exit and express the molar flow rate at that exit as

115