Introduction to Mass Transfer and Diffusion

1A Similarity of Mass, Heat, and Momentum Transfer Processes 

Introduction. In Chapter 1 we noted that the various unit operations could be classified into three fundamental transfer (or transport ) processes momentum transfer, heat transfer, and mass transfer. The fundamental process of momentum transfer occurs in such unit operations as fluid flow, mixing, sedimentation, and filtration. Heat transfer occurs in conductive and convective transfer of heat, evaporation, distillation, and drying. 

The third fundamental transfer process, mass transfer, occurs in distillation, absorption, drying, liquid-liquid extraction, adsorption, and membrane processes. When mass is being transferred from one distinct phase to aiiother or through a single phase, the basic mechanisms are the same whether the phase is a gas, liquid, or solid. This was also shown in heat transfer, where the transfer of heat by conduction followed Fourier s law in a gas, solid, or liquid. 

General molecular transport equation. All three of the molecular transport processes of momentum, heat, and mass are characterized by the same general type of equation given in Section 2.3A. 

This can be written as follows for molecular diffusion of the property momentum, heat, and mass  

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