Chemical Engineers' Portable Handbook
Emphasizing practice-oriented design and approaches, this guide offers a presentation of the salient and important aspects of chemical engineering for practicing professionals.
TABLE OF CONTENTS Chemical Engineers' Portable Handbook
Chapter 5: Mass Transfer
Introduction
The transfer of mass is an important and integral part of chemical and petroleum processes. It is, for example, the basis of the many separation technologies used in these processes.
In heat transfer the driving force was a temperature difference. For ordinary (concentration based) diffusion a concentration difference brings about a molar or mass flux. Unlike heat transfer, there can be other driving forces, as shown in Table 5-1.
| Category | Driving Force | Example |
|---|---|---|
| 1. Ordinary diffusion | Concentration difference | Absorbers distillation |
| 2. Forced diffusion | External force | Electrochemical devices |
| 3. Pressure diffusion | Pressure differences | Ultra-centrifuge |
| 4. Thermal diffusion | Temperature difference | Clusius-Dickel column |
The basic equation describing ordinary diffusion is Fick's first law
| (5-1) |  |
where i, j, and k are unit vectors in the x, y, and z direction and J A is the mass flux vector.
In defining J A, it is stipulated that this flux must be referred to a plane across which there is no net volume transport (i.e., the plane moves with respect to the fixed apparatus).
For process work we define a new flux N which moves relative to the apparatus. The relation between J A and N A is given by equation 5-2.
| (5-2) |  |
where v A and v B are partial molal volumes. The mass transfer equivalent to the equation of energy, equation 4-3, is the equation of continuity of species.
| (5-3) |  |
where R A
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