文档介绍：(P511)
Features:
1. ~ What’s diffusion ?
Diffusion is the movement, under the influence of a physical stimulus(物理驱动), of an ponent(单独的组份) through a mixture.
Molecular diffusion in binary mixtures
(扩散和单向传质)
§3 Diffusion and mass transfer
within single phase
(P511)
2. ~ Why reason?
The most cause of diffusion is a concentration gradient of the ponent.
3. ~ Purpose?
A concentration gradient(浓度梯度) tends to move ponent in such a direction as to equalize concentration and destroy the gradient.
Molecular diffusion in binary mixtures
(扩散和单向传质)
§3 Diffusion and mass transfer
within single phase
(P511)
4. ~ Steady-state flux
When the gradient is maintained by constantly supplying the ponent to the high-concentration end of the gradient and removing it at the low-concentration end, there is a steady-state flux of the ponent.
This is characteristic of many mass-transfer operations.
Molecular diffusion in binary mixtures
5. ~ Other reasons
Although the usual cause of diffusion is a concentration gradient, diffusion can also be caused by an activity gradient(能动梯度), as in reverse osmosis(反渗透), by a pressure gradient, by a temperature gradient, or by the application of an external force field(外场力), as in a centrifuge(离心力场).
Molecular diffusion in binary mixtures
6. What’s thermal diffusion?热扩散
Molecular diffusion induced by temperature gradient is thermal diffusion.
7. What’s the force diffusion? 强制扩散
Molecular diffusion induced by the application of a force from an external filed is forced diffusion.
Molecular diffusion in binary mixtures
Diffusion in a direction 扩散方向
Diffusion in a direction is perpendicular to the interface between the phases and at a definite location in the equipment.
9. Steady state
Steady state is assumed, and the concentrations at any point do not change with time.
Molecular diffusion in binary mixtures
二. Fick’s first law of diffusion for a binary mixture
(P514)(双组份扩散时的费克