气体吸收课件.pptxVIP

相际扩散与传质原理  概 述; A concentration gradient tends to move the component in such a direction as to equalize concentrations and destroy the gradient. ;When the gradient is maintained by constantly supplying the diffusing component to the high-concentration end of the gradient and removing it at the low concentration end, there is a steady-state flux of the diffusing component. This is characteristic of many mass-transfer operations. ;例如，当在填料塔中用水吸收气体中的NH3时，在塔内的任意一点上，在气相浓度梯度的驱使下，NH3由气相主体扩散至气～液界面，在界面上溶解，然后，在液相浓度梯度驱使下，扩散至液相主体。 ;In stripping a solute from a liquid the gradients are reversed; here diffusion brings solute from the bulk liquid to the interface and from there into the gas phase. ;In some other mass-transfer operations such as leaching and adsorption, unsteady-state diffusion takes place, and the gradients and fluxes decrease with time as equilibrium is approached. ;Although the usual cause of diffusion is a concentration gradient, diffusion can also be caused by an activity gradient, as in reverse osmosis, by a pressure gradient, by a temperature gradient, or by the application of an external force field, as in a centrifuge.;Molecular diffusion induced by temperature is thermal diffusion, and that from an external field is forced diffusion. Both are uncommon in chemical engineering. Only diffusion under a concentration gradient is considered in this chapter.;Diffusion is not restricted to molecular transfer through stagnant layers of solid or fluid. It also takes place when fluids of different compositions are mixed. ; The first step in mixing is often mass transfer caused by the eddy motion characteristic of turbulent flow. This is called eddy diffusion. The second step is molecular diffusion between and inside the very small eddies. ; Sometimes the diffusion process is accompanied by bulk flow of the mixture in a direction parallel to the direction of diffusion.;In all the mass-transfer operations, diffusion occurs in at least one phase a