第17章相际扩散与传质原理 概 Diffusion is the: movement 扩散是一种 under the influence of a 在物理驱动下单 physical: stimulus, of an 组分通过混合物 individual component through 的运动。扩散最 a mixture. The most common cause of diffusion is a 通常的原因是扩 concentration gradient of the散组分浓度梯度 diffusing: component 的存在
第17章 相际扩散与传质原理 概 述 扩散是一种 在物理驱动下单 组分通过混合物 的运动。扩散最 通常的原因是扩 散组分浓度梯度 的存在。 Diffusion is the movement, under the influence of a physical stimulus, of an individual component through a mixture. The most common cause of diffusion is a concentration gradient of the diffusing component
concentration浓度梯度驱使扩 gradient tends to move散组分朝着使浓 the component in such a 度均匀化的方向 direction as to equalize 扩散,从而使浓 concentrations and destroy the gradient. 度梯度趋于消失
A concentration gradient tends to move the component in such a direction as to equalize concentrations and destroy the gradient. 浓度梯度驱使扩 散组分朝着使浓 度均匀化的方向 扩散，从而使浓 度梯度趋于消失
B::When: the gradient: is 当不断地向 maintained: by constantly supplying the: diffusing 高浓度一端提供 component: to: the high 扩散组分,从低 concentration: end::of the 浓度一端除去扩 gradient and: removing: it at the: ow: concentration 散组分时,扩散 end, there: is: a: steady-state组分便具有稳定 flux: of the diffusing 的传质通量。这 component:: This is 就是许多传质操 characteristic: of many mass-transfer: operations 作的特点
❖ When the gradient is maintained by constantly supplying the diffusing component to the highconcentration 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. 当不断地向 高浓度一端提供 扩散组分，从低 浓度一端除去扩 散组分时，扩散 组分便具有稳定 的传质通量。这 就是许多传质操 作的特点
For example, when ammonia is例如,当在填料塔 removed from a gas b 中用水吸收气体中 absorption in water in a packed的NH3时,在塔内 column, at each point in the 的任意一点上,在 column a concentration gradient气相浓度梯度的驱 in the gas phase causes ammonia使下,NH3由气相 to diffuse to the gas-liquid 主体扩散至气~液 interface, where it dissolves,and界面,在界面上溶 a gradient in the liquid phase 解,然后,在液相 causes it to diffuse into the bulk浓度梯度驱使下, liquid 扩散至液相主体
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。 For example, when ammonia is removed from a gas by absorption in water in a packed column, at each point in the column a concentration gradient in the gas phase causes ammonia to diffuse to the gas-liquid interface, where it dissolves, and a gradient in the liquid phase causes it to diffuse into the bulk liquid
In stripping a solute from a 当从吸收液中 解吸溶质时, liquid the gradients are 浓度梯度与吸 reversed: here diffusion 收过程相反 brings solute from the bulk在此,扩散作 liquid to the interface and 用使溶质从液 相主体移至界 from there into the gas 面,再从界面 phase. 移至气相
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. 当从吸收液中 解吸溶质时， 浓度梯度与吸 收过程相反。 在此，扩散作 用使溶质从液 相主体移至界 面，再从界面 移至气相
In some other mass 在其它一些诸如 transfer operations such浸取和吸咐操作 as leaching and 中发生的是不稳 adsorption, unsteady- 定扩散,随着过 state diffusion takes 程趋于平衡,扩 lace, and the gradients pl and fluxes decrease with散组分的浓度梯 time as equilibrium is度和通量随时间 approached. 而降低
In some other masstransfer operations such as leaching and adsorption, unsteadystate diffusion takes place, and the gradients and fluxes decrease with time as equilibrium is approached. 在其它一些诸如 浸取和吸咐操作 中发生的是不稳 定扩散，随着过 程趋于平衡，扩 散组分的浓度梯 度和通量随时间 而降低
Although the usual cause of 虽然扩散的一般 diffusion is a concentration 原因是浓度梯度,但 gradient, diffusion can also be 能动梯度也能引起扩 caused by an activity gradient. as in reverse osmosis, by a 散如反渗透过程,压 pressure gradient, by a 力梯度、温度梯度和 temperature gradient,orby外场力都可引起扩散, the application of an external如离心分离。 force field, as in a centrifuge
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. 虽然扩散的一般 原因是浓度梯度，但 能动梯度也能引起扩 散如反渗透过程，压 力梯度、温度梯度和 外场力都可引起扩散， 如离心分离
Molecular diffusion induced由温度引起的分子 by temperature is thermal 扩散称为热扩散, diffusion, and that from an 由外场力引起的扩 external field is forced 散称为强制扩散。 diffusion. Both are 这两种扩散方式在 uncommon in chemical 化学工程中并不常 见。本章仅讨论由 engineering. Only diffusion 浓度梯度引起的质 under a concentration 量扩散。 gradient is considered in this chapter
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. 由温度引起的分子 扩散称为热扩散， 由外场力引起的扩 散称为强制扩散。 这两种扩散方式在 化学工程中并不常 见。本章仅讨论由 浓度梯度引起的质 量扩散
Diffusion is not restricted 扩散并不局 to molecular transfer限于分子通过固 through stagnant layers体或液体停滯膜 of solid or fluid. t also的传递。当不同 takes place when fluids组成的流体相混 of different compositions合时也会发生扩 are mixed 散
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. 扩散并不局 限于分子通过固 体或液体停滞膜 的传递。当不同 组成的流体相混 合时也会发生扩 散
The first step in mixing 混合过程首先 is often mass transfer 是因湍流的涡流特 caused by the eddy motion 性引起的质量传递。 characteristic of turbulent 这一过程被称为涡 flow. This is called eddy diffusion. The second step 流扩散。其次是在 is molecular diffusion 微小的漩涡间及其 between and inside the 内部的分子扩散。 very small eddies
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. 混合过程首先 是因湍流的涡流特 性引起的质量传递。 这一过程被称为涡 流扩散。其次是在 微小的漩涡间及其 内部的分子扩散