过程装备与控制工程专业英语翻译13_控制专业英语翻译

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Reading Material 13

Principles of Ma Transfer

1.General Remarks

Some of the most typical chemical engineering problems lie in the field of ma transfer.A distinguishing mark of chemical engineer is his ability to design and operate equipment in products is prepared, chemical reactions take place, and separations of the resulting products are made.This ability rests largely on a proficiency in the science of ma transfer.Applications of the principles of momentum and heat transfer are common in many branches of engineering, but the application of ma transfer has traditionally been largely limited to chemical engineering.Other important applications occur in metallurgical procees, in problems of high-speed flight, and in waste treatment and pollution-control procees.Eddy diffusion is apparent in the

diipation of smoke from a smokestack.Turbulence causes mixing and transfer of the smoke to the surrounding atmosphere.In certain locations where atmospheric turbulence is lacking, smoke originating at the surface of the earth is diipated largely by molecular diffusion.This cause serious pollution problems because ma is transferred le rapidly by molecular diffusion than by eddy diffusion.4.Convective Ma-Transfer Coefficients

In the study of heat transfer we found that the solution of the differential energy balance was sometimes cumbersome or impoible, and it was convenient to expre the rate of heat flow in qh(tstm)Aterms of a convective heat-transfer coefficient by an equation like

The analogous situation in ma transfer is handled by an equation of form

NAkP(AsAm)

The ma flux NA is measured relative to a set of axes fixed in place.The driving force is the difference between the conversation at the phase boundary(a solid surface or a fluid interface)and the concentration at some arbitrarily defined point in the fluid medium.The convective coefficient kPmay apply to forced or natural converction;there are no ma-transfer counterparts for boiling, condensation, or radiation heat-transfer coefficients,the value of kP is a function of the geometry of the system and the velocity and properties of the fluid, just as was the coefficient h.3.Eddy Diffusion

Just as momentum and energy can be transferred by the motion of finite parcels of fluid, so ma can be transferred.We have seen that the rate of these transfer operations, caused by bulk mixing in a fluid, can be expreed in terms of the eddy kinematics viscosity, the eddy thermal diffusivity, and the eddy diffusivity.This latter quantity can be related to a mixing length which is the same as that defined in connection with momentum and energy transfer.In fact, the analogy between heat and ma transfer is so straightforward that equations developed for the former are often found to apply to the latter by a mere change in the meaning of the symbols.Molecular diffusion also occurs in liquids and solids.Crystals in an unsaturated solution diolve,with subsequent diffusion away from the solid-liquid interface.Diffusion in solids is of importance in metallurgical operations.When iron which is unsaturated with respect to carbon is heated in a bed of coke, the concentration of the carbon near the surface is increased by inward diffusion of carbon atoms.The above remarks apply only in an approximate and qualitative way.The quantitative prediction of the diffusivity, thermal conductivity, and viscosity of a gas from a knowledge of molecular properties can be quite complicated.The consideration of such relations forms an important part of the subject of statistical mechanics.2.Molecular Diffusion

Molecular diffusion occurs in a gas as a result of the random motion of the molecules.This motion is sometimes referred to as a random walk.Acro a plane normal to the direction of the concentration gradient(or any other plane), there are fluxes of molecule in both directions.The direction of movement for any one molecule is independent of the concentration in dilute solutions.Consequently, in a system in which there is a concentration gradient, the fraction of molecules of a particular species(referred to as species A)which will move acro a plane normal to the gradient is the same for both the high-and low-concentration sides of the plane.Because the total number of molecules of A on the high-concentration side is greater than on the low-concentration side, there is therefore a net movement of A in the direction in which the concentration of A is lower.If there are no counteracting effects, the concentrations throughout the mixture tend to become the same.In the analogous transfer of heat in a gas by conduction, the distribution of hotter molecules(those which have a higher degree of random molecular motion)tends to be evened out by random mixing on a molecular scale.Similarly, if there is a gradient of directed velocity(as distinguished from random velocity)acro the plane, the velocity distribution tends toward uniformity as a result of the random molecular mixing.There is a transfer of momentum, which is proportional to the viscosity of the gas.In discuing the fundamentals of ma transfer we shall consider mainly binary mixtures, although multicomponent mixtures are important in industrial applications.Some of these more complicated situations will be discued after the basic principles have been illustrated in terms of binary mixtures.The analogy between momentum and energy transfer has already been studied in some detail, and it is now poible to extend the analogy to include ma transfer.By ma transfer is meant the tendency of a component in a mixture to travel from a region of high concentration to one of low concentration.For example, if an open test tube with some water in the bottom is placed in a room in which the air is relatively dry, water vapor will diffuse out through the column of air in the test tube.There is a ma transfer of water from a place whereits concentration is high(just above the liquid surface)to a place where its concentration is low(at the outlet of the tube).If the gas mixture in the tube is stagnant, the transfer occurs by molecular diffusion.If there is a bulk mixing of the layers of gas in the tube by mechanical stirring or because of a density gradient, ma transfer occurs primarily by the mechanism of forced or natural convection.These mechanisms are analogous to the transfer of heat by conduction and by convection;there is, however, no counterpart in ma transfer for thermal radiation.阅读材料13

传质原理

1.概述

一些最典型的化学工程问题存在于质量转移领域。化学工程师一个显著地特征是他的设计和操控设备的能力。在产品的准备，化学反应的发生和最终产品的分离。其他能力主要在对质量转移学科的精通。动量和热量转换定理通常被运用于工程的各个分支。但是质量转移一惯被限制在化学工程。其他重要的运用是在冶金过程，高速飞行的问题，废物处理和控制污染过程。

传质就是混合物某种组分有从含量高的区域向含量低的区域扩散的趋势。例如，如果在一个相对干燥的房间里，放置一个底部带有水的开口试管中，水蒸气将通过试管中的空气柱扩散出来。这就有一个质量转移，水从水从高浓度地方（仅在液体表面之上）传递到低浓度地方（在管的出口处）。如果管中的混合气体是不流动的，就通过分子扩散来转移。如果通过机械搅拌的方式搅和管中的气体层，或者由于密度梯度，质量专递首先是通过机械力或正常的对流。则这些机理就与通过传导和通过对流的热传递相类似；但是，在传质中没有热传质的对应物。

动量和能量装换的类似之处已经在一些细节上辈研究了，现在把这种类似性延伸到传质上已经成为可能。

在讨论传质的基本原理时，我们将主要考虑双组分混合物。尽管多组分混合物在工业应用中很重要，在对双组分混合物进行基本原理阐明之后，我们将对部分这些更复杂的情况进行讨论。

2.分子扩散

分子扩散是大量分子任一运动的结果。这种运动有时被称为自由运动。穿过垂直于浓度梯度方向的一个平面（或任何其它平面），在两个方向上都有分子扩散。在稀溶液中，每个分子的运动方向是独立的。因此，在一个存在浓度梯度的系统中，某一特定种类（把它当 作种类A）的小部分分子，穿过平面的高浓度和低浓度两侧是相同的，该种分子将运动穿过垂直于浓度梯度的平面。因为A的分子总数目，在高浓度一侧比在低浓度一侧的大，所以就存在A在一个方向上的单方向运动，在A的浓度更低的方向上。如果没有抵消作用，则混合物的浓度将趋向相同。与气体中热量以传导方式的传递相类似，较热分子（那些分子具有更高程度的自由分子运动）经过分子级别的随意混合，其分布将趋向平坦。类似地，如果有穿过平面方向上的速度梯度（该速度有别于随机速度），速度分布将趋向一致，这是分子随机混合的结果。动量传递与气体的粘度成比例。

以上论述只是应用了相似和定性的方法。仅运用分子性质的知识，对扩散系数、热传导系数和粘度进行定定性假设是相当复杂的，而这些关系的考虑形成了统计力学学科的一个重要部分。

分子扩散同样发生在液体和固体中。结晶体在未饱和溶液中溶解，随后从固-液界面扩散出来。固体中的扩散在冶金操作中非常重要。当在焦炭床上加热未饱和的铁时，经过碳原子的内部扩散，铁表面附近的碳含量将会增加。

3.涡流扩散

正如动量和能量一样，质量同样可以通过限定的流体部分的运动来传递。我们已经看到这些因流体中的体积混合而产生的传递操作速率，可以从涡流运动学粘度、涡流热扩散系数和涡流扩散系数方面来阐述。后者的数量与混合时间长短有关系。这如同阐述动量和能量传递的关系一样。实际上，传热和传质之间的类似是如此简单以致于经常发现前者的方程经过稍微改变符号就适用于后者了。

烟囱里冒出的烟的显然就是旋转扩散。涡流引起混合并使烟尘传递到周围的空气中。在某些缺乏空气涡流的地方，来自地表的烟尘主要以分子扩散的形式扩散。这会引起严重的污染问题，因为物质以分子形式的扩散比以涡流形式的扩散传递得更慢。

4.对流传质系数

在传热过程研究中，我们发现能量微分平衡方程的解答有时是很难的甚至是不可能的，用对流传热系数来表达热流是很方便的，其方程如下所示：

4.对流传质系数

在传热过程中，我们发现，能量微分平衡方程的解答有时很麻烦甚至是不可能的，用对流传热系数来表达热流速率是方便的，所用的方程如下所示：

qh(tstm)A

在传质中，类似的情况使用方程形式表示为：

NAkP(AsAm)

物质流量NA的度量涉及一套固定在空间的坐标轴。推动力是流体中在相边界（一个固体表面或液体表面）和某个任意规定的点之间的浓度差。对流系数kP适用于强制或自然对流；传质没有相对应的沸腾系数、冷凝、传热辐射系数。kP的值是系统的几何学和流体速度与性质的函数，跟系数h一样。

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