Reading Material 13
Principles of Mass Transfer寒食东风
跑步机1. General Remarks
Some of the most typical chemical engineering problems lie in the field of mass transfer. A distinguishing mark of chemical engineer is his ability to design and operate equipment in products is prepared, chemical reactions take place, and parations of the resulting products are made. This ability rests largely on a proficiency in the science of mass transfer. Applications of the principles of momentum and heat transfer are common in many branches of engineering, but the application of mass transfer has traditionally been largely limited to chemical engineering. Other important applications occur in metallurgical process, in problems of high-speed flight, and in waste treatment and pollution-control process. Eddy diffusion is apparent in the dissipation of smoke from a smokestack. Turbulence caus 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 dissipated largely by molecular diffusion. This cau rious pollution problems becau mass is transferred less rapidly by molecular diffusion than by eddy diffusion.
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4. Convective Mass-Transfer Coefficients
In the study of heat transfer we found that the solution of the differential energy balance was sometimes cumbersome or impossible, and it was convenient to express the rate of heat flow in terms of a convective heat-transfer coefficient by an equation like舞蹈教学视频大全
The analogous situation in mass transfer is handled by an equation of form
The mass flux NA is measured relative to a t of axes fixed in place. The driving force is the difference between the conversation at the pha boundary (a solid surface or a fluid interface) and the concentration at some arbitrarily defined point in the fluid medium. The convective coefficient may apply to forced or natural converction; there are no mass-transfer counterparts for boiling, condensation, or radiation heat-transfer coefficients,the value of 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 mass can be transferred. We have en that the rate of the transfer operations, caud by bulk mixing in a fluid, can be expresd 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 mass transfer is s
o 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 dissolve, with subquent 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 incread by inward diffusion of carbon atoms.
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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
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Molecular diffusion occurs in a gas as a result of the random motion of the molecules. Thi
s motion is sometimes referred to as a random walk. Across 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. Conquently, 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 across a plane normal to the gradient is the same for both the high-and low- concentration sides of the plane. Becau 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 (tho 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) across 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.
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In discussing the fundamentals of mass transfer we shall consider mainly binary mixtures, although multicomponent mixtures are important in industrial applications. Some of the more complicated situations will be discusd 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 possible to extend the analogy to include mass transfer.
推动高质量发展By mass 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 diffu out through the column of air in the test tube. There is a mass 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 becau of a density gradient, mass tra
nsfer occurs primarily by the mechanism of forced or natural convection. The mechanisms are analogous to the transfer of heat by conduction and by convection; there is, however, no counterpart in mass transfer for thermal radiation.
