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   1 Introduction A substantial number of the unit operations of chemical engineering are concerned with the  problem of changing the compositions of solutions and mixtures through methods not necessarily involving chemical reactions. Usually these operations are directed toward separating a substance into its component parts. For mixtures, such separations may be entirely mechanical, e.g., the filtration of a solid from a suspension in a liquid, the classification of a solid into fractions of different particle size by screening, or the separation of particles of a ground solid according to their density. On the other hand, if the operations involve changes in composition of solutions, they are known as the mass-transfer operations. The importance of these operations is profound. There is scarcely any chemical process which does not require a preliminary purification of raw materials or final separation of products from  by-products, and for these the mass-transfer operations are usually used. Frequently the major part of the cost of a process is that for the separations. These separation or  purification costs depend directly upon the ratio of final to initial concentration of the separated substances, and if this ratio is large, the product costs are large.   2 The mass-transfer operations are characterized by transfer of a substance through another on a molecular scale. For example, when water evaporates from a pool into an airstream flowing over the water surface, molecules of water vapor diffuse through those of the air at the surface into the main portion of the airstream, whence they are carried away. It is not bulk movement as a result of a “pressure  difference, as in pumping a liquid through a pipe. In the problems at hand, the mass transfer is a result of a concentration difference, or gradient, the diffusing substance moving from a place of high to one of low concentration.   3 CLASSIFICATION OF THE MASS-TRANSFER OPERATIONS Direct Contact of Two Immiscible Phases This category is by far the most important of all and includes the bulk of the mass-transfer operations. Here we take advantage of the fact that in a two-phase system of several components at equilibrium, with few exceptions the compositions of the phases are different. The various components, in other words, are differently distributed between the phases. In some instances, the separation thus afforded leads immediately to a pure substance because one of the phases at equilibrium contains only one constituent. For example, the equilibrium vapor in contact with a liquid aqueous salt solution contains no salt regardless of the concentration of the liquid. Similarly the equilibrium solid in contact with such a liquid salt solution is either pure water or pure salt depending upon which side of the eutectic composition the liquid happens to be. Starting with the liquid solution, one can then obtain a complete separation by boiling off the water. Alternatively, pure salt or pure water can be produced by  partly freezing the solution; or, in principle at least, both can be obtained pure by complete solidification followed by mechanical separation of the eutectic mixture of crystals. In cases like these, when the two phases are first formed, they are immediately at their final equilibrium compositions, and the establishment of equilibrium is not a time - dependent  process. Such separations, with one exception, are not normally considered to be among the mass-transfer operations. In the mass-transfer operations, neither equilibrium phase consists of only one component. Consequently when the two phases are initially contacted, they will not (except fortuitously) be of equilibrium compositions. The system then attempts to reach equilibrium by a relatively slow diffusive movement of the constituents, which transfer in part between the phases in the process. Separations are therefore never complete, although, as will be shown, they can be brought as near completion as desired (but not totally) by appropriate manipulations. »   The three states of aggregation, gas, liquid, and solid, permit six possibilities of phase contact.     4 Gas-gas    –    Since with very few exceptions all gases are completely soluble in each other, this category is not practically realized. Gas-liquid If all components of the system distribute between the phases at equilibrium, the operation is known as  fractional distillation (or frequently just distillalion). In this instance the gas phase is created from the liquid by application of heat; or conversely, the liquid is created from the gas by removal of heat.    For example, if a liquid solution of acetic acid and water  is partially vaporized by heating, it is found that the newly created vapor phase and the residual liquid both contain acetic acid and water but in proportions at equilibrium which are different for the two  phases and different from those in the srcinal solution. If the vapor and liquid are separated mechanically from each other and the vapor condensed, two solutions, one richer in acetic acid and the other richer in water, are obtained. In this way a certain degree of separation of the srcinal components has been accomplished.      Both phases may be solutions, each containing, however, only one common component (or group of components) which distributes between the phases. For example, if a mixture of ammonia and air is contacted with liquid water, a large portion of the ammonia, but essentially no air, will dissolve in the liquid and in this way the air-ammonia mixture can be separated. The operation is known as  gas absorption . On the other hand, if air is brought into contact with an ammonia-water solution, some of the ammonia leaves the liquid and enters the gas phase, an operation known as desorption   or  stripping . The difference is purely in the direction of solute transfer.    If the liquid phase is a pure liquid containing but one component while the gas contains two or more, the operation is humidification or dehumidification  , depending upon the direction of transfer (this is the exception mentioned earlier). For example, contact of dry air with liquid water  results in evaporation of some water into the air ( humidification   of the air ). Conversely, contact of very moist air with pure liquid water  may result in condensation of part of the moisture in the air ( dehumidification ). Gas-solid  Classification of the operations in this category according to the number of components which appear in the two phases is again convenient.
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