Mass-transfer operations design principles

My objective in writing this book is to provide a means to teach undergraduate chemical engineering students the basic principles of mass transfer and to apply these principles, aided by modem computational tools, to the design of equipment used in separation processes. The idea for it was bom out of my experiences during the last 25 years teaching mass-transfer operations courses at the University of Puerto Rico. 

The transfer of mass as well as heat from one material phase to another is quite commonly encountered in chemical process flow sheets. The same physical laws, rate equations, and design principles can be applied to mass-transfer operations as occurring in absorption, adsorption, crystal-lization, distillation, drying, extraction, jluidization, and humidification Equipment is designed to obtain intimate contact between phases, in either a stagewise or continuous manner, and many special types of equipment have been developed for any given operation. This discussion will be limited to the conventional types of equipment. 

It is, of course, impossible to treat every mass transfer operation in existence-there are too many variations. And new techniques of separation are being developed daily. But by means of the typical examples cited in these last two chapters, the reader should be able to fit the basic principles of control to his own operation. Do not try to apply the equations given herein as all-encompassing formulas. They are intended to demonstrate a point the evolution of a control system out of mass and heat balances. Writing these balances on your own process should always be the first step in designing its control system. 

In this chapter, consideration will be given to the basic principles underlying mass transfer both with and without chemical reaction, and to the models which have been proposed to enable the rates of transfer to be calculated. The applications of mass transfer to the design and operation of separation processes are discussed in Volume 2, and ihe design of reactors is dealt with in Volume 3. 

Modules Every module design used in other membrane operations has been tried in pervaporation. One unique requirement is for low hydraulic resistance on the permeate side, since permeate pressure is very low (0.1-1 Pa). The rule for near-vacuum operation is the bigger the channel, the better the transport. Another unique need is for heat input. The heat of evaporation comes from the liquid, and intermediate heating is usually necessary. Of course economy is always a factor. Plate-and-frame construction was the first to be used in large installations, and it continues to be quite important. Some smaller plants use spiral-wound modules, and some membranes can be made as capillary bundles. The capillary device with the feed on the inside of the tube has many advantages in principle, such as good vapor-side mass transfer and economical construction, but it is still limited by the availability of membrane in capillary form. 

Volumes 1, 2 and 3 form an integrated series with the fundamentals of fluid flow, heat transfer and mass transfer in the first volume, the physical operations of chemical engineering in this, the second volume, and in the third volume, the basis of chemical and biochemical reactor design, some of the physical operations which are now gaining in importance and the underlying theory of both process control and computation. The solutions to the problems listed in Volumes 1 and 2 are now available as Volumes 4 and 5 respectively. Furthermore, an additional volume in the series is in course of preparation and will provide an introduction to chemical engineering design and indicate how the principles enunciated in the earlier volumes can be translated into chemical plant. 

Chemical reactions carried out on an industrial scale are subject to control by processes such as mass transfer and heat transfer, which, in small-scale work, may often be reduced to negligible proportions by judicious choice of conditions. Volume 23 deals with these aspects of reaction kinetics, which must always be considered when significant scale-up is contemplated and which may be expected to be of paramount importance in industrial operations. The principles of chemical reactor design are treated in a form digestible by chemists and there is some emphasis on the way in which available kinetic data may be utilised by the chemical engineer. 

Before developing specific relationships to describe cooling tower operations, it is worthwhile to review some elementary principles in developing material and energy balances. In addition, we need to review heat and mass transfer analogies before tackling design problems. The more experienced reader may wish to proceed to Chapter 4 or try the example problems at the end of the chapter as a refresher. 

Our discussion up to now has concerned the cooling of hot process waters exclusively. However, we insisted back in Chapter 1 that a cooling tower is nothing more than a device that transfers heat from one mass to another. Therefore, gas coolers are governed by the same theory of operation and design principles as are water cooling towers. 

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