T he topic of mass transfer has a long and distinguished history dating to the 19th century, which saw the development and early applications of the theory of diffusion. Mass transfer operations such as distillation, drying, and leaching have an even earlier origin, although their practice was at that time an art rather than a science, and remained so well into the 20th century. Early textbook publications of that era dealt mainly with the topic of diffusion and the mathematics of diffusion.
The development of mass transfer theory based on the film concept, which began in the 1920s and continued during two decades of intense activity, brought about a shift in emphasis. The first tentative treatments of mass transfer processes dealing primarily with distillation and gas absorption began to appear, culminating with the publication, in 1952, of Robert Treybal’s Mass Transfer Operations. It was to serve generations of students as the definitive text on the subject.
The 1950s and the decades that followed saw a second shift in emphasis, signaling a return to a more fundamental approach to the topic. Mass transfer was now seen as part of the wider basin of transport phenomena, which became the preferred topic of serious authors. The occasional text on mass transfer during this period viewed the topic on a high plane and mainly within the context of diffusion. For the most part, mass transport was seen as one of three players on the field of transport phenomena, and often a minor player at that. In the 1980s and 1990s, it became fashionable to treat mass transfer as part of the dual theme of heat and mass transfer. In these treatments, heat transfer, as the more mature discipline, predominated and mass transfer was usually given short shrift, or relegated to a secondary role. This need not be and ought not to be.
The author has felt for some time that mass transfer is a sufficiently mature discipline, and sufficiently distinct from other transport processes, to merit a separate treatment. The time is also ripe for a less stringent treatment of the topic so that readers will approach it without a sense of awe. In other words, we do not intend to include, except in a peripheral sense, the more profound aspects of transport theory. The mainstays here are Fick’s law of diffusion, film theory, and the concept of the equilibrium stage. These have been, and continue to be, the preferred tools in everyday practice. What we bring to these topics compared to past treatments is a much wider, modern set of applications and a keener sense that students need to learn how to simplify complex problems (often an art), to make engineering estimates (an art as well as a science), and to avoid common pitfalls. Such exercises, often dismissed for lacking academic rigor, are in fact a constant necessity in the engineering world.
Another departure from the norm is the organization of the material according to mode of operation (staged or continuous contact), rather than the type of separation process (e.g., distillation or extraction). Phase equilibria, instead of being dispersed among different operations, are likewise brought together in a single chapter. The reader will find that this approach unifies and strengthens the treatment of these topics and enables us to accommodate, under the same umbrella, processes that share the same features but are of a different origin (environmental, biological, etc.).
The readership at this level is broad. The topic of separation processes taught at all engineering schools is inextricably linked to mass transport, and students will benefit from an early introductory treatment of mass transfer combined with the basic concepts of separation theory. There is, in fact, an accelerating trend in this direction, which aims for students to address later the more complex operations, such as multicomponent and azeotropic distillation, chromatography, and the numerical procedures to simulate these and other processes.
Mass transport also plays a major role in several other important disciplines. Environmental processes are dominated by the twin topics of mass transfer and phase equilibria, and here again an early and separate introduction to these subject areas can be immensely beneficial. This text provides detailed treatments of both phase equilibria and compartmental models, which are all-pervasive in the environmental sciences. Transport, where it occurs, is almost always based on Fickian diffusion and film theory. The same topics are also dominant in the biological sciences and in biomedical engineering, and the text makes a conscious effort to draw on examples from these disciplines and to highlight the idiosyncrasies of biological processes. Further important applications of mass transport theory are seen in the areas of materials science and materials processing. Here the dominant transport mode is one of diffusion, which in contrast to other disciplines often occurs in the solid phase. The reader will find numerous examples from these fascinating fields as well as a considerable amount of preparatory material of benefit to materials science students.
The text starts in an unconventional way by introducing the reader at an early stage to diffusion rates and Fick’s law and to the related concepts of film theory and mass transfer coefficients. This is done in Chapter 1, but the topics are deemed of such importance that we return to them repeatedly in Chapters 3 and 4, and again in Chapter 5. In this manner, we develop the subject matter and our grasp of it in successive and complementary stages. The intervening Chapter 2 is entirely devoted to the art of setting up mass balances, a topic that is all too often given little attention. Without a good grasp of this subject we cannot set about the task of modeling mass transfer, and the many pitfalls we encounter here are alone sufficient reason for a separate treatment. The balances include algebraic and ordinary differential equations (ODEs). The setting up of partial differential equations (PDEs) is also discussed, and some time is spent in examining the general conservation equations in vector form. We do not attempt solutions of PDEs but instead provide the reader with known solutions and solution charts, which we use in Chapter 3 to solve a range of important problems. That chapter also considers the simultaneous occurrence of mass transfer and chemical reaction.
Chapter 6 deals with phase equilibria, which are mainly composed of topics not generally covered in conventional thermodynamics courses. These equilibria are used in Chapter 7 to analyze compartmental models and staged processes. Included in this chapter is a unique treatment of percolation processes, which should appeal to environmental and chemical engineers. Chapter 8 takes up the topic of modeling continuous-contact operations, among which the application to membrane processes is given particular prominence. Finally, in Chapter 9 we conclude the text with a brief survey of simultaneous mass and heat transfer.
The text is suitable for a third-year course addressed to engineering students, particularly those in the chemical, civil, mechanical, environmental, biomedical, and materials disciplines. Biomedical and environmental engineers will find topics of interest in almost all chapters, while materials science students may wish to concentrate on the earlier portions of the text (Chapters 1 to 5). The entire text can, with some modest omissions, be covered in a single term. The professional with a first-time interest in the topic or a need for a refresher will find this a useful and up-to-date text.
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