I&EC REPORTS AND COMMENTS Four new thermo books reviewed Sonic energy for synthesis
Thermodynamics Books Keep Coming The high annual output of books on thermodynamics doesn’t necessarily reflect progress. Most of the books are texts of the classical variety for use in particular curricula. The large number of new texts is prompted primarily by the difficulties of introducing the student to and perfecting him in thermodynamics, for, as Prof. Denbigh has correctly observed, “Thermodynamics is a subject which needs to be studied not once, but several times over a t advancing levels.” T h e principal justification for many of the new texts seems to be the various ways in which the material is presented rather than any substantially new concepts. Recently four books have appeared which, for different reasons, are unusual and worthy of note; two of them are texts. M . H. Everdell, of the University of Aston in Birmingham, England, attempts to exficplain thermodynamics instead of authoritatively describing it (“Introduction to Chemical Thermodynamics,” W. W. Norton, New York, 1966). Despite its simplicity and instructional utility, classical thermodynamics is built around the central idea of the equilibrium state which, like the perfect gas, is a convenient fiction. Therefore, anyone explaining thermodynamics must discuss nonequilibrium states and natural irreversibility, the genesis of thermodynamics. Everdell does this in a way that appears to compress several “rounds” of study into one presentation. I n doing so he is one of the few authors of introductory texts to employ de Donder’s affinity and extent of reaction. These items are elegantly presented and are heady stuff for the student who has already gone a
round with classical thermodynamics. They may well be too much for the novice. Everdell places great emphasis on practical problems of industrial interest and, for this reason, should have great appeal to chemical engineers. If the lengthy text proves to be digestible by the novice, it will become a familiar book to many people. I t is well written, though very long. The second text is part of a series on thermal and transport sciences, primarily for students of mechanical engineering. T o those of us who have imagined the mechanical engineer’s interest in thermodynamics to be confined to heat engines, this book comes as a surprise. The composite picture of the mechanical engineer formed by the text is, in many respects, indistinguishable from that of the chemical engineer. I n treating such topics as plasmas, rarefied gases, magnetohydrodynamics, thermoelectric generators, ion engines, lasers, semiconductors, etc., the mechanical engineer has more than ample exposure to statistical methods. However, it appears that authors Sonntag and van Wylen (“Fundamentals of Statistical Thermodynamics,” Wiley, New York, 1966) are a bit optimistic about the capabilities of the beginning student. Since no prior study of quantum mechanics or advanced mathematics is required, these tools must be supplied. Consequently most of the book is devoted to preparation for thermodynamics rather than to the thermodynamics itself. While such a procedure might be necessary, it is also somewhat confusing a t times. The book must also provide the student with a thumb-
nail introduction to a good bit of general science that further dilutes the emphasis on thermodynamics. If, however, we consider the book as one of a series, then its merits increase. O n the other hand, the series seems to be tailored to a specific course of study and may not receive wide use for this reason. The third book is a collection of papers presented a t a symposium on nonequilibrium thermodynamics in May 1965 a t the University of Chicago (“Non-Equilibrium Thermodynamics, Variational Techniques and Stability,” edited by R. J. Donnelly, R. Herman, and I. Progogine, University of Chicago Press, Chicago, 1966). As an identifiable discipline, nonequilibrium thermodynamics is about 20 years old and resulted from the union of two schools of study. The school of Duhem and de Donder was primarily interested in the extension of conservation ideas. The school of Onsager was more concerned with reciprocity relations derived from the theory of fluctuations. For most of its life, nonequilibrium thermodynamics was, literally, the thermodynamics of a linear world which was almost as fictitious as the perfect gas and the equilibrium state. As a result of the difficulties with nonlinear problems, many came to the conclusion that nonequilibrium thermodynamics was of little use except as a teaching device. If one were required to demonstrate any utility, he would have great difficulty indeed. But this state of affairs doesn’t always have to remain so. The number of cases where nonequilibrium thermodynamics has already (Continued on page 74)
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demonstrated technical utility is sufficiently large to at least cause us to rethink the criticism of the subject. I n a sense, this was the purpose of the symposium at Chicago-to assess the present status of nonequilibrium thermodynamics and to try to foresee the general paths of development in the future. The symposium succeeded admirably. While those who believe that nonequilibrium thermodynamics has a useful future will be encouraged by these proceedings, there is also no doubt it will be a long time before the methods of nonequilibrium thermodynamics seep into the consciousness of designers and researchers. The last book we note is, by far, the most entertaining. Howard Reiss (“Methods of Thermodynamics,” Blaisdell, New York, 1965) is more concerned with why we do the thermodynamic things we do than he is with what we actually do. His book is, therefore, more or less a summary of his personal thermodynamics philosophy which he recites in the hope that it will help elevate understanding to the level where the reader can ultimately apply thermodynamics to an arbitrary s) stern with confidence. With this in mind, the book concentrates entirely on the physical concepts involved in applications. Presumably this procedure is more pictorial than any other; it also seems to be a bit overdone at times. I n many respects Reiss’s book complements Everdell’s. Both are. apparently, not satisfied with the conventional explanations and have decided to at least indicate that the thermodynamic world extends beyond a set of axiomatic laws. Bridgman once wrote a philosophical critique of classical thermodynamics that has yet to be surpassed when it comes to deflating scientists. Reiss has a different purpose, but his is one of the few attempts since Bridgman’s effort that makes us re-examine our thermodynamic surroundings. 14
Sonolysis The growing use of ultrasonic devices for cleaning and for fracture analysis has provided a vehicle for a new synthetic method that has much more appeal, in a number of ways, than either the plasma or the molten salt medium. It has long been known that sonic vibrations of liquids can cause their breakdown through cavitation and the presence of a volatile component in the liquid. I n reporting on his sonochemical investigations, Prof. M. Anbar of the Weizmann Institute of Science ( T h e New Scientist, p. 365, 1 2 May 1966) has probably succeeded in defining the primary mechanism for sonochemical reactions. The mechanical energy of sound waves can energize individual molecules to velocities sufficiently high to break chemical bonds but in insufficient quantities to explain the amounts of reaction products frequently formed. The key is the rapid nucleation and formation of bubbles under the influence of the periodic sonic fields. This cavitation is the feature of sonolysis that provides the necessary energy. Bubbles formed in a liquid are subjected to very rapid compression by the sonic wave with consequent adiabatic heating to several thousand degrees. The heating is too rapid for dissipation via liquid conduction and there results a source of ions, radicals, and molecules that are unusual or unknown at room temperature. The hot gas may react with the surrounding liquid or with other fragments within itself. During the final half of the sonic wave the cavity will collapse, leaving some of the fragments behind in the liquid for further reaction in postcycle surroundings. O n the next cycle the proccss is repeated. At high frequencies, cavities are active during the greater part of the cycle. Sonolytic conversion of mechanical energy to chemical energy is highly
INDUSTRIAL AND ENGINEERING CHEMISTRY
inefficient but does provide a unique combination of high temperature shock waves and free-radical chemistry in solution. A sonochemical system also permits rapid “freezing” of part of a system in equilibrium at a very high temperature. This allows the utilization of reactions which are thermodynamically impossible at room temperature-e.g., the oxidation of ammonia by water. I n most of the common examples of sonolysis there is a net “destruction” involved, much like corrosion. Sonolysis, therefore, has held little interest for most investigators. However, the possibilities for sonically induced synthesis make the subject at least as attractive as plasma arid molten salt syntheses. The simplicity of obtaining ultrasonic energy within liquids is the principal attraction of the sonic syntheses. However it remains to devise more efficient ways of providing energy inputs greater than a kilowatt per square centimeter where the product yield is usually of the order of millimoles per liter per minute but the heat generation is sufficient to require cooling for the system. Professor Anbar is obtaining the basic informa tion on sonochemical systems to help explain some of the hitherto ignored phenomena that constantly go on about us. However, he may also be providing us with yet another synthetic tool. Of more immediate interest is the potential of ultrasonics in emulsion technology and as an auxiliary to reactions in fluidized beds. In the latter case, it appears that the absorption-reaction-desorption cycle associated with adsorbents can be made more selective with ultrasonics. The selectivity may result more from controlling the energetics of the reaction in the adsorbed phase than from controlling the concentrations through increasing the diffusive transport rates in the fluids.
