APPLIED KINETICS AND CHEMICAL REACTION ENGINEERING ROBERT L. GORRING
hemical reaction engineering may be defined as the C analysis of all those phenomena involved in chemical conversion process and the synthesis of those
a
elements into a descriptive theoretical framework suitable for the design, control, and optimization of the environment in which the reaction is to be carried out. Although it is usually considered to be a relatively new discipline (dating roughly from the appearance in 1930 of “Der Chemie Ingenieur” by Damkohler), the history of chemical engineering provides numerous examples of concern with what we now call reaction engineering. h’otable among these is a remark of the Russian physical chemist M. V. Lomonosov on the occasion of the founding of a chemical laboratory at the Imperial Academy of Sciences in March 1745: “I not only saw from other authors, but am convinced from my own art, that chemical experiments, combined with physical, show peculiar effects” (2). The special distinguishing characteristic of reaction engineering lies in its treatment of situations in which the coupling between physical phenomena, such as diffusion and pure chemical kinetics, leads to a higher order interactive system of considerable complexity. I n 1908, Langmuir published a paper (3) entitled, “The Velocity of Reactions in Gases Moving Through Heated Vessels and The Effect of Convection and Diffusion.” This work, which seems to have been largely overlooked in the literature, contains a mathematical analysis of the influence of axial diffusion on the conversion in plug flow tubular reactors. Langmuir stated that an object of this paper was “TO develop formulas from which to calculate the velocity coefficient in those cases where. . ., in the mixing of gases, diffusion plays a more important part than convection.” The spirit of rational synthesis and the interest in quantitatively analyzing complex phenomena in this early contribution are themes that constantly recur during the evolution of reaction engineering. During the early development of chemical engineering and, indeed, up to the middle 1950’s, emphasis was very much on purely physical phenomena such as extraction, heat transfer, etc. Reaction engineering, with its focus on the chemical transformation itself, lays
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I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
VERN W. WEEKMAN, JR.
some claim to being the discipline that uniquely differentiates chemical engineering from the other branches of engineering. The recently developing interest in reaction analysis has served to reorient chemical engineering research toward the reactor itself and increasingly to consider unit operations from the viewpoint of their interaction with the chemical transformation. The analysis of reaction systems has increasingly concentrated on questions of selectivity and reaction path. Industrial experience has indicated that overall conversion in a chemical process can often be adjusted by reasonably straightforward manipulation of space velocity, temperature, etc. However, selectivity in a complex system is usually a much more difficult problem-sometimes requiring fairly complex analysis to clarify the basic yield structure. The development of mathematical models of conversion processes has received considerable attention recently, although the works of Wegscheider (5), Bodenstein and IVolgast ( I ) , and Langmuir (3) indicate that the subject has been of interest as long as has chemical engineering itself. Among the functions of reaction engineering are the uncoupling of the transport and flow phenomena from the kinetic phenomena and the exhibition of the pure reaction kinetics in their true scale-independent form. Construction of a theoretical model may then proceed along lines desired by the designer. With the increasingly widespread use of process models and the possibility of developing various alternative models based on differing assumptions, model discrimination techniques become important. Controversy between the German chemists G. Lunge and F. Raschig in 1907 ( 4 ) concerning the correct mechanism of the chamber sulfuric acid process is an early example of the need for objective model discrimination techniques. This persistent concern among engineers is currently receiving intensive examination, as some of the papers in this symposium will show. If the papers presented at this symposium may be said to reflect the current status of reaction engineering, we might speculate about the challenges facing us in the future. We know that we are a long way from designing
a n industrial reactor with anything like the confidence with which we design heat exchangers and distillation columns. With a few exceptions, our knowledge of the catalytic surface is not at present adequate to enable us to design a catalyst for a specific task. Most successful industrial catalysts are still discovered by Edisonian methods, although the work that follows is being based on somewhat more fundamental principles. The papers in this symposium that deal with model development point to a research area of great importance for reactor analysis-that of mixing and residence time distribution. I n fact, insufficient experimental research has been done on residence time distribution even for homogeneous reactors. Industrial reactors that feature two-phase flow over a solid or porous catalyst are common, yet experimental and theoretical analyses of these systems are scarce. The long-standing problem of reactor yield and conversion from residence time distribution data for reaction orders other than unity has yet to be satisfactorily solved. The complexity of these problems has resulted in a tendency for reactor analysts to move away from purely deterministic methods toward stochastic approaches. The advent of high speed digital and analog computers has had an enormous impact in reaction engineering. Complex interactions of kinetics and transport phenomena may now be studied in detail, when a decade ago only educated guesses or correlative approaches were possible. The control computer has spawned a large number of jobs for reaction engineers in the fields of mathematical models, optimization, and control. I n the beginning of this computer revolution most of the practitioners were “retreads” with some experimental experience ; thus, their new-found ability to reason with 38-place precision was tempered by their previous confrontations with nature. Today, however, experimental work is lagging behind the theoretical, and the continued need for good experimentalists, so that reactor engineering may continue to develop,
suggests that we should encourage the next generation of university graduates to consider the problems and rewards of experimentation. I n all healthy applied scientific endeavors we must eventually submit our abstractions and theories to a trial by nature. I n reaction engineering this trial is most logically found in the application of its methods and concepts to the design and utilization of industrial reactors. More data are needed to evaluate what we might call the industrial effectiveness factor; that is, how effective have been the tools and methods of reaction engineering in industrial applications. This effectiveness factor might be defined as the rate of application of reaction engineering divided by the rate of generation of reaction engineering papers. Although this effectiveness has never been close to one in magnitude (nor should it necessarily be), it is disturbing that its slope with time may currently be negative. Only by continually evaluating the efficiency of our tools and methods can we keep our efforts in a healthy direction. Hopefully, more people engaged in the applied areas of reaction engineering can publish such effectiveness data. A splendid challenge exists for those who would translate the tools of reaction engineering to the firing line of industrial application. I t is clearly a n exciting and rewarding field in which to be working. The present symposium is, therefore, offered in the expectation that it will provide a review of past and current work, and will, above all, stimulate further developments in chemical reaction engineering.
LITERATURE CITED (1) Bodenstein, M., Wolgast, K., Z. Phys. Ckem. 61,422-36 (1908). (2) Frank-Kamenetskii D. A “Diffusion and Heat Exchange in Chemical Kinetics” (Transl.), h$wxeton)Univ. Press, Princeton, 1955. (3) Langmuir, I . , J.Am. Chem. Soc. 90,1742-54 (1908). (4) Raschig, F., Angelu. Chem. 20,694-722 (1907). (5) Wegscheider, R., Z.Phyr. Chem. 95,513-87 (1900).
Robert L. Gorring and Vern W . Weekman are both with the Systems Research Group of the Ajplied Research and Development Division at Mobil Oil Corp.’s Paulsboro, N . J., laboratory. Both work with applied kinetics and catalysis as related to chemical reaction engineering and process simulation and control. The co-chairmen wish to acknowledge, with thanks, the contributions of Rutherford Aris and J . M. Smith who chaired two of the symposium sessions. Thanks are also due to Mrs. Bonnie Watkins of the staf of INDUSTRIAL A N D ENGINEERING CHEMISTRY for her invaluable he& in organizing all physical arrangements for the symposium. AUTHORS
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Gorring
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Weekman
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