I&EC REPORTS & COMMENTS Thre&‘Golden Ruler” and “The Pettimum” yclic processing promises improved performance
Chemical Rwctien Engineering At long last, the Proceedings of the
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Third European Symposium on Chemical Reaction Engineering, held September 15-17, 1964, in Amsterdam, have been published (Pergamon Pres, as a supplement to Chemical Engineuing sciaca). The most interesting part of these Proceedings, since the papers themselves have h e a d y received wide circulation, is c h a i i D. w. van Krevelen’s summary of the sympwium’s content and significance. Chemical reaction engineering draws on a wide variety of knowledge and experience. Ita componenta, accolyling to van Krevelen, can be organized into a type of structure, as Shown at right. By this structure, the product of chemical reaction engineering is reactor and pmceas design. There are eight aspecta to this product, of which only t h r e m b i l i t y , selectivity, and optimization-merit specific comment in the context of this symposium. Of these, optimization is of greatest immediate concern, because the semantics, and therefore the thought pmcesses, can readily cause confusion. During a discussion of this point, van Krevden recalls, Dr. W. F. Gruater of Ciba coined a new term-the “p&um”-to aid in clari6cation of the meaning of optimization. This term comes about as follows: Optimization, the s i ence of producing an optlaurn, Sometimes produces it through a maximum and sometimes through a minimum. In all cases, though, the optimum is restricted to technical variables and is achieved by maximizing the denirable and m i n i i g the undesirable. While it is always helpful to h o w how well off you can be, it sometimes is equally valuable to know how
badly off you can get, which is where the pcarimUm and pessimizatiOn come in. The pessimum is to pessimization as the optimum is to optimization. The pessimum is wnh e d to economic parameters correaponding to the technical parameters being optimized. Golden Ruler. To van Krcvelen, the sympasium’s organization and the structure of chemical reaction engineering demonstrate the generality and value of the three working rules of schocncmann [ C h . Eng. Sei. 18, 565 (1963)], which van Krevelen c a b the three Golden
Rules of chemical thought. They are:
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engineering
Think in tdrmr of balances. Maar, heat, and momentum balances form the starting point for any analysis. Know the lainits of II~CUT(ICY.Thac is no use in calculating or expsimenting to a greater accuracy than warranted by the basic data. Incidentally, this is why many of the available modeling techniques cannot be used mmessfully; data d the required accuracy are simply not available. (Continudm#ag# 76) VOL 5 8
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I&EC R E P O R T S
Mail Coupon for Czechoslovak Technical Paper in English translation How does an Eastern Reviewer, conditioned by Eastern literature, see the aqueous chemistry of zirconium and hafnium? TAM-manufacturer of zirconium compounds for industry-now offers you a complete English translation of a Paper by J. Hala of the Czechoslovak Academy of Sciences. Unlike many Western reviews (which often suffer from insufficient detail because of the language barrier), this Paper provides an intriguing Eastern-oriented insight for broadening your scope of the aqueous chemistry of solution. TAM also hopes this Paper will provide stimulating research data for evaluating the potential of zirccnium for a remarkably broad range of applications ,..and for your industry i n particular. For example, zirconium forms complex anions and cations; "ties" together diverse molecules in amazingly strong chemical bonds; and promotes the effectiveness of other materials i n reactions it does not itself enter. . LEAD-
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Speculatively project and precalculate. It is useful to scan the influence of parameters with a simple model as design proceeds. If we compare these Golden Rules with the structure of chemical reaction engineering shown in the chart, we note that the rules are the simplified relations between key positions in the structure. Thinking in terms of balances and models forms the starting point for reactor and process design. The influences of the variables can be estimated with simulation models, thus drastically reducing the number of experiments necessary to prove the design. This entire process is beautifully demonstrated in the now-celebrated paper by Boreskov and Slik'ko, first presented at this symposium. I n conclusion, Dr. van Krevelen notes that "to design is to decide." Every design is associated with a problem having a specific structure of its own. I n this structure there are more parameters than can possibly be measured or controlled. There are also objectives that can be formulated quantitatively. A technical problem, however, always involves a number of uncontrolled variables as well. Further, one is always concerned that the reactor or process does, in fact, correspond with the model. The art of decision making is always either algorithmic or heuristic. I n the algorithmic case, one obtains numerical information about the variables. I n the heuristic case, the solution obtained is intrinsically inaccurate. After reading the summary, we get a strong impression that chemical reaction engineering is a far-fromfinished piece of business. Each of the aspects of reactor design contains unknowns of considerable significance. Very little in the way of practical application of the fundamentals has found its way into the literature. But the subject is alive, as a cursory scan of current publications clearly demonstrates.
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
Demise of the Steady State Most chemical engineers are trained to accept the steady state as a desirable operating condition for chemical plants. We habitually strive to design for steady-state operation, even though we know that there are invariably fluctuations in every stream. This is justified by the usual necessity to overdesign around average stream properties and trust to a control system to maintain the average values. With the advent of periodic processing as a distinct subject for study, primarily through the efforts of Prof. F. Horn and his associates, we are now re-examining the desirability of the steady state. The prospect of improving chemical process performance through unsteady-state operation may indicate that the design and control criteria now used are not necessarily optimal. I n a recent paper [Chern. Eng. Sci. 21, 305-15 (1966)], J. M. Douglas and D. W. T. Rippin have examined the steady state and offer convincing evidence that unsteadystate operation may be more generally desirable than formerly suspected. If this is true, then process dynamics must be given more general attention than it now enjoys. Two instances for which the unsteady state is obviously necessary are pulsed extraction and parametric pumping, recently introduced by Prof. R. H. Wilhelm. I n general, the performance index for a process operating at steady state is different from that for unsteady-state operation; this is the principal justification for operating under transient conditions. This does not necessarily mean, however, that unsteady state is always necessary or desirable. The method of operation should always be that for which the performance index is higher. Now that process dynamics has reached the age of puberty, there are available the means properly to describe and design unsteady-state
processes with greater confidence than ever before. In their paper, Douglas and Rippin have considered two cases illustrating the value of the unsteady state. The first is the familiar isothermal CFSTR in which a second-order reaction occurs ; this is the classic case for a variable input. The second case is the chemical oscillator-Le., a continuous reactor which is periodically perturbed to vary the output. The oscillator has long been of interest, particularly to the control people. However, there doesn’t appear to have been any conclusive investigation of the oscillator with the object of improving performance. Investigating naturally oscillating systems, or systems in which the oscillations are induced, also introduces formidable analytical complexities. However, the methodology of Prof. Horn seems to be one way of treating some systems. As a result of the work of Horn and the provocative article by Douglas and Rippin, we should consider anew the status of steady-state processing. Douglas and Rippin have demonstrated that some of the accepted indicators for plant performance are suspect, at least, even though their inquiries are still in the formative stage. Should experimental attempts to demonstrate validity of these theoretical considerations not immediately do so, however, we should not lose heart but continue to ‘(chase the rabbit,” for the gains to be realized appear to be substantial. The key to subsequent investigations is the rational application of optimization theory in a manner that will not only accurately predict performance but also allow integration of technical and economic considerations. Provided the researchers can develop a consistent and comprehensible mathematical methodology for handling the great variety of processing problems, we may, in fact, be on the threshold of “the era of the unsteady state.”
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