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Thirty-Five Years of BSL Gianni Astarita Department of Materials and Production Engineering, University of Naples “Federico II”, Naples, Italy
Julio M. Ottino* Department of Chemical Engineering, Northwestern University, Euanston, Illinois 60208
Few engineering books remain influential for 35 years; even fewer can be said to have affected undergraduate and graduate education. Transport Phenomena (BSL)accomplished both and it brought fundamental changes to the way chemical engineers t h i n k BSL can be arguably regarded as the most important book in chemical engineering ever published. In this essay we place BSL in the context of its times and surrounding paradigms, review and comment on the early reception of the book, offer comments on style, and speculate on its possible revision.
Preliminaries Transport Phenomena by R. B.Bird, W. E. Stewart, and E. N. Lightfoot (BSL henceforth) is undoubtedly a ground-breaking, landmark book and we are honored to have the opportunity t o write about it. Given the stature of the book, this is not an easy task. We try to first present the book in historical context: which were the books in tangentially close areas available at the time; what had been the cultural paradigm of the profession of chemical engineering before BSL appeared? We then proceed to review what was the early reception of the book, and in what way it became an essential book for the profession. Early reviews were not without criticisms, and we analyze those from the vantage viewpoint of 35 years of hindsight. Finally, we discuss in what sense, if any, the book could be improved. Both of us have a strong personal relationship with BSL which drove us t o write this essay. In order not t o disrupt the flow of the paper, our own experiences with the book are briefly described in the Appendix.
BSL and Its Times Books that leave a mark belong to two classes: permanent books, those that “close a chapter”, that age well and that are as valid now as when they were written. H. Goldstein’s Classical Dynamics (1950)and R. C. Tolman’s The Principles of Statistical Mechanics (1938) belong to this class. Classic books can be permanently consulted and they have a long lifetime as reference material. Some books become classics; others appear to have been designed as such. S. Chandrasekhar’s Hydrodynamic and Hydromagnetic Stability might be an example of the latter. Almost by definition, any good mathematics book-they are not as many as one would think-belongs to this class: G. H. Hardy, J. E. Littlewood, and G. P6lya, Inequalities, is a good example of this kind. The other category is new paradigm books. These books chart new territory and suggest new problems. If successful, they can contribute to their o w n destruction by sparking many new advances in the field. Mandelbrot’s delightful, but often frustratingly impenetrable, The Fractal Geometry ofNature (19811,and the deceptively simple and trend-setting Scaling Concepts in Polymer Physics (de Gennes, 1979)unarguably belong
* To whom correspondence
should be addressed.
to this class as well. Only a few books manage to belong to both classes; L. Pauling‘s The Nature of the Chemical Bond (1939),a classic by virtue of permanence (three editions between 1939 and 1960) is most definitely a new paradigm book. The archetypical new paradigm book bursts onto the scene seemingly unannounced. Sometimes, to those familiar with the area, things may be a bit less dramatic; a new paradigm book connects in a coherent whole ideas that they had already seen as almost connected.’ New paradigm books can be messy and open new avenues; permanent books are error free and become classics. An evolving subject, by definition, should not be written about as a classic, but a new paradigm book is only successful to the degree that some organization appears present. The remarkable thing about BSL is that it is unquestionably a classic, but at the same time, and perhaps by appearing a t precisely the right time,2 it produced the fingerprint for a new paradigm in chemical engineering. How was BSL received when it f i s t appeared? Was it received as an unqualified success? Was it uniformly seen as a positive influence on the profession? Were there concerns about its utility? Before going into these questions it may be useful t o place BSL in the context of surrounding books. By the mid 1950s T. K. Sherwood and R. L. Pigford’s Absorption and Extraction was already in its second edition and W. R. Marshall and R. L. Pigford, The Application of Differential Equations to Chemical Engineering Problems, was already a decade old.3 Applied Mathematics in Chemical Engineering, by H. S . Mickley, T. K. Sherwood, and C. E. Reed, had its Asian edition in 1957,Bird’s Molecular Theory of Gases and Liquidswith J. 0. Hirschfelder and C. F. Curtiss-appeared in 1954;H. Schlichting‘sBoundury-Layer Theory appeared in English in 1955; J. Crank’s The Mathematics of Difusion came out in 1956;and both, E. R. G. Eckert and R. M. Drake, Heat and Mass Transfer and H. S . Carslaw and J. C. Yaeger, Conduction of Heat in Solids, had second editions in 1959. The first English translations of D. A. Frank-Kamenetskii’s Difision and Heat Transfer in Chemical Kinetics and L. Landau and E. M. Lifhshitz’s,Fluid Mechanics, appeared, respectively, in 1955 and 1959. Shortly thereafter, in 1962,another Russian classic, V. G. Levich’s Physicochemical Hydrodynamics, made its appearance. Meanwhile, also in 1962,in nearby Minnesota, R. Aris produced his Vectors,
0 1995 American Chemical Society Q888-5885/95/2634-317~~Q9.QQIQ
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Tensors, and the Basic Equations of Fluid Mechanics. That was the scene when BSL burst onto the stage. Tools coming from physics, physical chemistry, and mathematics were making inroads in chemical engineering; the discipline of transport phenomena, with its intertwined juxtaposition of molecular and continuum viewpoints, served as a confluence point for many of them. There was, however, no sudden paradigm shift and the prevailing paradigm of the time, unit operations, survived for quite some time. Like transport phenomena, the unit operation concept had also once been a revolution. It may be convenient to review a bit of its history as well. It can be argued that unit operations as the cultural paradigm of chemical engineering, replacing that of industrial ~ h e m i s t r ywas , ~ established in 1923, with the publication of the book by Walker, Lewis and McAdams (WLM). Of course, some elements of unit operations were taught before 1923, and some industrial chemistry was taught after 1923 (for example, Shreve’s The Chemical Process Industries was published in 1945). However, it is unquestionable that the paradigm of the profession had changed with the appearance of WLM: in the preface to the Badger and McCabe Elements of Chemical Engineering book of 1930, one reads. “The viewpoint that chemical engineering is based on the unit operations and their application is relatively recent ... It is therefore not surprising that thus far only one important book has appeared to cover this field, namely, WLM.” The Badger and McCabe 1930 book was not the only one of the WLM type published after 1923; the first phase of the wave of such books probably culminated with the 1950 book by Brown et al. which, interestingly, and as noted in the preface, is the first one which was called Unit Operations? After 1950, transport phenomena started creeping with increasing importance in books dedicated to unit operations: almost none of it can be discerned in the 1955 book by Treybal, but it is clearly, if latently, present in the McCabe and Smith book of 1956, and even more clearly in the Foust, Wenzel, Clump, Maus and Andersen (FWCMA) book of 1960. All these books were meant to be improvements on WLM, and indeed in many senses they were that-one can see the subject matter undergoing a significant evolution. FWCMA was probably the first book in the traditional mold where the idea of transport phenomena can clearly be identified; part I1 (166 pages out of a total of 754) is entitled “Molecular and Turbulent Transport”, and it deals with transport of momentum, heat and mass. “Newton wrote that Galileo had discovered that the constant force of gravity produces a motion proportional t o the square of time ... But Galileo said nothing of the sort” (Kuhn, 1970, p 139). Often the proponents of a new paradigm-Isaac Newton, in the above passage-will attempt to pave the way by finding early backers for the viewpoint. Thus Bird (1958)finds precedent for the viewpoint in Newton, Fourier, Fick, and many others. Later, Bird et al. (1965) advise readers “who wish t o pursue [transport phenomenal” beyond the level of [BSLI to consult a few books that tie closely with “ours both as to notation and viewpoint”. Landau and Lifshitz’s Fluid Mechanics is endorsed as “... a Nobel Laureate’s view of advanced transport phenomena.. .”. Nevertheless, in spite of all of these signals paving the road, efforts by proponents, and a general willing-
ness to adopt a transport phenomena viewpoint, it is apparent that there was already some concern in the minds of many about the balance between engineering and engineering science. “The transfer of allegiance from paradigm to paradigm is a conversion experience that cannot be forced” (Kuhn, 1970, p 151). Reviews of BSL make this clear. P. V. Danckwerts’s and T. K. Sherwood‘s reviews appeared within a month of each other. P. V. Danckwerts reviewed BSL for Endeavour in October 1961. He said “... this is a first rate book of its kind”. However, he qualified that statement by saying that “The current tendency for the engineering syllabus in U. S. universities t o ‘go scientific’ is reflected here in an avoidance of topics ... I believe that this trend may go too far, and that we ought to produce more powerful teaching methods for developing both insight and the qualitative analysis of problems”. T. K. Sherwood, a month earlier, in Chemical Engineering Science, September 1961, had expressed a similar viewpoint. Sherwood said “This is probably the most important textbook on chemical engineering t o appear in many years.’’ His endorsement, like that of Danckwerts, was not unqualified either, for he said “the worked examples are ... selected from those problems which can be solved, rather than those which need t o be solved” (Sherwood’sitalics). Further he sounded an ominous warning: “if perspective is lost through enthusiasm for scientific and mathematical analyses, the engineer will be less effective in industry.” The last review we mention, but one that preceded both Danckwerts’s and Sherwood’s reviews, is that of John Butt in the June 1961 issue of AIChE Journal. Butt was then at Yale and only 2 years out of his Ph.D. John Butt’s opening sentence was “In the opinion of this reviewer this book is probably one of the most important to appear in the field of chemical engineering in many years.” In the closing section of his review, afier going through a somewhat standard listing of the contents of the book and alluding to their importance, he tackled the ”engineeringlengineeringscience” debate: ‘. . . the text leads t o a good idea of the meaning of engineering science, particularly to those who may be convinced that the term is only a combination of two words sitting in somewhat uneasy proximity to one another.” By 1965 BSL had already undergone its fourth ~ r i n t i n g .But ~ the rise to stardom had not been without reservations. Efforts to spread the transport phenomenon viewpoint, and its widespread utility, continued unabated (for example, a t the May AIChE Meeting in San Francisco (Bird et al. 19651, there were six talks grouped under the title Selected Topics in Transport Phenomena). Two years later there were still questions. Witness, for example, the title of a six-paper session at the 1967 May Salt Lake City AIChE Meeting: “Transport Phenomena: Have We Gone Too Far?”, which included presentations with titles ‘We Have Not Gone Far Enough” and ‘Yes, We Have Gone Too Far”.
Analysis vs Synthesis? “Get the habit of analysis-analysis will in time enable synthesis to become your habit of mind”. (Frank Lloyd Wright) Let us now consider the nature and merits of the criticisms raised by the BSL reviews alluded to earlier. Sherwood said “In a sense [BSLI is a dangerous book, for it is so well done that it will probably accelerate the
Ind. Eng. Chem. Res., Vol. 34,No. 10,1995 3179 trend towards emphasis on analysis in chemical engineering curricula” (our italics). Danckwerts made, somewhat more strongly, the same point, and here we are completing the previous quote: “The current tendency for the engineering syllabus in U. S. universities to ‘go scientific’ is reflected here in an avoidance of topics, such as heat transfer to boiling liquids, that cannot be dealt with analytically. I believe that this trend may go too far, and that we ought to produce more powerful teaching methods for developing both insight and the qualitative analysis of problems.” The last idea is found again in Shenvood‘s review: “The book poses a challenge for someone to procure an equally good text dealing with the engineering aspects of chemical engineering“ (Shenvood’s italics).8 The comments quoted above were in essence predicting that the publication of BSL would reinforce the trend toward analysis in engineering curricula. That indeed has happened, and it is therefore logical to ask oneself whether such an effect has been beneficial or not. Wright’s quote provides a clue. Engineering, in the end, is synthesis, not analysis. However, the analysis part is the one that provides the foundation on which the synthesis ultimately rests. While it is difficult to give a broadly acceptable definition of “engineering science”, an important aspect of it is that it is cast in the language of mathematics-hence the problems dealt with are those which can be solved. This seems (as Shenvood implied) to be in stark contrast with engineering, which has t o do with real-life problems which need to be solved even if they cannot be analyzed rigorously. The Romans built beautiful arches and aqueducts without any knowledge whatsoever of solid mechanics. But there is also the “art” of engineering science: the ability to move from a real-life, inexorably complex problem which cannot be solved but needs to be, to an elegant problem which can be solved, a meta-problem. The art of engineering science is the art of seeing which steps should be taken t o erase from real life the (hopefully)irrelevant details which make it so complex, to look at the essential aspects of it, or, in other words, to decide what is the appropriate meta-problem to look at: t o conceive the appropriate chain of idealization. There are three points of criticism implied, more or less explicitly, in the Shenvood and Danckwerts reviews: (i) BSL is “perfect” in dealing with analysis, but it gives little if anything in terms of engineering art. That is undoubtedly true; and the authors-certainly a t the moment of the writing-meant it to be that way. In the preface to the 1958 Notes which preceded the final version of the book (Bird et al., 19581,they wrote “The engineering science background of direct interest t o the undergraduate chemical engineer consists of two major areas: the first is the theory of equilibrium (or thermodynamic) processes, and the second is the theory of nonequilibrium (or transport) processes ... The theory of transport processes has not heretofore been recognized as one of the key engineering sciences, although much of the subject material of these notes has long been taught within the framework of elementary problems of unit operations and design.” In the preface of the final version, they wrote “Because of the current demand in engineering education to put more emphasis on understanding basic physical principles than on the blind use of empiricism, we feel there is a very definite
need for a book of this kind.” The choice of the adjectives “elementary”and “blind” may have been interpreted by some as disdainful of engineering art; but that is irrelevant. The authors did not intend to do more than present the analysis, and that they did beautifully. That can in no way be construed as a shortcoming of BSL; when the authors clearly state that they are going to deal with subject A, nobody should expect that the book should discuss subject B. Subject B may be quite important, even essential to engineers, but that is an entirely different matter. However, it may be argued that lack of emphasis on qualitative analysis is perhaps a shortcoming of BSL. It is true, for instance, that approximately equal signs almost never appear in BSL: the reader is not taught in any way how to perform a preliminary order of magnitude analysis, which is clearly one of the steps of engineering art. The authors did not intend to teach that; and anybody who has tried to teach this subject knows how difficult it is: there is no algorithm, every new problem has its own twists, it is, after all, an art, not a science. Books where the analysis is discussed at a high level of scientific competence, but which also include numerous preliminary order of magnitude estimates and scaling ideas are not impossible: the 1972 book by Tennekes and Lumley, Landau and Lifshitz (19591,and the 1985 book by Pearson come to mind; in a decidedly chemical engineering setting one might mention Churchill (1979).The instructor using BSL as a textbook has to provide instruction on the qualitative estimates; and this does tend to partly reduce the status of BSL to that of a reference book. (ii)By being “perfect”in the analysis, BSL may result in less emphasis being given to the engineering art. This is somewhat of a preposterous claim. If one writes a perfect book on thermodynamics, one cannot possibly be accused of thereby resulting in less emphasis being given to, say, fluid mechanics. It is up t o the experts on fluid mechanics to come up with an equally perfect book on their subject. Shenvood himself published in 1963 his book on process design; that it has not achieved for engineering art the status that BSL has achieved for engineering science is certainly not a fault of BSL. Indeed, this brings up another interesting point. As said before, the book by Brown et al. (1950)was the first one in the WLM tradition to be called Unit Operations. WLM itself, and the later 1937 edition of it of which Gilliland was a coauthor, had the title Principles of Chemical Engineering; the Badger and McCabe book was called Elements of Chemical Engineering. In other words, books in the WLM tradition were conceived, written, and titled as books which gave (and arguably tried to capture) the essence of chemical engineering. By the very fact of making this claim they were liable to the criticism that they did not. The authors did not in any way (either in their preface or in their title) claim that BSL represented the essential conceptual content of chemical engineering; by the same argument, BSL is not liable t o the criticism that there is more t o the essential basis of chemical engineering than transport phenomena. It is, however, true that nowadays the emphasis given to engineering art in the curriculum is perhaps significantly less than would be desirable. We do not believe that this can be imputed to the publication of BSL, but neither do we think that this is a good state of affairs. But we would be digressing too much if we started a
3180 Ind. Eng. Chem. Res., Vol. 34, No. 10, 1995 discussion of what is good and what is not so good in today’s typical curriculum. (iii) By being so accurate and careful in the analysis, and excluding entirely the art, B S L sends subliminally the message that the art is not important. This is perhaps the most serious criticism, because there is more than a grain of truth in it. The choice of the adjectives ”elementary” and “blind” in the two prefaces clearly shows that the subliminal message was, perhaps somewhat subconsciously, in the mind of the authors themselves. This is perhaps why one of us (GA) finds himself more and more using BSL as a reference book rather than a true textbook. For example, in teaching about effectiveness factors in isothermal porous catalysts, one may point out first that the catalyst dimension, L , must be a relevant parameter, and that (at least for a first-order reaction) the only dimensionless group one can construct which contains L is the Thiele modulus, Th. Since when L is very small the effectiveness is obviously unity, the low Th asymptote is readily established. Now if L is very large (say infinity), there is no external length scale, and so the system must choose its own length scale, and a trivial dimensional analysis argument shows that can only be the square root of the ratio of diffisivity to kinetic constant. Hence everything happens over such a length scale, and if one doubles the catalyst size one is simply adding more useless catalyst, so the effectiveness factor must be inversely proportional to Th, and the proportionality constant is likely to be of order unity. Having established that way the behavior in the two asymptotic limits, one ends up by telling the students to carefully study pages 542-546 of BSL. The “potentially dangerous subliminal message” has been canceled that way, and the worth of BSL as a reference book is preserved. Going back to the argument that engineering art is the ability to reduce a nasty real problem to a beautiful one which engineering science can deal with, such an art is intrinsically impossible unless one knows which problems engineering science is capable to deal with. Having (hopefully) rebutted three of the criticisms explicitly or implicitly put forward by Sherwood and by Danckwerts, it is not to say that BSL is totally flawless. No book, when looked under the perspective of 35 years, could possibly be. This “rebuttal”,however, should not be taken literally; in a broader context, Sherwood’s and Danckwerts’s comments were precisely on the mark. All current discussions about reintroducing engineering into engineering-a problem t o which chemical engineering is certainly not immune-are a vivid manifestation of this fact (see, for example, Moses, 1994). Undoubtedly things went too far. As early as 1968 Bird felt compelled t o express that “The publicity associated with the development of our transport phenomena course seems to have misled some people into thinking that we have abandoned all reason. We regard the transport phenomena course as a third semester of physics, made necessary by the fact that elementary physics includes almost no material on fluid dynamics, heat conduction, and diffusion. We still include in our curriculum two 3-credit lecture courses in unit operations, a five credit unit operation laboratory course, as well as courses in chemical reactor operation, process dynamics, and process design”. Several years later Danckwerts (1982), and in a different context, let his feelings on the general state of the profession be known again: “I have felt for some years that chemical
engineering is weighted-down with more mathematics than it can support...”. The question is not whether or not transport concepts and their underlying mathematical foundations should be taught; they most certainly should, although, now that the ideas have been accepted, it is not clear if it should be as a unit or as separate entities (i.e., separate courses for fluid mechanics, heat transfer, and mass transfer, possibly involving different books). The question is how the ideas and viewpoints put forward in BSL should be reinterpreted and meshed in today’s curriculum.
&-Engineering BSL (or Other Books Other People Should Write?) “We must not let daylight upon the magic” (Walter Bagehot) “Form follows function” is the architectural dictum. In BSL’s case it could be argued that things are the other way around. Most books are one-dimensionaland designed around a simple principle: it is permissible to refer to past material and only sparingly to what lies ahead. However, as anybody who tried to write a book can testify, putting an entire field together and conforming to this simple precept is not easy. Let us compare the style of BSL with other books of its time, say Landau and Lifshitz (L&L)and Levich, which could be regarded as linear books.g Old books, for example, H. Lamb’s Hydrodynamics (18791, are composed of articles, in Lamb’s case, 385 of them. So are Landau and Lifshitz’s and Levich‘s. Articles are the unit of this kind of book, groups of articles forming cogent chapters. At the end the product looks perfect, yet one may naively assume that the process could have been carried to completion by starting at the beginning and finishing at the end, writing one article every other day, never looking back. Levich follows the same architecture and runs for 136 articles, just three more than L&L’s first edition. BSL deviates radically from this architecture, each part being fixed by a two-dimensional arrangement; only at the end the entire product can be seen. L. M. Milne-Thornson’sTheoretical Hydrodynamics (19501, not a much older book, still retains Lamb’s flavor: chapters have subsections, but in spite of a rather unusual numbering, none of them seems to be more important than another. Structure is the most salient aspect of BSL, and it is what gives the book its solidity ant its undeniable appeal. The interlocking structure may, however, turn out to be a problem as well. This symmetry being in place, it makes changes problematic; a slight imbalance might destroy its symmetry. BSL, like Kafka, seems non-editable. This tidy organization might have pervaded other aspects as well. Approximation, ordering, and intuition play an essential role in Levich and Landau and Lifshitz (witness the Turbulence chapter in L&L and the Coagulation chapter in Levich); solutions always have ranges of validity. If there is a recipe, it takes a while to grasp. In BSL, on the other hand, there are no ‘louts” or ”ifs”;everything is made t o look crisp and clear. Not even the sections on turbulence escaped this influence; in contrast with this, Tennekes and Lumley (19721, a good book on the subject written about a decade later, when much more had been accumulated, has a note
Ind. Eng. Chem. Res., Vol. 34,No. 10,1995 3181 about the use of symbols, one for errors of the order of 30%, another for prefactor ranging from 1/5 to 5 , etc. In many respects one gets the feeling that if one follows the rules one can hardly go wrong. There are virtually no approximately equal signs, =, proportionality signs, =, little or big O(.)symbols, inequalities, ’or
