I/EC SPECIAL FEATURE
RICHARD FLEMING Sun Oil Co., Marcus Hook, Pa.
Making the Most of Bench-Scale Experimentation in Process Development It has been wisely said that the only truly stable and changeless thing in our world of material things is the continuing fact of change itself. If the past is any guide, w e may be certain that one of the things that will characterize the future, as it does the present, will be the continually changing face of the pattern of things
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CHANGE is one of the real blessings of life, and one of its greater challenges. As a matter of fact, it has so fascinated most of us in technical pursuits that we have dedicated our working lives to the study, creation, and fostering of change, in a practical, technological sense. It is fitting that we should occasionally devote some time to considering the changing relationships of our own work techniques. Only in this way can we use those techniques most effectively and efficiently. Recent years have seen the development of methods and conditions which require a re-examination of the traditional role of bench-scale experimentation in process development. To understand what is meant by this "traditional role" we need a general description of bench-scale as a starting point. We then must discuss the kind of information such work was expected to provide, and 48 A
look at several jobs normally left to other phases of the process development effort. To put things into a more up-to-date framework, we will consider some of the factors affecting the role of bench-scale experimentation in development today, and what these have led to, and may lead to, in changing that role. Finally we will attempt to assess the impact of these changes on the bench-scale experimenter himself. Clearly, if it is our desire to make the most of bench-scale operations in present-day circumstances, such considerations are pertinent. Whet Is Bench-Scale Experimentation?
Have you ever counted up the ways in which various people decide what work they will call "benchscale?" Here are a few to indicate why a generally acceptable definition seems difficult to formulate.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1. Materials of construction of equipment used—if it is done in glassware, it is bench-scale. 2. Size of equipment—if equipment dimensions are smaller than some specific size, it is benchscale. 3. Space requirements—if equipment can fit into a certain area, it is bench-scale. 4. Capacity limitations—less than xxx lb./day is bench-scale. 5. Workers' qualifications— chemists do bench-scale work, engineers run pilot plants, or miniplants, or micropilot plants. 6. Philosophy of approach— bench-scale work is basic work, applied work is pilot plant. There are many other criteria that people apply. Obviously, depending upon what each of us thinks of when he says bench-scale, the role of bench-scale operation is somewhat different. For the purposes of discussion let us agree that traditional bench-scale work might be characterized as: A preliminary to pilot plant work, done with just enough material to permit reasonably accurate heat and material balances, with the objective of determining gross process characteristics.
In the pilot plant studies, specific information leading to a basis for commercial equipment design, the selection of economic alternatives, the preparation of sample quantities of the products, and the determination of workable solutions of commercial scale problems generally, were the principal goals. All these problems, and many more, were delegated to the follow-up work which took up where the benchscale operation, designed only to rough out the process, left off. Lest this rather impressive list of things not handled by traditional bench-scale research leads some of us to believe that, as some pilot plant engineers claim, bench-scale researchers create more problems than they solve, let us recognize that a great deal of skill and ingenuity were required to create the need for the more careful development effort to follow. Furthermore, this picture is now a little out of date and we will come back to look at this Work was pursued just far enough to define the major process problems and to devise useful approaches to them. Generally speaking, the results sought from such experimentation were: A tentative process flow sheet. Certainly this was far from the final form; usually subject to simplification in some spots, expansion in others. A basis for decision on pilot plant design. This required sufficient definition of the process characteristics to identify critical factors in deciding on which operations to pilot and on what scale. A basis for an economic analysis, limited pretty much to the material and heat balances and the flow sheet. A definition of the major problems for further basic research, and for market research and development activity. This latter information usually took the form of the definition of probable ultimate product qualities. For the most part these lookedfor results determined the place of bench-scale work in the over-all development operation. It provided the necessary scientific preamble to the more detailed and practically oriented work carried out in the pilot plant operation.
"Traditional" b e n c h - s c a l e experimentation rarely concerned itself with: selection of materials of construction raw material and finished products specifications continuous operation problems detailed study of parameters and their optimization scale-up in such operations as heat transfer and filtration effects of trace impurities recycling streams preparation of evaluation quantities of product automatic control of commercial unit transient effects of changes in flow of materials or heat or other variations in conditions catalyst poisoning, catalyst life, and other special effects of long-continued running cost reductions, alternative operations, and commercial equipment selection integration with plant operations by-product recovery solvent re-use
list again when we assess the modern position of bench-scale experimentation. Before we do this, however, let us try to identify some of the basic factors which have brought about, and are still causing, changes in the role of bench-scale operations in process development. We then can contrast past practices with current and probable future ones with somewhat greater clarity. Factors Causing Change in Role of Bench-Scale Work
Pilot Plant Costs. Building a pilot plant is expensive. Running a pilot plant is, ordinarily, even more expensive, and by a considerable margin. For example, a small catalytic reforming pilot plant might cost $50,000 to build and $100,000 to run for 6 months when operators, technical people, and analytical, maintenance, and overhead costs are considered. These are minimum numbers. Of course, more adequate automation of pilot plants is tending to reduce piloting costs somewhat. Nonetheless, a very sharp and significant rise in development costs is incurred when a project proceeds from the bench to the pilot plant scale. While this has always been true to some degree, this disparity in costs is receiving much more attention now than it has before. In the early postwar years such costs were paid largely out of excess profits otherwise lost to taxes. In more recent years, when profits were still relatively plentiful and the economy was rapidly expanding, development funds were still relatively easy to obtain. The current highly competitive situation requires much more definitive work, however, to justify the next big expense. Furthermore, recent technological trends toward the extended use of higher temperatures and pressures have widened the disparity in costs when comparing pilot plant scale with bench-scale work. T h e net result is increased interest in, and pressure on, getting not only more information but more practical information from bench-scale work. Research Competition. Increasingly stringent competition in the market place has its impact on research and development. More and more chemical process industry firms are looking to expanded and VOL. 5 1 , N O . 11
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I/EC
SPECIAL FEATURE
intensified research to protect their established markets and to provide the basis for diversification into new ones. This increased emphasis by competitors has forced a new sense of urgency into the development picture. The more competitive the circumstances, the more valuable is the time advantage gained by rapid development and commercialization. Because the design, construction, and initial operation of pilot plants usually represent a significant time lag, 6 months to a year frequently, there is a large incentive, sometimes of controlling importance, to bypass the pilot plant and to build the commercial unit on the basis of benchscale data. Again this greatly increases the requirement for more comprehensive bench-scale experimentation. In fact, in competition with pilot plant work, the speed and flexibility of bench-scale operation are among its biggest advantages. Growing Utility of Scale-up Theory. There has always been some incentive to bypass the pilot plant, but an important limitation has been the relative difficulty of scaling up bench-scale results to commercial operations with any assurance. In recent years we have considerably broadened our understanding of the theoretical and mathematical bases for such scale-up, and these are gradually permitting reductions in the scope of piloting required in some cases, and its elimination in others (5). There is, of course, copious literature in this field, and it is likely that a short literature study will turn up much useful information on scaling u p virtually every unit operation (3, 4). Availability of Computers. The availability of high-speed digital and analog computers has had an important impact on the ultimate capability of bench-scale experimentation. Unfortunately, this has not progressed in a practical sense nearly as fast as it theoretically could. The most apparent reason for this is the inability, or unwillingness, of the technologist to equip himself with the skills necessary to take fullest advantage of these very versatile tools. Even in such simple things as helping to digest the
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copious quantities of data obtainable from well set up bench and pilot equipment we are only now seeing frequent use of computational equipment. Use of available data to improve our understanding of the relationships between bench-scale and commercial unit performance is another seemingly simple application which justifies more attention than it receives. While some may argue that we shall ultimately be able to construct mathematical models of our systems and carry out our experiments on these without using the actual materials or items of equipment involved, I do not expect to see this during my lifetime. There seems to be little reason to doubt, however, that increased use of such models as adjuncts to and extenders of our more physical studies will permit greatly extended use of bench-scale studies without the necessity of pilot plant verification Availability of Scalable BenchScale Equipment. There was a time when every bench chemist was an expert glass blower. He had to be, to do his job. Our age of specialization has changed all this, and it is doubtful that many chemists do much of this anymore. However, for many years we have been limited to a relatively narrow line of standard laboratory equipment, most of which was a scale-up nightmare to any self-respecting engineer. Recently equipment manufacturers have begun to appreciate the fact that the beginnings of their applications problems are on the laboratory benches of the research labs and they would be well advised to reach down to that level with scalable equipment. This growing appreciation has had happy results for the bench researcher, for he now can use mixers, pumps, filters, centrifuges, flowmeters, extractors, automatic controls, and even reactors for which reasonable scale-up techniques are available and on which extensive data have been taken (7, 6-9). With this kind of equipment available, the extension of bench-scale data to commercial design becomes much more likely. Development of Microanalytical Techniques. Every new process
INDUSTRIAL AND ENGINEERING CHEMISTRY
seems to carry with it at least one new and different analytical problem, and most process developers realize the vital importance of careful, creative analytical support. So necessary are proper analytical data that not infrequently such work represents a substantial proportion of the total development cost. The desire to use bench-scale results for commercial application has created the need for many new analytical methods capable of rapidly yieldingresults of a high order of accuracy on samples of very small size. Without such methods the savings of time and money potentially realizable on the experimental program are lost by the need to produce large samples for analysis. Much progress has been made in this area. In fact, some microanalytical techniques arc of such accuracy that they are being used even when sample size is no limitation in the choice of the method. Particularly noteworthy have been the rapid advances in chromatographic and spectroscopic methods, which not only work effectively with small samples but yield a wealth of analytical detail that has heretofore been available only at great cost and with much tedious work. Not all of these problems are solved, however. Some are surprisingly difficult when the ease with which larger samples are handled is considered. For example, there is need for a method for making sharp distillation separations rapidly on very small samples. In a recent study of an extraction process we had trouble in getting effective stripping of the solvent from 10- to 15-cc. samples of product. We had neither the time nor the money to develop a stream analyzer or some other suitable technique, so we simply had to generate more sample. Fortunately, this was not a serious problem, but it illustrates the importance of analytical technique to the expanded use of bench-scale equipment. How Far Have We Come?
In sum, what do these factors mean to the role of bench-scale experimentation? Clearly they call for the bench-scale unit to solve many of the problems formerly left for the
pilot plant. In fact, the demand is for the bench unit to be the pilot plant. As is so often the case, the growing pressure of economic factors has been accompanied by increasing technical ability to handle the problem. If that is the case, where do we stand today? Let us return now to that list of jobs left for somebody else by traditional bench-scale work, and see. I am sure that you have recognized that the list, even as used before, was an oversimplification, since in some specific cases certain of these jobs could be and were done on a benchscale adequately some time ago. In further simplification, one might say that, generally speaking, most of these jobs can be and are being done adequately on a bench-scale in a much wider variety of development operations today. Beyond question, many can think of specific developments in which bench-scale could not or did not adequately handle certain of these tasks. Nonetheless, only in the case of the following does bench-scale experimentation fail to compete on a reasonably favorable basis with pilot operations : Preparation of evaluation or market development samples Effects of trace impurities Certain problems of scale—i.e., on operations for which no scalable bench equipment is available Organizational Means to Effect Changes
In addition to the basic factors outlined above, two important organizational moves have been used to assist in changing the posture of bench-scale experimentation : Moving the engineer into the earlier phases of process studies at the bench Providing closer economic direction of the development effort In general, these have accompanied a closer liaison of all the necessary technical skills with the bench-scale operation. The engineer and the economic analyst have introduced an element of practicality to bench-scale work that had been largely missing heretofore. Only a few improvements of major dimensions can be attributed to this new participation, but its chief value is found in the multitude of seemingly little things that are avoided which
have been known to cause headaches later. Such basic things as the selection of agitator design, reactor configuration, and reagent specifications used in the earliest experiments can mean much saved time in the long run. Experience seems to indicate that once the ice is broken, a new spirit of friendly cooperation, starting in the early phases of the laboratory work, has major benefits throughout the development. Problems in Further Development
Do not form the impression from the foregoing that the time has come to desert pilot plants for more secure places to work. Unfortunately, from a cost standpoint, pilot plants will be necessary as far into the future as most of us can see, because there are many problems in the way of our quickly realizing the ultimate development of bench-scale technique. There is a great catalog of problems of this sort, and rather than attempt to list them all, let's consider a few examples. We have said that problems involving the effects of trace impurities are difficult to handle on a small scale and that in some unit operations there currently does not appear to be suitable equipment for scale-up from bench-type operations. Examples of these are agglomeration, crushing and grinding, extrusion, and drying. A rather awkward problem in bench-scale experimentation at times is that of proper sampling. We are all familiar with the problem of obtaining representative samples from larger scale equipment. On the small scale this becomes the problem of obtaining samples of analyzable size which do not represent a major portion of the stream being sampled or do not unduly upset the system in some way. This is particularly troublesome when long-term time effects or recycle streams are involved. The developments in microanalytical chemistry have helped considerably here, but applicable techniques are not always available. Handling slurries on a bench scale is particularly difficult when these have a tendency to separate or deposit in the small tubing that is used. Heat losses per unit volume become large and are difficult to control in bench-scale equipment, especially when temperature gradients are involved or close control of high temperatures is necessary. In studying
reactions in which agitation and heat transfer through the vessel walls are important, there may be considerable scale-up difficulty. Phase contacting units which are subject to disturbance by small variations in heat supply or flow rate are difficult to operate on an extremely small scale to obtain meaningful results. Control of exceedingly small liquid and gas flows poses complicated problems at times, although some very useful items of equipment are now coming on the market. Small units are characterized by very high surface-volume ratios and this may become of great importance in specific cases quite apart from heatflow problems. The classical case is the difficulty of demonstrating arsenic poisoning of platinum reforming catalysts, even on a pilot scale, because of the adsorption of this poison on the metal walls of the system. There are significant problems in equipment capacity matching in small scale operation. For example, some difficulty has been experienced in catalytic cracking bench-scale units in which the catalyst regenerator is not properly sized relative to the reactor. Oversizing leads to much more efficient regeneration of the catalyst than is normally to be expected on a commercial basis and this can yield very significantly different results in the cracking step. Undersizing leads to similar difficulties, in that cracking takes place in the presence of improperly reactivated catalyst. Similar disparities must be watched when bench-scale units are assembled from relatively standard items of equipment. For example, the use of unpractically large filter surfaces may obscure the effects of cake compressibility or gelatinous impurities. Some problems occur only on the larger scale. Typical of these might be the surging and bubbling which greatly reduce the intimacy of contact in fluidized reactors of large size. Such effects are frequently damped out entirely by the wall effects in small scale units. In the petroleum industry many operations produce coke, and there is great difficulty in forecasting how much coke production will limit the operating time of the large scale unit. A relatively obscure factor here, such as surface roughness or the physical properties of the cokelike material VOL. 5 1 , NO. 11
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itself, m a y m a k e all the difference in t h e w a y t h e deposited material adheres a n d therefore eventually plugs the pipe or t u b i n g involved. While corrosion m a y be studied in a small scale unit with t h e use of corrosion samples, it is possible in the large unit to r u n into difficulties with bimetallic electrolytic corrosion because of the contact of two dis similar metals in the reaction m e d i u m . Different residence times, fluid velocities, a n d mass velocities can be troublesome if n o t properly evaluated. These examples should indicate clearly that bench-scale experimenta tion m e a n t for use in t h e direct design of commercial units must be very carefully done. A whole host of things, which a r e n o t normally con sidered in great detail in pilot plant work, become significant a n d must be thought a b o u t when o n e bypasses the pilot plant, just because of the greater scale-up ratio involved. I n such bypassing we a r e doing without one of the principal advantages of the pilot plant—testing of o u r scaleu p know-how in connection with the process involved. For this reason we must be confident of o u r ability to foresee the problems we will encounter in large-scale e q u i p m e n t t h a t d o n o t show themselves on t h e small scale. While the trend toward bench-scale experimentation a n d its use in the direct commercialization of processes is clear, it is even more clear that t h e h a z a r d s involved in this kind of operation must be fully a n d maturely appreciated before the step can be taken with a n y real assurance. O n the other h a n d , we must keep the need for scale-up d a t a in proper perspective. Such things as t h e requirements of timing of the commercial unit, its size, a n d t h e probable costs of excessive safety factor or commercial error m a y be more significant than t h e technical niceties of the problem. A com promise of some kind is almost always necessary.
The Future for Bench-Scale Experimentation
W h a t then is the future for benchscale experimentation? I t seems clear that more a n d m o r e it will be necessary for us to extend t h e scope a n d function of bench-scale work. W e will have to develop t h e skills, knowledge, a n d experience which will permit t h e application of all 52 A
available tools toward more direct conversion of bench results to com mercial designs. Application of digital computers a n d a n a l o g d e vices should e x p a n d tremendously in t h e area of scale-up a n d permit i m p o r t a n t savings of both time a n d money. T h e currently rapid de velopment of scale-up techniques should also accelerate. M a n y more scalable items of bench e q u i p m e n t should be forthcoming, as e q u i p m e n t manufacturers recognize that com mercial application of their products m a y be importantly d e p e n d e n t o n how well commercial performance m a y be predicted from small-scale testing. As these trends increase o u r abilities a n d o u r confidence, pilot planting as we currently think of it will diminish greatly in i m p o r t a n c e . T h e r e seems every reason to believe that ultimately t h e necessity for pilot scale operations will disappear in all b u t the most unusual cases. W h e n that happens, bench-scale units will be used for virtually all commercial design studies, a n d large, multipurpose plants will be used for the preparation of m a r k e t evalua tion a n d testing quantities. Now, w h a t d o these changes m e a n to the individual concerned with process development a n d benchscale experimentation? Well, first they seem to indicate clearly t h a t h e will have to be a highly skilled person, even more t h a n is currently the case. W e m a y expect, in fact, t h e emergence of w h a t might be called a scale-up specialist. H e will neces sarily require a n extensive back ground in chemistry, mathematics, and engineering with a n unusual comprehension of design a n d operat ing problems. T h i s c o m b i n a t i o n of skills will permit h i m to bring to t h e j o b a m u c h d e e p e r a n d m o r e basic insight into t h e problems involved t h a n c a n be obtained by combining individuals with the separate skills alone. Because of his m u c h broader comprehension of t h e total problem and his higher level of skills, o u r development m a n of t h e future will be a very valuable m a n . For these reasons, too, it is likely t h a t we will sec h i m devoting a m u c h longer period of his career to development activity in all of its phases. His j o b will be m o r e complex, better paid, a n d more i m p o r t a n t to t h e organiza tion with which h e is associated t h a n a n y o n e of t h e individuals currently working in these areas.
INDUSTRIAL AND ENGINEERING CHEMISTRY
These References Will H e l p Y o u Decide W h a t Route to Take in D e v e l o p i n g a Process f r o m BenchScale
(1) Biribauer, F. Α., Oakley, H. T., Porter, C. E., Staib, J. H., Stewart, Joseph, IND. ENG. CHEM. 49, 1675
(1957). (2) Fleming, Richard, éd., "Scale-up in Practice," Reinhold, New York, 1958. (3) Johnstone, R. E., Thring, M. W., "Pilot Plants, Models, and Scale-up Methods in Chemical Engineering," McGraw-Hill, New York, 1957. (4) Jordan, D. G., "Chemical Pilot Plant Practice," Interscience, 1955. (5) Michel, Laurent, Beattic, R. D., Goodgame, T. H., Chem. Eng. Progr. 50,332(1954). (6) Roth, E. R., IND. ENG. CHEM. 46,
1428 (1954). (7) Schcibel, E. G., Ibid., 49, 1679 (1957). (8) Stockman, C. H., Lynn, R. E., Jr., Ibid., 50, 585 (1958). (9) Sutherland, J. D., McKenzie, J. P., Ibid., 48, 17 (1956).
As a m a t t e r of fact, such individuals a r e needed now, a n d this is the biggest challenge of all to y o u n g men currently working with pilot plants, bench units, a n d scale-up for design. If you a r e able to master these disciplines, to combine t h e m in a t h o r o u g h u n d e r s t a n d i n g of t h e development problem in all its facets, a n d have the energy to convert t h a t u n d e r s t a n d i n g into workmanlike performance, there a r e wonderful opportunities a h e a d . This is n o t a n easy thing to d o a n d it is evident t h a t there a r e m a n y in this field w h o a r e not willing to m a k e the extra effort required. N o w a d a y s most technical people seem to think t h a t t h e only w a y to higher incomes a n d greater responsibility is to move into t h e already crowded managerial competition. I would think that, if you really prefer technical work, but a r e seeking opportunities to improve your own abilities a n d your future earning power, you might look a r o u n d in your own backyard. T h e r e is every indication that, if you a r e now working in process development, your opportunities a r e manifold right where you a r e . T h e only question a p p e a r s to be w h e t h e r or n o t you h a v e the interest a n d the ability to take a d v a n t a g e of t h e m . NORTH Jersey Section, ACS, April 27, 1959.
