SCALING UP A PROCESS A Panel Discussion Given ; a hypothetical process, its materials, reactions, equipment, unit operations
Pmbkm; which areas can be scaled up directly, which should be pilotplanted, what are the obstacles?
You saw the Row sheet on the cover You saw the questions
You saw the experts
h e r e are the Answers I..
The Process Liquid A is allowed to react with a gas in the presence of a solid catalyst held in liquid suspension in reactor I. The reaction product is then pumped to a centrifuge where the catalyst is removed, washed, and sent to a regenerative process for recycle. The wash water from the centrifuge contains both reaction product and unreacted A which are recovered by distillation. The effluent from the centrifuge is sent to precipitation tank 11, where liquid B is added. The reaction product is a solid which is recovered by filtration, washed, dried, ground, packaged, and stored. The filtrate is sent to a distillation column for recovering B. Primary Reactor I. The solid catalyst is held in suspension by liquid A which is present in excess. The gas is sparged into the reactor. A Hr. is highly exothermic, reaction temperature is 210" F., operating pressure is 150 p.s.i., and k is known from laboratory experiments in a 2-liter apparatus. No appreciable change in density occurs, but fluid viscosity increases by a factor of 2 during the reaction. Centrifuge 11. Catalyst pellets, about '/la inch, are removed from the reaction mixture in an existing batch centrifuge. Both the reaction product from I and A are water soluble and will be removed by water washing. The wash will be frac-
578
tionated to recover both A and the reaction product which are introduced into the process at appropriate points. Distillation Column VIII. Wash water from the centrifuge will be held in a hold tank until sufficient has accumulated to operate the column. It is proposed to use a bubble-cap column. Vapor-liquid data for the system are available. Precipitation Tank 111. The reaction mixture passing through the centrifuge is pumped into the precipitation tank where reactant B is added in excess. The A present is inert at this point. The reaction product obtained in the precipitation tank is a solid. A H r . is highly endothermic, reaction temperature is 80" F., reaction pressure is atmospheric, side reactions begin at 100' F.! and k is known from laboratory experiments in a 2-liter apparatus. Little change occurs in density and viscosity of the fluid. The final reaction mixture contains 15% solids by weight. Filter IV. The product from the precipitation reaction is to be recovered by filtration. The solids content is 15% by weight, and the slurry, which has the consistency of a clay slurry, is thixotropic. The cake is compressible and some difficulty was experienced in the laboratory using a Buchner funnel. The cake will be washed with water which is discarded. The filtrate comprising A and R will be fractionated.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Distillation Column V. Several packed columns from a defunct process are available for separating A and B. I t was found in the laboratory that a 2-inch diameter column, packed with 2 feet of l/s-inch carbon rings, would do the separation satisfactorily. Drier VI. The product to be dried has a softening point of 100' F. It was easily dried in the laboratory by placing overnight in a vacuum oven at IO" F. Grinder VII. Since the product has a softening point of 100" F., a cooled grinder will probably be needed. Good grinding was done in the laboratory by chilling the product to 40 'F. and using a chilled mortar and pestle. Agitation
Shelby A. Miller. For the purpose of this discussion, I am an agitator. I have worked with agitation and mixing problems for several years; hence my discussion will be confined to those parts of the process which involve this operation. Here there are two agitators-a primary reactor, I, which the flowsheet shows as some sort of an agitated pot and which the process description says is actually a 2-liter reactor. That is all we are told at present. Also the flowsheet shows a precipitation tank, 111, which is really another reactor. The process description tells you what goes on there.
At this point the question should be asked: Do we need two other agitated pots? As it stands now, each of these two pots must do double duty, as a reactor and as a slurry feed tank. From our friends the chemists who have done this on a bench scale, we know a catalyst is used and that its particles are baseballs inch in diameter. This catalyst is slurried into a presumably uniform suspension with a reactant liquid. A gas is admitted t o the reactor, reacts catalytically with the liquid (mechanism unstated) and, during the reaction, viscosity of the mass is doubled. The reaction, said to be highly exothermic, is carried out at a temperature of 210: F. and at an operating pressure of 150 p.s.i. We have a specific reaction rate constant k as determined in this 2-liter pot. Thus, the problem is one of reactor design which involves an agitator. Nevertheless, let us first look a t the problem from the standpoint of agitation. Immediately three major problems are apparent, one of which will be the important one, but a t the moment we do not know which. First, there is a solids suspension problem-density is unknown and big chunks must be kept suspended. Second, because the gas must dissolve before it can react, a gas solution problem is assumed. Third, there is a heat transfer problem because we are told that the reaction is highly exothermic. Also, there are other question-raising items. Because a gas is introduced into a material whose viscosity is increasing, we wonder: Will difficult frothing or perhaps a gas-disengaging problem be encountered? We have the viscosity increase to contend with, and because the chemists have not told us enough, I must go back to them and ask: What is the viscosity of this stuff? If you do not know, then give me a sample so that I can find out myself. Doubling viscosity at 1 cp. may be trivial, but doubling at 1000 cp. may present serious problems, indeed. Also, the process involves a relatively high temperature and a respectable
Shelby A. Miller, Chairman of Chemical Engineering at the University of Rochester, has been in chemical engineering education since 1946. With Du Pont from 1940 to 1946, h e worked in development a n d equipment specifications for agitation, gas dispersion, a n d filtration. Author of the subsection on gas dispersion in Perry’s Handbook, third edition, a n d various articles on agitation, Miller writes the annual review on filtration‘ for Industrial a n d Engineering Chemistry. H e is a former Fulbright lecturer.
pressure of 150 pounds; therefore, operating with a conventional stirred reactor will involve a seal problem. Added to all this is the usual material of construction problems. What can we do about design? Perhaps piloting this process is not necessary, but if not, then it is because the bench-scale operations were done in a way so that some procedures, of which agitation is one of the most important, can be scaled up, based on information given us by the chemists. I n other words, call it a pilot plant or what you will. The model exists now, and if it is a good model, perhaps we can operate from it right up the line-€ortunately in this instance, from a 2-liter rather than a microgram level. However, the answer to the piloting problem may be intermediate scale up. In my opinion this is necessary because information about these agitated reactors is insufficient. We may be gradually moving away from piloting, but in many operations the goal described years ago by MacAdams and Hanks is still remote. This paraphrased is, “If we had our druthers, we would sit down with a pencil and paper and from properties of materials involved and the way nature works, we would calculate with complete assurance, a workable design.” For agitated tanks, this is not yet possible. For the first reactor, if solids suspension is a problem and if the model is good, scale-up can be done with fair assurance, probably on the basis of equal power input per unit volume. If gas absorption depends on beating u p the gas and absorbing it from a dispersed mass, this can be scaled up also, but on a somewhat different basis. Actually, it would probably be based also on equalpower input per unit volume, but if so, equal performance in the usual sense will not be obtained. Instead, an equal coefficient will be obtained which in this case will surely mean a longer reaction cycle as the scale size increases. If heat transfer at the wall of the precipitation tank or a t any other surface in the plant reactor becomes important, this can also be scaled up, but by a well worked out correlation which is summarized in a number of design publications. Now it is apparent that a big “if” underlies this which requires consultation with the chemist again. How was the reaction constant k determined? O n what does it depend? Is it a function of the gas absorption rate? Is it a function of the uniformity of suspension of the catalyst? Need the reaction rate be controlled so that heat can be removed? If these facts are known, perhaps a good design can be scaled up. However, if this 2-liter reactor is of the usual kind-a round bottom, a neck, creases in the side, and a bendable
glass or stainless steel half-moon that drops down inside-it is probably about 8 inches in diameter and would be filled a t its deepest point with 4 inches of liquid. This, of course, will resemble nothing in the hypothetical scaled-up plant. I would hesitate to scale up such a flask, but I have both done it and seen it done. Sometimes it works. But because of unknowns involved here, particularly the gas-disengaging problem, this probably requires an intermediate step-not necessarily of the whole process but of this reactor at least. I have discussed mainly the first reactor because this seemed the toughest job. A more or less pure liquid product is brought from the first stage into the second reactor for blending with another liquid which produces a precipitate. Not much is known about the product except that it forms fine particles that agglomerate-we are told that the solids form a thixotropic mass similar to a clay slurry or a clay slip. Blending the two materials will probably not be difficult, but local excesses may be vitally important. Since we have no information on this, the chemist will have to be consulted again. If 15% solids are made into a thixotropic mass, there will be a tankful of high viscosity material. If the material is added gradually to completion and if the agitator is efficient during the first stage, the product added last will have little chance for rapid intermixing. However, if local excesses are not a problem, agitation will not be difficult; it could be scaled up with fair assurance. There is a final quirk in this second reactor-Le., getting the material out of the tank. This can be just as critical as the reaction process itself. I n my opinion, the primary reaction vessel needs a n intermediate, properly designed prototype. But before deciding about the second reactor, the precipitation tank, I would have to consult the chemist.
Distillation Edward G . Scheibel. The flowsheet presents two distillation problems. I n the first, corresponding to item VIII, all vapor-liquid equilibria data are available. Performance of large scale distillation equipment has been studied more fully than that of the small scale type, and therefore can be designed with more coinfidence from vapor-liquid equilibria data. Specifications for degree of separation, determined by the chemist, depend on requirements of the process for recycling solvents. Using the equilibria data, the number of theoretical plates can then be determined by standard methods. Thus, the pilot plant for this distillation operation could be omitted. Nevertheless, VOL. 50, NO. 4
a
APRIL 1958
579
some distillation equipment may be required by other operations of the pilot plant. This could be done batch-wise or continuous, depending on the equipment available. This distillation operation, of course, requires separation of three compounds-the product going overhead which is more volatile than water, water, and product A, which is less volatile than water. T o obtain pure materials, this must be done either in batch operation with intermediate fractions or in two continuous columns. t\rrangement of the columns will depend on vapor-liquid equilibria and boiling points to permit the process design engineer to decide whether the product and water or whether the water and A are separated first. The second distillation occurring in column V is said to perform satisfactorily in a packed column, 2 inches in diameter? with 2 feet of packing. However, scale-up of packed columns is uncertain. Correlation data suggest that the H E T P varies with about the 1.5 power of the column diameter. With frequent redistribution, of course, this variation can be restricted to a 1.2 power.
I Edward G. Scheibel, Director of Engineering for the York
Process Equipment Corp. and Adjunct Professor at the Newark ’ College of Engineering, joined the H. C. Bugbird Co. in 1937 and rose from assistant chemist to plant superintendent. Later he was employed by the M. W. Kellogg Co. as a chemical process design engineer, by Hydrocarbon Research as a chemical engineer, and by Hoffman-La Roche as head of its chemical engineering department. Scheibel has published over 40 articles on design of diffusion operations and application of physical chemistry and thermodynamics in chemical engineering. He has been granted 12 patents.
580
If the 2-inch column is scaled up a hundredfold in capacity-Le., to a 20inch column, the scale-up factor is 10. Assuming we design for frequent redistribution, using packed beds of 6 to 8 feet in height, 32 feet of packing would be required to duplicate the performance of the 2-foot column. Thus, columns larger than 20 inches will obviously rise to considerable height. There are indications that most of the efficiency in a packed column occurs in the entrance effect, and this contributes a n uncertainty to the correlation of H E T P for packing. Experimenters have in some cases reported that more theoretical plates can be obtained with half the height of packing than with the full height. This suggests that all efficiency occurs at the entrance. However, the best correlations call for a scale-up of the packing height more than proportional to the diameter. This, of course, will make a large diameter column uneconomic; therefore, the reported performance on 2 feet of packing in the 2-inch column must be translated to a plate-type column-either perforated or bubble plate. One method is to estimate an H E T P on this 2-inch column by selecting a number a t random-e.g., 4 inches-and designing the large unit on that basis. However, going back to the laboratory for vaporliquid equilibria data would be more reliable for determining whether 6 theoretical plates are needed, or more or less. Then why pilot a distillation process? We certainly don’t need the performance data. However, there are frequently many justifications. So far, only capacity and efficiency have been considered. Invariably, solvents are recycled. All reactions have side reactions and byproducts which accumulate in these recycle streams and interfere with performance of the auxiliary equipment. They may inhibit a catalyst, or finally build u p to the point where they are expelled in the product stream and thus throw such streams off specification. T o demonstrate a chemical process, it must be carried out in its entirety with all operations ultimately used in the full scale plan. This can be done only by setting up a pilot plant to recycle the streams and examine the process in detail. Centrifugation
John B. Tepe. The design or development engineer frequently is introduced to a new problem in concise terms. The telephone rings and an urgent voice says, “We want to move that old centrifuge in A factory over to B factory and we use it in making some of this new polymer X that research has been working on.” You can learn readily that the plant does not want to spend money and that the new facility has to be in operation yester-
INDUSTRIAL , A N D ENGINEERING CHEMISTRY
day. But little is learned about the old centrifuge in A factory. The hypothetical problem we are discussing here is realistic in many respects -e.g., the process is a batch unit and we must use an existing centrifuge, but beyond that, little is known. Manufacturer must be identified along with type of centrifuge, size, speed, kind of cycle controls, and materials of construction. Its general condition for re-use is frequently one of the hardest factors t o determine. Assurance is needed that these characteristics fulfill requirements sufficiently for investigating some of the details. The catalyst particles to be removed by centrifugation are coarse, and if the liquid is particularly viscous, it is assumed we would be told. Also it seems safe to assume that the initial separation is fairly easy. However, a good catalyst usually has a large surface area and the particles are probably porous. Therefore, thorough washing prior to recycle may be the cycle-controlling consideration. But is thorough washing required? Because centrifugation is followed by surge holdup in both the product-containing liquor and the wash water streams and batches are mixed in these holdups, batch integrity may not be needed-Le., perhaps a small portion of one batch may be mixed with the next as it comes through and washing sufficient merely to recover most of the product from the catalyst pellets is required. Of course, that recycle of product and prolonged exposure to reactor conditions does not damage product quality or cause catalyst poisoning, must be ensured. Such conditions are too frequently uncovered only when the plant-scale process is operating. Most of us are familiar with the prohibitive cost of full scale development conducted to solve problems of this type, using what we thought would be the final plant. Good intermediate scale development work will show up critical conditions such as an accumulation of contaminants and buildup of adhesions that seldom appear or are overlooked in the laboratory. I n the matter of selecting centrifugal equipment, it seems to me that consider-
John B. Tepe, Assistant Design Project Manager for Du Pont’s engineering department, has been associated with the Office of Scientific and Research Development and was a group leader for the Manhattan Project’s metallurgical laboratory in Chicago. He has written several articles on distillation, absorption, extraction, and machine computation; since 1945 he has worked in engineering research and plant design for Du Pont.
able information is available. However, a specialist in this field has emphasized that selection of centrifugal equipment is not well documented. Certainly, the best method of solving this problem before us now is to make a few tests using the centrifuge proposed for re-use. If not already set up, perhaps it can be, or perhaps the manufacturer has a similar unit available for tests. If none of these procedures are possible, or if material for large tests is not available, bench scale centrifuge tests are useful. Interest in the compression permeability type of small scale test, reported in the literature, is increasing. In this type of test, the cake is put under mechanical rather than fluid pressure and porosity and other characteristics can be determined under various conditions, over a range of pressures. These tests can be made quickly and cheaply. Armed with information from these tests, it must be decided whether or not the existing centrifuge is suitable for re-use. If not, what is needed? Can it be modified perhaps by changing speed, or can the process materials be modified by changing factors such as feed treatment? Perhaps after all, using the old centrifuge in A factory is not advisable. In filtration operation IV, much is left to imagination again. However, for the design and development engineer, this is not unusual. First, whether a batch or a continuous operation is needed has to be established, and whether the pumping operations surrounding the filter shown on the sheet are deleterious to the product. Probably in the laboratory a pump was not used. In my experience, a filtration problem is preferably attacked by leaf tests if a vacuum operation is indicated, and by a pressure cell, compression permeability, or leaf tests in pressure filtration. Leaf tests are simple and quick. To an experienced filtration man they give much information, not only for vacuum filtration but for pressure filtration. As in the case of centrifugation, after tentatively selecting the type of equipment and approximate cycle pattern, consultation with a manufacturer will be useful. In closing, I wish to make one or two brief points. First, although pilot plants range from a few pieces of standard equipment tied together with temporary piping to intricate, continuous integrated units which are prototypes of the full plant, they all have one thing in common-they cost a lot of money and waiting for results consume many valuable months. Yet, when pilot plant work is needed, it should be done. Second, aside from the usual process which simulates a pilot plant, scale model units are available today where factors such as mixing, drag forces, or web tensions can be studied. Valuable informa-
tion is obtainable without any chemical reaction whatsoever. The third point is process control and the necessity for understanding the dynamics of modern, large scale continuous processes which has brought intermediate scale development of process control and controllability importantly into the picture. Small semiworks or pilot plants often include continuous analyzers such as optieal and physical-measurement types which are developed simultaneously with the process itself. Also, use of analogs fob studying process dynamics and controllability is, of course, common. Today reactor design involves more than the classical consideration such as temperature, pressure, thermodynamic equilibrium, residence time, or degree of mixing. The role of kinetics and physical and chemical phenomena is becoming increasingly important. As never before, thought is in terms of over-all environment, phenomena such as molecular mixing, surface influences, efficient handling of fragile suspensions and extremely viscous liquids such as in extruders where thousands of pounds of pressure is needed for these materials to flow.
Thermal Process Techniques
D. Q. Kern. My viewpoint is that of an independent design engineer, the fellow who has no personal stake in the process and gets but one chance to do a design job at an immediate profit. We cannot afford errors because we operate on a margin which does not allow them. We do what we can mathematically in pre-designing a chemical plant to avoid fruitless experimental research. My remarks do not apply to demonstration plants for making products used by a sales department for market surveys, but only to actual research and scale-up which produces the best engineered plant for the least money in the least time. There is no intent to arrive at the cheapest plant, because this usually is far from the best engineered plant; here, initial capital investment and modern payout philosophy, which has some falla-
cies, are considered rather than continuing cost of operation and production. In our office, we rarely see the type of flowsheet illustrated here because we seldom meet a process in the bench stage of its development, The people we meet are either responsible for purchasing a plant and want it scaled up, or have themselves scaled-up a plant which fails to work. I do not wish to convey the impression that I am prejudiced against experimental research. Three examples I have encountered will preclude such a possibility. However, I am opposed to wasteful experimental process research which crudely reproduces the type of information we already have, or professional design data which never had a chance of proving anything. I believe that a process should be postulated mathematically with reasonable completeness before experimentation is undertaken. There are a number of tools for this purpose: dimensional analysis, analogies between heat, mass, and momentum transfer, Legendre polynomials,
Donald Q. Kern is head of his own organization engaged in mathematical development a n d engineering of thermal processes a n d components. H e has been employed for 15 years in managerial capacities in thermal process design a n d thermal equipment engineering. For 10 years, K e r n has headed the graduate chemical engineering courses in thermodynamics a n d heat transfer a t Brooklyn Polytechnic Institute; also, h e is Professorial ,Lecturer in chemical engineerlng in the graduate school of Case Institute of Technology. His firm, located in Cleveland, is.retained b y numerous companies such as Union Carbide, Calumet & Hecla, a n d Vicker’s Ltd. of England. Kern’s book, “Process Heat Transfer,’’ is now in its sixth printing.
VOL. 50, NO. 4
APRIL 1958
581
Gauss’s equation and hypergeometric functions, Bessel’s equation, the NavierStokes equation, Fourier’s equation, Fick’s law, and many others. These equations tell us what we need to studynot what we find easy to study. Experiments should then be designed to validate or modify assumptions and hypotheses introduced into the equations. Instead of being the history of a tiny plant operated on a particular afternoon, research can just as easily solve the design fundamentals for most extrapolation needed for the commercial plant. I also question experimental research predicated on design for which there are no criteria for scale-up similarity, either geometrically, thermally, chemically (including recycle), or mechanically. This occurs not infrequently. Nor can all situations be approached on the basis of complete similarity scale-up techniques. Often, availability of pilot equipment or controls makes it necessary to waive similarity procedures in design of experiments. But this cannot be done where a point is focal to the plant scale-up and essential to engineering design methods. Empiric equations can be evaluated only by experiment. One of the three examples I have already mentioned goes back to the pioneering period in fluid catalytic cracking of petroleum. An expensive pilot plant was built by the company which employed me. Many months were spent in attaining continuous operation, because innumerable problems of high temperature-pressure operation had to be solved, which were incidental only to the small pilot plant. These problems were not encountered in larger plants. The expense of this development was fabulous. Little needle valves, which deformed at high temperature and made precise control impossible, changed insignificantly in the commercial plant. However, to establish characteristics of a recycle stream it was thought necessary to operate the process for a long time. This was a point on which I disagreed. You do not have to demonstrate New-
582
ton’s law of gravitation to prove that a process stream will flow downhill. You merely have to obey it when you engineer the plant. The second example involved a plant for ore beneficiation, operated jointly by the Government and a private corporation. A small, continuous bench-scale model having a capacity of about 50 to 100 pounds per hour was operated for some time. O n the basis of this, a pilot plant was built having a capacity of a ton an hour, from which a commercial plant was constructed with an increase of 40 to 1 over the pilot plant. However, it could not be said that the commercial plant was scaled up, although that was obviously what the development engineers intended. Let us just say that a commercial plant was constructed. In this ore beneficiation process, a chemical atmosphere was created by combustion inside a number of successive kilns as the ore proceeded through a series of extractive solid-tovapor reactions. In the bench-scale model and pilot plant an economic degree of ore beneficiation was readily obtained by opening the valves and throwing the switch. The commercial plant, however, produced only a barely detectable amount of ore beneficiation. The reason was that in neither the bench-scale nor pilot plant was it possible to identify a particular critical stage-only that part of the flame a t the interface between the flame envelope and the ore as it rolled about the lower portion of the kiln, had an effective concentration of reactants. Large lifter bars to increase ore surface only dropped ore through the flame envelope without appreciably increasing the surface or time of contact with the envelope. Ore surface on the lower portion of the kiln roughly increased with diameter of the kiln, but volume of both the ore and chemical components which had to be burned in the large diameter kiln to produce the envelope increased with the square of the diameter. Not only was the cost of operating the commercial kiln to produce even an inadequate amount of flame envelope prohibitively high, but the plant was physically unable to make the appropriate product on a commercial basis. My comments reflect no hindsight. Here, optimum size of the kiln should have been studied independently and resolved somewhere between the pilot and commercial scale. Analyzing the process by differential equations would have made this apparent, even though the experimental data did not. Here was an obvious problem of geometric and chemical dissimilarity which should have alerted the engineers. Even the old rule of tenfold scale expansions was Edisonian at best, and had neither the rational advantage of being right and useful nor wrong and discarded. This
INDUSTRIAL AND ENGINEERING CHEMISTRY
plant is now overgrown with brush on a hillside. A third example involved a chemical company in the South, which was merged with a very large company because of a novel chemical plant it was building. During the merger, the engineers for the smaller company preferred to seek other employment and resigned when the plant was nearing completion. We were not permitted to visit the plant because time was insufficient to clear everyone-the project was classified. We did not even know the compounds involved-merely key names such as A , B, and C plus any properties we requested. But we had to ask; no properties were volunteered. When we were retained for the project, some 6 months had already elapsed since completion, and despite the owner’s large staff, the plant simply could not be made to operate. The key to the process involved a distillation separation with the primary heat input coming from a thermosiphon reboiler. The pilot plant data showed no inconsistency, and similarity criteria indicated the scale-up was consistent with mathematical analytic methods. Because we were not permitted to see the plant. we next requested erection drawings to see if some fundamental physical law had been violated. These drawings showed immediately that to make servicing easier, the bottom of the column was located at the second floor level of the support structure, and the thermosiphon reboiler at ground level some 24 feet below. The static head on the discharge and feed sides of the reboiler was so great that the tower bottoms could not recirculate. Vapor in the reboiler could not develop sufficient volume and hence, reduce gravity of the discharge leg for recirculation. This point could be readily computed. When steam was fed to the reboiler, it would simply blow through the tubes without significant condensation. The shell-side fluid only picked up a trickle of heat which it dissipated as it rose up into the column and started to heat the mass of metal comprising the piping and toiter. After each several days of attempted operation, the plant was shut down, and naturally the svstem cooled. Lifting the reboiler to a platform just below the second floor (space was not available on the second floor itself) put the plant into operation quickly. In this third example, I mish to point out two common problems. First, when a plant fails to operate uithin a reasonable time, it is frequently referred back to the pilot or bench stages for additional research. This often wastes even more money, because the original research was inadequately designed for successful scale-up. Second, in many organizations a missing link between develop-
’
ment and scale-up often prevents early recognitions of deficiencies. Also, a plant may fail to operate because of faulty engineering; in these cases the development engineers usually defer to the design and construction staffs. This problem should have been solvable by anyone willing to approach the situation mathematically instead of following hunches and speculation. I do not mean to infer that no problems of fundamental dissimilarity exist between the model and the commercial scale or that every aspect can be solved mathematically. Good human intuition is always an advantage and despite our extensive use of computers, we realize that this quality has not yet been incorporated into computer lines. Often, some design factor will have to be pulled out of the air, either because precise, scalable data is too expensive or because of fundamental dissimilarity such as existed in the kilns. We have an engineer in our office who is an exceptionally good guesser; in fact, he worked his way through college in a rather elegant style by playing the horses. When we have exhausted mathematical analysis, similarity and dissimilarity criteria, we ask him to guess. When he is not needed a t the office, we send him to the race track with all our spare change and then wait to see what he turns up with the next morning. Several times we have used the term, recycle similarity. This means operating a plant for a long time in which there is volumetric hold up, to determine if products, by-products, or impurities are thrown into the recycle stream. Frequently, this is used as an excuse for continuing research on the entire syntheses when both scope and continuation are often unnecessary. If prior to scale-up, a pilot plant has not been operated for long, there are other ways around the recycle problem. If this has not been studied, it is merely a matter for alertness during commercial plant start up and operation. As soon as the smallest deviation is observed in the commercial operation, the entire recycle system can usually be purged a t insignificant cost to remove the undesired polymer, catalyst poison, or dirt. U1timately, a recycle purge frequency can be established to avoid sudden freeze-up and clogging which sometimes occurs in polymeric and high viscosity systems. In the flowsheet, reactor I is the primary reaction vessel. The information we are given about the 2 to 1 change in viscosity which occurs here means little because we do not know whether the change was from 1 to 2 cp. or from 500 to 1000 cp. The former change is insignificant in fluid dynamics but the latter is vitally important in complex reactions, particularly in reactor design. For removing heat of reaction, the only issue here, we need no new experimental data except on the chances of abnormally
fouling the cooling element; this can be readily determined on the bench scale. If the fluid is Newtonian throughout its entire range, the reactor design is simple unless narrow limits of heat and time sensitivity are imposed. If it is nonNewtonian, it will have a t least enou,gh fluidity for obtaining an operating viscosity by increasing shear stress. We could extend the scope of the original research by using design methods available for studying, without additional experimentation, use of an external heating or cooling element. What occurs in the precipitation tank (111) is similar to that in the reactor, but a point relative to both vessels must not be overlooked-namely, I trust that viscosity at some intermediate stage did not change to several thousand centipoises even though the gross change from initial to final viscosity was only 2 to 1. This can be frequently overlooked in bench scale research when too large a piece of equipment has beep used for a relatively small job. The distillation column and its auxiliaries include no new elements and require no additional experimentation. Only the dryer remains. A prerequisite here is obtaining enough sample for a representative picture of how the process will operate. A better job of leaching coagulants can be done on the bench than in the large scale. A nice, fine powder resulting from experimental drying can be quite different in the commercial plant. After the drying cycle has been established, it is disconcerting when a product, supposed to be a porous powder, becomes a piece of hard rock with a soft center. By itself, this is not too bad, because a leaching cycle can probably be changed by varying the period of operation or quantity of fluid rather than by rebuilding a portion of the plant. Another situation is more alarming: when a development engineer, confronted with the product from the commercial plant exclaims, “Gee, it never looked like this before!” At this point, the time is way past for undertaking appropriate fundamental research.
Transport, Grinding, Economics
,
Donald G . Jordan. For materials transport, I can see no complicated problem here-i.e., no hot, corrosive, finely divided solid is moved, such as vanadium pentoxide fluidized catalyst. Hygroscopic solids can be difficult to move, but again I see no problem here. The kind of transport equipment needed depends on the degree of freedom which the designer had in laying out the plant. Part of the contents in the reactor can be allowed to drop by gravity into the centrifuge or other places. Sometimes, of course, the designer may not have this freedom and materials must be moved by another procedure-e.g., the instance of the centrifuge previously discussed. The thixotropic slurry presents an interesting problem because it can cause strange effects in agitators, pumps, and flow-in pipe lines, Shear stress and viscosity-temperature relationships should be measured in the laboratory. The chemist does not ordinarily consider these factors, but the engineer should certainly ask that it be done. Perhaps the slurry may refuse to flow by gravity out of the precipitation tankif its consistency is 10 to 15% solids, it may be quite stiff and thus require inert gas pressure, a pump, or both. Perhaps shear stress imposed by the agitator will break down the viscosity to the point where, if the agitator is left on while the material moves from the reactor, flow-out can be obtained readily enough. But the technique of moving the slurry from the reactor can be readily studied in the laboratory-certainly when the slurry was first made, whether or not it could be gotten out of the reaction vessel was apparent to the chemist. The softening point of the product, 100’ F., is important in determining the grinding technique. Also, how will the product be stored and shipped? If it must have a certain particle size and distribution of the solid, we may be embarrassed by finding that it has re-fused. This shipping and storage problem may be sufficiently acute for us VOL. 50, NO. 4
APRIL 1958
583
MEN
I
1
I
MONEY
I
1I
MATERIALS
I
1
DEVELOPMENT ACTIVITY
A. PRODUCT INFORMATION AND PRODUCT
Purity Applications
B. PROCESS INFORMATION
Temperatures Pressures
C. SCALEUP OR DESIGN INFORMATION
Heat and Mass Transfer
Concentrations
Coefncient
Toxicological Activily
Yields Conversion
Pressure Drops (Friction Factors)
Markets
Thermodynamics Kinetics
Agitation Data Filtration Data
Corrosion
Solids Transport
By-produds
Phaae Equilibrio
Safety
Physical Properties of Process Streams
Physical Properties Storage and Transporl
Catalyst
to restrict the material to captive use; thus, by interplant discipline, the particles can be maintained in a finely divided state. A grinder (VII) is available. Because the material was ground satisfactorily with a mortar and pestle, apparently no great amount of energy is required. And, because it is a filter cake of precipitated material dried a t a relatively low temperature, the mass is probably bound together with no great amount of cohesiveness. The kind of a grinder chosen will depend on several factors-e.g., how fine must the final product be? We have not been told, but if grindable in a mortar and pestle, probably it need not be very fine. Perhaps 10- or 20-mesh is satisfac-
Donald G. Jordan joined the Durez Plastics and Chemical Co. in 1940 and worked in its synthetic phenol plant. In 1942 Jordan was engaged by Arthur D. Little, Inc., for a number of industrial chemical research projects mostly related to the war effort, and in 1944 by MIT for other projects sponsored by the Office of Scientific Research and Development. Since joining American Cyanamid in 1949, Jordan has worked mostly on fluidized catalytic oxidation of naphthalene to phthalic anhydride and synthesis of methylstyrene from toluene and acetylene, He is the author of "Chemical Pilot Plant Practice," published in 1955.
584
tory. As to size distribution, the mortar and pestle grinding leads us to believe that distribution resulting from beating the product is satisfactory, but this may not be true. The customer may require certain specifications, and the narrower his range the more difficult the problem. If only 100,000 to 200,000 pounds are made annually, batch milling may be justified. However, if production amounts to several million pounds, then a continuous process must be devised. The softening point poses another problem. Even though operations are conducted at an ambient temperature of 60" or 70" F. stress necessary to break the solid will cause the temperature to exceed 100" F. Thus, a cooled grinder is needed. As a tentative design for estimating cost, I would propose a rough pressure roll, set to break the material into */8- or '/ls-inch pieces. These rolls should be cooled by refrigerated water and all crushing, classification, and packaging should be done in a room held at 40" to 60" F. Lastly, a finisher roll could be used, with rolls operating at different speeds to bring the material down to about 10-mesh. If size distribution is specified and it is within a narrow range, closed circuit grinding is needed, using classification screens which reject heavier material back to the grinder and smaller material back into the precipitation tank. However, to select the proper grinding equipment, tests must be run on models of commercial grinding equipment. As with the centrifuge, vendor tests are useful. But if these tests require 40 or 100 pounds of material, a difficult problem arises because only laboratory size
INDUSTRIAL AND ENGINEERING CHEMISTRY
equipment is available for making the product. T o my knowledge, there is no way t o collect data on a small scale for crushing and grinding. However in this instance, no difficult problems are apparent. In any development activity, men, money, and materials are needed in varying amounts and from these, essentially three streams arise. Stream A concerns the product. We are in the business of selling at a profit and much about the product must be known, such as toxicity, uses in articles of commerce, basic considerations and much more. T o learn these things, the product must be made by a process similar to that planned. Stream B concerns process information much of which can be gathered in the laboratory-e.g., kinetics. thermodynamics, and by-products. Stream C concerns scale-up, design, heat and mass transfer coefficients, agitation, viscosity, and physical properties of all streams. Generally, knowing more about the product than the process is desirable, but both are important. These things concern economics of the project and if unfavorable, the process will not be commercialized. Usually, stream C, scale-up or design, is of lesser importance-a chemical plant built and operated with little information may (not will) perform well enough. The importance of extensive scale-up data depends on the unit operation used and economics of the process. Plant design without this will probably have many uncertainties which later will cause considerable trouble and expense. This may or may not be justified. A process employing liquids or gases in the associated unit operations such as distillation, heat transfer, and fluid flow, needs less scale-up information than one involving solids, solid-fluid mixtures, or solid separation processes such as filtration and centrifugation. A new process for an established product requires more scale-up information because correction of mistakes and consequent delay in production will be costly. I n this case it is usually true that the economic picture is only moderately promising-Le., there is competition and narrow profit margins. O n the other hand, if profit margins are high and competition is negligible, more risk is permissible because high profits from early commercialization can absorb costs. Consequently, less scale-up information is required. Scale of production is important. A plant producing less than 1,000,000 pounds of product annually usually cannot justify high development costs, whereas one designed for 50,000,000 can. In my opinion, development costs exceeding 20y0 of proposed capital investment are too high. Carrying out a high cost program is a complicated matter; scale-up data can be either the most or least important factor.
