Building and operating a p i l o t plant i s one step i n an orderly plan of chemical process development. Its usual purpose i s t o obtain the quantitative data necessary for the design o f the production plant. The p i l o t step i s not always necessary, and its function i n a specific development program should b e carefully studied. Before proceeding, it must b e definitely established that the %
profit t o b e gained from commercial operation i s commensurate with the cost of the p i l o t plant step and construction of the commercial plant. The production unit should b e visualized, the quantitative data required for the final design listed, the p i l o t unit designed and built to give these data. Operation of the p i l o t equipment i s by chemical engineers rather than research chemists, and a valuable opportunity b presented to train future plant operators. The greater an organization’s experience i n a l l phases o f process development, the more successful w i l l b e the interpretation of data and translation into final plant The photograph shows design. a small synthetic rubber p i l o t plant.
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CHEMICAL PROCESS PILOT PLANTS H. L. Barnebey BLAW-KNOX COMPANY, PITTSBURGH, PA,
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departmental oraanization and the particular type of develoumint under consideration. For the purpose of the present discussion, let us split the work into six separate steps, as follows: 1. LABORATORY RESEARCH.Starting with the conception of the idea, trying it out in ordinary laboratory equipment, and ending with a general background of facts relative to the process. 2. PRODUCT EVALUATION.A cost and market study to determine whether the new product or process is worth developing. 3. PROCESS STUDY.An experimental study of the individual processing steps to obtain qualitative indications and determine equipment types. 4. PRELIMINARY ENGINEERING. Tentative approximate design and cost estimate of the commercial plant, based on present knowledge to decide whether it will make a good profit and war. rant proceeding further. 5. PILOT PLANT STUDY.The design, construction, and operation of a pilot unit to obtain the quantitative data required to build the production plant visualized in the previous step. 6. COMMERCIAL PLANT.Building the large plant and conversion of development efforts into money to pay the stockholders and finance other development programs. Such an apportionment of the development work has many advantages. Each step in itself is relatively simple and straight-
PILOT plant is a diminutive production unit for obtaining quantitative data to build a commercially successful chemical process plant. It represents a normal logicalstep in the transition from thelaboratory to the finishedproduct-astep not always necessary, often a waste of money, frequently worth one thousand times its cost. Problems are solved and equipment sized on a reduced scale because the work can be done faster and less expensively. The term “pilot plant” does not mean the same thing to all engineers. To some it’represents a scale model of the future commercial plant, just like it except for size. To others it is an overgrown laboratory setup of largesized glassware. It is hard to define a pilot plant in a few words; its nature and function are better presented as part of a development philosophy. Since the pilot step is but one phase of a development program, let us take a quick look at the over-all picture to orient ourselves. The progression from an idea to a successful commercial enterprise can be divided for convenience into a number of definite parts’. The exact number would depend upon the company’s Barnebey, H. L., Trans. Am. Inst. Chem. Engrs., 40,689 (1944).
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foryard. The completion of one phase demonstrates the conimercial feasibility of progressing to the next and yields the background of information required to proceed. PURPOSE
What is the purpose of a pilot plant? When the laboratory research work is finished, the individual steps are tentatively worked out, and the process shows gufficient promise to warrant the expense of further development, we start planning a pilot plant. Kow let us ask ourselves a simple serious question, not easy to answer: Why are a e building this pilot plant? Is it just for the satisfaction of seeing the process operate on a larger scale? Is it to make more product so that we can satisfy the sales department's demand for larger samples? Do we wish t o determine how to operate our present plant equipment to make a new product? Or is it to satisfy a superior that the process will work in steel pipe as readily as in glass tubing? The usual reason is to obtain quantitative data for the design of the large commercial plant, and this is the basis for its place and scope in the development program. I n a specific case any one of a hundred reasons might be sufficient cause for the pilot step, and we should definitely recognize the particular purpose so that it will dictate the type of pilot plant to build, if any. For example, if the main purpose is that of demonstration, the emphasis is placed on showmanship. The equipment must be neat and pleasing to the eye. Everything must be carefully painted, preferably in several colors. It must be immediately obvious to the observer where the raw materials enter and where the product leaves the apparatus. And most important, the
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operating data. After his laboratory work is conipleted and it is decided that the new product has a demand, he runs several batches in the pilot kettle to determine how to use his commercial equipment, He must know what conditions of temperature, pressure, time, and agitation give the desired results. It must be done right the first time in the large kettle because the ingredients are expensive and a batch cannot be wasted. As a second example, imagine a reaction involving three ingredients each dissolved in a different common solvent-thy1 alcohol, acetone, and water. The product precipitates out completely and is filtered off, and the main problem becomes recovery of the alcohol and acetone individually in the required degree of dryness. We might carry out pilot tests on the reactiofl, precipitation, and filtration, but probably not on the distillation since the commercial plant can be designed and built from existing data and knowledge. As a third example, let us imagine a reaction where a gas must be whipped into a viscous, sticky, semiliquid mass at 2000 pounds per square inch pressure and 700" F. temperature. The answer to this problem is not obvious. Even if we can accomplish the result on a small scale, we are not going to change to a large commercial autoclave without trying the process carefully in one or more intermediate steps. Equipment types are to be settled in the process study phase, and different styles should not be tried for a process requirement in the pilot plant whose purpose is to produce quantitative data As an example, suppose we are considering the distillation of a solvent from a viscous mass; although it works fairly well in a glass flask, we have a notion that agitation of the still pot is desired in order to obtain the required heat transfer. This should
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be investigated in the process study; the general shape of the vessel should be settled and we should determine whether a turbine, an anchor, a scraping txpe, or a double-motion agitator is needed. In the pilot plant we will determine optimum agitator speeds and heat transfer rates, The pilot plant is not an extension of laboratory work. There ought to be no question of whether the pilot plant will operate. When it is designed, we have enough qualitative data to know that it will work. Its purpose is to show the optimum conditions. Of course a tentative pilot plant design can be made at any time. On complicated problems this might help us to decide what must be done in the process study. It is always well to keep one eye on the next step. We frequently hear the objection that pilot plant operation takes too much time. This is often true, but it need not be so. A program, properly organized and employing skilled personnel and up-to-date equipment, can be developed rapidly if necessary, A systematic method of handling such problems will speed up the work by eliminating false moves and making each step count for the utmost. There are cases where speed is so essential that the pilot step must be eliminated. I n this case +e should fully realize that the commercial plant will assume many of the aspects of the pilot unit, and we must anticipate working out many problems on the large scale. When the pilot step is eliminated, the commercial plant must be designed differently. It should be built so that various features can be readily changed if it is later found that bad guesses were made on some of the items. A fractionating column is preferably built so that sections can be added or removed. Agitator drives should be made so that speeds can be readily changed. It may be necessary to replace some motors and gear reducers with heavier ones. I n general, the piping should be handled so that additional connections can be made here and there. One of the prime advantages of a complete pilot plant is that all of the steps operate together. Even though most of the individual parts have been tentatively worked out in the process study, running these simultaneously or in sequence brings out additional problems, especially in control instrumentation and piping. DESIGN
Having decided that a complete pilot plant is desirable and worth while, we now consider its design, construction, and operation. We have already visualized the commercial plant, because this is a necessary part of deciding whether we can afford t o build a pilot plant. We must now design the pilot plant to give the quantitative data for the commercial machinery. If any questions remain, the pilot unit must supply the answer, or else the commercial installation is in doubt. A continuous pilot apparatus must be built for a continuous process. Batch results cannot ordinarily be translated into continuous production. Operations which will be controlled automatically in the commercial plant should be handled in the same manner in the pilot step. If this is not done, we will be actually experimenting with automatic control on the commercial scale. Regulation of processes on the pilot scale may be easier or more difficult than in large equipment. Because of small flow rates, restricted space for instrument bulb location, and small volumes which will not properly cushion irregularities, i t is frequently difficult to obtain smooth control for small equipment. On the other hand, certain types of control are more difficult on the large scale because of temperature or time lag-for example, the automatic control of temperature *in a thick-walled autoclave where the heating is done through a jacket. The instrumentation of a pilot unit requires even more careful consideration than a commercial plant since we must obtain information and data for future design work. After the operating plant is once built, there is little requirement for obtaining
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heat transfer rates, pressure drops, and flow rates since these will have been established in the operation of the pilot equipment. One reason for preparing a tentative design of the large plant before we build the pilot apparatus is to force ourselves to rerognize what information is needed. We list the data required t o design the commercial installation and then work out the pilot equipment to give these results. The selection of the pilot plant size is frequently difficult, and there are no set rules to assist us. This must come largely as a result of experience. I n a processing operation, a step-up of 5 to 1 might be risky to a novice, whereas an experienced development engineer might safely step up the ratio of 100 or 1000 to 1. If one of the main remons for building the pilot plant is t o obtain larger quantities of product, it may dictate the size. To obtain design data for a vapor-phase catalytic reaction, we might use a single-tube reactor, the tube being about the size and length that would be employed i’n amultitubular commercial unit. This may, however, produce only one tenth the amount required to satisfy the immediate demand for product, so a ten- or twelve-tube converter must be employed. Care should be exercised to employ pilot equipment which will give results that can be duplicated. For example, a Soxhlet type extractor should not be used in the pilot step if the heat required will be too costly on the large scale. A kettle or autoclave which is agitated more vigorously than is physically possible in a large unit should not be used. When difficult equipment problems arise in the pilot plant design and operation, manufacturers who are expected to furnish the commercial plant equipment may be called upon to assist in the solution. I n this way they will become acquainted with the requirements and will be in a better position to fill them when the large plant is constructed. A common error is to make the piping too large. Velocities and pressure drops should be about the same as would be used in a commercial unit. It is difficult to obtain fittings, controls, and valves in special materials and very small sizes. Frequently the proper size of connection is smaller than standard pipe. In such cases tubing and tubing fittings may be employed. Well water should not be employed for a difficult cooling problem if recooled water must be used in the plant. Water used in pilot work should, if possible, be identical to that to be employed in the plant. This will bring to light troubles which may arise from scale accumulation, stress corrosion, caustic embrittlement, sludge deposits, and the effect of minor process water impurities on the reaction. Waste treatment problems are frequently affected by the type of water employed in the process. On small scale work, sources of electricity, steam, compressed air, and vacuum should closely resemble those planned for the production installation. Steam pressure of 300 pounds per. square inch should not be used for heating if only 100 pounds will be available on the large scale. A mercury diffusion pump used for evacuation, which can produce an absolute pressure of 1 micron or less, might lead to false conclusions if steam jets are planned for the commercial size. Good judgment must be used in deciding where to concentrate efforts and where no investigation is needed. The results should be worth the cost of obtaining them. It would be folly, for cxample, to spend a thousand dollars to secure complete heat transfer data on an operation where only a small amount of heat is involved, control is not critical, and the heat exchanger in the commercial plant will cost only two hundred dollars. Our desire to do a good job of investigating should be tempered by a sense of values. I n converting pilot data into commercial design or when designing a pilot plant to obtain commercial data, i t is necessary t o
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A pilot plant ought to be designed with engineering care, equal to that employed on commercial work. It should not be a tangled mass with wires, pipes, and levers running in all directionp The design should be neat, trim, and businesslike. It is preferable to use commercial equipment types wherever possible Special provisions should be made for starting up and shutting down, because this will be done frequently. A well built pilot plant is usually an expensive toy and it is good policy to estimattthe cost accurately before proceeding. CONSTRUCTION
The pilot plant ought to be constructed with the same attrii tion to safe operation as would be given in a large commerrle’ plant. I n fact, because of operating variations which will be ir.troduced and because of frequent starting up and shutting down, there may be certain additional precautions to be taken ~ i t h the small equipment. Also, since the process is being operated for the first time on long runs, unexpected conditions might ariw Whenever possible these should be anticipated so as not to C ~ I I W danger to the equipment and its operators, The pilot plant should be oonstructed by ordinary workint~i. mechanics, and pipe fitters; if there are any unusual difficultirb in the construction, they will report them. Each complete pilor plant is best set up by itself, not in close proximity to other operations. The pilot plant must operate very much like a commercial unit-that is, continuously 24 hours a day rather than j u s t during the day shift. Working conditions must be favorable 44th no interference in the form of noise, fumes, or obstacles A number of pilot plant operations should not be carried out in tila same room. It is good policy to employ the same type of housing for thc pilot equipment that is anticipated for the final plant. If the large equipment is to be located out of doors, do the same with tlic pilot apparatus. I n such capes, if time permits, the test runs should extend through a complete cycle of seasons so that t h e effect of hot weather and cold, sun, wind, rain, and snow call bf observed. The pilot equipment should be provided with suitable platfornih and stairways so that access to the various portions of the eqiiil’ ment, either for observation or maintenance, is facilitated. OPERATION
Pilot Plant Assembly for Liquid-Liquid Extraction Process
correlate small scale and large scale operations. For example, with a jacketed kettle or a tank with an internal coil, the heating surface in two sizes of units should represent about the same number of square feet per gallon of liquid capacity. A small jacketed kettle does not look like a large one when the same ratio of heating surface to volume is maintained. When distillation or evaporation is part of a process, the vapor release area per gallon of liquid capacity or cubic feet of vapor per minute per square foot of surface should be as nearly the same ratio as can be conveniently obtained in both cases. It is always difficult to correlate agitated equipment. A particular type of agitator should have about the same peripheral speed (not r.p.m.) in the pilot and commercial equipment. The commercial plant should use approximately the same amount of power per gallon of liquid capacity or per square foot of wetted surface as does the pilot plant. It is convenient t o consider both ratios and sometimes decide upon an intermediate figure. Another useful method of comparison where turbine agitators are used is to correlate them on the basis of the time required to turn over the liquid charge completely.
The pilot plant is operated by chemical engineers and regular process operators, not by research chemists. An excellent op portunity is here presented to train operators for the commeicial plant. Careful records should be kept of all runs, not just the g00t1 ones. A sufficiently large operating crew must be employed w that the process can be controlled and the required data takrri simultaneously. The control tests t o be employed in regnltir production should be determined and used a t this stage. A sufficient number of runs should be made to ensure that I I I I data represent what can be expected in day after day operation The tests ought to use exactly the same grades of raw material. as will be supplied to the plant. Tests should be run for a sufficiently long period to indicate any corrosion troubles. I n addition to observing the effect on the equipment, small corrosioi, test samples may be placed at strategic points. Each sampit. should be made from commercially available material and contain a weld if fabrication is to be by welding. These can be removed from time to time for weighing and yield numerical results. A complete detailed record should be kept of all the pilot plalirt work, including drawings, curves, tables of data, d-escriptionfi of runs, maintenance troubles, and photographs of the ins tallation, since the design and construction of the commercial plant will probably be carried out by other personnel. Results of niaii? months of work and many thousands of dollars expenditure n i i i i l not be transmitted by word of mouth.
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Flow Sheet of a Pilot Plant for Continuous Liquid-Phase Reaction a t 4000 Pounds per Square Inch Pressure During the course of the pilot work, meetings should be held et least once a week so that the engineers assigned to the job can discuss the progress and future plans with the department head, specialists who are working in a consulting capacity, a representative of the research department that carried the project through the laboratory stage, and a representative of the engineering department that will design the commercial plant. The proper interpretation of the pilot plant data calls for care and experience. The more development work an engineer or a company does, the easier and more effective will be the utilization of the data. The results should be translated into usable form and commercial plant implications should be determined each day or each week, rather than a t the end of a long series of runs; otherwise we may find that a month’s work was aimed in the wrong direction. The report on the pilot plant operation will be considered by management in the light of the cost involved. What did they get for the money spent? The general appearance of the report, the method of data presentation, goes a long way toward making the project appear worth while. Let us never become so involved in engineering that we forget to use salesmanship now and then. A thorough knowledge of equipment design is essential to reproduce successful pilot operation with equally successful commercial machinery. Large companies frequently have sufficient experience within their own organization. Smaller concerns can often employ equipment specialists t o advantage.
Good work can be done only by capable personnel. The best brains in the company should not be used for production, with development left to men just out of school as a training for production. A man should be trained for development as for any other important function. The development department should be organized with care and must be supervised by competent executives. Development is just as much a part of money making as production. The management must be just as efficient and practical. The proper utilization of the pilot plant for the development of s chemical process calls for straight, logical thinking. I n this connection we must be very critical of our own actions and be sure that every move is for a definite purpose. The pilot plant should always be considered a definite step in a program of chemical process development rather than as an isolated operation. For one reaaon or another it is not always possible to do everything in the ideal way, but by considering the ideal before taking action, troubles are frequently avoided just because they have been considered. By having a set of rules, our thinking is clarified and we make departures from the rule with full knowledge of the consequences.
