Au+
1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
program was carried out on pilot unite up to 12 inches in dismeter and 175 feet high and resulted in the evolution of reliable design methods, the development of a nuitable control system, and the attsinment of sttrition ratas wiihin acceptable limits.
162s
losophy of approach to pilot plant studies r d t s in the maximum quantity of useful procese design data in a minimum of time and with a minimum cost. Conclusion
Visual “Models” Help Solve Flow Problems
As indicated before, many other problems occurred during the development of the Houdriaow catalytic cracking process. For example, problems relating to catalyst flow and the disengaging of vapors from beds of solids had to be solved. Solutions to problema of this type were developed in visual “model” pilot units which were designed to scale to represent a segment of a commercial unit. In another example, heat of cracking and regeneration values were required for the design of commercial cracking reactors and kilns. Determination of these heat quanti&~ by actual heat balance around pilot units resulted in large errors becsuse the heat of reaction is obtained by ditlerence from heat quantities which have mors that rue large in relation to the heat of reaction. Therefore, u8e was made of modem calorimetric apparatus and techniques of proved high precision to determineexperimentallytheheatof combustionof thechargeandproducte from catalytic cracking. By combining these data with accurate sensible heat data, the heat of cracking and regeneration were calculated directly (8). The basic approach used in planning these development programs was the same aa that hereinbefore deemibed. Experience has shown that the use of this phi-
In planning pilot plant development work, considerationshould be given to the method of attack to obtain the solution of the problems. Three m e t h c d e - d y , fundamental, e m p i r i d , and a combination fundamental-empirical-can be uaed. The unitized concept ahould be followed in execUting pilot plant design and operation. Expdence has shown that this approach to pilot plant studies will result in the maximum quantity of useful infomationper unit of time or cost. Litarotun Cited
T.A,. Dart, J. C.. Kirkbride. C. G.. Peapy. C. C.. C h . E&?. Prov., 45, No. 2. 97 (1949). (2) Dart, J. C., Obiad, A. G., M . ,45, No. 2. 110 (1948). 13) Dart. J. C.. S a w - . R. T.. Kirkbride. C. G.. M . .45. No. 2. 103 (1) Burtis.
..
rn94-J).
.-__ W,
(4) Maerker.
I1 (1.51 .,I .
J. B., Sohall, J. W.. Dart, J. C:,
Bid.. 47, No. 2, 95
J. W.. Dart. J. C., Pdrolmm Refinei. 31. No.3, 101; No. 4.173 (19621. (6)Schall. J. W.. Dsrt, J. C., Kirkbride, C. G..C h . Eng. Picm., 45. No. 12.746 (1949). (5)
R~mxrroforreview Anpril 15, 1953.
ACCSPTEDMay 21. 1963.
basic Factors A. 1. CONN Standard Oil Co. (Idhm), Whiling, I d .
P
ILOT plant work may be defined as small d e experimental work &ed out to simulate projected conditions of a commercial p r o m . The work provides information for design and economic evaluation of a new proceas or for investigstion of troubles or proposed changes in an existing operation. The product is manufactured only in amounts d c i e n t for evaluation. A pilot plant is thus dktmguiihed from a memiworks unit, which can produce quantities large enough for initial marketing. It also differs from a bench scale unit, which is too small to simulate all the conditions of a commercial p r o m . The continued increase in she and cost of commercial unit8 has required more p r e c k definition of all the factors involved in d& and operation. As a rasult, pilot p h t work has aswmed a role of inneasing importance in the petroleum and chemical industries. The apecialined techniques of construction and operation have received much a h u t i o n . Very little has been written, however, about the more basic decisions that must be faced in every development program. when is pilot plant work warranted? What are the objectives of the work? How long should the work be continued? Because pilot plant workisitself costly, such basic questions must be carefully considered by all those concerned with the tschnioal details of a project. The major purpoee of pilot plant work is to reduce the area of uncertainty in the design and operation of commercial unite. The larger the area of uncertainty, the greater is the meentive forthework Beforeanypilotplantpmgramisundert9ken,these incentives must be weighed against the probable costs. Because
forward and as the area of uncartainty is reduced, the program must be r d n e d at frequent intervals to determine whether the work should continue as planned or whether emphasis should be s i f t e d . Pilot plant work is a v i b l connecting link between bench scale research and manufacturing, as shown in Figure 1. To be effective, pilot plant work mnst be continually coordinated with research, economic evaluation, procem design, engineering design, and manufacturing. I n large industrial research organiaations, each of these functions is usually handled by a separate group. The pilot plant group must have a thorough underatanding of the broad aspects of the project at all stages of the development to e n m e proper coordination. When Should Work Be Undemked on Pilot Plant Scale?
certainly the most impatant decision in pilot plant work is whether or not the project should he studied on this s d e at all. Projects that require pilot plant work fall into two p u p : thoee that rue desirableon the basis of current knowledge and those that show promiee on the basis of predictions of the future. In most cases, the time required to develop a new process through the pilot plant, de&, and coostruction stggeg is measured in years. Consequently, if the need is obvious M o r e the development work is started, the pro* may have been started tao lata. The most profitable ideas for development work are, there-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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fore, those requiring predictions of the future. Long-range predictions must be made of product quality and of market demand in order to determine the product improvements that must be made and the expansion in manufacturing capacity that will be required. Trends in product quality are sometimes difficult to predict because of changing requirements, such as jet fuels, but such trends can often be judged by simple extrapolation of the past-for example, the increase in octane number of motor gasolines. Predictions of expansion in manufacturing capacity help determine whether new units will be required or whether expansion of existing units will be adequate. New ideas in processing can most readily be incorporated into new units, and the incentive for process development is greatest in this case.
ENGINEERING
Figure 1.
Pilot Plant Functions
After preliminary economic evaluation has shown pilot plant work to be desirable, there are other questions that should be answered before work should be started: 1. Has Bench-Scale Work Been Carried Far Enough? Frequently the detailed design of a pilot plant raises questions that can best be answered by a few laboratory experiments. Lack of such data may make it difficult to place emphasis on the most important factors. Occasionally, additional bench scale data may be adequate t o define changes in design or commercial operation without the need for pilot plant work. 2. Can Results Be Calculated within Required Range of Accuracy? I n continuous distillation work, for example, pilot plant operation may not give results as accurate as can be obtained by calculation. I n one such experience, continuous fractionation of a complex mixture was carried out because the available equilibrium data were not considered sufficiently accurate. Unfortunately, it was difficult to determine the number of theoretical plates in the packed column and to obtain smooth operation. Hence, the results were not as good as those obtained by assuming a range of accuracy for the equilibrium constants and calculating the results within these limits. The uncertainties in operating the pilot plant were greater than those in the calculations. 3. Can Money and Time Required Be Spent More Profitably in Experimentation on Commercial Scale? I n one very small plant, for example, certain design changes were tested on the commercial plant at no greater expense than would be involved in constructing a pilot plant; although there was some uncertainty of success, this was offset by the fact that successful experimentation would permit immediate commercial operation. I n another instance, the uncertainties in the design of a small plant were found t o involve less money than would be required for a pilot plant program to resolve them. Liberal safety factors were provided in the commercial unit, and provision was made for possible changes after initial operation was started. 4. Can Information Be Obtained Elsewhere at Reasonable Price? Although there are many reasons for a company to carry out its own development work, the possibility of obtaining the information elsewhere at a smaller expense should not be overlooked. For example, it may be possible t o obtain a new process at a relatively low royalty fee. Or it may be desirable to have the experimental work carried out by another organization better equipped to do the job.
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After all these considerations have indicated that work on the pilot plant scale is warranted, the project is normally carried through the steps shown in Figure 2. Fundamental to all of these steps is the formulation of the program for the entire project. Design of the pilot plant is followed by construction, operation, and correlation of the data. This cycle may be repeated if correlation of the data suggests process modifications that require further experimental work. Planning and reporting are necessary at all stages of the project. Objectives Must Be Clear in Planning Pilot Plant Programs
In planning pilot plant programs, the objectives of the work must be well defined. These objectives differ as the project involves development of a new process or investigation of proposed changes in an existing process. I n a new process, primary concern is with the development of information for design of a commercial plant and for economic evaluation of the process. For an existing process, usual objectives are the testing of process changes or the investigation of process variables, feed stocks, catalysts, or impurities. Crystallizing the over-all objectives requires frequent contacts with all the groups concerned with the development project. First, it is necessary t o become thoroughly familiar with the earlier research work. Questions may arise that require further experiments before a pilot plant can be designed. Preliminary economic evaluation will indicate which factors should be emphasized in the pilot plant study, so that the unit can be designed for maximum utility. I n the general plan of operations, the work is first directed to determining the feasibility of the process or the changes necessary to make it feasible. This may require extensive work. Once feasibility has beeu demonstrated, it is necessary to investigate process variables, feed stocks, catalysts, and impurities in processing materials and to prepare products for evaluation.
(
)
OR DESlBN CHANBE
PROQRAM
CORRELATION
CONSTRUCTION
\
MODIFICATION
Figure 2.
1
A
Steps in Commercializing a Research Idea
During these studies, active work starts on the detailed process design of a commercial unit. Coordination with the design group makes it possible to so schedule the work that the most important data will be obtained first. After the major points in the process design have been settled, questions relating to new types of equipment and materials of construction must be answered. This may require discussion with the engineering-design group. If new types of equipment or new unit operations are involved, experiments on a larger scale may be required to obtain adequate design data. A complete new pilot plant of considerably larger size may be needed, but the necessary information can often be gained by studies on separate items of equipment, such as cyclone
August 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
separators for recovering fluidized solids or packed columns for liquid-liquid contacting. Pilot plant work often confinues after the commercial unit has been completed. During initial operations, close cooperation between the pilot plant and manufacturing groups results in better use of pilot plant data in commercial operations and better understanding of commercial operations by the pilot plant group. For processes in which a large background of experimental information is needed, the pilot plant group may become a consulting organization for manufacturing groups. In one such case, a group working on catalytic cracking coordinated the information from a number of commercial units. Attempts to develop correlations for general use led to further pilot plant studies that rather completely defined the variables in this process and made it possible to apply pilot plant data directly to commercial operations. Design of Pilot Plant Determined by Many Factors
After the objectives have been crystallized and the experimental program outlined, detailed design of the pilot plant can be started. In the early stages of the design, careful consideration should be given to such basic factors as size, space, safety, instrumentation, simplicity, and corrosion. Size. The size of a pilot plant will depend on the objectives of the work, the availability of processing materials, the cost, and general operability. If process data are the major requirements, a relatively small unit may be adequate; recent developments in equipment and analysis have made it possible to obtain reliable data on a smaller scale than heretofore. If equipment-design data are required, the size of the ultimate commercial plant may determine the size of the pilot plant, such that the ratio of scaleup will be within the limits of experience. The amount of product required for evaluation and further processing may be a determining factor in pilot plant size. I n many cases, product evaluation has been so thoroughly developed that only small quantities of materials are needed. For new products, however, larger amounts may have to be examined in order t o define quality with any degree of accuracy. The processing materials that have the greatest effect on pilot plant size are catalysts and feed stocks. If a number of catalysts are to be tested, determination of size should include consideration of facilities for catalyst preparation. For example, a unit having a 400-ml. reactor could use catalyst prepared in a small laboratory, whereas a 20-liter reactor would require catalyst preparation on the pilot plant scale, which might be unwieldy in an extensive catalyst-development program. Moreover, if catalyst samples are obtained from outside sources, 1liter samples can usually be obtained much more readily than 300-pound drums. Similar considerations apply also to feed stocks. Even if the pilot plant is erected where large quantities of feed stocks are readily available, the testing of special fractions can often be facilitated by the use of quantities less than 5 gallons, which can be prepared in relatively small equipment. The cost of operating the pilot plant markedly affects the optimum size. If the unit can be built so small and compact that it can be operated by one man, operating costs are obviously cheaper than for two-man operation. If the equipment is small enough to be readily fabricated in laboratory shops, as opposed to outside shops, the cost may be decreased and less time may be needed to erect the unit and make subsequent changes. Cost factors thus tend to favor the smallest size of equipment that can be feasibly operated. Space. The pilot plant area should always be planned so that adequate space will be available when a new unit is to be built. Otherwise, there is considerable tendency to squeeze the new unit into an existing small area, with resultant cramped operating space and lack of flexibility. Lack of space may often force the
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dismantling of pilot plants before the end of their useful service life. A good policy on dismantling may be stated as follows: When the pilot plant represents a commercial process that is still being operated, it should be kept in stand-by condition; when it represents a process that is obsolete from both a developmental and a commercial point of view, it should be torn down. The desirability of leaving useful pilot plants intact is most forcibly recognized when changing commercial situations require process changes or evaluation of new feed stocks or catalysts. The cost of building a new unit for this purpose may be prohibitive, but an existing one may be put into operation cheaply. Safety. As in all manufacturing operations, the importance of safety cannot be overemphasized in pilot plant work. This work often involves the first handling of a new raw material or a new product on a relatively large scale. Toxicity of these materials must be determined as early as possible, either through literature surveys or by experimentation in the laboratory. Direct application of plant safety rules to pilot plants can make the work so involved as to cause costly delays. For example, plant rules often require that in hazardous areas there be no equipment that is not explosion-proof. I n one instance, explosion-proof transtats for a pilot plant in a hazardous area were eight times as expensive as regular exposed transtats and were so bulky as to require much additional space. The problem was solved by enclosing regular transtats in a vaportight housing continuously purged with air. A positive pressure was maintained in the housing, and a sensitive switch was installed to give an alarm if the pressure dropped below a predetermined point. This equipment expedited completion of the unit, reduced the cost, and demonstrated the intelligent application of safety t o the job. Instrumentation. The subject of instrumentation of pilot plants will be considered in other papers in this series. It is fundamental, however, to recognize critical items and measure them accurately and to provide enough instrumentation to give the necessary process control. Because a pilot plant is used to investigate a much wider range of operating variables than a commercial plant, more instrumentation may be justified than on a commercial unit. Temperature measurement and control are partioularly important in pilot plant instrumentation. Because the small equipment has a much higher rate of heat loss than a commercial plant, this loss must be compensated by external heating. Unless temperatures are accurately measured and carefully controlled, external heating can result in conditions substantially different than those found in commercial reactors. Simplicity. I n order to cover the wide range of variables encountered in pilot plant work, it sometimes appears desirable to include all equipment that might be required in any conceivable situation. An excessively complicated setup may result that is difficult to operate and that introduces many possible sources of error in the data. It is often better to achieve flexibility by building the simplest possible unit first and by making required changes later. Although the unit may not a t first perform properly, this risk is offset by the many advantages of simplicity. Flexibility requires sufficient space for additional equipment and instrumentation, the use of equipment that can readily be ' enlarged or replaced, and adequate craft facilities for making these replacements. Sometimes it is desirable to build a pilot plant so that it is portable. I n one such instance, a small treating unit was built on a framework and set up near the source of feed stock; when the study was completed, the pilot plant was readily moved into the laboratory for other experimental work. After the design of the pilot plant is well advanced, additional information may indicate that simplicity can be achieved by major design changes. Although a delay in completion may result, this disadvantage must be weighed against the advantages to be gained by the proposed change. Sometimes a pilot plant can be simplified most readily on the drawing board.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Corrosion. The materials of construction for both the pilot plant itself and for the commercial unit must be examined from the standpoint of corrosion. Building the pilot plant out of the materials proposed for the large scale unit rarely gives accurate information. I n the small unit, the large ratio of surface to volume, the use of external heating, and operation under conditions beyond the range of ultimate application all result in much more severe corrosion than is experienced commercially, Consequently the pilot plant should be considerably more corrosionresistant than the anticipated commercial unit, and materials proposed for ultimate use should be tested by insertion of corrosion strips. I n some cases, special equipment may be built specifically for corrosion testing. Basic Factors in Operation Are Manpower and Maintenance
I n the operation of a pilot plant, basic factors to be considered, in addition to program planning, are manpower and maintenance. Most organizations use both technical men, who are college graduates and often hold advanced degrees, and nontechnical men, who are usually high school graduates. It is best to assign technical men to planning, supervision of the technical aspects of operations, and interpretation of results. Nontechnical men are assigned to work that can be reduced to some sort of routine. This practice is most consistent with the policy of developing in the technical men an understanding and appreciation of the broadest aspects of the project. Liberal use of nontechnical manpower makes time available for contacts with other groups and for study of the literature. Nontechnical men may be divided into four general classesoperating, analytical, clerical, and maintenance. The importance of proper training and selection should not be minimized. A good pilot plant operator can often make observations that lead to real process improvements. He can also keep a process in satisfactory operation should the instrumentation be faulty or inadequate. Similar advantages may be gained by properly training other types of nontechnical men. Because manpower is the most costly item in pilot plant operation, adequate supervision should be provided to keep it to a minimum. Supervision of the nontechnical aspects of operations is best handled by nontechnical supervisors on all shifts. If these men can be called on for assistance during starting u p and shutting down, substantial savings in manpower requirements may result. Optimum supervision of analytical personnel is achieved by handling the work a t a central laboratory. Where unusual on-the-spot test work requires analytical personnel to be located at the pilot plant, supervision is provided by the men in charge of operations. Clerical and maintenance personnel generally carry
Vol. 45, No. 8
out their work during the day shift under the direct supervision of technical men. Efficient maintenance requires careful planning. Because shutdowns frequently occur without warning, complete lists of maintenance items prepared well in advance permit most effective use to be made of the time during the shutdown. It is also necessary to have maintenance personnel of all types available for work on the pilot plant on short notice. This is best achieved by having a pool of maintenance men who have a backlog of less urgent work to do when the pilot plants are running smoothly. Written Reports Are Essential at All Stages of Pilot Plant Work
I n the long run, the only tangible results of a pilot plant development program are written reports. Reporting, however, serves an even more important function a t the beginning and a t intermediate stages of a program; i t stimulates the broad thinking necessary for proper orientation and for drawing sound conclusions. Before experimental work is begun, reporting starts with the issuance of a program memorandum in which the objectives of the work are stated and the operations are projected in considerable detail. After operations have been started, progress reports require re-evaluation of the objectives of the program and also stimulate contacts with other groups in the organization. When the vork is completed, a final report is prepared to present the data and conclusions of the entire project. This report is often neglected because it is time consuming and is written after the major objectives of the work have been achieved. One experience will illustrate the importance of complete reporting of the data. It was found necessary to double manufacturing capacity for a certain product. Although the existing unit was operating near capacity, reference to the report on the original pilot plant work showed that the unit could be operated at twice the capacity if only small design changes were made. Tests on the unit confirmed this conclusion, and the capacity was doubled a t only a fraction of the cost of a second plant. I n this case, the report on the pilot plant work was worth a lot of money. Pilot plant work is most effective when the broadest aspects of the development are kept constantly in mind. This viewpoint requires frequent evaluation of the objectives of the work, coordination with other groups working on the project, and careful study of the data obtained. Proper reporting at all stages is the most effective means of ensuring that this broad thinking is actually carried out. RECEIVEDfor review April 15, 1953.
ACCEPTED May 21, 1953.
