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stricted flow of solid particles-Le., 5 t o 10 particle diameters. I n this instance if the gas should start to flow more rapidly through one side of the tower than the other, the fast-flowing gas will not be cooled as quickly by the solids. Therefore, it will have a higher viscosity and a greater resistance to flow and will be self-correcting for uniform flow. If the process is reversed and the hot solids are cooled by means of a cold gas, any nonuniformity of gas flow aggravates itself because the more rapid flow of gas through one vertical section will develop a cool path through which abnormally large quantities of gas will flow. For the latter type of operations, the diameter of the pilot plant should be as near commercial size as possible. I n a process in which a bed of granular solids is heated or cooled by means of a li uid, the conclusions would be reversed because the viscosity of t i e li uid decreases with increasing temperature, the cooling step wou18 be self-correcting, and heating would give nonuniform flow.
A parallel case can be drawn for a granular solid extraction process with a liquid flow rate sufficiently low that pressure drop is negligible. If the solvent becomes more dense as i t flows through the bed the solvent automatically seeks uniformity of density at all points at any given level in the extractor. A downflow of solvent will give uniform flow, and a very small diameter pilot plant reactor is sufficient. If an upward flow of solvent is used in this operation, the denser solvent at the higher level in the extractor attempts to return to the bottom, and internal recycling and mixing result, particularly in large diameter vessels, and pilot plant data cannot be extrapolated. Suppose we have a process in which heats of reaction are involved. An example would be a reaction in a fixed bed of catalyst. The magnitude of the heat effects should be estimated, and a probable commercial design should be calculated. A pilot plant with an adiabatic shell need encompass only one representative full-length element of the commercial design. The pilot plant may indicate the necessity of more or less heat transfer surface, but when the final surface-volume relationship is established, the diameter can be expanded to commercial size without the necessity of building a larger pilot plant. Most of these examples illustrate types of processes in which one phase does not adversely affect the physical movement of a second phase that is involved in the process. I n such processes the equipment can usually be arranged so that either there is positive control of movement or random nonuniformity is selfcorrecting. In these processes a full-height, small-diameter, adiabatic-shell pilot plant is sufficient. There are also many processes in which one phase adversely influences the movement of a second phase. Examples of this type are contact of liquid with liquid as in solvent extraction, gas
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with liquid as in fractional distillation, gas with solids as in fluidized catalytic cracking, or liquid with solid as in contact decolorization. I n such processes countercurrent operation cannot be obtained directly, so we must either be content with one contact, often at a low degree of effectiveness, or resort to stage operation in which the phases are contacted, separated, and recontacted in a countercurrent sequence, as in a bubble tray fractionation tower or in many of the well-known liquid-liquid solvent extraction processes. Most of these latter processes give good countercurrent operation in small diameter vessels (up to a few inches) but as vessel diameter gets larger the results get poorer, and we begin to add internals to direct flow. What we should realize is that a material flowing through a vessel likes to follow the easiest path, and two phases never flow countercurrently if both have freedom of movement; they start internal cycling or by-passing each other. If it is decided that stage contact, separation, and recontact will be practiced commercially, then a small pilot plant is ample. But if for economic reasons it is desirable to build a commercial unit in which random flow will take place, then the bigger the pilot plant the better, because only a full commercial size will give the final answer. Enough examples have been given to illustrate types of processes that require only a small pilot plant and also those for which a pilot plant can only indicate how the next larger should be built. There is one fairly common fallacy regarding pilot plants: this is that the pilot plant vessels should have the shape of the proposed commercial unit. The fact that a commercial vessel, for design reasons, might have a diameter one half its height should have no influence whatever in establishing the shape of the pilot plant vessels. The pilot plant vessels should be designed to duplicate commercial velocities, heat transfer coefficient, contact (residence) times, and heating or cooling surface distribution. This does not imply that the size and shape of a commercial vessel has no effect on the type of contact obtainable. I n processes in which there is random movement and in which one phase adversely affects the uniform flow of another phase, the size and shape of a commercial vessel are very important, and the nearest approach to countercurrent cortact or uniform contact can be obtained with tall thin vessels. However, a pilot plant with the same shape factor must sacrifice either flow velocity, transfer coefficients, contact time, or other critical factors, which invalidate the data for use in commercial design. I n summary, i t is usually good economy to build pilot plants; and for developing a new process, a careful study of process characteristics will reveal the size that should be built. RECEIVED for review April 15, 1953.
.iCCIPTED
M a y 21, 1953.
An Approach to Pilot Plant Studies J. B. MAERKER AND J. W. SCHALL Houdry Process Corp., Marcus Hook, Pa.
T
HE great strides which have been made in the chemical and petroleum industries today and which are continuing a t a rapid pace are the result of new and improved processing methods. These changes are brought about by demands for new products, upgrading of existing products, and improving yields and quality of existing products. I n addition, the motivation for improving existing processing methods in many cases is the necessity for de-
creasing new plant investment and operating costs. Economic pressure forces reliable proof of the feasibility of new or improved processes and processing methods. This proof is one of the principal functions of the pilot plant. There are many misconceptions of the role of pilot plant studies in the development of new or improved processes. Pilot plant studies are carried out to obtain the necessary product yield and
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quality data, to develop the optimum processing steps and conditions, and to obtain engineering design data applicable to commercial plants, Pilot plants are not necessarily small replicas of the entire proposed commercial installation nor are they necessarily replicas of individual parts of the commercial unit. There may be many parts of a process which either cannot be or need not be studied in a pilot plant. Some parts of a process can be developed adequately only on a commercial scale. Careful study of the proposed process will indicate where pilot plant work is applicable. I n the parts of the process that require pilot plant work, the engineer should determine exactly what individual points need clarification and design the pilot plant specifically to obtain that information. In this discussion it is assumed that the justification, need for, and scope of the pilot plant work have already been established. This discussion is directed primarily to the “how” of pilot plant work. One method of approach to pilot plant studies is presented. Consideration of this and other approaches should enable industry to realize the greatest benefits from pilot plant studies. The approach to pilot plant studies described herein can be termed the “unitized” approach, This means that the pilot plant development work is divided into a number of basic parts or units. Each unit is then studied individually with no interference from the other parts of the process. This philosophy is maintained during the planning stages of development work as well as during the design and operation of the pilot plants. The subject matter of this presentation is made in two sections. In the first section the methods for attacking problems which are to be solved in pilot plant studies are described. The manner in which both the problems and the methods used for solving them can affect pilot plant design is discussed. The second section presents a practical application of this approach to pilot plant studies. The Houdriflow moving bed catalytic cracking process has been selected to exemplify the discussions. The Approach to Pilot Studies M a y Be Fundamental, Empirical, or Both
Pilot plant studies can be carried out using two general methods of attack-namely, fundamental and empirical. Certain of the problems that mkst be solved may lend themselves readily to a theoretical solution. If so, the pilot unit is designed to facilitate obBining fundamental data regarding material balance, energy balance, static equilibria, and the rates of transfer and transformation of mass and energy. The data are then correlated using these fundamentals as a basis. However, in many instances the mechanism of the process is so complex that pilot plant results cannot be correlated readily solely on the basis of fundamentals. Then, an empirical attack is often the best method of reaching a solution to the problems, and the basic pilot plant material and energy balance data are correlated on an empirical basis. A third attack t o pilot plant studies comprises a combination of the empirical and fundamental methods. I n using this combination, the pilot plant results might first be correlated on an empirical basis. This correlation is then modified by the application of the fundamental attack. The result is a correlation that is basically empirical but supported and checked by theory. In many instances the method of attack to a specific problem may be selected before the design of the pilot plant is started. In some cases, however, the possibility of theoretical interpretation of the pilot plant results is not realized until after correlation of the data has started. For this reason pilot plants should be designed to provide as large an amount of fundamental data as is practicable. If this is done, information will be available for the application of any of the three methods in the correlation of the pilot plant results. These considerations lend themselves to the philosophy of a unitized approach to pilot plant studies. I n executing the
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unitized approach, one of the first steps concerns the planning of the pilot plant design. In order to accomplish this, the problems to be solved in the pilot plant studies are divided into groups. Each group should contain those problems that can be solved by a single step or relatively few processing steps. Pilot plants should be designed so that each group of problems can be studied independently of the others, This may result in several pilot plants, each for a different part of the process. However, this plan results in simplification of pilot plant design, operation, and analysis of the results. Moreover, the individual pilot plants may be designed for greater flexibility since the design will not be limited by the piloting operations of the other parts of the process. Examples Point Up Effective Use of Each Method
This approach to pilot plant studies can be more readily understood by considering a practical application. The following paragraphs exemplify the approach by applying it to the development of the Houdriflow moving bed catalytic cracking process (1). This process is applicable to the processing of all petroleum distillate fractions for the production of high octane number motor and aviation fuels. The processing is accomplished by catalytically cracking the oil charge in a bed of downwardly gravitating catalyst particles. The catalyst is regenerated with air in a second position of the processing vessel, the kiln. Regenerated catalyst is transported to the top of the catalytic cracking reactor vessel by means of a gas lift. It follows, then, that the Houdriflow process can be divided conveniently into three principal parts-namely, the catalytic cracking step, the catalyst regeneration step, and catalyst transportation. Each of these parts will be considered from the standpoint of pilot unit studies. The problems concerning the catalytic cracking step may be divided into several classes. One phase concerns the effects of the process variables such as space rate, catalyst to oil ratio, and temperature on product distribution and product quality. The effects of variables such as catalyst type and source and boiling range of the charge stock on the process results are included in this phase. Other classes of problems concern heat of reaction, catalyst flow, disengaging of vapors from catalyst, distribution of charge to the catalyst, and seal leg operation. I n line with the unitized approach previously outlined, each of these classes of problems was investigated in separate pilot units. For the purposes of the present discussion only the effects of process variables will be considered. A considerable background of information concerning the effects of process variables on the process results was available from previous commercial and pilot plant operations in the general category of catalytic cracking. However, correlations of these data were of limited value primarily because the data did not cover a wide range in process variables on any one particular stock and catalyst. Therefore, a study was initiated to obtain these data for use in the design and operation of commercial catalytic cracking units. Consideration of the complexity of the reactions involved in catalytic cracking indicated that a theoretical attack of this part of the process would not be practical. Therefore, an empirical method was used. The design of the pilot plant was planned to segregate the study of the cracking reactions from studies of the regeneration and catalyst transportation parts of the process. Regeneration of spent catalyst from the pilot unit was handled in a separate “service” unit kiln. The only data taken during regeneration were those necessary to ensure that the catalyst was regenerated and that temperatures in excess of those permitted for the catalyst were not reached. The catalyst was transported to the top of the reactor and the kiln by means of a simple lift system or hoist. No data were taken for this operation. I n this way the study of the catalytic cracking step was not limited by design or operating
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features of the lifting or regeneration portions of the process. This same unitized approach was carried further in the separation of reactor products into gas, gasoline, and gas oil cuts. Whenever possible, the reactor effluent was not fractionated on the tower system associated with the cracking reactor. Instead, separation was accomplished in auxiliary distillation apparatus designed to handle only the quantity of material necessary to obtain the required yield and quality data. This resulted in considerable simplification of the pilot unit operation. The reactor was designed for a relatively low throughput of 1 to 4 barrels per day. This permitted the use of small equipment and reduced the cost of the pilot plant installation. The pilot unit was designed for a wide range of flexibility. This enabled the determination of the effects of the process variables on the process results over ranges even beyond those contemplated for the commercial unit. The design of the pilot plant was predicated on precise control. Accurate material balances were essential in order to develop reliable correlations between process variables, product yields, and product quality. The design was also considered in the light of probable continued pilot plant operation even aftel: the desired correlation of process data had been accomplished. Thus, the pilot plant was available for evaluations of different charge stocks, catalysts, or processing schemes as required. The pilot unit was operated at a number of conditions of space rate, catalyst to oil ratio, temperature, and recycle ratio, The resulting data were empirically correlated to show the interrelationships existing among the products from the cracking reaction as well as the relationship between the process variables and product distribution and product quality (4-6). Although the data were obtained only with one charge stock and one catalyst, the correlations were devised so that they would be applicable to other charge stocks and/or catalysts b y determining the cracking characteristics of the stock or catalyst from a small number of properly chosen pilot plant runs. These correlations not only assist the process engineer in the design of commercial units but also provide a basis for adjusting the operation of commercial units to meet the individual refiner’s requirements. The piloting of the regeneration step was considered from an entirely different viewpoint from that of the cracking reaction. As indicated, the regeneration of catalyst for the cracking unit was accomplished in a service unit. The fundamental information concerning catalyst regeneration characteristics which were required for kiln design were obtained more efficiently in a small pilot unit operated independently of the cracking pilot unit. This represented a further manifestation of the unitized philosophy of approach to pilot plant studies and resulted in a type of study that could not have been accomplished in a unit designed to operate in conjunction with the cracking pilot plant. Basically, the problems in kiln design concern the amount of regeneration gas, temperature, pressure, and kiln volume required to effect catalyst regeneration. Consideration of the regeneration problem indicated that the more important variables might include initial carbon content of the catalyst, initial hydrogen content, regeneration temperature, pressure, oxygen concentration in the regeneration gas, gas velocity, and catalyst pellet diameter. The reactions involved are simple and concern only the reaction of oxygen with carbon and hydrogen. These considerations indicated that a fundamental study of reaction kinetics might be used as the approach t o this pilot plant problem. A small regeneration pilot unit operable under a wide range of conditions was designed to determine the effect of the primary process variables on catalyst regeneration. The unit was designed to operate under essentially isothermal conditions. Operations were conducted to establish the relationship between carbon content and burning time. The effect of each process variable was determined. Analysis and correlation of the results indicated that the primary variables in regeneration were instantaneous carbon content, reaction temperature, and oxygen partial
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pressure. Burning rates were found to have second order dependency on carbon content and first order dependency on oxygen partial pressure. The effect of temperature was found to follow the Arrhenius equation. Comparison of the energy of activation for the carbon-burning reaction with that for a diffusional controlled step indicated that the rate controlling step in regeneration was actually the rate of chemical reaction. These data provided a sound fundamental basis for the design of the kiln section of Houdriflow catalytic cracking units ( 3 ) . The third section of the Houdriflow process concerns solids transportation. I n early moving bed units catalyst was transported b y elevators. This limited the quantity of catalyst that could be circulated. It was recognized that the elimination of the elevator limitations on catalyst circulation would result in tremendous added flexibility to the process and in considerable simplification to both the process and mechanical design. Consequently, the development of a simple pneumatic lift for circulating catalyst was started. The problems involved in this development included those of determining methods for introducing catalyst and lifting medium to the lift and separating these materials after the required lifting had been accomplished. A fundamental understanding of the principles involved in gas lift transportation was needed to apply the basic engineering data to design. I n addition, information concerning attrition of catalyst and erosion of the materials of construction was required. These considerations indicated that a combination of empirical and theoretical methods in pilot plant work probably would be necessary to develop pneumatic lifting techniques. It was anticipated that the lift development would progress through several sizes of pilot units. This fact plus the consideration that circulation rates considerably in excess of those required for the pilot unit cracking reactor and kiln would be investigated indicated that the lift techniques should be developed in units entirely independent of other parts of the Houdriflow system. Thus, the unitized concept of pilot plant studies resulted in a considerable reduction in pilot plant costs since cracking and kiln pilot units t o utilize the catalyst circulated in the lift studies were entirely unnecessary. As a result of these considerations a 11j2-inchdiameter glass lift was built and operated to study the vertical lifting of catalyst and to develop methods for introducing catalyst into the vapor lift and separating the catalyst from the vapor stream a t the top of the lift. Later a 3-inch diameter lift, 30 feet in height, was constructed. Operation of these pilot units indicated that lifting performance was satisfactory. Sufficient information was obtained during the operation of these units to permit the design of a larger sized pilot unit. Preliminary attempts were made to relate the basic variables affecting the performance of gas lifts. Preliminary catalyst attrition values were obtained and correlated with the operating variables. The development program was extended by the design and operation of a pilot unit utilizing a 6-inch diameter lift which was 175 feet high. This unit also operated satisfactorily. Theoretical correlations based on fundamental concepts of fluid dynamics were developed. The experimental data correlated well with the theoretical concepts. Therefore, these correlations formed a sound and fundamental basis for commercial lift design. These operations extended the empirical correlation between catalyst attrition and operating variables. Additional development work included the design and operation of a 175-foot, 12-inch diameter lift. Results from the operation of this unit at both ambient and elevated temperatures indicated that the d$gn methods developed from previous work were satisfactory for the design of commercial lifts. Catalyst attrition correlations were extended and design changes were developed to reduce catalyst attrition. Lift pipe erosion was found to be within acceptable limits for commercial operation. Methods for the control of circulation rate were developed. Thus, the gas lift development
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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.
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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-
