Pilot Plant Reactors Part of the Panel Discussion on Pilot Plants Presented before the Division of Petroleum Chemistry, 123rd Meeting, American Chemical Society
This is the third article in the series on -pilot pionh in the petmleum Industry.“ The present article concludes the presentotion Of the Popen delivered at the meeting. It is planned to present a review of these p o p e n incorporoting
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at the meetina ond comments SuDDlied by those reading these articles. Your comments to the editors will be appreciated.
F. D. Mors S h l l Dewlopnmt to., Entewilh, Cdif.
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ROBABLY no single wordin the chemical lexicon hss such a varied connotation aa the word “reactor.” A reactor, as a place to carry out B chemical reaction, aasumes a wider variety of shapes and forms than equipment for any other unit operation or unit process. The refinery engineer, dealing as he does with the treatment of materials normally found in crude oil or at least cloaely related to naturally occurring hydrocarbon fractions, attaches a different meaning to the word than does the chemical plant engineer. The latter sometimes more nearly resembles an organic chemist, and his resotom tend to assume more varied forma. All reactors have one thing in common. Care must be used in the selection of t _. m e and in the d e s..k .. or the result will be, if not failure, at leant disx~i~~uinting perfurmanre. In the discussion to follow. it is wauined that the pilot i.h n t is . related IO otherstqw in thc development procedure in n mamer customary to functionallyorg-anirrd eompanirs aurh aa Shrll V c velopment Co. Processes may ansp in a number of ways and to All B variety of needs. Preliminary work on R prul,lem is d ~ n o s t always carried out iri the lahoratory. This Pxpluratory xork asseasen tlie poR3ibility but not necessarily the practicality of the proeesa reachingcommercial utilization. If a process shows proinisr, the tempo of the laboratory nork is inmssrd, ronsistent with the over-all program, and data are ol> t&eJ on which to hase one or more preliminary commercial deeigos. Thin is d l y done even bcfore the pilot plant design is started, sinre at leaei a preliminary economic evaluation is required prior to a management decision to proceed with the remainder of the program. If the outlook is favorahle, pilot plant design is started and d e d through the construction and o w s t i o n campsbs. Simultaneously laboratory work is continued to improve the
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quality of the design data even to the extent of major changes in the process, and the commwcial design is kept up to date 88 laboratory and pilot plant results are reported. The latter action decreaaea the possibility of overlooking an important procesa variable that should be checked in order to support the final design. Examination of Reaction Conditions l a First Step in Selecting D Reactor
The Emt step in the selection of a pilot plant reactor is a critical examination of the characteristics of the reaotion. Many points must be either specifically determined to a high degree of accuracy or considered to an extent sufficient to ensure that the application of generalized correlations will not result in a serious error of omission. Reaction condit,ions that will affect the choice and design of the reactor are pressure, temperature, energy balance, reaction order and rate, reaction equilibrium, phase, homogeneity if catdytio, and whether mixing should be induced or minimized. Taking the above variables in order, the pressure range t o be studied must first be determined. This will affect not only the mechanical design of the reactor but also many other related deciaiom. Higher pressure may mean that more work must be done to raise the resctants to a pressure level that will sllow them to be fed to the reactor. On the other hand, it may be advantageous to place the point of energy expenditure before the reactor, even though the reaction itself might well be carried out at a lower pressure. For instance, if B unit in the recovery system is run at elevated pressure, it is normally cheaper and easier to pump a liquid to a high pressure than to compress to the 893338 pressure a gas that evolves from the liquid during treatment or reaction. Pressure may also vitdly affect the equilibrium and the resotion rate; this may be deduced from the&odyuamic or kinetic studiesor it may have l o be determined experimentslly iu the lslmrstory. M&hanical design problems, -safety considerations, or equipment cost may be incentives to keep the pressure as low 8 8 possible. In pilot plant construction i t is well to remember that preliminary results may lead to operation in pressure or vacuum ranges outside that st first thought optimum. The reactor and its accessories should be designed to allow this sort of departure from calculated conditions. The general temperature level of the reaction must be koown. Temperature 88 well as pressure governs both the rate of reaction and the equilibrium. In cases where a change in temperature has opposite effects on the two variablee--i.e., favors one while adveraely affecting the othef-the temperature rangeleading to maximum conversion will have been determined in the Iaboratory. Here again it is well to remember that temperature flexibility is a critieal requirement for any pilot plant resotor. The laboratory i n v e s t i t i o n may have missed completely the most desirable t e m p r a t w e range.
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ENGINEERING AND PROCESS DEVELOPMENT The energy balance of the reaction is another critical fpature. Apart from the temperature level, the endothermic or exothermic quality of the system must be considered in the design of the reactor. Usually a calculated figure for the heat of reaction, supported by judicious observation during laboratory development work, offers a iirm enough foundation for design of the pilot plant reactor. On the other hand, anomalous conclusions drawn from data from different sources may have t o be resolved by a special analytical determination of the heat of reaction. Fortunately, the flexibility, which is built into a pilot plant as standard practice, will provide allowance for a moderate error in the heat of reaction. Then if the problem seems critical in connection with commercial design, it may prove economical and convenient to make such a determination in the pilot plant. Temperature and flow measurement of process streams and heat transfer fluid will give two values which may be compared for proof of accuracy. Heat losses can either be estimated from calculations involving surface temperature measurements or calculated from a heat balance taken a t the operating temperature of the heat medium without reactant flow. I n addition t o heats of reaction, other thermodynamic, equilibrium, and reaction kinetic information is important, albeit possibly less so for the pilot plant design than for determining the direction of the early laboratory work. Experimental data are usually available for the design of the pilot plant reactor, The rates of reaction and the equilibrium over a range of temperatures and a t other different physical conditions are usually determined experimentally. This is expressed as yield and conversion against space velocity or as fraction of equilibrium conversion against reaction time. These data are then used t o size the reactor on the basis of throughput for a given production rate. Of course, confidence in the data is increased if correlation shows a consistent reaction kinetic pattern. Typical investigative procedures are (1) plot functions of the concentration a t constant temperature against time 01 ( 2 ) plot of the log of the specific rate constant against the reciprocal of the absolute temperature. A straight line in the first case will establish the apparent order of the reaction and in the second case will confirm the applicabili t y of the Arrhenius equation. Such things as initiation temperature and the need for influencing the course and rate of the reaction by catalysis are determined empirically by observation in the laboratory. At the time of pilot plant design there may not be any firm data on the activity history of a fixed or recirculating catalyst, since this is often one of the primary functions of the pilot plant. The pilot plant designer may have t o make a consistent allofi9ance for gradual changes in catalyst activity. The physical state of the reactants will exert an influence on the choice of reactor type. If the phase requirements indicate a heterogeneous catalytic vapor phase reaction, the choice must be made between a fked bed of supported catalyst or a fluidized bed of microgranular catalyst. One basis for this choice is the frequency and type of regeneration required. Any catalyst whose period of high activity between regenerations is measured in days or weeks is usually employed in a supported state in a fixed bed. Those that must be regenerated or reactivated after a few minutes operation are more conveniently handled in a fluidized bed. One exception is a highly exothermic gaseous reaction where advantage is taken of the manyfold higher heat transfer from a fluidized bed as compared t o a fixed bed. The resulting reduction in peak temperature allows reaction control a t higher reactant concentrations. Not only may this decrease the cost of the reactor for a given production rate, but also savings may be carried through into the recovery system because of higher crude product concentration. The final decision must be based on concurrent consideration of mechanical and economic studies of the proposed commercial scale plant, the reaction characteristics as influenced by possible back mixing of gases and catalyst and the relative difficulty of staging if this cannot be
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tolerated, and the experience background and the probable desires of the operating departments who will run the commercial plant. This last is often underestimated or even overlooked, but it may contribute t o the ultimate success of the project, Reactor Must Be Adaptable to Collection of Process Data
A great many types of reactors are available to the designer. The selection of a particular style is not always clear-cut even after a study of the reaction conditions. Possibly the designer may know of an analogous reaction being carried out successfully in a commercial reactor. Lacking this information, his choice will be the type of reactor that seems t o lend itself best t o the objectives. In selecting the reactor type, the designer is often guided by the requirement of simplicity. This is a goal t o be sought even at the expense of a sacrifice of maximum reactor efficiency. For instance, the construction and operation of an adiabatic reactor is so much preferred over t h a t of an isothermal reactor that it often pays if possible t o take a lower conversion by diluting the feed with recycle and thereby make the reaction controllable. Of course, almost all highly exothermic reactions require continuous heat exchange and are therefore usually impossible t o control in an adiabatic system. Having chosen the reactor type, the designer is faced with specifying a design t h a t will not only operate but will be particularly adapted t o the collection of pertinent data. Intuition is not a substitute for sound engineering principles, but imagination and experience must be employed t o foresee possible courses the research may take. Pilot plants are inexpensive neither to construct nor t o operate. A fine balance must be maintained between simplicity leading t o low initial cost and maximum utility t h a t often may be built in at the start of the plant for very little additional expense. Perhaps the most important decision t o be made that bears on the cost and the operability of the reactor is its size. This means in eff‘ect the fixing of the throughput, since kinetic data are used t o relate the actual physical size t o the required production rate. This may already have been fixed by factors beyond the control of the designer. Often in a new petroleum-derived chemical the experience of the marketing or sales division has been used t o specify the production rate required for customer evaluation or even t o hold interest over until full-scale production is under way. Other subsequent test steps, either within the designer’s company or by a customer, may require a certain minimum production. Lacking these restrictions, the minimum reactor size may be determined by the scale-up factor t h a t the commercial designer is willing t o use. The accepted scale-up may range from zero t o many hundred times. A typical example of no scale-up is a fixedbed catalytic reaction involving high heats of reaction. Where the commercial reactor may be several thousand packed tubes surrounded by a heat transfer medium, the pilot plant reactor is often one or a few tubes of exactly the same dimensions. The reason for this is t h a t it is considered difficult t o design such reactors close t o the economic production rate per unit rost without the danger of finding the reactor uncontrollable or a bad bottleneck. Experimental support of even the most exhaustive calculations is considered worth while. It may be observed that the assignment t o this example of the description “no scale-up” refers only t o the catalyst tube itself. The even distribution of feed and coolant t o a large number of tubes is quite another matter. The other end of the scale may be observed in the catalytic cracking pilot plants found throughout the industry. The scale-up here is so large that it is expedient t o establish a pragmatic correlation that will include in one experimental set of constants all the differences in results due t o all the size effects. This is of course only possible because there are already com-
INDUSTRIAL AND ENGINEERING CHEMISTRY
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PILOT PLANTS
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mercial reactors of this type, a circumstance not ordinarily obtaining in design work. Other factors in the process may dictate the reactor size. Cost and availability of the feeds may limit the design throughput. Means available t o pump and meter the feeds may exert their influence; if plant type instrumentation is used, the orifices employed with differential flowmeters must be large enough t o be nonclogging, depending upon the cleanliness of the fluid stream. This iii effect sets the lower limit of the reactor volume. Often one of the prime functions of a pilot plant is t o study the effect of contaminant buildup in a recycle that might have been overlooked in the laboratory and the pilot plant must be large enough not t o obscure this effect. No convenient formula can be given for this determination. The designer simply must consider all the facts and then make his best decision. Once the reactor size is determined, the process design becomes somewhat more straightforward. The critical process calculations, where applicable, are heat transfer and pressure drop. Correlations are available for heat transfer in stirred vessels (3, 18), in packed tubes (8, 12, IS, 16),in fluidized beds (6, Id), in finned and bare tube annuli (4,11,do), and in many other types of vessels in which a reaction might be carried out. These must be combined with transfer coefficients for the heat transfer medium, which may be steam, circulating, or boiling water or hydrocarbon, flue gas, or molten salt or metal. Here again it is well t o remember that a complete pilot plant investigation may include throughputs several times the tentative design, and unless the cost is prohibitive it is well t o build in extra heat transfer capacity. Pressure drop through the various portions of the reactor and accessories must be investigated. For instance, pressure drop through a packed catalyst bed may determine either the length of the bed or the size of the catalyst pellets, or it may influence both. Pressure drop is especially important if work must be done t o recover the pressure loss for subsequent steps or if unduly high feed pressures are required. By observation or measurement during the laboratory work, the requirements of the reaction for induced mixing must be determined and, if present, the type of mixing must be specified. Mixing is one of the most difficult of the unit operations t o translate in scale, involving careful analysis t o ensure equivalent fluid motion (17). Sometimes there are no comparative data except that mixing should be. "mild" or "vigorous." Often the requirement is much higher than it seems. A stirred liquid phase reaction with two phases of widely different specific gravities, for instance, requires a well-designed agitator with plenty of power t o effect proper mixing, Pilot plant stirred vessels should have continuous or stepwise variable control over agitator speed or at least should be adapted t o speed change by a simple field replacement of a gear or pinion. I n a circulating or tubular reactor it may be t h a t turbulence brought about by an orifice in a line or by baffles in a vessel will be sufficient. Such a device can often be combined with a pump which is a normal requirement of the process. Mechanical Design of Reactor and Accessories Follow Process Design
Mechanical design and construction of the reactor follow the process design. This is usually a cooperative venture between a mechanical engineer skilled in t h a t field and the pilot plant designer, who has a more intimate knowledge of the process as a whole. With t h e two working together the plant designer may be able t o choose between alternatives on the basis of operability and future utility. Both must keep in mind such features as cost of construction, adaptability of the unit in the collection of experimental data, ease of field maintenance, and flexibility of control of operating variables. Normally the process designer will specify the desired physical arrangement of the parts of the
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reactor, and the mechanical designer will specify the manner in which t h a t arrangement is achieved. Vessel design is one of the most verdant fields for building future utility at very little extra cost. For instance, if the process requirements call for a vessel to be equipped with 300-pound standard flanges, proper design of the shell will allow a rated working pressure of 415 pounds per square inch gage at 650" F.;
Cooling U a t e r to Shaft t h r u Rotating Joint
1 8 ' Six-bladed Turbine impellers
Shell:
44-1/2" 42" ID 20% Type 318
OD x
Stn'l Steel Clad
Spray Ring: I" Sch. 40 P i p e , 1 I?. 3/32" Holes SDaced 3' A p a r t , Inclined 30' Toward Shell
i\lF a-
Figure 1.
300-Gallon, 600-Lb.-per-Sq.-lnch-Gage Autoclave
Heat transfer surface: shell, 74 square feet; coil, 67 square feet. Head nozzles not shown: two each, 2, 2'/2, and 3 inches
the codes recognize a sliding scale of temperature versus working pressure for ASA pipe flanges but a constant shell allowable stress up to 650" F. It behoove&all concerned t o insist on having the vessel stamped with its actual maximum working pressure and not just the working pressure required by the process designer. Often substitution of material by the fabricator on the basis of availability will result in a stronger vessel than originally specified. That this should be accounted for in the final rating seems self-evident, but experience has shown that anyone lacking primary interest will simply fail t o get i t in the record. Also, now is the time t o get in the extra connections for the unforeseen sample and temperature points, for the changed feed flow, for the unpredicted steam injection, for the vital pressure tap, and for all the other plant changes made in the past where the presence or absence of an unused connection has saved or cost a great deal. The required mixing or stirring must be built into t h e reactor. Here again money not used t o ensure overdesign can never be spent as effectively t o correct underdesign. Especially in stirred reaction kettles, used t o make a variety of polymers, one agitator failure due t o structural weakness or underpower will usually more than make up for any slight initial saving realized by cutting corners. We customarily press autoclave manufacturers t o offer extra heavy drives and agitators, assuring them that their competitive position will not suffer since all proposals will be examined on the same basis. Likewise, when we are designing
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ENGINEERING AND PROCESS DEVELOPMENT Mechanical Design Includes Choice of Materials of Construction
The philosophy that normally governs the choice of materials of construction is one of speriiying better materials than it is hoped are really needed. The reason for this is t,hat continued operation of the exq~erimentalplant without breakdox-ns due to evcessive corrosion is ensured, and corrosion information can be obtained by the use of coupons of several metals a t critical points in the pilot plant (6, 7 ) . Pilot p h n t materials can be specified on the basis of corrosion reports and tables ( 1 , 91, observation of lttboratory Tvork on the procesq, or laboratory corrosion tests made n i t h synthetic mixtures u n c h plant flow conditions. The pilot plant designer is not rcivarcled by any tremendous
7
7
LO \i, eI1
Ten 1 I 3 ga.
10" Sch. 20.
S t e e l Pipe
Figure 2.
300-Gallon Stainless-Clad Autoclave
our o\Tn kettles, it is good pi:~cticeto rely on agitator manufacturers for valid advire as to the size and design of the stirrer, but t o call on our own past e\l)erience for the details of the shaft, stuffing box, and drive. The result of this poliry has been on occasions a motor several tinirs R S heavy as that originally recommended by the miser manufacturer. Operating experience has shown the wisdom of this course. An assembly draxing of a stirred autoclave developed for use a t our labointories is shown in Figure 1. I t s size of 300 gallons working volume talles it out of the bench-scale class, and the vessel is used mostly for preparation of batches of material for product evaluation. It was built with the thought in mind that some day an entire autoclave charge might polymerize to a solid mass. The stuffing box and head can be lifted off the shaft, after cutting loose the cooling coil, leaving the vessel interior completely open. It will be noted that we have avoided the flexible self-centering shaft construction, which is often satisfactory for specific services, in favor of a rigid shaft 3 inches in diameter. The high friction loss due to the large shaft is compensated for by internal cooling of the shaft. The drive is a two-speed constant horsepower 20-hp. motor. This was chosen as a compromise betmeen some control over the vigor of the stirring and a continuously variable speed changer with its maintenance and space requirements. Figure 2 is a view of the installed autoclave.
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Figure 3.
Tubular Catalytic Reactor
szviiigs if he skimps on materials. Seither doe^ hc have the background of firm data availahle t o tlie coinniercisl dcsigner (as tJhe result of the pilot, plant work) on which t o base an economic choice of materials. Often gross corrosion is not present, but small amount's of contaminants may have an adverse oatalytic effect or may result in objectionable color in a final product. Consequently it behooves tlie designer, within reason, t'o specify mat.erials that he linon-s \vi11 resist inroads by the reartants or the products. One design problem not covered by the codes, but peouliar to the problem at, hand, is thermal expansion relief. The chemical engineer must use his reaction data t o predict temperature differences in different parts of the reactor. The mechanical engineer then must take steps t o keep the resulting stresses u-ithin bounds. A typical trouble spot is in stirred autoclave jackets a h e r e the jacket is attached t o the s h 4 a t the top of the
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
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PILOT PLANTS straight side and is also pierced by a bottom discharge valve. Differential thermal effectsmay not be excessive in steam-heated kettles, but expansion bellows may be required for Dowtherm heating. An exothermic reaction in long tubes surrounded by a circulating or boiling liquid is another example. In one instance this was solved at Shell Development by including a section of stainless steel in the outer shell of such a reactor, The vessel consisted of packed catalyst tubes 45 feet long. Calculations showed that the exothermic reaction taking place within the tubes would cause a temperature difference of 30" F. between the tubes and the shell. This would cause a length difference of 0.125 inch, a t the operating temperature of 600" F., which would take four t o six expansion bellows t o obviate. In lieu of the expansion bellows, a section of Type 304 stainless amounting t o 20% of the length was built into the shell. This gave a zero differential expansion at operating conditions. The maximum stress occurred during start-up when the coolant was externally heated to initiate the reaction, but this stress put the tubes in tension rather than compression. The tendency of the tubes to bend and buckle was thereby eliminated, and the maximum stress was easily designed for, as a long hollow metallic cylinder is much stronger in tension than in column loading. Figure 3 shows the details of this reactor. Figure 3 also s h o w one solution of another of the reactor designer's problems-that of procefis variable measurement and control. It will be noted that means have been provided for seven simultaneous temperature measurements within the catalyst bed, these being made a t any seven related points a t the discretion of the operator. It is possible t o take periodic profiles of the bed temperature and a t the same time keep a continuous watch for any overt movement of the hot spot. The number of thermocouple points installed in the probe is determined only by the limitations on the size of the sheath. We try to keep the sheath in catalyst tubes down to as small a diameter as possible. To this end we use 30-gage glass-wrapped thermocruple wire. For those couples in a probe within a sheath, the probe is 9-gage hypodermic tubing with an inside diameter of about l / 8 inch. Seven pairs of wires are pulled into this probe. The sheath is 1/4 inch by 22-gage tubing. This combination has proved t o be quite satisfactory and movement of the probe is accomplished without excessive binding. The disruptive effect of the presence of the axial thermowell is determined a t least qualitatively before the data are certified for use in the desigp of a commercial reactor where most or all the tubes are without thermowells. Exothermic reactions are studied by relating yields t o bed temperature until the corresponding coolant temperature is established. Then before the problem is considered complete the thermowell is removed, the tube is repacked with catalyst, and the reaction is carried out a t the apparent optimum coolant temperature. Runaway reaction or deviation in yield and conversion correlations establishes the fact that the influence of the thermowell was excessive, and the problem is reassessed. Many methods of temperature control are available for the varied forms the reactor may take. Some will be known t o the reactor designer and many more will be familiar t o staff members who specialize in process instrumentation. One basic principle is to try to have a thermally stable installation no matter what the condition of the reaction. In other words, the ideal system mill automatically switch from heating to cooling and vice versa
October 1953
without any action on the part of the operator. We accomplished this in one autoclave installation by having a spray ring built into the vessel jacket. In this n a y we were able t o avoid the usual difficulty of having t o reverse the flow of cooling water and steam in order t o change the heat transfer cycle. Figure 4 is a diagram of the installation. Steam flow is proportional t o the
Cooling Water Condensate
~
Figure 4.
Instrumentation of Utility Autoclave
deviation below the set point of the controller and the three-way valve sends the condensate to the drain. As the set point is approached, the cooling water flow is started just before the steam flow is shut off, and the three-way valve diverts the outlet flow to the cooling water return. From then on, as the temperature rises above the set point, more and more cooling water is turned on, and the steam flow is stopped. Of course this means that a t about the neutral point the cooling water is being heated with steam, but the cooling tower will not usually be affected by this very small artificial load. We found this system to be very effective when controlled by an instrument that has rate response and also has a sensitive element with low dead time. The utility is especially apparent in autoclaves used for a variety of preparations, each with its individual heat characteristics. An emergency cooling arrangement is provided by the use of interlocked valves which can be operated manually in the event of an overtemperature not readily controllable by the instrument and its pneumatic diaphragm valves. Overpressure relief is a related problem that is part of the installation design. The various types of overpressure are well known: hydrostatic overfilling, external conflagration, malfunction of process pressure control, oversupply of heat medium, uncontrolled exothermic reaction, or air-hydrocarbon explosion. Pressure-vessel codes specify the basis for sizing relief devices once the maximum rate of discharge is established. The designer
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ENGINEERING AND PROCESS DEVELOPMENT will have t o examine critically all the sources of overpressure peculiar to the installation and provide relief for the worst case. Correlations are available t o assist in protecting against pressure generation from common sources such as external fire (19) and vapor explosion (2). Other relief rates will be calculable by inspection of the process flow diagram or by chemical engineering principles.
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Two examples of reactor types have already been shown in Figures 1 and 3. The former is the typical stirred reaction kettle in which so many batch and constant composition or cascade continuous r e a c t i o n s may be carried out. Inclusion of a gas injection line leading below the lower impeller adapts the unit t o liquid phase Oxidation, h y d r o genation, or other gas-liquid reactions, A draft tube l e a d i n g from the vapor space t o the eye of the top impeller is said t o induce redispersion of unreacted gas. A s u s p e n d e d solid, Figure 5. One-Barrel-per-Day s u c h as R a n e y Catalytic Cracking Pilot Plant nickel hydrogenation catalyst, may be kept in the vessel through successive reaction batches by the use of an edge or disk filter in conjunction with the bottom drain valve. The tubular jacketed reactor with axial thermowell and catalyst support is useful for vapor phase oxidation, chlorination, hydrogenation, isomerization, and other similar reactions. Temperature levels from boiling refrigerant t o Dowtherm vapor are practical. Other forms of reactors are in comnion use. For instance, pilot plant catalytic cracking of petroleum fractions is carried out by a variety of methods. These range from the fixed or static bed bench-scale reactors t o pilot plants whose flow duplicates that of a commercial plant. The former, along with fluidized fixed-bed reactors, have been developed as an aid t o catalyst development and as a control apparatus for checking plant feed stocks (10,16). The larger and more complicated pilot plants are used for closer investigation into the effects of feed stock variation, for study of changes in operating conditions, and for observing the reaction of the catalyst t o long and continuous use. Some pilot plants have been built with discontinuous regeneration of the catalyst-that is, the spent catalyst is regenerated batchwise and then charged t o a preheating vessel where it is heated t o operating temperature before being picked up by a carrier gas and fed to the reactor. Figure 5 is a photograph of a completely continuous fluid catalytic cracking pilot plant. The picture was taken during the course of construction before the insulation was applied. Illustrated are the regenerated catalyst standpipe and control valve, the catalyst feed line, the bottom of the reactor and the catalyst stripping leg, and the spentrcatalyst control valve and transfer line. The electrical heating elements required for heat compensation are shown attached to some of the
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lines and vessels; in others they have already been covered by the metallic sheath on which the lagging is t o be mounted, Such a pilot plant will run continuously over a long period provided catalyst flow can be maintained without bridging or and provided coking of the feed preheater and of the product exit filters can be avoided. Conclusion
The design of a pilot plant reactor may be merely the application of established principles t o a standard design, or it may be the novel development of an apparatus t o meet special requirements. I n any case, it must be based on the best knowledge availahlc Crom laboratory studies and from theoretical considerations of the problem. It must be capable of being expanded in design t o a full-scale plant, and while it may not duplicate the eventual plant in detail, the operation must point out difficulties and pitfalls t o be avoided in the h a 1 design. It must be as inexpensive as possible, yet there must be built into the reactor a flexibility seldom if ever required of a commercial plant. Finally, and here the pilot plant reactor often differs from those in the laboratory, it must be provided with automatic control instrument. which will ensure the collection of valid data in the shortest possible time. This insistence on a design that will allow start-up with a minimum of mechanical changes and operation with no untoward drifts in run conditione stems from two factors. The labor and raw materials cost of operating a pilot plant is a major factor in the lvhole research program. This expenditure must not be made without returns in the form of design information. In addition, there is a particular urgency about pilot plant opeiation which is the result of the fact that once the decision is made t o enter the field with a nea product, each succeeding day x itliout commercial production represents money lost beyond recovery. literature Cited (1) Chem. Eng., 57,107 (November 1950). (2) Cousins, E. W., and Cotton, P. E., Chem. Eng., 58, 133 (.iugust 1951). (3) Cummings, G. H., and West, 8 . S., IND. EXG.CHEM.,42, 2303 (1950). (4) DeLorenzo, B., and Anderson, E. D., Trans. Am. Soc. Mech. Engrs., 67,697 (1945). (5) Dow, W. M., and Jakob, 31. Chem. Eng. Progr., 47,637 (1951). (6) Fontana, M. G., IND.ENG. CHEM.,41, 101 A (March 1949); 41,103A(April1949); 41,95A(May1949). (7) Holmberg, E. G.,Chem. Eng. Proor., 48,377 (1952). (8) Hougen, J. O., and Piret, E. L., Ibid., 47,295 (1951). ENG.CHEM.,43,2197 (1951). (9) IND. (10) Ivey, F. E., and Veltman, P. L., PefroleumRefiner, 31, 93 (June 1952). (11) Knudsen, J. G., and Katz, D. L., Chem. Eng. Progr., 46, 490 (19501. , ---, (12) Leva, M., IND. ENG.CHEW,39, 857 (1947). (13) Leva, M., I b i d , 42, 2498 (1950). (14) Mickley, H. S., and Trilling, C. A , , Ibid., 41,1135 (1949). (15) M o h o , D. F., and Hougen, J. O., Chem. Eng. Progr., 48, 147 (1952). (16) Rackley, C. W., and Shelley, C. W., Petroleum Refiner, 31, 89 (June 1952). (17) Rushton, J. H., Chem. Eng. Progr., 47,485 (1951). (18) Rushton, J. H., Liohtmann, R. S., and Mahony, L. H., ISD. ENG.CHEM., 40,1082 (1948). (19) Sylvander, N. E., and Katz, D. L., Petroleum Processing, 3, 642 (1948). (20) Wiegand, J. H., Trans. ?i Inst. ?% Chem. Engrs., 41, 147 (1945).
RECEIVED for review April 15, 1953.
ACCEPTED June 23, 1953.
PLANT SECTION
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 45, No. 10
