ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Fluidized Solids. Any solid m:iterial stable under the conditions used, and of the pr0pc.r p:trticle size for fluidization, should be satisfactory for heating baths. The material obviously should be inert, nonvolatile, refrac-tory. attrition resistant, and noncorrosive. For use in heating baths, particle sizes Lvithin {lie range 325 to 100 mesh (40 to 150 microns) are suggested. Thci preferred size range will be part'ially dependent on particle density. Small light particles are difficult t o retain in the bat,h, whilc large dense particles require excessive amounts of air for fluidization. Baerg, Klassen, and Gislilet ( 1 ) reported on the effect#01 particle size and density on heat transfer coefficients and concluded that small dense particlrs alloir- m;txiinum heat transfer. Alumina, silica gel, and sand :trc examples of solitls rratlil). available for use in heating b:tt~hsafter grinding and/or sizing. It has been found in this 1ak)ortitory t,hat cracking catalj.st produced commercially in a fluidizuhle size range is particularl?, suitable when available. This type of material is supplied t o petroleum refineries by such manufa ctmers as American Cyaniimid Co., Davidson Chemical Corp., Filtrol Corp., Sat,ion:il &\luminateCorp., and Morton Salt Co. Equipment Installations. For ni:+simum utilization of the bath, the equipment to he heated should be designed for , removal from the bath. This pitii be accomplished by caonstructing the equipment to fit in a n opening in the top of the bath. Thc, equipment is then mounted under or t,hrough a plate made to close the opening when the asseinhl?, is in place. ;tIachined $eats which utilize the weight of the equipment are usual1~adequate to effect the closure required. Feed lines, outlet linos, thermocouple wells, and othcr :wsiliary lines are introducwl through the closure plate so t,litit thc complete assembly can be removed as a unit. Efficient Iieiiting is obtained only within t,he dense fluidized solids bed, hence port'ions of the equipment requiring transfer of heat should extend well into the bed. h e heating of feed streams and similar operations can be carried out in the same bath by a sufficient extension of t,hese lines int,o tho bath. In certain instances d i e r e feed lines extend into the bed tmd full preheabing is not, desired, thrsc lines can be jacketed and insulated. Operation over Wide Temperature Range Is Feasible
Operation of fluidized d i d < lit~atingbaths required coutrol only of temperature, fluidizing gas rate, and bed level. Temperature control is ohtairied hv t~onventionalprocedures. The
(Heating
Literature Cited (1) Baerg, A., Klassen, J . , arid Gishlor, 1'. E., Can. J . Research, 28F, 257 (1950). ( 2 ) Ceiringer,P. L., Chem. E ~ Q 57, . , No. 10, 136 (1950). ( 3 ) Leva, M., Weintraub, >I., and Gmmmer, M., Chem. Eng. P w g r . , 45, 563 (1949). (4) hlatheson, G . L., Hcrbst, W. A., and Holt, . '1 H., 211d, 1x11.Eso. CHEX..41,1099 (1949). ( 5 ) Mickley, 13. S., and Trilling. C:. .A, I b i d . , p. 11338. RECEIVEDfor review Julie 8, 1954. ACCE~TED October 18, 1954. Presented before the Division of Petroleu~iiChemistry at the 125th Meetin!: of the .4hlERICAN C I I E U I C A L EOCILTY. Kansas ('its, >Io.
Small Reactors)
L. 1. GRIFFIN, JR., Esso
AND
J. F. MOSER, JR.
laboratories, Esso Standard Oil Ca., Louisiana Division, Baton Rouge, La
N ItECEXT years the fluitiizud solids tcchriique has heen applied more and mow frequently to industrial processes. This is particularly true in the petroleum industry mhere tho fluid catalytic cracking process is so widely used. More recently, wch processes as fluid adsorption, fluid hydroforming, and fluid coking show promist, of considerably broadening t,lie use of the fluidized solids technique iii industry. These, as well as future processes, require suitable small scale pilot p1aiit.s both i n the original process developments arid in their ultimate iniprovexnent. Since there are obvious differences between fixed hcd, 01' 2460
temperat'ure of the bath can be measured a t alii within the dense phase. Care should he exerci thermocouple wells adjacent to heating surfaces. Therniocouplcs wells extending a t least 1 inch through walls where heat ifi UJIplied, however, have been found to measure the true t,empt?rat,ure of the bed. The isothermal character of a fluidized bed is o f t w criterion of proper operat,ion and design. Teniperittu t'tl measurement is also the preferred method for indicating thc luvrl of the fluidized bed. A sharp drop in temperature is noted : h o v e the level of the dense bed. Temperature readings at the l ~ t . 1 a t which solids are to be maiiitained will thus show by a d r o p iri temperature when addition of make-up solids is required. There is a minimum velocity of gas that will give satisfactor>mixing of tthe solids ( 1 , 4, Or) and proper heat transfer. This g;t" rate is usually expressed as a h e a r or inass velocity based on the cross-sectional area of the empty enclosure. A gas velocity of about 0.3 foot per second (corrected for temperature) through the bath is ronsidered best for operation with cracking catalyfit or similar materials, Denser solids require a somewhat higher velocity. This recommended velocity is sufficient to give good fiuidization and heat transfer while allowing a minimum carryover of solids. Higher velocities result in higher heat transfer coefficient,s for the fluid bed, hut simultaneously more heat is removed from the bed in heating the additional gas. The rate of flow of the fluidizing gas does not require any special control and is conveniently indicated by means of a rotameter or orifice meter. The fluidized solids heating bath system has been found to he very Satisfactory for maintaining isothermal conditione. Furiher, operability over a wide temperature range is feasible. I3ecause of these qualities, and also because of the safe and simple nature of the system, the fluidized Polids bath merits (Ionsideration whenever small wssrl heating systems are plannrd.
nonfluidized reactors, and fluidized unitt?, there is a strong i 11centive t o devclo:, flexible fluidized solids piiot plants capable of closely duplicating commercial units. This paper descrihs :i unique reactor construction which is particularly adaptalile to many fluid catalytic processes. General Design Considerations Are Determined by Speciflc Use of Pilot Unit
In the design of any fluidized solids pilot plant, there are it number of factors which must he considered. Such items a s
INDUSTRIAL AND ENGINEERING CHEMISTRY
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PILOT PLANTS size of unit, throughput, range of operating conditions, etc., are specific for each case. Once these basic factors are set, the design is guided by the general principles of fluidized solids operation. Vessel diameters are set so that the superficial gas velocities are adequate t o maintain good fluidization over the entire range of operating conditions. Vessel heights are normally determined by the throughput or size of solids charge desired. However, other factors, such as length t o diameter ratio of the fluidized bed and disengaging space required above the bed, must also be considered. Means for recovering entrained solids and for properly distributing the fluidizing gas must also be provided. These are only a few of the more important considerations in design, and a more complete discussion of such factors is given in the accompanying paper and the literature ( I , $ ) . Air Cooled Reactor A l l o w s Rapid a n d Responsive Temperature Control
tion is shown in Figure 2. I n this modified reactor, half-pipes are simply welded t o the outside of the reactor vessel to form the cooling ducts. Both reactors have proved satisfactory in operation. I n Figure 3, a diagrammatic sketch of the unit with the attenda n t instrumentation is shown. Basically, temperature control is achieved by supplying a constant amount of heat greater than that required by the process while simultaneously removing the excess heat by a variable flow of cooling air supplied continuously t o the ducts. Although air is used in this example, any gas t h a t
COOLING AIR OUTLET-
One of the most important design features of a pilot plant is that of temperature control. I n some cases, i t is desired to maintain completely isothermal conditions for rather long periods of time-say, from one hour to several weeks. I n other cases, it is desired to change temperature level rapidly in a cyclic manner over a wide range. This latter type operation is often particularly desirable in catalytic processes. I n many cases, the catalyst activity decreases rapidly with time and frequent regenerations are required. Since regeneration temperatures are normally higher than reaction temperatures, the temperature of the
OOLING AIR PIPES. ANIFOLOED A i TOP
M
W
Figure 2.
COOLING -AIR OUTLET
-COOLING
:I:
AIR I N L E T
M Y
COOLING AIR SLOTS. MANIFOLDED AT TOP AND BOTTOM
--FLUIDIZED
-METAL
AIR OUT
i.2
Modified Reactor
is available under pressure can be used. Heat is applied continuously to the reactor by the strip heaters on the wall, and the rate of heat input is preset by the voltage regulator in the power supply line. A thermocouple located within the reaction zone actuates a conventional temperature recorder controller equipped with derivative control. This controller in turn regulates the amount of cooling air admitted t o the ducts in order to maintain the desired temperature.
BED
ELECTRICAL STRIP HEATERS
a iAi
COVER
THERMOCOUPLE CONTROL
kVALVE %ROL
\
COOLING AIR SLOTS
Figure 1.
COOLING -AIR SUPPLY
Air-Cooled Reactor
,
VOLTAGE
pilot unit reactor must be raised and lowered frequently. I n pilot units with conventional heating equipment, the time spent in changing temperature levt.1 is a very significant amount of the total operating time. Thus, throughputs are reduced and operating efficiency is low. This is particularly true if the actual reaction times or cycles are in the order of only a few minutes. T o overcome this difficulty, a special air cooled reactor has been developed. Although the reactor is adaptable to many processes, it was originally designed for catalytic cracking and will be described as used in this process. A schematic diagram of the reactor is shown in Figure 1. The reactor is constructed of heavy wall, stainless steel pipe, and four slots are milled in the vessel wall. These slots are covered with metal strips, so as to form a continuous duct for cooling gas. As shown in the top view, four strip heaters are clamped around the circumference of the vessel in the space between the cooling slots. The bed level of fluidized solids is maintained below the top of this slotted section. The disperse phase temperature above the slotted section can normally be maintained by manual control with electrical heaters. An alternate design t h a t is somewhat simpler in construc-
December 1954
,FLUIDIZED
BED
7
ELECTRICAL STRIP HEATERS
REACTOR
Figure 3.
Reactor Temperature Control System
With this system, isothermal conditions ( F.) under steady operating conditions can be maintained indefinitely. I n addition, temperature profiles obtained during normal operation show less than 5' F. variation in temperature from 2 inches above the bed bottom to the bed top. However, the real advantages of the system lie in the capacity to operate isothermally under varying process conditions or varying heat loads, and to change temperature level rapidly. As a n example of the first advantage, just after feed is admitted to the reactor, the heat load is increased considerably and the bed temperature begins t o drop. This, however, instantly reduces the air supplied to the cooling ducts
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING. QESIGN. AND PROCESS DEVELOPMENT so t h a t the large excess of heat supplied by the electrical heaters compensates for this trend. After the initial change in heat load, the bed temperature quickly returns t o the control point, and the controller now admits a somewhat smaller amount of air to maintain isothermal conditions. Similarly, if there is a sudden decrease in heat load, the amount of air supplied is quickly and automatically increased in order t o hold a constant temperature.
Figure 4.
Air-Cooled Reactor Cyclical Temperature Operation
The second advantage-the ability to rapidly change from one temperature level to another-is extremely important in iniproving operating efficiency. I n catalytic cracking, for example. it is generally desired to carry out a 2- to 30-minute cracking cycle a t around 850" t o 950" F., followed by a 10- t o 2O-nlinute regeneration cycle a t about 1 1 0 0 O F. It is important that the time lost in changing temperature level be minimized if a reasonable throughput of feed oil per day is to be obtained. Such rapid changes in temperature are easily accomplished in the manner described and may be scheduled by an automatic cycle timer. If the temperature is to be increased, raising the temperature controller index t o the desired temperature cuts all air out of the ducts and permite rapid heating. Conversely, lowering the controller index supplies a large excess of cooling air. I n the catalytic cracking unite a t Baton Rouge, heating and cooling rates of 15" F. per minute have been obtained easily. A typical temperature chart from one of these cyclic units is shown in Figure 4. This chart illustrates clearly the rapid change in temperature level and excellent control of temperature a t a given level. I n addition, there is virtually no "overshoot" or "undershoot" as temperatures are changed. This smooth approach t o the control temperature was obtained by installing a derivative controller on a standard temperature control instrument. Without derivative control, some difficulty was experienced with cycling a t the control point. It will be appreciated that this type of temperature control depends on rapid heat transfer to and from the reactor contents.
2462
Unless this is obtained, hot spots will occur a t the wall adjacent t o the heaters and cool zones will lie along the cooling ducts. With fluidized-solids processes or even with well stirred liquid processes, heat transfer is rapid enough so that the reactants are essentially isothermal. I n designing the temperature control system for a n air cooled reactor, several factors should be considered. The amount of heat to be supplied over and above the normal heat load is determined by two factors-the maximum heat-up rate desired and the maximum change in heat load anticipated while an a.ttempt is made t o hold a constant temperature. I n most cases, the first item is controlling and determines the size of the voltage regulator and elect,rical heaters needed. For maximum flexibility, it is generally advisable to size the voltage regulator so that it' is normally operating at' 50 to 75% of capacity. The cooling ducts are generally made as large as practical in order to supply the greatest amount of heat transfer surface. This, of course, reduces the amount of air required as Tvell as the pressure drop through the ducts. Minimum pressure for the cooling gas supply for good cont,rol is determined by the rate of cooling desired and thc maximum change in heat load expected. The control valve is sized to operate a t about half the maximum capacity under normal heat load conditions. Act'ually, the flesibility of the system is such that no one factor is so critical in design t h a t it cannot easily be compensated by somc other factor. Conclusions
The air cooled reactor has proved to be a flexible pilot unit. reactor for studying both catalytic and noncatalytic processes by the fluidized solids technique. The ease with which tempersture can he conbrolled and the rapidity n i t h which it can be changed has markedly increased operating efficiency of pilot units used in cyclic process studies. I n addition, niechanicai efficiency has proved to be excellent, and very little time has been lost because of mechanical failures. Because of the simplicity of construction, the turnaround time required to changc catalyst or solids in the reactor is short. Another feature of the design is that i t is adaptable t o scheduled automatic control of temperature level. Tlowever, since temperature control depends on rapid heat transfer achieved with a fluid bed, the air cooled reactor is not well suited for fixed bed or nonfluidizetl operation. literature Cited (1) IKD. ENG.CHPM, Review of Chemical Engineering Unit Oi-ierations, 45, 74 (1953); 44, 68 (1952); 43, 90 (1951); 42, 55 (1950) ; 41,44 (1949). (2) Sicholson. E. W., Noise, J. E., and Hardy, R. L., I b i d . , 40, 2033 (1948). RECEIVEDfor review June 8, 1954. ~~CCEPTED October 18, 106i Presented before the Division of Petroleum Chemktry at the 125th Meeting of the A M E R I C A N C H F U I C A L SOCIETY, Kaneas City. >IO
INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
Vol. 46,Ne. 12
