HEATING SMALL REACTORS High Temperature Fluidized Solids Bath for Continuous Systems C. E. ADAMS, M. 0. GERNAND,
AND
C.
N. KIMBERLIN, JR.
Esso Laboratories, Frso Standard Oil Co., Louisiana Division, Baton Rouge, La.
of liquid or molten material are frequently used ior t t l l o ~s uniform heating of equlpmmt of irregular shapes 1301 B A heating THS laboratory anti coxnmerc:ial equipment because of example, separate prrhcsting of various feed streamp ma\ \ , t the t’emperature control and even heating afforded. WatJrr. carried out in the h t h used to hest tht. wac’tion zone. various organic compounds, petroleum fractions, salt mixtures, and metals have been descrihed ( 2 ) as media for different apFluidized Solids Bath I s Simple to plications. These media, however, itre often unsuitable for Design and Construct general use, particularIy a t high trmpcratures, because of thc The absence of critical liinitat,ionc; with fluidized solida k x l , t l ~ ~ limited ranges of temperature and the special equipment required cliiniiiat,es complicated design. Baths uith widely differtwt for handling pressure and expensive or hazardous niatcrials. details of construction have d l operated datisfactorilg. Tile I t has been found that baths utilizing fluidized solids offer it solufollowing discussion of general oonstruct,ion considerations illtion to these problems and provide an excellent means of heating eludes t.he necessary infotmatioii for t,hose who are unfa,tnilj;ir. laboratory and small scale pilot plant units and equipment. \\-it,h fluidized solids techniques. These baths of granular solids, when properly agitated by ii stream of gas, have many of the properties of baths of ordin:it,y liquids. Like liquid baths, the Auid-solids baths have thc atlASPIRATOR vantage of being substantiall)- isothermal (I, 9, 6),are subject VENT E t,o close temperature control, have a high heat capacity, and maintain uniform temperature on t’he surface of the equipment PRAT -DANIEL heated. A broad temperature range in the fluidized solids ba,th PRODUCT OUT is possible with the proper choice of granular material. Temperatures from room temperat,ure t o l60O0 F. have been used iii t,hese baths, and much higher values are doubtless feasible. The heat transfer properties of the fluid solids system are superior to to those of many other heat trsnsfer media. Hpat t>ransfcr coefficients of 80 to 200 H.T.U. per (square foot) (hour) ( O F,), depending on the solids and fluidizing conditions used: havr been reported ( 1 , 9, 5 ) for the transfer of heat. from fluid solids to internal tubes. Maximuni values for heat transfer with ii given solid are obtained at, the highest practical fluidizing velocities. These heat transfer coefficients have not been checked in the applications of the fluidized solids heating baths for sm:tll scale equipment described in this paper. However, limitations of heat transfer have not been encountered over a xide rmge of conditions and reactions tcssted. Considerable experience with fluidizrti solids heat,ing baths has demonstrated the easc of operation and versatilit’j- of thesc baths. The absence of critical factors, such as pressure, safct,!hazards, expensive media, and temperature limitations: eliminates complicated design, construction, and operation. OnljFigure 1. Fluidized Solids Heating simple generalizations need to be considered in the construction Bath of a fluidized-solids heating bath. Conventional methods of’ heating and temperature control ttre applicable tu these baths. Simple methods of placing the equipment in the batch permit Figure 1 is a cutaway sketch, drawn approximately to sc.a,lr, easy removal and replacement of the equipment even while the of ii typical fluidized solids heating bath installation. ‘Flie itssembled installation is shown iii Figure 2. bath is in operation. The fluid nature of the heating bat,h also
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 46, No. 12
PILOT PLANTS
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Bath Design. The outer shell of the bath may be made from a section of pipe of an appropriate size. There should be a minimum annular space of about 2 inches between the wall of the bath and the tube to be heated. For heating larger equipment, it may be desirable t o have thc hcat capacity afforded by a bath with about twice thc dinmeter of the equipment heated. Tcmperature gradients may be expected in the operation of fluidized baths having unusual or very asymmetric shapes, although such extremes will not normally be encountered in heating bath applications. A very shallow bed allows relatively little lateral mixing of the solids because of difficulties in the proper distribution of the aeration gas into the bed. Uniform heat application to long, narrow beds is more difficult because of relatively less vertical mixing of the solids. When air is readily available, it is used t o fluidize the solids except in special circumstttnces. The fluidizing gas is introduced into the bottom of the bath, preferably through a distributor plate. If proper distribution of the fluidizing gas is not provided, a zone of unfluidised solids forms around the gas inlet and allows negligible heat transfer. The use of distributor plates is particularly recommended for baths with a diameter greater than about 6 inches. Smaller equipment may use an inlet section ing of a n inverted acute cone, but this is less efficient. X trouble-free design for distributor plates utilizes a number of small holes drilled in the center of equal areas of a plate welded into the bottom of the bath. These holes should be large enough t,hat they will not be plugged by the solids. For example, when solids passing through a 100-mesh screen are used the holes inch in diameter. A rule of thumb for disshould be about tributor plates is a minimum of five holes, or one hole for every 2 to 3 square inches of area, while still allowing a pressure drop through the plate. A definitc pressure drop through the distributor is important and should be equivalent to about 10% of the pressure drop through the system. Closures a t the top of the bath may consist of flanges or other standard pipe fittings. Simple seats with machined faces are satisfactory for most purposes. The weight of the fitting is usually sufficient to maintain the seat, but bolt's or added weight.s may be used if desired. The use of an air or steam aspirator in the vent line from the bath is a practical means of reducing the escape of dust into the room, even when the bath is opened for removal of equipment. The fluidizing gas leaving the bed carries some of the solid particles with it and thus should be vented away from occupied areas. Because of the availability and cheapness of many of the solids, this loss need not he a serious factor. However, maintenance of the bed level may become troublesome for long periods of operation. Solids carry-over may be materially reduced by leaving a "disengaging" sone of several inches above the level of the fluidized bed. The height of this disengaging space is dependent in part on the design of the equipment to be heated, as reduced temperatures and heat transfer are found in this zone. .i disengaging zone equal to one half the diameter of the bath is recommended if possible. The use of an expanded section above the bath further enhances the retention of solids by reducing the velocity of the gas flowing through this zone. Filters may bc used to retain these solids. Cyclones with return dip-legs terminating underneath the level of the bed may also be used to recover the entrained solid particles. Cyclones commercially available a t moderate cost, such as the Prat-Daniel dust collectors (Standard HC or Valmont S) supplied by the Thermix Corp., have been found suitable. An installation of one of these cyclones is illustrated in Figure 1. Although more troublesome, the cyclones may be mounted in the vent line away from the bath. I n this case, the solids may be returned to the bath continuously by means of pipes, or periodically from a recovery chamber. When the cyclone is located externally, aeration of the solids return line should bc provided. This is accomplished by introducing a small flox of gns into the bottom of this line counter-
December 1954
Figure 2.
Pilot Plant Installation of Fluidized Solids Heating Bath
current to the flow of solidc;, so that the downward nioverneiit oi the solids is maintained. Heating of external cyclones and lines may be required to prevent plugging due t o condensation of moisture released when the bed is heated after a shutdown. Methods of Heating. For operation of fluidized solids baths between room temperature and about 1100" F., electrical heating is used. This heat may be supplied by means of commerciallyavailable electrical heating units immersed in the fluidized solids or attached to the outside of the metal wall of the bath. Resist,ance heating wire protected by insulating beads and wrapped around the outside of the bath also has been used, as shown i n Figure 1. Internal heaters are more efficient since the heat is taken up directly by the fluidized solids. The turbulence and high heat conductance of the fluidized solids avoid danger of local overheating, thus allowing the heat to be applied from a small heating surface. Isothermal conditions are maintained throughout the bat,h except for a short distance of less than 1 inch from the point where the heat is applied or lost ( 1 , S , 6). Control of electrical hcat ia accomplished in conventional ways through temperature regulators and variable transformers. Temperatures above 1100' F. can be obtained by combustlion within the fluidized bedof a light gas oil such as Essoheat Medium. The gas oil is introduced into thc fluidized bed just above t,he distributor plate and R i ignited readily by the fluidizing air atid hot solids a t temperaturcs above 900" F. Burning of oil may also be utilized as an auxiliary source of heat for rapid heating above 900" F. When oil is burned. temperature control is obtained by regulat,ing the oil feed rate. Also, when oil is used, care should be exercised to prt:vcnt leakage of carbon monoxide into occupied areas.
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
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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
Vol. 46, No. 12
