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
June 1949 Table 11.
Energy Balances
Run Feed rate, tons/hour Feed moisture % No. 2 fuel oil, &l./ton feed Air rate (standard conditions), cu. feet/min. Stack temp. C. Recorded wihdbox temp;, O C. Calcd. windbox temp., C. B.t.u. input from oila B.t.u. to evaporate feed water B.t.u. i n sized stone a t 110’ C.
B.t.u. /n B.t.u. in exhaust dust at 75‘ gas C. a t 75’ C. Total B.t.u. requirement Difference a
1 33 0.8
2 36.5 0.9
4800 75 330-390 365 99,000
0.77
Fr60K 61,200
l::ii: 99,300 300
0.8
3 29 1.2
0.88
4 28 1.9 1.07
5100
4900
6000
75 350 370
75 290-390 340 113,000
75 230-330 380 137*500
l%ooo
19,85F 26,400 60,800 61,000
41,700 60,500
5,160
exit gas temperature was only 75 C. This nontypical phenomenon of fluidization is attributed t o moist feed entering above the fluid bed level with resulting countercurrent drying. FIELD
O F APPLICATION
Laboratory tests indicate separations from 20 to 100 mesh as possible with this unit (mesh of separation is defined as t h a t mesh on which the undersize product registers 1.5%). Separations finer than 100 mesh require relatively low space velocities with resulting low sizing and drying capacities, For fine meshes of~ separation, the feed must be crushed to a relatively fine size, ~ as the largest feed particle must be maintained fluid at the space velocity required for the fine mesh of separation for satisfactory operation.
3;;:;
1 ~ : ~ 100,710 107,840 126,300 2,290
1249
11,200
Oil a t 128,500 B.t.u./gal., net heat.
RECEIVED January 3, 1949.
Fluidization Nomenclature and Symbols F l u i d i z a t i o n as a field of o u r science has grown so rapidly so recently t h a t it is having vocabulary problems. T h e audience a t t h e Cambridge symposium last December was presented m a n y examples of overlapping t e r m i n o l ogy in t h e papers given a t t h a t t i m e . T h e editors were f o r t u n a t e in securing very effective aid in t h i s problem before t h e symposium papers in t h e present issue were set in type. A t o u r urging, a discussion group was formed t h a t assembled a l i s t of recommended t e r m s a n d symbols f o r use by those authors w h o were in agreement w i t h t h e m . T h i s group has given unsparingly of i t s energies and m u c h has been accomplished in a r i g i d l y l i m i t e d time. Participants were: Leo Friend, M. W. Kellogg; M a x Leva, Bureau of Mines; R. D. Morse, D u Pont; D. 0. M y a t t , I n d u s t r i a l and Engineering Chem-
istry; H. J. Ogorraly, Standard O i l Development Co.; J. H. Perry, Du P o n t ; R. L. Taylor, Chemical Industries; R. H. Wilkelm, Princeton University; and F. J. V a n Antwerpen, Chemical Engineering Progress. H. J. Hall, Standard Oil Development Co., was secretary of t h e group. Nomenclature is never static a n d there is n o t h o u g h t t h a t t h e t e x t below is either complete o r perfect. However, as a compilation t h a t is generally acceptable t o a n u m b e r of experienced workers in t h e field, it has served well i t s orlginal purpose of m a k i n g t h e published symposium u n i f o r m in terminology. We present it here p a r t l y as a c o m m o n glossary f o r t h e symposium papers and p a r t l y f o r w h a t benefit it can be in establishment of a permanent nomenclature in t h e field of fluidization.
T
further increases in fluid velocity give a progressive separation of the particles which remain individually and uniformly dispersed in particulate fluidization. With many gas-solid systems the bed expands to only a limited extent, and a portion of the gas tends t o accumulate in gas pockets or bubbles passing through the fluidized bed in aggregative fluidization. In small tubes, these bubbles may grow t o a size substantially filling the cross section of the vessel, resulting in slugging. Normally, however, the bubbles remain small with respect to the vessel and the mass becomes a turbulent fluidized bed. Such a turbulent, bed can even be maintained above the free-falling velocity for the solid particles in R gm-solid system if the rate at which solids are fed is kept high enough. Finally, if the fluid velocity is still further increased or if the solids feed rate ia too low, the surface of the fluidized bed disappears and the whole mass becomes a dispersed. suspension. Proposed definitions for these terms are given below.
HIS discussion applies primarily to systems in which a mass of solid particles is in contact with or supported by a stream of gas or liquid of lower density rijing relative to the particles. It is recognized, however, that many of the terms suggested for such systems may be equally applicable t o the broader field, which may include gaseous or liquid matter in particulate form or fluid motion in any direction. When a stream of gas or liquid is passed upward through a mass of solid particles, reproducible changes in physical behavior are observed which pass successive stages as t>hevelocity of the fluid is increased. The definitions given have been chosen to describe the most important of these stages or types of behavior. Some of the various relationships observed between them may be summarized as follows, recognizing that other, transition, condibions also may exist. Whcn the velocity of a gas or liquid flowing up through a mass of solid particles is insufficient to lift or support any of the solid, the mass is called a fixed bed or moving bed, depending on whether the solid is stationary or moving with respect t o the containing vessel. With increasing fluid velocit,ies, in the absence of channeling, the pressure drop through the bed rises until it approaches the net effective weight of the solid per unit area, when the packing arrangement of the particles becomes more open so that the bed expands; with slight further increases in velocity the particles are fully supported and the expanding bed becomes fluidized. Just at the point of fluidization the maSs may form a quiescent fluidized bed. With liquid-solid systems,
W A L T E R J. M U R P H Y , E d i t o p
DEFINITIONS
1. A “fixed bedSJis a body of motionless solid particles supported by direct contact with each other and the retaining walls. 2. A “moving bed” is a similar body in which the particles remain in direct contact and are substantially fixed in position with respect to each other, but move with respect to the retaining walls. 3. A “fluidized mass” of solid particles is one which exhibits the mobility and hydrostatic pressure characteristic of a fluid.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1250
(This condition may be achieved through suspending the particles by means of a stream of gas or liquid rising past the particles.) 4. A “fluidized bed” is a mass of solid particles which exhibits the liquidlike characteristics of mobility, hydrostatic pressure, and a n observable upper free surface or boundary zone across which a marked change in concentration of particles occurs. (In a fluidized bed, the random motion of the particles increases with increasing velocity of the supporting medium.) 4a. “Particulate fluidization” of a bed refers to a condition in which the particles are individually and uniformly dispersed. (Particulate fluidization is commonly observed in beds fluidized by a current of liquid. The term “teetering” as used in the oredressing industry refers to a relatively high-density suspension of this type.) In contrast, coexistence of dense and dilute suspensions (bubbles) within a fluidized bed is termed “aggregative fluidization.” (Aggregative fluidization is commonly observed in beds fluidized by a current of gas.) 4b. A “quiescent fluidized bed” is a dense fluidized bed which exhibits little or no mixing of the solid particles. (Such a bed is analogous t o a body of liquid a t rest, having a well-defined upper free surface.) 4c. A ‘%urbulent fluidized bed” is a fluidized bed in which mixing of the mass of solids takes place. (The degree of turbulence, increasing from the lower limit of quiescent-bed conditions to violent mixin , de ends upon the dynamics of the system. The passage of bubbyes tlrough the bed may give rise to such turbulence and mixing. While such a bed may operate a t a gas velocity below the free-falling velocity for the bulk of the solid particles, it can also be maintained at a velocity materially above the freefalling velocity if a continuous feed of solids is supplied t o the bed. The boundary zone or interface a t thc free upper surface of a turbulent fluidized bed is generally diffuse, as in the surface of a boiling liquid.) 5. A I‘dispersed suspension” is a mass of solid particles or aggregates sui ended i n a current of li uid or gas riskg past the particles, whicR differs from a fluidizedqjed in that an upper level or interface is not formed under conditions of continuous solids entrainment and uniform superficial velocit,y. (This is usually observed under conditions of low solids concent,ration and either high fluid velocity or low solids feed rate. Thus, in general, a dispersed suspension is analogous to a vapor, whereas a fluidized bed is analogous to a liquid. One example of this condition is observed in pneumatic transport. I n a vessel containing a fluidized bed a dilute suspension of entrained particles above the bed also is such a dispersed suspension, and is frequently referred to as the “disperse phase” while the bed itself is referred to as t’he “dense phase.”) 6 . ‘LChanneling’)is the establishment of flow paths in a bed of solid particles through which a disproportionate quantity of the introduced fluid passes. 7. ‘Wugging” is a condition in .ir.hich pockets or bubbles of the supporting fluid grow to the diameter of the containing vessel, and the mass of particles trapped between adjacent pockcts moves upward in a pistonlike fashion. (This condition is usually limited to vessels of high length-to-diameter ratio.) LETTER
SYMBOLS
Most of the previously established standards (1)for the fundamental physical magnitudes seem suitable without chango. In a few instances, deviation seems advisable or a new symbol seems required through lack of precedent. The list at’ the end of this section contains est,ablished symbols of special pertinence to the field of fluidization as well as the new ones recommended. Kew symbols or elements are indicated by an asterisk in parent’heses. While these symbols are fundament,al t o many physical operations, frequently it may be necessary to apply them t o the characteristic states of fluidization systems. The following subscripts are suggested as modifiers for the primary symbols:
a = hypothetical bed with zero interstitial voids s = settled bed (conditions of settling must be specified) f = fluidized bed
mf e d
= =
=
M =
X
=
p
=
F =
Vol. 41, No. 6
minimum fluidized bed expanding bed (between s and mf) dispersed suspension total mixture, solid plus supporting fluid solid constituent supporting fluid individual particle
Such subscripts are obviously applicable, for example, to symbols L, G, a, and p . T o illustrate, L = height of bed (general), La = height of hypothetical bed at no interstitial voids, L, = height of settled bed under specified settling conditions, L , = height of fluidized bed, p p = bulk density of the individual particle, including occlusions, P S = true density of solid constituent, U M = surface per unit volume of bed, u p = surface per unit volume of particles, etc. Although these terms are recommended after careful consideration of all viewpoints expressed during their development, the immature state of knorvledge in the field of fluidization may make subsequent alteration advisable. If the majority of these terms afford a satisfactory base from which a permanent notation can evolve as knowledge of the field grows, the objectives of this effort will have been achieved.
Magnitude Area Density Diameter Diameter, particle (method of measurement must be specified) Distance above datum plane Drag coefficient Efficiency, fluidization Energy dissipation, rate Expansion ratio Flow rate, mass Friction factor Frictional resistance Gravity (Newton’s law conversion factor, 32.17) Gravity, acceleration Length or height Pressure drop Radius, hydraulic Reynolds number Shape factor Surface per unit mass (of solids*) Surface per unit voluine Velocity Velocity, mass, superficial Velocity, superficial Velocity, terminal (method of measurement must be specified) Viscosity Void, fraction Volume
*
Typical Units, English System 8s. ft. Lb./cu. ft. Ft.
Ft.
Ft. KOunits No units (Ft.)(lb. force)/sec. No units Lb./sec. No units Ft. X lb. force/lb mass (Lb.)(ft.)/(sec.)* (lb. force) (Ft./sec.)/sec. Ft. Lb./sq. f t . Ft. 3-0 units No units Sq. ft./lb. sq. ft./cu. ft. Ft./sec. Lb./(sec.)(sq. ft.) Ft. /see. F t .)see.
Lb./sec. ft.. N o units
cu. ft.
Indicates new recommandation.
LITERATURE CITED
(1) American Standards, 210.12-1946 (1946); IND. EXQ.CHEM., 39, 438 (1947); Chem. Eng., 54, 112 (1947); Chem. Eng. Pmgresu, 43, 20 (1947).
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