June 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
A platform for installation and maintenance was built around the stack (page 1098). On this platform the air flow regulator for the Pitot tube and the filter for the purging air are located. The Brown potentiometer and the flowmeter were installed in the control room which is near ground level. MA1NTENANCE
The zero point of the catalyst concentration meter is being checked about once every 2 days. For this purpose the sliding tubes are pushed into the stack and left in this position for about 10 t o 15 minutes to remove finely divided catalyst which may have entered the tubes during the sliding operation. I n order to leave enough space for the air to escape, it is advisable to retract one of the tubes by about 0.5 inch after contact is made. When cleansing of the tubes is complete, the voltage input into the Brown potentiometer should be recorded for about 5 minutes to determine its average value. If the average deviates from the voltage which was chosen to correspond to zero concentration, it may be adjusted by means of ‘a 200-ohm potentiometer which is connected in series with the thermopile. In a check run of the instrument which cqvered a period of 3 weeks, the zero point was adjusted 16 times t o a value of 4.5 mv.
1247
The average deviation of the voltage output between adjustments was A0.17 mv. or &3.8%. This deviation would produce an error of *lo% in the value of the instantaneous catalyst loss. While the sliding tubes are in the stack, the condition of the light source window and of the lens in front of the thermopile is examined. As a rule, cleaning of these parts is only necessary about once every 2 months. At the same time the air flow rates in the Pitot tube purging system are checked and, if necessary, adjusted. After the sliding tubes have been retracted, the instrument is ready for use. ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of P. J. Satinsky in testing and calibrating the catalyst loss recorder during its trial period. LITERATURE CITED
(1) Hagan Corp., Hagan Bldg., Pittsburgh 30, Pa., Bull. 1M 48. ( 2 ) Rhodes, T. J., “Industrial Instruments for Measurement and Control,” 1st ed., pp. 269-70., New York, McGraw-Hill Book Co.. 1941. RECEIVED January 3, 1949.
Fluid-Solid Air Sizer and A u n i t for t h e continuous drying and sizing of dolomite employing t h e fluid-solid technique has been developed and i s operating on a commercial scale for the preparation of dry, dust-free rotary kiln feed. Removal and recovery of marketable fines from the feed give an improved product and effect a fuel saving in t h e kiln operation. T h e unit, 5 feet 8 inches in diameter by 15 feet 6 inches high, has a sizing and drying capacity of 45 tons of -4-mesh feed per hour. For 2.070 water feed entering a t 18’ C., drying air enters a t 400” C., dry sized product leaves a t
110’ C. and 0.0270 water, and moist dust-laden air leaves a t 75” C. Air i s heated by an oil burner using No. 2 fuel oil a t a rate of 1 gallon per ton of feed; 8000 cubic feet of free air compressed t o 1.2 pounds per square inch gage are required per t o n of feed. One t o n of feed a t 15% -65 mesh results in 0.9 t o n of sized product a t 670 -65 and 2% -100 mesh and 0.1 ton of dust a t 1.570 +65 mesh, most of which is recovered f r o m cyclones as a dry marketable product. Separations f r o m 20 t o 100 mesh are indicated as possible with this unit.
CLARENCE J. WALL, THE D O R R COMPANY, WESTPORT, WILLIAM J. ASH, NEW ENGLAND L I M E COMPANY, CANAAN,
I
N T H E customary application of fluidization, dust entrain-
ment is merely a nuisance. This paper describes a commercial application of fluidization wherein entrainment is a desirable feature of the operation: in a unit that dries and sizes the feed for two rotary kilns of the New England Lime Company a t Canaan, Conn. The principles of operation of the sizer-dryer are: the use of hot air t o evaporate the moisture and the sizing of the feed by differential entrainment from a fluidized bed. By discharging the sized product a t the bottom of the bed, short-circuiting is minimized and advantage is taken of the maximum possible scrubbing effect of the fluid bed. Limestone or dolomite for lime or dolime production is generally obtained by a quarrying operation with its attendant variable moisture conditions. Some stones decrepitate on heating to such an extent that it has been found expedient to precrush to - 3 / l e inch. Such a fine feed is bound to result in a high percentage of dust carried out of the rotary or FluoSolids kiln. This dust, which is a mixture of lime and unburned stone, constitutes a waste of fuel and of otherwise useful agricultural stone and presents a disposal problem. Preliminary laboratory test work a t the Dorr Company, Westport, Conn., laboratories established that dry limestone could be
CONN. CONN.
successfully sized in a fluidizing unit. An air sizer unit 14 inches in diameter was then installed a t the New England Lime Company’s plant a t Adams, Mass., to prepare a dust-free feed for the FluoSolids limestone calcining pilot reactor. The air sizer operated satisfactorily up to a capacity of 1 ton of feed per hour. When the lime reactor was operated using feed prepared in this sizer, the dust loss from the lime reactor was reduced 50% and the lime reactor fuel requirement was reduced from 42 to 39 gallons of Bunker C oil per ton of lime produced. The New England Lime Company had another plant a t Canaan, Conn., using rotary kilns for calcining dolomite, where it was felt that a sized feed might prove advantageous. The Dorr Company therefore designed a combined dryer and sizer unit for this plant to handle up to 50 tons per hour of feed of variable moisture content. The unit was installed by the New England Lime Company and is in satisfactory commercial operation. DESCRIPTION OF T H E U N I T
Figure 1 is a diagrammatic sketch of the sizer-dryer unit. The welded steel shell is 5 feet 8 inches in inside diameter and the over-all height ia 15 feet 6 inches. The air heater compartment and the windbox are lined with insulating refractory brick.
INDUSTRIAL AND ENGINEERING CHEMISTRY
1248
-4
Vol. 41, No, 6
The feed, primary dust, and secondary dust tonnages were obse ved results and the sized stone and unrecovered dust figures were calculated from the screen analyses a t 65 mesh. Similar calculations at 200 mesh gave 0.35% unrecovered dust. This figure is therefore to be considered as only approximate. Fuel requirements for drying vaiied with the feed rate and feed moistuie content. Feed stone entered at 18" C. and was discharged as a dry (less than 0.05% water) sized stone a t 110" C and as dust a t 75 C. T a h k I1 presents energv balances on four runs. These figures show a discrepancy in the hext input and the heat requirement for thefouroperating conditions presented. Considering the fact that cvery truck load of stone coming from the quarq- was of different moisture content and that the sizer-dryer unit was operated a t an evrrchanging feed rate dependent upon the crushing and screening operation, these figures are aq reliable as can be expected. I t will be noted in Table I1 that the stone was didiarged from the fluid bed a t 110" C. whereas the
MESH T O
O
CO W S T I O N
cay
SIZED
SECONDARY DUST
Ir - r r r : % T -325
HEY(
FUEL OIL
Figure 1.
Air Sizer and Dryer
Equipment used in conjunction with the unit includes a proportioning oil burner, a turbine air blomr, a vibrating f e d e r , and temperature, pressure, and flow indicating and control equipment.
Table I .
OPERATI O N
A flow sheet for the whole operation from quarry to kiln is shown in Figure 2. Because operations are on a one-shift basis, the unit must be started up each morning. As soon as the first truckload of stone arrives, the blower is turned on and the air rate is adjusted to about 5000 cubic feet per minute, The burner is then lighted and the apparatus warms up. When the freeboard thermocouple reaches 75 C., the vibrating feeders to both the hammer mill and dryer are turned on. As soon as the fluid bed builds up to 1-foot depth, the automatic discharge gate actuated by the differential pressure across the fluid bed takes over and holds the bed depth a t 1 foot, As the feed rate and moisture content of the stone vary, the oil rate is automatically adjusted by a temperature controller to hold 75 C. in thc freeboard.
Marcrinl Sized Primary Secondary stone dust dust Wt. % of Feed (Dry Basis) 9 0 .6
Feed 7-
100
8 20 35 G5 100 150 200 +325 -325 (1
4.6 43.3 71.3 84.4 88.7 91.5 93.3 95.0
The range of oil rates t o the burner which come within the control of the automatic controller can be quickly adjusted up or down t o take carc of unusual loads. One man operates both the crushing plant and the dryer, including all conveyers. The pressure a t the blower under the conditions described is 1.2 pounds per square inch gage.
-
Unrecovered dust (0.61)
...
5.2 43.8 77.2 93.7 97.5 98.7 99.2 99.4
...
% cumulative
Material Balance
... ...
1.8 11.9
...
28.2
,..
...
100: 0
..,
43.0 55.4
..I
..
...
...
,..
determined 1-0t
+. QUARRY
STONE
J. MILL
HAMMER
1
t 4 MESH
VIBRATING
SCREENS
RESULTS AND DISCUSSION
As soon as initial operating difficulties were smoothed out, data were collcctcd and correlated for determining the operating characteristics of the unit. The dryer-sizer unit was operated up to a capacity of 45 tons of feed per hour. Operating capacity vas limited by the capacity of the crushing and screening circuit ahead of the unit. Space velocities of 3.6 to 4.25 feet per second were employed. A space velocity of 4 feet per second was found to be sufficient for a 65-mesh separation of this dolomite feed stone. Space velocity is defined as the velocity of the gases, including evolved water vapor, in the freeboard area above the fluid bed. The removal of dust present in the feed was determined from a material balance and screen analysis of the feed. sized product, and dust products. Removal of fines is on the order of 60, 79, 85, and 89% of the -65-, -loo-, -150-, and -200-mesh fines, respectively. Table I is a material balance for the dryer-sizcr unit with typical screen analysis of the feed stone and resulting products.
- 4
MESH T O DUST
+-k7 DORRCO F L U 0 SOLIDS AIR SlZER A N D DRYER
PRIMARY
SECONDARY
CYCLONE
CYCLONE
GASES
SECONDARY
TO
SCWBBER
Figure 2.
WST
PRlWRY DUST
TRucns
TO
OR BAGGER
OR
TO
TRUCKS BAGGER
--
DRY SIZED PRODUCT
T O KILN FEED STOPAGE W N
Flow Sheet for Continuous Drying and Sizing of Dolomite
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.
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
exit gas temperature was only 75 C. This nontypical phenomenon of fluidization is attributed to moist feed entering above the fluid bed level with resulting countercurrent drying.
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.
1 33 0.8
0.77 4800 75 330-390 365 99,000
2 36.5 0.9 0.8 5100
4 28 1.9 1.07
3 29 1.2 0.88 4900
FIELD
6000
75 350 370
75 75 290-390 230-330 340 380 l%ooo 113,000 137*500 F r 6 0 K 1 9 , 8 5 F 2 6 , 4 0 0 41,700 60,800 60,500 61,200 61,000
l::ii:
1
~
2,290
5,160
O F APPLICATION
Laboratory tests indicate separations from 20 to 100 mesh as possible with this unit (mesh of separation is defined as that 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;;:; :
99,300 100,710 107,840 126,300 300
1249
11,200
Oil a t 128,500 B.t.u./gal., net heat.
~
RECEIVED J a n u a r y 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 to accumulate in gas pockets or bubbles passing through the fluidized bed in aggregative fluidization. In small tubes, these bubbles may grow to 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 to 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 to 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.
