Removal of Mists and Dusts from Air by Beds Q luidized Solids range of f r o m 20 t o 120 pounc of acid (10070 basis) per 1,000,000 cubic feet. For a given fluidized bed, removal ef-
M i s t s composed of sulfuric acid droplets, 2 t o 14 microns in diameter, were filtered f r o m a i r a t substantially atmospheric temperature and pressure by passing this a i r up through beds of solids fluidized in a 2-inch tube. Beds of nonporous materials like silica and glass beads showed a n impractically short life, b u t porous materials like commercial microspheres, silica gel, and a l u m i n a picked u p over 5'7" by weight of acid before sticking destroyed fluidization. Removal efficiency, defined as t h e percentage (by weight) removal of t h e acid m i s t f r o m t h e air, was substantially constant during t h e life of t h e beds a n d independent of t h e entering concentration over t h e
ficiency improved w i t h increasing bed weight per u n i t area and w i t h increasing superficial gas velocity. W i t h porous solids of -170 mesh and a bed weight per u n i t area of 32 pounds per square foot, a m a x i m u m removal efficiency of over 90% was obtained w i t h a superficial veloci t y of 3 feet per second, corresponding t o a pressure drop of about 6 inches of water across t h e bed. Removal from air of a m m o n i u m nitrate smokes by fluidized solids beds, now under study, shows results somewhat analogous t o acid m i s t removal.
H.P. MEISSNER
AND
H.S. MICKLEY
M A S S A C H U S E T T S INSTITUTE O F TECHNOLOQY. C A M B R I D G E . M A S S ,
R
EMOVAL of dusts arid mists from gases by passing these through fixed beds of broken solids is well known. For ex-
ample, dust and mists are removed in many sulfuric acid plants by the use of coke boxes, and similarly fibrous mats coated with viscous liquids of low volatility are used t o free air from dust in ventilation systems, The use of moving beds also is not new; one interesting application is presented by Hall and Munday(2). On the other hand, the possibility of removing mists and dusts from gases with a bed of fluidized solids does not seem t o have been investigated. T h e object of this paper, therefore, i8 to describe the degree of success attained in preliminary experiments investigating the removal of sulfuric acid mists from air by use of fluidized solids beds. Some mention of exploratory work on ammonium nitrate dust removal also is made. APPARATUS AND PROCEDURE
Sulfuric acid mists of small droplet size were generated by introducing acid vapor into a stream of cool air. This mist-laden air was then passed through a mist meter, next up through the fluidized bed under test, and finally through a second mist meter. Study was made of the influence on mist removal of the following variables: the nature of the fluidized bed material, bed age, bed weight per unit cross-sectional area, gas velocity, and initial mist concentraFLUIDIZA~ION. tion. The apparatus used is shown in Figure 1. Air was taken from the room, and was either used directly or first dried by bubbling through a strong calcium chloride solution. This air was divided into three streams A , B,and C; each stream was metered separately. Stream B entered the mist generator, which consisted of a n electrically heated Pyrex flask. After becoming almost saturated with acid vapor in this flask (by Figure 1. bubbling up through the
98% sulfuric acid a t about 270" to 280" C.), stream B then entered the main air stream A through a tee connection as shown. The turbulent mixing and sudden cooling which occurred here resulted in the formation of small stable mist droplets. The mist-laden air, after cooling t o room temperature by passing through several feet of glass tubing, entered the first of the two mist meters. Each meter consisted of a cylindrical chamber 1.25 inches internal diameter with a 50-cp. bulb a t one end separated 10 inches from a photoelectric cell (Wcston photronic) a t the other end. The mist-laden air entered near one end of this cylindrical chamber and left near the other end. Both the light and the cell were protected from the mist by heavy glass windows sealed with lead gaskets. Although the mist was quite stable, fogging was encountered, necessitating occasional cleaning of the uindows. A standard light intensity was obtained by adjusting the light bulb's voltage to give it reading of 25 ma. on the photocell, whereupon the mist concentration (in pounds per 1,000,000 cubic feet) could be obtained from the current output of the photocells by use of calibration curves. These calibration curves, in turn, were prepared by filling a 5-gallon bottle with mist-laden air corresponding to a certain photocell reading, settling this mist by permitting the bottle to stand idle for 24 to 48 hours, and titrating the acid on the walls and bottom of the bottle. Due to the method of analysis, the acid concentration in the gas is reported on a 100% HsSOl basis as pounds of acid per 1,000,000 cubic feet of air. The fluidizing column operated a t all times a t substantially atmospheric pressure and temperature ( 2 2 " to 25" C.). This column was made of Pyrex and was fitted with txessure taps as shown. 'The mist-laden air entered at the bottom of the column through a simple cone, and then passed up through the bottom section (4 feet high and 1.85 inches internal diameter) into the disengaging section at the top of the column (18 inches high 8 - VALVES and 4 inches internal diameter), Most of the solids carried over in the air froin the disengaging section were separated in the first cyclone, which consisted of an inverted 1liter Erlenmeyer flask into which the air stream entered tangentially. The recovered solids were reApparatus
1238
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
cycled from here to the column, with the aid of air stream C, as shown. A second smaller cyclone, made out of a 500-mm. Erlenmeyer flask, removed any remaining solids, whereupon the air passed through the second mist meter before being exhausted from the apparatus. The acid mists generated were found t o be highly stable, so that negligible mist removal resulted on passage through the empty fluidizing tube and cyclones. On the other hand, if the column was not cleaned thoroughly after a fluidization run of long duration, it was found that the solid particles which collected on the walls of the system caused some knockout to occur. MISTS T E S T E D
The sulfuric acid mists tested had an original concentration calculated as 100% sulfuric acid varying from about 15 to 150 pounds per 1,000,000 cubic feet of air a t room temperature and pressure. This range in concentrations was chosen t o include certain mists encountered industrially-for example, those from the cooking liquor system of a paper mill were found t o run 70 pounds per 1,000,000 cubic feet ( 1 ) . Photomicrographs of slides exposed t o the mist were made t o determine mist particle size. It was found that average droplet diameter ranged from about 2 to 6 microns, depending on the velocity of streams A and B a t the mixing tee, with the majority of the particle diameters lying between 2 to 14 microns. It was originally expected that the reading of the mist meters would depend not only on the concentration of the acid present but also on the average particle size. However. it was found that the particle size variations encountered here were too small to a!Tect the calibration curve, which therefore appeared to be a function only of the concentration of the acid present, within the limits of experimental precision. BED MATERIALS T E S T E D
The five bed materials tested for sulfuric acid mist removal were as follows: microspheres (aluminum silicate), silica gel, activated alumina, silica sand, and glass beads. Four different size fractions of microspheres were studied: as received, - 170 mesh, 100 to 150 mesh, and 150 to 200 mesh. Screen analyses of the other materials are presented in Table I. This table shows the microspheres ( - 170 mesh) and silica gel to have approximately the same packing density and particle size distribution. Equal bed weights of these two materials, therefore, will contain approximately the same number of particles and also the same total
1239
Table I . Bed Materials Tested 1. Microspheres (Universal Oil Products Cracking Catalyst) As received (packing density, 41.9lb./cu. ft.) % 22.2 100 34.2 100-150 10 150-170 10.8 170-200 8.7 200-325 14.0 - 325 - 170 mesh 32.2 170-200 26.0 200-325 41.8 -325 100-150 mesh, all within this size range 150-200 mesh, all within this size range
+
2. Bctivated alumina (Alorco, Grade A) Packing density, 56 lb./cu. ft. after grinding
%
After grinding
29.3 42.1 11.1 17.5
100 100-170 170-200
- 200
3. Silica gel (Davison Chemical Co Commercial Grade, 3913X) Packing density, 43 lb./cu. ft.’hfter grinding After grinding % 170 5.6 39.3 170-200 30.3 200-325 24.8 -325
+
4. Silica sand (Central Scientific Co.)
% 6.5 47.4 29.2 17.9
+::;-zoo 200-325 - 325
5. Glass beads (Minnesota Mining and Manufacturing Co.) One bead size of 0.01 inch in diameter was employed.
particle surface area (assuming these particles to have similar geometrical shapes). Alumina has a greater density and a larger average particle size than the silica gel and microspheres ( - 170 mesh). Hence, for a fixed bed weight, alumina exhibits a lower specific surface and a smaller number of particles than the silica gel and - 170-mesh microspheres. It was found that all these materials, when fluidized, could filter acid mist from gases. Figure 2, which shows graphically the data of Table 11, contrasts the fraction of the mist remaining after passing misbladen air through fluidized beds of silica gel, microspheres (-170 mesh), and alumina. The bed weight in these runs was 32 pounds per square foot. Inspection shows t h a t in the best case, the fraction of mist remaining was only 0.07,
Table II. Run No. 1.
Effect of Solid Type on M i s t Removal
Alumina as ground: inlet mist concentration, CI = 74 ib./ 1,000,000 CU. f t . ; bed weight, W , 32 lb./sq. f t . Ratio Outlet to Inlet Superficial Gas Velocity, Mist Concentration, Ft./Sec. c2/m
1.16 1.66 2.00 2.30 2.57
52.8 42.6 37.9 25.0 28.4
Run No. 2. Silica gel as ground. inlet mist Concentration, ci = 74 lb./ 1,000,000 cu. ft.; bLd weight, W , 32 lb./sq. ft.
1.23 1.66 2.02 2.32 2.59 2.79 0.06
I
I
I
I
I
I
I
0
0.4
0.8
1.2
1.6
20
2 4
I
2.8
SUPERFICIAL GAS VELOCITY, F T / S E C .
Figure 2.
Effect of Type of Solid on Acid M i s t Removal
Constant I n i t i a l bed weight, 32 ib./sq. ft.; i n i t i a l mlst concent r a t i o n , 70 ib./l,OOO,OOO cu. f t
Run No. 3. Microspheres
28.5 19.5 18.3 12.8 7.7 7.2
-170 mesh. inlet mist concentration, c i 70 lb./1,000,000’ou.ft.; bed ;eight, W , 32 lb./Sq. ft. 1.05 28.7 1.11 27.0 1.34 23.4 1.39 28.6 1.39 26.3 1.75 15.7 12.4 2.03
-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
sume that precautions are taken t o prevent excessive deposits from forming on the tube walls. I n experiments in which the gas velocity was varied, the scatter of the data was sometimes as great as *2*5%. The reason for the greater scatter under these conditions is under further study, but currently it appears t h a t fogging and reflections caused by acid accumulation in the mist meter are responsible for the loss of reproducibility.
B 08
f
B
t
9
'2
0.6
L
04
EFFECT O F VELOCITY
CI
3
0.2
Run 111 112 113 114
Vol. 41, No. 6
Material Microspheres, a s received Microspheres, 100-150 mesh Silica gel Alumina
Bed Weight, Lb./Sq. F t . 20 20 28 32
Gas Velocity, Ft./Sec. I .58 1.58 2.33 2.33
corresponding to a 93% removal. Consideration of the results obtained to date indicate that with suitable design, even higher removal may be attained. I N I T I A L M I S T CONCENTRATION AND BED L I F E
The effect of inlet mist concentration on mist removal in four different types of beds is shown in Figure 3. I n each of the four cases shown, all variables were held constant cxcept the inlet mist concentration and bed age. The bed age increased during these runs, as acid was slowly accumulating in the bed, and so the later points of a series were made with a n older bed than the earlier points. Inspection shows t h a t within t h e precision of the measurements, the mist removal was independent of the initial mist concentration and bed age over the range of variables studied. It was found t h a t the microspheres, silica gel, and alumina tested enjoyed a relatively long life and could pick up at Ieast 5% by weight of acid before sticking and agglomeration destroyed the bed. Silica sand and glass beads, on the other hand, showed a life of only a few minutes. This difference in behavior was presumably due to the low porosity of the latter two. It seems likely that the nonporous granules quickly became coated with acid which rendered them sticky enough to destroy t h e bed through agglomeration. T h e porous material, on the other hand, was able to imbibe a considerable amount of acid whiIe leaving the granule surfaces dry. As a result sticking did not occur until the granules had taken up enough acid t o become saturated. It was found further t h a t the age of the bed had no effect on percentage removal within the precision of the tests, provided other factors remained constant. For example, in a 3-hour test of a silica gel bed initially weighing 28 pounds per square foot, a t a gas velocity of 2.3 feet per second and an entrance mist concentration of 70 pounds per 1,000,000 cubic feet, the ppicentage removal remained constant a t 7 2 * 8%. During this time, the solid bed increased in weight by about 1.5y0. similar results were obtained on microspheses and alumina-namely, t h a t percentage mist removal remained substantially constant for any given set of operating conditions, regardless of the weight of acid in the bed, until the time when agglomeration destroyed fluidizing, ;\loreover, it was found t h a t a bed which had been used in a variety of runs could duplicate performance within the experimental error of * 8q10 for any given set of operating conditions. even when acid accumulation had increased its weight by 5q10 and more. These generalizations regarding reproducibility as-
Figure 2 shows that, for a given bed, increasing the superficial gas velocity improves the mist removal because it reduces the fraction mist remaining in the gas. This is illustrated further by the data of Table 111; when plotted as in Figure 4 these show hoiv the mist remaining in the gas decreases with velocity for microsphere beds of different weights. Figures 2 and 4 also show that. for a n y given bed, a st'raight. line may be draJ5-n through the points plotted as the logarithm of C ~ / C I (the fraction mist remaining) against, the velocity. No points were obtained a t velocit,ies belou 1 foot per second because fluidization was incomplete a t such low velocities. Similarly, no data points were taken a t velocities above about 2.75 feet per second because of excessive bed carryover. E F F E C T OF B E D W E I G H T P E R U N I T CROSS-SECTIONAL AREA
Increasing the weight of bed material preqent tended to increase the percentage mist removal, as is evident from an inspection of Figure 4. The relation between the percentage mist rcmoval and bed weight is further illustrated by Table I\' and Figure 5 , where the term
[-
1
log c,/c,]
is plotted as ordinate
against bed weight as abscissa. Each datum point presented is an average of from six to twelve velocity runs made a t a constant value of the bed u-eight per unit cross-sectional area. Line A was drawn through the points for microspheres (-170 mesh) and silica gel. Line B passes through the data points for two types of inicrosphere beds-namely, the 100- to 150-mesh material and the 150- t o 200-mesh material. Both lines A and B are straight and can be represented by an equation of the form:
For line A , n=0.34 and k=O.142, whereas for line R, n=0.157 and k is again 0.142. The data are too meagre to permit any generalization regarding the universality of the value of the constant, k. %
I o
3
0.8
$
0.6
%
f
0 4 h
B
$
;0 2 5. 0.1
08
0 4
I P
I 6
PO
2 4
2 8
SUPERflClAL CAS Y€LOC/TY, F X / S € C .
Figure 4.
Effect of Velocity on M i s t Removal
Microspheres, -170
mesh: initial m i s t concentration, 75 Ib./ 1,000,000 cu. ft.
INDUSTRIAL AND ENGINEERING CHEMISTRY
lune 1949 10
1241
differential thickness traveling with a velocity uo up through a bed of total thickness, N :
0.6
O b
0.4
312 u
-1;
s I
0.2
0. I
4
6
a i 0
Figure 5.
40
20
BE0 WEIGHT, LBS./SQ
60
eo
F%
Table I I I.
Effect of Velocity o n M i s t Removal
-
(Microspheres, 170 mesh) Ratio of Outlet to Inlet Superficial Gas Velocity, Mist Concentration, Ft./Sec. ca/a Run No. 3. See Table 11. Bed weight, W 1.37 1.40 1.81 1 . ,55 1.73 1.89 2.06
Run S o . 5 .
- log? = kuoW
Effect of Bed Weight on Performance
I n i t i a l mist cohcentratlon, 75 lb./1,000,000 cu. ft.
Run No. 4.
The constant k’ is presumably a function both of the shape and the surface area of the mist particles and of the bed particles. It is probably also a function of the average kinetic energy of the particles, since increased kinetic energies probably enforce collisions which otherwise would not occur. For a given mist-bed system, therefore, k’ can probably be expressed as k‘=ku;. It is further evident t h a t ud%=dL,and that f p f d L = W , where W is the total bed weight per unit cross-sectional area. Integrating the foregoing equation and making the appropriate substitutions results in the following expression:
14.5 Ib./sq. it.; inlet mist concentration, ci = 70 lb,/1,000,000 cu. f t . 0.412 0.470 0.309 0.326 0,300 0,228 0,203 =
Bed weight W = 4 . 3 Ib./sq. ft.; inlet mist concentration, el’= 7C lb./1,000,C00 CU. ft. 0.589 1.05 0.530 1.33 0.507 1.4G 0,456 1.73 0.441 1.87 0.317 2.08
-~
Bed weight is the parameter which was used in these correlations because of its ease of measurement. It clearly does not follow t h a t bed weight is the only variable involved, since Figure 2 shows t h a t for equal bed weights, silica gel and microspheres perform about equally, whereas alumina performs more poorly. It has been pointed out already t h a t the first two materials had about t h e same packing density and size distribution, while the latter had a higher packing density and a coarser size distribution. It may be, therefore, t h a t parameters like total surface area in the bed, or number of particles would be more pertinent. To date, insufficient evidence is availablc for an adequate test of these parameters. It does not appear, however, t h a t surface area alone can be used, since microspheres of 100 to 150 mesh and 150 to 200 mesh perform equally, even though the latter material has a greater specific surface. M E C H A N I S M OF REMOVAL
Mist removal in a fluidized bed is presumably a result of collisions between mist particles and bed particles. An equation similar to Equation 1 can be derived by a simple treatment based on this mechanism. Collision likelihood, although influenced by particle size distribution and shape, is assumed t o vary with the number of particles present in any given system. If, in a given system of bed and mist particles, N is the number of these collisions in a unit bed volume per unit time, then i t is clear that N will be a function of c, the mist concentration, and pf, the bed concentration. It is further evident that N must be proportional t o dc/dO, the rate of change of mist concentration with time. The following equation therefore applies to a horizontal slice of gas of
(3)
c1
The similarity between Equations 1 and 3 lends some support to the collision mechanism postulated as being the cause of mist removal. On the other hand, no explanation of the exponent on the term W in Equation 1 can be presented a t this time. C O M M E R C I A L APPLICATIONS
It appears likely t h a t fluidized systems could be used for achieving mist removal in installations larger than those described here. Scale-up would presumably involve keeping bed height constant, but increasing the bed’s cross-sectional area t o accommodate the increased volume of gas while maintaining t h e superficial velocity constant. If necessary, the bed material could be withdrawn, continuously washed free of acid, dried, and returned to the system. Costs of bed material revivification or replacement obviously would be an important factor t o be considered in determining the economics of a process such as this. Another cost element would be the power consumed in blowing the contaminated gases through the fluidized bed. The pressure drop to be overcome would be approximately equal to the weight of t h e bed per unit area, amounting in the typical case of run 1, Table 11, t o about 0.25 pound per square inch, independent of gas velocity. A M M O N I U M N I T R A T E DUST R E M O V A L
Exploratory runs on the removal of ammonium nitrate dusts from air by use of fluidized beds of microspheres have been made. The dusts, having a particle size range of from 0.25 to 2.5 microns, were generated by passing streams of ammonia and of nitric acid vapor separately into an air stream. This dust-laden air then traveled through a test apparatus very similar to t h a t used with acid mists. ~
Table IV. Run No. 3 4 5 6 7 8
Material Used
Microspheres
- 170 mesh
10 11 12 13
ii
16 17 18 19 20 21 22
~~
~
Effect of Bed Weight on Performance
Silica Gel
Miorospheres, 100-150 meeh Miorospheres, 150-200 meah
Bed Weight Lb./Sq. Ft.’ 14.5 4.3 4.2 14.5 32.0 35.0 24.2 18.8 9.15 12.0 20.0 10.0 20 0 130.0 10.0
1
-uo log cz’cl 0.446 0.307 0.210 0.249 0.319 0.440 0.450
0.410
0,364 0.285 0.276 0.406 0.411 0.433 0.201 0,234 0.229 0.219 0.222 0.258
INDUSTRIAL AND ENGINEERING CHEMISTRY
1242
It was found, as before, that high percentage removals could be obtained, and that the removal was independent of the initial dust concentration and of the bed age. The removal again increased with bed weight. Only preliminary investigation has been made of the velocity effect, so that no generalizations can be presented at this time.
TI'
e
Q
Vol. 41, No. 6
= Bed weight per unit area, pounds of solid per square foot = Time, seconds
= Concentration of fluidized solids, pounds per cubic foot ACKNOWLEDGMENT
It is a pleasure t o acknowledge the experimental contributions of J. Byrne. W.E. Jensen. C. W.Larkham. L. J. Martin .T. Shrier, J. D. Spalding, R. H. Thena, J. H. Thomas, and W. W. Twaddle. E. A. Hauser very kindly made the photomicrographs of the mist and dust particles. H, H. Carter prepared the drawings for this article. I
NOMENCLATURE
= Mist concentration expressed as pounds
c c1
c
k, L
= = k' =
2
=
N
=
n
=
ug
=
Of loo% H2S04 per 1,000,000 cubic feet of air Mist concentration before passing through bed Mist concentration after passing through bed Constants in Equations 1, 2, and 3 Bed height, feet Number of collisions between bed particles and mist droplets per second per unit bed volume ExDonent in Eauation 1 Superficial gas ?elocity, feet per second
-
LITERATURE CITED
(1) Byme, Spalding, and Thomas, Mass. Inst. Tech., Practice S C ~ O O E Rept. N8 (June 1947). (2) Hall and Munday, U. S. Patent 2,411,208 (Nov. 19, 1946). RECEIVED January 3, 1949.
Evaluation of Fluid Catalyst Development of Laboratory Scale Units T w o types of units have been developed for evaluation of fluidized catalyst on a laboratory scale. A mechanically stirred u n i t w i t h helically finned rotor makes i t possible t o simulate fluidization independently of t h e gas velocity below 1 foot per second. A steel stirred reactor is used i n research where great flexibility i n regard t o reactant flow rates i s desired. A baffled nonstirred u n i t is less subject t o mechanical difficulties t h a n t h e stirred type.
ROLANQ A. BECK' T H E T E X A S C O M P A N Y , BEACON, N. Y .
NE of the problems that mere faced in the early stages of developing the American process for synthesizing liquid hydrocarbons from carbon monoxide and hydrogen was the lack of a synthesis reactor suitable for use with fluidized powdered catalysts for research on a laboratory scale. Preliminary work with heavy metal powders in glass tubes showed that in order to obtain good fluidization-that is, a minimum of large gas bubbles and catalyst slugs-a linear gas velocity of the order of 1 foot per second was required. It was also observed, with the type and grind of catalyst powder used, that if tube diameters of less than about 2 inches were used, excessive bridging occurred and cylinders of catalyst were formed that traveled unbroken to the top of the tube. However, if a 2-inch tube were to be used at, for example, 600" F. and 400 pounds per square inch gage, more than 1000 standard cubic feet per hour of total charge gas would be required t o maintain the desired superficial linear velocity of 1 foot per second in the tube. Because this gas requirement could not conveniently be met in the laboratories at the time, an effort was made to develop techniques t h a t would a low fluidization work to be done with smaller quantities of gas. It appeared that there were two paths to follow. The first involved the use of a mechanical stirrer that would produce fluidization independently of gas velocity. The second alternative was to devise for use in small tubes a type of baffle that would minimize their inherent bridging and slugging characteristics. S T I R R E D F L U I D REACTORS
For the mechanically stirred units a reaction tube 1.625 inches in inside diameter and 26 inches long was employed. 1
The three
Preaent address, The Texas Company, Box 300, Montebello, Calif.
types of stirrers tested are shown in Figure 1 as A , B , and C. Rotation rates used ranged from 270 to 480 r.p.m. x-ith catalyst powders in the 60- t o 200-mesh size range. Stirrer A , with flat projections, satisfied only one requirement of fluidization-Le., the catalyst particles were kept in constant motion. I n order t o cause aerated suspension of the catalyst a pitched blade stirrer, B , mas tested. Even a t a rotation rate of 480 r.p.m., however, the catalyst lifting was slight. Stirrer C, a continuous screw or helically finned rod with a wall clearance of the order of less than inch, proved highly successful in accomplishing the dcsired expansion of the catalyst bed along with satisfactory particle motion. Rotation rates as low as 270 r.p.m. resulted in an elevation of loo'% of the dense phase catalyst level. Further expansion to 300% with a corresponding decrease in fluid bed density vias readily obtained by increasing the rotation rate to 480 r.p.m. BAFFLED F L U I D REACTORS
I n order t o prevent some of the mechanical dicculties that might occur in the stirred type of fluidization reactor and also to simulate more closely the mass gas velocities characteristic of commercial scale fluid reactors, the development of the smalldiameter baffled type of unit was undertaken. A tube 0.5 inch in inside diameter and 9 feet long was selected for the initial attempt with catalyst powder in the 200- to 325-mesh size range. Air was used as the fluidizing gas. Operation of an open tube of this diameter had proved to be impossible. The tube was therefore equipped with simple baffles consisting merely of cross wires supported at 4-inch intervals along a central rod, all made of 12-gage wire, as shown in Figure 2, A . Fluidization appeared optimum a t a superficial gas velocity of 1 foot per second. At 0.5 foot per second the lo%-erfew inches of the catalyst bed did not fluidize actively, whereas at 1.5 feet per second excessive carry-over of catalyst fines occurred.
