Literature Cited
Amis, E. S., “Solvent Effects on Reaction Rates and Mechanisms,” Academic Press, New York, 1966. Baekelmans, P., Gielen, M., Nasielski, J., Ind. Chim. Belge 29, 1265 (1964). Berson, J. A., Hamlet, Z., Mueller, W. A., J . A m . Chem. SOC.84, 297 (1962). Bjerrum, N., 2. Phys. Chem. (Leipzig) 108, 82 (1924). Brdnsted, J. M., 2. Phys. Chem. (Leipzig) 102, 169 (1922). Cram, D. J., Rukborn, B., Kingsbury, C. A., Haberfield, P., J . A m . Chem. SOC. 83, 3684 (1961). Eckert, C. A., Ind. Eng. Chem. 59 (9), 20 (1967). Eckert, C. A., Boudart, M., Chem. Eng. Sci. 18, 144 (1963). Frisch, H . L., Bak, T. A., Webster, E . R., J . Phys. Chem. 66, 2101 (1962). Frost, A. A., Pearson, R. G., “Kinetics and Mechanism,” 2nd ed., Wiley, New York, 1961. Glasstone, S., Laidler, K. J., Eyring, H., “Theory of Rate Processes,” McGraw-Hill, New York, 1941. Grieger, R. A., personal communication, 1969. Grieger, R. A., Eckert, C. A., A.1.Ch.E. J . , in press, 1969. Grieger, R. A., Eckert, C. A., IND.ENG. CHEM.PROCESS DESIGNDEVELOP. 6, 250 (1967). Harkness, J. B., Kistiakowsky, G. B., Mears, W. H., J . Chem. Phys. 5, 682 (1937). Harris, H. G., Prausnitz, J. M., Ind. Eng. Chern., Fundarnentals 8, 180 (1969). Hartmann, H., Kelm, H., Rinck, G., 2. Phys. Chern. (Frankfurt) 44, 335 (1965). Hildebrand, J. H., Scott, R. L., “Regular Solutions,” Prentice-Hall, Englewood Cliffs, Fi.J., 1962.
Hildebrand, J. H., Scott, R. L., “Solubility of Nonelectrolytes,” 3rd ed., Dover, New York, 1964. Kondo, Y., Tojima, H., Tokura, N., Bull. Chem. Soc Japan 40, 1408 (1967). Kondo, Y., Tokura, N., Bull. Chem. SOC. Japan 37, 1148 (1964). Kondo, Y., Tokura, N., Bull. Chem. Soc. Japan 40, 1433, 1438 (1967). Kondratev, V. N., “Chemical Kinetics of Gas Reactions,” Pergamon Press, New York, 1964. Marcus, R. A., J . Chern. Phys. 46, 959 (1967). Mills, T. R., Eckert, C. A., Ind. Eng. Chem. Fundamentals 7, 327 (1968). Raistrick, B., Sapiro, R. H., Newitt, D. M., J . Chem. SOC.1939, 1760.‘ Schmid, H., Kubassa, F., Herdy, R., Monatsh. Chem. 79, 430 (1948). Shuikin, N. I., Naryshkina, T. I., Zh. Fiz. Khirn. 31, 493 (1957). Snyder, R. B., M.S. thesis, University of Illinois, Urbana, Illinois, 1968. Stefani, A. P., J . Am. Chern. SOC.90, 1694 (1968). Szmant, H. H., Roman, M. N., J . A m . Chem. SOC.88, 4034 (1966). Wassermann, A,, Monatsh. Chem. 83, 543 (1952). Wells, P. R., Chem. Rev. 63, 171 (1963). Wong, K. F., M.S. thesis, University of Illinois, Urbana, Ill., 1967. RECEIVED for review Pu’ovember 8, 1968 ACCEPTED April 14, 1969 155th Meeting, ACS, San Francisco, Calif., April 1968. Work supported financially by the National Science Foundation.
EFFECTIVENESS OF A FLUIDIZED BED IN REMOVING SUBMICRON PARTICULATE FROM A N AIR STREAM C H A R L E S
H .
B L A C K ’ A N D
R I C H A R D
W .
B O U B E L
Oregon State University, Coruallis, Ore. 97331
ONE of the difficulties associated with any filtration system is removal of the collected material from the filter without shutting down the operation. Cyclones and scrubbers simply drain the collected material from the bottom of the system. However, most filtration systems require periodic regeneration of the filter media; this usually necessitates taking that part of the filtration system “off the line.” If the elements of a filter can be moved readily, an arrangement becomes possible for cycling them continuously through a dust-laden stream and a cleaning system. This may make it possible to use certain materials, having desirable properties, which would not otherwise be feasible. The fluidized bed offers an opportunity to maintain moving elements in proper position with respect to each other and the dust stream, thus producing a suitable filter medium and the opportunity to regenerate the filter on a continuous basis. The purpose of this study was to investigate the factors contributing to removal
’ Present address, Sorthern Arizona University, Flagstaff, Arizona 86001
efficiencies of small-diameter aerosols in a bed of fluidized glass shot. Experimental Program
The effectiveness of a 2-inch diameter fluidized bed in removing airborne particulate from an air stream was investigated a t superficial gas velocities of 8.75 t o 25.0 feet per minute. Higher velocities resulted in excessive bed carryover. Bed height-to-diameter ratios were varied from 2 to 6. Concentrations of aerosol ranged from 0.03 to 8.3 mg. per cubic meter. Ambient temperature conditions prevailed, normally 20” to 30”C. The apparatus used is shown in Figures 1 and 2. Room air, after passing through a filter, entered the aerosol generating flask, where sublimated ammonium chloride particles were picked up and carried to the stirred settling chamber. The chamber entrance valve was shut off and the vacuum pump pulled air from the stirred settling chamber through the fluidized column and a SinclairPhoenix Model J M 2000 photometer. The photometer was calibrated before and after each series of measurements in accordance with manufacturer’s instructions. The fourway reversing valve allowed the column to be shunted VOL. 8 NO. 4 OCTOBER 1969
573
Submicron particulate (mean size 0.52 micron) was filtered from air at substantially atmospheric temperature and pressure by passing air up through a bed of fluidized glass shot in a 2-inch column. Removal efficiency (percentage removal of particulate from the air stream) was essentially constant during the life of the bed and independent of the entering concentration over the range of 0.03 to 8.3 mg. per cu. meter. Varying superficial gas velocity from 8.75 to 25 feet per minute and bed heights from 5 to 12 inches, resulted i n filtration efficiencies ranging from about 50 to 90%. Removal efficiency of the fluidized bed improved with increased bed height and decreasing superficial gas flow velocities. Analysis of filtration mechanisms indicated that inertial impaction forces may be considered negligible and that the predominant effects are Brownian diffusion, direct interception, and induced electrostatic attraction. An equation describes effective filtration efficiency as a function of mean sizes of challenging aerosol and bed material, air viscosity and density, bed height-to-diameter ratio, and superficial gas velocity.
out of the system and the air stream to go directly to the photometer, bypassing the column. Thus, mass concentrations of the aerosol penetrating the system could be compared with the mass concentration of the challenging aerosol by switching the reversing valve position. Experimental Procedures
Upon charging the stirred-settling chamber with an aerosol, a suitable period (5 to 15 minutes) was allowed to lapse for the larger particles of the aerosol to settle out. Then the fluidized bed was challenged by the aerosol from the chamber. A transition period followed until the fluidized bed and column reached a steady state. Figure 3 shows a typical transition period where the per cent penetration of the column by the aerosol increased from background noise level to a steady state. The entire effluent gas stream from the fluidized bed was carried through Tygon tubing to the photometer, which measured the concentration of the aerosol remaining in the gas stream after passing through the fluidized bed. The concentration of the challenging aerosol was measured by diverting the flow from entering the bed to the photometer by means of a four-way reversing valve. As concentration of the aerosol in the stirred chamber was a function of time, a series of measurements of challenging and penetrating aerosol concentrations was made over
a period of time for each variation of bed height or gas flow rate. This provided the data required for determining the mean aerosol size and the effectiveness of the fluidized bed as a function of challenging aerosol concentration. Results
Light field microscopy (1000 X) was used to determine the challenging aerosol particle size distribution. Samples of the aerosol were collected on membrane filter and sized, using a Porton eyepiece. The mean size with respect to count was 0.52 micron, with a geometric deviation of 2.32. Mean size with respect to mass was 4.3 microns as determined by the following relationship suggested by Hatch (1957).
LnN, = LnN,
+ 3 LnLmp
Filtration efficiencies of the fluidized bed in removing ammonium chloride particles of submicron size ranged from approximately 50 to 90% on a mass basis. Lowest efficiencies were encountered a t highest gas flow rates and lowest bed heights. Highest efficiencies resulted from low gas flow rates and high bed heights. N o effective changes in filtration efficiencies of the fluidized bed were found as a result of bed age or changes in challenging aerosol concentration. Figure 4 shows efficiency data for several combinations of bed height and gas flow rates
I 1 I:'
i i '
V a c u u m Pump
Fluidized Column
Figure 1. Schematic flow diagram 574
I & E C PROCESS D E S I G N A N D D E V E L O P M E N T
(1)
0
,
0
I
5
10
15
20
25
Time [mmutes)
Figure 3. Recording trace of photometer readout vs. time cI. Reading while sampling challenging aerosol c2.
Reading while sampling penetrating aerosol
in an exponential decay of the penetrating aerosol as in the following relationship. -
Figure 2. Apparatus for determining filtration efficiencies of a fluidized bed as a function of inlet concentration of the aerosol. The following equation fits data for all runs within experimental accuracies.
ho-' = 0.565 V,".' where h is the bed height-to-diameter ratio and V, is ^....^..f^:^l ~ - ..-l--:L.. n - ~ nnm . ~ ~ r superficial gas velocity. Over 90% of measured efficiency values are within 5% of values predicted by Equation 2 . Figure 5 presents efficiency data (means) as a function of h"/ V:',
^.I+the
~
Conclusions Referring to Equation 2, it is to he expected that as the height-diameter ratio of the fluidized bed, h, was increased, efficiency would increase. The bed height resulted
Ln(1 - 7)
i i
h
The fact that gas velocity appears in the denominator of Equation 2 offers some interesting aspects of the mechanisms of filtration by the fluidized bed. If the filtration mechanism is presumed to he inertial impaction between ammonium chloride particles and bed particles, the following equation can be derived by a simple treatment based on this mechanism. Ile = 1 - eK~iBh (4) . This is essentially the relationship that Meissner and Mickley found to apply to a fluidized bed filtration system (1949), hut it does not apply in the present study. The basic difference between the two studies is that the average particulate mass used in the present study is approximately two orders of magnitude smaller than that used by Meissner and Mickley. Therefore, inertial forces in the present study would he much smaller. In Meissner and Mickley's work, inertial impaction was undoubtedly the predominant filtration mechanism, but in the present study, impaction was not the major contributor. Figure 6 shows photomicrographs (1000 X) of bed material before and after being used as filter media. The aerosol is clearly shown collected upon the bed material. The
01 1
10
concentration (gr.m/meter
0 3 )(.
10
4
1
Figure 4. Typical results of fluidized bed filtration efficiency measurements for different levels of challenging aerosol concentration Single observation
VOL. 8 NO. 4 OCTOBER 1 9 6 9
575
Figure 5. Efficiency data ond curve representing Equotion 2
0.7
0.6
: 0.1 OO.
9
1.0
1.1
1.4
1.3
1.2
ho.4
0.3
/vn
r
1
question arises: If impaction is not the major filtration mechanism, by what means did the bed material remove the aerosol from the air stream? Each of the pertinent theoretical target efficiencies was evaluated to determine which filtration mechanisms were effectivein the present study (Table I). Only the interceptive mechanism shows “strong” target efficiencies for this study. However, it was felt that other filtration mechanisms were operative, as the interceptive mechanism is independent of velocity, and efficiencies in the study showed a slight velocity effect. Furthermore, interception never occurs alone, hut always as a limiting case of another form of filtration. The inertial impaction parameter is 0.004, far below the critical value for N,,which theory states corresponds to the minimum particle size, below which impaction cannot take d a c e (American Petroleum Institute, 1961a,b). Therefore, it is assumed that any filtration in the system under study by inertial impaction may be considered negligible. The most satisfactory quantitative correlation of effective filter efficiency, VI#, with the combined effects of Brownian diffusion and interception is that proposed by Friedlander (1957, 1958, 1967, 1968). Assuming Lamb’s solution for viscous flow for values of N i x 1, the following relationship was suggested:
v,NnNp, = ~ ( N R N ~ ’ ~+ N 3(h’~“’~Nd’‘))” ~’~) (5)
Figure 6. Photomicrograph of bed material used as filtration medium 1000 x
Parameter Dimensionless Parameter
Upper.
Before use
N, (interception
Lower.
After use
Ni (inertialimpaction) N R (Browing diffusion) Ki (induced electrostatic attraction) K E (charged particles electrostatic attraction)
Dark ving i s p ~ r of t gloss shot out of depth of focusing field
576
Table I. Filtration Mechanism Parametric Values and Calculated Target Efficiencies
I & E C PROCESS D E S I G N A N D DEVELOPMENT
VdUe
Efiiency, 7
2 x 10-2 4 x 10-~ 2 x io3
0.83 0.00 0.063
io-‘
0.088
2x
3 x IO-’
0.0094
As N R 0 (pure diffusion), the second term goes to (pure interception) the first term zero and as Np, goes to zero. Extensive data taken from Chen (1955), Ranz and Wong (1952), and others covering ranges of 5(10 ') < N c < 1 and NR
