November 1949
*
+
INDUSTRIAL AND ENGINEERING CHEMISTRY
2417
exactly the same periods were tabulated for Farrell. The individual half-hour Cu to M ratios were calculated for each recorder and the average values per period were compared at the two stations. The Cu to M ratios associated with the maximum ground concentrations in each period were also compared. Table VI1 is a summary of these data. Because the same periods were considered a t the two recorders, the comparisons show the relative ground concentrations at 3.2 and 5.1 miles from the stacks. Linear dilution a t these distances would mean a ground concentration a t Veck 1.6 times that a t Farrell. The factor for dilution, according to the inverse square law, would be 2.5; this factor is obtained for the average of all the data in Table VII, though there is a great deal of variation among the component comparisons. The average dilution factor for the maximum concentration in each period is 2.1. On a number of occasions, Farrell has registered higher concentrations than Veck. Also there have been occasions when crop damage has occurred beyond Farrell when neither recorder showed a significant concentration. It cannot be said a t this time whether the gas passed over the nearer area and came down later or whether it traveled along the mesa beside the agricultural area before striking the crops beyond the recorders.
tion arises as to whether n might not vanish from the equation with tall stacks. I n any case there is no suggestion in the data considered here (except for the two short stacks) that n is ever greater than zero, unless there is compensation because of lowering of the value of cz.
DISCUSSION I t seems clear from the observations a t Selby, Tacoma, Garfield, and El Paso that maximum ground concentrations under the tall stacks show excellent agreement with the values predicted by the Bosanquet-Pearson formula or the Sutton formula using n = 0, if the diffusion coefficients lie between 0.05 and 0.07. With the short Selby stack and the short El Paso stack, it would be necessary to assume a value of n up to 0.25 with diffusion coefficients in the range from 0.05 to 0.10. Since the supporting data for Sutton’s formula, requiring appreciable values of n, were obtained largely from smoke clouds generated near the ground, the ques-
(7) Holmes, J. A., Franklin, E. C., and Gould, R. A., U. S. Bur,
ACKNOWLEDGMENT The authors acknowledge the assistance of Louis V. Olson, 19. A. Somerville, Glen Aase, and Thomas Bunkall in the collection of the data and of Milton R. Berntson, Rodney Walker, and S. R. Irvine in the calculation and tabulation. LITERATURE CITED (1) Beers, N. R.,Nucleonics, 4, 28--38(1949). (2) Bosanquet, C. H.,and Pearson, J. L., Trans. Faraday Soc., 32,1249-64(1936). (3) Church, P. E.,private communication (1949). (4) Church, P. E., and Gosline, C . A., Jr., U. S. Atomic Energy
Commission. Document MDDC-73. ( 5 ) Cunningham, 0. C., Addington, L. H., and Elliott, L. T., J,
Agr. Research, 55, 381-91 (1937). (6) Hill, G. R., Thomas, M. D., and Abersold, J. N., Industrial
Hygiene Foundation, Pittsburgh, Pa., Medical and Enpineering Section, Ninth Annual Meeting, Trans. Ser. Bull.,-11-28 (1944).
Mines, Bull. 98 (1916). (8) O’Gara, P. J., and Fleming, E. P., American Smelting and Re-
fining Company, unpublished data.
(9) Sutton, 0. G., Proc. R o y . Soc. (London), 135, Ser. A,pp. 143-65 (1932). (10) Sutton, 0.G.,Roy. Meteorological SOC.Quart. J., 73, 426-36 (1947). (11) Thomas, M. D., Ivie, J. O., Abersold, J. N., and Hendricks, R. H., IND.ENG.CHEM.,ANAL.ED., 15, 287-90 (1943). (12) Thomas, M.D.,Ivie, J. O., and Fitt, T. C., I b i d . , 18, 383-7 (1946). RECEIVED March 26, 1949.
Deposition of Aerosol Particles from Moving Gas Streams H. F. JOHNSTONE AND M. H. ROBERTS1 University of Illinois, Urbana, I l l .
,
T h e efficiency of deposition of aerosol particles on water droplets moving by centrifugal force across a rotating gas stream is calculated on the basis of Sell’s theory of impaction. For accelerations of 100 X gravity the maximum efficiency is obtained with droplets of about loop diameter. Impaction falls off rapidly for particles below 2~ diameter and deposition by diffusion becomes important for very small particles. The development of the new Venturi scrubber for collection of dust and absorption of gases during the atomization of a liquid in a high velocity gas stream is described. Large scale installations of this device have given efficiencies of removal of fume from industrial gases between 92 and 99%. The efficiency is a function of the specific drop surface developed per unit volume of gas and depends also on the nature of the fume.
1
Present address, Rohm & Haas Company, Bristol, Pa.
T
HE theory of deposition of aerosol particles is important in the development of improved methods for the removal of dust and fume from industrial gases. The present paper discusses the deposition by impaction and by Brownian diffusion, and omits separation due to electrostatic charges. The latter has been dealt with extensively and may be regarded as a phenomenon resulting from forces superimposed on the natural properties of the aerosol itself. THEORY OF IMPACTION Any theoretical treatment of the deposition of aerosols must take into account the tendency of the particles to follow the streamlines of the gas and thus be carried around the surfaces of obstacles in their path. Some interesting conclusions concerning the efficiency of various shapes of bodies for collecting dust particles from gas streams have been reached by Sell (25)from a simple analysis of the motion of dust particles. These have been usefu in predicting the amount of deposition by impaction on drops, wires, hairs, filaments, stems, leaves, and other surfaces. As a first approximation, Sell assumes that the flow lines in a gas mov-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2418
ing around an object remain parallel. It is then possible to derive the equations of motion of a particle crossing the streamlines due t o the centrifugal or inertial effect. For very small particles the force resisting the motion of the particle may be represented by Stokes’ law, so that, a t any point, the components of the force system, as shown in Figure 1, are:
-m
dtiy
= k ( v , - u sin a ) dt
Vol. 41, No. 11
Y
/E
(2)
Integration of these equations, to find first the components of the velocity, and then the displacement of the particles a t any bime, gives
Equations 3 and 4 may be used for a step-by-step calculation of the path of a particle, provided the velocity and direction of the streamlines are known for each small increment of distance consistent with the assumption of uniform air flow. Sell actually measured these quantities for bodies of three different shapes, a circular cylinder of 100-mm. diameter, a flat plate of the same diameter placed normal to the lines of flow, and a cup-shaped body also of the same diameter placed so that the axis was parallel to the main flow lines. For spheres, the flow lines were calculated using the equation for ideal potential flow which was considered to be a suitable approximation. The results of Sell’s calculations are shown in Figure 2, in which the efficiency of impaction, q , representing the ratio of the area of the tube from which all particles are removed to the projected frontal area of the object, is shown as a characteristic function of the group m u / k D . In aerosol terminology, the efficiency of impaction is called “the fraction of the area dose removed by the object.” For a cylinder placed normal to the flow lines, the efficiency is equal to ratio b / D , where b is the limiting width of the initial streamlines in which all particles collide with the cylinder, and D is the diameter of the cylinder. For a sphere, the efficiency is equal t o b2/D2. The group m u / k D may be called the “impactibility” of the aerosol, as it determines the deposition efficiency. For small spherical particles in the Stokes’ law range k
=
3rpd
(5)
and
E = -1 -d2up / kD
18 p D
(6)
For spherical particles of uniform density the efficiency of impaction is a function of the square of the diameter of the particle and of the reciprocal of the size of the impacting object. This suggests that the size distribution of particles in an aerosol might be found from the amount of material deposited on a series of vertical wires of different diameters. Such an arrangement has been used for rough assessment of aerosol clouds. For best results the wires should be coated with a sticky material which can be washed off for analysis. For impaction on a single object placed in an aerosol stream, the efficiency is a function of dzp. Because the density of a cluster is inversely proportional t o the cube of the diameter, loose clusters, or aggregates of low density, may have a lower impactibility than that of the unitary particles comprising the aggregate. Sell’s derivations do not take into account the existence of a boundary layer. For distances close to the surface of an object, therefore, his assumption of uniform flow lines is not valid. Albrecht ( I ) has shown that, for a cylinder, the deposition becomes zero for values of m u / k D = 0.09. This indeed mould be
Figure 1. Motion of Particle across the Streamlines of a Cas, after Sell (25)
expected if a boundary layer exists. Albrecht’s curve for EI cylinder is shown in Figure 2 by the dotted line for low values of mu/kD. For larger values of the dimensionless group his curve coincides with that of Sell. Thus, for very small dust particles, or for large objects placed in a slowly moving gas stream, there will be no deposition by impaction because of the existence of the boundary layer. Other mechanisms of deposition enter when the particle diameter is very small. For most practical problems, therefore, the deviations from nonideality which would invalidate Sell’s calculation would be negligible. DEPOSITION BY DIFFUSION For aerosols of very small particles the diffusion due to the Brownian movement plays an important part in the rate of deposition. In fact, when the particle diameter is less than 0.5 micron
November 1949
*
4
INDUSTRIAL A N D ENGINEERING CHEMISTRY
2419
the rate of deposition by diffusion may exceed that by impaction, even though the velocity of the gas past the object is very high. The rate of diffusion of the particles up to the surface may be calculated in a manner analogous to the rate of absorption of a solute gas from a gas mixture-that is, the particles are considered to reach the surface entirely by diffusion through a stagnant layer, the effective thickness of which depends upon the gas velocity, the shape of the object, and properties of the gas and diffusing substance. Outside of the stagnant layer, the concentration of the aerosol particles remains uniform, while, a t the surface itself, the equilibrium concentration is zero-that is, the collision of the particles with the surface is assumed to be completely inelastic. The rate of collection of the aerosol particles by diffusion to the surface of a sphere of diameter D, therefore, will be proportional to the concentration gradient across the film and will be given by the usual absorption equation, dN = ,?rD2Dv(Ce - 0) (7) dB X
-
where 2 is the effective thickness of the film and D, is the diffusivity of the aerosol particles. The number of particles collected by diffusion in the time required for the gas to travel unit distance then is ?rD2DvC~/ux. The number of particles contained initially in a tube of unit length and of diameter b is ?rbeCo/4. Equating these quantities and solving for the ratio ba/Da, the efficiency of deposition by diffusion becomes
*4'
The effective film thickness, 2, is a function of the Reynolds number and the Schmidt number. For small spheres and low rates of flow, x is approximately equal to the radius of the sphere. Eads (6) has correlated the data of Frossling (7) on rates of evaporation of small droplets in moving air streams as well as Kramers' data on the analogous process of heat transfer t o small spheres (11) and has found that the following general dimensionless equation holds over the entire range of the measurements:
Here D, is the molecular diffusivity of the evaporating substance. For heat transfer, the Schmidt number is replaced by the Prandtl number. The diffusivity of dust particles due to Brownian movement ' may be estimated from the Stokes-Einstein equation
D, =
k T (1
+
1.72 A/d) 3rpd
The Cunningham correction factor is important for particles less than 1 micron in diameter. Table I shows the values calculated for Dv a t 25' C., and the Schmidt number for dust particles of various sizes compared with the same quantities for sulfur dioxide molecules. From Equations 8, 9, and 10, the efficiency of deposition of aerosols by diffusion may be found for spherical objects placed in streams moving a t any velocity. DEPOSITION OF DUST PARTICLES ON SPRAY DROPLETS The removal of small dust particles from a gas stream by means of a simple spray is relatively inefficient. If an atomizing nozzle is used to provide a large number of drops with a large surface, the smaller droplets quickly lose their kinetic energy and do not penetrate into the gas. Deposition of the dust particles by impaction becomes negligible when the drops reach the terminal velocity and are carried along by the gas stream. Attempts to overcome this difficulty by using the centrifugal force of a spinning gas to hurl the small droplets a t a high velocity across the gas stream have been successful (9, 10). The cyclone spray
I
A
2!
"
a '4 I
1 1
1 1 1 !lk
DIAMETER
I
'
I I I i i l l 4;
' Id0
I
?.L
'6b' OF D R O P , MICRONS.
20
i
I I I l l l
'4d0'6bd 'lob0
Figure 3. Velocity of Water Droplets in Air under Various Centrifugal Forces
scrubber is an efficient device for removing dust from gases when the particles are larger than about 2 microns. For smaller dusts, the efficiency of the cyclone spray falls off rapidly as the particle size decreases, unless high ratios of the scrubbing liquid to gas are used. This decrease in efficiency in the deposition of the small dust has been correctly ascribed to streamlining of the dust particles wound the drops (10). It is of interest to compare the efficiency of drops of various sizes for the collection of dust particles by impaction and by diffusion using the equations derived above. We shall consider drops less than 400 microns in diameter and velocities under centrifugal accelerations up to 100 times gravity. The distortion from the spherical shape will not be serious. The velocity of drops may be calculated from the equation for the terminal velocity of spherical bodies in a centrifugal field using a drag coefficient which is a function of the Reynolds number ( l a ) . Figure 3 shows the velocities calculated for water droplets in air for constant radial accelerations of 100, 10,and 1 times gravity. From these values the efficiencies of deposition by impaction were calculated for various sizes of drops and dust particles. The results are shown in Figure 4 by the curves which have a maximum a t a drop diameter of approximately 100 microns. The curves to the left in the same figure show the efficiency of collection hy
Table I.
.
~
~
~
Diffusivities of S m a l l Particles in Air a t 25' C. Schmidt No.,
Diameter,
Microns
Du,6q. Cm./Seo.
l*/paDv
Baaed o n mean free path of molecules of 10-6 om. 0.5 6 . 4 X 10-1 2 . 4 X 100 0.1 6 . 5 X 10-6 2 . 3 x 104 0.01 4 . 4 x 10-4 3.4 x 10% 0,001 4.1 X 10-2 3.7 80s molecules 11.8 X 10-2 1.28
INDUSTRIAL AND ENGINEERING CHEMISTRY
2420 Table 11.
Comparison of Nozzles for Dust Collection by Impaction and Diffusion and C a s Absorption in a Cyclone Scrubber (Constant radial acceleration of drops = 100 X gravity)
Do.
Nozde
>lean Drop Diameter, Microns
~ ' f b D
Si& Distribution Factor
A B C D
'/a
E
150 100 100 100 50
A B C D E
150 100 100 100 60
1/3 I/K 1/3
__
'/e 1/3
1
1
B.
1 1
Dust Collection, Diameter of Dust, Microns
2
1 Impaction
54.3 76.3 81.7 84.8 161.0
27.6 36.9 40.9 43.4 79.2
0 5
0.5
8.4 10.7 12.4 13.3 23.8
0.02 0.16 0.06 0.03 0.15
Relative Efficiencies, E = 1 0,419 0,534 0,558 0.672 0.800
0,241 0,309 0,336 0.352 0,547
0.081 0.101 0,116 0.124 0.212
-e 0,0002 0.0016 0,0006 0,0003 0.0016
0.1 Diffusion
o,ol
0.10 0.74 0.28 0.12 0.70
1.6 12.1 4.6 2.0 11 6
100 0.0010 0.0073 0,0028 0.0012 0.0070
0.016 0.114 0,046 0.020 0.110
so2
Absorption, Diffusion 82.5 882.6 261 6 8x1 553.8
0.562 0.999+ 0,927 0.564 0.996
C . Relative Quantities of Liquid Required in Cyclone Scrubber for Constant Percentage Removal 326 128,000 I/s 51 100 22,700 1670 A 150 33 '/a 36 75 ' 258 17,200 3,740 B 100 226 3.1
c
D
E
100 100 50
'/Q 1 1
34 32 17
67 63 35
222 207 116
45,200 105,000 18,200
9,800 22,700 3,920
505 1370 238
10.5 33 5.0
Vol. 41, No. 11
where r is the length of the path of the drop which is talcen as the radius of the cyclone, W is the volume rate of the liquid through the nozzles, G is the voIume rate of the gas through the cyclone, and D is the diameter of the drops. The term 3vrW/2DG represents the ratio of the volume of gas swept out to t h e total volume of gas. I n order to obtain 95y0 efficiency thisexponent must have the value of 3 . The drops obtained from an atomizing nozzle are not uniform but vary over a wide range of diameters. Lewis ( I S ) has shown that the distribution of drops from hydraulic nozzles follon-s a statistical equation suggested by Kukiyama and Tanasawa for air atomizing nozzles (14): dn = dD
aDae-CDq
Exponent q is a measure of the degree of uniformity of the dropfor hydraulic nozlets. Its value is usually between 1 / ~and zles operating a t moderate pressures (widest distribution rangc). For air atomizing nozzles with large ratios of air to liquid q is approximately 1. T h e value of constant c, which may be found from a plot of the drop size distribution data as log 1 / 0 2 X A n / A D vs. 0 4 , is related to the statistical mean volume-surface drop diameter, DO. A comparison of several nozzles of different types and characteristics, that may be used for the collection of dust and the absorption of gases in cyclone scrubbers, is shown in Table 11.
DROP S I Z E , M I C R O N S
Figure 4.
Those listed as A, B, and C are typical of good commercial hydraulic nozzles operating at 150 to 200 pounds per square inch pressure. Nozzles D and E are typical of air atomizing nozzles in which a liquid jet is injected into a high velocity gas stream. The comparisons are made on several bases for dust collection by impaction and diffusion and for gas absorption. Impaction is the principal mechanism for the removal of particles larger than 0.5micron diameter. I n Table 11, A, the effective area of the drops for the various mechanisms is compared by summation of the product of the collection efficiency and the projected area of alI the dro s between 5 and 450 microns in diameter in unit volume of the iquid. Because the projected area per unit volumc is 3 / 2 0 , the summation is expressed as 2 ( 3 7 / 2 D ) . I n Table
Efficiency of Individual Drops for Absorption of Sulfur Dioxide a n d Dust Collection
diffusion for various submicron sizes of dust and for the absorption of sulfur dioxide. Contrary to the case of deposition b y impaction, the maximum efficiency of collection by diffusion for individual droplets is given by the smallest droplets which move at the lowest velocities. This conclusion agrees with the experimental results on absorption of sulfur dioxide in a cyclone spray tower, in which the efficiency of removal, expressed as the number of transfer units due to the spray droplets was approximately inversely proportional to the spin velocity of the gases (8). Inasmuch as the total amount of dust removed in a cyclone scrubber is proportional to the concentration of the dust in the gas and to the volume of the gas swept out by the drops ( 9 ) , the efficiency of dust removal may be represented by Figure 5.
Experimental Equipment Showing Venturi Throat
INDUSTRIAL A N D ENGINEERING CHEMISTRY
November 1949
2421
11, B, the relative efficiencies of the nozzles are shown. These were calculated from the effective areas of the drops using Equation 11 and keeping the ratio G/rW a t a constant value of 100. The third basis of comparison is shown in Table 11, C, in which the relative quantities of liquid required in a cyclone scrubber to give a constant percentage removal of approximately 95% are shown. I n all cases the relative difficulty of removing dust particles between 0.1 and 0.5 micron is evident. The collection of extremely small fume with particles around 0.01 micron is much easier, and gas absorption by molecular diffusion is accomplished with less difficulty than the collection of larger dust particles by impaction. It appears that considerable improvement in the removal of fine dust by cyclone spray scrubbers can be obtained by usingnozzles that provide good atomization of the liquid. The data shown were calculated for a cyclone in which the centrifugal acceleration is 100 times that of gravity. Somewhat better results can be obtained in removing small dust by using lower spin velocities, but the difficulties of throwing out the small liquid droplets would then require an increase in the height of the tower. DEVELOPMENT OF VENTURI SCRUBBER
Figure 6.
Experimental ~~~i~~~~~ and Tower
Showing Venturi
The discussion above has considered only the deposition of aerosols on drops moving across a gas stream as a result of a centrifugal force. Comings has shown that extremely high rates of evaporation and heat transfer may be obtained during atomization of a liquid through injection into a high velocity hot gas stream ( 5 ) . The high transfer rates are consistent with the thin effective layer a t the surface of the liquid, the greatly extended surface of the liquid as films, filaments, and drops, and the extreme turbulence existing in both the gas and the liquid in the vicinity of the jet. High velocity vaporizers were used in several devices developed for military purposes during the war. The possibility of applying these principles to the removal of aerosol particles from gas streams has been suggested. Although it was considered a t first that the transfer of the extremely small particles from the gas to the water surface would take place mainly by d ffusion, it was recognized that electrostatic effects due to charging of the water droplets by atomization, impaction, and enmeshing of the larger aerosol particles by the liquid films would extend the usefulness of the device to particles far beyond the range of Brownian diffusion.
Table 111. Absorption a n d Humidification in Venturi Scrubber G, Cu. Feet/ Min.
Run NO.
23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39
'
Til
3
Feet/ Sec.
L Gal./i030 Cu. Feet
Jet Diameter, Inches A.
1060 995 1003 1021 1056 1070 846 852 86 1 855 856 856 855 853 626 630 630
230 216 218 221 223 232 184 185 187 185 186 186 185 185 136 137 137
2.85 4.50 2.56 2.04 0.93 1.46 3.10 2.46 1.82 1.22 2.Q3 3.06 4.00 5.01 5.45 2,82 4.20
0.122 0.122 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.070 0.094 0.09% 0.094 0.094 0.094 0.094 0.094
1101 1101 1103
239 239 240
0 71 1 40 2 13
0.122 0.122 0.122
SO2 Absorption, Jet % Velocity Feet/Sec: Inlet Outlet NI Eight Vertical Liquid Jets a t Top of Throat
Humidification Temperature, C. Outlet
td
tw
td
tu
NJ
0.1060
0.1091 0.1439 0.1419 0.1456 0,1271
0.0212 0.0171 0.0252 0.0257 0.0536 0.0412 0.0444 0.0476 0.0611 0.0730
1.63 1.89 1.51 1.47 0.68 0.98 1.18 1.09 0.87 0.56
23.02 31.38 35.33 36.39 34.58 38.19 29.23 33.48 34.12 33.74
17.15 16.55 20.28 21.05 22.66 24.20 14.61 17.18 17.88 17.87
21.75 20.73 23.98 24.72 27.47 28.48 19.78 21.92 23.62 26.38
20.00 19.73 22.37 23.13 23.90 25.85 17.88 19.67 20.39 20.30
3.11 3.91 3.04 3.10 1.68 2.36 2.91 2.68 2.28 1.68
0,1427 0,1275 0,1398 0.1370 0.1928 0.1941 0.1959
0.0501 0,0321 0.0252 0.0221 0.0602 0.0370 0.0690
1.04 1.38 1.71 1.82 1.16 0.80 1.04
33.59 33.48 32.86 32.86 31.78 33.17 34.48
19.33 19.25 19.02 18.70 15.29 15.57 16.17
25.53 24.40 23.68 23.10 22.00 23.75 23.52
22.40 22.49 22.24 21.75 19.38 19.43 20.67
2.48 3.18 3.58 3.58 2.85 2.18 2.94
27.80 28.78 28.01
13 29 14.18 14.20
21.41 19.89 18.85
15.92 17.12 17.37
1.43 2.37 3.20
10.3 15.2 24.4 19.7 9.9 14.8 24.8 19.8 14.8 9.9
0.1089 0.1130 0.1143 0.1121
10.4 15.2 19.8 24.8
19'8 10.3 15.4
Inlet
B. One Horizontal J e t a t Each E n d of Throat 40 41 42
10 7 21.0 31.9
0 1052 0 0911 0,1051
0 0641 0.0441 0,0307
0.50 0.72 1.23
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
2422 1 . 0
rates of absorption of sulfur dioxide by alkaline solutions sprayed into the high velocity gas stream, as this was considered to be the ultimate in the rate of diffusion of small aerosol particles. These results were then compared with the performance of the pilot plant operating on sulfate fume. T h e measurements were made in a rectangular Venturi 1.5 X 8 inches in cross section at the throat. The apparatus, shown in Figures 5 and 6, was constructed of Plexiglas sheeting. Movable vanes were used in the divergent section of the Venturi in order to study the effect of the angle of divergence on the efficiency and on the pressure drop. I n most of the runs the scrubbing solution was injected through eight tubes spaced along the top side of the throat. Several runs were also made injecting the liquid through two tubes, one in each end of the throat. Three different sizes of tubes were used to obtain a range of liquid jet velocities. Gas velocities through the throat ranged from 135 to 240 feet per second and liquid rates from 0.7 to 5.5 gallons per thousand cubic feet of gas. The results of the experiments on absorption of sulfur dioxide and humidification of the gas are shown in Table 111. A11 the data have been correlated on the basis of the assumption that the efficiency of gas absorption and of dust removal is a function of the amount of surface developed during t h e atomization. Thus, the number of transfer units given by the equation
0.9
0.8 0.7
0.6 Q5 -
0.4 -
0.3 -
0.2 -
>
V 2
w
u
.I -
.09-
--
.07W
- .ObI
.05
--
.04
-
.03-
.oa
N , = 2.3 log 1/(1
-
.01-
10 S,CALCULATED SPECIFIC AREA OF DROPS, SQ.FT/CU.FT.CAS
Figure 7.
Vol. 41, No. 11
-
E)
(13)
should be proportional to the surface of the fully developed and partly formed drops. Using the equation given by Lewis (IS)relating the mean drop diameter t o the properties of the liquid, the velocity of the gas at the throat, and the liquid-gas ratio, the specific surface was estimated a,s follows:
Efficiency of Venturi Scrubber for Absorption. Humidification, a n d Dust Collection
The use of a Venturi tube for accelerating the gas and obtaining maximum recovery of the kinetic energy through the divergent section, and a cyclone scrubber to remove the spray droplets after atomization and to improve the efficiency of dust removal was first tried in 1946 in a pilot plant operating on stack gases from a kraft recovery furnace (4). The fume of sodium sulfate and sodium carbonate in these gases is less than 1.5 microns in diameter and probably averages about 0.5 micron. The efficiency of recovery obtained at gas velocities through the Venturi throat of 200 feet per second was surprisingly high, and at times exceeded 92% removal. The efficiency of the Venturi scrubber alone was about 5% less than the combination of the Venturi and cyclone scrubbers, whereas the efficiency of the cyclone scrubber alone was only 60% of the combined efficiencies. On the basis of the pilot plant results, a full scale unit was designed and installed to handle all t h e gases from the recovery furnace in a 15Bton kraft mill. This unit has a Venturi throat 25 inches in diameter and handles 40,000 to 50,000 cubic feet of hot gases per minute. Approximately 10 tons of salt cake a day are recovered from the gases. Details of the development of the Venturi scrubber and its application to kraft furnace gases have been published by Collins (3). Application of the scrubber to other types of aerosols has developed rapidly. Large scale installations are now in operation on gases from an oxygen open-hearth furnace where removal efficiencies are reported to be better than 98.5% (photo page 2426). Successful results have been obtained on several small installations handling various organic fumes, and mist from sulfuric acid plants (8). I n this laboratory, a study is being made on the rates of mass transfer and deposition of aerosol particles during atomization of the scrubbing liquid. Measurements were made first on thP
245L s=-DO
wheie D Ois the calculated mean volume-surface diameter of the drops in the atomizer in microns; Vt is the velocity of the gas in the throat, feet per second; L is the liquid-gas ratio, gallons per 1000 cubic foot; and S is the calculated specific area of the drops, square feet per cubic foot of gas. Although Equation 15 may not give ail accurate estimate of the actual surface formed, the correlations shown in Figure 7 are suiprisingly good. As expected, the rates of the humidification and of absorption of sulfur dioxide are considerably higher than the rate of collection of the dust. Likewise, from the slope of tho lines, the number of transfer units for humidification is greater than that of absorption of sulfur dioxide b y a factor of 2.20, m hich corresponds to the ratio of diffusivities of water vapor and sulfur dioxide in air. No difference was noted in the location of the nozzles or in the method of injection of the liquid into the small Venturi throat used in the experimental work. Attempts to correlate the data from another pilot plant installation operating on an entirely different type of fume on the same basis have not been so successful, and there are definite indications that certain dusts are more difficult to remove than others. Whether or not this is due to electrostatic effects, or to a higher percentage of elastic collisions, or to the effect of the size of particles actually existing in the gas cannot be stated at this time. All the information available on the efficiency of the Venturi scrubber so far indicates that the method of injecting the liquid into the gas stream IS not important, provided that completely uniform coverage of the entire throat area is obtained. From a practical standpoint the most important item to be conyidrred in the operation of the Venturi >crubbPr i s the amount of
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
November 1949
2423
injected into a high velocity gas stream takes place a t high eficiencies consistent with the high velocity of the gas past the distended surfaces of the drops, films, and filaments. The rate of gas absorption is again somewhat higher than that of dust collection. Correlation of the data on dust collection, gas absorption, and humidification indicates that the number of transfer units is directly proportional to the surface of the drops formed per unit volume of gas. ACKNOWLEDGMENT
The work presented here is.part of an investigation on treatment of stack gases in the Engineering Experiment Station. The authors wish to express their appreciation to A. W. Anthony, Jr., for many helpful suggestions. NOMENCLATURE a, c, q 7 constants h = limiting width of streamlines from which particles
are deposited CO = concentration of particles (or solute gas) d = diameter of aerosol particle D = linear dimension of object; diameter of sphere, or spray drop D, = diffusivity, sq. om. per second DO = statistical volume surface mean drop diameter, microns E = efficiencv of removal of aerosol or eas G = volume i a t e of gas flow I C = Stokes' law resistance coefficient k = Boltzmann constant, 1.37 X lo-'* erg per O K. L = liquid-gas ratio, gallons per 1000 cubic feet m = mass of aerosol particle n = number of drops in s ray with diameter less than D N = number of articles moles) diffusing to surface N. = number o? transfer units obtained during atomization r = radius of cyclone specific drop surface in Venturi scrubber, square feet per cubic foot of gas t = time u = gas velocity initial velocity of aerosol particle a t x 0 vo = velocity components of aerosol article v51 Vt = gas velocity a t throat of Venturi, g e t per second volume rate of liquid flow effective thickness of film angle of streamlines with z-axis angle of particle direction with z-axis effiqiency of deposition of particles mean free ath of gas molecules viscosity oFgas density of particles; subscript g refers to gas
-
A-
EXPERIMENTAL SCRUBBER 7'DIVLRCLNCE I
I
I
I
(%
As=
power required by the over-all pressure drop across the unit. Correlation of the data obtained on a number of Venturi atom izers, varying in size from 0.1-inch up to 25-inch diameter at the throat, indicates that most of the power is consumed in accelerating the liquid to a velocity somewhat less than that of the gas at the throat. If the resistance of obstacles placed in the gas stream ahead of the throat is held to a minimum, the over-all pressure drop of a dry Venturi is about 15% of the differential head across the convergent section. If the pressure drop is expressed in terms of the throat velocity head, it is expected that it would be a function of the liquid-gas ratio. A plot of the data obtained on the experimental unit with the rectangular throat is shown in Figure 8. It appears that one throat velocity head is expended by the injection of approximately 6 gallons of water per thousand cubic feet of gas, regardless of the velocity of the gas at the throat. CONCLUSIONS
The theory of the deposition of aerosol particles from moving gas streams by impaction and by diffusion has been applied to the removal of dust and fumes from industrial gases. For a cyclone spray scrubber the impaction efficiency of individual drops for removing dust particles of various sizes should be a maximum when the drop size is approximately 100 microns. A comparison of the efficiency of various types of nozzles with different operating characteristics indicates that, for a given mean drop diameter, a nozzle giving a wide distribution of drop sizes should be more efficient for removing small particles by diffusion than a nozzle which gives more uniform drops. However, greater efficiency for dust removal and absorption of gases can be obtained by improving the atomization of the liquid over that given by commercial hydraulic nozzles. T h e absorption of gases with relatively high diffusivities compared with the diffusivities of extremely small aerosol particles takes place a t much higher efficiencies than dust removal. The removal of dust and fine fumes by atomization of a liquid
-
-
w=
LITERATURE CITED Albrecht,F., Physik.Z.,32,48 (1931). Anthony, A. W., Jr., paper presented before Smoke Prevention Assoc. of America, June 10,1948. Collins, T. T., Jr., Paper I n d . Paper World, 28, No. 5, 685, No. 6,830, No. 7,984 (1947). Collins, T. T., Jr., Seaborne, C. R., and Anthony, A. W., Jr., Paper Trade J . , 124, No. 23, 45-9 (June 5, 1947). Comings, E. W., Adams, C. H., and Shippee, E. D., IND.ENQ. CHEM.,40,74 (1948). Eads, D. K., M.S. thesis in chemical engineering, University of Illinois, 1947. Frassling, N., Gerlands Beitr. Geophys., 52, 170 (1938). Johnstone. H. F.. and Silcox, H. E., IND. ENG.CHEM.,39, 808 (1947) Kleinschmidt, R. K . , Chem. Met. Eng., 45, 487 (1939). Kleinschmidt, R. K., and Anthony, A. W., Jr., Trans. Am. Soo. Mech. Engrs., 63, 349 (1941). Kramers, H. A., Physica, 12, 61 (1946). Lapple, C. E., and Shepherd, C. B., IND. ENQ.CHEM.,32, 605 (1940). Lewis, H. C., Edwards, D. G., Goglia, M. J., Rice, R. I., and Smith, L. W., Ibid., 40, 67 (1948). Nukiyama, S., and Tanasawa, Y., Trans. SOC. Mech. Eng. ( J a p a n ) , 5 , No. 18,63 (1939). Sell, W., B'ororschungsheft, 1931, 347. I
RECEIVED June 30, 1948. Preaented in part before the Division of Physiod and Inorganic Chemidtry, Symposium on Aerosols, at the 113th Meeting of the AMERICANCHEMICAL SOCIETY, Chirago, Ill.
