I
K. T. SEMRAU, C. W. MARYNOWSKI, K. E. LUNDJ, and C. E. LAPPLE Stanford Research Institute, Menlo Park, Calif. t
Influence of Power I put on Efficie Dust Scrubbers Efficiency on a given aerosol may be correlated as a function of total theoretical power input per unit of gas flow rate. Performance is comparable for different types of scrubbers
ONE
'
of the principal methods for removing fine particulate matter from gas streams is by contact with a liquid, usually water. Many scrubbers for this purpose are available commercially or have been constructed by users. Few data are available for rationally designing a scrubber or comparing the performance of different devices. With fine aerosols, high scrubber efficiencies are associated with high power consumption-power requirement ini creases as the particle size of the dust or mist decreases. I n a recent investigation (75) of typical devices in which power consumption takes the form of gas pressure drop, all contactors gave substantially the same efficiency for a given pressure drop. It was concluded that the controlling factor in scrubber performance is turbulence, and the total power input (or power dissipated) per unit of gas flow rate might be a general criterion of efficiency for devices in which power is introduced not only from the gas stream but also from liquid sprays or mechanically driven rotors. In the work reported here, the totalpower-input method of correlation suggested by Lapple and Kamack (75J has been applied to three types of pilot plant scrubbers-Venturi, cyclonic spray (Pease-Anthony), and pipeline.
single unit (Chemical Construction Corp.) having a nominal capacity of 500 cubic feet per minute, and arranged for use either singly or in combination. Water was fed through four radial taps to the 2.5-inch-diameter throat of the Venturi, and then discharged with the gas to a cyclone separator where water droplets and collected dust were removed. The cyclone was equipped with a central spray tree fitted with spray nozzles which discharged radially from the axis of the cyclone (7, 74). aned gas from the cyclone passed h an orifice meter and then to a blower which -discharged to the atmosphere. Feed water, supplied to the Venturi throat and to the spray tree by a pump, was measured with eters, and its pressure was m with Bourdon gages. Gas pressu across the units were measured tube manometers. The pipeline scrubber (Figure l), which had a capacity of 50 to 75 cubic feet per minute, consisted basically of a pipe to which water was fed gible pressure, followed by a cy trainment separator for removing water
s the
chamber sur-
Experimental
Aerosols. The scrubbers were operated on fume from a kraft (sulfate) process black-liquor recovery furnace and on dust and fume from a lime kiln burning lime mud (Table I).
Table 1.
(Pressure, 1 atm.) Water Vapor, Dust % of Loading, Stack Gas Temp., Total Grains/ Vol. Cu. FtSa Stream O F. Recovery furnace Upstream of precipitator 280 30 2.36-3.27 Downstream of precipitator 280 30 0.0535-0.202 Lime kiln Raw gas 280 40 0.898-3.36 Prewashed gas 175 45.8 0.122-0.462 a Gas volume measured at stack (scrubber inlet) conditions.
The fume from the recovery furnace (conventional B&W-Tomlinson model) consisted primarily of sodium sulfate, a smaller amount of sodium carbonate, and a minor amount of sodium chloride, Photomicrographs indicated the ulti-
Equipment. Venturi and cyclonic spray scrubber were combined in a
Literature Background Subject Ref. No. Venturi, orifice, pipeline, and sieve-plate scrubbers give similar emciencies for same total pressure drop (16) Emciency of Venturi scrubbers can be correlated on basis of Venturi pressure drop (3, 11,$1) Large and small scrubbers give similar performance at same total pressure drop (16) Performance is same for large and small Venturi scrubbers operated at same Venturi pressure drop (2)
Stack Gas Conditions and Dust Loadings
2"IPS
line. The rate of water make-up determined the proportjon of water recycled at a given gas rate and pressure drop. The unit could be operated without recycle by closing the line.
-CLEAN GAS TO ORIFICE METER AND EJECTOR
EQUALIZING IPS CONTACTOR LINE 14'
2" I P S
Make-up water rate was
Y
was constructed from Schedule-40 pipe and fittings. An ejector using compressed air at 100 p.s.i. supplied draft.
Figure 1. Except for cyclone, pipeline scfubber was constructed of standard pipe and fittings Wt. 50, NO. 11
NOVEMBER 1958
1615
02
16 16
INDUSTRIAL AND ENGINEERING CHEMISTRY
-
-
DUST SCRUBBERS 80
I Ventur~
I I Downstream of Ppti.
70 - 0
Vantur,
Uprtreom of Pptr.
6.0 - 0
Cyclonic spray Downstrom 01 Pptr.
0
55
50
g45 a w I
- 40
In
* *
t f 4.0 - 0
+
Pipe.line
Downstream 01 Pptr.
K
~npe-lms
Upstream of ~ p t r .
=-
-
5.0
W
In
5 -
3.0
E
-
I
I
I
I
I
l
l
Ventwe + Downa,sm 01 Pptr. Cqclon,c *pray
Vmtur, + uprlr..m Cqslon4s rproy
of Pptr.
-
-
U
-
-
0
K
y 2 0 -
-
5 O,e,a,A,A COLD WATER Q,+SOLN.
-
0
Figure 4. There was an apparent - increase in pressure drop expressed in velocity heads through pipeline scrubber with decrease in 80 velocity
70
20 30 40 50 60 WATER LOADING, ga1./1000 cu. ft.
.
Contoctor and cyclone
Collection Efficiency. The conventional basis for expressing the degree of collection is the efficiency, q. However,
Table 11. Relation of Efficiency to Number of Transfer Units
the efficiency shows only small percentage changes in the high range and can give a misleading impression, particularly when plotted on a linear scale. The efficiency is generally an exponential function of the process variables for most types of collection equipment. Hence, a more fundamental basis for expressing the effectiveness of aerosol
No. of No. of Transfer CollectionTransfer Collection Unitsn, Efficiency, Units, Efficiency, Nt % Nt" % 0.l o b 9.52 5.0 99.33 0.50 39.35 6.0 99.752 1.0 63.21 7.0 99.909 2.0 86.47 8.0 99.967 3.0 95.02 10.0 99.9955 4.0 98.17
collection in the number of transfer units, Nt, here defined by: N# = In or
7 = 1
1
(G) - e-Nt
(l) (2)
This is the definition used in gas absorption calculations for a solute gas which exhibits no back pressure above the solvent liquid (78). The number of transfer units bears a direct relation to the process variables over the complete range of efficiency from 0 to 100%. The numerical relation of efficiency to number of transfer units for some salient values of N , is given in Table 11. The decontamination factor, DF, sometimes used in reporting collection of radioactive materials, is defined in the same manner as the number of transfer units, but in terms of the logarithm to the base 10:
(I
Based on N t
=1
In ___ ,
1,)
=
(1 fractional efficiency. b For values
+
71
IO
I
30 40 5 0 60 70 8 0 9 0 100 POWER INPUT, thew. hp /(IO00 cu. ft./min
20
Figure 5. Differences in performance of pilot plant scrubbers on sulfate recovery furnace fume are believed due to condensation effects. 0 Hot solution
102
o
1
10 10
47
AIR (64.F)
-
-
z
-
