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Harold Doyle and Alan F. Brooks, Consolidated Mining and Smelting Co. of Canada Ltd.
The Doyle Scrubber Doyle scrubber utilizes principles of wet-impingement collection at relatively high gas velocities with high recovery efficiency and low installation and maintenance cost
IXECOVERY of dusts and fumes from process gases and ventilation systems is an important phase of m a n y industrial operations. T h e proper selection of wet or dry collection equipment for a specific operation is most important. Not only must adequate performance be realized, but installation costs have to be realistic, and operating and maintenance charges must be at a minimum. A new wet-impingement scrubber, developed from a research program initiated in 1945 by Consolidated Mining a n d Smelting Co. of C a n a d a Ltd. (Cominco), has been widely adopted by the c o m p a n y for dust and fume collection, gas ab-
sorption, and gas cooling. Over 40 units are in successful operation in Cominco's metallurgical and chemical plants, and several installations have been completed u n d e r license in C a n a d a , the United States, a n d elsewhere. T h e scrubber, known as
Power Requirements:
the Doyle scrubber, is the subject of patents, issued and pending, in a number of countries, including C a n a d a and the United States. T h e performance of the scrubber has been outstanding; very high recovery efficiencies have been ob-
2.2 to 3.0 hp. per 1000 c.f.m. of gas.
Scrubbing Liquid Requirements : 2 gallons per 1000 cubic feet of gas. On some applications, the rate is as low as 0.5 gallon per 1000 cubic feet of gas. Temperature:
Dust-laden streams up to 1660° F. have been handled directly.
Installed Cost :
Per 10,000 c.f.m.—$0.60 per c.f.m. for steel unit and SI.00 per c.f.m. for a rubber-lined steel unit.
GMS
INLET
GAS OUTLET
DOWNCOMER DUCT
AIR-LOCK RELEASE
SPRAY ELIMINATOR
CONE OVERFLOW WEIR BOX
SCRUBBING LIQUID LIQUID INLET
SEAL POT
Two-stage Doyle scrubber installation in operation on wet process phosphoric acid plant acid agitator ventilation system. Units are rubber-covered mild steel and have a total capacity of 6 4 , 0 0 0 cubic feet per minute
An enclosed tank is provided with means for admitting and discharging a scrubbing liquid at a steady rate of flow. The liquid overflow is maintained at a constant level. The incoming gas, containing fume or dust, is brought down to the surface of the scrubbing liquid by means of a downcomer duct, which is so designed that the gas velocity is greatly increased immediately before the gas leaves the downcomer and enters the liquid. The increase in gas velocity is normally achieved by concentrically positioning a stream-lined cone at the bottom of the downcomer to give an annular gap, as shown above. The desired annular gas velocity (impingement velocity) is obtained by providing a cone of appropriate base diameter. An adjustable cone, to give variable annular gap and therefore a variable impingement velocity, can be used if a wide variation in gas volume is anticipated VOL. 49, NO. 12
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tained a n d maintenance needs a n d costs have been very low. Major economies in plant space a n d construction costs have been realized by its adoption. T h e gas stream (see figure) with its suspended particulate m a t t e r or proportion of contaminant gas impinges at high velocity into the liquid b a t h a n d then undergoes a 180° change in direction within the bath. T h e cleaned gas leaves the impingement tank at the top and enters a spray eliminator section, which, for compactness, is usually a n integral p a r t of the complete scrubber. W h e n the spray or suspended liquid droplets of scrubbing medium have been removed, the gas stream m a y be exhausted or used in other processes. Factors Influencing Performance
I m p i n g e m e n t velocity is the important design factor governing collection or removal efficiency of the Doyle scrubber. Normally, the annular velocity used with experimental a n d operational units is from 90 to 240 feet per second with 150 feet per second representing the usual velocity for plant scrubbers. Pressure d r o p across the nozzle annulus, impingement tank, and
A Workbook Feature
spray eliminator is about 2.0 times the velocity pressure equivalent to the velocity of impingement. The velocity of impingement a n d the particle size and density of the gas stream contaminant establish the kinetic energy a n d the magnitude of inertial forces tending to drive the particulate m a t t e r downward into the liquid. At impingement velocities of 130 feet per second a n d higher, a n extremely high degree of turbulence exists within a correctly proportioned t a n k ; the liquid a n d gas a r e in a violent stage of agitation, the turbulent region extending to a level just below the gas outlet. Properties of particulate m a t t e r and/or gases—particle size distribution, density, and chemical reactivity with the scrubbing medium —influence the performance of any wet scrubber. T h e more that is known about such factors, the better the possibility of choosing a suitable collector. Particle size alone—at constant density—is not necessarily an indication of the recovery efficiency to be expected; the most satisfactory approach has been to carry out tests with small-scale or pilot installations on each specific dust, fume, or mist collection j o b . T h e technique of using empirical
test d a t a from development programs on industrial applications was the approach used to determine the suitability of the scrubber for Cominco installations. Dust loadings u p to 1000 mg. per cubic foot of gas (15.4 grains per cubic foot of gas) have been handled by the scrubber. Experimental evidence has indicated that percentage recovery efficiency, for a n y given dust, is not significantly affected by the dust concentration. S u m m a r y of Performance
A s u m m a r y of the performance of the scrubber on seven different industrial dusts, fumes, mists, or gases is presented in the table. T h e seven applications represent installations on which test a n d evaluation programs have been carried out. Suspension Roasting Plant Dust Collection System
Suspension roasters of Cominco design are in operation a t Kimberley, B. C , roasting pyrrhotite concentrates to supply feed gas containing 6 . 8 % sulfur dioxide to a 270-ton-perd a y sulfuric acid plant. T h e hot dust-laden gases from the roasters
Doyle Scrubber Performance Concentration of Solids in Scrubber Gas Stream, Grains/Cu. Ft. N T P Inlet Outlet
Source and Collection Equipment
Impingement Velocity, Ft./Sec.
Liquid R a t e , Gallons/1000 Cu. Ft. of Gas
Removal Efficiency" (Inlet-Outlet) X 100 Inlet weight basis, %
O N INDUSTRIAL D U S T S AND F U M E S
Phosphate rock dryer, cyclone followed by 1stage 16,000 c.f.m. c , scrubber Coal dryer, cyclone followed by 1-stage 12,000 c.f.m. scrubber Pulverized coal boiler, 1 -stage 500 c f . m . scrubber Lead sinter preparation wet-mix dryer, 1-stage 30,000 c.f.m. scrubber Lead sinter crusher, 1-stage scrubber
7.63»
0.205
177
2.4
97.4
1.93
0.0278
170
1.7
98.6
10. l 2.08
0.150 0.044
160 to 175 185
5.0 1.7
98.4 97.9
0.833
0.00306
140
1.0
99.6
3.5 3.5
97.2 99.4
18.5
99.5
d
I N W E T PBOCESS PHOSPHOBIC ACID P L A N T
Acid agitator ventilation s y s t e m containing acid mist, phosphate rock fines, and volatile fluorides Total fluorine P 2 Oi 2-stage scrubbers 64,000 c.f.m. installation Fluorine stripper exit g a s e s containing SiF 4 gas (total fluorine) 2-stage 500 c.f.m. pilot scrubber a
Removal efficiency across D o y l e scrubber only. Particle size, 9 5 % less than 5 microns. Cubic feet per minute. d Particle size, 4 5 % less than 10 microns. ' T o t a l pressure drop, 15 inches of water. 6
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0.392 0.116
12.6
0.0105 0.000586
0.0555
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are cooled to 550° F. in passing through waste-heat boilers. Then the gases from each roaster pass through, in turn, d r y cyclones, a onestage Doyle scrubber, and a cooling tower. T h e cleaned gases are then passed through acid-mist electroprecipitators before entering the acid plant. T h e scrubbers operate at nozzle pressure drops of 12 to 14 inches of water and scrub dust loads that are about 8 6 % by weight less t h a n 10 microns in size. Each cooling tower and scrubber is essentially a single structural unit, and the water flows downwardly through the cooling tower then across the scrubber. Cooling requirements govern the water flow rate. U p o n leaving the scrubber, the scrubbing and cooling water flows by gravity through a stripper, where most of the small amount of absorbed sulfur dioxide gas is removed and returned to the process. Operation and performance of the scrubbers since start-up has been satisfactory.
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