Development of the Venturi Scrubber
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WILLIAM P. JONES Chemical Construction Corporation, New York, N . Y.
A recently developed method of wet gas scrubbing is discussed. High efficiencies are obtained in removal of mists and dusts by utilizing high gas velocity in a Venturi tube for collision with and atomization of a stationary sheet of liquid. Several commercial installations where submicron dust is removed from gas streams are reviewed. A pilot plant handling 1000 cubic feet per minute of gas has been used on a variety of applications involving removal or recovery of low micron and submicron dust and sulfuric acid mist. Removal efficiencies of 98 to 99.8% on sulfuric acid mist can be attained economically.
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IIE I-enturi scrubber is based on the principle of fine atomization of liquids by high velocity gas streams. This principle has heen used for various purposes in the past, such as the steam desuperheater, and was developed further during the war for special military devices such as the production of smoke screens. T h e theoretical considerations of these principles have been discussed in recent literature ( 4 ) and this paper will deal primarily with practical applications as applied to the removal of dusts, fogs, fumes and odors, and smoke from gas streams. Thc Venturi scrubber is being used successfully for the removal of submicron particulate matter, both in solid or liquid state, from gas streams. This collection is thought to be due mainly to collision and impaction; but diffusion or molecular bombardment, electrostatic charge effects, and condensation may all be involved. Johnstone ( 3 ) discusses a theoretical treatment of some of the aspects of the S7enturiscrubbing principle. PRINCIPLES OF OPERATION
Figure 1 is a schematic drawing of a typical Venturi scrubber installation. T h e scrubbing liquid is introduced into, or just ahead of, the Venturi throat under low pressure and is distributed t o give a fairly complete apparent curtain of liquid across the throat. The upstream end of the Venturi is connected to the source of dirty gas. The gas is drawn through the system by an induced-draft fan, not shown here, but normally placed after the cyclonic separator. This puts the fan on the cool end where it handles a reduced volume of cleaned gas. T h e gas, at a velocity in the range of 200 to 400 feet per second, collides with the curtain of liquid at the Venturi throat, where the liquid is briefly but
LLLXFigure 1.
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Pease-Anthony Venturi Scrubber
violently accelerated and disrupted. The gas is decelerated in the diverging section; the fine particulate matter in the gas is wetted by the finely atomized liquid droplets and considerable coalescense of droplets occurs downstream from the throat, resulting in drops of liquid large enough t o be removed from the gas stream by centrifugal force. The auxiliary equipment shown is typical but may be different for each individual problem. There are three design variables which have important effects on the results of the Venturi scrubber. These are: the velocity of the gas in the throat; the ratio of liquid to gas; and the distribution of the liquid in the throat. While all of these have a bearing on the scrubbing efficiency, the first tR.0 variables have an appreciable efTect on the pressure drop across the Venturi. Most of the power required for operation of this scrubber is expended in gas pressure drop, since relatively small quantities of liquid are required and at low pressure. A properly designed Venturi tube will recover about 85% of the differential pressure encountered between the inlet and the contracted throat section. However, here a liquid is introduced at the throat a t zero velocity with respect to the flow of gas. T h e work done in accelerating and atomizing this liquid is reflected in pressure drop. It is therefore reasonable t o expect t h a t t h e pressure drop will vary with the amount of liquid used. With limited experience i t is indicated that increased throat velocity results in higher scrubbing efficiency with less liquid, and with somewhat less pressure drop and fan power. Figure 2 is a set of curves made by Collins ( 1 )on salt cake fume. These curves bear out the above statement. Starting with the curve farthest to the left, it will be seen that a throat velocity of about 215 feet per second required 9 gallons of scrubbing liquid per thousand cubic feet of gas to get 9070 removal efficiency. This same efficiency is indicated with 3 gallons per thousand cubic feet, at a velocity of 310 feet per second,
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T h e lower liquid rate also lowered the pressure drop by 2 inches water gage; this, converted to power, amounts to about 0.5 hp. saving per thousand cubic feet of gag, in fan power alone. The pressure drop and scrubbing liquid requirements will vary widely with different scrubbing problems, depending on the characteristics of the particulate matter in the gas and the efficiency of removal required. Some commercial installations are operating satisfactorily using only 2 to 3 gallons of water per thousand cubic feet of gas with pressure drops of 9 to 12 inches water gage. Other variables (such as temperature, density, and viscosity of both liquid and gas) may also have a bearing on the efficiency of the scrubber and on the power requirements.
E F F E C T O F T H E V E L O C I T Y OF GAS THROUGH THE VENTURI THROAT A T VARIOUS L I Q U I D TO GAS R A T I O S UPON THE
COMMERCIAL INSTALLATIONS d
Large scale commercial installations are in operation on three different types of problems, all of which were preceded by pilot plant work. The first commercial installation was made a t a kraft pulp and paper mill and is used for the recovery of sodium compounds from the stack gas of the chemical recovery furnace. Complete data on this installation have been published (I,2 ) . Figure 3 is a general view of the installation showing the large diameter duct leading to the Venturi section and on to the cyclonic separator. The duct from the top of the separator leads to the induced-draft fan (not visible) which discharges the scrubbed gases into a horizontal flue and to the stack directly behind the separator. Figure 4 is a close-up of the Venturi throat section. Figure 5 is a photograph looking downstream inside the converging section of the Venturi, and shows the complete coverage of the Venturi throat by scrubbing water as air was being pulled through the throat a t high velocity. This is a 25-inch diameter throat and the entire coverage is made by the use of internal jet pipes; the ends of these are tapered for streamlining effect. Radial jet holes are drilled in each pipe so that the water is injected a t right angles to the gas flow. This unit handles 50 to 60 thousand cubic feet per minute of gases and recovers 7 to 10 tons of valuable sodium compounds per day. T h e particle size range of this fume is from below 0.1 to about 1.5 microns. Two large Venturi scrubber installations have been in operation for over a year on open hearth steel furnaces. These furnaces in which an oxygen lance is dipped into the molten metal give off a dense red smoke composed mainly of iron oxide. Each furnace has two Venturis in parallel; the throats are 19.125 and 12 inches in diameter, and recent gas flows were between 70 and 80 thousand cubic feet per minute a t about 700 O F. Figure 6 shows a close-up of the larger Venturi throat. The liquid is injected partially from outside radial jets and partially by seven jet pipes inside the converging section. Figure 7 shows an electron micrograph of the small end of the iron oxide particle size spectrum from this operation. Particle counts on a number of micrographs of this smoke showed the mean diameters of the particles to be in
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I l l VENTURI THROAT ( F E E T P E R srcoro)
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the range of 0.25 to 0.33 micron on a weight basis, with many particles down in the 0.05 to 0.10 micron range. Efficiencied between 97 and 99.7% on a weight basis, have been reported by the steel company for normal operating conditions. One of the large chemical companies is now operating several commercial Venturi scrubbers on exhaust gases containing vaporized fatty material. This is a complex problem as the material is initially in the gas phase but condenses to a bluish-white smoke on contact with the atmosphere and soon evaporates again, leaving an odor. The particles are unstable, making it impossible to accurately determine particle size, but they are thought to be under 0.25 micron. Efficiencies are measured in terms of optical density and odor elimination. All the commercial installations described here were preceded by pilot plant investigations using both the Venturi and older Pease-Anthony cyclonic spray scrubber. However, all of the final installations consist of Venturi and cyclone without sprays. In each instance, the efficiency of the commercial Venturi unit has equaled or exceeded the pilot plant results; in two cases, i t was possible to operate at higher gas velocities but a t lower pressure drops, because of lower water rates, while maintaining the high efficiencies. PILOT
PLANT WORK
The Pease-Anthony cyclonic spray scrubber has been used for difficult scrubbing jobs for about 16 years. This equipment has been fully reported in previous literature, but a brief description is given here, as it has been used in all the pilot plant work. Figure 8 shows a schematic outline of the cyclonic spray scrubber. The dirty gas comes in tangentially at the bottom of the tower and spirals up through a fine spray made by suitable noz-
COURTE8Y PAPER TRADE JOURNAL
Figure 3. Large Venturi Scrubber
Figure 4.
Venturi Throat Section
Figure 5. View inside Venturi Looking Downstream toward Throat
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COURTESY POWER GENERATION
Figure
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Venturi Throat o n Open Hearth Furnace Cas Scrubber
z l q mounted on an axial manifold. The centrifugal force of the rotating gas spins the fine liquid droplets out to the wall of the tower, and the dust particles are wetted b y collision as they pass through the spray zone. The finest wetted particles and spray droplets are thrown out in the upper section of the tower. This scrubber with suitable nozzles, liquid pressures, and quantities is highly efficient for removing particulate matter down to about 1 micron in size. Its use, following the Venturi, gives an added clean-up of the gas. Because of the many unknowns encountered in difficult gas cleaning problems and the limited experience with the Venturi, a portable testing unit was constructed and is shown in Figure 9. It is designed to scrub 1000 to 1500 cubic feet per minute, measured a t saturated temperature, The unit is a combination of both Venturi and cyclonic spray scrubbers, both shoxn in the toreground of Figure 9. With this Combination i t is possible t o use either the Venturi or cyclonic spray scrubber separately, or they can be used together. I n the background there is a skid which carries the exhaust fan and pump together with motors, starters, drives, and piping. The unit i s equipped with flowmcaters and manometers to give complete operating data
Figure 7.
Electron Micrograph of Iron Oxide Dust from Oxygen-Lanced Open Hearth Furnace
Vol. 41, No. 11
This portable test scrubber was installed at a West Coast steel plant last summer to treat the stack gases from an open hearth steel furnace in which only cold metal and scrap are charged. A substantial amount of the scrap was galvanized and the stack gases contained from 0.5 to 1.5 grains of solids per cubic foot 01 gas, corrected to standard conditions. About 50y0 of these solids was zinc oxide; the remainder consisted largely of iron oxide and dag. The gas, initially a t about 1200' F., was cooled by water saturation ahead of the Venturi to ensure the condensation of all condensables. Figure 10 is an electron micrograph of the fines in this fume. The large dark particles are iron oxide; the lightei large particles, slag; and the small ones, mostly zinc oxide. The 50%, by weight, of zinc oxide does not show up on the micrographs, but it is suspected that it is masked by the larger particles of iron oxide and slag. Most of the weight of the material in this picture is in the size range of 0.2 to 0.85 micron, with fines down to 0.08 micron. The Venturi scrubber removed 90 to 98% of this material, the efficiency varying with the inlet loadings However, the total exit dust loadings were fairly constant in the range of 0.02 to 0.05 grain per cubic foot, with zinc oxide loading. a11 below 0.018 grain on the eight different analyses made. Thew exit loadings were well below the local code specifications arid there was practically no visible fume, other than condensed water vapor. Most of these tests were made using 4 to 4.5 gallons of water per thousand cubic feet of saturated gas in the Venturi throat. Another interesting problem encountered was the scrubbing of the fume from the baking of enamel insulations onto copper wire. This fume contains some acrolein, aldehydes, and phenolic resins The stacks from these baking ovens emit a very thin bluish haze 1'nder certain atmospheric conditions, the fumes come down close
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Figure 8.
Pease-Anthony Cyclonic-Spray Scrubber
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I N D U S T R I A L A N D ENGINEERING CHEMISTRY
Figure 9. Portable Pease-Anthony Pilot Plant for Handling 1000 t o 1500 Cu. Ft./Min. of Gas
t o the ground with an offensive odor and also cause serious eye irritation in the immediate vicinity of the plant. The pilot plant Venturi scrubber was successful in removing all the visible haze. As long as fresh cool water or a weak caustic solution was used as the scrubbing liquid, a high percentage of the offensive odor was removed, but recirculated water soon became saturated with the odor. The most difficult part of this problem was trying to determine what materials were present and how to measure results. Attempts to collect samples for analysis were unsuccessful. Attempts were made to collect the gases in vacuum bottles, on microscopic slides, and for electron microscope examinations, but it was not possible to obtain sufficient concentrations to identify the components. Obviously, a large part of the offensive odors came from substances in the gaseous state. There are many sources of sulfur trioxide fume, or sulfuric acid mist, around chemical plants, and this has long been a difficult problem t o eliminate. T h e Chemical Construction Corporation has been faced with this nuisance for years, in the design of contact plants, acid concentrators, and acid-recovery plants. T h e initial cost of electrostatic precipitators in some instances has heen prohibitive.
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The Venturi scrubber will efficiently rrmove acid mist from a gas stream. Extensive test work has been carried out on acid mists from several sources, such as acid concentrator and contact plant tail gases, and copperas roasting operations. Although the mist loadings and the character of the mist varied widely, high removal efficiencies were obtained in every case. Total sulfur trioxide and sulfuric acid mist content of the gases varied from 50 to 730 mg. per cubic foot. The Venturi scrubber gave exit loadings in the range of 0,5 to 3.0 mg., when using 4.3 to 5.7 gallons of scrubbing liquid per thousand cubic feet of gas. The pressure drop range across the Venturi was from 9 to 15 inches water gage. The exit mist loadings were practically constant, regardless of the inlet loadings, when the throat velocity and liquid rate were held constant. An increase in liquid rate gave lower exit mist loadings but a t an increased pressure drop across the system. Both water and weak sulfuric acid were used with equal efficiency as the scrubbing liquid. All of the applications discussed have been on problems involving submicron dust or fume and mists. However, the indications that this principle may be economically adapted to wet scrubbing of coarser dusts have not been overlooked. The Pease-Anthony cyclonic spray scrubber has been used for about 10 years for the cleaning of blast furnace gas. The dust is relatively coarse, being mainly 1 t o 20 microns following the dust catcher. With the advent of the Venturi scrubber, one of the steel companies u$ng the Pease-Anthony cyclonic spray units gave the Venturi a trial on blast furnace gas, hoping to get combined primary and secondary cleaning efficiency in one step. They installed a pilot plant Venturi, having an 8-inch diameter throat, on the “dirty gas” line and ran only enough tests to satisfy themselves that the results justified a plant size unit, which has since been constructed. All efficiencies with the test Venturi were above 99.9% on a weight basis, as the dust loadings were high in the dirty gas. A number of clean gas loadings were as low as 0.003 to 0.009 grain per cubic foot, which compares favorably with exit loadings from precipitators preceded by primary washers. Indications were that exceptionally high efficiencies can be obtained on this coarser dust, with relatively lower gas velocities and pressure drops. Consequently, the Venturi principle will be investigated further on coarser dusts. Experience bears out earlier beliefs that the Venturi scrubber is a new and useful tool for t h e benefit of industry, not only for abatement of nuisances and air pollution, but, in many cases, i t will be found useful for the recovery of valuable materials. Its practicability has been demonstrated for cleaning gas by wet methods and its efficiency is evident particularly on low micron and submicron dusts and fumes. Simplicity of the equipment leads to low initial cost and economical maintenance and operating costs. ACKNOWLEDGMENT
The author wishes to express his appreciation to A. W. Anthony, Jr., for his experienced advice and assistance in the preparation of this paper. LITERATURE CITED (1) Collins, T. T., Jr., PapwIndustry & Paper World, 28, No.6,830
(1947) (2) Collins, T. T., Jr., Seaborne, C. R., and Anthony, A. W.,Jr., Paper Trade JmTWE, 28, N O . 3, 55-8 (1948). (3) Johnstone, H. F., and Roberts, M. H., IND, ENQ.CHE~M., 41, I
2417 (1949). (4) Lewis, H.C., et aL, Ibdd., 40,No. 6, 67 (1948). REC~IVE Marcth D 21,1949.
Figure 10. Electron Micrograph of Iron Oxide and Zinc Oxide in Scrap Open Hearth Furnace Fume
Paper 10 in the Symposium on Atmospheric Contamination and Purification, “Determination of Free Sulfur in the Atmosphere,” by Paul L. Magill, Myra Rolston, and R. W. Bremner, will be published in the November issue of Analytical Chemistry,
