ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT (15) Kunkel, W. B., J.A p p l . Phys., 21,833-7 (1950). (16) Lehmicke, .D. J., Am. DyestuffReptr., 38, 853 (1949). (17) Loeb, L. B., Science, 102, 573-6 (1945). (18) Lopez, J. A., and Hewson, J. K., presented at 30th annual convention of Amer. Assoc. of Textile Chemists and Colorists, New York, October 1951. (19) Natl. Bur. Standards, Circ. C438 (1942). (20) Pohl, H. A., J.Appl. Phys., 22,869-71 (1951). (21) Rani, W. E., “The Impaction of Aerosol Particles on Cylindrical
and Spherical Collectors,” U. S. Atomic Energy Comm. SO-1004, Univ. Ill., March 31, 1951. (22) Richards, H. F., Phys. Rev., 22, 122 (1923). (23) Rossano, A. T., Jr., Sc.D. Thesis, Harvard University, 1954. (24) Rossano, A. T., Jr., and Silverman, L., Heating and Ventilating, 51, 102-8 (May 1954).
(25) Shaw, P. E., and Jex, C. S.. Proc. Roy. Soc. (London), A l l l , 339 (1926). (26) Zbid., A118, 97-108 (1928). (27) Silverman, L., and Viles, F. J., J . Tnd. Hug. Toxicol., 30, 124 (1948). (28) Thomson, E., J . Amer. Znst. Elec. Eng., 4 1 , 3 4 2 (1922). (29) Walton, W. H., “The Electrical Characteristics of Resin
Impregnated Filters,” Chemical Defense Experimental Station, Porton, England, Report No. 2465, December 1942. RECEIVED for review November 10, 1984. ACCEPTEDFebruary 25, 1955. This s t u d y was made as part of the work done under Contract AT(30-1)841 between Harvard University and the U. S . Atomic Energy Commission. Opinions expressed are those of the authors a n d do not necessarily represent the views of the Commission.
Design Considerations in Filtration of Hot Gases C. A.
SNYDER AND R. T. PRING
American Wheelabrafor and Equipment Corp., Mishawaka, Ind.
Because of their efficiency, cloth filters are widely accepted in hot gas cleaning applications for air pollution control. The success of a hot gas filtering installation depends on incorporating in the collector the most suitable design features for the application and on the proper sizing of the equipment. Mechanical design features that have proved best suited for the filtration of hot corrosive gases are described. Factors in sizing equipment (selection of the proper cloth area) are also discussed. The pressure drop-dust burden relationships advanced by Williams and his coworkers, Lapple, and Hemeon do not apply because of changes in the characteristics of the dust layer on the fabric as its thickness increases. Pilot scale testing under field conditions is indicated wherever operating experience in similar installations i s lacking.
T
HE unquestioned efficiency of cloth filtration for the removal of fine particulates from gases has created new interest in its application to hot gas cleaning problems in air pollution control work. Cloth filters or baghouses have been employed for many years in the nonferrous smelting industry for the recovery of values from stack gases. Their application to t h e recovery of nonvaluable contaminants from hot gases was restricted by the high cost and operating limitation of t h e then available natural fiber filter fabrics. Commercial production of several synthetic textile fibers in the middle forties greatly increased the applicability of cloth filtration to the more difficult gas cleaning jobs previously monopolized b y electrostatic precipitators. For example, modern continuous automatic bag filters are now economically operated in cleaning hot, corrosive gases from carbon black production furnaces (Figure I), iron foundry cupolas (Figure 2), electric steel furnaces, electric copper casting furnaces, and directfired dryers. Successful pilot scale operations of baghouse systems for open hearth steel furnaces have indicated a definite place for filtration in that field also.
Gas Cooling The application of cloth filtration to the cleaning of hot gases requires in many cases that t h e gases be cooled in order to protect the filter fabric and to ensure economical bag life. A discussion of gas cooling methods is given b y Pring ( 3 ) . I n t h e case of evaporative cooling, i t is desirable to design spray cooling towers to operate dry-i.e., with no runoff of unevaporated water a t the bottom of the tower. T o permit the design of such coolers, a method has been developed to estimate the vertical travel distance of a given water droplet size to ensure complete evaporation. I n a stream of hot gas, the evaporation dis-
960
tance for a water droplet is a function of t h e square of the particle size, indicating the desirability of employing spray nozzles that produce extremely fine spray droplets. Not only must the spray droplets be sufficiently fine t o avoid water runoff from the tower, but also the tower must be operated with dry walls. This necessitates locating the required number of spray nozzles, one above the other, on the vertical center line of the tower. Such a configuration of spray nozzles favors t h e use of hollow cone sprays. Spray cooling control requires t h e use of a temperature controller operating a full modulating valve on the water supply line leading to t h e spray nozzles. Where cooling requirements vary greatly from time to time, it is desirable to install, in addition, solenoid valves on each individual spray line, providing staggered snap on-off control to maintain a sufficient nozzle pressure in t h e nozzles that remain in operation.
Bag Filter The basic components in the modern bag filter as employed in hot gas cleaning (Figure 3) are
1. Structure (housing, hoppers, and supports) Bag cleaning mechanism Cell plate to which the bottoms of the filter bags are sealed Filter bags 5. Accessorv items-damper valves, materials handling equipment a n a conveyors, repremuring. system, and control instruments 2. 3. 4.
Structure. The housing and hoppers are usually fabricated from heavy plate with welded joints and flanges. Provisions for thermal expansion are required, and the square feet of external housing surface per square foot of filter cloth is kept to a minimum to prevent overcooling the gases and resulting condensation. Special access doors, as well as internal and external walkways
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
AIR POLLUTION
COURTESY AMERICAN WHEELABRATOR A \ O
Figure 1.
EaUlPKEhT CORP.
Continuous automatic cloth filter
Twelve-compartment filter recovering carbon black from 101,000 cubic feet per minute production gas a t 250' to 275' F.; collector follows electrostatic agglomerator and cyclones
tension easily is important and favors the use of strap-type suspension. All else being equal, the smaller the filter bag diameter] the greater the area of the filter surface which can be compressed in a given floor or ground space. There is a practical lower limit t o tube diameter if bridging of falling filter cake a t the mouth of the bag is to be avoided. The useful range is from 5 to 12 inches in diameter, the bag length being in rough proportion t o the diameter. A c c e s s ori e 6 . Continuous automatic, compartmented filters are required for most hot gas cleaning applications and must be equipped with damper valves and operators for interrupting gas flow through filter bags during shaking, the sequence of operating and shaking being controlled by 8; program timer. I n some instances, a repressuring system is used t o create a slight air motion through the filter fabric in a reverse direction during shaking. Repressuring or a t m os p h e r i c venting also counteract possible leakage of gas through closed dampers. The s e l i tion and sizing of accessories, including materials handling equipment] will depend on the characteristics of the individual application.
and galleries, are important where ease of inspection and mainSelection of Filter Media tenance are factors. The entire structure is designed t o faciliFactors affecting the selection of filter fabric include the gas tate economical and effective insulation. To permit isolating a temperature] composition, and dew point, and the particle size portion of the filter bags from the gas stream a t one time for and properties of the dust or fume to be collected. For hot gas shaking (necessary for continuous operation), the interior of the filtration] it is desirable to operate the collector a t a temperature housing is divided into a number of separate compartments. as high as is commensurate with economical filter fabric life in Bag Cleaning Mechanism. The tubular bag filter employing order to minimize the amount of gas cooling required. Particua mechanical bag shaking mechanism is preferred for most hot larly in installations where evaporative cooling from initial gas applications, particularly where fine fume particles are encountemperatures in the order of 2000" F. is employed] the filtration tered in the gases. The preferred shaking action is that which transmits a rippling motion down the length of the loosely suspended tubes without snapping or stressing the fabric. Fine fume particles are removed with difficulty from envelope or panel-type filter bags .that are stretched over internal screens and ' cleaned by a bumping or jarring action. Cell Plate. Tubular filter bags are supported from above and the open bottoms through which contaminated gases enter the interior of the bags are sealed to the openings in the cell plate or floor separating the dirty and clean gas portions of the collector housing. A desirable construction .employs tube bottoms that expand into flush cell plate holes, thus eliminating pockets in which dust t h a t dislodges from filtering surfaces during shaking can accumulate. This is important, particularly when the filtered material is acidic, hygroscopic, abrasive, or subject to spontaneous combustion. Filter. Bags. Actual filter bag construction varies according to the dust collector design employed. Tubes m a y b e suspended Figure 2. Foundry cupola fume control system a t a Los Angeles gray iron from hooks by loop straps sewn to the foundry tops of the bags, from buckle-type fasteners Fume and dust from cupola are collected in 5-compartment continuous automatic tubular cloth by open-ended Or by 'lamping to filter employing Orlon bags (foreground); volume filtered is 16,990 cubic feet per minute at bells or inverted thimbles. Adjusting bag 270' F.; spray cooling reduces gas temperature from initial maximum of 1790' F. Y
May 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
961
ENGINEERING, DESIGN, A N D PROCESS DEVELOPMENT
( d ) , filter resistance comprises the sum of the resistance. to air flow of the filtering medium (cloth) and of the accumulated layer of particles on the surface of the cloth, which affects a major portion of the filter action of the apparatus. At filtration velocities commonly encountered, flow through filter cloth is streamline or laminar, and resistance is a linear function of velocity, thus, for clean air
SHAKER MECHANl
RI FILTER BAGS
=
KoV
(1)
Permeability of a given cloth is readily determined by laboratory or field experiment and may be corrected for variations in viscosity of the gas. The maximum resistance R f a t the end of any given operating period, T, may be expressed
CELL PLATE
Rf = Ri
+R
(2)
The authors, in applying t o gas filtration the fundamental relationships of fluid filtration through granular filter beds, suggest that
On the premise that the variables enclosed within the brackets in the above expression would be constant for any given aerosol, it is proposed that they be combined in a single factor K1
Rf
- Rr
Figure 3.
Cutaway sectional view of cloth filter dust collector
temperature should be 75" t o 100' F. above dew point to prevent condensation on interior surfaces of the filter housing. The present day conception of the ideal filter fabric would be one which 1. Is dimensionally and chemical1 stable and relatively unaffected by gas temperatures t o 450' 2. Possesses tensile strength and resistance to the flexing associated with the mechanical shaking mechanism of the same order as the cotton filter fabrics employed a t room temperatures. 3. Possesses moderately high permeability, yet is highly retentive to the dust or fume being filtered and holds a light residual dust load after shaking.
5.
At present the three types of filter fabrics most commonly used for hot gas filtration are wool, Orlon acrylic fiber, and Fiberglas. None of these fabrics fulfills all three requirements. For example, wool, while moderately resistant to acid gases, will withstand temperatures only t o 215' F., whereas Fiberglas can be operated a t 450' F. but possesses poor flexing resistance and, in certain weaves, is relatively inefficient against very fine fume particles. The technology of Fiberglas fabric is improving rapidly, however, and considerable hope for future developments is assigned t o this material. Orlon, while limited t o 275" F. in operating temperature, fully satisfies the other two requirements, and, a t the present moment, enjoys wide popularity for hot gas filtration. Table I compares specifications for various Orlon fabrics.
KiLTV'
= ___
COURTESY AMERICAN WHEELASRATOR AND EOUIPMENT CORP.
7000
(4)
This formula implies that 1. For any aerosol, R is a linear function of LTV, weight of dust per square foot of cloth. 2. K1 is independent of cloth characteristics. 3. K1, and consequently R, is a linear function of kinematic viscosity.
Lapple (2) states that the pressure drop across the loaded cloth is a function of the absolute viscosity of the gas but supports the previous reference in suggesting a proportionality constant unaffected by cloth characteristics or thickness of dust cake on the filtering surface. American Wheelabrator and Equipment Corp. operates various laboratory test dust collectors ranging in size from 1 square foot of cloth and upwards for the purpose of evaluating the filtration characteristics of various dusts and fabrics. I n addition, much work of this type is performed on full size Dustube collectors.
Determination of Required Cloth Area The cloth area required in an industrial air filter is determined by the air flow rate-volume, cubic feet per minute-and the velocity of filtration-air to cloth ratio-necessary for satisfactory filter operation, and the requirement for good filter performance can be reduced to one consideration-operation of the filter a t an acceptable pressure drop over the normal cycle period. This presumes that cleaning or collecting efficiency does not vary significantly with filtering velocity or ratio; therefore, this statement is accepted to apply only to fabrics providing collection efficiencies in the range of 95% and upward. Filter Resistance. According to Williams and coworkers
962
Figure 4. Test apparatus used in determining filtration characteristics of various dusts, fabrics, and gases
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
Table 1.
Properties of Various Orlon Filter Fabrics
Dust: fine petroleum coke Results: average of 5 cycles Dust Loading Weight, Permeability Oz./Sq. Grams/' Yd. New Used Sq. Ft. Cloth 10 29.00 3.9 35 Napped filament Orlon, 3/1 twill 58 15.40 7.6 85 Knit Orlon, napped 33 18.21 7.5 60 Orlon spun staple Orlon spun staple, napped both sides 9.0 100 50 19.95 Orlon spun fiberstock, 3/2 twill 4.9 110 62 17.96
Experimental. The laboratory equipment includes a low rate volumetric feeder with infinitely variable-speed drive for feeding the test dust to a compressed air ejector used for particle dispersion. The air-dust mixture is usually passed through a settling chamber (duplicating the action of the inlet chamber of the Dustube collector) so only the representative, finer-sized particles reach the cloth being tested. Air flow through the system is provided by a positive displacement gas pump. The 12 X 12 inch (effective size) fabric in question is tightly clamped between flanged halves of the filter box which is provided with a dirty gas inlet a t the top, cleaned gas outlet at the bottom, and pressure connections to a large inclined manometer where R/ is observed a t timed intervals. The clean air side of the box includes a pressure channel adjacent t o the fabric, which channel is connected to a second air pump (adjustable flow rate). A second manometer is connected across the pressure channel and the box, and pump flow is regularly adjusted to zero the manometer. Thus, gas leakage edgewise of the fabric is offset and does not enter the volume calculation. The cleaned gas passes through a rotameter flowmeter with mercury manometer on t h e inlet side so that volume can be corrected to standard conditions. A paper extraction thimble can be introduced between the cloth and the rotameter if desired EO that rejected dust can be accounted for and fabric collecting efficiency determined. Figure 4 illustrates the laboratory test apparatus. The cloth is weighed before and after each test cycle, and R, is observed at timed intervals starting with aero time. Several cycles are usually run after shaking the fabric for a timed period in a prescribed manner. The value of K1is calculated from
R/ - Ri K --= - L~. Tv
R Pounds dust/sq. ft. X V
7000
Pressure Drop (R)Versus Dust Burden. K I is supposedly a constant for a specific aerosol. Certain dusts have been tested which conform to this theory-that is, a graph of R versus LTV is a straight line on linear paper. However, few dusts behave in this manner; exponential curves are more apt to be found, especially for extremely fine materials and high surface area fabrics. Generally speaking, the linear curve is seen only with
Table II.
Filtration Characteristics of Napped and Unnapped Sides of Orlon
Orlon: 4-02.napped filament, 76 X 72 count, 3/1 twill Reeults: average of 4 runs Fume: freshly generated magnesium oxide Dust Loadin Used Residual Collecting PermeGramst Dust, Efficiency, ability" Sq. Ft. Grams c % Ki Napped side 4.2 12.0 15 89.4 234 Unnapped side 8.1 13.8 4.5 84.0 254 Permeability after shaking CU. ft./min./sq f t a t 0.5 inch w.g. Corresponds J o weight of dust removed by $hiking, grams/sq. Et. C Dust remaining on cloth after shaking.
May 1955
Residual Dust, Grams/ Sq. Ft. 4.18 4.58 1.48 2.15 0.99
Collecting Efficiency 99.91 99.70 95.19 99.64 93.42
Filtration Constant,
KI
17.1 12.5 15.0 7.4 13,O
relatively coarse materials. Experience in this laboratory indicates, therefore, that K1 varies with time at constant loading and constant ratio with properties of the fabric, with porosity, and with pounds of dust per square foot. Effect of Fabric. A lightly napped fabric shows a lower value of Kl than an unnapped fabric (no other,mechanical difference between the two fabrics), in addition to showing a higher cleaning or collecting efficiency (Table 11). The greater is the fiber surface area in square feet per square foot of filter media, the lower is the value of K I . 5
Figure 5. Values of R versus LTV in filtering fine petroleum coke dust 1. 1-1 2 Fiberglas fabric, low flber surface area 2. Napped 8-27 Orlon 3. 8-26 staple Orlan fabric napped both sides, high fiber surface area
Tests also indicate that, for a given fiber surface area per square foot of filter cloth, the higher the permeability, the lower is K I . When speaking of this proportionality factor, therefore, one is concerned with a particular dust-fabric combination since Ka varies with the cloth. Hemeon (f) has also suggested that the pressure drop through a layer of dust supported on a filter cloth is proportional to t h e thickness of the dust layer which can be expressed in terms of weight per unit area of cloth (dust loading) and that the pressure drop across the loaded filter is the sum of the pressure drop through the cloth after shaking plus the pressure drop through the layer of dust deposited on the fabric surface. The pressure drop across the loaded filter is the sum of the residual pressure (clean cloth plus residual dust, often expressed as residual pressure per cubic foot per minute for consideration of performance a t other ratings) plus draft loss increase due to the weight of dust filtered per unit area, per cycle. I n conducting tests, i t is advisable to duplicate cloth style, grain loading, and time cycle as nearly as possible. Thereis Iittle doubt that the variable 8 (fractional voids), which Williams and coworkers have included in the proportionality factor Kl is not necessarily constant for any given aerosol. If a certain dust is deposited on a filter media having a surface area of 1 square foot per foot (plane surface) and if the porosity of the
INDUSTRIAL AND ENGINEERING CHEMISTRY
963
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT Table 111.
Values of Filtration Constant" Covering Indicated Portions of Dust Deposit Residual Dust,
Cycle 1 2 3 4 5. 6
grams/ sq. ft.
Constant
7 Average, except 1
..
7.5 4.6 5.0 3.4 4.3 4.7
5.1 8.4 8.2 8.8 8.6 8.7
4.9
8.0
Added Dust Grams/ Constant sq. f t . 37.4 16.1 48.2 14.8 55.2 13.0 49.6 8.9 6.7 47.6 43.0 8.6 46.9 8.2 48.4
10.0
Total Dust Grams/ Constant sq. f t . 37.4 16.1 37.7 19.9 35.3 21.4 28.2 17.1 22.6 15.5 23.7 17.2 25.0 16.9 28.8
18.0
Calculated from corresponding inches resistance/cu. ft./min./sq. ft. divided by the weight of dust on the fabric ,(a) after shaking (residual), (b) a t end of test run (total), and (c) added during test run (increase). The resistance in (a) was corrected for the resistance of new cloth. a
filter cake remains constant regardless of weight of cake per unit area, then K I would be a constant value. A curve of LTV versus R would appear as a straight line on rectilinear paper. However, cloth filter mediums have a high ratio of fiber surface area to projected area, which increases the porosity of the initial dust deposit necessary to obliterate the structure of the filter medium by holding the weight per unit area to a greater depth. I n addition, our understanding of the mechanics of filtration indicates that, a t the beginning of a cycle, relatively coarse particles adhere to the filter mediums, gradually bridging over with finer and finer particles as time goes on. At the same time, the ratio of voids t o solids (porosity) is constantly decreasing. This action is probably progressive until rupture of the filter cake occurs from weight and/or pressure differential, or from mechanical disturbance of the cake. When a loaded filter bag is shaken, a portion of the deposited dust contributing to the variable R (the increase in pressure drop due to the dust layer) falls away, leaving on the cloth a varying amount of so-called residual dust which, with the filter fabric employed, influences the magnitude of the residual pressure, R,. This residual dust, being intimately associated with the fibers of the filter medium, takes the form of a more porous deposit than does the dust added to the filtering surface during normal operations. Perhaps an equally important factor, however, is that whenever the residual dust layer is disturbed by handling or shaking the filter fabric, there is a sharp increase in the porosity
of the residual dust deposit. A typical example of this condition is illustrated in Table 111. Here, values of the filtration constant are shown for the residual dust deposit remaining on the cloth after shaking and for the total dust burden before removal. Canton flannel was the filter medium and its heavy nap is the principal factor influencing the character of the dust layer. Figure 5 shows the effect on filter resistance of variations in porosity of petroleum coke dust deposits due to differences in properties of the filter fabrics. 1-12 Fiberglas fabric has a low, B-26 Orlon, an exceptionally high, and napped B-27 Orlon fabric, a medium total fiber surface area per square foot of cloth. The curve of R versus LTV approaches a straight line for the Fiberglas fabric, indicating that K I is nearly constant. On the other hand, the high fiber surface of the B-26 Orlon results in changes in dust porosity and K I as the thickness of the deposit increases. Figure 6 shows curves obtained by filtering a very fine dust obtained in Wheelabrating used oil paint drums on a high twist, unnapped Orlon (low fiber surface area) and on fiberstock Orlon (relatively high fiber surface area). I n the case of the high twist Orlon, K I was nearly constant throughout the test run as evidenced by the straight line curve. The curve obtained using fiberstock Orlon is hyperbolic, indicating that the porosity of the dust deposit changed as the dust burden increased. Effect of Dust Layer. T h a t different dusts have widely varying filtration characteristics is an accepted fact. Some factors influencing K1 are 1. Particle size distribution 2. Particle shape 3. Surface properties, including electrostatic charges
Values of K1have been observed as low as 0.50 for a mediumfine fibrous paper dust, and as high as 500 for de-agglomerated carbon black. Figure 7 illustrates curves of R versus LTV for three different dust6, all on standard cotton sateen filter cloth. Here, t h e only differences among the three tests were in the particle size, shape, and surface properties of the three dusts used.
Effect of Gas Viscosity Because many atmospheric pollution control installations involve the filtration of hot gases, i t is important that proper
1
I
I
Figure 6. Values of R versus LTV covering fine Wheelabrator dust generated in abrasive blasting of used steel paint drums on high twist, unnapped Orlon with an extremely low fiber surface area per square foot and fiberstock Orlon 964
Figure
7. Effect of particle size and shape on filter resistance of cotton sateen cloth
Silica gel cotalyst dust, 43% below 10 microns Wheelabrator steel scale dust, moderately coarse, 2% less than 10 microns Ground limestone dust, coarsest, 32.3% minus 325 mesh
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 5
+ '
AIR POLLUTION
'
corrections be made to the estimated pressure drop through t h e bag filter to compensate for changes in gas temperature and gas composition over that usually encountered in laboratory practice. Williams and coworkers state in Equation 3 that resistance through a filter is a function of kinematic viscosity p / p . On the other hand, Lapple states that the pressure drop through a cloth filter is a linear function of the absolute viscosity. T o determine the effect of gas viscosity and density on filter resist,ance, the procedure as outlined was adapted to t h e use of compressed gases in cylinders filtered under controlled conditions of flow. Control tests were run using air, and from the curve of R versus V for air, corresponding curves for several other gases were calculated by correcting for kinematic viscosity and absolute viscosity of the given gas. Argon, helium, acetylene, and sulfur dioxide were used in testing, and the observed pressure losses for these gases were plotted against their corresponding filtration velocities for comparison to calculated values. Figure 8 shows typical results obtained using sulfur dioxide and Figure 9, argon.
ing many combinations of particulates and filter fabrics has shown that the porosity of the deposited dust on a filter fabric is not constant throughout the filtering cycle because of variations due t o the fabric structure, possible compression of the dust layer under the influence of high differential pressures, and a significant change in the residual dust layer following any disturbance of the filter, as in shaking or other handling. As a result of variations in the dust layer due to one or more of the above influences, curves of resistance versus weight of dust per unit area of fabric are seldom linear as Williams and coworkers and Lapple propose; instead, exponential or irregular curves are more frequently observed.
2 0
3
x, I
U
I
0
Figure
2
3
4 5 6 V FT PER MINUTE
7
8
9
IO
9. Effect of gas viscosity on filtering resistance with variation in filtration velocity
Observed data for air and argon compared with calculated values obtained by correcting values observed for air by ratios of kinematic and absolute viscosities of argon V
Figure 8.
FT
PER
MINUTE
Effect of gas viscosity on filtering resistance with variations in filtration velocity
Observed data for air and sulfur dioxide compared with calculated values obtained b y correcting values observed for air by ratios of kinematic and absolute viscosities of sulfur dioxide
I n all cases, experimental data on the cylinder gases are more nearly in agreement with curves computed from absolute viscosity corrections of test data based on air. These results support Lapple's contention that the pressure drop across the cloth in a fabric dust filter is a function of the absolute viscosity of the gas.
Conclusions Cloth filtration has become a popular method of removing particulate solids from hot gases. Experience dictates certain design modifications to adapt the conventional standard fabric dust collector to the filtration of hot gases. I n conjunction with any of several available methods for gas cooling, synthetic fabrics, mainly Orlon and Fiberglas, have provided economical cloth life in filtering corrosive aerosols at temperatures well above those a t which natural fiber filter fabrics can be employed. The determination of cloth area for a filter t o be used in hot gas cleaning depends on one factor-operation a t an acceptable pressure drop across the cloth throughout a predetermined cycle. Both Williams and coworkers and Lapple suggest that the increase in pressure loss with time be computed from formulas containing a proportionality factor which includes variables for porosity of the dust layer. Each of these authors suggests that the proportionality factor, determined empirically for any given aerosol, will remain a constant independent of the thickness of the dust layer or fabric construction. Experimental work cover-
May 1955
Because of marked changes in gas viscosity at elevated temperatures, it is necessary to apply a viscosity correction in estimating filter resistance. Williams and coworkers conclude that the pressure loss through a filter is a function of the kinematic viscosity of the gas; whereas, Lapple advances the theory that the absolute density governs. The resistance of a given gas in passing through a filter bed is a function of the absolute viscosity and not the kinematic viscosity. Because of the failure of the resistance-dust loading relationships advanced by both Williams and coworkers and Lapple to compensate for variations in porosity of the deposited dust on a filter surface, their use must be approached with caution. Particularly in the design of filtration equipment for hot gas cleaning, it is desirable to conduct pilot scale tests at the job site as an aid in estimating operating resistance of the equipment under consideration. I n such tests, it is advisable to duplicate anticipated operating conditions insofar as possible in order to eliminate variables. Where such tests are not possible, as in new plant design, i t is necessary to depend on experience with approximately similar applications in sizing filtration equipment.
Nomenclature gravity constant, English units
5
=
IC
= constant depending on particle shape, surface properties
= increase in filter resistance due to dust load, feet of fluid,
air
of dust, and units employed, dimensionless permeability constant of filter mediums, including residual dust, if any, inches water gage/cu. ft./min./ sq. ft./min. (always determined with clean air) K1 = specific resistance or resistance factor of the de osit, inches water gage/pound dust/sq. ft. cloth a r e a i o o t / minute filtering velocity L = dust loading in air coming to filter, grains/cu. ft. Permeability = air volume/sq. ft. of cloth a t 1/2-inch water gage
KO
=
INDUSTRIAL AND ENGINEERING CHEMISTRY
965
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT R Rf R,
= = =
t
=
T v
= =
V
=
VIA
=
w.g. = 6
=
increase in filter resistance in time, T maximum filter resistance, inches water gage initial filter resistance, inches water gage time in seconds to reach H feet resistance time in minutes to reach R inches resistance average filtration velocity, feet/second filtration velocity, ft./min., or cu. ft./min./sq. f t . cloth, or air to cloth ratio ratio of volume to surface area of the particles on the filter, feet water gage porosity of the filter bed = volume of voids divided by total or bulk volume of deposit
p po
=i
gas density, pounds/cu. ft.
= real density of dust particles, pounds/cu. ft. = gas viscosity, English units
Literature Cited (1) Hemeon, W.C.L., Heatingand Ventilating, 37,191 (August 1940). (2) Lapple, C. E., Chemical Engineer's Handbook, 3rd ed., p. 1013,
McGraw-Hill Book Co., New York, 1950. (3) Pring, R.T., A i r Repair, 4, No.1, 1-6 (1953). (4)Williams, C. E., Hatch, T., and Greenburg, L., Heating, Piping A i r Conditioning, 12,259-63 (1940). R~CEIVE forD review September 29, 1954. ACCEPTED March 10, 1955.
Catalytic Reactivation of Air Purification Systems AMOS TURK,
in
Tarrywile Lake Road, Danbury, Corm.
The cost of operating an activated carbon system to purify heavily contaminated air i s high because of rapid carbon saturation necessitating frequent steam reactivation. To reduce this cost, a new method has been developed for rapid, inexpensive reactivation in place of activated carbon without the use of superheated steam. The carbon is impregnated with a small amount of catalyst of moderate activity, without impairing its adsorptive properties. When the carbon i s saturated, reactivation is effected b y warming it in an air stream until the adsorbate is completely oxidized on the catalyst-carbon surface. An activated carbon with high kindling temperature permits quantitative partition in oxidizing all the adsorbate but none of the carbon. Successive reactivations do not alter the distribution of pore diameters of the carbon, nor diminish its adsorptive capacity. Recycling of the air at the start of the process prevents atmospheric discharge of any desorbed pollutants. Data are given for various sorbates.
E
XTRANEOUS atmospheric vapors t h a t constitute a nuisance to man because of odor, irritation, toxicity, or other reasons can be controlled by a variety of techniques. These include suppression of the source, dilution with pure air, modification of an unpleasant olfactory stimulus by admixture of the offending vapors with other odorants, and physical or chemical decontamination methods. Recently this subject has been reviewed by the author (6, 8). Many such pollution problems are caused by complex mixtures of vapors, with the result that selective methods for control of individual substances or simple mixtures cannot be used. When it is desirable or necessary to purify an air stream by removing from it such a complex mixture of contaminating vapors, a practical, nonselective abatement method must be used. The two methods of this type which are available fbr commercial application are oxidation and adsorption. Oxidation. The complete incineration of a stream of odorous vapor t o its ultimate oxidation products is a theoretically satisfactory method of odor nuisance abatement. Some exceptions include organic sulfur compounds, whose oxidation products include sulfur dioxide and sulfur trioxide, which are odorous gases. A more serious objection t o this method, however, lies in the capital and operating costs required to heat large volumes of contaminated air t o incineration temperatures. Partial incineration is often a poor solution because intermediate products of oxidation of organic vapors may be as odorous or more odorous than the unoxidized material. The addition of a catalyst to aid the process of incineration by reducing the ignition temperature necessary for its oxidation is a very valuable technique in industrial odor nuisance abatement ( 4 , 7 ) . By this method, a suitable cata-
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lyst will initiate oxidation a t a temperature as low as 500" F. When the heat content of the contaminated air is sufficiently high, the process, once started, will sustain itself without further addition of heat from an outside source. This method is, therefore,. most economical when the temperature of the contaminated air is close t o 500" F. a t its source or where its heat content is sufficiently high SO that outside addition of heat is unnecessary once catalysis has been initiated. The method is not as economical, but still often practical, when the heat content of the odorous vapors is sufficient to provide a significant portion of the energy required to maintain the temperature needed for efficient catalysis. I n such case, the necessary size and operating cost of the heating installation will be proportionately reduced, but not eliminated. I n practice, catalytic combustion is practical a h e n the concentration of organic vapors in the contaminated air is one hundred to several hundred p.p.m., but safely below the lower explosive limit. Adsorption. Adsorption of organic vapors by activated carbon is a satisfactory method of air purification (6). When the odor concentration is high, however, this method suffers from the practical disadvantage that frequent reactivation of the carbon is necessary. The method for reactivating carbon used for air purification usually involves removing the containers which house the carbon and returning them to the manufacturer. The carbon is then subjected t o superheated steam a t 1000" to 1300' F., after which it is ready for service again. Reactivation of the carbon in situ with superheated steam is another possibility but usually involves a prohibitively expensive installation. The service life of an adsorption system between reactivations is given by the Formula 1.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47,No. 5
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