INDUSTRIAL AND ENGINEERING CHEMISTRY
860
signal. The signal can he picked up by an appropriate recording instrument. Thus, the grease-filled gage may be readily converter to a recording instrument. Conclusion
The grease-filled pressure gage is a cheap, sensitive, and accurate means of measuring polymer pressures. I t is virtually unaffected by changes in temperature, its range may be changed quickly, and its maintenance is simp1e’ It can be wherever there is room for a half-inch hole, it, can be placed very close to moving parts, and it occupies no volume inside the proeessing equipment. The gage is easily adapted for continuous recording of pressure.
Vol. 45, No 4
Acknowledgment
The author is indebtcd to many of his fellow workers for suggestions for improving the gage, and to H. G. Ryan, Jr., ‘who drew thP figures. literature Cited (1) Gilmore, G. D., and Spencer, K. S., Modern Plastics, 27, 143 ff. (1950). (2) Rhodes, T , J , , “Industrial Instrumentsfor Measurement and Control,” 1st ed., p. 31, S e w York, McGraw-Hill Book Go., 1941. ACCEPTEDXovember 2 2 , 1952.
RECEIVED f o r review .Sugust 19, 1952.
8
c e e
icrofiltratio Irnpregnated
e EDWARD
D. KANE
Cuno Engineering C o r p . , Meriden, Conn.
F
ILTRATIOY, the separation of solids from a liquid suspending medium by the passage of the fluid through a membrane or barrier, has long been recognized as a chemical enginering unit operation. The chemical engineer today, faced with a problem of solid separation, has a great many methods a t his disposal. A recent summary of the field was presented by hIiller (10). As the first step toward the selection of filtration equipment for a particular application, the engineer must consider the following important factors ( 4 ) : Maximum allowable percentage of suspended solids in the filtrate Physical characteristics of the suspended solids to be removed (size, shape, and nature) Amount of material to be removed These considerations apply broadly to the entire field of filtration. This paper, however, is concerned only with the narrower field of clarification-Le., the removal of a small amount (less than 2%, generally) of a micronic-size contaminant, to obtain a filtrate that is essentially clear. If the solid suspended matter iE not a semicolloidal material which under a pressure will distend and “slime” over a media, or a true colloidal material which exhibits a Brownian motion, the use of a clarifying element without a filter aid can be considered. Examples of these easily filterable solids are sand. silt, pigments, and automotive sludges. In fact, any rigid material present in rather small amounts and in a particle size range below the commercially available screen or cloth sizes (approximately 50 microns) can be filtered with these clarifying units. Screens or cloths can be used to filter in the micronic range with the use of filter aids, but large and expensive equipment is required. The author has recently completed a literature survey in connection with micronic filtration for the Ordnance Department of the Army, and, a t that time, the lack of emphasis on the use of the inexpensive types of micronic filters was noted. The purpose of this paper is to point out the uses of these filters and, in particular, to discuss in detail the application of a woolen fiber resinimpregnated type of cartridge. Types of Filter Elements. In the field of micronic filtration or clarification with inexpensive throw-away types of filter elements,
there are three general classes. Surface or edge-type filters are generally paper or resin-treated paper. They will handle particle sizes dom-n to the range of 1 micron or thereabouts. They operate very much like ordinary filter paper in that once a particle of contaminant has lodged on the surface, the useful life of that portion of the filter has ended. To increase the filter life to an economic length of service, the manufacturers have resorted to pleating, stacking a number of paper disks, or spiral winding of the paper to form a tube, the thickness of which is equal to the width of the ribbon wound. Another type consists of the assortment of powdered metal or porous ceramic media, These materials are made to filter to controllable limits of particle sizes of an assortment of materials suitable for most chemical applications. The last type are called depth-type filters and are either randompacked material or rigid, oriented structures. The randompacked filters would, it is believed, be unsuitable for fluid clarifi-
A
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Figure 1.
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Physical and Chemical Structure of Wool Fibers from Which Filters Are M a d e ( 7 )
April 1953
Figure 2.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Cross-Section Photomicrograph of Flow through a Woolen Micro-Klean Filter Element (5X)
Flop path bottom to top
cation. These xvaste-packed filters were designed as automotive lubricating oil filters, where there is usually a hy-pass system, so that only a portion of the oil is filtered continuously; therefore, any channeling is not considered serious. I n the oriented depth-type filters, there is a filter constructed of a yarn wound helically around a central supporting screen, the direction of the windings being reversed for each layer. The external surface has an imposed diamond shaped pattern, as does each successive layer of crossed yarns. As the yarn is wound, it is napped; the filtration occurs through the napped fibers in these parallelograms and not through the yarn itself as may be supposed. The material most commonly employed in these filters is a virgin cotton. The second type in this class of oriented fibrous filters is represented by a filter made of a resin-impregnated woolen fibrous mags. This paper is devoted to these woolen fibrous impregnated media, as no prior flow data have been offered and the bulk of the experience of this laboratory has been with these units. Resin-Impregnated Wool Filters
Manufacture. The process of manufacture of these woolen resin-impregnated filters is as follows:
Wool of a grade from 64 to 80 is mixed with various amounts of either smaller or larger diameter cellulose fiber, depending on the ultimate particle size retention rating of the unit manufactured. These fibers are accreted on a brass mandrel from a liquid medium under vacuum. The thickness of the batt so formed is a function of fiber crimp, fiber bending strength, and vacuum. The original develo mental work did not overlook other fibers, but wool was the on?y material avallable at that time suitable for felting, which allowed a workable thickness to be obtained. The fact that adjacent fibers do not lie in complete contact and seal off the vacuum helps to explain the thickness. The first fractional thickness has upon it the full vacuum, the next increment has a lesser amount, equivalent to the pressure
861
drop through that first iiection, neglecting for the moment the effects of the perforations around the. mandrel which would cause velocity gradients from the centers of the holes to the solid section6 around them. Ultimately, a point is reached where the available vacuum is insufficient to hold the wool onto the outer edges of the batt. The inches of felting vacuum versus thickness curve is of the general shape of a parabola, y2 = Kx. The value of K and the relative position of the curve is a function of fiber crimp level, fiber stiffness, felting temperature, woo1 consistency, fiber diameter, and water pH. By varying the crimp level a cartridge with a higher dirt holding capacity and the same particle size retention is obtained. The fiber diameter and the fiber length determine the mean minimum particle size retention,. The pH and temperature are important in relation to the chemical structure of wool. Figure 1 shows the chemical composition of the wool protein (7, 9, 13). By varying the p H the degree to which these coils unravel and the extent to which they swell volumetrically is affected. The effect of temperature is more important as it relates to the fluid viscosity than as it speeds up the rate of any chemical reactions. When the consistency is lowered the vacuum is maintained for a longer period of time and thus the packing of the felted fibers is mechanically increased. These batts are impregnated with a phenolic or modified phenolic-type resin. To complete the cure which is limited by the charring temperature of the wool at 285' F., the cartridges are placed in an oven at 280 to 285" F. This process has been patented ( 1 ) . T o make the various density filters rated as lo-, 25-, or 50micron particle size retention elements the furnish-mixture of fibers used in pulp and paper industry--is altered. Thus, by increasing the amount of cellulose fibers approximately 10 microns in diameter, the mean minimum size of the dirt particles retained in the filter is reduced; by including a larger amount of a coarser fiber, the mean minimum size of the dirt particles retained in the filter is increased. Graded Density Filters. In graded density-type filters the ex'ternal surface of the filter has macropores which open into winding, curvaceous, ever-decreaPing diameter channels. Thus, a particle of dirt entering such a cartridge will penetrate until a depth is reached so that it is physically impossible for the particle to continue, because the diameter of the particle is greater than that of the section immediately ahead of it. A second, smaller particle will enter the cartridge, proceed randomly into the filter, and perchance may enter the channel just behind the first lodged particle. The pressure will force it to move into an intersecting channel and it will move along this newer channel until a section is reached that will cauae this dirt particle to lodge because of the decreased diameter of the flow path imposed by the closer packing of the woolen fibers as the inside diameter is approached. These tortuous, decreasing diameter paths are shown in Figure 2. This photomicrograph demonstrates visually the progressively greater fiber density encountered by a fluid as it passes from the outside t o the inside discharge surface. The pressure drop through these filters at flows from 0 to 3 gallons per minute is shown in Figure 3. These flow data constitute a relatively low pressure drop €or the degree of filtration imposed. The dirt-holding capacity, when rated with a fine grade air dleaner road dust (furnished by A. C. Spark Plug Div., General Motors Corp., Flint, Mich.) to a 20 pounds per square inch pressure drop a t 3.0 gallons per minute is: 10-micron, 80 to 100 grams; 25-micronJ 120 to 140 grams; 50-micron 160 to 180 grams. The Arizona road dust used for these tests was a fine grade with the following analysis: Diameter, Microns 0-5
-5-1--n
10-20 20-40
40-80
Wt. % 39 =t 2 18 f 3 16 =t 3 18 i 3 9=t3
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
862
The size of the test filters was 23/4-in~houtside diameter, 1-inch inside diameter, and 73/4 inches long. These capacity figures are for air cleaner road dust only. The dust is added in 20-gram increments; each successive increment is added after the system is visually clean. Cleaning Efficiency Test. A cleaning efficiency test is run using various grades of American Optical Co. graded dust. Thus a 10-micron cartridge is rated with a No. 3031/2 (9-16 microns), 25-micron cartridge with a No. 302l/2 (22 t o 32 microns), and the 50-micron cartridge with a KO.302 (29 to 46 microns). A 100gram sample of the dust is added to the system all at once. Samples are taken after 1, 2, 5, and 10 minutes recirculation. If the 10-minute sample shows nil per cent by volumeof the contaminant, then the filter has met its specification. The actual capacity is a function of both particle size and particle size distributions.
Figure 3.
Flow Curves for Filter Elements
Vol. 45, No. 4
A partial list of fluids currently filtered with these woolen impregnated cartridges includes acetic acid (any strength), acetone (50% aqueous), alcohols (methanol through butyl alcohol), ammonia (anhydrous), benzene, butyl acetate, carbon tetrachloride, cellulose acetate, chloroform, cyclohexane, diethylene glycol, ethyl acetate, ethylene dichloride, ethylene glycol, Freon, hydrochloric acid (10% concentration), inorganic salt solutions (pH below lo), monocyclic terpenes, monoethanolamines, odichlorobenzene, perchloroethylene, naphtha, natural gas, oleic acid, p-dichlorobenzene, petroleum ether, polyvinyl alcohol, styrene monomer, sulfuric acid ( 12% concentration), vinyl acetate, and vinyl chloride. From this list it can be generalized that these resin-impregnated woolen cartridges are suitable for use with most common organic solvents and aqueous solutions of salts, up to a pH of 10, with a lower tolerance toward organic fluids of similar alkalinity. These cartridges are stable with nonoxidizing acid solutions. They can be used in a moderately destructive medium if the dirt load is such that the cartridge will end its useful filter life prior to softening because of chemical action. It is generally advisable, if there is some doubt, to run a laboratory filtration test on the material and definitely determine these facts. These cartridges have the incidental advantage of being water separators in a two-phase system. The hydrophobic surfaces of the cartridges are used to maintain instruments in a dry condition, as they will not pass water droplets; the surfaces are ineffective against water vapor, however. It is also possible to coat these cartridges with a hydrophilic material; thus a hydrophobic filter followed by a hydrophilic filter will serve to arrest both water and oil droplets from an air supply line.
I1
Velocite oil at l o o o F. Average dala for ten elements of each size
The test stand used for the tests of dirt-holding capacity and degree of filtration and for subsequent data is shown in Figure 4. When operating to determine the dirt-holding capacity of a particular filter, the system containing 5 gallons of Velocite C is first brought to operating temperature of 100" F. when the specific gravity is 0.865 and the viscosity is 25 centipoises, with a new filter in place. Valves G and H are closed and F opened. 20 grams of air cleaner road dust are added to sump I . The v a h are then reversed, forcing the flow through the sump and.carrying the road dust to the filter a t K . The pressure drop is noted. After 5 minutes recirculation, the procedure is repeated. This continues until the pressure drop reaches the specified limit of 20 pounds per square inch. T o determine the cleaning efficiency, valves G and H are kept closed and the American Optical Co. graded dust is added directly to the svstem a t sump AT. The system with Velocite C is kept at 100 F."and the samples taken as noted. The stand can also be used to measure flow and pressure drops by keeping valves G and H closed and using valves D and F to regulate the flow through the filter. The resultant pressure drops are measured a t the gages, J . O
Applications The factors limiting the use of these cartridges from a chemical standpoint is the structure of the wool fiber and the chemical stability of the phenolic-type resin. I n this laboratory the adsorption of active chemicals from the filtrate is not a problem. I n automotive lubricating oil filtration this is important, since the oils contain special additives. A study was carried out by Bridgeman ( R ) which showed that wood pulp, cellulose, and paper were adsorptive in that order. Rodman demonstrated with motion pictures the adsorptive characteristics of cellulose and the lack of adsorptiveness of wool ( l a ) .
0 Figure 4.
Test Stand
Used for Evaluating Filter Performance
A. Sample drain line valve B. No. 1 Brown and Sharp gear pump C. Line heater D. By-pass control valve E. By-pass return line F. Valve to control Row path G,H. Valves to operate dirt addition
I
1. 1.
Did addition sump Pressure gager
K. Test filter assembly L. Rotameter M. Flow return line N. Test stand sump
The size of the filter unit can be varied by changing the felting mandrel, sawing to any desired length, or trimming the outside diameter to whatever size is necessary. The standard size is 1-inch diameter center hole, 23/4-inch diameter over-all, and 93/4 inches long. These units can be mounted in housings holding 1to 215 cartridges a t the present time. The major limitation is the mechanical design of the housing. The size unit recommended for any specific application is dependent generally on laboratory tests or previous field experience. A series of equations is being formulated which will predict the relationship of fluid flow, fluid viscosity and density, dirt load, and filter density to the ultimate retention efficiency and filter unit size.
Theory The available data on flow through porous media are not directly applicable to this system. They refer either to porous spherical packings (5),oriented fiber systems (6, I C ) , air flows through random fibers (8)or air flow through fabrics (11).
April 1953
INDUSTRIAL AND ENGINEERING CHEMISTRY
The approach of Rainard (11) to his problem of air flow through fabrics parallels this problem. He begins with Poiseuille's law stating that in streamline flow of fluids the differential pressure is in direct proportion to the flow. Hagenbach (6) proposed a correction to account for the deviations which he thought were attributable t o energy changes caused by an increase in fluid velocity in moving from a reservoir to a tube.
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2
3
863
holding capacity and the amount by which the average radius changes from the outside t o the inside discharge surface, but not to the surface configuration. This being the case, the length of travel of the fluid, or the effective cartridge thickness, should remain essentially constant, as should the number of channels, n; but the average radius of the channels should decrease as dust is added and the cartridge loaded. With a 25-micron cartridge a flow test was conducted in the stand previously described using clean Velocite C oil a t 100' F. (specific gravity 0.865, viscosity 25.0 centipoises). A 50-gram sample of fine grade air cleaner road dust was added (no attempt was made to measure the filtration efficiency in these tests). When the system appeared visually clean, a similar flow test was repeated. This procedure was followed, with the exception that the last addition was only 25 grams. At the completion of the test, there were approximately 125 grams of dust in the cartridge. The flow data are presented in graphical form in Figure 5. From these curves the slopes, K1, were calculated, and the intercepts, Kz, were measured. These data are given in tabulated form in columns one and three of Table I. Table I also includes the relative slopes and intercepts, calculated by dividing the slopes and intercepts by the 50-micron element figures, in the one series, and the clean 25-micron element in the dirt-loading series.
GAL./MIN.
Figure 5.
Flow Curves Showing Effects of Increased Dirt Load and Particle Size Retention
The final equation as derived from Rainard's paper (11), is given as
where AP = pressure drop across the element, lb./sq. inch q = flow through filter element, cu. inches/sec. m = a constant to account for the variation in velocity gradient through the diameter of the tube p = fluid density, lb./cu. inch = conversion factor, 386 (lb./lb, force) (in./sq. see.) gc R = radius of the channel, inches n = the number of channels through which the Auid moves L = the length of the channel, inches q = fluid viscosity, Ib./(in.)(sec.) The equation can be simplified by combining the constants in any given system t o
'.
This equation is a straight line; as the radius of the channel is increased, the slope of the line will decrease. I n addition, K Zis a function of L, the length of travel through the cartridge, and this quantity does not appear in Kl. Thus, if the test filters were loaded with various amounts of contaminants, and if the constants were t o change by the same relative amounts, then it is logical to conclude that the changes are due primarily to changes in R, the average radius, which appears in both expressions rather than to changes in L or even n, the number of channels. I n Figure 5 are plotted the flow data for lo-, 25-, and 50micron cartridges. Inspection shows that as the average radius of the channels is increased, demonstrated by considering the lo-, 25-, and 50-micron elements, in that order, the slope lessens and tends toward zero. The intercepts have changed despite the identical value of L in all cases. These facts can all be explained on the basis of changes in the average radii of the channels. The 10-micron cartridge has the highest values of both slope and intercept, and these values decrease as the radii increase. With these facts as a background, the flow characteristics of a 25-micron cartridge progressively loaded with air cleaner road dust caa be studied. The cartridge performance ie a function of the internal dirt-
Table 1.
Slopes, KI, and Intercepts, Kz, Calculated from Figure 5 Ki K2 Filter Density Rating, Microns K1 K I Reference Kz K2 Reference 10 25 50 (Reference) 25 125 Grams 100 Grams 50 Grams Clean (Reference)
-0.4 -0.15 -0.1
4.0 1.5 1.0
3.2 1.3 0.65
5.0 2.0 1.0
-0.18 -0.15 -0.10 -0.07
2.3 2.0 1 3 1.0
2.7 2.2 1.7 1.2
2.2 1.8 1.4 1.0
The two K's change by the same relative amount in each horizontal row. This indicates that only the values of the average radii of the channels are changing, and that L and n remain essentially constant. (As the cartridge plugs up, the average L should increase somewhat, because the fluid must take a longer path to reach the inside discharge surface.) The agreement is not good enough to warrant any stronger conclusion than an indication a t the present time. A more precise measurement on a new test stand currently being designed should afford a better correlation. As Rainard (11)points out in his remarks relating to air permeability of fabrics, from a strictly logical point of view the data obtained simply justify an empirical equation,
It is impossible to give exact meaning to terms K1 and Kz because it is not possible to experimentally determine the number of macropores on the surface of a cartridge, nor to measure to the radius and length of each channel. A certain property of this filter has been measured which can be used to identify a filter cartridge. There is a great deal of work yet t o be done in relating KI and Ks to actual filter performance in the field. Fortunately, these coefficients can be obtained without destructive tests, which will allow the correlation K1 and Kz with performance data. Summary The major points dictating the use of clarifying filter elements are: (1)the dirt load must be relatively low, (2) the solids should not require a filter aid to prevent sliming, (3) the degree of clarification desired should be relatively high, and (4) the particle sizes of the contaminant should lie in the micronic range. The graded density cartridges afford a large dirt-holding capacity,
INDUSTRIAL AND ENGINEERING CHEMISTRY
864
relativelh- lo\\ pressure drop, and are priced lo^ enough to he discarded nhen they reach the end of their useful filter lifp, as indicated by the pressure differential. The initial cost of any of these t: pes of c*artridge clarifiers is relatively IOT, as they are generally installed in the pipe line and the fluid is filtered in transit. These filteis are not the aim\-er to any and all filtration clarification problems but do have a distinct function in clarifying fluids where it is important to have a clean, clarified fluid in the chemical process plant. literature Cited (1) Anderson, L. E. (to Cuno Engineering Corp.), U. S. Patent 2,539,767 (Jan. 30, 1951). ( 2 ) Bridgeman, 0. C., e l ul., Trans. SOC.Automotite Engrs., 1, 309 (1947). (3) Brownell, L. E., and Kate. D. L Chem. Eng. Progr., 43, 537 (1947).
a a a a
a
Vol. 45, No. 4
(4) Dickey, G. D., and Bryden, C L., “Thcory and Practice of Fil-
tration,” New York, Reinhold Publishing Corp. 1946. (5) Emersleben, O., P h y s i k , 26, 601 (1925). (6) Hagenbach, J. E., “Viscosity of Liquids,” London, Bell (New York, D. Van Nostrand C o . ) , 1928. (7) Hopkins, G. E., Teztile Research J . , 19, 816 (1949). (8) Iberall, A., J . Research iVut2. Bur. Standards, 45, No. 6 (1950). (9) Lewis, W. K., Squires, L., and Broughton, G., “Industrial Chemistry of Colloid and Amoiphous Materials,” S e w York. Macniillan Co.. 1942. (10) Miller, 5 . h.,Chem. Inds., 6 6 , 38-48 (1950). (11) Rainard, L. W., Textile Research J . , 17, 167 (1947). (12) Rodman. C., and Pansey A. K., Ibid., 20, 873 (1950). (13) Stoves, J. L., Fibers, 302 (Sept,ember 1947). (14) Sullivan, R. R., J . A p p l . Phvszcs, 11, 761 (1940); 12, SO3 (1941); 13, 725 (1942). RECEIVED for review September 17, 1951. - 4 C C E P T E D October 1, 1952. Presented at the XIIth International Congress of P u r e and Applied Chemistry, S e w York, N Y . , September 1951.
Temperature Gradients in Turbulent Gas Measurement of Temperature, Energy, and Pressure Gradients ‘4, G. SCHLINGER, N. T. HSU, S. D. CAVERS’,
AND
6. H. SAGE
Colifornio lnrtitute of Technology, Pasadena, Calif.
T
HERMAL flux and shear a t thz boundaries of a two-diniensional stream must be known in order to determine values of the eddy conductivity and eddy viscosity as a function of relative position in thtl flow channel. I n addition, the distribution of velocity, thermal flux, and tempwature within the body of the stream must be established. The present discussion is concerned with the measurements of temperature, thermal flux and shear in a uniform, steady air stream. In this discussion, the term “uniform flow” implies that properties of the fluid and conditions of flow remain unchanged dong the length of the channel. The measurement and control of temperature offer problems of wide scientific and industrial interest. The basic information concerning the mrasurement of this undefined concept (4,I S ) has been reviewed and assembled in a systematic form ( I ) . No attempt is made here to enlarge or improve upon the basic methods of measurement. The equipment utilized in the determination of values of eddy conductivity and eddy viscosity (18) under conditions of steady, uniform flow has been described (6), and the results are available (68-31). In measurements of this type it is desirable to establish the temperature gradient in the vicinity of the solid boundary of the stream as well as in the turbulent core. The attainment of steady conditions requires careful attention to the control of temperature, thermal flux, and rate of flow. Measurement of the temperature of such a flowing stream with stationary instruments is not in itself a simple problem. The arrangement of the apparatus ( 6 )is shown schematically in Figure 1.
made was 11 inches in width, 0.75 inch in height, and 13 feet in length. The air stream was circulated by means of the blower, F , driven by a variable-speed motor. -4 refrigeration coil, G, and electric heaters, H , were provided to remove or supply energy at a steady rate. After passing the Venturi meter and its approach section, I , the air flowed through the control heater,.J, and a grid-type resistance thermometer, K . After passing through a calming section and a set of screens a t L, the air reentered the channel a t A . The oil circulating systems, not shown in Figure 1, for the closed ducts, D and E, included axial-flow pumps, refrigeration coils, and electric heaters. These heaters supplied energy a t a steady rate. Control was established by small grid-type heaters mounted in the oil circuit.
4
r$r:& -
-V4CUUM
CONNECTION
C ALORIIVIE TER
Essentially it involved a circulating air stream at A which passed between two parallel plates, B and C, each maintained a t substantially uniform temperature by the longitudinal circulation of oil in ducts D and E. The channel where measurements were 1
Present address, University of Saskatchewan, Saskatoon, Saskatchewan,
Canada.
Figure
1, Schematic Arrangement of Flow Channel
