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Fibrous Filters for Air Sterilization. Small Seale Equipment.. Role of Turbine Impellers.. Testing of Filters,.
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Control of Oxygen Uptake.. Aeration Effectiveness.. Microbiologieal Transformalion OF Sferoids
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ELMER L. GADEN, Jr., and ARTHUR E. HUMPHREY’ Department of Chemical Engineering, Columbia University, New York 27, N. Y.
Fibrous Filters for Air Sterilization Design Procedure Proper design of sterilizing filters makes possible considerable reduction in size, accompanied by greater rdiability ,To design a sterilizing filter, information i s needed, from either observation or specification, on concentration of organisms in the inlet air (maximum expected value, if estimated), operating period between filter sterilizations, and allowable penetration probability (the acceptable chance that a single entering organism will penetrate during filter life). With these points established, fibrous media may be tested in the laboratory to establish the relationship among penetration (efficiency), pressure drop, and air velocity. A number of possible filter designs may be proposed and the most economical selected. Filter design must avoid channeling (by providing maximum bed uniformity and effective seal between bed and wall) and bed movement (by constructing the bed to prevent shifting and settling and to minimize fragmentation).
CERTAIN
chemical process applications require substantial amounts of air which is sterile, or nearly SO. T h e aeration of fermentation broths is the 1 Present address, School of Chemical Engineering, University of Pennsylvania, Philadelphia, Pa.
2 172
most important now, but others are under development which may also need large volumes of pure and sterile air. Many schemes for sterilizing air have been proposed and some have been employed commercially with success (7, 74). At present, however, the most common method is filtration through beds of fibers (cotton, glass, and steel wool), granular solids (carbons), or specially prepared filter media. Despite their widespread use, the design of fibrous filters is still largely a matter of judicious guesswork tempered with experience. About the only generalization that can be offered with respect to current sterilizing filters is that they are usually greatly “overdesigned” from the standpoint of size, yet are unreliable in performance. A major difficulty has been the great concern over bed depth as the focal point of filter design. Little or no attention has been given to the manner in which the filter bed is made u p and maintained during operation. Yet experience indicates that these factors are a t least as important as the physical dimensions of the filter in determining its performance. In recent years a great deal has been learned about the mechanics of filter action and the roles of design and operat-
INDUSTRIAL AND ENGINEERING CHEMISTRY
ing variables, particularly with respect to aerosols (3, 76). A number of studies (4, 8, 70, 75) with bacterial aerosols have given information applicable to design of sterilizing filters. In one of these (8) the effect of air stream velocity on the efficiency of specially compounded glass-fiber beds in removing bacterial spores (B.subtilis) from air was examined. From the data obtained in these experiments and the generalizations already available in the literature, a general procedure for the design of fibrous, airsterilizing filters has been established.
Principles of Air Filter Design Any filter design represents a compromise between two somewhat incompatible criteria : high efficiency in removing organisms and low pressure drop ( 8 ) . Practical considerations add to these a third: reasonable service life at the specified efficiency. I t is, of course, impossible to guarantee complete removal of organisms from an air stream with any filter bed whose interstices are larger than the organisms to be removed. Only a statistical prediction of penetration can be made and no design can be better than the “calculated risk” involved in arbitrary
.
Information Essential for Design
Certain essential information must be on hand for any design job; if these data are not known, they must be measured or estimated. The primary items are:
1. Concentration of organisms (contaminants) in the inlet air supply 2. Operating period between sterilizations 3. Allowable penetration probability; the acceptable chance that a single organism will penetrate during the operating period of the filter
Figure 1.
pressure drop across a filter bed is also a function of air velocity. Data for I M F filter beds 0.01 foot thick, expressed in inches of water. are also qiven in Figure 2. Pressure drops for ;his filter material are characteristically low because of its relatively low bulk density (high porosity), Empirical pressure drop equations for filter design may also be found in the literature. Typical of these is the expression proposed by Wong and Johnstone in connection with their study of fibrous filter efficiencies (76) :
Table I.
Bacterial Content of Air
Location A. Large cafeteria during lunch (winter) Hospital surgical room Public schools (summer) Dairy barn
Colonies Enumerated per Cubic Foot of Air 44 13
47 2500
B. Clerical offices
16-66 Large machine shops engaged in engineering 5-28 Normal machine shops 12-28 Special workshops 0.5-334
Table
II.
Filter mat
where P
= =
Of
linear
gas (air) Of gas
An "effective" fiber drag coefficient, CDe, incorporating the effects of fiber orientation, distribution, etc., in a real filter bed, was employed as a basis for correlation. Similar relationships have been presented by Iberall(9) and Kozeny ( 7 7). Kozeny's equations have been modified by Carman (2). Filter Design. The design of a fibrous filter for any specific job involves two distinct but interdependent considerations :
1. What bed depth, or thickness of filter material, is needed to remove a specified fraction of the entering organis, isms a t various air velocities-that what is the velocity-efficiency relationship (Figure 2) for the filter bed? 2. What air stream velocity will yield the most economical filter design consistent with the specified removal requirement? T h e first requires only physical data for the aerosol-filter combination in question; the second involves costs of filter installation and operation.
Penetration-Bed Depth Relationship for Fiber Filters
Penetration Fractional penekration (101) or 0 . 1 (lo2) or 0.01 (lo3) or 0.001 (IOs) or 0.000001 ( 107) (los) (109)
(10'2) a
Organisms out per 106 organisms in 105 104
103 la
1/10 1/100 1/1000 1/1,000,000
Equizalent Remoaal Eficiency 0.9 0.99 0.999 0.999999
etc.
Bed Depth, Required, Multiple of 1
2 3
6 7 8 9
12
From here on, values represent probability of 1 organism penetrating per million entering.
2 1 74
INDUSTRIAL A N D ENGINEERING CHEMISTRY
C o n t a m i n a n t Concentration. Air contaminants arise from the microflora of the soil attached to dust particles which are air-borne by the winds. Generally speaking, except for congested spaces, the microbiological population of the immediate atmosphere is rather small and almost all common species may at times be found. The types actually present in any air supply will depend on the near-by sources of organisms, air currents, temperatures, and humidities. Obviously. only species reasonably resistant to desiccation and sunlight can long persist in the air. Data on the qualitative and quantitative nature of air-borne contaminants in any locality are normally obtained by standard air-sampling techniques. The enumeration of these organisms is, however, no simple task. T h e concentration varies greatly not only from place to place but also at any one location on different days. For example, microorganisms are more numerous in the air during dry weather than just after a rain. Considerable data on the concentration of organisms in air may be found in the literature of air hygiene. Some values given by Luckiesh and Taylor (72) are listed in Table 1,A. Perhaps the most exhaustive enumeration of air-borne microorganisms was carried out under the auspices of the Medical Research Council in England ( 7 ) . These investigators sampled over 10,000 cubic feet of air above the roof of a hospital under all climatic conditions during every season of the year. T h e mean number of bacteria-carrying particles was found to be 2.02, with a low of 0.38 and a high of 14.6. A number of tests were also carried out on factory air; these data are given in Table 1,B. Decker and others (6) have suggested 85 to 230 microorganisms per cubic foot as typical of the concentration range encountered in metropolitan areas. Samplings of the outside air about various pharmaceutical and food-canning plants have generally fallen in the range 25 to 100 organisms per cubic foot. Microorganisms constitute only a very small fraction of the total number of particles found in air. hlthough the
I M P R O V E D FERMENTATION EQUIPMENT & DESIGN count varies widely with location and season, there are roughly from 5,000,000 to 30,000,000 individual particles per cubic foot of air (73). This count includes only particles readily visible under the regular light microscope, mostly soot, ash, and industrial debris in the size range from about 0.2 to 5 or 10 microns. I n setting design specifications it must also be remembered that a sterilizing filter rarely draws fresh air directly. Any number of air-cleaning mechanisms, deliberate or inherent, may precede the filter unit. T h e most important of these are conventional dust filters a t the air intake, wherever it may be, fall-out due to velocity changes in ducts and surge chambers, and air compressors themselves. I n most cases, the actual air concentration a t the filter inlet will, therefore, be far below that of the general surroundings. T h e real problem is that contaminants reaching the filter are probably the smallest and most resistant members of the original population. Operating Period. The longer a filter may be operated between sterilizations, the better. Not only are there obvious savings in steam and labor, but the life of the filter bed material is greatly extended by minimizing the degradation that normally results from repeated steam sterilization. Actually, stwilizing filters, properly designed, can be run for much longer periods than present industrial practice demands. The operating period is often determined by the over-all equipment cycle, as in batch fermentations. I n continuous processes, however, the reliable filter operating period is a determining factor. Allowable Probability of Penetration. Because fibrous filters cannot be absolute, some allowable probability of penetration must be elected as a basis for design. This probability represents, in a sense, the reasonable calculated risk which we are willing to take. A filter may, for example, be designed to have sufficient depth to provide a l-in10 chance that a single organism will penetrate from the total number entering during the filter’s operating period; another may be designed for a I-in-106 chance. I n terms of the log-penetration relationship, a penetration of 0.1 for a given bed thickness (log Cout/Cln= -1) means that, on the average, 10% of the total number of entering organisms will penetrate. This is equivalent to a removal efficiency of 0.9 (90%). T h e data of Figure 2 are given in these same terms; bed thickness gives a penetration of 0.1 or removal of 0.9 a t any given air velocity. Each succeeding equivalent thickness then reduces
the concentration of contaminants in the air stream by another 90% and the penetration decreases exponentially. The total filter bed depth required, X , is then given by the product of exponent of the desired fractional reduction in number of organisms, expressed as a power of 10, and the apprQpriate value of bed thickness ( X g o ) for a 90% reduction, or: X = X g o (-log COut/Cln). T o illustrate the penetration-bed depth relationship, a typical set of values are given in Table 11. Fractional values for the number of organisms penetrating really indicate the probability of penetration for one organism.
001 01
I
I I 8 8 I t I I
,
I 1 1 1 1 1 1 1 1
01
I
ID
AIR VELOCITY - f l /,IO
Filter Selection An infinite number of specific filter dimension combustions may be proposed, once the job has been defined in terms of the three essential specifications: inlet organism (contaminant) concentration, period between sterilizations, and allowable penetration probability. This is the case because depth and cross-sectional area of the filter bed are both functions of air velocity. T h e characteristic removal efficiency a t any’ air velocity (Figure 2) determines the required bed depth. Similarly the volume to be handled (capacity) determines the cross-sectional area of the filter. T h e problem then becomes one of selecting, from the many possible, the most economical filter design. I n essence this selection involves a compromise between “operating costs,” primarily dependent on pressure drop, and “first cost,” primarily a function of filter size or volume. Operating costs should include factors like frequency of replacement of bed materials. T h e primary operating and .fixed cost items, however, usually are pressure drop and filter volume. Design Example. To illustrate the approach to selection of an economic filter design, a sample case may be worked out, using the experimental data for the removal of spores from air streams presented in Figure 2. I n the absence of real cost data, no particular answer may be obtained ; nevertheless, the method can be set forth. Consider as a n example the design of a sterilizing air filter, with the following specifications, for a 10,000-gallon plant fermentation tank: 1 volume of air per volume of batch (min.) = 1200 cubic feet per minute at standard temperature and pressure Contaminant con- 100 per cu. foot centration in (average) entering air
Figure 2. Experimental efficiency and pressure drop data for filter mat
Operating period 100 hours between sterilizations Allowable penetra- 1/106 = 10-6 tion probability The number of organisms entering the filter during its operating period may then be computed :
N,, =
=
air volume X time number inlet organisms X air volume operating time 7.2 X 108, or approximately 10s organisms
This number indicates the total “loading” to be absorbed by the filter during operation. T h e depth of filter bed required may now be calculated for any air velocity from Equation 6:
x=
X90( --log
co“dCln)=
xgd --log N m d N m ) ( 6 ) The ratio NOut/N,, may be substituted for COut/Ci, because the volume of air
Air rate
Figure 3. design
Summary of sample filter
VOL. 48, NO. 12
DECEMBER 1956
2175
considered is constant. I n the example. then : x = xg0(-log 109/10-~) = 15 (xgo) Values of bed depth required a t various air velocities for I M F filters ( 8 ) are listed in Table 111. T h e operating pressure drop across the filter bed required for each velocity may be computed from the data of Figure 2. These values are in “inches of water” for 0.01 foot of the I M F filter mats. Computed pressure drops for the design example are also given in Table 111. Now the cross-sectional area and filter volume for a n air flow of 1200 cubic feet per minute a t various air velocities may be computed (Table 111). T h e volumes shown are those of the bed only (bed depth X cross section) and d o not allow for end spaces, etc. They are, however, proportional to the first cost of the filter unit. I n summary, filter volume and pressure drop for the sterilizing filter postulated in the design example are plotted in Figure 3. I t is now necessary to bring in economic considerations in order to choose a n operating velocity. The curves indicate two regions of minimal operating costs. When power cost (pressure drop) is important, the low velocity region is the more desirable, its only limitation being equipment size. When first cost is important, the high velocity region, a t the point of highest rate of efficiency increase, gives the most economical filter design. Actually, with real cost data a t hand, the total cost of the filter may be differentiated or plotted against air velocity to obtain the minimum value. A third criterion of filter design has been mentioned : reasonable service life. This consideration has not been considered so far. When filter life becomes an important factor, low velocities are better chosen for two reasons. First, the loading (organisms) per unit volume of bed goes down because the same number of particles is being absorbed in a greater volume of filter material. Second, a more uniform distribution of bacterial particles through the filter is obtained with low velocity than with high velocity operation.
T h e consideration of filter life brings up a n interesting point. I n operating a t conditions where inertial effects predominate, the fraction of particles remove by any one thickness of filter is proportional to the air stream velocity in that thickness. If the velocity increases with the depth of the bed, because of changes in effective area or other peculiarities in fabrication, the fraction of particles removed by the layers increases. I t would be possible, therefore, by designing a conical filter bed, to distribute more evenly the loading of the particles on the filter fibers. Such a filter would have the longest life in terms of the amount of fibers used. O n the other hand, the fabrication complications introduced by conical design may be too great to warrant this alternative. I t is interesting to compare the voiumes calculated in Table I11 with those of commercial sterilizing air filters now installed. Many will question their validity, as it appears possible to sterilize a rather heavily contaminated air supply for a large vessel with a very low penetration probability with a filter volume of only a few cubic feet. T h e marked contrast with current practice is really only a n indication of the almost total lack of rational design procedures in the past. Actually, the general procedure, and part of the data (Figure 2), outlined in this paper have been successfully employed for the design of sterilizing filters by a t least one major antibiotic producer. T h e filters designed represent a severalfold reduction in size and have been operated much more reliably than their predecessors. Practical Aspects of Filter Fabrication
In setting u p a filter bed, two practical matters become paramount:
1. Channeling must be prevented; otherwise the bed will be literally shortcircuited over all or part of its depth. 2. Internal movement of the fibrous bed must be eliminated. All too often these matters have been neglected, with the result that filters,
Table 111. Summary of Sample Filter Design Calculations I;MF Experimental Data” XQQ. bed dewth Pressure Pressure drop across for 90% reDrop across Required 0.01 foot of moval (10% Required Velocitg,
Ft./Sec. 0.1 0.5 1.0 3.0
5.0 10.0 a
penetration), inches 1.60 3.60 4.60 2.30 0.60 0.15
filter mat, inches HzO 0.008 0.052 0.125 0.48 0.90 2.15
Depth, Feet 2.00 4.50
5.75 1.87 0.75 0.188
From Figure 2.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Depth, Lb./Sq. Inch 0.058 0.843 2.59 3.24 2.43 1.46
Filter Area, Volume, sq. ft. cu. f t .
200 40 20
6.7 4.0 2.0
400 180 115 12.5 3.0 0.37
though much larger than necessary, are rapidly penetrated. One of the simplest and the most effective solutions to both these problems is the use of a fiber mat of the type used in the experiments reported here and earlier ( 8 ) . By breaking the bed u p into many separate layers of a bonded fiber mat, many of the problems of filter bed fabrication are avoided-for example, uneven fiber distribution, shifting and pulsing of the bed, poor sealing, and channeling along the wall. Individual mat layers may be packed and compressed to give any desired bed (packing) density with suitably designed holddown rings or screens. Furthermore, the bed so established will be far more immune to the settling and fiber fragmentation which often take place after repeated cycles of sterilization and air flow. Bonding prevents local motion and fiber breakdown, and ensures a firmer seal of the fiber ends in each mat against the fiber wall. T h e I M F mats used in practice have even given good service with steam sterilization. literature Cited
(1) Bourdillon, R. B., “Studies in Air Hygiene,” Medical Research Council, London, Spec. Rept. 262 (1951). ( 2 ) Carman, P. C., Trans. Inst. Chem. Engrs. (London) 15, 150 (1937). (3) Chen, C. Y . ,“Filtration of Aerosols by Fibrous Media,” Chem. Rev. 55, 595 (1955). (4) Cherry, G. B., McCann, E. P., Parker, A,, J. Appl. Chem. 1, Suppl. 2, S 103 (1951). (5) Davies, C. K.,Proc. Inst.Mech. Engrs. (London) B1, 185 (1952). (6) Decker, H. M., Geile, F. A., Moorman, H. E., Glick, C. A,, Heating Piping Air Conditioning 23, 125 (1951). (7) Gaden, E. L., Jr., “Sterilization,” Encyclopedia Chem. Technol., vol. 12, Interscience, New York, 1954. ( 8 ) Humphrey, A. E., Gaden, E. L., Jr., IND.ENG.CHEM.47, 924 (1955). (9) Iberall, A. S., J . Research Natl. Bur. Standards 45, 398 (1950). 10) Kluyver, A. J., Visser, J.? Antonie van Leeuwenhoek J . Microbiol. Serol. 16, 311 (1950). 11) Kozeny, J., Wasserkraft u. Wasserwirtsch. 22, 67, 86 (1922). 12) Luckiesh, M., Taylor, A . H., Heating Piping Air Conditioning 19, 113 ( 1 947). (13) Smith, W. J., Stafford, E., “Dry Fibrous Filters for Dust-Free -4ir,” U. S.Technical Conference on Air Pollution, Washington, D. C., 1951. (14) Stark, W. H., Pohler, G. M., IND. ENG.CHEM.42, 1789 (1950). (15) Terjesen, S. G., Cherry, G. B., Trans. Inst. Chem. Engrs. (London) 25, 89 (1947). (16) Wong, J. B., Johnstone, H. F., “Collection of Aerosols by Fiber Mats,” Eng. Expt. Sta., University of Illinois, Tech. Rept. 11, AEC Contract .4T(30-3)-28 (1955).
RECEIVED for review August 9, 1956 ACCEPTED October 23, 1956
