WILLIAM D. MAXON The Upjohn Co., Kalamazoo, Mich. ELMER 1. GADEN, Jr. Department of Chemical Engineeri
Fibrous Filters for Air Sterilization Experimental Studies with a Pilot Scale Filter Pilot plant tests show good correlation with laboratory data for a fibrous filter supplying air for fermentation processes
,A
fibram filter for the sterilization
of air, d e i g n e d on the basis of laboraahdies, w a s checkd in pilot scale The mulh carrdated well with kbomroy data, .although Ihe preaure drop wos Ggher than thof
apparatus.
padid. Steam Mliz&bn hod little affect on Alter efficiency, but caused sanwtllterdeteriororion. loss
of packing material is a function of bdh total length of tMrHiratim time and number of sterilizing cycles.
,
“minimum efficiency”of the filter should not be dangerously low-that is, short periods of operation a t l a than the d b sign air rate muat not result in contamination. D a b n Sp.cMwIion8. with thaw general requirement8 artabliahed, the following design SpKifications were elected.
From this the total conaminant “load” anticipated during an operating period is:
Au flow, atandard cu. feet/hour (manured at 70’ F. and 1 atm.) Tank p m x , lb./sq. inch gage Operating period?b u m Inlet au mntammahon, organisms/
bars for design, the- will be a 3.5 X 10- chance of penetration by a single
cu. foot
A amof fibrour film for
PnOcBDuIlB
100
10
for the design
air atcrihation (7) made me of data obtained in an exputmenurl study of the filtration of hactalal aMloL by An-bonded, glass fiber mam. Facilities were lacking,
’
350 10
b m r , for extrapolation of the pro@ method to lager d e film. A general investigation of the perfarma n a of air filters a t the Upjohn Co. dud an opportunity for testing both the design method and the data on filtercaickncy. Thisnportautnmariaa &e exprrimental reault8 obtained with a pilot sale, fibrour bed filter nrpp1ying air for fermentationproceaaes.
PAP
350 standard cu. fect/hour (100 b u m ) X lo erg= 3.5 cu. hoot
If a penetration of 10”‘ is sclected as a organi~ during ~ the 100-hour run.
operatins ~ndlliom.The operating air velocity bared on the empty filter shell (all air velocities are expnased on this basis) is determined by the c h m filter diameter and the air flow specified in the design. It was calculated to be 3.83 feet per sccond, with an mum4 air temperature of 80’ F. and an average filter prrasure of 11.7 p u n d s per quare in& gage. For this velocity the predicted efficiency [from Figure 3 ( 2 ) ] is 0.22 for six layers of the filter mat. Stated differently, the value of X.0 (bed thickPCM in inches requircd to remove 90% of the entering organisms) is 1.2 inches Figure 2 (7)]. A total bed thickness of 11 (12) = 13.2 inches is then needed for the specified penetration. Actually 600 lavers of IMF filter mat, a nominal bed &ckncas of 12 inches, were used. The prudicted drop for thi bed, again uaing the data of Figue 2 (I), is 2.4 pounda per square inch gage. The dimensions and pnssure drop of this filter design are rulMlnable and thus the original &ice of filter diameter appears to be j u a W . T h e minimum efkiency of the filter was calculated to be 99.98% for 600 layers (at 1.2 f a t per aecond) from Figure 2 (I). This was considered autficicntly great to cantaminadon during short pcri OE o p t i o n at low air rata. F N c F o M r d l o n . Adiagramofthc f i l a i s &own in pisurc 1, The filter body is ‘‘/%+dlstainla s u d *, borcdasnoath to an aaual inaidc diam-
T3
Figure 1.
Air fliter
x 1w organisma
”iAM
F l L L l N G FUNNEL
v ?
8
EXHAUST *
SPORE SUSPENSION AIR FOR STIRRING”
70 PSIGAIR
30 PSlG
+
AIR
DRAIN
t
Figure 2.
8
1
DRAIN
Schematic diagram of air contamination and filter test apparatus I
I
I
AIR VELOCITY
Y
2
I
I
- 3.7 FTJSEC.
6
P
a w 0.
0
z
a
eter is used to set and measure the air flow, but is bypassed at all other times. Air passes up through the filter; steam for sterilization passes down. This is the arrangement most generally used in actual plant installations. Method of Operation. Flowing steam a t 30 to 45 pounds per square inch gage is first passed through the filter for 0.5 hour to sterilize it. Steam pressure is then released and air is passed through the filter for an hour in order to dry out the bed. In actual filter installations air used for drying is not generally passed into the fermentation tank. The air flow is then adjusted and the test begun. Sampling. A sample of contaminated air is withdrawn through a capillary impinger just upstream of the test filter (see Figure 2 ) . The capillary impingers used were of the type recommended by Rosebury ( 3 ) . As the air leaves the sampler, its volume is measured by an accessory rotameter not shown in Figure 2. Aliquots of water from the samplers are diluted appropriately and plated out to determine the number of organisms trapped. Penetration of spores through the filter bed is measured in the manner suggested by Humphrey and Gaden ( 2 ) . The filter bed is removed from the test setup, and disassembled, and the individual filter mats (or successive groups of them) are transferred to sterile Waring Blendor jars containing 200 ml. of water. After blending, aliquots are removed, diluted, and plated out. If desired, a 50-ml. sample may be plated out directly in a 2-quart milk bottle laid flat.
0
-10 Results
-2 NUMBER OF DISKS FROM INLET Figure 3.
Distribution of 6. subfilis spores in
eter of 15,’s inches. I t was packed with 600 I M F filter disks cut to a diameter of 111/16 inches, slightly larger than the filter inside diameter in order to eliminate channeling along the walls. In certain cases, less than 600 disks were used, in order to ensure measurable penetration. Spacer rings, “slip-fit” inside the filter body, are placed in the bed a t intervals to hold the filter disks flat and press their edges even more firmly against the walls. Two dispersion plates, a t inlet and outlet ends, are used to distribute the air stream. These, too, are slip-fits in the filter body, and the upper (outlet) end is provided with a hold-down screw. With this adjustment the upper dispersion plate may be po2 178
IMF filter bed
sitioned to compress the bed to any degree to prevent shifting. In the experiments described here, the bed, normally 12 inches deep (600 mats), was compared to 9 to 10 inches.
‘Test Methods Apparatus. A general diagram of the whole test unit is shown in Figure 2 . Spores of Bacillus subtilis are suspended in distilled water (concentration about 108 per ml.) and fed to a small metal atomizer (Paasche, Type VL, air brush), where they are sprayed into the holding chamber. I n this chamber the spore aerosol is mixed with secondary air and passed to the test filter. A rotom-
INDUSTRIAL AND ENGINEERING CHEMISTRY
Effects of Air Velocity. A number of runs were made a t different air velocities and bed thicknesses in order to permit comparison with the data of Humphrey and Gaden ( I , 2). Some typical results are summarized in Table I. In each case the bed depth considered was only a part of the total packing in the filter. The remaining packing was plated out by the methods described above to determine the number of spores leaving any section. The results are expressed here in terms of k, the average collection efficiency per unit thickness of filter bed, assuming logarithmic penetration. Correlation with the laboratory data is fairly good. However, such factors as high entrance velocity, tending to give higher apparent k values, and the true depth of the packed bed have not been considered. Penetration. The resultsgivenin Table I involve only the inlet and outlet spore concentrations and a “log-penetration’’ relationship is assumed to give values of k. T o check this, the penetration pro-
I M P R O V E D FERMENTATION EQUIPMENT & DESIGN file was actually measured by taking the bed apart in the manner previously outlined. In this experiment the filter was operated under the following conditions: Air rate, standard cu. feet/hour 345 Total air volume, standard cu. feet
Air velocity, feet/sec. Downstream pressure, lb./sq. inch Pressure, drop, lb./sq. inch gage Total air contamination, spores
33,200 3.7
10 3.5 lo9
Artificially contaminated airwas passed through the filter for about 2 hours at a time, at three separate times during the 96-hour operating period. The observed penetration pattern is shown in Figure 3. The fact that penetration was logarithmic (after the first section) shows that the spores did not migrate. They were irreversibly absorbed on the filter bed material. The first section of the filter removes proportionately more spores than succeeding sections. This is undoubtedly due to the high entrance velocity, with resultant higher filtering efficiency, caused by the dispersion plate. In instances where the filter was not dried before use, penetration was much deeper and was not logarithmic. The filters must be dry for effective operation. I n actual operation, contamination may occur during drying. There are, however, only a few minutes before the first layers are dry and even a filter of poor efficiency is adequate for this short period. As a precaution, the air used for drying should not be passed into the fermentor. The pressure drop is higher than predicted from laboratory data: 3.5 against 2.4 pounds per square inch gage. The difference is caused in part by entrance and exit effects but mostly by the tighter compaction of filter disks required for stability of the bed. Effect of Steam Sterilization. In the
previous work ( 2 ) I M F filter beds were sterilized with ethylene oxide. Autoclave tests had shown that this filter material was stable under general steam sterilization conditions, but these are not typical of the rigorous treatment encountered in a commercial filter installation. T o check this point a number of runs were made with different periods of steam sterilization prior to operation (Table 11). Steam sterilization had little effect on filter efficiency, k, as measured by this method. I t did, however, cause loss of the resin (phenol-furfural) binder. After 12 hours of steaming most of the binder remained; after 48 hours it was removed. There are, however, other manifestations of filter deterioration with sterilization. There appears to be a breakdown of the filter bed starting with the end at which steam enters. This results in a loss in filter material from this end but does not affect the packing of the remainder of the bed. This type of deterioration will, of course, decrease the efficiency in proportion to the amount of material lost or disrupted. The amount of material lost can be measured either by weighing before and after sterilization or by determining the pressure drop across the filter under constant conditions of air flow and back pressure. The latter technique gives variable results but was used because it does not involve unpacking the filter. Sixty hours of continuous sterilization with steam a t 40 pounds per square inch gage caused a 20% decrease in pressure drop, while GO hours of intermittent sterilization with 15 operating cycles caused ,a 45y0 decrease. Evidently the loss of packing material is not alone a function of the total length of sterilizing time or of the number of sterilizing cycles but a combination of the two.
Acknowledgment The authors wish to acknowledge the assistance of R. G. Pearson in collecting the experimental data and of T. H. Elferdink and C. W. Means in the design and construction of the filter and test apparatus. They wish to thank H. A. Nelson and D. R. Colingsworth for helpful suggestions and support.
literature Cited (1) Gaden, E. L., Jr., Humphre A. E., IND.ENC.CHEM.48. 2172 2 9 5 6 ) . (2) Humphrey, A. E., Gaden, E. L., ’Jr., Ibid., 47, 924 (1955). ( 3 ) Rosebury, T., “Experimental AirBorne Infection,” Williams & Wilkins, Baltimore, -1947.
RECEIVED for review August 9, 1956 ACCEPTED October 2 3 , 1956
Correct ions In the article on “Difunctional Acids by Petroleum Hydrocarbon Oxidation” [IND.ENG.CHEM.48, 1938 (1956)] on page 1938 footnote 2 should read: “Present address, Food Machinery & Chemical Corp., Princeton, N. J.” In Table V I the structure of dimer g (right center of table below “Flash distn. a t 350’ C.”) should be: (CHz)s-CH-O-CH=CH-(
On page 1946, third column, the structural formulas should be : CHz-CHz-CH2-CHO.
4
booH CH2-CH 2-CH 2-CHOH
I
I co
0
On page 1947, top of first column, the structural formulas should be : R-CH-CH
b-0 .
2-CH 2-CH 2-R
Table II.
LO-OH I
Effect of Air Velocity on Efficiency of IMF Packed Air Filters
(Back pressure, Velocity, feet/second Test filter bed, No. of layers No. of spores entering section No. of spores leaving section - iog (fractional penetration) k = bed depth - inches (nominal) Expected k, from laboratory data (8)
10 lb./sq. inch gage) 1.22 2.75 600 470 1.7 X lo’ 8.6 X lo7 21 140 0.50
-1
0.31
3.70 375 9.0 x 108 12
0.65
1.05
0.56
1.0
Effect of Sterilization Time on Efficiency of IMF Packed Air Filters
Sterilizing steam pressure, 40 lb./sq. inch gage. Time of sterilization, hours Test section, No. of layers No. of spores entering section No. of spores leaving section k
12 375 9.0 x 10s 12 1.05
Air velocity, 3.7 feet/sec. 30 48 360 385 2.5 X lo8 2.3 X 25 12 0.97 0.94
lo8
I
COOH
COOH OH
R-CH-CH
Table 1.
CHz)z
I
’
+
2-CH z-CH-R
’
CARLN. ZELLNER FREDLISTER Th‘e senior author “Heterogeneous Acid tive Cellulose Fibers” 48, 1183 (1956)l is E.
of the article on Hydrolysis of Na[IND.ENG.CHEM. H. Immergut.
Frederick S. Mallette, coauthor of the Workbook Feature on “StandardizaGon Aspects of Air Pollution Control” [IND. ENG.CHEM. 48, 69 A (September 1956)] is no longer with Resources Research, Inc., but is a consultant with headquarters a t 56 Winthrop Drive, Riverside, Conn. VOL. 48, NO. 12
DECEMBER 1956
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