Contamination Free Air Supply

7/24/2019 Contamination Free Air Supply

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Contamination Free Air Supply(With Diagram)

The methods destroying microorganisms from air to supplycontamination free air in bioprocess which however are not inpractice now include the following:

1. Dry heating by gas fired or electrically heated system

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2. Adiabatic-compression

3. rradiation

The methods which remove particles or microbial load from airstream and of which one of them being industrially!commerciallypracticed include:

". #crubbing

$. %lectrostatic precipitation in cyclone separator

&. #ieving

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'. (iltration through fibrous beds

). (iltration through granular beds


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*f these methods industrially applicable and fruitful are those of+2, +', and +),. (iltration through fibrous materials such as glass

wool is by far the most common of the three.

Basic Requirements of an Air Sterilizer:For sterilization of air for inustrial fermentationpractices the sterilizer must satisfy the follo!ing ma"orrequirements:+a, Design of the system should be simple.

+b, The operation cost of the euipment should be cheap.

+c, t should remove or destroy air-borne contaminants to the

e/tent necessary for satisfactory fermentation performance.

+d, n case of repeated steam or chemical vapor sterili0ation itshould be stable.

+e, t should condition the air i.e. it should remove any oilentrained during compression and adusting the temperature andhumidity to a satisfactory range for fermentation.

+f, t should be able to supply sterile air continuously.

Air Sterilization #sing $eat:any years ago many investigators showed that acillus subtilisspores can be sterili0ed by a passage through an electrically heatedfurnace. They also concluded that the e/it air temperature of22$45 from the furnace was sufficient to 6ill all the spores of astrain of . subtilis with an e/posure time of 7." 8 7.& sees.

Disa%antage:9ery costly method. Also it reuires proper cooling of air beforesending to fermenter which imparts e/tra cost.

Sterilization &y Air Compressor:


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n some fermentation processes the air sterili0ed by adiabaticcompression was used. #tar6 and ohler designed a smallreciprocating compressor to compress the air to 177 p.s.i forsupplying sterile air for seed tan6 fermenter for pilot plant studies

of the aerobic 23 butylene glycol process for aerobic culture of A.ory0ae and for some other purposes.

They demanded reciprocating compressors operated at pressureup to 177 psi. might be preferable for smaller installations but atlow pressure turbo-compressors with suitable filters would bemore economical for large capacities.

5hain et al. also investigated the sterility of air compressed to 3-"

atm. by a reciprocating piston compressor and reported it to bepractically sterile in agreement with the findings of #tar6 andohler. They used this air in conunction with glass wool forlaboratory scale fermenter and claimed not to have e/periencedany contamination.

Disa%antage:9ery costly method to be applied to large industrial fermentation.

Air Sterilization &y 'ranular Filters:Amongst all the granular materials granular carbon has beenfound to be the best granular filter material for air sterili0ation.

Adsorptive power of charcoal may be the cause of its selection tofiltering medium.

A typical carbon filter si0e for 27777 gallons fermentor was asfollows:

aterial of construction ; #teel cylinder Diameter ; &< to =lass wool does not pose any ha0ard.

+e, 5ollection efficiency is high.

+f, @ow pressure drop of air flow through the fibrous filter bed.


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+ Filtration effecti%eness of fi&rous filters:t was revealed from earlier observations that penetration ofmicroorganisms into a filter was logarithmic in nature. That is thelog ratio of the number of organisms entering a filter +1, to those

leaving +2, a particular depth of filter @ was a function of thedepth i.e.

where 6 ; filtration constant and is a function of the followingfactors:

+a, Air velocity

+b, (ilter density

+c, (ilter si0e

+d, Density of organisms to be removed.

The filtration constant 6 is usually e/pressed in terms of thedepth of filter necessary to remove =7B of the entering organism+@=7,:

This way of representation ma6es it easier to visuali0e the filterperformance in terms of physical si0e.

, -ffect of air %elocity on filtration:


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9arious wor6ers performed e/periments in terms of @=7and thesignificances of their e/perimental data are illustrated in (ig. ".1.

(rom this it is seen that the nature of bacterial collection is such

that for a particular filter there is an air velocity at which filtrationefficiency +1!2, is a minimum.(or glass fiber 1& C diameter.

hen air velocity ; 1 fps

then 1!2; minimuminimal efficiency occurring at inter-medial air velocity is due tothe action of different forces in collecting air borne particles atdifferent air velocities.

-ffect of .o! Air /elocity:At low air velocities particles are e/erted by gravitationaldiffusional and electrostatic forces and their effect is inverselyproportional to the air velocity.

-ffect of $igh Air /elocity:

At high air velocities inertial forces act on the particles. Theseinertial forces are directly proportional to the air velocity. Thenature of inertial effects is such that below a certain critical air

velocity collection due to inertial forces is 0ero. or6ers havereported that this air velocity corresponding to inertial impactionis given by


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Ca ; Air viscositydf; (iber diameterpp; Density of air borne organismdp; article diameter5 ; A correction factor for deviation from #to6eEs law called slipflow factor.

(or collection of unit density 1C bacterial particles from airstream at room temperature and pressure the velocity is eual to

Fegardless of air velocity some collection always occurs due to thefact that air borne particles possess a finite si0e and will beintercepted by some fiber bloc6ing an air stream along with whicha particle moves. 5ollection must always be greater than that dueto interception as it represents the minimum collection physicallypossible.

n case of collection of phage only diffusion effects are importantat reasonable air velocities. #o phages are most efficiently collectedat low velocities. 5ollections of phage are not as efficient as

bacteria. ut if phage are unusually small i.e. less than 7.7$C orthe filter fiber are uite small less than 2C in diameter theirefficiency of collection approaches that of bacteria.

ow uestion may arise whether the filter design is based on

bacteria or on phageG

#ince little is 6nown about the loading of microorganisms in airstream and since loading can be determined for any plant locationit seems most practical for the present to base filter design oncollecting bacteria.


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0 Design proceure:*he current thin1ing on the esign of a pac1e filter forsterilizing air inclues the follo!ing steps:+a, roper assessment of filtration ob

+b, %stimation of the filter effectiveness for the particular filtermedium

+c, 5hoice of filter si0e from cost consideration

Assessment: t involves

+i, Determination or setting of contaminant loading of air

+ii, 5hoice of what allowable penetration of these contaminantspermit.

+iii, 5ontaminant loading of air varies depending on variousfactors. A good sound design figure when e/perimentalobservations are lac6ing might be $7 microorganisms +m.o, percubic foot of air.

+iv, The allowable penetration of m.o. must be less than one m.o.during the course of fermentation. A figure involving 1 to 1777chance of single m.o. penetrating the filter during particularfermentation should be amply safe.

2 -conomic esign of fi&er filter:Design shoul &e &ase on:+i, (i/ed height to diameter ratio of a filter bed i.e. height should

be fi/ed for a given diameter.

+ii, 5onstant height where diameter is variable.

asic euation of log penetration law for designing of a fiber filterfor air sterili0ation is as given by euations ".' to ".=


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3 -%aluation of &e epth for sterilization:*ne way of e/pressing the efficiency of air filter is

here

1; o. of organisms in entering air2; o. of organisms in leaving air5 ; 5oncentration of microorganisms in incoming air

H ; 9ol. flow rate of air in ft!minute

t ; eriod of operation minute

p ; Allowable chance of penetrating the filter

5ombining euations +".=, and +".17, one gets

4 .og penetration la! an its assessment:


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f air borne microbes number passing through a cylindrical bedof fibrous materials are collected uniformly along the bed depth+collection efficiency ; IJ, of single fiber whose vol. fraction inthe bed is a and leaving 2number of microbes in aerosol the

relation between overall collection efficiency I and bed depth @ islogarithmic and is e/pressed as

n addition the following correlation was used by Aiba based on

the empirial correlation presented below

I ; I7+1 K ".$ L, +".1$,here I7is overall collection efficiency of a single fibre and thevalue of a being bound in the limit 7ML M7.17.5 Single fi&re efficiency:n collection of air borne particles!microbes by a single fibre thefollowing mechanisms are presented and supported bye/perimentation to play maor role.

+a, nertial impaction

+b, nterception

+c, Diffusion


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n the relation 5 is 5unnighamEs factor for slip flow ppis thedensity of the particle dpis its diameter v7is the velocity of up-steam air pais air viscosity and dfis fibre diameter. t has beenobserved by @ongmuir that when ; 1!1& the value of r,E is 0eroat the critical air velocity for this condition to e/ist is given ineuations ".$ and ".&.t shows the relationship of radius of dpwith 95with Caas aparameter for a given df. Nowever if the flowing air borneparticles had no mass the flow of air across the fibre would have

been closed to streamline. The entrained particles in thisstreamline air would have been collected by direct contact or

interception with the fibres. (or such condition @ongmuirprovides the appro/imate relation to compute IO7+the single fibreefficiency due to interception, as


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here F is universal gas constant is Avogadro number +&.72 /17 molecules!g.mol, and P is mean free path of the particles.


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6 7ac1ing of the filter:The si0e of the filter may be ascertained using the followingrelation.

9 ; @.A

here 9 is filter bed volume @ is its height and A is cross sectionalarea of the bed. The amount of fibre material +, to be pac6ed inthis bed volume is computed by the following euation.

n which ; eight of the housing fibre material pf;Density offibre material lf;Thic6ness of fibre and f ; Allowable free space

above and below the pac6ing material.8 Cost consierations:

An economical fibre filter design as one in which total cost will beminimum. The total cost +5, for the filtration ob may bee/pressed by

n these relations 5e; 5ost of euipment!?nit weight A1; Annualdepreciation allowance 5f; 5ost per unit weight of fibre material

Ar; Annual freuency of pac6ing the filter and 5m; 5harge perman-hour. Design should be based on minimum cost. inimumload corresponding to minimum cost must be ta6en intoconsideration for design purpose. The minimum cost is


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ascertained as shown in the (ig. ".3.

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Contamination Free Air Supply