DESIGN OF TYPICAL FERMENTOR

DESIGN OF TYPICAL FERMENTOR (AERATED STIRRED TANK BATCH

FERMENTOR)

1) Fermentor Vessel:

A suitable vessel is used to carry out the whole fermentation process. The vessel is

capable of being operated aseptically and meets the control requirements. The vessel is

designed in such a way that it requires minimal labour operation and maintenance. It

has smooth internal surfaces and a similar geometry. Depending upon the fermentation

process two types of vessels are used. For small scale fermentation, glass vessels are

used and for industrial scale stainless steel vessels are used.

Stainless steel is the most satisfactory material for large scale fermentations. These

vessels can be sterilized in situ and hence have the capability to withstand pressure and

corrosion. The corrosion resistance of stainless steel is dependent upon the thin

hydrous film on the surface of the metal. This film is stabilized by chromium and is

continuous, non-porous, insoluble and self-heating. Corrosion resistance can be

improved by tungsten, silicone and other elements.

Capacity:

The capacity of the fermentor may range from a few hundred to several thousand

gallons. The capacity of the fermentor is usually stated on the basis of the total volume

capacity of the same. Thus, based on total volume capacity the fermentors are of

following types:

i) Small Laboratory fermentors (range from 1-2 liters with a maximum up to of 12 –15

liters.)

ii) Pilot plant fermentors (range from 25 –100 gallons up to 2000 gallons) (100 to 400

liter)

a. to 1, 00,000 gallons.) (20000 to 40000 liter)

iv) Horton spheres fermentors (range from 2, 50,000 to

Actually the working volume in a fermentor is always less than that of the total volume.

In other words, a ‘head space’ is left at the top of the fermentor above the level of

fermentation media. The reason for keeping a head space is to allow aeration,

splashing and foaming of the aqueous medium. This head space usually occupies a fifth

to a quarter or more of the volume of the fermentor.

2) Heating and cooling apparatus (Temperature control):

Temperature control is achieved by a water jacket around the vessel. This is often

supplemented by the use of internal coils, in order to provide sufficient heat-transfer

surface. Heat is provided by passing hot water through these coils. During fermentation

process, heat is generated due to microbial activity and mechanical agitation which is

maintained by observing readings on thermometer fitted to fermentor.

3) pH Control:

pH control is achieved by acid or alkali addition, which is controlled by an auto- titrator.

are available and are used according to the requirements. The shaft passes through a

bearing in the lid of the fermentation tank. It is rotated with the help of an electric motor

mounted externally at the top of the tank. The liquid medium is thrown up towards the

walls of the fermentor while rotating the impeller blades at a high speed.

Impellers are classified as:

Disc turbine: It consists of a disc with a series of rectangular vanes. The vane disc

again has a series of rectangular vanes attached vertically on the underside.

Air from the sparger hits underside of the disc and is displaced towards the vanes

where the air bubbles are broken up into smaller bubbles.

Variable pitch open turbine: It also contains vane disc, the vanes of which are attached

to the blades of a marine propeller on the agitator shaft. The air bubbles in this turbine

do not hit any surface before dispersion by vanes or blades.

The liquid medium is thrown up towards the walls of the fermentor while rotating the

impeller blades at a high speed. This results in the formation of a vortex, which is

eliminated, usually by four equally spaced baffles attached to the walls of the fermentor.

the aerobic microorganisms for metabolic requirements. Sparger introduces air into

liquid in the fermentor.

Usually, the aerating device consists of a pipe with minute holes, through which

pressurized air escapes into the aqueous medium in the form of tiny air bubbles. The

size of the holes in a sparger ranges from 1/64 to 1/32 of an inch or larger, holes

smaller than this requires too high air pressure for economical bubble formation. One

should always remember that the smaller the air bubbles, the greater is the bubble

surface area. It is desirable to adjust the size of the air bubbles to give the greatest

possible aeration without greatly increasing the overall cost of the fermentation process.

The reason for this is that sterile air is a costly item for large-scale fermentation.

The cheapest means of sterilization of air is to pass it through a sterile filter composed

of glass wool, carbon particles or some other finely divided material that will trap

microorganisms present in the air. Spargers in fermentors for growth of mycelium

forming organisms often utilize 1/4 inch holes to prevent plugging of the holes by hyphal

growth. Pipes crimped at the end or with a single small hole to produce a stream of air

bubbles also are employed in some instances.

The air bubbles from the sparger are picked up and dispersed through the medium by

the action of the impeller blade mounted above the sparger. In some very large

fermentation tanks, an impeller is not utilized. The medium is stirred by the directed rush

of air bubbles from a sparger at the bottom of the tank. These tanks are specially

designed and usually do not contain baffles.

7) Feed ports:

Feed ports are silicone tubes connected to the nutrient reservoir. They are used to add

nutrients and acid/alkali in the fermentor. They are sterilized in situ with stem after

connection has been completed and before any additions are made.

8) Foam Control (Antifoam agents):

Foams are dispersions of gas in liquid. Aeration and agitation of a liquid medium can

cause the production of foam. This is particularly true for the media containing high

of the tank and be forced from the tank along with the exit valve. This condition often

causes contamination of the fermentation from organisms picked up by breaking of

some of the foam which then drains back into the tank. Excessive foaming also causes

controlling foam is to add an antifoaming agent, although a supplementary impeller

blade mounted high in the tank may at times be effective.

When antifoam is required in a tank, it is added either manually or electrically.

Obviously, manually addition requires that someone continuously observe the tank so

that the antifoam can be added as required. addition of antifoam is usually preferred. To

accomplish this automatic addition, a sensing mechanism is employed to determine

when the foam has risen into the head space of bioreactor. Such a device is provided

with two electrodes mounted in the top of the fermentor. These electrodes are

connected to a pump associated with a reservoir of sterile antifoam and as the foam

rises in the reactor it touches the two electrodes in the process allowing current to flow

between them so as to activate the pump for addition of antifoam. The foam then

collapses away from the electrode thus breaking the electrical connection between them

9) Valves:

Valves are required to control the flow of gases and liquids in a variety of way. Five

types of valves are used.

Globe valves: They are suitable for general purposes on stem or a water line for use

when fully opened or fully closed. They do not regulate flow.

Butterfly valves: They are used in large diameter pipes operating under low pressure.

They are not suitable for aseptic operation.

Ball valves: They are suitable for aseptic condition. They can handle mycelial broths

and can be operated under high temperature.

Diaphragm valves: They are used for flow regulation and for stem services within

Safety valves: Safety valve is incorporated in every air or stem vessel and pipe layout

which is subjected to work under pressure. These valves ensure that the pressure never

exceeds the safe upper limit of the specified value. In simple safety valves, a spindle is

B) Multiple-stage continuous bioreactors

This consists of a series of fermentation tanks. The medium is led into the first and the

out flow into the second, third, or fourth as the case may be. This is most frequently

used for the fermentation involving metabolites. The first tank may be used for the

growth phase and subsequent tanks for production.

c) Recycled single or multiple stage continuous bioreactors

The out flowing broth may be feed of the organisms by centrifugation and the

supernatant returned to the system. This system is particularly useful where the

substance is difficult to degrade or not easily miscible with water such as in

hydrocarbons. Recycling can be applied in a single stage fermentor. In a multiple stage

fermentor, recycling may involve all or some of the fermentation vessels in the series

depending on the need.

perforated pipes or plates, or metal microporous spargers. The vessel used for bubble

4-6. The rate of flow of gas affects the O2 transfer and mixing.

iv Airlift bioreactors – Airlift bioreactors are commonly used for aerobic bioprocessing

technology. In the airlift bioreactors, the medium of the vessel is divided into two

interconnected zones by means of baffle or draft tube. The air/gas is pumped into one

of the two zones referred to as ‘riser’ and the other zone that receives no gas is known

as ‘downcomer’. The dispersion flows up the riser zone while the down flow occurs in

the downcomer. Further there are two types of bioreactors.

1) Internal loop bioreactor - These bioreactors have a single container with a central

draft tube that creates interior liquid circulation channels which keeps the volume and

from one bioreactor (maintained at temperature 300C are pumped into another

bioreactor (at temperature 420C). This is done because it is very difficult to increase the

temperature quickly from 300C to 420C in the same vessel. The cells are grown in the

first bioreactor and with the help of the fitted valves and a transfer tube and pump, they

are transferred into the second bioreactor, where the actual bioprocessing takes place.

4) Tower bioreactors - In this type of bioreactor, a high hydrostatic pressure is generated

at the bottom of the reactor which increases the solubility of O2 in the medium. Since

the top is expanded, the pressure is reduced which helps in the expulsion of CO2. The

cycle completes with the medium flowing back into the downcomer. The advantage with

Tower bioreactor is that it has high aeration capacities without having moving parts.

v) Fluidized bed bioreactors–

These bioreactors are mainly suitable to carry out reactions involving fluid suspended

biocatalysts such as immobilized enzymes, immobilized cells, microbial flocs etc. The

design of the bioreactors is such that the top is extended and the reaction column is

narrow which retains the solids in the reactor and allows the liquids to flow out. To

maintain an efficient operation of fluidized beds, gas is sparged to create a suitable gas-

liquid-solid fluid bed. The recycling of the liquid ensures continuous contact between the

reaction contents and biocatalysts which increases the efficiency of bioprocessing.

vi) Packed bed bioreactors-

A packed bed bioreactor consists of a bed of solid particles, with biocatalysts on or

within the matrix of solids, packed in a column. The solids are generally porous or non-

porous gels which may be compressible or rigid in nature. The nutrient broth

continuously flows over the immobilized biocatalyst and the products are released into

the fluid from where they are removed. However, due to poor mixing, it is difficult to

control the pH of packed bioreactors by the addition of acid or alkali.

iv) Photobioreactors–

These bioreactors are specialized for fermentation that can be carried out either by

exposing to sunlight or artificial illumination. The photobioreactors are made up of glass

or transparent plastic which are the solar receivers. The cell cultures are circulated

through the solar receivers by using centrifugal pumps or airlift pumps. These

bioreactors work in the temperature which ranges from 25-400C. In these bioreactors,

the microorganisms e.g. microalgae, cyanobacteria etc. grow during the day time while

the products (e.g. beta-carotene, asthaxanthin) are produced during the night.

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Scale up and scale down of fermentation

Two of the most common phrases one often met in fermentation technology research

are 'Scaling up' and 'Scaling down' studies. However, the phrase 'scaling up' is more

commonly understood and practiced during the designing of industrial scale fermentors.

Whereas 'scaling down' studies are rarely heard that frequent.

A good example is when we intend to start with a fermentation process with the ultimate

objective of producing the fermentation products on the level of industrial scale.

Products need to be produced at large volume so that the process is economically

fermentation process is technically and economically viable to be produced in the end at

a large scale.

SCALE UP STUDIES

INITIAL SCALE UP STUDIES

Most scale up studies are usually carried at different phases involving different scales of

fermentors. Preliminary work is carried out at the level of petri dishes and small scale

laboratory fermentors to establish whether the process is:

Technically viable, meaning it is possible to produce such fermentation process and the

products on the small scale. Additional parameters not provided by petri dishes studies

and for more confidence are obtained by carrying further studies using submerged liquid

fermentation using various sizes laboratory scale fermentors and even a pilot plant

fermentor.

There are a few rules of the thumb followed when doing scale up studies such as: a)

Similarity in the geometry and configuration of fermentors used in scaling up b) A

minimum of three or four stages of increment in the scaling up of the volume of

fermentation studies. Each jump in scale should be by a magnitude or power increase

and not an increase of a few litres capacity. Slight increase in the working volume would

not yield significant data for scale up operation.

It must be appreciated as the size of fermentation increases during scale up various

parameters measured might not show a predictable linear co relationship. Certain

parameters change some remained constant. Some parameters need to be modified

and adjusted during scale up studies. The objective is to try to get the same

values.

The exercise in scaling up involved a number of programmed research or steps that has

to be established so as to predict the final behaviour of the final large scale production

4. Shear conditions, foam production.

SCALE DOWN STUDIES