Design Features of Stirred Tank Bioreactor

Stirred tank bioreactor and its main component

• Most important bioreactor for industrial application (Low capital and operating cost)

• However, no single system adequately meets the needs all biological systems can be constructed.

• Laboratory scale bioreactors liquid volume < 10 L constructed out of Pyrex glass.

• For larger bioreactors, stainless steel is used. Stainless steel = refers to various alloys of primarily iron, nickel and chromium.

•Different grade of SS = 302, 304, 316, 318. (higher the number, the greater the resilience of the steel.

•316 L – most widely used (L indicates the steel has low carbon content)

•The stainless steel used in bioreactors are polished to a mirror finish (makes cleaning and sterilization easier).

•Components joined in an oxygen-free environment to avoid corrosion, displace by argon (TIG technique, Total Inert Gas)

STANDARD GEOMETRY OF A STIRRED TANK BIOREACTOR

Geometry measurement of stirred tank bioreactor

• A mechanically stirred tank bioreactor fitted with a sparger and a Rushton turbine impeller will typically have relative dimensions (Table 1)

Ratio   Typical values

Remarks

Height of liquid in reactor to height of reactor

HL/Ht ~0.7-0.8 Depends on the level of foaming produced during the fermentation

Height of reactor to diameter of tank

Ht/Dt ~1 - 2 European reactors tend to be taller than those designed in the USA

Diameter of impeller to diameter in tank

Da/Dt 1/3 - 1/2 Rushton Turbine reactors are generally 1/3 of the tank diameter. Axial flow impellers are larger.

Diameter of baffles to diameter of tank

Db/Dt ~0.0.08 - 0.1

 

Impeller blade height to diameter of impeller

W/Da 0.2  

Impeller blade width to diameter of impeller

L/Da 0.25  

Distance between middle of impeller blade and impeller blade height

E/W 1  

Table 1: Geometry Dimension of Stirred Tank Bioreactor

A tank's height:diameter ratio is often referred to as its aspect ratio. A stirred tank bioreactor is approximately cylindrical in shape. It has a total volume (Vt) of 100,000 litres.

Dt:Ht 0.50

Da:Dt 0.33

Db:Dt 0.10

The geometry of the reactor is defined by the following ratios:

•Ex:

A cylindrical reactor has a liquid volume of 100,000 L. It has an aspect ratio of 1:1. The height of the liquid in the reactor will be approximately….??

Head space volume•A bioreactor is divided in a working volume and a head space volume.

•A working volume

= fraction of the total volume taken by the medium, microbes and gas bubbles

= 70- 80% of the total fermenter volume

= but depending on the rate of foaming formation during fermentation.

•The remaining volume is called the head space volume.

A modern mechanically agitated bioreactor will contain:

        An agitator system         An oxygen delivery system         A foam control system         A temperature control system         A pH control system         Sampling ports         A cleaning and sterilization system.     A sump and dump line for emptying of the

reactor.

Main components and parts of stirred tank biorector

AGITATION SYSTEM (Agitator and Baffles)

Function: provide good mixing and

thus increase mass transfer rates through the bulk liquid and bubble boundary layers.

provide the appropriate shear conditions required for the breaking up of bubbles.

Stirrer's shaft seal

- subject to high mechanical loads- Important for good operation; to avoid

risk of contamination; shaft jamming, possible leak.

- All component with contact to the product must have smooth surfaces and may not have any recesses

- < 1 L bioreactor = plastic coated magnetic rods are used located on bearings on the bottom (associated with possible aeration problems) or suspended for rotational movement.

Magnetically coupled agitator from ELECTROLUX for pilot scale.

•Stirred vessels > 10 L volume, mechanical drive coupling together with rotating mechanical seals.

•Seal components: carbon and ceramic.

•Laboratory scale: simple rotating mechanical seal.

•Larger scale: double-action rotating mechanical seals; achieving longer operation times.

•The rotating mechanical seal must be free from cracks and the cavities between the packing must be steamable.

Rotating mechanical seals

Drive Configuration (the drive for the agitator shaft)

Can be installed either above through the reactor cover or from below through the bottom flange.

Bottom drive; - leaves cover free for the installation of other components and connection, the agitator shaft can kept shorter.

Top drive; - easily protected against leakages; sterility is easier to maintain.

Agitator (has to fulfill the following tasks)

-   Dispersion of culture air in the form of bubbles and the creation of higher transfer rates at the gas/liquid interface for supplying oxygen to the microorganisms and extracting CO2.

- Creation of constant living conditions (substrate, pO2, pH, temperature, etc.) by homogenous distribution.

-   Improvement of the heat transfer at the heat transfer surfaces for dissipating the heat generated by the biological reaction and mechanical work.

Different types of impeller

EKATO INTERPROP impeller

Multi-blade disc impellers- produce radial flow and high energy dissipation density in the proximity of the agitator.

Propeller impellers- create an axial flow (for mammalian cell cultivation), low shear forces in connection with low oxygen requirements are applicable

INTERMIG impellers- create both radial and axial flows, for high viscous media or products

Turbine – normally for loop systems(mixing effect is achieved by circulating the liquid)

EKATO INTERPROP – improved heat and mass transfer, high dispersion of drops and bubbles, homogenous distribution of solid particles as well as low investment costs.

The function(s) of the agitation system include;

increasing mass transfer rates, especially KLa, through the bulk liquid

providing appropriate levels of shear increasing heat transfer rates reducing the size of boundary layers

KLa can be influenced in 3 ways by agitation;

a) The impeller can break up the air into smaller bubbles thus increasing the gas/liquid interfacial area

b) Agitation can delay the loss of air from the bioreactor.

c) Turbulence shear can reduce film thickness at the gas/liquid interface.

Effect of Impeller Spacing on Flow Patent and Power Absorption

a) Spaced too closely – multiple impellers tend to behave like a single large impeller.

b) Spaced too far apart – appearing regions in the liquid (Stagnant area).

- Optimal spacing is about one impeller apart, with the lowest impeller about one impeller above the bottom of the vessel.

Impellers have 2 distinct function

I) To provide mixing by pumping liquid round the vessel. For this function, it requires a large diameter, low speed impeller with a small number of blades.

II) To dispersed the injected gas steam as small bubbles and re-disperse coalesced bubbles. It requires a high speed, small diameter impeller with a large number of blades

Agitators characteristics:

Radial Flow

- the liquid flow from the impeller is initially directed towards the wall of the reactor

Axial Flow

- The liquid flow from the impeller is directed downwards towards the base of the reactor

Radial flow impellers

Radial flow impellers - Shear characteristics

Flow pattern using radial flow impellers.

Radial flow impellers

Radial flow impellers - Rushton turbine

Generation of high shear conditions by radial flow impeller

Six bladed Rushton turbine impeller

Function of Rushton turbine impeller.

Radial flow impellers - Rushton turbine

•Mixing is achieved with the use of baffles.

•higher input energy

•Mixing is not efficient as axial flow mixing

Axial flow impellers

Marine and hydrofoil impellers

Flow pattern of axial impeller

•More energy efficient, effective at lifting solids from the base of the tank, low shear properties

•Use for shear sensitive process such as crystallization, precipitation reactions and culture of animal cells.

•Not suitable for bacterial and fungal aerobic fermentation (ineffective breaking up bubbles).

Axial flow impellers - Intermig Impeller

Flow pattern created by INTERMIG impeller

•- for microbial fermentation

•Bottom: has large axial flow section

•The tips of impeller contain finger like extensions, create a turbulence wake for breaking bubbles.

•Overall shear conditions in the reactor are lower than would be generated by a radial flow impeller (Rushton turbine).

Top entry and bottom entry impellers

Top driven impeller

Bottom driven impeller need higher maintenance due to damage of the seals by particulates in the medium and by medium components that crystallize in the seal when reactor is not in use

 Baffles (Aid in satisfactory mixing, heat and mass transfer)

Liquid mixing;

a) Baffled

b) Unbaffled

Unbaffled bioreactor. Note the presence of a

large vortex. The liquid is circulating around the

impeller.

Baffled bioreactor. Note the presence of small

bubbles from gas entrainment and the absence of a large

vortex.

Formation of eddies by baffles

Baffles break the liquid flow lines causing the formation of turbulent eddies.

OXYGEN DELIVERY SYSTEM

Consists of: a compressor inlet air sterilization

system an air sparger exit air sterilization system

A compressor – forces the air into the reactor, need sufficient

pressure to force the air; for large reactors, produce air at 250kPa.The air should be dry and oil free so as to not block the inlet air filter (not to use “instrument air”)

Air sterilisation system – to prevent contaminating organisms from

entering the reactor as well as to prevent the microorganism in the reactor from contaminating the air.

Common method: filtration Smaller bioreactor (<5 L) – disked shape

hyrophobic Teflon membranes housed in a polypropylene housing.

Teflon – tough, reusable and does not readily block.

Membrane filter house with

Teflon for air sterilization

Membrane filters housed in

polypropylene cartridges are used.

•Large bioreactor (> 1000L) – pleated membrane filters housed in polyproplylene cartridge.

•By pleating the membrane, it is possible to create a compact filter with a very large surface area for air filtration. Large scale membrane filtration is very expensive process.

•filtration is not possible; too expensive; Heat sterilisation (steam is use to sterilize the air)

Condenser – simple heat exchanger through which cool water is passed; minimize water evaporation and the loss of volatiles; Drying the air also prevents blocking the exit air filter with water

•For small reactor, the exit air system, will include a condenser.

AIR SPARGER

Ring Sparger

•The air sparger breaks the incoming air into small bubbles

•The sparger must located below the agitator to facilitate bubble break up

Formation of bubbles from ring sparger

•During emptying of a fermenter, it is important that the air feed valve is closed. This will minimize contamination of the inlet air line.

Slow impeller speed

Fast impeller speed

Effect of agitation speed on bubbles distribution in liquid

•Slow impeller speed – bubbles will not be broken, rise directly to to the surface, accumulate and coalesce under impeller leading to from large bubbles and low oxygen transfer rates.

•Fast speed – smaller bubbles will be generated, move throughout the reactor increasing the gas hold up and bubble residence time.

AIR FLOW RATES

•Volume per volume per minute, vvm.

•The airflow rate and liquid volume must have the same basal unit. The air flow rate must be expressed in terms of volume per min.

CONDENSOR TEMPERATURE

A cold condenser temperature can help to control the foam. The density of the foam increases when it moves from the warm headspace volume to the cold condenser region. This causes the foam to collapse

THANK YOU