Department of Biotechnology

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AGITATORS(IMPELLERS)

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Dr. Jitender Kumar

Head, Biotechnology Department

HMV,Jalandhar

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AGITATOR (IMPELLER)�

• The agitator is required to achieve a number of mixing objectives, for example, bulk fluid and gas-phase mixing, air dispersion, oxygen transfer, heat transfer, suspension of solid particles, and maintaining a uniform environment throughout the vessel contents.
• It should be possible to design a fermenter to achieve these conditions; this will require knowledge of the most appropriate agitator, air sparger, baffles, the best positions for nutrient feeds, acid or alkali for pH control, and antifoam addition.
• There will also be a need to specify agitator size and number, speed, and power input.

AGITATOR (IMPELLER)

• It should be possible to design a fermenter to achieve these conditions; this will require knowledge of the most appropriate agitator, air sparger, baffles, the best positions for nutrient feeds, acid or alkali for pH control, and antifoam addition.
• There will also be a need to specify agitator size and number, speed, and power input.

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Types

• Agitators, depending on their type will impart either axial flow (parallel to the impeller shaft) or radial flow (perpendicular to the impeller shaft).
• Agitators may be classified as disc turbines, vaned discs, open turbines of variable pitch, and propellers together with more recent designs.
• The disc turbine (or Rushton turbine) is the most widely use fermenter agitator and consists of a disc with a series of rectangular vanes set in a vertical plane around the circumference and the vaned disc has a series of rectangular vanes attached vertically to the
• underside.

Features

• Air from the sparger hits the underside of the disc and is displaced toward the vanes where the air bubbles are broken up into smaller bubbles.
• The vanes of a variable pitch open turbine and the blades of a marine propeller are attached directly to a boss on the agitator shaft. In this case, the air bubbles do not initially hit any surface before dispersion by the vanes or blades.

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Major Types

• Disc turbine
• Vaned disc
• Open turbine, variable pitch
• Marine propeller

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History

• Since the 1940s a Rushton disc turbine of one-third the fermenter diameter has been considered the optimum design for use in many fermentation processes.
• It had been established experimentally that the disc turbine was most suitable in a fermenter since it could break up a fast air stream without itself becoming flooded in air bubbles.
• This flooding condition is indicated when the bulk flow pattern in the vessel normally associated with the agitator design (radial with the Rushton turbine) is lost and replaced by a centrally flowing air-broth plume up the middle of the vessel with a liquid flow as an annulus.

History

• The propeller and the open turbine flood when Vs (superficial velocity, ie, volumetric flow rate/cross-sectional area of fermenter) exceeds 21 m h–1, whereas the flat blade turbine can tolerate a Vs of up to 120 m h–1 before being flooded, when two sets are used on the same shaft.
• Besides being flooded at a lower Vs than the disc turbine, the propeller is also less efficient in breaking up a stream of air bubbles and the flow it produces is axial rather than radial . The disc turbine was thought to be essential for forcing the sparged air into the agitator tip zone where bubble break up would occur.

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Working

• In other studies it has been shown that bubble break up occurs in the trailing vortices associated with all agitator types, which give rise to gas-filled cavities and provided the agitator speed is high enough, good gas dispersion will occur in low viscosity.
• It has been also shown that similar oxygen-transfer efficiencies are with them also.

Working

• In high-viscosity broths, gas dispersion also occurs from gas filled cavities trailing behind the rotating blades, but this is not sufficient to ensure satisfactory bulk blending of all the vessel contents.
• When the cavities are of maximum size, the impeller appears to be rotating in a pocket of gas from which little actual dispersion occurs into the rest of the vessel.

Design

• Another is the Prochem Maxflo agitator. It consists of four, five, or six hydrofoil blades set at a critical angle on a central hollow hub.
• A high hydrodynamic thrust is created during rotation, increasing the downward pumping capacity of the blades.
• This design minimizes the drag forces associated with rotation of the agitator such that the energy losses due to drag are low.
• This leads to a low power number and the recommended agitator to vessel diameter ratio is greater than 0.4. when the agitator was used with a 800-dm3 Streptomyces fermentation, the maximum power requirement at the most viscous stage was about 66% of that with Rushton turbines.
• The fall in power was also less in a 14,000-dm3 fermentation.
• The oxygen-transfer efficiency was also significantly improved

Scale up

• The Scaba 6SRGT agitator is one, which at a given power input, can handle a high air flow rate before flooding .
• This radial-flow agitator is also better for bulk blending than a Rushton turbine, but does not give good top to bottom blending in a large fermenter which leads to lower concentrations of oxygen in broth away from the agitators and higher concentrations of nutrients, acid, or alkali or antifoam near to the feed points.

Mixing

• Recently, a number of agitators have been developed to overcome problems associated with efficient bulk blending (mixing) and oxygen mass transfer in high viscosity.
• One approach is to combine two classes of impeller—one for mixing, the other for oxygen transfer.
• The second approach is to use a novel impeller type, which may also be used in combination.

References

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• Biotechnology: Expanding Horizon – B.D. Singh (Kalyani Publication)
• Biophysical and Biochemical Technology – Wilson and Walker (Cambridge University Press)
• Principle of Gene Manipulation and Genomics – Primrose (Blackwell Publication)
• General Microbiology by Stainier
• Biotechnology: Expanding Horizon – B.D. Singh (Kalyani Publication)
• Biophysical and Biochemical Technology – Wilson and Walker (Cambridge University Press)
• Principle of Gene Manipulation and Genomics – Primrose (Blackwell Publication)

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• THANK YOU