Static Seals: Silicone is the best choice for the head plate and ethylene propylene diene monomer (EPDM) for all other static seals. Teflon has better temperature resistance, but it cold



Axial flow (Low viscosity)

Axial flow (Low viscosity)

Curved blade
Axial flow (high viscosity)
Helical ribbon
Straight blade

Pitched blade

Marine propeller Figure 3.9 Impellers used in stirred tank reactors flows and does not stretch. Viton™ has good temperature resistance and is quire serviceable, but it hardens.

Rotating Seals: There are two types of rotating seals used in agitated vessels, stuffing box packings and mechanical seals

Stuffing box packings: Stuffing box packings have been used successfully in large fermentors (size 100 m3 and more), because there is a difficulty in the manufacture of large mechanical seals of size 0.2 m in diameter. The sealing rings in stuffing box packings are compressed in the axial direction first. With subsequent radial deformation, the installed sealing rings are pressed against the stuffing box boring and the spindle.

Mechanical seal: The best choice is a double mechanical seal. Tungsten carbide seats are more expensive than the commonly used ceramic and carbon faces, but they give longer seal life and less maintenance. The static shaft seals should be EPDM, except for hydrocarbon fermentations, for which Viton is recommended. In the conventional design of double mechanical seals, which derives from the chemical process industries, the seals are mounted back to back. Potential problems with this design are that the housing pressure for this design must be kept higher than the vessel pressure. If the top seal leaks, coolant flows into the vessel. Finally, solids can accumulate in the pocket between the vessel flange and the shaft static seal.

The back to back design works very well for chemical reactors, but in chemical reactor design asepsis is not a consideration. For aseptic operation, the top seal is reversed, and the seal housing is operated at atmospheric pressure. This design eliminates pockets where solids can accumulate, does not allow flow into the vessel, compensates automatically for head pressure variations, and results in lower temperature operation of the seals. Steam flows through the housing during sterilization, and sterile steam condensate flows through it during fermentation to provide lubrication and cooling. This design is simpler than the one which must be used for back to back seals.

In addition to the above seals, magnetic couplings have also been used in small fermentors. Magnetic couplings enable noncontact torque transmission to the vessel without any penetration of the vessel. For fermentation with recombinant organisms or mammalian cells, magnetic couplings are usually employed. Since power transmission ceases whenever the design load is exceeded, magnetic couplings are protected against overload. Magnetic couplings require no external bearings; however, the bearing in the reaction chamber may be susceptible to failure due to particulate solids in the form of cells or media.

Table 3.9

Operating range of impellers with respect to viscosity

Impeller Type Viscosity Range (mPa s)

Turbines 1-50000

Propellers 1-10000

Anchors 100-5000

Paddles 100-50000

Gate anchors 1000-100000

Helical screw 5000-500000

Helical ribbon 10000-5000000

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