Design Criteria

The objective of fermentor design is to produce fermentation systems necessary to build economical production facilities that satisfy well considered performance criteria. Reactor design and scale up considerations are driven by the need to provide the organism optimal conditions for producing the desired product uniformly in the reactor (188). Mechanical Aspects

Mechanical design aspects are important for the successful operation of any fermentation plant. A few design ratios, which have gained significance irrespective of the fermentation system employed, include: vessel diameter to height ratio (should be in the range of 1:3-1:4), and impeller diameter to vessel diameter ratio (should be in the range of 0.3-0.5).

The mechanical design aspects with reference to the geometrical requirements of a fermentor are discussed in Standard Dimensions (Section 3.9.2).

Some practical aspects before getting on to the technical details of vessel design are:

1. Space requirements: The vessel dimensions must be chosen to meet plant space limitations. Poor choice of equipment sizes can cause inordinately high building costs.

2. Transportation: Shopbuilt vessels are usually less expensive and of higher quality than fieldbuilt vessels.

3. Special heads: A hemispherical bottom gives better mixing and less shear in tissue culture vessels than does a standard dished head. Process Aspects

With the lack of adequate information on fermentor design for a particular fermentation process, design has been an applied art rather than a science. It requires a careful consideration of a host of interrelated factors, many of which are risk assessments and value judgments. Therefore, it is imperative that the design engineer applies experience and judgment in reaching reasonable compromises.

The fermentation process guidelines commonly employed without proper consideration of the process aspects do very little to promote good design. These include:

1. Aeration rate: The airflow rate to the fermentor must be generally one volume of air per liquid volume per minute (1 vvm). However, in practice, the aeration rates depend on oxygen solubility in media, and 1 vvm may be too high for a particular fermentation process when operated at a larger scale.

2. Impeller tip speed: The tip speed of a fermentor impeller must not exceed 7.6 m/s.

3. Arrest of fermentative metabolism: At the end of fermentation, the fermentor broth must be cooled immediately and stored at 4°C, to arrest the fermentative metabolism.

4. The maximum production rate cell mass theory: Process optimization is achieved by obtaining very high cell mass concentrations at very high growth rates.

5. Oxygen transfer rate: The consequences of increasing OTR by increasing air flow rate and agitation could lead to foaming, increased gas holdup (189), higher gas velocities, higher vessel pressure, and oxygen enrichment.

6. Heat transfer rate: Heat transfer usually is the limiting constraint for highly aerobic large scale fermentors. Oxygen and heat transfer rates are coupled closely in aerobic fermentations. The total heat generated during growth (Qtotal) is given by:

Qtotal = Qmetabolic ^ Qmechanical (3.12)

where Qmetabolic is the metabolic heat generated and Qmechanical is the heat generated by the agitation required to provide adequate oxygen transfer and mixing.

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