DESIGNING FERMENTOR PART 1: BASIC REQUIREMENTS

The evolution in the design of fermentors over several decades not only reflect the change in understanding of the process of fermentation itself, but also reflect the challenges in the technology of producing new and novel products using the fermentor.

The earliest design of the fermentor as exemplified by the fermentation of penicillin showed how simple the design and function of the fermentor in the forties. Today’s fermentors are more sophisticated as reflected by the various monitoring and control facilities with computer interface for a more efficient fermentation process. In reality, the fermentors have not really underwent a radical design of yester years, only that the monitoring and control get more sophisticated. The basic fermentor hardware still basically remains the same.

However, in the past few years we have seen novel designs in the fermentors it selves. This trend towards a customized and functionally built fermentors are in response to the “new” kind of bioproducts that were not even imagined years ago. The new fermentors of today have to produce such as biopharmaceuticals, enzyme and other biotechnology-derived compounds.

The limitation of specific conditions imposed by such products has resulted in the paradigm shift in the design of fermentors. The engineers who built the fermentors of the past now have to wrestled to find out solutions in the design of new breed of fermentors
As it is now we are beginning to see fermentors whose design is dedicated to a single product, or changed out frequently to process many different ones.

DESIGNING THE COMPLETE FERMENTATION SYSTEM AND NOT THE FERMENTOR ALONE

We cannot design the fermentor alone detached from the supporting fermentor system. For a successful fermentation process to occur we must match for compatibility the design of the fermentor with the design of its ancillary support equipment

In the design of modern fermentation system great emphasis are placed upon the role of the fermentor either for multipurpose operation or whether the fermentors will be used for scale ups.

The critical areas that will have to be considered strongly are vessel design, process piping, jacket service piping, agitation and mixing and instrumentation and control.

Thus in planning a custom built fermentor various options must be considered to ensure what ever features are necessary or needs to be added and whether they are beneficial or cost effective. We simply just can’t buy fermentors off the rack! (Although, many do!)

The first consideration in the design of fermentor or retro fitting is to define properly the system that is needed. Are we using bacteria, mammalian cells or fungi? Each type of cell has its own specific requirements or limitations. Even within one particular cell type such as in the case of cell cultivation there is the option of using pneumatically mixed air lift fermentors or by mechanical stirring.


VESSEL DESIGN

GEOMETRY VOLUME
The importance of vessel geometry and volume cannot be understated. While the geometry more refers to the shape of the vessel the volume refer to the capacity of the fermentor. Both geometry and volume of the fermentor will affect the performance of the fermentor

The most popular range of ratios of height-to-tank diameter ratios of 1:1 to 3:1 are common for stirred tanks.

The expected range of the vessel working volume, typically 60-85% of the fermentor total volume. However this must be adjusted with consideration to any expected fed batch additions. It is important that so that the impellers can be spaced appropriately.

SURFACE FINISHING
The level and type of tank polish is selected based on the acceptability of irregularities in the tank surface. The most popular method of finishing is by mechanical and electropolishing and passivation. The choice depends on the quality requirements of the product

PORTS SENSORS
The number, location and type of tank nozzles and ports influences the ability to add materials to the vessel and insert instrumentation probes. It does gives flexibility but at the expense of increase in fabrication costs, setup time and the risk of damage associated with repeated disassembly for cleaning.

There are so many things that enter and leave the fermentor, that in a way the fermentor can’t afford to accommodate all the requirements. It simply does not have the areas to accommodate it. Thus prioritization of usage of ports is essential

Location of the port is important to ensure no damages incurred during insertion and withdrawal.

The weld procedure used to attach these fittings must be reviewed carefully to ensure a sturdy design and to maintain roundness. A compromise has to be achieved between minimizing dead legs and room for sturdy attachment.

MATERIAL
The consumables such as gaskets and O rings must be evaluated based on the expected frequency and expense associated with each change. Sometimes it is more efficient, practical and economical to use less expensive, single-use approach.

Factors such as compressibility and memory of the material should be evaluated to ensure that leaks will not develop when the vessel is heated and cooled repeatedly. All materials in product contact must be suitable for sanitary use.

Materials should be selected that are resistant to high temperatures depending upon the specific application.

STERILITY
For improved sterility, steam tracing should be carried out in pipings and equipments. A tube or small-diameter pipe attached to the pipe and carrying a heating medium for the addition of heat all along the process pipe is commonly referred to as “heat tracing.” If the system of heat tracing utilizes steam in the heat delivery process, it is then termed “a steam-tracing system.” Steam tracing requires a highly thermally resistant gasket or O-ring material, due to the constant steam exposure.

The cleanability of the fermentor component is important, thus the number, location and mode of attachment of internals are also important to define. Welded internals improve cleanability by eliminating crevices,

The choice of valves is important. Complete drainability is often a criterion for bottom-valve installation

HEADPLATES

For very large fermentors there must be allocation on the head plate for cleaning and repair .While this size is sufficient for a person to enter the tank, impellers often need to be designed with a split hub to permit their entry.

Removable headplates introduce the design feature of a headplate O-ring seal. Smaller head plates can be hinged and easily removed by hand; larger ones are often bolted and may go several years without disassembly. Consequently, sealing gaskets and groove locations must seal well for extended periods and be constructed for sanitary, high-temperature service.






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