TYPES TYPES OF FERMENTER

Types of Fermenter Aerobic fermenters may be classified depending on how the gas is distributed Stirred Tank Reactor Airlift Loop Reactor Immobilised System

Basic Fermenter Design Criteria (i) Nature of microbial cell (a) Hydrodynamic characteristics (b) Mass and Heat Transfer (c) Kinetics (d) Genotype and Phenotype (ii) Environmental Control and Monitoring of the process (a) pH, temperature, dissolved oxygen etc. (b) Asepsis and avoiding contamination (iii)Process factors (a) Effect on other unit operations (b) Economics (c) Potential for scale-up

Air lift reactors In such reactors, circulation is caused by the motion of injected gas through a central tube with fluid re-circulating through the head space where excess air and the by-product CO2 disengage. The degassed liquid then flows down the annular space outside the draught tube Advantages Low shear Easier to maintain sterility Increased oxygen solubility Can allow large vessels Disadvantages High capital cost High energy costs Hard to control conditions Foaming hinders gas -liquid separation

Stirred Tank Reactors Most commonly used fermenter Made from stainless steel Height to Diameter ratio 2:1 and 6:1 Baffles prevent a large central vortex Also used to carry coolants in large systems

Basic features: stirred tank bioreactor Agitation system The function of the agitation system is to 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. The agitation system consists of the agitator and the baffles. The baffles are used to break the liquid flow to increase turbulence and mixing efficiency.

Stirred Tank Reactor

STR - Control systems An oxygen delivery system A foam control system An agitator system An oxygen delivery system A foam control system A temperature control system A pH control system Sampling ports A cleaning and sterilizing system. A sump and dump line for emptying of the reactor.

Agitator design and operation Radial flow impellers - Rushton turbine The most commonly used agitator in microbial fermentations Like all radial flow impellers, the Rushton turbine is designed to provide the high shear conditions required for breaking bubbles and thus increasing the oxygen transfer rate.

Aeration and Agitation The transfer of energy, nutrients, substrate and metabolite within the bioreactor must be brought about by a suitable mixing device. The efficiency of any one nutrient may be crucial to the efficiency of the whole fermentation. For the three phases, the stirring of a bioreactor brings about the following: Dispersion of air in the nutrient solution Homogenisation to equalise the temperature and the concentration of nutrients throughout the fermenter Suspension of microorganisms and solid nutrients Dispersion of immiscible liquids

Sterilization Sterilizing the feed solution is essential because the media cannot contain foreign microbes because this could severely hinder the growth of the production microbe Most popular method is heat sterilization of the feed solution

MODIFICATIONS (i) Important in tank reactor design: 1. Continuous flow (activated sludge waste treatment) · Suitable when substrate at low conc. · Allows greater control on growth rate\ cell physiology 2. Immobilised cells - may be membrane (e.g. hollow fibre reactor), immobilised onto support such as ceramic (e.g packed-bed) or in polymers (e.g alginate beads) · Increases rate of reaction · Microenvironment created protects cells e.g. from shear damage 3. Low energy aeration\ mixing Air-lift, draft-tubes, loop reactors etc. · Increase height to diameter ratio. Increased path length of bubble, improves mass transfer · Results in decreased shear levels, important in floc systems.

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