2 Types of FermenterAerobic fermenters may be classified depending on how the gas is distributedStirred Tank ReactorAirliftLoop ReactorImmobilised System
3 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
4 Air lift reactorsIn 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 tubeAdvantagesLow shearEasier to maintain sterilityIncreased oxygen solubilityCan allow large vesselsDisadvantagesHigh capital costHigh energy costsHard to control conditionsFoaming hinders gas -liquid separation
5 Stirred Tank Reactors Most commonly used fermenter Made from stainless steelHeight to Diameter ratio 2:1 and 6:1Baffles prevent a large central vortexAlso used to carry coolants in large systems
6 Basic features: stirred tank bioreactor Agitation systemThe function of the agitation system is toprovide 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.
8 STR - Control systems An oxygen delivery system A foam control system An agitator systemAn oxygen delivery systemA foam control systemA temperature control systemA pH control systemSampling portsA cleaning and sterilizing system.A sump and dump line for emptying of the reactor.
9 Agitator design and operation Radial flow impellers - Rushton turbineThe most commonly used agitator in microbial fermentationsLike 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.
10 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 solutionHomogenisation to equalise the temperature and the concentration of nutrients throughout the fermenterSuspension of microorganisms and solid nutrientsDispersion of immiscible liquids
11 SterilizationSterilizing the feed solution is essential because the media cannot contain foreign microbes because this could severely hinder the growth of the production microbeMost popular method is heat sterilization of the feed solution
12 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 physiology2. 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 damage3. 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.