FERMENTER DESIGN

INTRODUCTION Fermenter Guidelines for fermenter designRequirements of a fermenter Parameters for fermenter designGas transfer Heat transfer Nutrient transfer Aeration & agitationReferences

FERMENTER

Fermenter provides a suitable environment in which an organism can efficiently produce a target product, that might be -1. Cell biomass2. Metabolite3. Bioconversion product

The sizes of bioreactor can vary over several orders of magnitudes1. The microbial cell (few mm³)2. Shake flask (100-1000ml)3. Laboratory fermenter (1-50L)4. Pilot scale (0.3-10m³)5. Plant scale (2-500m³)

PARTS OF FERMENTER1. Vessel2. Impellers3. Baffle4. Sparger5. Drain point6. Shaft7. Aseptic inoculation pipe8. Sampling point

GUIDELINES FOR FERMENTER DESIGN AND OPERATION 

Material : Stainless steel

Height to diameter ratio of the vessel: 2 to 1 or 3 to 1Impeller Two or three disk turbine impellersDiameter: 0.3 to 0.4 of tank diameter Agitation speed: 50 ± 200 rpmImpeller - shaft enters either from the top or bottom.Baffle - Four equally spaced to prevent vortex formationWidth : one tenth of the tank diameter SpargerRing sparger (Single orifice for a small Fermenter)Heating or cooling coilFor sterilization or to control the temperature - Agitation and aeration - Cell suspension

-Enhance the aeration (oxygen limitation problem)-Mixing-Problem with shear sensitive cells-Heating and cooling-Sensors- pH Control

DESIGN OF A FERMENTERFactors to consider when designing a fermenterAseptic and regulator capability,Long term reliabilityAdequate aeration and agitationLow power consumptionTemperature and pH controlsSampling facilitiesLarge volume & low value productsHigh value & low volume productsProductivity & yieldProduct purificationWater managementEnergy requirementsWaste treatment

REQUIREMENTS OF AFERMENTORFERMENTORThe vessel must be strong enough to withstand pressureThe vessel should not corrupt the fermentation productPrevention of growth of contaminating microorganism must be provided

Efficient O2 Supply if fermentation is aerobicAddition of anti-foaming agent as demanded by the foaming state of the mediumFermenter should posses temperatureControlFermenter should posses a mechanism for detecting pH values of culture mediaThere must be drain in the bottom of the fermenter

MIXING RELATED DESIGN  ISSUES  ISSUES Agitator selectionPower draw and torque calculationsScale-upMechanical designBlending performance (scale of agitation, turnovers- per-minute, blend time, homogeneity)Heat removal, temperature field, and possible heat damageSolid-liquid mixing (just-suspended speed, settled solids fraction, cloud height)Reaction performance (productivity, selectivity) Surface motion, solids and gas drawdownShear rates and impact velocities, possible shear damageOptimum feed locationsSubstrate concentration field, nutrient starvationOxygen starvation or poisoning (local or global)CO2 or other product poisoning (local or global)pH controlGas-liquid mixing (mass transfer, gas holdup, power factors)

PARAMETERS FOR THE FERMENTER DESIGN FERMENTER DESIGN Physical ParametersChemical ParametersBiochemical ParametersBiological Parameters

PHYSICAL PARAMETERS Agitation power & speedBroth volumeColor DensityFoamingGas flow rate & humidityHeat generation rate & transfer rateLiquid flow rateTemperatureOsmotic pressure

CHEMICAL PARAMETERS Amino acidCO2Cation levelConductivityIonic strengthMalliard reaction productsNitrogenNutrient compositionO2Phosphorous

B IOCHEMICAL PARAMETERS Amino acidsATP/AD P/AMPCarbohydratesCell mass compositionEnzymeNAD/NADHVitamins & nucleic acid

B IOLOGICAL PARAMETERS Age distributionAggregation & contaminationGenetic instabilityMutationTotal cell countDegenerationDoubling time

GAS TRANSFERThe theory of gas transfer refers to a process where the gaseous form of a compound is eventually dissolved into or driven out of the water.The rate at which the gas is transferred from air to water or vice versa is proportional to the area of the gas-liquid interface and the difference between saturation concentration and the actual concentration in the water.These indictate both the direction and rate of gas transfer at the gas water interface

The availability of the oxygen to the biological system depends upon:SolubilityMass transfer rate of oxygen in the fermentation broth3Rate of utilization of DO by microbial biomassSolubility decreases as temperature or salinity increasesFor example, the saturation level of dissolved oxygen is lower in warm water than coldwater systems

Gas solubility increases as the total pressure (sum of atmospheric and hydrostatic pressure) or mole fraction increases.In other words, the saturation level increases as the total pressure of the system increases.The rate of dissolution of a gas into water is proportional to the difference between actual and saturation concentrations of the gas in solution,

Cs ± C The area of the gas liquid interface, a(m²)The thickness of the liquid film, dThe diffusion coefficient, DdC/dt= KL.a . ( C s   ± C )WheredC/dt = rate of gas transferKLa = overall mass transfer coefficient

DETERMINATION OF OXYGEN TRANSFER COEFFICIENTS TRANSFER COEFFICIENTS KLa=no2/(C * ± C  L )-------1.)Sodium sulfite oxidation methodDynamic methodDirect methodOxygen yield coefficient method

SODIUM SULFITE OXIDATION METHOD METHOD

This method employs the oxidation of sodium sulfite by oxygen in the presence of copper or cobalt which act as catalyst.Na2SO3+½O2=Na2SO4To findKLa by this method, air is sparged through a1N Na2SO3 solution in the presence of copper ions at a conc. of 10¯³ M & mechanical agitation.The conc. of dissolved oxygen in the liquid is nearly zero since the oxidation reaction is extremely fast.Therefore, KLa = no2/C*Where,no2 = rate of oxygen transferC* = saturation dissolved oxygen conc. in solution

DYNAMIC METHODThis method is the simplest one since it requires only the dynamic oxygen balance in a batch culture, which has the following formdC L /dt = KL . a . ( C *   ± C  L  ) - Qo2 .  X WhereQo2X = rate of oxygen consumption per unit mass of cellThis method contains three versions:Addition of lethal agentThe gassing out methodDynamic oxygen balance

Frequency response techniqueTherefore,KLa can be calculated asKLa = Q o 2 X/C *   ± C DIRECT METHODThis method based upon oxygen balance in inlet & outlet gas streams around a fermenter.oxygen balance for the sparged air yieldsno2.T=1/VL(no2.ino2.o)Whereno2.T= rate of oxygen transfer VL=volume of fermentation brothThe dissolved & saturated oxygen conc., CL & C * ,need to be measured experimentally in order to determineKLa C* can be calculated from the partial pressure of oxygen in the exit gas stream in small scale fermenter But in large scale fermenter, the assumption of a perfectly mixed gas stream may not be valid.So, the log mean of the driving force i.e., ( C *   ± C  L )log. mean, between the inlet & outlet of the fermenter should be used in the following equation determiningKLa, KLa = no2.T/( C *   ± C  L  ) log. mean

OXYGEN YIELD COEFFICIENT  METHOD METHODThe rate of oxygen transfer can be related to the growth rate of microorganism using the Oxygen yield coefficient according to the following eq.no2.T= uX/Yo2WhereU= specific growth rate of microorganismX=cell conc.Yo2=yield coefficient of oxygenno2.T=oxygen transfer rate Onceno2.T is determined by this equation then equation 1.can be used to calculate theKLa provided that the dissolved oxygen conc. is measured.

HEAT TRANSFEREfficient heat transfer is important in controlling the temperature during sterilization operationsHeat generated in the fermentor needs to be dissipated by coolingOptimum temp. should be maintained in the fermentor This can be achieved by:External jacketsInternal coilsExternal surface heat exchanger 

TRANSPORT OF NUTRIENT Transport of reactants to & from phase to the biological component has a significant impact on the performance of the reactor.

It is affected if the rate of the transport of the limiting nutrients is slower than the rate of utilization by the cells.

AERATION AND AGITATION The primary purpose of aeration is to provide microorganism in submerged culture with sufficient oxygen for metabolic requirements.Agitation should ensure that a uniform suspension of microbial cells is achieved in a homogenous nutrient medium.The structural components are:-- Impellers-- Baffles-- Spargers-- Stirrer gland

IMPELLERS Impeller types can be radial, mixed flow, axial and peripheral and are selected on the basis of the pump design and the application.The number of vanes will affect the efficiency; in general more vanes are more efficient.The agitator is required to achieve a no. of mixing objectives ex. bulk fluid & gas phase mixing, oxygen transfer, heat transfer, air dispersion & maintaining a uniform environment throughout the vessel contents.

SPARGER

What is a Sparger?Sparger is a technical term for injecting gas into a liquid or for spraying a liquid onto a solid.Spargers are porous disc or tube assemblies that are also referred to as Bubblers, Carbonators, Aerators, Porous Stones and Diffusers.Porous Metal Spargers are widely used in Gas-Liquid contacting applications that affect everyday living.A common example of sparging can be seen in an aquarium where air is bubbled into the tank through a fine porous stone in order to maintain the level of the dissolved oxygen in the water .

Types of spargers:Porous sparger OrificeNozzleCombined sparger-agitator

B

 BAFFLES Four baffles are normally incorporated into vessel of all sizes to prevent a vortex & to improve aeration efficiency.The agitation increased with wider baffles but drops sharply with narrow baffles.

STIRRER GLANDThe satisfactory sealing of the stirrer shaft assembly has been one of the most difficult problems to overcome in the construction of fermentation equipment which can be operated aseptically for long periods.Types of stirrer glands:Stuffing boxSimple bush sealMechanical sealMagnetic drive

REFERENCES 

Principles of fermentation technology, 2nd edition by  P.F.STANBURY & A.WHITAKERBioprocess engineering, basic concepts, 2nd edition by M .L.SHULER & F.KARGI ebook.comwww.google.com

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