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Types of Bioreactors

Types of Bioreactors. Unit - 2. Continuous Stirred Tank Reactors. A Bioreactor is a device that is used to carry out one or more biochemical reactions to convert substrate into product The product of conversion may be biomass, metabolites,etc.

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Types of Bioreactors

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  1. Types of Bioreactors Unit - 2

  2. Continuous Stirred Tank Reactors • A Bioreactor is a device that is used to carry out one or more biochemical reactions to convert substrate into product • The product of conversion may be biomass, metabolites,etc.

  3. Many different kinds of bioreactors are available for different types of products . • A bioreactor is used in all kinds of bioprocesses like • food processing, • treating of waste water, • for making vaccines, antibiotics etc.. Bioreactors are designed both for submerged culturing and solid state fermentations

  4. Only 70-80% of the volume of stirred reactors is filled with liquid. This allows adequate head space for disengagement of droplets from the exhaust gas and to accommodate the foam which may develop. If foaming is a problem, an impeller called a foam breaker may be installed. Alternatively, chemical antifoam agents are added to the broth, but they reduce the rate of O2 transfer ,so mechanical foam dispersal is generally preferred.

  5. The aspect ratio of stirred vessels,i.e the ratio of height to diameter ,can be varied over a wider range but the least expensive shape to build has an aspect ratio of 1. When aeration is required the aspect ratio is usually increased. this provides longer contact times between the rising bubbles and liquid. Stirred fermenters are used for free and immobilized enzyme reactions and for culture of suspended and immobilized cells. Care is required for sensitive cells, particularly in plant and animal cell culture especially when impellers are run at high speeds.

  6. Stirred tank bioreactors consist of a cylindrical vessel with a motor driven central power shaft that supports one or more agitators. • Microbial cultures are generally provided with four baffles placed equidistant around peripheral of the tank. • Baffles run the entire height of the tank to prevent vortexing

  7. Baffle width is 1/10 or 1/12 of the tank diameter. • The aspect ratio of vessel is between 3 and 5. • The number of impellers depend on aspect ratio of the vessel. • The lower most impeller is locted about one third of the tank diameter above the bottom of the tank.

  8. Rushton turbines are used for microbial cell cultures. • Marine propellers are used for animal cell cultures. • Aspect ratio is less than 2 for animal cell culture vessels. • Animal cell culture vessels are unbaffled.

  9. In microbial fermenters ,gas is sparged below the lowermost impeller using a perforated ring sparger with ring diameter slightly smaller than impeller diameter • Aeration velocity is generally kept less than 0.05ms-1. • For animal cell culture vessels aeration velocity is less than 0.01ms-1.

  10. Stirred tank are commonly used for • Aerobic or Anaerobic (air-sparged if aerobic) • bacterial cells

  11. In ALB the fluid volume of the vessel is divided in to two zones by means of a baffle or a draft tube The sparged zone is known as riser,the zone that receives no gas is the downcomer Airlift bioreactors

  12. The bulk density of gas liquid dispersion in the riser tend to be less than that in the downcomer. Consequently dispersion flows up in riser and downwards in downcomer. Sometimes riser and downcomer are two separate vertical pipes that are interconnected at top and bottom to form an external circulation loop

  13. External loop configurations are used less when compared with internal loop. • AFB are generally used in • Bacterial and yeast cultures • Fermentation of mycelial fungi • Animal and plant cell culture • Wastewater treatment

  14. AFB are highly energy efficient relative to stirred fermenters. • Heat and mass transfer characteristics are good and it is efficient in suspending the solids than bubble columns. • For optimal mass transfer ,the riser to downcomer cross section area ratio must be in between 1.8 and 4.3

  15. All performance characteristics of AFB are related with gas injection rate and resulting rate of liquid circulation. • The liquid circulation rate depends on the difference between gas holdup (vol fraction of gas in gas-liq dispersion) between riser and downcomer. • Liquid velocity is also affected by geometry of the vessel and viscosity of fluid.

  16. In general ,liquid circulation velocity increases with square root of height of the airlift device. • So aspect ratios are generally high between 6 and 8 or even in hundreds. • Circulation is enhanced if there is little or no gas in down comer. • ALBs can be used for both free and immobilized cells.

  17. The advantages of Airlift reactors are the elimination of attrition effects generally encountered in mechanical agitated reactors. • It is ideally suited for aerobic cultures since oxygen mass transfer coefficient are quite high in comparison to stirred tank reactors

  18. Which type of reactor is the world's largest industrial fermenter?

  19. A STR can never be very large because of the heat-exchanger, mixing and aeration problems. • The world's largest fermenter is Air-lift Bioreactor! The fermenter is 200' high and 25 ft diam. The first photo shows the fermenter being transported on vehicles with tank treads.

  20. The next photo shows the fermenter being raised into position at the ICI, Ltd. factory, Billingham, UK.

  21. Suited to reactions involving a fluid suspended particulate biocatalyst such as immobilized enzyme or microbial flocs An upflowing stream of liquid is used to suspend or “fluidize” the relatively dense solids. Fluidized bed bioreactors

  22. The reactor is expanded in the top to reduce the superficial velocity of the liquid to a value below that needed to keep the solids in suspension • Consequently solids sediment in the expanded zone and drop back into the narrower reactor column • Hence the solids are retained in the column and liquid flows out.

  23. A liquid fluidized bed may be sparged with air or any gas to produce a gas-liquid-solid fluidised bed. • If the solid particles are too light they may have to be artificially weighted, for example by embedding stainless steel balls. • A high density of solids improve solid-liquid mass transfer and denser solids are easier to sediment.

  24. But the density should not be too high, otherwise fluidization itself may be difficult. • Even with lighter solid particles, superficial velocity needed to suspend the solids may cause the liquid to leave from the reactor too quickly. • This leads to insufficient solid liquid contact time for the reaction and liquid may have to be recycled to obtain sufficiently long cumulative time with the biocatalyst.

  25. The minimum fluidization velocity –the superficial liquid velocity needed to just suspend the solids from a settled state depends on • Density difference between two phases • Shape and diameter of the particles • Viscosity of the liquid.

  26. A photograph of a fluidized bed bioreactor (inside volume 450 cm3) used to produce butanol from whey permeate using C. acetobutylicum P262.

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