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Fermentation is the process of Breaking down of organic substances and compiles it into other substance using a method of respiration without oxygen (anaerobic), but on industrial scale it is sometimes done in the presence of oxygen. Since many years, fermentation has been utilized in making beer, and wine, cheese production, baking bread, yogurt, soy sauce, and many other foodstuffs.
Fermentation is done under specific conditions which are to be maintained and it can only be achieved in the fermenters, therefore Fermenters are the apparatus that maintains optimal conditions for the growth of microorganisms, used in large-scale fermentation and in the commercial production of products like antibiotics and hormones.
Although many micro-organisms have been used in the production of amino acids, nucleic acids, enzymes, alcohols but the "modern Biotechnology" is to use recombinant DNA techniques to produce physiologically active substances like growth hormones, interferon and insulin which are now present in the very less in plants and animals.
Fermenter setup for commercial production
Fermenters which are used for commercial production are very huge in size with the capacity of approximately 10000 litres to 200000 litres depending upon the scale of production, with more production, the cost of running the plant also increases and it also increases the scale-up problems
In general, commercial large-scale fermentation is made up of the following steps:
selection of organism with respect to
â€¢ Substrate versatility
â€¢ Characteristics of by-product formation
â€¢ Cell recycling Viability, vigorness of the organism.
â€¢ Physiological characteristics like growth rate, requirements for aeration.
â€¢ Genetic convenience.
Cellular and Metabolic engineering:
â€¢ Improve existing properties of the organism
â€¢ Introduce innovative functions like, by simplifying recovery of the product, increase substrate and product ranges and allowing fermentation to occur under nonstandard conditions.
Fermentation process development:
â€¢ Augmentation of cultivation parameters for product recovery and purification (minimize chemical inputs, minimize by-product formation, and develop High-cell-density cultivation)
â€¢ Culture and media accumulation (from complex to defined minimal media)
â€¢ Inclusion of cell holding and recycling.
Downstream processes within a fermentation process:
â€¢ Examples are separative fermentation, electro dialysis and in-line membrane separation technologies.
General fermenter diagram
There are various types of fermenters which can be classified in the following system:
Internal mechanical agitation reactor
Stirred vessel with suction tube
Stirred vessel with draft tube
(b) Bubble column and air-loop reactors
Air lift reactor
Pressure cycle reactor
Bubble column with fritted disc
Bubble column with a draft tube for rotation motion flow
Sieve plate cascade system
(c) External circulation reactor
Water jet aerator
Recycling aerator with fritting disc
Forced water jet aerator
Design of the fermenter (based on aeration)
On laboratory scale we use standard rates of volume of air at standard volume of liquid per minute or standard cubic feet of air per hour per gallon, whereas in large scale fermenters for saving energy in production, the air agitated fermenters are used.
The cost of savings is not supposed if we compare the cost of operating a fermenter agitator to the cost of operating increased air pressure one. Although if we compare the total cost i.e. Cost of capital and operating cost, the non-mechanically agitated fermenter design is cheaper. Mainly, top impellers are meant to circulate the fluid and helps very little to bubble dispersion and oxygen transfers, most of the power of agitator is used in mixing the fluid, whereas primary function is to increase the surface area of air bubbles to lessen the diameter of the bubble.
Top entry impeller
Unlike chemical reactors, fermenters does not have first or second order of reactions with the initial reactants, oxygen dissolving rate is monitored by diffusion. An organism consumes the oxygen in the fluid which is irreversible reaction and for aerobic fermentation to continue, the sufficient amount of oxygen should always be diffused in. Diffusion of air can be done in various methods like, more interstitial area, higher air pressure, reduced cell volume, by controlling metabolism rates by reduced carbohydrate feed rates, but the major concern of most of these methods is shear, foaming and availability of proper devices.
Another concept which is very important for oxygen transfer efficiently is the height-to-diameter ratio of fermenter. If we compare tall, narrow tank to the short, broad tanks fermenters, tall and narrow tanks have more advantages like, residence time for bubbles is more in tall and narrow to that in short and broad ones, higher oxygen dissolution due to higher air pressure at sparger in taller vessels. Greater vertical height can also be used for large volume tanks, therefore fermenter height is most important Geometric factor in designing the fermenter, contrariwise, shorter vessels need more mechanical agitation or more air to effect the same.
"Majority of industrial fermenters are in the H/D range of 2-3", the larger sizes are about "106 Litres". It is therefore to be noted that power consumption for tall fermenters is less than the short and broad fermenters because cost of compressing air agitation is expensive.
Effect of air requirement on design of the fermenter
Bubble Residence time
Fermenter design, 1997 By Henry C. Vogel, Celeste L. Todaro
(Scfm-Standard cubic feet meter) Const 30,000 gal tank; 24000 gal run vol; 0.4ft/sec SLV .
Mixing horsepower by aeration
Agitation effect of aeration can be easily determined, as two forces at inside the fermenter i.e. first, which is caused by rise in bubbles. These bubbles raise to the surface when pressure of rise of bubbles from sparger is equal to the hydrostatic pressure of the liquid, and these pressures remains at equilibrium with each other until the bubbles escapes out from the liquid surface. Air inside the bubble has equal temperature to the fermentation temperature to the fermentation temperature and remains constant. This condition of fermentation reaction is known as "isothermal expansion of air", where gas pressure and gas volume changes at constant temperature".
Horse power for isothermal expansion of the air can be calculated by the following expression given by "Perry and Chilton"
Hp/1000scfm = 4.36pâ‚‚ In pâ‚‚/pâ‚
Pâ‚‚: hydrostatic pressure
pâ‚: pressure above liquid
"The intermingling of two or more dis-similar portions of material, resulting in the attainment of a desired level of uniformity either physical or chemical in the final product", is called as mixing. Mixing is very essential step in fermentation, for e.g. Liquid/Liquid mixing i.e. liquid phase continuous like liquids, solids and or gases dispersed. Mixing is done by agitators or impellers or stirrers, Rotating component which are usually made up of steel, iron, bronze, aluminium etc metals. It is driven by the motor, from which, it transfers the energy by accelerating the fluid from the centre. Impellers are used in agitated tanks to stir the slurry in the tank, which is used to mix the materials like liquids, solids and gas. There are various gradients in conditions like temperature and concentration while mixing. Mixing gives rise to disruptive action called Shear, extension and impactation.
Mainly agitators can of following types depending on their mixing properties:
Mechanical Agitators and Impellers
Huge mixers which are introduced from the bottom or sometimes sideways, these are consist of blades made of steel, iron, aluminium and alloy which are joined to shaft motor of high horse power, used for mixing high viscosity fluid by rotation, the flow pattern of agitators and impellers is determined by dimensionless quantity called Reynolds numbers(Re or Nre). If the impeller have high Reynolds number of impeller is high i.e. (>10000) Flow fills the vessels and the system is said to be turbulent.
There are two main types of impellers, depending upon the flow pattern created by them
Axial flow impeller
The axial flow draws the liquid from the top and discharges toward the bottom by imposing essential bulk motion. This is more efficient impeller than the radial flow, and are used on standardized processes, in which is important to increase fluid volumetric flow rate.
Radial flow impeller.
The radial-flow impeller generates a "butterfly" flow pattern, in which impeller draws material from both the top and bottom and discharges radially toward the sides. Radial flow impellers impose essentially shear stress to the fluid, and are used, for to mixing immiscible liquids or in general when there is a deformable interface to break. Another application of radial flow impellers are the mixing of very viscous fluids.
Pulp and paper publication 1985
However, successful agitator selection is based on impellers design, diameter and its delivered horse power. There are many types of impellers which are used in fermentation industry, depending upon their shapes and power, for example propeller type is the best in power dissipation, axial flow turbine, turbine, disc turbine.
Propeller Rushton (disc-turbine)
Normal type of agitator (propeller, blade, or turbine) is "not effective for mixing viscous (>100 poise) liquids" because the mixed volume of viscous liquids is limited to the area swept by the body of the agitator. This area is very limited in the above mentioned types of agitators. Low heat-transfer rates result from this poor quality of mixing
Design and Dimensions of impellers
Rushton turbine blades Chemineer HE-3 LightninA6000
Np and Nq of several Lightnin impellers: A6000, A310, A200 or PBT, C102, A315, C104, R100 or RT6, R510 or bar turbine, and R500 or sawtooth
Maximum and average shear rates and shear rate correlations for A200 or PBT, R100 or RT6, and A310
Np and Nq of A315, R100 or RT6
Maximum and average shear rates for Lightnin A315
K-factor of Lightnin A310, A315, A200 or PBT-D, R100 or RT6
Np and Nq of Lightnin A320 and C104 =f(viscosity), Re
Flow discharge angles for A320 and A200 or PBT-D
Mixing and blending times for A320 and A200 or PBT-D
Np of 3,4,5,6,8,10, an 12 bladed Rushton Turbines =f(viscosity, Re, geometry, baffles)
Np of 3-bladed propellers with 1,1.5,2, and 2.5 pitch =f(viscosity, Re, geometry, baffles)
Np of Chemineer HE-3 and PBT.
Maximum liquid levels for HE-3, PBT, RT4 and RT6
Mixing rate constants for above impellers and =f(viscosity, Re) for HE-3
Nq for HE-3
Heat transfer coefficients for above impellers
Np of RS6
Dimensions of impeller design include, number of blades, turbulent power number, turbulent flow number, non-turbulent power numbers (Np), flow numbers (Nq) and discharge angles as a function of Reynold's Number (Re).
Baffles are a kind of the walls that are required to prevent the swirl in the tank. Most impellers rotate in the clockwise or counter-clockwise direction. "Without baffles, the tangential velocities coming from any impeller(s) can cause the entire fluid mass to spin". It may look good from the surface seeing that vortex all the way down to the impeller, but this is the worst kind of mixing. There is very little shear and the particles go around and around like in a Merry-Go-Round. It looks more like a centrifuge than a mixer. But, Baffles are important part of mixing in fermenter, without baffles most impellers styles impose a tangential flow pattern, whereas with the baffles, most impellers show their true flow characteristics. Most vessels have three baffles around the wall, adding more baffles can sometimes limit the impeller speed. Baffle width, (wB), is a function of the viscosity. Baffles are not required for high viscosity fluid and broths, because there is enough resistance to flow at the walls. With the decreasing viscosity, baffling becomes important and the width of the baffles becomes larger.
Air lift fermenters or bioreactors are the tower shaped bioreactors, which are run under optimized aerobic conditions for efficient mixing by injecting air from bottom of the tank with the help of the pump, avoiding destruction in shear sensitive organisms, and requiring low energy input and simple construction. The hydrodynamic force is induced by bubble motion and associated with wake interaction; these are the key factors responsible for heat and mass transfers. Because bubble-induced flows in the airlift reactor are identified to be dynamic in nature, the time averaged flow properties cannot well represent the dynamic governing mechanisms of flow structures. Hence instantaneous, rather than time- or volume-averaged, it is obvious that the approximation of the, hydrodynamic flow phenomena is required to provide further insight into the design and scale-up of airlift reactors
Erzeugung von Zitronensäure : Zitronensäure ist eines der wichtigsten biotechnologisch hergestellten Produkte und wird in der Kosmetikindustrie , zum Entrosten und Reinigen von Metalloberflächen oder im Lebensmittelsektor verwendet. Production of citric acid: Citric acid is one of the major biotech products and is in the cosmetics industry, used for rust removal and cleaning of metal surfaces or in the food sector. Die Erzeugung erfolgt mittels Schimmelpilzen ( Aspergillus niger ) oder Hefen ( Candida spp. , Hansenula spp. ). The production takes place by means of molds (Aspergillus niger) and yeasts (Candida spp., Hansenula spp.). Letztere weisen eine kürzere Prozessdauer (ca. 4h) auf, während die Herstellung durch Schimmelpilze 6-10 Stunden in Anspruch nimmt. The latter have a shorter process time (approximately 4 h), while the production by molds takes 6-10 hours to complete. Bei der ( aeroben ) Fermentation wird vorrangig Rüben- oder Rohrzuckermelasse als Energie- und Kohlenstoffquelle eingesetzt. At the (aerobic) fermentation is mainly beets or cane used as an energy and carbon source.
Single Cell Protein (SCP) : Als SCP bezeichnet man Proteine , die von einzelligen Mikroorganismen wie Bakterien , Hefen oder Algen erzeugt werden. Single Cell Protein (SCP): The SCP is known proteins that are produced by unicellular microorganisms such as bacteria, yeasts or algae. Der Begriff umfasst sowohl rekombinant erzeugte als auch natürlich von den Organismen produzierte Proteine. The term includes both recombinant and natural proteins produced by organisms. Airlift-Reaktoren werden aber vor allem mit Anwendungen aus der Nahrungsmittelindustrie in Verbindung gebracht. Airlift reactors but are associated primarily with applications in the food industry combined. Als Nährstoffquelle für die betreffenden Mikroorganismen kommen alle fermentierbaren Kohlenstoffverbindungen in Betracht, die preiswert und verfügbar sind- zB: Kohlenhydrate , Lignine , niedere Alkohole , Fettsäuren sowie langkettige n- Paraffine , Methan oder Ethan . As a nutrient source for the micro-organisms, all fermentable carbon compounds are concerned, are inexpensive and available, eg carbohydrates, lignins, lower alcohols, fatty acids and long-chain n-paraffins, methane or ethane.
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