Relative Merits Of Airlifts And Mechanical Agitators Biology Essay


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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

There are various types of fermenters which can be classified in the following system:

Internal mechanical agitation reactor

Turbine-stirring installation

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

General fermenter diagram

Fermenter consists of following components:



Sample line inlet

Culture /nutrient medium let

Temperature sensor

Cooling water outlet

Cooling jacket

Air filter

Harvest line(outlet)

Design of the fermenter (based on aeration)

Agitator effectiveness

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.

Fermenter height

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

Sparger pressure



















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 = 4.36p₂ In p₂/p₁

Pâ‚‚: hydrostatic pressure

p₁: 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.

Impellers depending upon the flow pattern created by them:

Axial flow

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

The impellers are designed on the basis of turbulent power number, number of blades, turbulent flow number. Some of the impellers are given below:


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

Non-turbulent power numbers -Np, flow numbers -Nq Reynolds'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 is assume to be working fine if we look at the swirl all the way down to the impeller, but it causes the worst type of mixing. It induces very less shear and the particles go around and around like in a Merry-Go-Round. This mixing looks like a centrifuge machine. But, Baffles are important part of mixing in fermenter; most impellers designs give rise to a tangential flow pattern without baffles, 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. It is 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 baffles should be increased

Airlift fermenter

ICI, Ltd. factory, Billing ham, UK, (Chem. Eng. News, 18-Sep-1978)

Air lift fermenters or bioreactors are the tower shaped bioreactors with 8:1H/D ratio, which are run under optimized aerobic conditions for efficient mixing by injecting air from bottom of the tank with the help of the pump which induces the bubbles, which increases the surface are for interaction of air and media and therefore increases the oxygen rate, 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. They are mainly used in production of citric acid, single cell protein and sometimes for waste water management by activated slurry. Largest air-lift fermenter used till now is 1.5 Million litres and was used for manufacturing single cell protein by ICI, Billing ham

Working of airlift fermenter

Informa UK Ltd, 2006.

(b)Gas flow behaviour, with re-circulation

(a)Gas flow behaviour, without re-circulation

When the gas velocity of (approx) 0.7 and 2.2 cm/s is applied, Regime I is made, the recycled flow starts to induce small bubbles into the down comer. With the increasing gas velocity the larger and deeper bubbles are entrained but without being re-circulated back to the riser. The flow structure comprises of a descending region, fast bubble region, vertical region, and central plume region. Clustered bubbles behave like a wave-motion. The intermixing in the central plume region is very less, and the bubble-bubble interactions are less significant. "The flow condition is characterized by a gross circulation of the liquid phase, wherein the liquid rises in the middle portion of the column and largely passes into the down comer". Whereas in the regime II two fast moving bubble flow regions merge together to form one central bubble region in the center of the column in the absence of the central plume region. The bubbles in this regime dominate the gas flow intermixing and break-up. The vertical flow region and descending flow regions can be seen. This produces the main difference in flow condition between Regimes I and II. Wake effect is induced by strong liquid flow by large bubbles rising from centre and circulated to the down comer and sometimes rise back again, which is the main difference in the flow of both the regimes.

Merits of airlift agitators

It is more beneficial over conventional mechanical agitator systems which

Requires Less maintenance

They do not have mechanical agitator blades, pumps, motors as in stirred tank fermenters, for that reason they don't need much electricity

Low shear rates, more options of growing cultures easily.

Sterilization is convenient

Allows more area of contact area.

More residence time, therefore allows maximum oxygen solubility

Relatives merits of mechanical agitator

Less chances of cell spoliation.

Lesser setup cost as compared to air lift fermenters.

Variety of choice of organism to work with.

Choice of different agitator blades according to the desired product

Monitors biomass concentration and production of secondary metabolite production.

Maintains oxygen and nutrient levels for the media.

Works well for food product agitation, like milk products


Fermentation is required for the production of various products and using different type of fermenters with different agitation properties totally depends on the desired product, for that reason it has to be more efficient, less labour intrinsic, with less energy consumption, less maintenance and most importantly cost effective for setup as well as operating cost, however in comparison with mechanical agitator airlift type of fermenters are more successful and cost effective type of agitation used in biochemical industry, as bioprocess engineers are trying to find more methods of agitation with less energy consumption, less setup cost and more practicality.

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