Mixing of liquids or solids

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  1. stirred tank type in which mechanical motor driven stirrers are provided or
  2. of air lift type in which no mechanical stirrers are used and the agitation is achieved by the air bubbles generated by the air supply.

The mixing of liquids or solids or any mixture within a container is achieved by agitator, which consists of a motor that have a fixed sparge ring for circulating liquid. The agitator consists of a motor driven shaft of to which solid disc is attached to one side and agitator blades on the other side of disc. The circular cover plate that is attached to the bottom end of agitator blades allows the induction of fluid within the inner area of the agitator blades and some area from the sparge ring. This helps the liquid to be expelled from all the outer ends of agitator blades which results in better distribution of a liquid around the sparge ring and also improves mixing of gas with a liquid without drawing the gas into the input area of the agitator.

Air Lift

The air lift agitators are the ones in which the liquid that is to be fermented is mixed equally with the help of air. These Agitators require no mechanical agitation and eliminates the need for a stirrer system. It is very efficient for large unit plants and for fermentations that require a high and constant aeration degree. The airlift fermenters are mostly applied in tissue culture. For tissues the normal mixing is impossible as they are sensitive to shear and with airllift the mixing is achieved as the shear levels are low. They are fitted in a long, thin vessel with an aspect ratio of around 10:1 (height to base diameter). A conical section is often used on the top of vessel to give widest possible area for gas exchange. In the steel base section the sensors are mounted. A stream of air enters at the base of the vessel and is passed through the fluid present in the vessel for both mixing and aeration. The air is being moved upwards with the help of a draft tube fitted in the center of the vessel. The double walled draft tube allows heating and cooling by thermo circulator system. When the agitator rotates the aerated fluid goes to the top of the draft tube and spills down through the spacing between the outer wall of the draft tube and inner wall of the vessel. There is an increase in the density of the fluid going down to the bottom by the gas transfer from liquid to gas phase at the headspace of the draft tube. This descending liquid, which goes down to the bottom, is again aerated and rises to the top. There are two types of air lift fermentors based on their circulation types: Natural and Forced. In natural circulation the liquid is recirculated by the gradient density generated by air whereas, in the forced circulation type fermentors an external mechanical power is applied for lifting the liquid by air which results in recirculation of the liquid in fermentor. In both these types of fermentors the liquid that is heated up while fermentation is cooled by the external refrigirator that is attatched to the fermentor itself. An aeration rate of 1.5 to3.0 liters of air per minute per liter of medium has been found effective.

The main features of airlift fermenters are Internal-loop vessesls, Draft tubes, External loop or Outer loop. The advantages of airlift fermentor are in low shear there is lowmixing which means the fermenter can be used for growing plant and animal cells, since there is no agitation sterility is easily maintained, in a large vessel the height of the liquid can be as high as 60m and the pressure at the bottom of the vessel will increase the oxygen solubility, extremely large vessels can be constructed in which the microorganisms will undergo a biochemical reaction and release large heat which is not achieved by conventional stirred tank design. The main disadvantages of airlift fermentors are high capital costs with large scale vessels, high energy costs although an agitator is not required for most of the fermenters, a greater air throughput is necessary and the air has to be at high pressure, particulary on a large scale. Also the efficiency of the gas compression is low, as the microorganisms circulate through the bioreactor, the conditions change and it is impossible to maintain consistent levels of carbon source, nutrients and oxygen throughout the vessel, the seperation of gas from liquid is not very efficient when foam is present. In the design of an airlift bioreactor, these disadvantages have to be minimised. If the feed comes in at only one location, the organism would experience continuous cycles of high growth, followed by starvation. This would result in the production of undesirable by-products, low yields and high death rates. Therfore, particularly on a large scale, multiple feed points should be used. Similarly, air should be admitted at various points up the column. However, the air must mainly enter from the bottom to circulate the fluid through the reactor.

Mechanical Agitators:

The are mainly usedfor drilling surface tanks. The components of mechanical agitator are a drive motor, a geared reducer (also called a gear box), a gearbox output shaft and impellers. The main objective of mechanical agitation system is uniform suspension of all solids, appropriate application of shear, homogeneous fluid proporties throughout the system, and economical application of applied power. Most mechanical agitators are driven by eletric motors. These motors must be rated for explosion-proof duty (to ensure that motor, starters, and wiring meet specifications for local codes and operating criteria) and may be mounted horizontally or vertically . Motors may be coupled to or direct-face mounted to a gear reducer that in turn drives the impeller shaft. Impellers are mounted on the shaft at a specific distance off the tank bottom to achieve desired results. The main advantages of mechanical agitators are it suids most need due to the variety of gearbox and impeller combinations, it is effective for large and deep tanks, it can also be designed based on more or less shear needed to induce, it helps in cooling the mixture by exposing the mixture to atmosphere. The various disadvantages associated with mechanical agitators are that they cannot blend fluids if the syetem has different compartments, Higher initial cost, Heavier than other agitators and requires more space, Electricity required to run the motors and it may also require installation of baffles.