Mechanical agitators in fermenters

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History of Fermentation is not having connection with this century, more over it's having connection with the era when ancient fermented food processes, such as making of cheese, wines, bread etc. can be dated to more than 5,000 years ago. They were developed when long before man had any kind of knowledge of the existence of the micro- organisms involved in fermentation. Also, fermentation is a very powerful economic incentive for small-industrialized countries, in their need to produce bio ethanol.


Fermenters are described as industrial fermentation it more loosely refers as the breaking of organic substances and re-assemble into other important substances. Somewhat culture of fermenter in industrial capacity refers to highly very oxygenated and aerobic growth conditions, where as fermenters are strictly anaerobic process


The history of fermentation is connected from many thousands of years. It has proven that ancient people used this process in making of breads, has been found in some in ancient Egyptian tomb dated 1400 BC.

It was used in production of wine and grape treading as depicted on the walls of Nakhte's tomb, Thebes, 1400.B.C.

In the year 1928, Sir Alexander Fleming (6 August 1881 - 11 March 1955) was famous biologist and pharmacologist due to poor lab bio-security practice accidently discovered penicillin from the fungus Penicillium notatum. Fleming was surprised by this inhibited new growth. Later, Fleming identified active agents but failed to isolate pure agent. He did not believe it would kill existing infections and would be stable enough for use in medical. Penicillin discovery was major step and basics towards the treatment in many dangerous diseases. Unfortunately that classic discovery was ignored for 10 years.

After this in the year 1939,Florey and Chain discovered that it penicillin is stable in the human body and it would kill existing infections if it is isolated from pure compounds .It was used in world war second to save lives which actually saved about 0.5million lives in this world war second. For this discovery of isolation they received Nobel Prize in the year 1945 for bioscience.


There are five major groups of commercially important fermentation technology:

  1. Microbial and biomass as the product as single cells, yeast, E.coli.
  2. Microbial enzymes are catalse, protease, pectinase, celluase, and lactase.
  3. Microbial metabolites are:
    Ethanol, glutamic acid, lysine, polysaccharides, all antibiotic fermentation
  4. Recombinant products are HBV, streptokinase.
  5. Biotransformations are steroid biotransformation etc.

Some nutrient sources for industrial fermentation technology

Some growth media are basically required for industrial fermentation,

  1. Microbe requires water.
  2. Oxygen as an energy source.
  3. Good carbon source.
  4. Good nitrogen source.
  5. Micronutrients for growth.

Carbon and energy source + nitrogen source + oxygen+ some other requirements ? Biomass + Product + byproducts + CO2 + H2O + heat The most basic requirements for fermentation are organism, water, substrate, oxygen.


Basic fermenters are of following types:

  1. Mechanically stirred / agitated
  2. Bubble columns

In bubble columns there are some basic functions:

  1. Mixing which usually maintain optimum substrate. and Biomass concentrations every place in columns.
  2. It maintains uniform temperature and avoids hotspots.
  3. It keeps solids suspended
  4. It disperse air oxygen and small bubbles which covers maximum total bubble surface area.
  5. It entraps basically recycle or retain air bubbles, but most important is to avoid air escape before all oxygen is dissolved.


It is nevertheless contentious process as to achieve as a high product concentration as possible. A well designed plan to investigate sources of variability will only indicate only expected sources of variation. Most of the deviations of directly controllable Variables such as pH, temperature. Some indirectly controllable

Variables such as nutrient concentration, some organisms specific growth factor may clearly improve its performance. The most obvious action will be to up rate the control system.It may be expensive but it is important to control that. After installing a sophisticated controlling system for a process variable that may have little effect on the some process object and the process variability may constitutes a waste of resources.

Well, a much planned statistical analysis of a process may be used with experimental established designs which overcome the shortcomings in individual control loops.

Such as variability parameter which is proposed by Manoranjan and Wrinkler proposed the idea for biological process in which:

It effectively treats the process variability as characteristics of the process it self, like the yield and the productivity.

As minimum variability can be made the objective of a statistical and other some optimising procedure in the same as minimum unit cost.

This kind of approach has been made to do removing some effect of the cause and rather than the cause of effect.


The agitator is basically made up of one or more than one type of impellers are mounted on drive shaft aligned with the vessel axis.

The shaft basically suspended on a type of a thrust bearing above the vessel and it enters the vessel through a gland or by mechanical seal. It is a major contamination risk point and one of major cause for non- mechanically agitated vessel for the contamination -sensitive fermentations. The role of impellers is actually to effect the dissipation of the powers transmitted by the agitator shaft by the drive motor.

The most important problem here is that basically the agitators have to perform two types of functions that are to some level conflicts in their requirements of design.

To maintain the homogeneity the mixing is required a type of large diameter and is slow speed impeller containing a small number of blades is used for obtaining the required liquid pumping action.

For dissolving the air , there is action of the agitator which breaks the flowing of air into bubble and it prevents converting into larger bubbles and for this a high shear kind of system which contains a small and high speed impeller with a number of large blades is requires. Some designs reached a compromise in which there are two types of functions can be performed with the help of one or more than one same impellers are mounted on a single common kind of agitator shaft. Diameter of the impeller is very important as a small change can have a very large effect on the impeller performance.

The impellers sitting on the shaft of a agitator which is multi impeller is very important. Some particular type of organisms are believed shear sensitive and it is other important reason for using some non -mechanically agitated vessels as it may cause extensive retrofitting practice and adaptation of existing or a particular type of requirements of the typical process under consideration.

The distance of the sparger and basically the impeller is lowest is important.

In all this above it contrasts with aerobic biological waste and water treatment procedure where mechanical agitation is used.

Dissolution of oxygen occurs by absorption at the surface liquid exposed to the atmosphere and some additional gas-liquid area has being created to prevent the disruption in the liquid surface by the mechanical agitation.

The effect of aeration throughout the surface free at the upper should not be neglected basically in small and low ratio system in which the free surface liquid gives a proper proportion of the liquid and gas interfacial area used for oxygen dissolution.

As in some water -waste treatment process surface free aeration is the main mechanism of mass transfer.


The airlift fermenters are found to be unsuitable for the development of high viscosity to prevent liquid circulation and produce a stagnant down flow section. As with bubble columns some airlift fermenters may be operated either in the batch and continues state. The circulation of internal removes most of the importance of co and counter - current operation. Some fluid flow pattern is determined principally with the help of gas sparge. Some draught tube design is mostly used in the fermentation but basically in the high ratio configuration, but when sparged tanks having no physical limits between the sparged and unsparged sections are mostly used in the water waste treatment.

As such there is no particular distinction between an airlift reactor and an unbaffled bubble column with having axial gas injection in it. It helps in developing a relation in stable annular circulating pattern. It resolves ratio question.


A type of version loop reactor system which is called by circulating bubble column is successfully used as water waste treatment as method of deep draft shaft procedure.

As very loops which are deep produce very high hydrostatic Pressure in the bottom of the loop it usually gives very high air dissolution. Originally this is a spin off from the combination of bubble -column or airlift fermenter. There is an advantage of the circulating bubble column is basically that once the liquid circulation pattern have been used and established by sparging air into the riser section. In theory also be done in a conventional airlift fermenter, the tubular configuration may start the estimation of fluid velocities to be made more reliable.


By using an energy balance for the estimation of liquid circulation velocity in airlift reactors and it is estimated that the liquid circulation rate go higher with the square root with height reactor.


Some problems occurs in designing a fermenter and basically object of the above is to discuss the mixing and agitation in linked to the work of fermenters and it characteristics by determining the distribution of rates of circulation. One of the big problem comes in the designing of a fermenter is the choice of the exact type of mechanical agitation system. The problem is not solved to the extent that what it requires for the agitator is not always correct, in the sense that the particular function of agitation for a particular fermentation is unknown. Further problem arises because performance character of impellers, and the differences of these characters with these rheological properties of broth, is not clearly understood. It is easy to consider systems in which mechanical agitation is given by a round stirrers and stirrers mounted on a shaft on the vertical mid-line of the fermenter. Basically it is not a fully restrictive assumption and much of what exactly follows is equal in application to other agitation system for example as air -lift fermenters, for example. It is very easy to take one exact form of system as a basis for a good discussion. In any type of fermentation process, agitation, sometimes but not always coupled with the aeration, it has to perform and gives a various types of functions. The skill of the designing is it should be able to define and show unequivocally of these functions is difficult and limited.


Fermenter is basically used in fermenting which is applicable in many industries in which it is use. This technology is widely used world wide to do large processes in that airlift and mechanical agitator's kind of fermentaters use. So as above discussion it clears the merits and demerits of this kind of fermenters.


  1. Notes by Professor Chris Hodrien
  2. Industrial Fermentation. (2009, December 5). Retrieved December 25, 2009, from Wikipedia:
  3. (1990). In M. Winkler, & M. Winkler (Ed.), Chemical Engineering Problems in Bio-Technology (pp. 250-275). Barking, Essex, England: Elsevier Science Publisher Ltd.