Fermenter

INTRODUCTION TO FERMENTER / FERMENTOR

Fermenter: Fermenter can be defined as an organism that causes fermentation.

Fermentor: Fermentor can be defined as an apparatus which provide physiological enviroment for the growth of microorganism or an apparatus in which fermentation takes place.

Bioreactor: Bioreactor is an apparatus that provides right environment for the growth of organism (Eukaryotic cells) OR Bioreactor is a device where cells are grow to produce desired products.

In general fermenter is a device used for the cultivation of prokaryotic cells while bioreactor is a device used for the cultivation of eukaryotic cells.

Fermentation: Fermentation is a metabolic process where organisms (yeast/microbes) convert glucose to acid, gases or alcohol in absence of oxygen.

Industrial Fermentation: Fermentation is a process where organisms used typically grown on a large scale to produce valuable commercial products or to carry out important chemical transformation.

Components of fermenter 

Vessel: Most of fermenter has vertical cylinders vessel with top and bottom plates or hemispherical. The height to diameter ratio is called aspect ratio and also a critical factor in vessel designing. Vessel used for bacterial fermentation having aspect ratio 3:1 while vessel having 1:1 aspect ratio used for mammalian fermentation. High aspect ratio provides more residence time to gas in vessel. In fermentation process vessel can be used only 80% of its gross volume.  
MOC of vessel: Lab scale fermentor made up of glass and stainless steel are widely used.Now a days AISI (American Iron and Steel Institute)grade 316 commonly used for fermentor and bioreactor construction, while for low pH (1-2) fermentation process AISI 317 and for brewing fermentation process AISI grade 304 is used.  

Agitator: Agitator is one of the important component of fermenter; it mainly used for mixing purpose and create homogenous condition. It can be top driven or bottom driven and having agitator shaft and impeller. The height of agitator shaft for bottom driven is from bottom to 80% of the vessel while for top driven from top lid to bottom hence power consumption for bottom driven fermenter is less compare to top driven fermenter. Agitator driven by the motor. There are four different kinds of seal to fix agitator in fermentation vessel.

   a)      Stuffing box (packed gland seal): Having layers of packing rings of asbestos or cotton yarn and pressed to shaft by gland follower. This stuffing box of packed rings are difficult to sterilize due to poor heat penetration.

   b)      Simple bush seal:

   c)      Mechanical seal: Mechanical seal composed of two parts one stationary fix to bearing housing another rotates on the shaft. Steam condensate is used to lubricate and make cool the seal. Mechanical seal may be single or double mechanical seal.

   d)      Magnetic drive: The main advantage of magnetic drive seal is shaft doesn't need to penetrate the vessel. There are two magnets one is driving and other is driven, driving magnet connected outside to drive shaft while driven magnet connected to inner side one end of the agitator shaft.
   
      Impeller: The selection of impeller in terms of Type and number is based on application. The flow pattern of fluid also depends on type of impeller. As per thumb rule impeller diameter is 1/3 of the vessel diameter. The distance of impeller on agitator shaft should be 1 to 1.5 of impeller diameter for proper mixing and preventing dead/unmixed space. 

      Types of impeller:

      Disc turbine: Disc turbine having disc and vanes are fixed vertically to the circumference of the disc. The flow pattern generated by this is radial.

      Vane disc turbine: It has series of rectangular vanes and fixed vertically to the underside.

      Marine propeller: It mainly used for cell line process and flow pattern generated by this is axial. 

Baffle: Baffle use to prevent vortex formation in the vessel. The baffle width should be 1/10 or 1/12 that of vessel’s diameter. The baffle width should be optimum because agitation slightly increase with wider baffle while drop sharply with narrow baffle. To prevent microbial growth on baffles and scouring action need to keep gap between baffle and vessel.  

Sparger: In fermenter air can be transferred two ways one via sparger and other without sparger (overhead). The main function of sparger is to provide oxygen to the growing cells. Air bubbling from sparger also helpful for mixing the culture.

Different types of sparger:

a)      Porous sparger: It is generally used for lab scale non agitated vessel and bubble formed 10 to 100 times larger than the pore size. Pressure drop occur across the sparger and the holes tend to blocked by the growth which is the limitation of it.  

b)      Orifice sparger (Perforated pipe) :  It is generally used in small scale fermenter with or without agitator. It is perforated pipe kept below the impeller in ring form and should have 3/4 of impeller diameter.

c)      Nozzle sparger (Opened or Partially closed sparger ) : It is mainly used for large scale fermenter. The single open or partially closed pipe centrally positioned below the impeller.

Level sensor: To determine the volume in the vessel different sensor used like, differential pressure sensor (DP sensor), Radar sensor and load shell.

DP (Differential pressure) sensor: Dp sensor having primary elements produce the pressure, Secondary element measure the pressure produce by primary element, Electronic housing receive signal from secondary element and produces current. It mainly used for flow measurement and level measurement. In this two-primary element/pressure sensor are kept one at top dead space as low pressure and another at the bottom of the vessel as high pressure and both elements/sensor connected to transmitter, it shows the level based on difference in pressure between these two sensors.

Radar sensor: There is a sensor rod dip to the bottom of the vessel to shows the level.

Load shell: it is weight based display whole vessel loaded on the load shell and display the value. It is applicable for vessel that have less vibration during operation.

Spray ball: It is located at the top lid of fermenter and used for effective cleaning of vessel. Effectiveness of spray ball checked with the help of riboflavin, In this case riboflavin sprayed by spray ball in the vessel and then cleaning is performed  as per defined procedure and then vessel is exposed to UV light.  

Condenser: It is located between vessel and outlet/exhaust filter. The main purpose of it is to prevent media evaporation loss and indirectly to prevent filter wetting/chock.