Body construction of bioreactor

1. Construction materials of bioreactors: 

Construction materials of bioreactors differ with small scale, pilot and large scale utelization. In small scale for bioreactor construction glass or stainless steel may be used. For pilot and large scale process, stainless steel (>4% chromium), mild steel (coated with glass or epoxy material), wood, plastic or concrete may be used as vessel construction material. Any vessel used should not have any corners and smooth surface is essential. The construction material must be non toxic and corrosion proof. The corrosion resistance of stainless steel is thought to depend on the existence of a thin hydrous oxide film on the surface of metal.

2. Vessel shape: 

Typical tanks are vertical cylinders with specialized top plates and bottom plates. In some cases, vessel design eliminates the need for a stirrer system especially in air lift fermenter. The top plates are of an elliptical or spherical dish shape. The top plates can be either removable or welded. A removable top plate provides best accessibility, but adds to cost and complexity. Various ports and standard nozzles are provided on the stainless plate for actuators and probes. These include pH, thermocouple, and dissolved oxygen probes ports, defaming, acid and base ports, inoculum port, pipe for sparging process air, agitator shaft and spare ports.

Tank bottom plates are also customized for specific applications. Almost most of the large vessels have a dish bottom, while the smaller vessels are often conical in shape or may have a smaller, sump type chamber located at the base of the main tank. 

Sealing between top plate and vessel is an important criteria to maintain airtight condition, aseptic and containment. Sealing have to be done between three types of surfaces viz. between glass-glass, glass- metal and metal-metal. There are three types of sealing. They are gasket, lip seal and ‘O’ ring. This sealing ensures tight joint in spite of expansion of vessel material during fermentation. The materials used for sealing may be fabricnitryl or butyl rubbers. The seals should be changed after finite time. There are two way of sealing in O ring type simple sealing and double sealing with steam between two seals.

3. Baffles: 

Baffles are metal strips that prevent vortex formation around the walls of the vessel. These metal strips attached radially to the wall for every 1/10th of vessel diameter. Usually 4 baffles are present. There should be enough gap between wall and baffle so that scouring action around vessel is facilitated. This movement minimizes microbial growth on baffles and fermentation walls. If needed cooling coils may be attached to baffles. The agitator (impeller) 

Agitation provides uniform suspension of cells in homogenous nutrient medium and agitation provides bulk fluid and gas phase mixing. It is achieved by: bulk fluid and gas-phase mixing, Air dispersion, Oxygen transfer, Heat transfer, Suspension of solid particles and maintain a uniform environment throughout the vessel contents. Enhancement of mass transfer between dispersed phases. There are four classes of impeller, namely Disc turbine, Vaned disc, Open turbine of variable pitch and Marine impeller. 

4. Aeration System (SPARGER): 

Sparger is a device for introducing air into fermenter. Aeration provides sufficient oxygen for organism in the fermenter. Fine bubble aerators must be used. Large bubbles will have less surface area than smaller bubbles which will facilitate oxygen transfer to a greater extent. Air supply to sparger should be supplied through filter. There are three types of sparger viz. porous sparger, orifice sparger and nozzle sparger. Valves and Steam Traps: 

Four types of valves are there on a bioreactor: 

1. Addition valves:There are four types of addition valves viz. 

    (a) Simple ON and OFF, 

    (b) For coarse control, 

    (c) Accurate adjustment and 

    (d) Safety valve-flow in one direction. 

2. Check valves: Valves used to prevent accidental reversal flow of liquid or gas due to break down. There are three types – swing check, lift check, combined stop and check. 

3. Pressure control valves: These types of valves are used for two purposes.(a) Pressure reduction (b) Pressure retaining. 

4. Safety valve: There are types of safety valve by which the increase in pressure is released. They are, 

    (a) A spindle lifted from its seating against the pressure – releases pressure 

   (b) Bursting / rupturing of discs to release pressure In case of releasing the gas, the escaping gas must be treated before release.

5. Steam Traps: 

Steam trap is important to remove any steam condensate. There are two components viz. valve and seat assembly and open / close device. The operation of the component is based on: 

(i) Density of fluid : A float (ball / bucket) float in water, sink in steam. When it floats it closes and when it sinks it opens the valve. 

(ii) Temperature of fluid : It has water / alcohol mixture which senses the change in temperature. This mixture expands in hot steam and closes the valve. When it contracts in cool water opens the valve. 

(iii) Kinetic effect of fluid in motion : If a low density steam is flowing it will be high velocity. Like wise high density will flow with low velocity. The conversion of pressure energy into kinetic energy control the opening and closing.