Design of a fermenter

Design of a fermenter

Construction materials

The fermenter must withstand repeated steam sterilization cycles. Glass and/or stainless steel are generally used.

On laboratory scale glass is mainly used. Glass gives smooth surfaces, is non-toxic, corrosion proof and it is usually easy to examine the interior of the vessel.

Two basic designs where glass is used as the fermenter body material are

1. A glass vessel with a round or flat bottom and a top flanged carrying plate

2. A glass cylinder with stainless-steel top and bottom plates

At pilot and industrial scale, vessels are normally constructed of stainless steel or mild-steel.  The corrosion resistance of stainless steel is an advantage. The inclusion of nickel, chromium, molybdenum, tungsten, silicone, etc in steel enhances their engineering properties.

In a vessel there will be joints between glass and glass or glass and metal or metal and metal joints such as between a fermenter vessel and a detachable top or base plate. A reliable aseptic seal should be made between such joints. With glass and metal, a gasket seal, a lip seal or an O ring seal may be used. In metal to metal joints only an O ring is suitable. O ring seal ensures a good liquid- and/or gas-tight joint despite the glass or metal expanding or contracting at different rates with changes in temperature during a sterilization cycle or an incubation cycle. Nitryl or butyl rubbers are normally used for these seals as they will withstand fermentation process conditions.

Temperature control

In a fermenter there should be provision for temperature control. Heat will be generated by microbial activity and also through mechanical agitation.  If the heat generated by these two processes is not ideal for the particular manufacturing process, then we may have to add or remove heat from the system.

On a laboratory scale extra heat is generally provided by placing the fermenter in a thermostatically controlled bath, or by the use of internal heating coils or a heating jacket through which water is circulated or by a silicone heating jacket.

In larger vessels internal coils are used and cold water is circulated to reduce the temperature to achieve the correct temperature. 

Aeration and agitation

Primary purpose of aeration is to provide microorganisms with sufficient oxygen for metabolic requirements, while agitation ensure the uniform suspension of microbial cells in a homogeneous nutrient medium. Some fermenters work with mechanical agitation system while some others employ non mechanical aeration-agitation system.

The structural components of the fermenter involved in aeration and agitation are:

(a) The agitator (impeller).

(b) Stirrer glands and bearings.

(c) Baffles.

(d) The aeration system (sparger).

The agitator or impeller

The agitator is required for mixing, for fluid and gas-phase mixing, air dispersion, oxygen transfer, heat transfer, suspension of solid particles and maintaining a uniform environment throughout the vessel contents.

Agitators are classified as disc turbines, vaned discs, open turbines of variable pitch and propellers.

The disc turbine consists of a disc with a series of rectangular vanes set in a vertical plane around the circumference.  The vaned disc has a series of rectangular vanes attached vertically to the underside. Air from the sparger hits the underside of the disc and is displaced towards the vanes where the air bubbles are broken up into smaller bubbles.

The vanes of an open turbine and the blades of a marine propeller are attached directly to a boss on the agitator shaft. In this case the air bubbles do not initially hit any surface before dispersion by the vanes or blades.

Stirrer glands and bearings

The satisfactory and aseptic sealing of the stirrer shaft assembly is difficult.  The stirrer shaft can enter the vessel from the top, side or bottom of the vessel. Top entry is most commonly used, but bottom entry may be advantageous if more space is needed on the top plate for entry ports, and also shorter shaft will be needed here which permits higher stirrer speeds without shaft whipping. But  bottom entry stirrers would be submerged

Four basic types of seal assembly used are the stuffing box (packed-gland seal), the simple bush seal, the mechanical seal and the magnetic drive.

Most modern fermenter incorporate mechanical seals instead of stuffing boxes and packed glands. Mechanical seals are more expensive, but are more durable and less likely results in contamination or leakage. Magnetic drives which are quite expensive, are being used in some animal cell culture vessels.

The stuffing box or packed-gland seal

The shaft is sealed by several layers of packing rings of asbestos or cotton yarn, pressed against the shaft by a gland follower. At high stirrer speeds the packing wears quickly and excessive pressure may be needed to ensure tightness of fit. The packing may be difficult to sterilize and it is necessary to check and replace the packing rings regularly.

The mechanical seal

The mechanical seal assembly is composed of two parts, one part is stationary in the bearing housing, the other rotates on the shaft, and the two components are pressed together by springs or expanding bellows. Steam condensate is used to lubricate and cool the seals during operation and the steam also serve as a containment barrier.

Magnetic Drives

In magnetic drive the impeller shaft does not pierce the vessel.  It consists of two magnets: one driving and one driven. The driving magnet is held in bearings in a housing on the outside of the head plate and connected to a drive shaft. The driven magnet is placed on one end of the impeller shaft, held in bearings in a housing on the inner surface of the headplate. Using magnets power will be transmitted.  This is ideal for minimizing contamination.

Baffles

Four to eight baffles are normally incorporated into fermenter to prevent vortex and to improve aeration efficiency.  These are metal strips roughly one-tenth of the vessel diameter and attached radially to the wall

Sparger

A sparger is a device for introducing air into the liquid in a fermenter. Three basic types of sparger are porous sparger, the orifice sparger (a perforated pipe) and the nozzle sparger (an open or partially closed pipe). A combined sparger-agitator are also used.

Porous sparger

The porous sparger of sintered glass, ceramics or metal, are used on a laboratory scale vessel. The bubble size produced from such spargers is 10 to 100 times larger than the pore size of the aerator.  There is a problem of the fine holes becoming blocked by growth of the microbial culture.

Orifice sparger

These are perforated pipes. In small stirred fermenters the perforated pipes are arranged below the impeller in the form of crosses or rings (ring sparger).

Nozzle sparger

Most modern mechanically stirred laboratory to industrial scale fermenter have a single open or partially closed pipe as a sparger which is positioned below the impeller.

Combined sparger-agitator

The combined sparger-agitator introduce the air via a hollow agitator shaft and emit it through holes drilled in the disc between the blades. The design gives good aeration.

Sterilization of the fermenter

The fermenter may be steam sterilized under pressure. The medium may be sterilized in the vessel or sterilized separately and subsequently added aseptically to the fermenter. Steam should be introduced through all the entry and exit points and steam should be allowed exit through the air outlet. Steam should reach all parts of the equipment and each drainage point in the pipework should be fitted with a steam trap.

Sterilization of the air supply

Sterile air will be required in very large volumes in many aerobic fermentation processes.  Filter sterilization is generally adopted.  Most fermenters are fitted with cartridge-type filters. 

Sampling

The sampling points should be working while maintaining sterility. A sterile barrier must be maintained between the fermenter contents and the exterior when the sample port is not being used and it must be sterilizable after use.

Feed ports

Additions of nutrients and acid/alkali to small fermenters are normally made via silicone tubes which are autoclaved separately. It can be aseptically connected to fermenter and pumped by a peristaltic pump. The ports which are used intermittently can be sterilized in situ with steam.

Sensor probes

Double '0' ring seals are used to provide an aseptic seal for glass electrodes in stainless steel housings.

Foam control

In any fermentation it is important to minimize foaming. When excessive foaming, the filters become wet and result in contamination. Also there may be loss of all or part of the contents of the fermenter. Antifoams may be used or Foam breakers are used.  Foam breakers break down foam by an impact mechanism by a rotating mechanism inside the fermenter.

Valves and steam traps

Valves attached are used for controlling the flow of liquids and gases. The valves may be:

1. Simple ON/OFF valves which are either fully open or fully closed.

2. Valves which provide coarse control of flow rates.

3. Valves which may be adjusted very precisely so that flow rates may be accurately controlled.

4. Safety valves which are constructed in such a way that liquids or gases will flow in only one direction.

The valves may open and close by

(a) raising or lowering the blocking unit with a screw thread (rising stem)

(b) a drilled sphere or plug or a disc rotating in between two bearings

(c) a rubber diaphragm or tube which is pinched.

Gate valves

In this valve, a sliding disc is moved in or out of the flow path by turning the stem of the valve. It is suitable for general purposes on a steam or a water line for use as fully open or fully closed.  It is not suitable for aseptic conditions.

Globe valves

In this valve a horizontal disc or plug is raised or lowered in its seating to control the rate of flow.  It is used for regulating the flow of water or steam.  It is not suitable for aseptic operation.

Piston valves

These are similar to a globe valve except that the flow is controlled by a piston passing between two packing rings. This is very efficient under aseptic operation. But there may be blockage problems with mycelial cultures.

Needle valves

The needle valve is similar to the globe valve, except that the disc is replaced by a tapered plug or needle fitting into a tapered valve seat. The valve can be used to give fine control of steam or liquid flow.  It is not generally used for aseptic applications.

Plug valves

In this valve there is a parallel or tapered plug sitting in a housing through which an orifice has been been machined. When the plug is turned through 90° the valve is fully open and the flow path is determined by the cross-sectional area of orifice. It can provide good flow control.

Ball valves

This valve has been developed from the plug valve. The valve element is a stainless-steel ball through which an orifice is machined. The ball is sealed inside a pipeline. The valve is suitable for aseptic operation, can handle mycelial broths and can be operated under high temperatures and pressures.

Butterfly valves

The butterfly valve consists of a disc which rotates about a shaft in a housing. The disc closes against a seal to stop the flow of liquid. It is not suitable for aseptic operation.

Pinch valves

In the pinch valve a flexible sleeve is closed by a pair of pinch bars or some other mechanism which can be operated by compressed air remotely or automatically. The flow rate can be controlled. The valve is suitable for aseptic operation and suitable with mycelial cultures.

Diaphragm valve

Like the pinch valve, the diaphragm valve makes use of a flexible closure.  This valve is very suitable for aseptic operation provided that the diaphragm is of a material which will withstand

repeated sterilization. The valve can be used for ON/OFF, flow regulation, and for steam.

The most suitable valve

Among these group of valves, globe and butterfly valves are most commonly used for ON/OFF applications, gate valves for crude flow control, needle valves for accurate flow control and ball, pinch or diaphragm valves for all sterile uses.

Check valves

The purpose of the check valve is to prevent accidental reversal of flow of liquid or gas in a pipe due to breakdown in some part of the equipment. There are three basic types of valve: swing check, lift check and combined stop and check.

Pressure-reduction valves

Pressure-reduction valves are incorporated into pipelines when it is necessary to reduce from a higher to a lower pressure.

Pressure-retaining valves

A pressure-retaining valve will maintain pressure in the pipeline upstream.

Safety valves

Safety valves must be incorporated into every air steam line and vessel which is subject to pressure

ensure that the pressure will never exceed the upper limit recommended by the manufacturer.

Steam traps

In all steam lines it is essential to remove any steam condensate which accumulates in the piping to ensure optimum process conditions. This is achieved by steam traps, which will collect and remove any condensate at appropriate points in steam lines automatically.