Fermenter Design PDF

Fermentation Fermentation is a metabolic process that converts sugar to acids, gases or alcohol. It occurs in yeast and bacteria, but also in oxygen- starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganism...

Fermentation Fermentation is a metabolic process that converts sugar to acids, gases or alcohol. It occurs in yeast and bacteria, but also in oxygen- starved muscle cells, as in the case of lactic acid fermentation. Fermentation is also used more broadly to refer to the bulk growth of microorganisms on a growth medium, often with the goal of producing a specific chemical product. The science of fermentation is known as zymology Range of Fermentation Processes To Produce Microbial cells or Biomass To Produce Microbial Enzymes To Produce Microbial Metabolites To Produce Recombinant Products To modify a compound which is added to the fermentation (Transformation) Steps to carry out a Fermentation The formulation of media to be used in culturing the process organism during the development of the inoculum and in the production fermenter. The sterilization of the medium, fermenters and ancillary equipment. The production of an active, pure culture in sufficient quantity to inoculate the production vessel. The growth of the organism in the production fermenter under optimum conditions for product formation. The extraction of the product and its purification. The disposal of effluents produced by the process Basic Design of a Fermenter Basic Functions of a Fermenter The vessel should be capable of being operated aseptically for a number of days and should be reliable in long-term operation and meet the requirements of containment regulations. Adequate aeration and agitation should be provided to meet the metabolic requirements of the micro-organism. However, the mixing should not cause damage to the organism. Power consumption should be as low as possible. A system of temperature control should be provided. A system of pH control should be provided. Sampling facilities should be provided. Evaporation losses from the fermenter should not be excessive. The vessel should be designed to require the minimal use of labour in operation, harvesting, cleaning and maintenance. Ideally the vessel should be suitable for a range of processes, but this may be restricted because of containment regulations. The vessel should be constructed to ensure smooth internal surfaces, using welds instead of flange joints whenever possible. The vessel should be of similar geometry to both smaller and larger vessels in the pilot plant or plant to facilitate scale-up. The cheapest materials which enable satisfactory results to be achieved should be used. There should be adequate service provisions for individual plants Hazard Assessment Systems Once the organism has been allocated to a hazard group, the appropriate containment requirements can be applied.  Hazard group 1 organisms used on a large scale only require Good Industrial Large Scale Practice (GILSP). Processes in this category need to be operated aseptically but no containment steps are necessary, including prevention of escape of organisms. If the organism is placed in Hazard group 4 the stringent requirements of level 3 will have to be met before the process can be operated. Structure of a Fermenter Materials for Body Construction of a Fermenter In fermentations with strict aseptic requirements it is important to select materials that can withstand repeated steam sterilization cycles. On a small scale (1 to 30 dm3) it is possible to use glass and/or stainless steel. Glass is useful because it gives smooth surfaces, is non-toxic, corrosion proof and it is usually easy to examine the interior of the vessel. Pilot scale and Industrial scale vessels are normally constructed of stainless steel or at least have a stainless-steel cladding to limit corrosion. The American Iron and Steel Institute (AISI) states that steels containing less than 4% chromium are classified as steel alloys and those containing more than 4% are classified as stainless steels. Mild steel coated with glass or phenolic epoxy materials has occasionally been used. The corrosion resistance of stainless steel is thought to depend on the existence of a thin hydrous oxide film on the surface of the metal. The composition of this film varies with different steel alloys and different manufacturing process treatments such as rolling, pickling or heat treatment. The film is stabilized by chromium and is considered to be continuous, non-porous, insoluble and self healing. The minimum amount of chromium needed to resist corrosion will depend on the corroding agent in a particular environment, such as acids, alkalis, gases, soil, salt or fresh water. Increasing the chromium content enhances resistance to corrosion, but only grades of steel containing at least 10 to 13% chromium develop an effective film. The inclusion of nickel in high percent chromium steels enhances their resistance and improves their engineering properties. The presence of molybdenum improves the resistance of stainless steels to solutions of halogen salts and pitting by chloride ions in brine or sea water. Corrosion resistance can also be improved by tungsten, silicone and other elements. At this stage it is important to consider the ways in which a reliable aseptic seal is made between glass and glass, glass and metal or metal and metal joints such as between a fermenter vessel and a detachable top or base plate. Types of Seals Gasket Seal Lip Seal O ring Seal With glass and metal, a seal can be made with a compressible gasket, a lip seal or an '0' ring. With metal to metal joints only an '0' ring is suitable. Nitryl or butyl rubbers are normally used for these seals as they will withstand fermentation process conditions. Aeration and Agitation Primary purpose of aeration is to provide microorganisms in submerged culture with sufficient oxygen for metabolic requirements. While agitation should ensure that a uniform suspension of microbial cells is achieved in a homogenous nutrient medium. Agitator (Impellers) The agitator is required to achieve a number of mixing objectives, e.g. bulk 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 may be 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. Stirrer Glands and Bearings The satisfactory sealing of the stirrer shaft assembly top plate has been one of the most difficult problems to overcome in the construction of fermentation equipment which can be operated aseptically for long periods. The stirrer shaft can enter the vessel from the top, side or bottom of the vessel.  A simple Stirrer Seal A porous bronze bearing for a 13-mm shaft was fitted in the centre of the fermenter top and another in a yoke directly above it. The bearings were pressed into steel housings, which screwed into position in the yoke and the fermenter top. The lower bearing and housing were covered with a skirt-like shield having a 6.5 mm overhang which rotated with the shaft and prevented air- borne contaminants from settling on the bearing and working their way through it into the fermenter.  The Mechanical Seal The seal 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. The two meeting surfaces have to be precision machined, the moving surface normally consists of a carbon- faced unit while the stationary unit is of stellite-faced stainless steel. Baffles Four baffles are normally incorporated into agitated vessels of all sizes to prevent a vortex and to improve aeration efficiency. Baffles are metal strips roughly one-tenth of the vessel diameter and attached radially to the wall. The agitation effect is only slightly increased with wider baffles, but drops sharply with narrower baffles. It is recommended that baffles should be installed so that a gap existed between them and the vessel wall, so that there was a scouring action around and behind the baffles thus minimizing microbial growth on the baffles and the fermenter walls. Extra cooling coils may be attached to baffles to improve the cooling capacity of a fermenter without unduly affecting the geometry. 46 Aeration System (Spargers) A sparger may be defined as a device for introducing air into the liquid in a fermenter. Three basic types of sparger have been used and may be described as the Porous sparger, the Orifice sparger (a perforated pipe) and the Nozzle sparger (an open or partially closed pipe).  Porous Sparger The porous sparger of sintered glass, ceramics or metal, has been used primarily on a laboratory scale in non-agitated vessels. The bubble size produced from such spargers is always 10 to 100 times larger than the pore size of the aerator block. There is also the problem of the fine holes becoming blocked by growth of the microbial culture.  Orifice Sparger In small stirred fermenters the perforated pipes were arranged below the impeller in the form of crosses or rings (ring sparger), approximately three-quarters of the impeller diameter. In most designs the air holes were drilled on the under surfaces of the tubes making up the ring or cross. Sparger holes should be at least 6 mm (1/4 inch) diameter because of the tendency of smaller holes to block and to minimize the pressure drop.  Nozzle Sparger Single open or partially closed pipe as a sparger to provide the stream of air bubbles. Ideally the pipe should be positioned centrally below the impeller and as far away as possible from it to ensure that the impeller is not flooded by the air stream. Sterilization of Air Supply for Fermentation Sterile air will be required in very large volumes in many aerobic fermentation processes. Heating and Filtration are the main methods for sterilization. Heat is generally too costly for full scale operation. Glass wool, glass fibre or mineral slag wool have been used as filter material, but currently most fermenters are fitted with cartridge-type filters. Valves and Steam Traps Valves attached to fermenters and ancillary equipment are used for controlling the flow of liquids and gases in a variety of ways. There are four main types of valves: Simple ON/OFF valves which are either fully open or fully closed. Valves which provide coarse control of flow rates. Valves which may be adjusted very precisely so that flow rates may be accurately controlled. Safety valves which are constructed in such a way that liquids or gases will flow in only one direction. 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 waterline for use when fully open or fully closed and therefore should not be used for regulating flow. Not suitable for aseptic conditions. there may be leakage round the stem of the valve which is sealed by a simple stuffing box. This means that the nut around the stem and the packing must be checked regularly. Needle Valve 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 body can be used to give fine control of steam or liquid flow. Accurate control of flow is possible because of the variable orifice formed between the tapered plug and the tapered seat. The aseptic applications are very limited. Ball Valve 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 between two wiping surfaces which wipe the surface and prevent deposition of matter at this point. The valve is suitable for aseptic operation, can handle mycelial broths and can be operated under high temperatures and pressures. 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 may be achieved by incorporating steam traps, which will collect and remove automatically any condensate at appropriate points in steam lines. A steam trap has two elements. One is a valve and seat assembly which provides an opening, which may be of variable size, to ensure effective removal of any condensate. The second element is a device which will open or close the valve by measuring some parameter of the condensate reaching it to determine whether it should be discharged. Air Lift Fermenter An air-lift fermenter is essentially a gastight baffled riser tube (liquid ascending) connected to a downcomer tube (liquid descending). The driving force for circulation of medium in the vessel is produced by the difference in density between the liquid column in the riser (excess air bubbles in the medium) and the liquid column in the downcomer (depleted in air bubbles after release at the top of the loop). Circulation times in loops of 45-m height may be 120 seconds. This type of vessel can be used for continuous culture. It would be uneconomical to use a mechanically stirred fermenter to produce SCP (single-cell protein) from methanol as a carbon substrate, as heat removal would be needed in external cooling loops because of the high rate of aeration and agitation required to operate the process. To overcome these problems, particularly that of cooling the medium when mechanical agitation is used, air-lift fermenters with outer or inner loops were chosen. Oxygen requirements of Fermentation A microbial culture must be supplied with oxygen during growth at a rate sufficient to satisfy the organisms' demand. The oxygen demand of an industrial fermentation process is normally satisfied by aerating and agitating the fermentation broth. Effects of Dissolved Oxygen concentration Hirose and Shibai’s (1980) investigations of amino acid biosynthesis by Brevibacterium flavum provides an excellent example of the effects of the dissolved oxygen concentration on the production of range of closely related metabolites. These workers demonstrated the critical dissolved oxygen concentration for B. flavum to be 0.01 mg dm-3 and considered the extent of oxygen supply to the culture in terms of the degree of 'oxygen satisfaction’, that is the respiratory rate of the culture expressed as a fraction of the maximum respiratory. A value of oxygen satisfaction below unity implied that the dissolved oxygen concentration was below the critical level. Oxygen Supply Bartholomew et at. (1950) represented the transfer of oxygen from air to the cell, during a fermentation, as occurring in a number of steps: The transfer of oxygen from an air bubble into solution. The transfer of the dissolved oxygen through the fermentation medium to the microbial cell. The uptake of the dissolved oxygen by the cell