Bioreactors (Fermenters): Function, Designs and types
A bioreactor is a device in which a substrate of low value is utilized by living cells or enzymes to generate a product of higher value. Bioreactors arc extensively used for food processing, fermentation, waste treatment, etc.
On the basis of the agent used, bioreactors are grouped into the following two broad classes: (i) those based on living cells and, (ii) those employing enzymes. But in terms of process requirements, they are of the following types: (i) aerobic, (ii) anaerobic, (iii) solid state, and (iv) immobilized cell bioreactors.
All bioreactors deal with heterogeneous systems dealing with two or more phases, e.g., liquid, gas, solid. Therefore, optimal conditions for fermentation necessitate efficient transfer of mass, heat and momentum from one phase to the other. Chemical engineering principles are employed for design and operation of bioreactors. But, in general, theoretical explanation usually lags behind technical realization.
A bioreactor should provide for the following: (i) agitation (for mixing of cells and medium), (ii) aeration (aerobic fermenters; for O2 supply), (iii) regulation of factors like temperature, pH, pressure, aeration, nutrient feeding, liquid level, etc., (iv) sterilization and maintenance of sterility, and (v) withdrawal of cells/medium (for continuous fermenters). Modern fermenters are usually integrated with computers for efficient process monitoring, data acquisition, etc.
The first truly large-scale aseptic anaerobic fermentation vessels were developed in the wake of the process developed (during the First World War, 1914-1918) by Weizmann and co-workers of U.K. to produce acetone by a deep liquid fermentation using Clostridium acetobutylicum.
For this, large cylindrical vessels of mild steel that permitted sterilization with steam under pressure were constructed, and piping, joints and valves were specially designed to maintain aseptic conditions, which were the major problem; mixing was achieved by the large volumes of gas produced during fermentation.
The large-scale aerobic fermentation vessels were first used in Central Europe during 1930s for the production of compressed yeast; these fermenters had large cylindrical tanks in which air was introduced at the base via a network of perforated pipes.
In later modifications, mechanical impellers were used to improve mixing of broth and dispersal of air bubbles. Fermenter design was considerably improved during 1940s to accommodate the requirements of strict aseptic conditions, and good agitation and aeration for penicillin production from submerged cultures; for this, steel fermenters with working volumes of 54,000 1 were built in U.S.A. In 1944, Cooper and co-workers (and several others) reported the findings from studies on agitation in baffled stirred tank fermenters, which had a major influence on the design of later fermenters.
Basic Functions of a Fermenter:
- It should provide a controlled environment for optimum biomass/product yields.
- It should permit aseptic fermentation for a number of days reliably and dependably, and meet the requirements of containment regulations. Containment involves prevention of escape of viable cells from a fermenter or downstream processing equipment into the environment.
- It should provide adequate mixing and aeration for optimum growth and production, without damaging the microorganisms/cells. The above two points (items 2 and 3) are perhaps the most important of all.
- The power consumption should be minimum.
- It should provide easy and dependable temperature control.
- Facility for sampling should be provided.
- It should have a system for monitoring and regulating pH of the fermentation broth.
- Evaporation losses should be as low as possible.
- It should require a minimum of labour in maintenance, cleaning, operating and harvesting operations.
- It should be suitable for a range of fermentation processes. But this range may often be restricted by the containment regulations.
- It should have smooth internal surfaces, and joints should be welded wherever possible.
- The pilot scale and production stage fermenters should have similar geometry to facilitate scale-up.
- It should be contrasted using the cheapest materials that afford satisfactory results.
- There should be adequate service provisions for individual plants.
A bioreactor is a device in which a substrate of low value is utilized by living cells or enzymes to generate a product of higher value. Bioreactors are extensively used for food processing, fermentation, waste treatment, etc.
On the basis of the agent used, bioreactors are grouped into the following two broad classes: (i) those based on living cells and, (ii) those employing enzymes. But in terms of process requirements, they are of the following types: (i) aerobic, (ii) anaerobic, (iii) solid state, and (iv) immobilized cell bioreactors.
All bioreactors deal with heterogeneous systems dealing with two or more phases, e.g., liquid, gas, solid. Therefore, optimal conditions for fermentation necessitate efficient transfer of mass, heat and momentum from one phase to the other. Chemical engineering principles are employed for design and operation of bioreactors.
But, in general, theoretical explanation usually lags behind technical realization. A bioreactor should provide for the following: (i) agitation (for mixing of cells and medium), (ii) aeration (aerobic fermenters; for O2 supply), (iii) regulation of factors like temperature, pH, pressure, aeration, nutrient feeding, liquid level, etc., (iv) sterilization and maintenance of sterility, and (v) withdrawal of cells/medium (for continuous fermenters). Modern fermenters are usually integrated with computers for efficient process monitoring, data acquisition, etc.
Agitation and Aeration:
In scaling up, both chemical (O2, pH, medium constituents and removal of wastes) and physical (the configuration of bioreactor and power supplied to the reactor) factors have to be optimised for good results.
The medium must be suitably stirred to keep the cells in suspension and to make the culture homogeneous; it becomes increasingly difficult with the scaling up. Various types of stirrers range from simple magnetic stirrers, flat blade turbine impellers, to marine impellers, to those using pneumatic energy, e.g., airlift fermenter, and those using hydraulic energy, e.g., medium perfusion.
Improved mixing can be obtained by changing the design of stirrer paddle or by using multiple impellers. The objective of stirring is to achieve good mixing without causing damage to the cells. Vibro-mixer achieves stirring by vertical reciprocating motion of 0.1-3 mm at a frequency of 50 cycles/sec of a mixing disc fixed horizontally to the agitator shaft. These stirrers cause random mixing, less foaming and lower shear forces.
It is important to supply sufficient O2 without damaging the cells. Mean O2 utilization rate by cells is about 6 mg O2/106 cells/hour. But O2 is only sparingly soluble in culture medium; the oxygen transfer rate (OTR) from gas phase into medium is about 17 µg/cm/hr.
Therefore surface aeration can support about 50 x 106 cells in I 1 culture vessel. Efficient aeration is achieved by bubbling air through the medium (sparging), but this may damage animal cells due to the high surface energy of the bubble and on the cell membrane.
The damage can be reduced by using larger bubbles, lower gassing rates and by adding non-nutritional supplements like Pluronic F-68 (polyglycol) and sodium carboxymcthyl cellulose (these protect cells from damage due to shear forces and bubbles, respectively). Silicone tubing (highly gas permeable) can be arranged inside the culture vessel (2-5 cm tubing of 30 111 length for a 1000 1 culture) and air is passed though the tube; however it is inconvenient to use.
Aeration may be achieved by medium perfusion, in which medium is continuously taken from culture vessel, passed through an oxygenation chamber and returned to the culture. The cells are removed from the medium taken for perfusion so that the medium can be suitably altered, e.g., for pH control. Perfusion is used with glass bead and, more particularly, with micro-carrier systems.
Where considered safe and desirable, O2 supply in the culture vessel can be enhanced from the normal 21% to a higher value and the air pressure can be increased by 1 atmosphere. This increases the O2 solubility and diffusion rates in the medium, but there is a risk of O2 toxicity.
The basic objective of aeration is to provide microorganisms growing in submerged cultures with adequate oxygen for their metabolic needs. Agitation, on the other hand, aims to ensure a homogeneous distribution of microorganisms and the nutrients in the broth.
The type of aeration- agitation system used in the fermenter is dictated by the characteristics of the fermentation process. For example, in processes based on low viscosity, low total solids broths, agitation may not be needed as aeration itself would create the necessary agitation.
Fine bubble aerators without mechanical agitation offer the advantage of lower equipment and power costs. Such fermentations are usually carried out in vessels having height/diameter ratio of 5 : 1, but a tall column of liquid would require a higher energy input for the compression of air used for aeration.
However, mechanical agitation is usually necessary for fermentation processes based on actinomycetes and fungi. The following components of the fermenter are required for aeration and agitation: (t) agitator (impeller), (ii) stirrer glands and bearings, (iii) baffles, and (iv) sparger (the aeration system).
- Agitator (Impeller):
Agitators achieve the following objectives; (a) bulk fluid and gas-phase mixing, (b) air dispersion, (c) oxygen transfer, (d) heat transfer, (e) suspension of solid particles, and (f) maintenance of a uniform environment throughout the vessel.
These objectives are achieved by a suitable combination of the most appropriate agitator, air sparger and baffles, and the best positions for nutrient feeds, acid or alkali for pH control and antifoam addition. Agitators are of several different types, e.g., (i) disc turbines, (ii) vaned discs, (iii) open turbines of variable pitch and (iv) propellers.
Disc turbine consists of a disc with a series of rectangular vanes set in a vertical plane around its perpheri (Fig. 14.1 A). The vaned disc turbine has a series of rectangular vanes attached vertically to the underside of the disc (Fig. 14. 1B). In case of variable pitch open turbine, the vanes are attached directly to a boss on the agitator shaft (Fig. 14.1C).
The marine propeller is similar to variable pitch open turbine, except that it has blades in the place of vanes (Fig. 14.1D). In case of disc and vaned disc turbines, the air bubbles from the sparger first-hit the underside of disc before being broken into smaller bubbles and dispersed by the vanes.
But in the case of the latter two types of agitators, air bubbles contact the vanes/blades directly and arc broken up and dispersed by them. These basic agitation devices have been variously modified. For example, the variable pitch open turbine scheme has been modified to develop four modern agitator types, viz., Scaba 6SRGT, Prochem Maxflo T, Lightning A315 and the Ekato Intcrmig.
The Rushton disc turbine, having a diameter of one-third the fermenter diameter, has been long considered optimum for many fermentation processes. The disc turbine was considered optimum because it was shown to be able to break up a fast air stream without itself becoming flooded in air bubbles; the latter situation seriously hampers oxygen dispersal in the broth.
In contrast, the impeller and open turbine were found to have the tendency to be flooded in air at higher aeration rates. In subsequent studies, it was found that in low viscosity broths, all the four agitator types can achieve good gas dispersion provided the agitator speed is high enough.
In such broths, agitator type does not appear to be a significant factor affecting oxygen transfer efficiencies. In high viscosity broths, however, gas dispersal presents problems and is greatly reduced. In view of this, a number of agitators have been developed for high viscosity broths, e.g., Scaba 6SRGT, Prochem Maxflow T, Lightning A315 and Ekato Intermig (Fig. 14.2).
These agitators are larger, require lower power input (they do not lose as much power as the Rushton turbines when aerated), are able to handle higher air volumes without flooding, and give better bulk blending and heat transfer in more viscous media.
But they can cause mechanical problems mostly of vibrational nature. Good mixing and aeration in high viscosity broths may also be achieved by a dual impeller combination in which the lower impeller primarily dispenses the air, while the upper impeller primarily enhances mixing of the broth.
- Stirrer Glands and Bearings:
The satisfactory sealing of the stirrer shaft assembly has been one of the most difficult problems; this is very important for maintaining aseptic conditions over long periods. Four basic types of seal assembly have been used in fermenters: (1) the stuffing box (packed- gland seal), (2) the simple bush seal, (3) the mechanical seal and (4) the magnetic drive.
Most modern fermenters use mechanical seals; these seals are more expensive, but they are more durable and less prone to leakage or contaminant entry. Magnetic drives, although quite expensive, are being used in some animal cell culture vessels.
The mechanical seal consists of two parts; one part remains stationery in the bearing housing, while the other rotates on the shaft. The two components of the seal are pressed together by springs or expanding bellows. Steam condensate is used to lubricate and cool the seals during operation and servers as a contaminant barrier.
Baffles are metal strips roughly one-tenth of the vessel diameter and attached radially to the fermenter wall (Fig. 14.3). They are normally used in fermenters having agitators to prevent vortex formation and to improve aeration efficiency.
Usually, four baffles are used, but larger fermenters may have 6 or 8 baffles. Extra cooling coils may be attached to baffles to improve cooling. Further, the baffles may be installed in such a way that a gap exists between the baffles and the fermenter wall. This would lead to a scouring action around and behind the baffles, which would minimise microbial growth on the baffles and the fermenter wall.
- Aeration System (Sparger):
The device used to introduce air into the fermenter broth is called sparger. Spargers are of the following three basic types: (1) porous spargers, (2) orifice spargers and (3) nozzle spargers. Porous spargers may be made of sintered glass, ceramics or a metal.
They are used primarily on a laboratory scale in non-agitated vessels. The bubble size from such spargers is always 10 to 100 limes larger than the pore size of the sparger. These spargers have low air throughput because pressure drops across the sparger, and the fine holes often become blocked by microbial growth.
Orifice spargers consist of perforated pipes arranged in various ways, e.g., the sparger pipe forming a ring below the impeller. In most cases, air holes are drilled on the underside of the pipe and the holes are arranged in the form of ring or cross.
It is desirable that the holes are at least 6 mm in diameter to avoid clogging by microbial growth. These spargers (without agitation) have been used to a limited extent in yeast manufacture, effluent treatment and in air-lift fermenters used for single-cell protein (SCP) production.
Nozle sparger consists of an open or partially closed pipe. Most modern fermenters (laboratory to production scale) have a single open or partially closed pipe as a sparger that is ideally placed centrally below the impeller.
It provides a stream of air bubbles. The sparger should be as far below the impeller as possible to avoid flooding of the impeller in a stream of air bubbles. These spargers cause a lower pressure loss than the other spargers and they are not easily blocked.
In small fermenters, a combined sparger-agitator may be used. In this case, the air is introduced via a hollow agitator shaft, and it comes out through holes drilled in the disc between the blades and connected to the base of the main shaft. This design gives a good aeration in baffled vessels over a range of agitator speeds.
The fermenter must have an adequate provision for temperature control. Both microbial activity and agitation will generate heat. If this heat generates a temperature that is optimum for the fermentation process, then heat removal or addition may not be required.
But in most cases, this may not be the case; in all such cases, either additional heating or removal of the excess heat would be required. Temperature control may be considered at laboratory scale, and pilot and production scales.
- In laboratory scale fermentations, normally little heat is generated. Therefore, heat has to be added to the system; this can be achieved in the following ways: (a) the fermenter may be placed in thermostatically controlled bath, (b) internal heating coils may be used, (c) water may be circulated through a heating jacket, or (d) a silicone healing jacket may be used. The silicone jacket consists of two silicone rubber mats, and heating wires between these mats. This jacket is wrapped around the fermenter and is held in place by Velcro strips.
- In case of larger fermenters beyond a certain size, excess heat is generated, and the fermenter surface becomes inadequate for heat removal. The size at which fermenter surface becomes inadequate for heat removal will depend on the fermentation process and the ambient temperature at which fermentation is being carried out. In such cases, internal coils have to be used to circulate cold water through them for removing the excess heat.
The cooling surface area necessary for temperature control will depend mainly on the following factors: (i) temperature of cooling water, (ii) the culture temperature, (iii) the type of microorganism, and (iv) the energy provided by stirring. The average cooling area for a 55,000 l fermenter may be considered to be around 50-70 m2; if the cold water temperature were 14°C, the broth temperature would cool down to 30°C from 120°C in 2.5 to 4 hours without stirring. The cooling water consumed during bacterial fermentation in a vessel of this size would range between 500 to 2,000 l h-1. Fungal fermentation, however, may need 2,000 to 10,000 l cooling water per hour as they have a lower optimum temperature for growth.
The heating/cooling requirements for a specific fermentation process can be accurately estimated by taking into account the overall energy balance of the process, which is described by the following formula.
Qmet + Qag + Qgas = Qacc + Qexch + Qevap + Qsen
where, Qmet – the rate of heat generated by microbial metabolism;
Qag = the rate of heat produced by mechanical agitation;
Qgas = the rate of heat generated by aeration power input; Qacc = the rate of heat accumulation in the system;
Qexch = the rate of heat transfer to the surroundings and/or heat exchanger, i.e., heating/cooling device;
Qevap = the rate of heat loss due to evaporation; and
Qsen = the rate of sensible enthalpy gain by the flow streams (exit-inlet). This equation may be arranged as follows.
Qexch = Qmet + Qag + Qgas – Qacc – Qsen – Qevap
In this equation, Qexch provides the estimate of heat that has to be removed by the cooling system. The values for Qmet are experimentally determined for different substrates, while those of Qag, Qgas, Qevap,and Qsen are computed using appropriate methods/formulae. For example, estimates of these values for Bacillus subtilis grown on molasses in one study are summarised in Table 14.7.
Among these values, contributions due to Qevap and Qsen are quite small. In a steady-state system, Qacc is zero; further, Qevup can be eliminated by using a saturated air stream that has the same temperature as the broth. In large fermenters, Qevap will depend on operating temperature and flow conditions. Similarly, Qag, will be determined by the choice of agitator and the speed of agitation, while aeration rate and sparger design will determine Qgas.
Once the value of Qexch is estimated, the cooling requirement (jacket and/or pipes) can be computed using appropriate formulae. The factors that will affect the heat transfer surface area may be summarised as follows: vessel geometry, fluid properties, flow velocity, wall material and thickness, the temperature difference between the cooling agent and the broth, and of course the value of Qexch.
Foam is produced during most microbial fermentations. Foaming may occur either due to a medium component, e.g., protein present in the medium, or due to some compound produced by the microorganism. Proteins are present in corn-steep liquor, pharma media, peanut meal, soybean meal, etc.
These proteins may denature at the air-broth interface and form a protein film that does not rupture readily. Foaming can cause removal of cells from the medium; such cell wills undergo autolysis and release more proteins into the medium. This, in turn, will further stabilize the foam. Five different patterns of foaming are recognized; these are listed below.
- Foaming remains at a constant level throughout the fermentation. Initial foaming is due to the medium, but later microbial activity contributes to it.
- Foaming declines steadily in the initial stages, but remains constant thereafter. This type of foaming is due to the medium.
- The foaming increases after a slight initial fall’, in this case, microbial activity is the major cause of foaming.
- The foaming level increases with fermentation duration; such foaming pattern is solely due to microbial activity.
- A complex foaming pattern that combines features of two or more of the above patterns.
Foaming may lead to several physical and biological problems. Some examples of physical problems are as follows:
(1) The working volume of the fermenter may decrease due to a circulation of oxygen-depleted gas bubbles in the system.
(2) The bubble size may also decrease, and
(3) The heat and mass transfer rates may also decline.
(4) Foaming may interfere with the functioning of sensing electrodes resulting in invalid process data, and incorrect monitoring and control of pH, temperature, etc. The biological problems of foaming include (1) deposition of cells in the upper parts of the fermenter, (2 problems of sterile operation as the air filter exits of the fermenter become wet, and (3) increased risk of contamination. In addition, (4) there may be product loss due to siphoning of the culture broth.
Whenever excessive foaming occurs, the following approaches may be used to resolve the problem:
(1) A defined medium may be used to avoid foam formation. This may be combined with modifications in physical parameters like pH, temperature, aeration and agitation. This approach will be successful in such cases where medium is the main culprit, but will fail whenever microbial activity is the main contributor.
(2) Often the foam may be unavoidable; in such case, antifoam should be used. This is the most standard approach to combat foaming.
(3) A mechanical foam breaker may also be used. Antifoams are surface active agents; they reduce surface tension in the foams and destabilize protein film by the following effects: (a) hydrophobic bridges between two surfaces, (b) displacement of the absorbed protein, and (c) rapid spreading of the surface film. Ideal antifoam should have the following properties.
- It should disperse rapidly and act fast on existing foam.
- It should be used at a low concentration.
- It should prevent new foam formation for a long time.
- It should not be used up or degraded by the microorganism.
- It should be nontoxic (to the microorganism as well as animals, including humans).
- It should not interfere with downstream processing.
- It should not cause problems in effluent treatment.
- It should be safe to handle.
- It should be cheap.
- It should not affect oxygen transfer.
- It should be heat stable for heat sterilization.
Several compounds meet most of these requirements, and have been found to be suitable for different fermentation processes; these compounds are as follows: alcohols (stearyl and octyl decanol), esters, fatty acids and their derivatives (especially, triglycerides like cottonseed oil, linseed oil, soybean oil, sunflower oil, etc.), silicones, sulphonates, and miscellaneous compounds like oxaline, Alkaterge C, and polypropylene glycol.
Many of the antifoams are of low solubility; therefore, they are added with a carrier like lard oil, liquid paraffin and castor oil. There carriers, however, may be metabolized, and they may affect the fermentation process. Further, many antifoams would reduce oxygen transfer by up to 50% when used at effective concentrations.
Antifoams are generally added when foaming occurs during fermentation. But foam control in fermentation industry is still an empirical art. Therefore, the best method of foam control for a particular process in one factory is not necessarily the best for the same process in other factories. Further, the design and operating parameters of the fermenters may affect the properties and the foams produced during the fermentation process.
Types of Fermenters:
A variety of fermenters have been described in the literature, but few of them have proved satisfactory for large scale aerobic fermentations. The most commonly used fermenters are based on a stirred upright cylinder with sparger aeration (Fig. 14.3).
The volumes ranging from 1 l to several thousand I. A general description of the following types of fermenter is given in the following sections: (1) stirred tank reactor, (2) airlift fermenter, (3) tower fermenter and (4) bubble up fermenter.
Stirred Tank Fermenter:
These are glass (smaller vessels) or stainless steel (larger volumes) vessels of 1-1,000 1 or even 8,000 1 (Namalva cells grown for interferon; but in practice their maximum size is 20 1 since larger vessels are difficult to handle, autoclave and to agitate the culture effectively).
These are closed systems with fixed volumes and are usually agitated with motor-driven stirrers with considerable variation in design details, e.g., water jacket in place of heater type temperature control, curved bottom for better mixing at low speeds, mirror internal finishes to reduce cell damage, etc. Many heteroploid cell lines can be grown in such vessels.
The needs for research bio-chemicals from cells are met from 2-50 1 reactors, while large scale reactors are mainly used for growing hybridoma cells for the production of monoclonal antibodies although their yields from cultured cells is only 1-2% of those obtained by passaging the cells through peritoneal cavity of mice.
Continuous-Flow culture systems, a type of stirred tank reactors, are either of chemostat or turbidostat type. In both the types, cultures begin as a batch culture. In a chemostat type, inoculated cells grow to the maximum density when some nutrient, e.g., a vitamin, becomes growth limiting.
Fresh medium is added after 24-48 hours of growth, at a constant rate (usually lower than the maximum growth rate of culture) and at an equal rate the culture is withdrawn. When the rate of growth equals the rate of cell withdrawal, the cultures are in a ‘steady state’, and both the cell density and medium composition remain constant. One of the constituents of the medium is used at a lower concentration to make it growth-limiting. However, chemostat is the least efficient or controllable at the cell’s maximum growth rate hence the steady-state growth rates in them are much lower than the maximum.
In contrast, in a turbidostat cells grow to achieve a pre-decided density (measured as turbidity using a photoelectric cell). At this point, a fixed volume of culture is withdrawn and the same volume of fresh normal (not having a growth-limiting factor) medium is added; this lowers the cell density or turbidity of the culture.
Cells keep growing, and once the culture reaches the preset density the fixed volume of culture is replaced by fresh medium. This system works really well when the growth rate of the culture is close to the maximum for the cell line.
The continuous-flow cultures provide a continuous source of cells, and are suitable for product generation, e.g., for the production of viruses and interferons. It is often necessary to use a two-stage system in which the first stage supports cell growth, while the second stage promotes product generation.
An airlift fermenter consists of a gas light baffled riser tube or draught tube (broth rises through this tube) connected to a down-comer tube (broth flows down through this tube). The riser tube may be placed within the down-comer tube as shown in Fig. 14.4, or it may be externally located and connected to the latter (Fig. 14.5). Air/gas mixture is introduced into the base of the riser tube by a sparger.
The aerated medium/broth of the riser tube has a lower density, while that in the down-flow tube it is relatively much less aerated and, as a consequence, has a higher density. This density difference drives the circulation of broth.
The lighter medium in the rise tube flows upward till it reaches the gas disengagement space of the fermenter. The O2 is continuously consumed by the cells and CO2 is generated by respiration.
The bulk of CO2 and other gases move out of the medium broth into the gas phase, and the un-aerated medium flows down through the down-flow tube. Circulation times in loops of 45 m height may be 120 seconds.
Single cell protein (SCP) production by Marlow Foods, U.K uses an air-lift fermenter in which the riser tube is externally placed (Fig. 14.6). Air and gaseous ammonia are introduced at the base of riser tube, while sterilized glucose, biotin and mineral salts are pumped into the fermenter at the base of the down-flow tube.
An internal heat exchanger coil is located at the bottom loop connecting the riser and down-flow tubes; it maintains the temperature at 30°C. The upper loop connecting the riser and down-flow tubes acts as an air outlet assembly through which CO2 is continuously extracted. The removal of CO2 and continuous consumption of O2 dissolved in the medium increases the density of the culture broth, which causes it to settle down through the down-flow tube.
SCP is harvested through a port at the base of riser tube, which leads into an RNA reduction vessel; steam is injected into the vessel to raise the temperature to 60°C, which reduces RNA content of SCP. After RNA reduction, SCP is harvested and processed.
This air-lift fermenter of 43 m3 volume is used in a continuous mode for the production of mycoprotein Quorn from Fusarium gaminareum grown on wheat starch-based medium. It allows production of long hyphae due to low shear, which is the preferred form of the product.
However, it gives lower biomass yields (only 20 g l-1) due to lower oxygen transfer rates in the high viscosity broth resulting from fungal hyphae. This fermenter is a modification of that designed by ICI pic, U.K. for SCP production using methanol as substrate.
The fermenter was developed to reduce production costs by minimising cooling costs since agitated vessels would generate additional heat. ICI pic used it in a continuous process to produce SCP for animal feed, but the process had to be discontinued because of high methanol cost and competition from animal feeds based on protein-rich crop produce. The mycoprotein production is, however, primarily for animal food.
Modifications of airlift fermenters include various modifications of draught (riser) tubes and multiple air-lift fermenters.
(1) In one modification of draught tube, stainless steel four-mesh tubes were placed at the top and bottom of the tube. This fermenter was used for growing Aspergillus terreus for itaconic acid production. The sieves modulated the fungal morphology so that the biomass was in an intermediate state between pellets and pulp. The type of culture gave double the yields of itaconic acid per unit volume per unit culture time.
(2) The multiple air-lift fermenters has three air-lift fermenters placed in a single vessel. The medium is fed into the central fermenter from where it flows in the middle one and then finally into the outer compartment from where it is eventually discharged.
(3) Another modification of air-lift fermenters is described in Section 188.8.131.52.
Animal cell cultures are also grown in such vessels that are both aerated and agitated by air bubbles introduced at the bottom of vessels (Fig. 14.7). The vessel has an inner draft lube through which the air bubbles and the aerated medium rise since aerated medium is lighter than non-aerated one; this results in mixing of the culture as well as aeration. The air bubbles lift to the top of the medium and the air passes out through an outlet.
The cells and the medium that lift out of the draft tube move down outside the tube and are re-circulated. O2 supply is quite efficient but scaling up presents certain problems. Fermenters of 2-90 1 are commercially available but 2,000 1 fermenters are being used for the production of monclonal antibodies.
A tower fermenter has been defined by Green-shields and co-workers as an elongated non-mechanically stirred fermenter that has an aspect ratio (height to diameter ratio) of at least 6 : 1 for the tubular section and 10 ; 1 overall, and there is a unidirectional How of gases through the fermenter. There are several different types of tower fermenters, which are grouped as follows on the basis of their design: (1) bubble columns, (2) vertical-tower beer fermenter and (3) multistage fermenter systems.
- Bubble Column Tower Fermenters:
These are the simplest type of tower fermenters; they consist of glass or metal tubes into which air is introduced at the base. Fermenter volumes from 3 / to up to 950 / have been used, and the aspect ratio may be up to 16 : 1. These tower fermenters have been used for citric acid and tetracycline production, and for a range of other fermentations based on mycelial fungi.
- Vertical-Tower Beer Fermenters:
These fermenters were designed for beer production and to maximise yeast biomass yields. A series of perforated plates are placed at intervals to maximise yeast yields. It has a settling zone free of gas; in this zone, yeast cells settle down to the bottom and return to the main body of the tower fermenter, and clear beer could be removed from the fermenter. Tower of up to 20,000 / capacity and capable of producing up to 90,000 I beer per day have been installed.
- Multistage Tower Fermenters:
In these fermenters, a column forms the body of vessel, which is divided into compartments by placing perforated plates across the fermenter. About 10% of the horizontal area of plates is perforated. In a variant of this type of fermenter (down-flow tower fermenter), the substrate is fed in at the top and overflowed through down spouts to the next section, and the air is supplied from the base. These fermenters have been used for continuous culture of E. coli, S. cerevisiae (baker’s yeast), and activated sludge.
It is a bubble column fermenter that is fitted with an internal cooling coil (Fig. 14.8). Air is introduced from the bottom of the column. In this vessel, the cooling coil effectively separates the column into an inner riser/draught tube and the outer down-flow tube. The cooling coil assembly functions as a leaky draught tube.
The culture broth rises in the compartment enclosed by the cooling coils and it moves down in the compartment outside the coil, although back- mixing also occurs through the coils. The region above the cooling coil shows good mixing, and there were no poorly oxygenated zones in the vessel. It can generate liquid velocities of 1 m sec-1, giving circulation times of 9-12 seconds and mixing times of 14-18 seconds.