Mechanically agitated fermenters
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Published: Mon, 5 Dec 2016
Traditional mechanical agitation fermenters have dominated the industry since the antibiotic era as needs changed new fermenter designs were created. As a result air lift agitated fermenters were created and have many merits in comparison to mechanical agitation fermenters. In this essay we will go through both systems merits in regards to mixing, aeration, practicality and energy costs
Agitators are mechanical instruments used to mix substances, Fermentation is an age old art in which organic substances are broken down and reassembled into other substances. Fermenters are large bioreactors in which fermentation occurs, fermenters are the instruments employed to manufacture economically viable biological products. Their basic function is to provide a controlled environment in order to achieve optimal growth and product formation of the particular biological product required. For biotech and pharmaceutical purposes the products from fermentation are microbial cells or biomass, enzymes, and microbial metabolites such as antibiotics and ethanol. The basic desired functional properties of all Fermenters are that they can create gas liquid interfaces without making foam a problem. They should sufficiently hold up dispersed phases and allow reasonable heat transfer. They should also be able to control bulk flow so no dead zones can form. In league with these functional requirements they should be cheap, robust and have a simple mechanical design additionally they should have low power consumption and be easy to scale up. In this essay we will compare two different types of Fermenters, airlift Fermenters and mechanically agitated Fermenters.
Both types of mixers within Fermenters results in the intermingling of two or more dissimilar portions of material resulting in the acquirement of either physical or chemical uniformity in the final product. In industrial fermentation reactions there is a basic requirement of substrate, organism, water and oxygen. Mixing within Fermenters usually causes equilibrium between, rate, purity and production yield. Mechanical agitators are used in traditional Fermenters for mixing they maintain optimum substrate biomass concentration everywhere, keeps solid suspended, disperse oxygen, and allow an upkeep of total bubble surface area and the recycling of air bubbles (figure 1).
Mechanically agitated Fermenters
Mechanically agitated Fermenters require a relatively high input of energy per unit volume. In these systems a large variety of impeller shapes and sizes are available to produce different flow patterns inside the Fermenter. The use of multiple impellers produces better mixing that works in addition with baffles that are normally used to reduce vortexing. Approximately 70-80% of the volume of stirred reactors is filled with liquid. Foaming may be a problem with this type of Fermenter. Foam breakers, may be necessary. It is better to use mechanical anti foamers over chemical anti foamers because the chemicals often reduce oxygen transfer rate. One of the limits of this system is the use of high speed impellers can damage and even destroy cells. Aspect ratios of these Fermenters vary over a wide range. For aeration to be increased a higher aspect ratio is needed (H/D rates). Increased aeration results in greater contact times between liquid and rising bubbles and produces hydrostatic pressure at the bottom of the Fermenter.
Bubble column /Air Lift Fermenters
In these systems aeration and mixing are achieved by gas sparging. Gas is sparged only into the riser. Decreased liquid fluid density and gas accumulation cause the liquid in the riser to mover upwards. Gas disengages at the top of the vessel leaving heavier bubble-free liquid to recirculate through the downcomer. This process needs less energy than mechanical stirring. This mixing, method is used in the production of beer and bakers yeast. The advantages of this method over mechanical agitation are, lack of moving parts, low capital costs satisfactory mass and heat transfer. Air lifted Fermenters produce heterogeneous and homogenous medium flows. In heterogeneous flow, Bubbles and liquids tend to rise up in the center of the column while a corresponding down flow of liquid occurs near the walls. In Homogenous flow, bubbles rise with the same upward velocity with no back-mixing of the gas phase. Foaming may also be a problem with these Fermenters. There are two kinds of air lift Fermenters internal loop and external loop Fermenters. Mixing is better in external loop Fermenters because the riser and downcomers are further apart in external loop vessels which cause the density difference between fluids in the downcomer and riser to be greater meaning circulation of the liquid vessel is faster due to fewer bubbles being carried to the downcomer. Airlift Fermenter are normally used for the culture of immobilized catalyst and the culture of plant and animal cells because of their low sheer level.
Stirred Fermenters and air lifted Fermenters both offer adequate mixing and mass transfer. However when a large Fermenter is required (50-500M3) for a low viscosity medium air lift vessels may be a better choice due to their advantages. These being they are cheap to install and operate. When scale up is required large mechanical agitators are impractical as the power required to achieve adequate mixing becomes very high. Mechanical agitators are used for high viscosity cultures. Mass transfer rates decline at viscosities greater than 50-100 cP. Mechanical agitation creates much more heat than sparging of compressed gas. This can become a problem when the reaction temperature is high for example when trying to produce single celled proteins from methanol, removal of frictional stirrer heat can be problematic this is where air-lift agitation is preferred.
In brief the conventional, stirred tank bioreactor has dominated the industry since its successful application in the antibiotic era and most fermentation processes today use Fermenters of this type because of this. However due to change in the industry in regards to products in demand. Such as the growth of hydrodomas cell and recombinant DNA technologies of genetically modified cells of plant, microbial and mammalian origin imposed new demands that traditional agitators could not provide at an economically viable level. For this reason new novel Fermenters where designed and put into use. The air lift Fermenter being one of them. The air lift Fermenter has no movable parts or motors the only power requirement comes from the air compressors that provide air through the sparging system. No mechanical agitation occurs, the air bubbles forced through the sparger cause induced turbulent liquid mixing and mass transfer in which mixing rates and aeration rates are coupled together. Their main advantage is low sheer and energy requirement along with aseptic seals not being required around the shaft which makes them highly suitable for producing single celled protein. Additionally in air lift Fermenters mixing is improved by the inclusion of a draught tube to impart a circulation loop which produces a higher oxygen mass coefficient (KLA). The Air lift Fermenters are ideal when there is need for gentle agitation. Whereas the conventional mechanical agitated Fermenters have a broader range of application but they have a poorly defined mixing pattern in comparison to airlift Fermenters. Additionally they cannot be aerated at a high enough rate due to impeller flooding. Practicality wise they have a long life, the mechanical agitation configuration has become too established in processes for new methodologies to replace them. It would be too expensive to do.
To provide aeration into a vessel means to supply or expose the medium to the circulation of air. Airlifted Fermenters provide a much greater aeration than mechanical agitators as gas is constantly pumped into the medium and consequently causes fluid circulation. Aeration within a mechanically agitated Fermenter is controlled by the type of impeller and baffle system. For example Turbines, propellers and paddles are generally used in low viscosity systems and operate at high rotational speed inside the Fermenter. Turbines are normally used for dispersion of gases in liquids. There are many types angled-blade turbines and retreating-blade turbines, the rushton/inclined six blade impeller. Similarly for large vessels with high aspect ratios it is common practice to mount more than one impeller of the same shaft. Baffles are of particular importance as they prevent gross vortexing which is detrimental to mixing/ aeration they are normally fitted on the walls of a vessel.
Depending on the product being produced in the Fermenter and the viscosity of the medium practicality of mechanical and airlift agitators differ. Mechanical agitators are very practical when it comes to mixing highly viscous non Newtonian mediums however the power for this can be very high and subsequently this increases the costs. Additionally the practicality of the Fermenter being used in regards to merits is determined by the type of product being produced, the microbiology of particular cell systems in use coupled with the morphology and nutritional requirements needed for optimal growth. The geometric configuration of the Fermenter play an important role. Effective mixing to minimise temperature, PH & concentration gradient are very important particularly with mechanically agitated Fermenters especially when a process is scaled up. Additionally the viscosity of the medium plays an important role, does the medium behave in a Newton or non Newton manner is it a solid or liquid state fermentation. The sheering effect of a particular agitation system dictates whether sheer sensitive cells can be cultivated.
All of this is taken into account keeping in mind what is best for economic performance. For example large mechanical agitators have better Practical use than air lift agitators for use with the following cell systems, these are immobilised Bacteria, yeast and plant cells and are used for the for the production of products such as ethanol, monoclonal antibodies, growth factors and medicinal products. This is because they can tolerate sheer at a level best for productivity. Resulting in large quantities of moderate quality products with good profit costs. Alternatively air lift agitators are generally used for the cell systems of bacteria yeast and other fungi producing products such as single celled proteins E.G. Quorn, enzymes, secondary metabolites and biosurfactants. This is because they are more economically practical due to them having low sheer values meaning they do not damage the cells, they have much lower running costs and they can produce higher value sheer sensitive GM products. Furthermore when it comes to scale up with airlifted Fermenters it can be difficult to alter stirring rates making it difficult to deal with important rheological changes and foaming. This is where mechanically agitated Fermenters are favoured. Also air lifted Fermenters are less flexible than mechanically agitated systems as Aeration is responsible for homogenization.
Energy use and Cost
Mechanical agitators use more energy have moving parts, seals and are more expensive to run than airlift fermenters.
The main benefit of air-lift Fermenters over mechanical agitators is that they can be constructed at much greater reactor volumes air-lift Fermenters can be built at volumes of several thousands cubic meters while mechanical operated agitators can be scaled up to a maximum of 800-1500 m3 (Ruitenberg et al 2001) As a consequence of this the investment costs of air-lift Fermenters is significantly lower when compared to mechanically operated agitators of the same capacity. At higher volumes mechanical agitators cause mechanical problems because of the large power requirements of the impeller. Furthermore, scale-up of air-lift Fermenters is much more straight forward than that of mechanical agitated fermenters. Scale-up from a 5 m3 pilot to 1500 m3 and larger is well defined. (Ruitenberg et al 2001) Figure 3 shows the Capital cost comparison of air-lift Fermenters vs. mechanical agitated fermenters. The cost for a mechanically agitated fermenter is defined as 1 for a 1500 m3 tank. The cost of a 1500 m3 air-lift fermenter is a bit lower than that of the equivalent mechanically agitated fermenter. However, the investment cost follows the 0.6 rule until 6000 m3 is reached. Above 6000 m3, more than one air lift fermenter may need to be used.
Another advantage of air-lift fementers over mechanical agitated fermenters is that the oxygen input efficiency is the same or better at considerably lower shear. Additionally Because no moving parts are present in air-lift Fermenters, the costs for maintenance will be lower as compared to mechanically agitated fermenters. The combination of high oxygen input efficiencies and low maintenance costs results in lower operational costs.
Shear rates are much lower in air-lift Fermenters than in mechanically agitated fermenters. Low shear rates facilitate growth of biofilms, which can increase the reaction rate. This advantage is thought to be greatest when thermophilic bacteria are used. Because a three-phase settler can be integrated on top of an air-lift fermenter, the solids retention time can be separated from the hydraulic retention time causing biomass retention, (Ruitenberg et al 2001)
Mechanically agitated Fermenters have been in use since the beginning of the industry however due to changes in demand that comes with time in regards to technology and products needed novel Fermenter ideals were designed and put into fruition the air lift Fermenter is but one. In many ways this air lift agitators have many advantages as was just discussed.
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