Aeration, fermentators power

Even for the simple stirred, aerated fermenter, there is no one single solution for the scale-up of aeration-agitation which can be applied with high probability of success for all fermentation processes. Scale-up methods based on aeration efficiency (kio) or power consumption/unit volume have become the standard practice in the fermentation field. 

Aerated power draw, during fermentation, 11 39-40 Aeration... 

An aerated stirred-tank fermenter equipped with a standard Rushton turbine of the following dimensions contains a liquid with density p = 1010kgm and viscosity n = 9.8 X 10 Pa s. The tank diameter D is 0.90 m, liquid depth Hl = 0.90 m, impeller diameter d = 0.30 m. The oxygen diffusivity in the liquid Dl is 2.10 X 10 5 cm- s T Estimate the stirrer power required and the volumetric mass transfer coefficient of oxygen (use Equation 7.36b), when air is supplied from the tank bottom at a rate of 0.60 m min at a rotational stirrer speed of 120 rpm, that is 2.0 s T... 

Some fermentation broths are non-Newtonian due to the presence of microbial mycelia or fermentation products, such as polysaccharides. In some cases, a small amount of water-soluble polymer may be added to the broth to reduce stirrer power requirements, or to protect the microbes against excessive shear forces. These additives may develop non-Newtonian viscosity or even viscoelasticity of the broth, which in turn will affect the aeration characteristics of the fermentor. Viscoelastic liquids exhibit elasticity superimposed on viscosity. The elastic constant, an index of elasticity, is defined as the ratio of stress (Pa) to strain (—), while viscosity is shear stress divided by shear rate (Equation 2.4). The relaxation time (s) is viscosity (Pa s) divided by the elastic constant (Pa). 

Power Consumption Power consumption sometimes becomes important in industrial bioprocesses, because the power used for aeration and agitation can be highly expensive. The cost of power consumption occupies approximately 15-20% of total production cost in aerobic fermentation processes. 

Many alternative fermenters have been proposed and tested. These fermenters were designed to improve either the disadvantages of the stirred tank fermenter-high power consumption and shear damage, or to meet a specific requirement of a certain fermentation process, such as better aeration, effective heat removal, cell separation or retention, immobilization of cells, the reduction of equipment and operating costs for inexpensive bulk products, and unusually large designs. 

Gas hold-up is one of the most important parameters characterizing the hydrodynamics in a fermenter. Gas hold-up depends mainly on the superficial gas velocity and the power consumption, and often is very sensitive to the physical properties of the liquid. Gas hold-up can be determined easily by measuring level of the aerated liquid during operation ZF and that of clear liquid ZL. Thus, the average fractional gas hold-up H is given as... 

Due to the special situation in the post-war era, the implementation of this endeavor was extremely difficult. With the help of a French chemist, Captain Rambaud, of the French occupation forces, a small team of scientists and engineers succeeded in producing sufficiently pure penicillin within a rather short period of time (1948). Problems of equipment were solved by using various redundant military materials, e.g. V2 missile containers as liquid vessels, self-produced fermenters stirred with the help of motors of submarines and aerated by compressors powered by motors of German Tiger tanks. The necessary pipes were obtained from a bombed Innsbruck cafe. Since corn-steep liquor was not available, yeast extract had to be used, and whey had to serve as a substitute for lactose. Even the necessary butanol for the preparation of the extractant had to be produced by installing a butanol fermentation. 

Fermentation biomass productivities usually range from 2 to 5 g/(l h). This represents an ojq gen demand in the range of 1.5 to 4 g 0/(1 h). In a 5(X)-m fermenter, this means achievement of a volumetric oxygen transfer coefficient in the range of250 to 400 h . Such oxygen-transfer capabilities can be achieved with aeration rates of the order of 0.5 WM (volume of air at STP/volume of broth) and mechanical agitation power inputs of 2.4 to 3.2 Kw/m (1.2 to 1.6 HP/ 100 gal). 

Deindoerfer (1960) and Richards (1961) have published excellent reviews on this subject, and presented an appraisal of the use of these properties in engineering and analytical correlations employed in fermentation practice. Although there are some cases where the operation and scale-up of these mycelial fermentations have been accomplished successfully using either the power input per unit volume or the overall oxygen transfer coefficient of Newtonian fluids as a basis, this does not preclude further study of the scale-up of non-Newtonian fermentation broths. On the contrary, much more information about properties such as oxygen transfer in bubble aeration, and mixing time in non-Newtonian fermentation fluids is needed to provide a better understanding of the operation and scale-up of fermentation processes. 

The available literature contains very few references to studies of the power consumption for agitation and the volumetric oxygen transfer coefficients of multi-stage impeller fermenters. Takeda and Hoshino (1968) showed experimentally that closely spaced impellers caused serious interference between the flow streams from adjacent impellers and an overall reduction in power consumption. On the other hand, Oldshue (1966) has indicated that within fairly large ranges of geometric variables, a similar oxygen transfer coefficient is obtained if the power per unit volume is maintained at similar values under a constant aeration rate. 

By varying the impeller blade dimensions variations in power per unit volume were made at a constant impeller speed. At low power per unit volumes there was a linear increase in k a which correlated with P/V with an exponent of 0.9 to 1.2. yond the breakpoint the exponent relating the P/V dependence was 0.53. In both regions the k,a dependend on the superficial gas velocity to the 0.3 power. Tnese results are similar to those reported earlier by Blakebrough and Sambamurthy (1966) and Hamer and Blakebrough (1963) obtained in smaller scale vessels also using paper pulp suspensions. Other references on the aeration of viscous non-Newtonian fermentation broth are Banks (1977) and Blanch and Bhavaraju (1976). 

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