Effects of Antifoam Agents and Surfactants

The volumetric coefficient kLa for oxygen absorption into oil-in-water emulsions is of interest in connection with fermentation using hydrocarbon substrates. Experimental results [7] have shown that such emulsions can be categorized into two major groups, depending on their values of the spreading coefficient s (dyne cm-1) defined as 

In the group with negative spreading coefficients (e.g., kerosene-in-water and paraffin-in-water emulsions), the values of kLa in both stirred tanks and bubble columns decrease linearly with an increasing oil fraction. This effect is most likely due to the formation of lens-like oil droplets over the gas-liquid interface. A subsequent slower oxygen diffusion through the droplets, and/or slower rates of surface renewal at the gas-liquid surface, may result in a decrease in kLa. 

The effects of broth viscosity on kLa in aerated stirred tanks and bubble columns is apparent from Equations 7.37 and 7.41, respectively. These equations can be applied to ordinary non-Newtonian liquids with the use of apparent viscosity a, as defined by Equation 2.6. Although, liquid-phase diffusivity generally decreases with increasing viscosity, it should be noted that at equal temperatures, the gas diffusivities in aqueous polymer solutions are almost equal to those in water. 

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 (see Equation 2.4). The relaxation time (s) is viscosity (Pas) divided by the elastic constant (Pa). 

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