Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Transport phenomena in biotechnological systems
Other factors affecting kia'
Surfactants

As mentioned in Chapter 2, many substances of biological origin are amphipathic by nature, meaning they contain both highly hydrophilic and hydrophobic regions, which tend to concentrate at gas-liquid and liquid-liquid interfaces. During various growth phases, Cells secrete substances with surface-active properties (e.g., Polypeptides); sometimes these substances cause foaming of the reaction mixture in aerated reactors. The addition of antifoaming agents also affects the interfacial resistance to mass transfer; this effect is typically opposite to that of surfactants.

The adsorption of surfactants at the interface is a spontaneous process; As a result, the Free energy and, consequently, the surface tension o at the interface decrease. From the empirical equations given in Example 8.2, it follows that in the presence of surfactants, The values of D_sm and D_c should decrease; this will lead to an increase in the interfacial area per unit volume a'.

The increase in a' may be offset by The Effect of surfactant films on the mass transfer coefficient k_l. The adsorption of surfactants increases the rigidity and reduces the mobility of the interface. The reduction in k_l considered here is believed to be due to one of two mechanisms (or both simultaneously): 1) the mobility of the liquid phase near the interface is reduced due to the relatively low mobility of the interface itself; therefore, various mass transfer theories based on estimating the rates of exchange of small fluid elements between the surface and the liquid phase predict a decrease in the mass transfer coefficient (for details, see [3, 4]); 2) like a Cell membrane, the molecular film must possess its own resistance, which can cause deviations from the assumed gas-liquid equilibrium at the interface.

The addition of 10 million^-1 sodium lauryl sulfate (SLS) reduces the oxygen transfer k_l by 56% (relative to pure Water). A further increase in surfactant concentration does not alter k_l. The interfacial area per unit volume a' slowly increases as the SLS concentration rises from 0 to 75 million^-1, while k_la' reaches a minimum at an SLS concentration of about 10 million^-1.

It was found that in a turbine-aerated Reactor, an increase in surfactant concentration is accompanied by a monotonic increase in k_la'. Analysis of published data on The ratio of a' in the presence of a surfactant to a' in its absence, and the corresponding ratio of k_la' values, showed that in the presence of 4 million^-1 SLS, a' increases fourfold, while k_la' increases by only 15%; it follows that k_l decreases by approximately 71%.

The reduction in k_l observed in these two series of experiments is consistent with the findings of other studies. For the mass transfer of several sparingly soluble gases, the addition of a surfactant up to a constant k_l value reduces this parameter by an average of 60%. A Study of the effect of SLS by Benedek and Heideger showed that turbine aeration in the presence of this agent corresponds to a region intermediate between the two regions described by equations (8.35) and (8.36) [33]. A similar result was obtained by Danckwerts for the case of aeration using grid trays. These results should once again caution the reader against overestimating the numerous empirical equations presented in this chapter; although the latter are undoubtedly useful, whenever possible, experimental data obtained under conditions and in equipment as close as possible to the process under study should be used.