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

Transport Phenomena in Biotechnological Systems
Mass Transfer Between Gas and Liquid Phases in Cellular Systems

The physical basis of the mass transfer phenomenon discussed in this chapter is shown schematically in Fig. 8.1. A sparingly soluble gas, most commonly oxygen, is transferred from its source (such as rising air bubbles) into The Cell-containing liquid phase. (A similar picture is observed in the case of any other sparingly soluble substrate, e.g., liquid Hydrocarbons in microbiological hydrocarbon Processing Methods.) Along this pathway, oxygen must overcome a series of mass transfer resistances, the relative magnitudes of which depend on bubble (droplet) hydrodynamics, Temperature, cellular activity and culture density, solution composition, various interfacial phenomena, and other factors.

The total mass transfer resistance is made up of individual resistances in the following subprocesses:

1. Diffusion of gas from the bulk of the gas phase to the gas-liquid interface.

2. Transfer across the gas-liquid interface.

3. Diffusion of the dissolved solute through the relatively unagitated liquid layer adjacent to the bubble into the agitated liquid phase.

4. Transfer of the dissolved solute through the liquid phase to the second, relatively poorly agitated liquid layer surrounding the Cells.

5. Transfer across the second, relatively unagitated layer surrounding the cells.

6. Diffusion into cell clumps, mycelia, or soil particles.

7. Transfer across The Cell wall/membrane into the intracellular space (reaction site).

All of these types of resistance are illustrated in Fig. 8.1. When organisms exist as single cells, the sixth type of resistance is absent. Microbial cells tend to adsorb at phase boundaries. For example, cells may concentrate predominantly near the boundary separating the gas bubble and the liquid phase. In this case, the diffusing dissolved oxygen has to cross only a single unagitated region; thus, transport through the liquid phase preceding the fifth type of resistance is also eliminated. Under such conditions, the concentration of O₂ dissolved in the liquid phase does not reflect its supply rate for microbial Respiration.

Similarly, in microbiological transformation processes involving other sparingly soluble substrates, such as hydrocarbon droplets, cell adsorption at (or near) the interface separating the hydrocarbon phase from the aqueous phase has frequently been observed. MATHEMATICAL MODELING OF bioreactor performance under such conditions will be discussed in Chapter 9.

Figure 8.2 illustrates various configurations of physical contact between the gas and liquid phases. All these configurations can be divided into two groups: in the first group, mass transfer occurs through the free rise of gas bubbles or the settling of liquid or solid particles, whereas In the second group, it is driven by liquid movement under the action of external forces other than gravity (forced convection). There is no sharp boundary between these two groups; for instance, gas-liquid mixing in a slowly stirred semi-continuous system can be driven equally by the free rise of gas bubbles and by mechanical agitation. Structure/19.html">The Importance of hydrodynamics in biochemical engineering processes necessitates studying the relationship between fluid flow and mass transfer. Before proceeding to this problem, however, it is necessary to make a few General Remarks and introduce several terms and Definitions commonly accepted in mass transfer theory.

Class="center">Table 8.1. Solubility of O₂ at a pressure of 1 atm O₂ in Water at various temperatures and in salt or acid solutions at 25 °C^a

Solubility of O₂ In aqueous solutions at 25 °C

^a From: International Critical Tables, vol. III, p. 271. McGraw-Hill Book Co., New York, 1928; Todt F., Elektrochemische Sauerstoffmessungen, W. de Gruyter & Co., Berlin, 1958.