No abstract

Mass transfer rates in liquid-liquid extraction processes can be seriously affected by the presence of surface-active contaminants. This is especially true of applications of a biotechnological origin, where the microorganism used in the process may produce the surface-active contaminants. An investigation into the effects of soluble and insoluble fermentation broth components on mass transfer using chloramphenicol extraction into octanol as the model system was conducted. Soluble components produced during fermentation were found to adsorb to the interface, where they reduced the overall mass transfer coefficient by up to 70%. After fractionation it was found that components in the weight range from 10-30 kDa had the greatest effect on mass transfer. Protein and phospholipid compounds of similar size were found to reduce the overall mass transfer coefficient to a similar extent to the broth components at concentrations around 0.001mg/l. The biomass produced during the fermentation also reduced mass transfer substantially, and it is likely that this was due to physical blockage of the interface.

Reactive liquid-liquid extraction can be used to recover hydrophilic fermentation products that would not otherwise partition into nonpolar solvents through an ion-exchange reaction at the two-phase interface. However, the ion-exchange reagent may not be specific to the solute of interest and other compounds present may also be extracted. In this study, the effect on solute extraction of other compounds present in the extraction medium was investigated for phenylalanine extraction using Aliquat 336. The extent of extraction at equilibrium was modeled using the equilibrium constants for the reactions present in the process. The interaction of different species within a multicomponent medium was examined using the model and experimental results. It was found that the extent of extraction and coextraction is controlled by the thermodynamics of each extraction reaction and, due to the formation of a common product, the interaction between each of the reactions. The main competition to reactive extraction will come from hydrophobic anionic compounds that will be present in fermentation broth.

Fermentation systems can contain may surface-active compounds that can interfere with downstream separation processes. This work examines the interactions that can occur between surfactants and biomass during solute mass transfer in a liquid-liquid extraction system. Adding the surfactants sodium dodecyl sulfate and dodecyl trimethyl ammonium bromide to the aqueous phase caused a substantial increase in the mass transfer of chloramphenicol between water and octanol. Further investigation of the interfacial region using an optical Schlieren apparatus revealed that these increases were due to interfacial turbulence that gave rise to a rapid surface renewal convective mass transfer mechanism. When microbial biomass was present with sodium dodecyl sulfate, an increase in the mass transfer rate was again found, however, to a lesser extent. In contrast, dodecyl trimethyl ammonium bromide did not promote mass transfer and it is postulated that electrical interactions between the surfactant and the cell surface prevented adsorption of either at the interface. The interaction between the antifoaming agent polypropylene glycol 2000 and extraction system components was also investigated, with both positive and negative effects being recorded under varying conditions.