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Oxygen mass transfer represents the most important parameter involved in the design and operation of mixing-sparging equipment for bioreactors. It can be described and analyzed by means of the mass transfer coefficient, k(L) a. The k(L) a values are affected by many factors such as geometrical and operational characteristics of the vessels, media composition, type, concentration and microorganism morphology, and biocatalysts properties. The efficiency of oxygen transfer could be enhanced by adding oxygen-vectors in broths, such as hydrocarbons or fluorocarbons, without increasing the energy consumption for mixing or aeration. The experimental results obtained for simulated broths indicated a considerable increase of k(L) a in the presence of n-dodecane, and the existence of a certain value of n-dodecane concentration that corresponds to a maximum mass transfer rate of oxygen. The magnitude of the positive effect of n-dodecane depends both on the broths' characteristics and operational conditions of the bioreactor.

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The complexity of downstream processes for biosynthetic products constitutes a particularity of industrial biotechnologies, especially because of the biosynthetic product high dilution in fermentation broth, their chemical and thermal liability and the presence of secondary products. For these reasons, new separation techniques have been developed and applied to bioseparations. Among them, reactive extraction, pertraction (extraction and transport through liquid membranes) and direct extraction from broths have considerable potential and are required for the further development of many biotechnologies. This review is structured on two parts and presents our original results of the studies on the separation of some biosynthetic products (antibiotics, carboxylic acids, amino acids, alcohols) by reactive extraction in the first part, and by pertraction and direct extraction from broths without biomass filtration in the second. For all the analyzed cases, these extraction techniques simplify the technologies by reducing material and energy consumption, by avoiding product inhibition, by increasing the separation selectivity, therefore decreasing the overall cost of the product

The study on mixing distribution for an aerobic stirred bioreactor and simulated (solutions of carboxymethylcellulose sodium salt), yeasts (S. cerevisiae) and fungus (P. chrysogenum pellets and free mycelia) broths indicated the significant variation of mixing time on the bioreactor height. The experiments suggested the possibility to reach a uniform mixing in whole bulk of the real broths for a certain value of rotation speed or biomass concentration domain. For S. cerevisiae broths the optimum rotation speed increased to 500 rpm with the biomass accumulation from 40 to 150 g/l d.w. Irrespective of their morphology, for fungus cultures the existence of optimum rotation speed (500 rpm) has been recorded only for biomass concentration below 24 g/l d.w. The influence of aeration rate depends on the apparent viscosity/biomass concentration and on the impellers and sparger positions. By increasing the apparent viscosity for simulated broths, or biomass amount for real broths, the shape of the curves describing the mixing time variation is significantly changed for all the considered positions. The intensification of the aeration induced the increase of mixing time, which reached a maximum value, decreasing then, due to the flooding phenomena. This variation became more pronounced at higher viscosities for simulated broths, at higher yeasts concentration, and at lower pellets or filamentous fungus concentration, respectively. By means of the experimental data and using MATLAB software, some mathematical correlations for mixing time have been proposed for each broth and considered position inside the bioreactor. These equations offer a good agreement with the experiment, the maximum deviation being +/-7.3% for S. cerevisiae broths.

The reactive extractions of acetic acid, HAc, with tri-n-octylamine (TOA), Q, dissolved in three solvents with different dielectric constants (dichloromethane, butyl acetate, and n-heptane) without and with 1-octanol as phase modifier have been comparatively analyzed. The results indicated that the mechanism of the interfacial reaction between acid and extractant is controlled by the organic phase polarity. In absence of 1-octanol, the structures of the extracted complexes are HAc.Q for dichloromethane, HAc.Q 2 for butyl acetate, and (HAc) 2 Q 4 for n-heptane. These structures are modified by adding 1-octanol and become HAc.Q for extraction in dichloromethane or butyl acetate and (HAc) 2 Q 2 for extraction in n-heptane, respectively. Although the presence of 1-octanol improves the extraction efficiency, it leads to the reduction of extraction constants for lower-polar solvents, influence that is more significant for n-heptane.

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