The distribution of mixing efficiency in a split‐cylinder gas‐lift bioreactor has been investigated for Y. lipolytica suspensions with cells concentration varying between 10 and 50 g/l d.w. The results indicated important variation of mixing time on the height of riser or downcomer, as well as different behaviour of suspensions flows in these two regions. Thus, the less intense mixing for the upward circulation has been recorded for the bottom region, while the most efficient mixing was reached at the top position at higher biomass concentration and aeration rate. This variation is similar to that observed for the downcomer, the intermediary positions being associated with the lowest efficiency of mixing for concentrated yeast cells suspension, due to the bubbles entrapping coalescence and stratification phenomena. The influence of air input rate on turbulence extent is positive only for the superior regions of the riser. For the other positions inside the gas‐lift bioreactor this parameter effect depends on the biomass concentration. By means of the experimental data and using MATLAB software, mathematical correlations for mixing time have been proposed for each circulation region, taking into consideration both the operational parameters and the distance from the bioreactor bottom. These equations offer a good concordance with the experiment, the average deviation being 5.1 % for the riser and 5.6 % for the downcomer zone.

The distribution of mixing in a split-cylinder gas-lift bioreactor has been investigated for suspensions of immobilized Y. lipolytica cells with particles diameter varying between 3 and 4.2 mm. The results indicated important variation of mixing time on the height of riser or downcomer, as well as different behavior of suspensions flows in these two regions. Therefore, for the riser, the mixing efficiency increases from its bottom to the top, the biocatalyst particles with intermediary size (3.6 mm diameter) allowing reaching the most intense circulation of suspension. The analysis of the suspension flow in the downcomer region revealed that the intermediary positions are associated with the highest mixing mainly for the largest immobilized yeast particles (4.2 mm diameter). In both cases, the influence of aeration on turbulence extent is positive only for air superficial velocity up to 1.05 -1.35x10 -3 m/s, the magnitude of this effect being correlated with the biocatalyst size and position on the riser or downcomer. By means of the experimental data, mathematical correlations for mixing time have been proposed for each circulation region, taking into consideration both the operational parameters and the Downloaded by [University of Otago] at 10:37 05 October 2015 2 distance from the bioreactor bottom. These equations offer a good concordance with the experiment, the average deviation being of 5.82 for the riser and 6.06% for the downcomer zone.

The efficiency of enzymatic hydrolysis of Penicillin G to 6-Aminopenicillanic acid under substrate inhibition has been analyzed for a bioreactor with mobile bed of immobilized penicillin amidase. The results indicated that the optimum values of temperature and pH remained the same as for homogeneous hydrolysis using free enzyme. The inhibitory effect induced at higher Penicillin G amount in liquid phase was attenuated by increasing the size of the biocatalyst particles. For this reason, at substrate concentration over 150 mol/m 3 , the highest volumetric productivity of hydrolysis process was recorded for the larger particles of immobilized enzyme. The proposed mathematical correlation between the volumetric productivity, Penicillin G concentration and biocatalyst diameter offers a good concordance with the experimental data, the average deviation being of 4.53%.

The distribution of mixing time for a split-cylinder gas-lift bioreactor has been investigated for simulated broths with apparent viscosities up to 26 cP. The results indicated important variation of mixing time on the height of riser or downcomer, as well as different behavior of dispersion flows in the two regions. Thus, the highest mixing efficiency for the ascending circulation has been recorded for the intermediary positions, placed above the sparger. Contrary, for the downcomer, the intermediary positions are associated with the lowest efficiency of mixing, due to the bubbles entrapping phenomenon. Moreover, at air superficial velocities over 1.05x10 -3 m/s, the influence of air input rate on turbulence extent is continuously positive for the riser, while it became negative for the downcomer. By means of the experimental data and using MATLAB software, mathematical correlations for mixing time have been proposed for each circulation region, taking into consideration both the operational parameter and the distance from the bioreactor bottom. These equations offer a good concordance with the experiment, the average deviation being of 3.3 for the riser and 6.6% for the downcomer zone.

No abstract