Distribution of mixing efficiency in a split‐cylinder gas‐lift bioreactor for Yarrowia lipolytica suspensions

Abstract

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.

Distribution of Mixing Efficiency in a Split-Cylinder Gas-Lift Bioreactor with ImmobilizedYarrowia LipolyticaCells Used for Olive Oil Mill Wastewater Treatment

“…According to the previous results, this behavior is the main consequence of the change of the relative importance of bubbles disengagement in the gas separation region and their entrapping into the suspension which circulates downward (Chisti and Moo-Young, 1988;Merchuk and Gluz, 1999;Caşcaval et al, 2015). Thus, at low air superficial velocity, the suspension flow velocity in the riser is low and allows separating completely the bubbles at the top of gas-lift bioreactor.…”

“…Due to the buoyancy tendency of the bubbles, as well as to their accumulation, stratification, followed by their coalescence, the descending velocity of the dispersion is reduced and, implicitly, the mixing efficiency is affected. The presence and increase of the amount of Y. lipolytica immobilized cells in the downcomer exhibit a more significant effect on inducing the bubbles stratification compared to that observed for simulated broths without biomass or for free yeast cells suspensions (Galaction et al, 2014;Caşcaval et al, 2015). In the same time, the descending movement of solid phase, more important for the biocatalyst particles than for the biomass, hinders the ascending flow of the bubbles, reducing supplementary the suspension flow velocity.…”

“…Although the previous studies on viscous media without biomass indicated that the variation of mixing efficiency on the riser height is directly controlled by the value of gas input rate, fluid phase viscosity, and biomass concentration, the presence of biocatalyst particles could generate the change both of the mixing intensity and of its distribution along the riser region Caşcaval et al, 2015).…”

“…The variation of mixing time along the riser height for Y. lipolytica immobilized cells suspensions differs from that previously observed for simulated broths and free biomass cultures. As it was concluded for free Y. lipolytica cells suspensions, the suspensions flow velocities in the regions of bottom clearance and top gas separation, namely positions 1 and 4, are controlled mainly by cells concentration in these regions, and less by flow section area, as in the case of simulated viscous media(Galaction et al, 2014;Caşcaval et al, 2015). For the alginate particles suspensions,Figure 3indicates that the mixing time generally decreases from the bottom to the top region of the riser.This variation is the result mainly of the decrease of the solid phase amount on the riser height, due to its deposition.…”

Formation of dead zone in catalytic particles in catalysis and biocatalysis—new alternative method of determination

“…They concluded that bioreaction rates in the pellets were limited by the dissolved oxygen concentration, and as a consequence cell growth occurred only in the outer layers, as was confirmed by scanning electron microscopy. Cascaval et al investigated the process of penicillin G enzymatic hydrolysis to 6‐aminopenicillanic acid in a bioreactor with a stationary basket bed. The results indicated that very low internal mass flow rates could be reached in the centre of the particles.…”