The magnetic tracer technique was used to obtain essential information on hydrodynamics, mixing and reactor design in a multiphase airlift reactor with an enlarged separator. The tracer method involving the use of a magnetic particle as a neutrally buoyant flowfollower allowed measurement of the residence time of the tagged particle in all reactor sections (riser, downcomer, separator and bottom sections). Knowing the settling velocity of the tagged particle, the liquid circulation velocity was also determined. The effects of the design of the enlarged separator (HDT/HC), of the riser to downcomer crosssectional area ratio (AD/AR) and of the liquid level in the head zone (HT) were investigated. The hydrodynamics of the ALR was affected by all design parameters. The results of experiments with different heights of draft tube demonstrated how easily various operating flow patterns can be achieved ranging from typical flow patterns for an internal-loop ALR with high downcomer gas holdup to those of an external-loop with low or nil one. On the base of histograms of the residence time of the tagged particle in the separator zone and visual observations, three flow patterns of the particle were described. The RTDs of the tracer particle were also used for assessment the mixing efficiency of reactor sections. The results showed that the best option for the design of ALR with an enlarged separator concerning mixing intensity is the use of reactor configuration with a dual separator and an area ratio AD/AR around 4.

In this article a hydrodynamic and rheological analysis of a continuous airlift bioreactor with high-celldensity system is presented. A highly flocculating recombinant strain of Sacharomyces cerevisiae containing genes for lactose transport (lactose permease) and hydrolysis (β-galactosidase) was exploited to ferment lactose from cheese whey to ethanol. The magnetic particle-tracer method was used to assess the effect of operational conditions (air-flow rate, biomass concentration) on hydrodynamic behavior of an airlift bioreactor during the fermentation process. Measurements of liquid circulation velocity showed the existence of a critical value of biomass concentration at which a dramatic deceleration of net liquid flow appeared with increasing biomass quantity. Rheological analysis revealed exponential increase of viscosity of the yeast floc suspension at the same biomass concentration of about 73 g/dm³ corresponding to 42.8% v/v of solid fraction. These facts have a particular importance for the successful processing of a high-celldensity airlift bioreactor as only a circulated flow regime will be favorable to keep the solid particles in suspension state and evenly distributed throughout the bioreactor.

In this paper a hydrodynamic and rheological analysis of a continuous airlift bioreactor with high cell density system is presented. A highly flocculating recombinant strain of Sacharomyces cerevisae containing genes for lactose transport (lactose permease) and hydrolysis (β-galactosidase) was exploited to ferment lactose from cheese whey to ethanol. The magnetic particle-tracer method was used to assess the effect of operational conditions (air flow rate, biomass concentration) on hydrodynamic behaviour of an the airlift bioreactor during the fermentation process. Measurements of liquid circulation velocity showed the existence of a critical value of biomass concentration at which a dramatic deceleration of net liquid flow appeared with increasing biomass quantity. Rheological analysis revealed dramatic changes in viscosity of the yeast floc suspension at the same biomass concentration of about 73g.dmˉ³ corresponding to 42.8 % v/v of solid fraction. These facts have a particular importance for the successful processing of high cell density airlift bioreactor as only a circulated flow regime will be favourable to keep the solid particles in suspension state and evenly distributed throughout the bioreactor.

One of the most common ways to improve the productivity of a fermentation process is the use of high cell density systems. In practice, such a system usually represents a threephase (gas-liquid-solid) dispersion operating in a continuous mode. The interest for these biosystems has been increasing because they seem to be a very promising alternative to the traditional batch fermentation with freely suspended cells. The cells are usually immobilised on a carrier or in a simpler and cheaper way, they are self-aggregated forming flocs. High cell density biosystems have many specific advantages: higher volumetric productivity, higher product concentration and substrate conversion, easy separation of biocatalyst (cells) from the liquid medium, utilization of the same biocatalyst (cells) for extended periods of process time and a minimised risk of contamination. A continuous airlift bioreactor (CALR) due to the advantageous combination of sufficient mixing, low shear stress and satisfactory flocs suspension at low power input is being often chosen for carrying out fermentations with high cell density. However, there is still a lack of reliable data on transport phenomena, which would allow bioreactor design and scale-up procedures to optimise a bioprocess performance at any bioreactor scale. In airlift bioreactors with a well-defined liquid circulation loop, the liquid velocity is the major hydrodynamic parameter, which considerably affects all physical phenomena. Most velocity measurement techniques are not suitable for use in fermentation processes (e.g. tagging of liquid elements with chemicals due to their interference with the exactly defined substrate pool and sterility problems, visual techniques as Laser Doppler Anemometry due to the opaqueness of the broth). The use of small flowfollowing particles with non-invasive detection of their movement is one of the promising methods. Detection techniques for opaque media include the use of radioactive counters, inductive coils and radio wave detectors. One of the attractive possibilities for a utilization of high cell density system is alcoholic fermentation of lactose from cheese whey using flocculating yeast. Cheese whey, as a by-product of dairy industry, represents a significant environmental problem due to very high values of BOD and COD. For this purpose, a flocculating recombinant strain of Sacharomyces cerevisae was developed enabling the hydrolysis of lactose to galactose and glucose, followed by sugar conversion into ethanol. The main goal of this study was to investigate the hydrodynamics of continuous airlift bioreactor during ethanolic fermentation using highly flocculating yeast. The magnetic particle-tracer method was used for hydrodynamic measurements. Different operation conditions (dilution and air flow rates and biomass concentration), bioreactor configuration and its scale (6 and 50 dm3) have been applied in order to assess their impact on bioreactor hydrodynamics and its operation and to study scale-up effects on the bioprocess. Measurements of liquid circulation velocity revealed one very important fact regarding to airlift bioreactor operation with high cell density system – the existence of a critical value of biomass concentration, at which a dramatic deceleration of net liquid flow appears when the biomass quantity increases (see Figure). Moreover, the magnitude of critical biomass concentration was found not to be dependent on gas flow rate.

The hydrodynamic study of a three-phase airlift (TPAL) bioreactor with an enlarged gas–liquid dual separator was carried out. Different lengths and diameters of the draft tube were tested to show how the design of the separator zone affects the hydrodynamic performance of the TPAL reactor. Ca-alginate beads with entrapped yeast biomass at different loadings (0, 7, 14 and 21% v/v) were used in order to mimic the solid phase of conventional high cell density systems, such as those with cells immobilized on carriers or flocculating cells. Important information on multiphase flow and distribution of gas and solid phases in the internal-loop airlift reactor (ALR) with high solids loading was obtained, which can be used for suggesting optimal hydrodynamic conditions in a TPAL bioreactor with high solids loading. It is finally suggested that the ALR with a dual separator and a downcomer to riser cross-sectional area ratio (AD/AR) ranging from 1.2 to 2.0 can be successfully applied to batch/continuous high cell density systems, where the uniform distribution of solid phase, its efficient separation of particles from the liquid phase, and an improved residence time of air bubbles inside the reactor are desirable.

The hydrodynamic study of a three-phase airlift reactor with an enlarged head zone was carried out. Effects of gas-solid-liquid separator design and riser to downcomer cross-sectional area using different lengths and diameters of the draft tube were investigated. Ca-alginate beads with entrapped yeast biomass at different loadings up to 21 % (v/v) were used in order to mimic the solid phase of conventional high cell density systems. The main hydrodynamic parameters – liquid circulation velocity, gas and solids holdups in all reactor sections have been measured and experimental values were satisfactorily predicted using an appropriate three-phase model. The results of this study may beapplied to suggest the optimal design (in terms of hydrodynamic behaviour) of a batch/continuous three-phase ALR for high cell density fermentations.