Hydrodynamic considerations in three-phase internal-loop airlift bioreactors – effect of dual separator and draught tube design

In recent years, there has been a growing interest in bioreactors, which utilise immobilised enzymes and cells in order to improve the bioprocess productivity. In fact, there are many specific advantages of the immobilised systems in comparison with the more conventional systems, in which the bio-agents are suspended freely (cells) or dissolved in a bulk aqueous medium (enzymes): • high cell densities per unit bioreactor volume, resulting in very high bioconversion rates; • performance of a bioreactor in the continuous mode of operation; reducing capital costs • employment of the same biocatalyst (cells) for extended periods of process time; • easy separation of biocatalyst (cells) from the liquid medium; • minimised risk of contamination. The immobilised system usually represents three-phase dispersion, where an intimate contact of gas, liquid and solid phases should be ensured. Three-phase airlift (TPAL) bioreactors provide such suitable environment and together with advantageous combination of controlled mixing and low shear rate, efficient suspension of solids, makes airlift system attractive for bioprocesses, where microorganisms are immobilised at solid carriers (e.g. biofilm particles) or flocculate (e.g. flocculating yeasts S. cerevisiae). In these high-cell density systems, may be solids loading as high as 30-40 % of the total reactor volume, what is necessary for achievement of a high conversion of the continuous bioprocess. This amount of particles can be completely suspended in TPAL with a lower energy requirement comparing to bubble columns [1]. This advantage of ALR results from existence of liquid circulation loop inside the reactor originating from the density difference between the riser and downcomer sections. The liquid circulation velocity is essential parameter in the design of the TPAL reactor because it has crucial effect on various processes – mixing, extent of bubble recirculation, efficiency of solids suspension and distribution of gas and solids holdups. Thus, the knowledge of liquid circulation rate is of particular importance. Together with another hydrodynamic parameters, such as gas and solids holdups, circulation and mixing time, is influenced by air flow rate, solids loading and their properties and reactor design. Most hydrodynamic studies on bioreactor design investigated the influence of basic reactor dimensions – downcomer to riser cross-sectional area ratio (AD/AR), height of draft tube and column height to diameter ratio (H/D). However, a gas-liquid separator, which may significantly affect the performance of TPAL, is still frequently overlooked. The head zone is usually designed at purpose of a control of the extent of bubble penetration into the downcomer, whereby it determines the magnitude of gas holdup and the driving force for liquid circulation. Consequently, it affects overall hydrodynamic, mass transfer and mixing characteristics of airlift reactors. The control of intensity of bubble separation is usually achieved by the change of size (i.e.