The effects of geometry and operational conditions on gas holdup, liquid circulation and mass transfer in an airlift reactor

Abstract

-In airlift reactors transport phenomena are achieved by pneumatic agitation and circulation occurs in a defined cyclic pattern through a loop. In the present work, the effect of geometrical relations on gas holdup and liquid velocity, and consequently on the gas-liquid mass transfer coefficient, was studied in a 6-liter airlift bioreactor with A D /A R = 0.63; A D , downcomer cross-sectional area, and A R , riser cross-sectional area. Measurements of the volumetric oxygen transfer coefficient (k L a) were taken in a water-air system using a modified sulfite oxidation method. Different conditions were examined by varying parameters such as superficial air velocity in the riser (U GR ), bottom clearance (d 1 ) and top clearance (d 2 ). It was observed from the experimental results that d 1 and d 2 have a remarkable effect on k L a values. The effect is due to their influence on gas holdup and liquid velocity, consequently affecting k L a. Superficial air velocity in the riser (U GR ) ranged from 0.0126 to 0.0440 m.s -1 and k L a varied between 40 to 250 h -1 , whereas gas holdup (ε) reached values up to 0.2. The volumetric oxygen transfer coefficient (k L a), gas holdup in the riser (ε R ) and downcomer (ε D ) and superficial liquid velocity in the riser (U LR ) for all the geometrical relations were successfully correlated with dimensionless numbers, namely, the Sherwood number (

Airlift Bioreactor Fluid-Dynamic Characterizationfor the Cultivation of Shear Stress Sensitivemicroorganisms

“…This, in turn, increases the ascending velocity of gas bubbles, and the circulation of the liquid, which will affect gas hold-up. Similar patterns are commonly observed, even for different bioreactor geometrical designs [22][23][24][25][26][27]. For the glass ELA prototype, the gas that enters the downcomer, due to its lower density, put forth a counter flux effect, which is detrimental to the circulation of the liquid, as observed in Fig.…”

“…Table 1 presents the values of kLa and other hydrodynamic parameters, obtained in the literature for different bioreactor designs. It is important to compare the results obtained in this study w ith those of Gouveia et al [27], who worked with an internal-loop 6-L airlift bioreactor. The differences in gas hold-up values between the bioreactor designed by Gouveia et al [27] and the bioreactor from this study, are due primarily to the fact that for the latter there is total gas separation, and the downcomer (absence of gas bubbles) occupies around 7% of the bioreactor entire volume.…”

“…Despite these differences, the kLa values were similar for both bioreactor designs, proving the efficiency of the current design. Furthermore, Gouveia et al [27] used the sulfite method instead of the nitrogen method, employed in the present study, for the determination of kLa. Thus, it is likely that the real kLa value obtained by those authors is lower than the reported value.…”

Hydrodynamic and Mass Transfer Characteristics of Annulus-Rising Airlift Reactor – The Effect of Reactor Scale

“…However, studies on AR-ALRs are quite scarce [15][16][17][18] although Koide et al. [17,18] have shown in their experimental studies that AR_ALRs have in fact higher gas holdups and mass transfer rates than CR_ALRs, especially for liquid phase systems which have frothing properties.…”

Investigation of hydrodynamics and transition regime in an internal loop airlift reactor using CFD

“…They have advantages over bubble columns due to the higher liquid circulation and higher intensity of turbulence (Chisti, 1989;Cao et al, 2007). The ALRs are widely used in chemical industries, biotechnological processes and wastewater treatment (Jianping et al, 2005;Gouveia et al, 2003;Vial et al, 2001;Couvert et al, 2001). Simple construction, low power consumption for agitation and aeration, high efficiency of homogenization and relatively constant as well as mild shear stress throughout the reactor are the advantages of airlift reactors compared to other gasliquid contacting devices (Vial et al, 2001;Couvert et al, 2001;Jin et al, 2006).…”