Hydrodynamics of non-Newtonian liquids in external-loop airlift bioreactors
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Gas holdup investigation was performed in two external-loop airlift bioreactors of laboratory (V L =1.189·10−3− 1.880·10−3 m3; H R =1.16 − 1.56 m; H D = 1.10 m; A D /A R = 0.111 − 1.000) and pilot scale (V L =0.157−0.170 m3; H R =4.3−4.7 m; H D =4.0−4.4 m;A D /A R =0.04−0.1225), respectively, using as liquid phase non-Newtonian starch solutions of different concentration with K=0.061−3.518 Pa sn and n=0.86−0.39 and fermentation broths of P. chrysogenum, S. griseus, S. erythreus, B. licheniformis and C. acremonium at different hours since inoculation and from different batches. The influence of bioreactor geometry, liquid properties and the amount of introduced compressed air was investigated. The effect of sparger design on gas holdup was found to be negligible. It was found that gas holdup depends on the flow media index, ɛGR decreasing with the increase of liquid pseudoplasticity, A D /A R ratio and H R /H D ratio. The experimental data are in agreement with those presented in literature by Popovic and Robinson, which take into account liquid properties, geometric parameters and gas superficial velocity, with a maximum error of ±30%. It was obtained a correlation for gas holdup estimation taking into account the non-Newtonian behaviour of the fermentation broths and the dry weight of the solid phase, as well. The concordance between the experimental data and those calculated with the proposed correlation was good, with a maximum error of ±17%. Also, a dimensionless correlation for gas holdup involving superficial velocities of gas and liquid, cross sectional areas ratio, dispersion height to riser diameter ratio, as well as Froude and Morton numbers, was obtained.
KeywordsFermentation Riser Superficial Velocity Starch Solution Media Index
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