Hydrodynamic characteristics and gas-liquid mass transfer in a biofilm airlift suspension reactor RID B-2738-2009 RID D-1669-2009

The hydrodynamics and mass transfer, specifically the effects of gas velocity and the presence and type of solids on the gas hold-up and volumetric mass transfer coefficient, were studied on a lab-scale airlift reactor with internal draft tube. Basalt particles and biofilm-coated particles were used as solid phase. Three distinct flow regimes were observed with increasing gas flow rate. The influence of the solid phase on the hydrodynamics was a peculiar characteristic of the regimes. The volumetric mass transfer coefficient was found to decrease with increasing solid loading and particle size. This could be predominantly related to the influence that the solid has on gas hold-up. The ratio between gas holdup and volumetric mass transfer coefficient was found to be independent of solid loading, size, or density, and it was proven that the presence of solids in airlift reactors lowers the number of gas bubbles without changing their size. To evaluate scale effects, experimental results were compared with theoretical and empirical models proposed for similar systems. (C) 1998 John Wiley & Sons, Inc

Mixing in large-scale vessels stirred with multiple radial or radial and axial up-pumping impellers: modelling and measurements

Mixing phenomena are regarded as one of the major factors responsible for the failure to successfully scale up some bioprocesses. Such phenomena have been investigated within the framework of an EC project `Bioprocess Scale-up Strategy'. Mixing in bioreactors depends on energy input, impeller type, reactor configuration and impeller geometry. Here, two different reactors of volumes 12 and 30 m3 were used, and they were equipped with either multiple Rushton turbines or with a combination of a Scaba 6SRGT radial impeller with multiple 3SHP axial up-pumping hydrofoils above it. Mixing time, power consumption, gas hold-up and liquid velocities were measured at different stirrer speeds and aeration rates in water. At the same total specific power input, aeration did not influence the mixing time much unless it changed the bulk flow pattern. A considerable reduction of mixing time was achieved if the upper impellers were axial instead of radial Rushtons at the same power consumption. The improvement with the axial impellers could be related to the reduction of axial flow barriers due to different circulation flow patterns. The Compartment Model Approach (CMA) was used to develop a flow model based on the general knowledge of the hydrodynamics of both unaerated and aerated stirred vessels. The model was successfully verified for different impeller and reactor configurations and different scales with measured pulse response curves, using either a fluorescent or a hot water tracer. The model can be used for process design purposes

Instabilities when using a Standard (T/3) Rushton Turbine for Stirring as Foam Disruption (SAFD)

The concept of using the upper stirrer for foam disruption in a bioreactor agitated by multiple impellers has recently been published by Hoeks et al. (1997). This concept, stirring as foam disruption (SAFD), was shown by them to be effective with a range of impellers. However, the commonly used (so-called) standard Rushton turbine of one-third the fermenter diameter was not included. This paper fills that important gap. By measuring the foam height, the holdup, the power draw and the velocities of the liquid in the dispersion just below its top surface, it is concluded that the SAFD concept does not work well with the standard Rushton turbine. This is because the amount of broth for which foam can be disrupted is less than that found with all the other impellers tested to date; and even when foam disruption occurs, significant flow instabilities and torque fluctuations are found. Perhaps the poor performance of this impeller, which has been used so frequently in industry and in academic studies, explains why the concept of SAFD was not developed earlier

Mixing in large-scale vessels stirred with multiple radial or radial and axial up-pumping impellers: modelling and measurements

Mixing phenomena are regarded as one of the major factors responsible for the failure to successfully scale up some bioprocesses. Such phenomena have been investigated within the framework of an EC project `Bioprocess Scale-up Strategy'. Mixing in bioreactors depends on energy input, impeller type, reactor configuration and impeller geometry. Here, two different reactors of volumes 12 and 30 m3 were used, and they were equipped with either multiple Rushton turbines or with a combination of a Scaba 6SRGT radial impeller with multiple 3SHP axial up-pumping hydrofoils above it. Mixing time, power consumption, gas hold-up and liquid velocities were measured at different stirrer speeds and aeration rates in water. At the same total specific power input, aeration did not influence the mixing time much unless it changed the bulk flow pattern. A considerable reduction of mixing time was achieved if the upper impellers were axial instead of radial Rushtons at the same power consumption. The improvement with the axial impellers could be related to the reduction of axial flow barriers due to different circulation flow patterns. The Compartment Model Approach (CMA) was used to develop a flow model based on the general knowledge of the hydrodynamics of both unaerated and aerated stirred vessels. The model was successfully verified for different impeller and reactor configurations and different scales with measured pulse response curves, using either a fluorescent or a hot water tracer. The model can be used for process design purposes

Comparing a range of impellers for "Stirring as Foam Disruption" (SAFD)

The effectiveness of a range of impellers for "stirring as foam disruption" (SAFD) is assessed in a vessel of 0.72 m diameter and an aspect ratio of 2: 1. Measurement of power drawn by the impeller achieving SAFD and of the three-dimensional flow field close to the dispersion surface are both used to explain the findings along with the global gas hold-up. A large radial flow Rushton turbine can disrupt foam at a great height but requires high power. Down-pumping hydrofoils are only effective when the ungassed liquid height is below the level of the impeller employed to disrupt foam. Up-pumping hydrofoils are the most effective because their flow pattern gives rise to high velocities across the dispersion surface, which are able to entrain foam in the downflow generated at the walls. © 2002 Elsevier Science B.V. All rights reserved

Comparing a range of impellers for "stirring as foam disruption"

The effectiveness of a range of impellers for "stirring as foam disruption" (SAFD) is assessed in a vessel of 0.72 m diameter and an aspect ratio of 2: 1. Measurement of power drawn by the impeller achieving SAFD and of the three-dimensional flow field close to the dispersion surface are both used to explain the findings along with the global gas hold-up. A large radial flow Rushton turbine can disrupt foam at a great height but requires high power. Down-pumping hydrofoils are only effective when the ungassed liquid height is below the level of the impeller employed to disrupt foam. Up-pumping hydrofoils are the most effective because their flow pattern gives rise to high velocities across the dispersion surface, which are able to entrain foam in the downflow generated at the walls. © 2002 Elsevier Science B.V. All rights reserved