Mass transfer, gas hold-up and cell cultivation studies in a bottom agitated draft tube reactor and multiple impeller Rushton turbine configuration

Abstract

Gas-liquid mass transfer is an important phenomenon in aerobic microbial cultivations, and the mass transfer performance of an industrial reactor strongly affects the overall process economics. Traditionally, industrial and laboratory bioreactors have been agitated with flat disc turbines (Rushton turbines) although there are many variants to this design. In addition, pneumatically agitated reactors such as bubble columns and airlift reactors have been studied and used by the industry. In this study we utilize an agitated draft tube reactor in cell cultivation and mass transfer studies. A standard reactor geometry agitated with three Rushton turbines was compared to Outotec OKTOP®9000 reactor which is a draft tube reactor agitated with a single impeller located just below the draft tube. The experiments included cell cultivation with Pichia pastoris yeast, determination of overall mass transfer coefficient by dynamic gassing in method and measurement of local gas holdup by electrical impedance tomography (EIT). In addition, agitation power was estimated from the power consumption of the DC-motor. OKTOP®9000 reactor was found to have higher kLa values than the STR with similar agitation power and gas flowrate. The overall gas holdup was similar in both geometries at same power inputs and gas flow rates. However, some significant differences were detected in the distribution of gas phase between the two geometries especially in the axial direction. Also changes in the gas dispersion regime can be detected from the spatial distribution of the gas holdup measured by EIT. The cell cultivation experiments showed the applicability of this type of agitated draft tube reactor to bioprocesses although a direct comparison with Rushton geometry is not straightforward.

Effect of oxygen transfer on yeast growth — Growth kinetic and reactor model to estimate scale-up effects in bioreactors

“…Laboratory scale batch cultivations were carried out using Pichia pastoris X33 yeast in batch cultivations with two reactor types: OKTOP9000® and stirred tank reactor agitated with three Rushton turbines. The main geometrical parameters are presented in Figure 1, and more information of the reactors including gas dispersion and mass transfer performance can be found from Tervasmäki et al (2016). For the cultivation experiments, inoculum was cultivated overnight in a liquid YPD-medium in baffled shake flasks (2 x 500 ml), and the bioreactor cultivations (Biostat C-DCU, Sartorius, Germany) were carried out in a basal salt medium supplied with 4.35 ml/l PTM1 trace salts at a temperature of 30 °C (Table 1).…”

“…The model was also applied to the batch cultivation in STR discussed in section 4.1. The initial conditions were the same as in the batch experiment and kLa was estimated to be 250 h -1 in the experimental conditions based on previous research (Tervasmäki et al, 2016). 30%.…”

“…The SDTR was found to have higher efficiency for mass transfer compared to STR but not to ALR. Recently, a scaled down model of industrial scale bottom-agitated draft tube reactor (Outotec OKTOP®9000) was shown to perform well in mixing and mass transfer experiments and was also suitable for cell cultivations (Tervasmäki et al, 2016). This reactor type has been used in the hydrometallurgical industry for direct leaching of zinc concentrate in scales up to 1000 m 3 , which is a rather large scale for multiphase stirred tank reactor.…”

Oxygen transfer and bubble sizes occurring in a pilot‐scale cultivation of Bacillus velezensis 83 for the production of poly(γ‐glutamic acid) under two schemes of power drawn

“…Several recent works focusing on bioreactor hydrodynamics and oxygen transfer (in terms of bubble size or k L a as a function of different parameters) have been performed using model fluids. [24][25][26][27][28] Although considerable basic information may be obtained from these model systems, extrapolation to more complex real systems is not always justified. Moreover, few works are available in which the power input is accurately measured in actual fermentations; representative works include those published by Rocha-Valadez et al 19 and Amanullah et al 29 PGA production is an attractive model for analyzing mass transfer and air dispersion, involving a fast fermentation which generates broths of low turbidity and exhibits non-Newtonian rheological behavior which affects the oxygen transfer.…”

“…For mycelial fermentations, in which the presence of solids is hydrodynamically relevant, pulp paper suspensions are used to simulate the actual fermentation behaviors. Several recent works focusing on bioreactor hydrodynamics and oxygen transfer (in terms of bubble size or k L a as a function of different parameters) have been performed using model fluids 24‐28 . Although considerable basic information may be obtained from these model systems, extrapolation to more complex real systems is not always justified.…”