S.D. Vlaev

A networks-of-zones analysis of mixing and mass transfer in three industrial bioreactors

Abstract The original version of the networks-of-zones (N-o-Z) model developed for the description of gas–liquid flow in stirred-vessel reactors (R. Mann, Gas–liquid stirred vessel mixers: towards a unified theory based on network of zones, Transactions of the Institution of Chemical Engineers 64 (1986) 23–34) has been extended and enhanced to cover distributed bubble sizes, gas–liquid mass transfer, bioreaction kinetics and multiple-impeller operation. In addition, a modified version of the N-o-Z model for bubble columns has been simply derived from the impeller version, assuming the existence of a two-loop axisymmetrical circulation pattern induced by the non-uniform distribution of gas holdup in bubble columns. The liquid circulation velocity has been expressed as a function of gas flow rate and the density difference between the gas and liquid phases, based on Zehners circulation model (P. Zehner, G. Schuh, A concept for the description of gas phase mixing in bubble columns, German Chemical Engineering 5 (1985) 282–289). These two variants of the N-o-Z model have been used for modelling three different industrial fermenters: 3 and 31 m 3 triple-impeller stirred reactors, and a 236 m 3 bubble column reactor. The performance of these three reactors, typical of the fine chemicals, bioprocessing and pharmaceutical process industries was evaluated and compared in terms of geometry/size, gas flows, power inputs, pressure, liquid mixing, oxygen mass transfer, reaction speed and spatial variability of behaviour. This provides potentially valuable insights into the relative factors influencing the selection of an appropriate reactor type.

A 3-D Analysis of Gas-Liquid Mixing, Mass Transfer and Bioreaction in a Stirred Bio-Reactor

Gas-liquid stirred vessel two-phase mixing accompanied by bioreaction has been analysed using a 3-D networks-of-zones, in which non-axisymmetric phenomena can be included. The effect of the feed of liquid nutrient from a single dip-pipe can be incorporated so that previous 2-D limitations of axisymmetry are avoided1. The turbulent swirl flow created by the impeller uses clock-wise and anti-clock-wise swirl coefficients, which can be estimated using image-reconstruction 3-D visual modelling8. The simulations can provide detailed predictions of the local gas hold-up distribution, the local mass transfer area, the partial segregation of both the dissolved oxygen and the nutrient and the extent of oxygen depletion of bubbles. The overall gas hold-up and mass transfer area are obviously summations of the local values and the local and overall reaction rates can be predicted as well as the local and overall oxygen absorption fluxes. Simulations are presented for a 3 X [2X (10 X 10)) X 60] configuration of networks-of-zones for a 3 m3 triple-impeller industrial pilot-plant bioreactor. The theoretical predictions are demonstrated using colour-augmented 3D contour maps and solid-body isosurface images created by AVS graphics. Severe non-uniformity of gas hold-up distribution and consequently spatially uneven oxygen mass transfer create significant partial segregation of both oxygen and nutrient. The simulated bioreactor is predicted to be far from perfectly mixed, so that Tylosin producing microorganisms will experience large variations in dissolved oxygen and nutrient concentrations as they circulate around the stirred fermenter.

Macro-mixing and Streptomyces fradiae - Modelling oxygen and nutrient segregation in an industrial bioreactor

Some illustrative results are presented for a 3 m3 triple-impeller stirred reactor producing tylosin (using Streptomyces fradiae) being fed with air and a concentrated aqueous ammonium nitrate solution. The analysis is performed for a 3 × 2 (10 × 10) configuration of a networks-of-zones model comprising 600 zones. The gas-liquid flows, giving rise to circulation and bubble dispersion, are calculated assuming that bubbles move independently at their rise velocity without distorting the liquid flows being generated by the impellers. In this way the gas hold-up distribution in 3-D is obtained from simple continuity principles. Mass transfer and a 2nd order bioreaction may then be combined with gas-liquid flow to predict the spatial distribution of the (pseudo-stationary) concentration fields. The observed overall gas voidage, overall oxygen mass transfer and a locally measured dissolved oxygen level are all exactly matched by the simulations. The predicted segregated concentration fields show remarkably complicated patterns. Micro-organisms following stochastic paths around the vessel will experience wide variations in chemical environments.

Experimental Visualization and CFD Simulation of Flow Patterns Induced by a Novel Energy-Saving Dual-Configuration Impeller in Stirred Vessels

The flow characteristics of a novel agitator, termed ‘Narcissus’ (NS), have been investigated by dye-tracer visualization and laser Doppler velocimetry; the experimental velocity data were also used to initiate and then validate the CFD simulation of the flow induced by the NS in a cylindrical stirred vessel, using a multiple reference-frame technique and an unstructured inner-volume mesh. This novel impeller is shown to have the characteristics of a radial agitator, with a double configuration: the flow loop structure in the vessel depends upon its normal or inverted insertion, with liquid either being drawn radially towards the NS or ejected outwards from it. This dual configuration, in conjunction with its very low power consumption, constitute interesting and important impeller features, especially for gas–liquid applications.

A Simplified CFD for Three-dimensional Analysis of Fluid Mixing, Mass Transfer and Bioreaction in a Fermenter Equipped with Triple Novel Geometry Impellers

A Three-dimensional networks-of-zones model has been developed to describe the performance of a novel geometry Narcissus (NS) impeller when used for gas–liquid mixing in a triple impeller bioreactor. The networks-of-zones can be simply adapted from an earlier version based upon the Rushton turbine. The adaptation is achieved by a conformal stretching of the zone volumes. This accommodates the complex up–down liquid flow pattern by a simple adjustment of the zone volumes matrix, whilst the underlying mass and component balances on every zone stay unchanged. Some results are presented for a pilot-scale Tylosin fermenter. Spatially complex fields in three-dimensions are visualized by solid-body graphics for gas hold-up, bubble phase oxygen, liquid-phase dissolved oxygen, nitrate nutrient and bioreaction rates for a 3 × 2 × (10 × 10) × 60 configuration comprising 36,000 zones. The triple NS impeller produces locally higher levels of dissolved oxygen than are achieved by triple Rushton turbines. In both cases, the behaviour predicted is completely different from that obtained by an oversimplified plug (gas)–backmixed (liquid) bioreactor model.

CELLULASE PRODUCTION BY TRICHODERMA SP. GROWN ON CORN FIBRE SUBSTRATE

Abstract Trichoderma sp. strain 414 was cultivated in Tanaka media based on corn fibre pulp from corn starch production to test the hemicellulase- and cellulase-inducing properties of the pulp when used as carbon source in nutrient dispersions. The pulp contained up to 70% hydrolysable matter. Cellulase and xylanase production were controlled and their activities reached compatible levels. Preliminary analysis of bioreactor performance showed that the process required moderate mixing and aeration, pH 4·5, and a temperature range between 28°C and 30°C. Cellulase activity reached 3·4 U/cm 3 , while xylanase activity achieved was 3·7 U/cm 3 . The time-course of enzyme production was determined.

CFD modelling of two-phase stirred bioreaction systems by segregated solution of the Euler–Euler model

An advanced study of a bioreactor system involving a Navier-Stokes based model has been accomplished. The model allows a more realistic impeller induced flow image to be combined with the Monod bioreaction kinetics reported previously. The time-course of gluconic acid production by Aspergillus niger strain is simulated at kinetic conditions proposed in the literature. The simulation is based on (1) a stepwise solution strategy resolving first the fluid flow field, further imposing oxygen mass transfer and bioreaction with subsequent analysis of flow interactions, and (2) a segregated solution of the model replacing the multiple iterations per grid cell with single iterations. The numerical results are compared with experimental data for the bioreaction dynamics and show satisfactory agreement. The model is used for assessment of the viscosity effect upon the bioreactor performance. A 10-fold viscosity rise results in 2-fold decrease of KLa and 25% decrease of the specific gluconic acid production rate. The model allows better understanding of the mechanism of the important bioprocess.

Some effects of rheology on the spatial distribution of gas hold-up in a mechanically agitated vessel

The effects of rheology on gas hold-up in non-Newtonian circulation flow has been rarely discussed in the literature, especially so far as local gas distribution in mechanically agitated vessels is concerned. This study analyses experimental gas dispersion behaviour for a wide range of complex-rheology conditions. Non-Newtonian aqueous solutions of xanthan gum with concentrations from 0.5 to 5 kg/m 3 and Newtonian glycerol solutions of 450 and 800 kg/m 3 containing also 5% electrolyte are studied. Hold-up is measured conductometrically at 36 spatial points within the stirred vessel mainly around and above the impeller for a conventional geometry (Rushton standard configuration with T = 0.2 m and D/T = 0.33). In addition to increasing viscosity, three aspects of rheology impact on gas hold-up and different dominances of component effects upon the overall gas dispersion mechanism are revealed. The results are presented as local and global gas hold-up vs. consistency, pseudoplasticity, and position. By comparing data obtained for Newtonian and non-Newtonian flow conditions and data obtained for the same polymer solution at constant plasticity and variable consistency, an attempt to reveal the net effects of consistency and pseudoplasticity is made. The boundaries of regions with large differences of mixing intensity reflected in terms of local gas availability are quantified.

Pressure distribution at impeller blades of some radial flow impellers in saccharose and xanthan gum solutions: A CFD visualization approach

Food-mixing operations often require distribution of components, e.g. sweeteners and thickeners, in various liquids, e.g. syrups or pulps. Generation of homogeneous texture and composition depends strongly on drag distribution at the impeller blades. In view of the fact that the substantial part of impeller power draw is due to pressure drag, pressure distribution across blades is a measure for impeller performance in food-blending operations. The study objective has been to reveal the pressure distribution profiles in the vicinity of some typical blades employed in blending operations and fermentation. Three typical radial flow impellers, namely, the conventional flat-blade disc turbine, an impeller with half-pipe cylindrical blades and an impeller with half-pipe parabolic blades, are studied. In order to gain more information, a CFD visualization has been employed for the analysis. The CFD analysis included RANS modelling and an MRF solving method. As a result, the pressure field in three typical liquids—water, a 64% saccharose solution and a 0.5% hetero-polysaccharide (xanthan gum) solutions—has been elucidated. The effects of blade design and fluid physical properties are discussed and interpreted in terms of power draw for mixing operations.

Rheology and oxygen transfer in starch suspensions during high temperature (35 °C) α-amylase fermentation

Abstract The rheological characteristics of a pseudoplastic starch suspension formed during high temperature α-amylase production from Bacillus licheniformis 44 MB82 were measured at apparent viscosities ranging from 250 to 20 mPa s. Parallel runs with model non-bacterial suspensions of similar rheological behaviour were used to evaluate the oxygen transfer coefficients K L a. A more than threefold increase in mass transfer rate followed the decrease in viscosity. Relationships for K L a vs. specific power input were generated for the various stages of the changes in the highly viscous media. On the basis of these results, two extreme operating regimes—a regime with a constant mixer revolution rate and continuously increasing K L a and one with a variable mixer revolution rate and constant K L a—are discussed in terms of power consumption. It is shown that a programme of variable mixer revolution rate leading to an invariable mass transfer rate is more conducive to equipment efficiency.