Characterization of the two-phase systems in airlift tower-loop bioreactors during the cultivation of E. coli

During the cultivation of E. coli in an airlift tower-loop bioreactor, the following properties were measured: transverse profiles of Sauter bubble diameter, d(S); local relative gas holdup, E(G); bubble rise velocity, u(BS); local mean velocity, ū turbulence intensity, u'; macrotime scale, T(EL); dissipation time scale, tau(E); power spectrum, E(n); and energy dissipation spectrum D(n) at different distances from the aerator. The influence, distance from the aerator, absence and/or presnece of cells, and batch and/or continuous-culture operation on the behavior of the two-phase system are discussed on the basis of these properties.     

Investigation of the structure of two-phase flow fermentation model media in bubble-column bioreactors

Summary Electrical conductivity microprobes have been used to estimate the transverse variation of bubble size, local gas holdup and local specific gas/liquid interfacial area in bench scale bubble column bioreactors containing fermentation model media. Inserted O₂-electrodes and plane parallel windows alter the structure of the two phase flow. Even slight tilting of the column strongly influences the transverse profiles of the bubble size and local gas holdup. The larger bubbles are collected at the wall, where they can be redispersed. These observations open up new possibilities for the construction of bubble column bioreactors.     

Investigation of the structure of two-phase flow model-media in bubble-column bioreactors

Summary By applying photographic, electrical conductivity, and electrooptical methods, the transverse variation of bubble size and velocity, the local gas hold up, and the local specific gas/liquid interfacial area were estimated in a bench scale bubble-column bioreactor containing model cultivation media. The liquid velocity profile, the transverse turbulence intensity variations, and the turbulence energy dissipation scale were also measured by a hot film turbulence probe and constant temperature anemometer technique.A significant relationship was found between the two-phase flow fluid dynamical state and the transverse variation of the various properties.Symbols M mass - L length - T time - a gas/liquid interfacial area L² - specific gas/liquid interfacial area with regard to the bubbling layer volume L^–1 - D transverse coordinate (measured from the wall of the column) L - d bubble diameter L - d mean bubble diameter L - d_e dynamic equilibrium (maximum stable) bubble diameter L - d_p primary bubble diameter L - d_s Sauter bubble diameter L - E specific energy dissipation rate with regard to the volume of the liquid ML^–1T^–3 - EV_L energy dissipation rate ML²T^–3 - , since =1 g/cm³, E has the same numerical value as E. Therefore, the symbol E is used everywhere in the present paper for E for simplicity and called energy dissipation rate (S.s^–2=Stokes.s^–2) L²T^–3 - E_G or local relative gas holdup - f (r) cross correlation function - g acceleration of gravity LT^–2 - h longitudinal distance from the aerator L - relative turbulence intensity - N_O number of u and crossings T^–1 - n_B bubble frequency T^–1 - r distance between two points 1 and 2 of the cross correlation function L - t time - u instantaneous liquid velocity LT^–1 - mean liquid velocity LT^–1 - mean square fluctuation velocity L²T^–2 - turbulence intensity LT^–1 - w_SG superficial gas velocity LT^–1 - w_SL superficial liquid velocity LT^–1 - or E_G local relative gas holdup LT^–1 - energy dissipation scale L - kinematic liquid viscosity L²T^–1 - liquid density M L^–3 - surface tension M T^–2 - dynamic turbulence pressure M L^–1T^–2 Indices p primary (at the aerator) - e equilibrium (far from the aerator)     

Investigation of the structure of two-phase flows. Model media in bubble-column bioreactors

Summary The following two-phase properties were evaluated in bubble column reactors with porous plate (5 m pore diameter) or perforated plate (1 mm and/or 3 mm hole diameter) gas distributors using distilled water or a 1% methanol solution: transverse profiles of the mean and Sauter bubble diameters, local gas holdups, true mean liquid and bubble velocities. Furthermore, swarm bubble velocity distributions were evaluted and compared with calculated values.     

A simplified model of hydrodynamics in airlift loop reactors

As a function of the gas throughput the following parameters were measured in an external loop reactor with a riser diameter of 0.6 m and a gassed liquid height of 8.6 m: integral and local values of gas hold-up; liquid velocities; mixing times and axial dispersion coefficients of the liquid phase. The height of the reactor could be altered by reconstruction. Measurements were also carried out with lower heights than 8.6 m. Besides pure water, aqueous solutions of coalescing, non-coalescing and viscosity-increasing substances were used as model systems. With the results a general relationship between superficial gas velocity, gas hold-up and liquid velocity was established. This hydrodynamic model uses the relative velocity between gas and liquid phase as the fundamental parameter. The generally valid model consists of one term for the homogeneous and of two additional terms for the heterogeneous flow regime.     

Velocity field studies at surgically imposed arterial stenoses on the abdominal aorta in pigs

In order to describe velocity profiles and the size of deterministic and non-deterministic velocity disturbances at arterial stenoses, symmetrical and asymmetrical stenoses with intended area reductions of 50% (‘moderate’) and 85% (‘severe’) were applied on the abdominal aorta in six pigs. Blood velocities were registered by hot-film anemometry in 21 measuring points distributed across the vessel cross-sectional area in one pre-stenotic and three post-stenotic positions. Signal analysis included ensemble averaging, the high-pass filtering technique, and three-dimensional visualization. None of the stenoses affected the pre-stenotic velocity field. Downstream moderate stenoses flow separation and vortex formation were present. Moderate asymmetric stenoses induced turbulence in the post-stenotic velocity field. Immediately downstream of severe stenoses a prominent post-stenotic jet was present. Farther downstream, a multitude of coherent vortices and turbulence dominated the flow field. The transverse distribution of turbulence intensity parallelled with the peak systolic velocity profile, whereas transverse profiles of the relative turbulence intensity (turbulence intensity/mean velocity) revealed peak values in flow field locations with high velocity gradients. Velocity parameters for symmetric and asymmetric severe stenoses were highly comparable. However, the exact degree of stenosis was significantly higher for symmetrical (85%) than for asymmetrical (76%) stenoses. Therefore, recalling that stenosis severity strongly influences the development of velocity disturbances, this indicates that asymmetry of a stenosis is a predictor for blood velocity disturbances.     

Fluiddynamic characterization of airlift tower loop bioreactors with and without motionless mixtures

In a 60 l airlift tower reactor with outer loop fluiddynamical measurements were carried out in presence of three motionless mixer modules (Type SMV, Sulzer) in water, 0.6%, 0.9% and 1.2% CMC solutions. The global mixing properties were determined in the liquid and gas phases by tracers. Detailed local measurements revealed differences of local flow patterns and mixing properties in the unhindered riser and in the immediate wake of the mixer module. The local liquid velocities were measured by the pseudorandom heat pulse technique. The local bubble velocities were determined by the ultrasound Doppler technique. The dependence of liquid velocity, gas velocity and gas holdup on the superficial gas velocity were determined. The radial profiles of the mean liquid velocities and their standard deviations were evaluated in water and CMC solutions upstream and downstream of the motionless mixer modules. The radial profiles of the mean large-bubble velocities and mean small-bubble velocities and their standard deviations were determined as well. The influence of the presence of the motionless mixers in the riser on these properties was evaluated.List of Symbols Bo _L w _L L/D _L liquid Bo number - Bo _G w _G L/D _G ,gas Bo number - c tracer concentration - CMC carboxymelthylcellulose - D _G gas dispersion coefficient - D ₁ local liquid dispersion coefficient - D _L liquid dispersion coefficient - D _r riser diameter - d distance between transmitter and detector of the heat pulse probe - E _G gas holdup - HBV horizontal bubble velocity - HLBV horizontal large-bubble velocity - HSBV horizontal small-bubble velocity - L length of the column - l relative distance of the detector position from the tracer injection with respect to L - LBV large-bubble velocity - n number of circulations - OHBV overall horizontal bubble velocity - OVBV overall vertical bubble velocity - P power input - Pe ₁ w ₁d/D₁, local liquid Peclet number - SBV small-bubble-velocity - V _L liquid volume - VBV vertical bubble velocity - VLBV vertical large-bubble velocity - VSBV vertical small-bubble velocity - w _G gas velocity - w ₁ local liquid velocity - w _L liquid velocity - w _SG superficial gas velocity - w _SL superficial liquid velocity - mean residence time of the liquid in the riser H.M.R. thank the Verband der Chemischen Industrie for a Fund der Chemie scholarship     

Local gas hydrodynamics in an external-loop airlift reactor: newtonian and non-newtonian fluids

Measurements of local gas phase characteristics are obtained in an external-loop airlift reactor filled with newtonian or viscous non-newtonian liquids. A double-optical fiber probe technique is used. It allows the determination of the axial and radial profiles of gas hold-up, bubbling frequency, bubble size and velocity. In the case of air-water system, the results show a strong effect of radial liquid velocity variation on the gas flow characteristics at the bottom of the riser. In the case of highly viscous non-newtonian solution, the gas flow is strongly affected by the gas distribution just above the gas sparger. This study also points out the bubble coalescence and the break-up phenomena in different liquids and levels in the reactor. Furthermore, the local measurements of bubble size and velocity allows to gain more detailed information on the dynamics of the bubble-flow and shows a tendency of large bubbles to circulate in the column center.     

Hydrodynamics in bubble column bioreactors with fermentation broths having a yield stress

Summary The hydrodynamics in a bubble column bioreactor with fermentation broths having a yield stress are studied. Specifically, the liquid velocity at the reactor axis, the axial dispersion coefficient, and the gas hold-up are examined. The liquid velocity at the reactor axis and the gas hold-up are measured in a 40-1 bench-scale bubble column fermentor using carboxypolymethylene (Carbopol) aqueous solutions as simulated broths. Theoretical correlations for the liquid velocity at the reactor axis, the axial dispersion coefficient, and the gas hold-up are derived on the basis of an energy balance and the mixing length theory. The correlations are compared with the present data and a reasonable agreement is found. The theoretical predictions are also in satisfactory agreement with the re-examined data for actual fermentation broths which are Chaetomium cellulolyticum and Neurospora sitophila cultured in a 1000-1 pilot-plant scale airlift fermentor.     

Computational fluid dynamics modeling of mass transfer behavior in a bioreactor for hairy root culture. I. Model development and experimental validation

A two-dimensional axisymmetric computational fluid dynamics (CFD) model based on a porous media model and a discrete population balance model was established to investigate the hydrodynamics and mass transfer behavior in an airlift bioreactor for hairy root culture.During the hairy root culture of Echinacea purpurea, liquid and gas velocity, gas holdup, mass transfer rate, as well as oxygen concentration distribution in the airlift bioreactor were simulated by this CFD model. Simulative results indicated that liquid flow and turbulence played a dominant role in oxygen mass transfer in the growth domain of the hairy root culture. The dissolved oxygen concentration in the hairy root clump increased from the bottom to the top of the bioreactor cultured with the hairy roots, which was verified by the experimental detection of dissolved oxygen concentration in the hairy root clump. This methodology provided insight understanding on the complex system of hairy root culture and will help to eventually guide the bioreactor design and process intensification of large-scale hairy root culture.     

Effects of airflow and liquid temperature on ammonia mass transfer above an emission surface: Experimental study on emission rate

The present study performed a series of experiments in a wind tunnel to investigate the impact of velocity, turbulence intensity and liquid–air temperature difference on ammonia emission rates. Decreasing velocity, turbulence intensity and liquid temperature are shown to reduce the ammonia emission rates. The emission rates are more sensitive to the change of velocity at a low velocity compared to change of velocity at a higher velocity range, which corresponds with the conclusion that the boundary layer thickness of velocity increases sharply when velocity is changed from 0.2 m/s to 0.1 m/s. In addition, the emission rates are more sensitive to the change of temperature at a higher temperature than at a lower liquid temperature range. The influence of velocity and liquid–air temperature difference on boundary layer thickness is also analyzed. The relationship between the emission rate and boundary layer thickness is presented.     

Enhanced uptake of dissolved oxygen and glucose by <Emphasis Type="Italic">Escherichia coli</Emphasis> in a turbulent flow

Laboratory experiments were conducted to study the effect of turbulence on Escherichia coli cells in an oscillating grid reactor under conditions of no oxygen transfer to the liquid phase. Fluid flow was quantified at a submillimeter resolution using a particle image velocimetry measuring technique. The root-mean-square estimates of the velocity gradient tensor components indicated the dominance of shear rate deformation in the fluid surrounding E. coli. The E. coli growth rate, dissolved oxygen (DO), and glucose uptake rates were facilitated by fluid-flow energy dissipation in the turbulent fluid. The Kolmogorov length scale (eta(K)) and velocity (u( K )) underlined characteristic scales at which enhanced DO and glucose uptake by E. coli were determined in a turbulent flow in comparison to still-water controls. A first-order power-law relation between the mass transport to the cells and the moving fluid is developed. The combined effects of the enhanced rate of strain at eta(K) scale and uniform velocity at u(K) determined the facilitated DO and glucose fluxes to E. coli. The mass transport to the E. coli was modeled by the Sherwood (Sh)-Péclet (Pe) number relationship by Sh = 1 + 1.08Pe(uK)(0.62) where Pe(uK) is the Péclet number defined by the u(K) velocity scale. The proposed first-order model described experimental data fairly well.     

Numerical simulations of the pulsating flow of cerebrospinal fluid flow in the cervical spinal canal of a Chiari patient

The flow of cerebrospinal fluid (CSF) in a patient-specific model of the subarachnoid space in a Chiari I patient was investigated using numerical simulations. The pulsating CSF flow was modeled using a time-varying velocity pulse based on peak velocity measurements (diastole and systole) derived from a selection of patients with Chiari I malformation. The present study introduces the general definition of the Reynolds number to provide a measure of CSF flow instability to give an estimate of the possibility of turbulence occurring in CSF flow. This was motivated by the fact that the combination of pulsating flow and the geometric complexity of the spinal canal may result in local Reynolds numbers that are significantly higher than the commonly used global measure such that flow instabilities may develop into turbulent flow in these regions. The local Reynolds number was used in combination with derived statistics to characterize the flow. The results revealed the existence of both local unstable regions and local regions with velocity fluctuations similar in magnitude to what is observed in fully turbulent flows. The results also indicated that the fluctuations were not self-sustained turbulence, but rather flow instabilities that may develop into turbulence. The case considered was therefore believed to represent a CSF flow close to transition.     

The influence of vessel height and top-section size on the hydrodynamic characteristics of airlift fermentors

Fermentations of the yeast Saccharomyces cerevisiae were carried out in a 90 to 250-L working volume concentric tube airlift fermentor. Measurements of liquid circulation velocity, gas hold-up, and liquid mixing were made under varying conditions of gas flowrate, vessel height, and top-section size. Both liquid circulation velocity and mixing time increased with vessel height. Liquid velocity varied approximately in proportion to the square root of column height, supporting a theoretically based relationship. The effect of vessel height on gas hold-up was negligible. The height of the top-section had a significant effect on liquid mixing. Mixing time decreased with increasing size of the top-section up to a critical height. As the top-section was expanded beyond this height, little improvement in mixing was seen. This indicated the presence of a two-zone flow pattern in the top-section. Liquid velocity and gas hold-up were essentially independent of top-section height. (c) 1994 John Wiley & Sons, Inc.     

Cell death in the thin films of bursting bubbles.

A sparged gas bubble floating at the liquid interface has a liquid film which drains and thins until the film spontaneously ruptures at a point. This causes rapid retraction of the film, forming a rim of collected fluid. This rim moves at a constant velocity of about 3 m/s and any cells in the bubble film are rapidly accelerated to this velocity in the moving rim. Half of the surface energy originally in the thin film is converted to kinetic energy of the rim, while the rest is dissipated in this rim. The rate of energy dissipation per mass of rim fluid is approximately 9000 m2/s3, which corresponds to a Kolmogorov eddy size of 3.2 microns in fully developed turbulence or a shear stress of 95 N/m2 in laminar flow. Either of these limiting cases presents an environment in which rapid cell death would be expected. Experiments with Sf-9 insect cells suggest that the cell concentration in these thin films is 0.6 times the bulk liquid concentration and that about 20% of these cells are killed when the film ruptures. An equation based on this mechanism accurately predicts the death rate.     

Liquid circulation velocity and overall gas holdup in a modified jet loop bioreactor with low density particles

Effect of low density particles on the apparent liquid circulation velocity and overall gas holdup was studied in a modified reversed flow jet loop bioreactor. Experiments were conducted using polyurethane beads, polystyrene particles which are comparable to bioparticles found in biological applications and glass beads. Influence of gas and liquid flow rates, draft tube to reactor diameter ratio and solids loading on these hydrodynamic properties were studied. The liquid circulation velocity was found to increase with an increase in liquid flow rate but decrease with an increase in gas flow rate or solids loading. The overall gas holdup increased with an increase in gas or liquid flow rate but decreased with an increase in solids loading. The range of optimum draft tube to reactor diameter ratio was found to be 04–0.5. The results obtained with low density particles were comparatively better than those with glass beads. Correlations were proposed to evaluate liquid circulation velocity and overall gas holdup in terms of operational and geometrical variables.     

Manipulating the intensity of near-bed turbulence in rivers: effects on benthic invertebrates

1. Flow conditions were modified over patches of river bed in three rivers in south-western Australia to determine the effects of turbulence on benthic invertebrate communities.
2. Artificial structures to increase downstream turbulence were developed in a laboratory flume. In the field, these increased turbulence intensity by 35% for a 20% reduction in velocity.
3. Patches of gravel were placed in each river and turbulence-generating structures allocated randomly to half of these, creating treatment patches. An acoustic Doppler velocimeter was used to measure flow conditions over both treatment and control patches at several heights above the bed. After 6 weeks, the invertebrate fauna of the gravel patches were sampled to examine the response to modified flow conditions.
4. The treatments increased relative turbulence intensity twofold for a reduction in velocity of between 3 and 5 cm s⁻¹, but turbulence intensity was significantly higher in only one of the three rivers.
5. There were no significant effects of increased relative turbulence intensity on any aspect of the invertebrate assemblage. This may be a result of the fairly small increase in relative turbulence intensity created during the experiment, the spatial scale of the manipulation or the types of stream community studied.     

Saccharomyces cerevisiae fermentations in a pilot scale airlift bioreactor: Comparison of air sparger configurations

Hydrodynamic and oxygen transfer comparisons were made between two ring sparger locations, draft tube and annulus, in a concentric pilot scale airlift reactor with a baker's yeast suspension. Sectional hydrodynamic measurements were made and a mobile DOT probe was used to characterise the oxygen transfer performance through the individual sections of the reactor. The hydrodynamic performance of the reactor was improved by using a draft tube ring sparger rather than the annulus ring sparger. This was due to the influence of the ratio of the cross sectional area of the downcomer and riser (A _D/A_R) in conjunction with the effect of liquid velocity and a parameter,C ₀, describing the distribution of the liquid velocity and gas holdup across the riser on the bubble coalescence rates. The mixing performance of the reactor was dominated by the frequency of the passage of the broth through the end sections of the reactor. An optimum liquid height above the draft tube, for liquid mixing was demonstrated, above which no further improvement in mixing occurred. The liquid velocity and degree of gas entrainment showed little dependency on top section size for both sparger configurations. Extreme dissolved oxygen heterogeneity was demonstrated around the vessel with both sparger configurations and was shown to be detrimental to the oxygen uptake rate of the baker's yeast. Dissolved oxygen tensions below 1% air saturation occurred along the length of the riser and then rose in the downcomer. The greater oxygen transfer rate in the downcomer than in the riser was caused by the combined effects of a larger slip velocity in the downcomer which enhancedk _La and gas residence time, high downcomer gas holdup, and the change in bubble size distribution between the riser and downcomer. The position of greatest oxygen transfer rate in the downcomer was shown to be affected by the reactor from the influence on downcomer liquid linear velocity. UCL is the Biotechnology and Biological Sciences Research Council sponsored Advanced Centre for Biochemical Engineering and the Council's support is greatly acknowledged.     

Flow rate dependent kinetics of urease immobilized onto diverse matrices

Gas holdup and liquid circulation velocity meassurements were made for a range of liquid viscosities in a 22 l external loop airlift column and 250 l pilot-scale concentric cylinder airlift bioreactor. The results showed that for a fixed superficial gas velocity, liquid circulation velocity decreased monotonically with increasing liquid viscosity. The gas holdup for a fixed gas flow rate showed an initial increase with liquid viscosity followed by a decrease when liquid viscosity increased beyond a critical value. These observations could not be described satisfactorily using the available models of gas holdup and liquid circulation.List of Symbols U _sg m/s Superficial gas velocity - U _sl m/s Superficial liquid velocity in the riser Greek Letters Pas Liquid viscosity - _g Gas holdup in the riser     

Liquid circulation velocity in the inverse fluidized bed airlift reactor

Results of the investigations of the liquid circulation velocity in the external loop airlift reactor with the fluidized bed in the downcomer are reported. Densities of solid particles are much lower than the liquid density. It is found that the curve of the liquid circulation velocity dependence on the gas flow rate consists of two different parts. A slow linear increase of the liquid circulation velocity is observed if the gas velocity is lower than the minimum fluidization velocity. An exponential course of the liquid circulation velocity curve is observed if the gas flow rate exceeds the minimum fluidization velocity. A theoretical equation for the prediction of liquid circulation velocity for gas velocities higher than the minimum fluidization velocity is presented.