Studies on transfer processes in mixing vessels: effect of particles on gas-liquid hydrodynamics using modified Rushton turbine agitators

The work presents the effect of solid particles having a mean diameter between 15–1000?μm, on the gas dispersion in a mechanically agitated vessel with standard and modified Rushton turbine agitators positioned singly or doubly on same shaft. For the dispersing and uniform distribution of the three phase (gas-liquid-solid) through the entire vessel section, the modified blade turbines, with the surface fraction of the perforations equal to 0.353, were found to be more efficient, the power consumption being reduced by approximately 50%in comparison with the standard Rushton turbines. The power number in the turbulent mixing of the three phase system is dependent on the aeration rate, the surface fraction of the perforations, the turbine number and the physical and rheological properties of the suspensions.     

Studies on transfer processes in mixing vessels: suspending of solid particles in liquid by modified Rushton turbine agitators

The modified blade turbines are attractive alternatives to the standard Rushton turbine as they do not require any modification in the electrical engine motor and drive assemblies are simple to manufacture and have a reduced power consumption.The modified blades were obtained through increase in the blade height of the Rushton turbine simultaneously with perforation of the blade surface. The field surface of the modified blade is equal to the blade surface of the standard Rushton turbine.In this study the modified blade turbine with the surface fraction of the perforations equal to 0.353 is used.The complete suspension speed and the power dissipation in transition and turbulent regimes using standard and modified Rushton turbine agitators positioned singly or doubly on same shaft, in five solid-liquid systems were investigated.The solid particles used have the mean diameter between 15–1000 m.The modified blade turbine, noted as TP3, was found to be more efficient than the standard turbine in complete and homogeneous suspension.List of Symbols A distance between turbine and the vessel bottom (m) - c dimensionless constant (-) - d agitator diameter (m) - d _p surface-to-volume mean diameter of the particle (m) - D vessel diameter (m) - (H _L )₁ suspension height for one turbine immersed (m) - (H _L )₂ suspension height for two turbines immersed (m) - K consistency index (Pa s ⁿ ) - l _k eddy-size characteristic (m) - N flow behaviour index (-) - N _p number of blades of the mixing system (-) - N agitator speed (s^–1) - N _js agitator speed that just causes complete suspension (s^–1) - Ne P_L/_LN³d⁵ power number in liquid system (-) - (Ne) _g P_g/_spN³d⁵ power number in solid-liquid system (-) - P _L power consumption in liquid system (W) - P _s power consumption in solid-liquid system (W) - r coefficient of correlation (-) - R distance between turbines (m) - Re _spNd²/ _a Reynolds number (-) - S suspension parameter in Zwietering equation (2) (-) - S _C full surface of the blade (m²) - S _G surface of the perforations applied on the blade (m²) - S _G /S _C surface fraction of the perforations (-) - X particle concentration (g/l) - w baffle width (m) - _js specific power input per mass at the complete suspension state (W/kg) - _a apparent viscosity under mixing conditions (Pa s) - _L kinematic viscosity of the liquid (m²/s) - _L density of liquid (Kg/m³) - _s density of solid (Kg/m³) - _sp density of suspension (Kg/m³)     

Enzymatic hydrolysis of sunflower oil in a standardized agitated tank reactor

This paper presents results on the kinetics of the hydrolysis of sunflower oil catalysed by the enzymes of Candida cylindracea. The influence of the agitation is focused, in a standardized reactor, with a Rushton turbine as the agitating element. It is proved that an increase of the agitation speed improves the kinetics of the reaction up to a certain value of the rotational speed where the agitation becomes turbulent. A large range of substrate concentrations is investigated and discussed compared to the usual conditions met in the literature.     

Dependence of penicillium chrysogenum growth, morphology, vacuolation, and productivity in fed-batch fermentations on impeller type and agitation intensity

The influence of the agitation conditions on the growth, morphology, vacuolation, and productivity of Penicillium chrysogenum has been examined in 6 L fed-batch fermentations. A standard Rushton turbine, a four-bladed paddle, and a six-bladed pitched blade impeller were compared. Power inputs per unit volume of liquid, P/VL, ranged from 0.35 to 7.4 kW/m3. The same fermentation protocol was used in each fermentation, including holding the dissolved oxygen concentration above 40% air saturation by gas blending. The mean projected area (for all dispersed types, including clumps) and the clump roughness were used to characterize the morphology. Consideration of clumps was vital as these were the predominant morphological form. For a given impeller, the batch-phase specific growth rates and the overall biomass concentrations increased with agitation intensity. Higher fragmentation at higher speeds was assumed to have promoted growth through increased formation of new growing tips. The mean projected area increased during the rapid growth phase followed by a sharp decrease to a relatively constant value dependent on the agitation conditions. The higher the speed, the lower the projected area for a given impeller type. The proportion by volume of hyphal vacuoles and empty regions decreased with speed, possibly due to fragmentation in the vacuolated regions. The specific penicillin production rate was generally higher with lower impeller speed for a given impeller type. The highest value of penicillin production as well as its rate was obtained using the Rushton turbine impeller at the lowest speed. At given P/VL, changes in morphology, specific growth rate, and specific penicillin production rate depended on impeller geometry. The morphological data could be correlated with either tip speed or the "energy dissipation/circulation function," but a reasonable correlation of the specific growth rate and specific production rate was only possible with the latter. Copyright 1998 John Wiley & Sons, Inc.     

Oxygen transfer efficiency in the biosynthesis of antibiotics in bioreactors with a modified RUSHTON turbine agitator

In this work, the oxygen mass transfer efficiency and power consumption in a non-biological system and an antibiotic biosynthesis process, using a modified RUSHTON turbine agitator, were investigated. It was demonstrated that a simple modification of the blades through the increase of the blade height, simultaneously with the discontinuation of the blade surface, could improve the oxygen transfer efficiency by about 30%. Experiments performed in stirred tank bioreactors with an overall volume of 20 m³, equipped with the modified RUSHTON turbine agitator, showed that the power consumption diminished by a factor of 1.18 to 1.6 during the fermentation processes of Streptomyces erithreus, Streptomyces griseus, Streptomyces noursei, and Nocardia mediaterranei, compared to the witness bioreactor. The use of the modified RUSHTON turbine for the antibiotic biosynthesis process may contribute to the decrease of the overall costs and the obtainment of better productivity, allowing an intensive utilization of power inputs for aeration and agitation.     

Mixing state of xanthan gum solution in an aerated and agitated fermentor—Effects of impeller size on volumes of mixing regions and circulation time distribution

The effects of the impeller diameter and width on the volumes of the micromixing and macromixing regions, and on the circulation time distribution were investigated at various agitation speeds to formulate the relationships of them in emperical equations. A fermentor was a 10-l capacity, which was equipped with a turbine impeller with six flat balades and aerated at 1 vvm. It was found that the volumes of the micromixing and macromixing regions depended on the tip speed of the impeller, ND, and the discharging performance of the impeller, ND²W, respectively, in the xabthan gum solution with concentrations of 0.9, 1.8, 2.7, and 3.9%. Empirical equations were derived to estimate the volume of each mixing region from the impeller diameter, D, impeller width, W, agitation speed, N, and consistency coefficient of the xanthan gum solution. On the other hand, the circulation time distribution could be estimated empirically from only the impeller diameter and agitation speed, regardless of variation in the impeller width and consistency coefficient of the xanthan gum solution tested.     

A special reactor design for investigations of mixing time effects in a scaled‐down industrial L‐lysine fed‐batch fermentation process

A specially designed model reactor based on a 42‐L laboratory fermentor was equipped with six stirrers (Rushton turbines) and five cylindrical disks. In this model reactor, the mixing time, Θ₉₀, turned out to be 13 times longer compared with the 42‐L standard laboratory fermentor fitted with two Rushton turbines and four wall‐fixed longitudinal baffles. To prove the suitability of the model reactor for scaledown studies of mixing‐time‐dependent processes, parallel exponential fed‐batch cultivations were carried out with the leucine‐auxotrophic strain, Corynebacterium glutamicum DSM 5715, serving as a microbial test system. L‐ Leucine, the process‐limiting substrate, was fed onto the liquid surface of both reactors. Cultivations were conducted using the same inoculum material and equal oxygen supply. The model reactor showed reduced sugar consumption (−14%), reduced ammonium consumption (−19%), and reduced biomass formation (−7%), which resulted in a decrease in L ‐lysine formation (−12%). These findings were reflected in less specific enzyme activity, which was determined for citrate synthase (CS), phosphoenolpyruvate carboxylase (PEP‐C), and aspartate kinase (AK). The reduced specific activity of CS correlated with lower CO₂ evolution (−36%) during cultivation. The model reactor represents a valuable tool to simulate the conditions of poor mixing and inhomogeneous substrate distribution in bioreactors of industrial scale. © 1999 John Wiley & Sons, Inc. Biotechnol Bioeng 64: 599–606, 1999.     

Polysaccharide production: Experimental comparison of the performance of four mixing devices

This investigation was undertaken with the objective to compare experimentally the performance of four different mixing devices for the production of the polysaccharide pullulan with Aureobasidium pullulans (2552). Fermentations were performed using identical bioreactors with respectively an assembly of three Rushton turbines (RTB), a helical ribbon impeller (HR) and two different reciprocating plates (RPB1, RPB2). Each mixing vessel had identical geometry and working volume (18 L). These four fermentations were performed with an equal level of power input per unit volume (1000?W/m³) and gas flow rate (0.5 vvm, 9 L/min). For each system, the evolution of biomass, polysaccharide concentration, dissolved oxygen and agitation speed or frequency were recorded as a function of time along with the rheological properties of the culture broths. The type of mixing device used had a significant impact on the rate of biomass production and on polysaccharide physical properties. However, the rate of polysaccharide production appears to be less sensitive to the bioreactor design. The overall productivity, which represents the ability of micro-organisms to convert rapidly substrate into biomass and polysaccharide, was maximised using the RPB2 system. The quality of the synthesised polysaccharide, in terms of viscosity producing power, was highest using the HR system but the overall productivity was very low. Thus, the best compromise between productivity and product quality was achieved with the RPB2.     

Studies on transfer processes in mixing vessels: power consumption of the modified Rushton turbine agitators in three-phase systems

In this work, the modified blade turbines, with the surface fraction of the perforations equal to 0.353, are used in the study of the relationship between the power consumption in three-phase systems and the superficial gas velocity, mixing intensity, agitator type, solid particles concentration, physical and rheological properties of the suspensions. The power consumption in three-phase mixing with modified turbines, single and double, is approximately 40% smaller than for the standard turbines at the same mixing intensity. The modified blade turbines are simple to manufacture and do not require any modification in electrical motor and drive assemblies.     

Performance of industrial scale bioreactors with modified RUSHTON turbine agitators

The data presented here with respect to the behaviour of industrial scale stirred tank bioreactors equipped with modified RUSHTON turbine agitators in the biosynthesis processes of antibiotics are valid for that case that the power consumption is the same as it is in standard RUSHTON turbine agitators. Each modified RUSHTON turbine agitator was obtained through the variation of the blade surface by adding perforations so that the ratio between the perforation surface area and the full surface area (or the surface fraction of the perforations) is 0.36. In the fermentations of Streptomyces aureofaciens, Streptomyces rimosus and Penicillium chrysogenum producing tetracycline, oxytetracyline and penicillin, respectively, in bioreactors equipped with modified RUSHTON turbine agitators, the relative antibiotic production is increased by more than 30% compared to standard bioreactors.     

kLa of stirred tank bioreactors revisited

By means of improved feedback control k_La measurements become possible at a precision and reproducibility that now allow a closer look at the influences of power input and aeration rate on the oxygen mass transfer. These measurements are performed online during running fermentations without a notable impact on the biochemical conversion processes. A closer inspection of the mass transfer during cultivations showed that at least the number of impellers influences mass transfer and mixing: On the laboratory scale, two hollow blade impellers clearly showed a larger k_La than the usually employed three impeller versions when operated at the same agitation power and aeration rate. Hollow blade impellers are preferable under most operational conditions because of their perfect gas handling capacity. Mixing time studies showed that these two impeller systems are also preferable with respect to mixing. Furthermore the widths of the baffle bars depict a significant influence on the k_La. All this clearly supports the fact that it is not only the integral power density that finally determines k_La.     

An oxygen supply strategy for the large-scale production of tissue plasminogen activator by microcarrier cell culture

An oxygen supply strategy involving agitation speed and aeration method for the large-scale production of tissue plasminogen activator (TPA) by a microcarrier cell culture was investigated by small-scale model experiments. A preliminary calculation indicated that diffusion limitation of dissolved oxygen (DO) could be caused in a microcarrier sedimentation layer more than 0.5 mm in thickness. Within an agitation speed range above 70 rpm, which was the critical speed for all of the microcarrier beads to remain suspended and thus for avoiding a deficiency of DO, the TPA productivity was higher at a lower agitation speed, while the cell concentration was not affected by the agitation speed. The addition of soluble starch to the culture medium prevented sedimentation of the microcarrier beads, even at the low agitation speed of 20 rpm, resulting in a TPA productivity higher than that at 70 rpm, which was the optimum speed without soluble starch. Use of an air spray system with an optimized air flow rate resulted in a k_La 2.35 times higher than that with simple surface aeration. Increasing the internal pressure of the culture from 0.2 kg/cm² (1209 hPa) to 1.5 kg/cm² (2483 hPa) had no effect on the cell growth but slightly increased the TPA production rates. However, based on the glucose consumption, both the cell and TPA yields were much improved by pressurization. As an optimum mixing and oxygen supply strategy for the production of TPA on a large scale, it is recommended that soluble starch be added to the culture medium to allow the microcarrier suspension to be maintained at a low agitation speed, while keeping a high oxygen transfer rate by means of an air spray system and pressurization.     

Modeling of gas–liquid mass transfer in a stirred tank bioreactor agitated by a Rushton turbine or a new pitched blade impeller

A combined computational fluid dynamics (CFD) and population balance model (PBM) approach has been applied to simulate hydrodynamics and mass transfer in a 0.18 m³ gas–liquid stirred bioreactor agitated by (1) a Rushton turbine, and (2) a new pitched blade geometry with rotating cartridges. The operating conditions chosen were motivated by typical settings used for culturing mammalian cells. The effects of turbulence, rotating flow, bubbles breakage and coalescence were simulated using the k–ε, multiple reference frame (MRF), Sliding mesh (SM) and PBM approaches, respectively. Considering the new pitched blade geometry with rotating aeration microspargers, $k_{\text{L}} a$ mass transfer was estimated to be 34 times higher than the conventional Rushton turbine set-up. Notably, the impeller power consumption was modeled to be about 50 % lower. Independent $k_{\text{L}} a$ measurements applying the same operational conditions confirmed this finding. Motivated by these simulated and experimental results, the new aeration and stirring device is qualified as a very promising tool especially useful for cell culture applications which are characterized by the challenging problem of achieving relatively high mass transfer conditions while inserting only low stirrer energy.     

Scale-up synthesis of lipase-catalyzed palm esters in stirred-tank reactor

Lipase-catalyzed production of palm esters by alcoholysis of palm oil with oleyl alcohol in n-hexane was performed in 2L stirred-tank reactor (STR). Investigation on the performance of reactor operation was carried out in batch mode STR with single impeller mounted on the centrally located shaft. Rushton turbine (RT) impellers provide the highest reaction yield (95.8%) at lower agitation speed as compared to AL-hydrofoil (AL-H) and 2-bladed elephant ear (EE) impellers. Homogenous enzyme particles suspension was obtained at 250 rpm by using RT impeller. At higher impeller speed, the shear effect on the enzyme particles caused by agitation has decreased the reaction performance. Palm esters reaction mixture in STR follows Newtons' law due to the linear relation between the shear stress (tau) and shear rate (dupsilon/dy). High stability of Lipozyme RM IM was observed as shown by its ability to be repeatedly used to give high percentage yield (79%) of palm esters even after 15 cycles of reaction. The process was successfully scale-up to 75 L STR (50 L working volume) based on a constant impeller tip speed approach, which gave the yield of 97.2% after 5h reaction time.     

Agitation,aeration and shear stress as key factors in inulinase production by Kluyveromyces marxianus

Factorial design and response surface analyses were used to optimize the production of inulinase (2,1-β-d-fructan fructanohydrolase, EC 3.2.1.7) by Kluyveromyces marxianus ATCC 16045, using sucrose as carbon source. Effects of aeration, agitation and type of impeller (disk turbine, marine, pitched blade) were studied in a batch stirred reactor. Two factorial designs 2² were carried out. Agitation speed varied from 50 to 550 rpm (revolution per minute), aeration rate from 0.5 to 2.0 vvm (air volume/broth volume·minute). It has been shown that the enzyme production was strongly influenced by mixing conditions, while aeration rate was shown to be less significant. Additionally, the increase in the agitation speed is limited by the death rate, which increases drastically at high speeds, lowering the enzyme production. Also, the impeller type has significant influence in the production, the disk impeller at 450 rpm and aeration at 1.0 vvm led to an activity of 121 UI/mL, while the pitched blade was shown to be the best impeller for this process, leading to the best production, 176 UI/mL, at 450 rpm and 1.0 vvm. The maximum shear stress for inulinase production was about 0.22 Pa, since higher values cause higher cell death rates, affecting the enzyme production. The same results were confirmed with another microorganism, which was also sensible to shear stress. Therefore, it has been concluded that in some cases, mainly when the microorganism is sensible to shear stress, the interaction between mass transfer and mechanical stress should be considered in scale up processes.     

Enhanced riboflavin production by recombinant Bacillus subtilis RF1 through the optimization of agitation speed

Dissolved oxygen is one of the most important bioprocess parameters that could affect cell growth and product formation, and it is easy to control by changing agitation speed. In this work, the effects of agitation speed on the performance of riboflavin production by recombinant Bacillus subtilis RF1 was investigated in fed-batch fermentation. The lower agitation speed (600 rpm) was beneficial for cell growth and riboflavin biosynthesis in the initial phase of fermentation process. While, during the later phase, higher agitation speed (900 rpm) was favor for cell growth and riboflavin biosynthesis. Thus, a two-stage agitation speed control strategy was proposed based on kinetic analysis, in which the agitation speed was controlled at 600 rpm in the first 26 h and then switched to 900 rpm to maintain high μ for cell growth and high q _p for riboflavin production during the entire fermentation process. However, it was observed that a sharp increase of agitation speed resulted in an adverse effect on cell growth and riboflavin synthesis within a short time. To avoid this phenomenon, a multi-stage agitation speed control strategy was set up based on the two-stage control strategy, the maximum concentration of riboflavin reached 9.4 g l^?1 in 48 h with the yield of 0.051 g g^?1 by applying this strategy, which were 20.5 and 21.4 % over the best results controlled by constant agitation speeds.     

Influence of bioreactor design on the rate and amount of curdlan-type exopolysaccharide production byAlcaligenes faecalis

Summary The amount of polymer recovered during lab-scale batch production of curdlan-type polysaccharide byAlcaligenes faecalis (ATCC 31749) was increased by 46% through the manipulation of the vessel configuration. When standard turbine impellers were used to provide mixing and agitation the specific rate of production, Qp, decreased significantly after 40–50 hours elapsed fermentation time, EFT. The Qp remained at a high level throughout the entire time course of production (90 hours) when (i) a propeller was substituted for a flat-blade turbine impeller in a conventional baffled stirred tank reactor, or (ii) agitation and mixing were accomplished in a non-rotary vibro-fermenter.     

Influence of Reynolds Number on Multi-Objective Aerodynamic Design of a Wind Turbine Blade

At present, the radius of wind turbine rotors ranges from several meters to one hundred meters, or even more, which extends Reynolds number of the airfoil profile from the order of 10⁵ to 10⁷. Taking the blade for 3MW wind turbines as an example, the influence of Reynolds number on the aerodynamic design of a wind turbine blade is studied. To make the study more general, two kinds of multi-objective optimization are involved: one is based on the maximum power coefficient (C _Popt) and the ultimate load, and the other is based on the ultimate load and the annual energy production (AEP). It is found that under the same configuration, the optimal design has a larger C _Popt or AEP (C _Popt//AEP) for the same ultimate load, or a smaller load for the same C _Popt//AEP at higher Reynolds number. At a certain tip-speed ratio or ultimate load, the blade operating at higher Reynolds number should have a larger chord length and twist angle for the maximum C _popt//AEP. If a wind turbine blade is designed by using an airfoil database with a mismatched Reynolds number from the actual one, both the load and C _popt//AEP will be incorrectly estimated to some extent. In some cases, the assessment error attributed to Reynolds number is quite significant, which may bring unexpected risks to the earnings and safety of a wind power project.     

Effect of the aeration rate and agitation speed on β-carotene production and morphology of Blakeslea trispora in a stirred tank reactor: mathematical modeling

The effect of aeration rate and agitation speed on β-carotene production and morphology of Blakeslea trispora in a stirred tank reactor was investigated. B. trispora formed hyphae, zygophores and zygospores during the fermentation. The zygospores were the morphological form responsible for β-carotene production. Both aeration and agitation significantly affected β-carotene concentration, productivity, biomass and the volumetric mass transfer coefficient (K_La). The highest β-carotene concentration (1.5 kg m⁻³) and the highest productivity (0.08 kg m⁻³ per day) were obtained at low impeller speed (150 rpm) and high aeration rate (1.5 vvm). Also, maximum productivity (0.08 kg m⁻³ per day) and biomass dry weight (26.4 kg m⁻³) were achieved at high agitation speed (500 rpm) and moderate aeration rate (1.0 vvm). Conversely, the highest value of K_La (0.33 s⁻¹) was observed at high agitation speed (500 rpm) and high aeration rate (1.5 vvm). The experiments were arranged according to a central composite statistical design. Response surface methodology was used to describe the effect of impeller speed and aeration rate on the most important fermentation parameters. In all cases, the fit of the model was found to be good. All fermentation parameters (except biomass concentration) were strongly affected by the interactions among the operation variables. β-Carotene concentration and productivity were significantly influenced by the aeration, agitation, and by the positive or negative quadratic effect of the aeration rate. Biomass concentration was principally related to the aeration rate, agitation speed, and the positive or negative quadratic effect of the impeller speed and aeration rate, respectively. Finally, the volumetric mass transfer coefficient was characterized by the significant effect of the agitation speed, while the aeration rate had a small effect on K_La.     

Dependence of mycelial morphology on impeller type and agitation intensity

The influence of the agitation conditions on the morphology of Penicillium chrysogenum (freely dispersed and aggregated forms) was examined using radial (Rushton turbines and paddles), axial (pitched blades, propeller, and Prochem Maxflow T), and counterflow impellers (Intermig). Culture broth was taken from a continuous fermentation at steady state and was agitated for 30 min in an ungassed vessel of 1.4-L working volume. The power inputs per unit volume of liquid in the tank, P/V(L), ranged from 0.6 to 6 kW/m(3). Image analysis was used to measure mycelial morphology. To characterize the intensity of the damage caused by different impellers, the mean total hyphal length (freely dispersed form) and the mean projected area (all dispersed types, i.e., also including aggregates) were used. [In this study, breakage of aggregates was taken into account quantitatively for the first time.]At 1.4-L scale and a given P/V(L), changes in the morphology depended significantly on the impeller geometry. However, the morphological data (obtained with different geometries and various P/V(L)) could be correlated on the basis of equal tip speed and two other, less simple, mixing parameters. One is based on the specific energy dissipation rate in the impeller region, which is simply related to P/V(L) and particular impeller geometrical parameters. The other which is developed in this study is based on a combination of the specific energy dissipation rate in the impeller swept volume and the frequency of mycelial circulation through that volume. For convenience, the function arising from this concept is called the "energy dissipation/circulation" function.To test the broader validity of these correlations, scale-up experiments were carried out in mixing tanks of 1.4, 20, and 180 L using a Rushton turbine and broth from a fed-batch fermentation. The energy dissipation/circulation function was a reasonable correlating parameter for hyphal damage over this range of scales, whereas tip speed, P/V(L), and specific energy dissipation rate in the impeller region were poor. Two forms of the energy dissipation/circulation function were considered, one of which additionally allowed for the numbers of vortices behind the blades of each impeller type. Although both forms were successful at correlating the data for the standard impeller designs considered here, there was preliminary evidence that allowing for the vortices would be valuable. (c) 1996 John Wiley & Sons, Inc.