Large Impeller Research Articles

The effect of impeller pumping and fluid rheology on solids suspension in a stirred vessel

AbstractPumping velocities of different impellers were measured using laser Doppler velocimetry. It was found that there is a link between impeller pumping capacity and the 5 parameter in the Zwietering correlation. Measurements showed that Njs increases with impeller spacing for dual impeller configurations. A dual‐impeller configuration shows less sensitivity to solids loading change than a single‐impeller configuration, and the change in Njs is larger for small impeller spacing than for large impeller spacing. An increase in slurry cloud height and a reduction in Njs resulted at a small increase of non‐Newtonian viscosity.

Emulsification in batch-emulsion polymerization of styrene and vinyl acetate: A reaction calorimetric study

Dispersion of liquid–liquid systems is commonly applied in industrial processes such as extraction, suspension, and emulsion polymerization. This article describes the influence of the quality of emulsification on the course and outcome of a batch-emulsion polymerization of styrene and vinyl acetate. From visualization experiments and polymerizations in combination with reaction calorimetric studies, a critical impeller speed, N*, can accurately be determined for a particular reactor setup and a given recipe. The results show that styrene–water emulsions are more difficult to emulsify than vinyl acetate–water emulsions. In general, a large turbine impeller appears to be more effective in emulsifying monomer–water dispersions than a pitched-blade impeller. In addition, for vinyl acetate emulsion polymerization, the possibility of premixing the reaction mixture has been investigated. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 79: 944–957, 2001

The application of a laser scanning vibrometer for dynamic characterization of large impellers

The application of a laser scanning vibrometer to detect resonance frequencies and mode shapes of large impeller rotors is illustrated. The dynamic analysis of the structure has been previously performed by means of a standard FEM code. The key to success was the ability of the laser vibrometer to detect very low vibration amplitudes (of the order of nanometers) with a high spatial resolution, inside an industrial environment, in very low testing time; all fundamentals for this application. The experimental and numerical activity presented in this work has been carried out at Nuovo Pignone together with the University of Ancona with the final purpose of establishing a new, fast measurement technique to identify, in a production industrial environment, the dynamic behaviour of large dimension impellers for centrifugal compressors.

Solids suspension with up‐pumping pitched‐blade and high‐efficiency impellers

AbstractThe use of axial‐flow impellers in the up‐pumping mode has been increasing, but little performance and design information is available. Up‐pumping pitched‐blade and high‐efficiency impellers have been studied in solids suspension applications, and their performance has been compared to and contrasted with that of the more conventional down‐pumping mode. Just‐suspended speed data has been interpreted in terms of two literature correlations that can be used for design purposes. Just‐suspended torque and power requirements are presented, as well as turbulent power number and flow pattern data. In general, the just‐suspended torque and power requirements of up‐pumping pitched‐blade and high‐efficiency impellers are substantially higher than those of the down‐pumping mode. However, if a large impeller diameter is required to avoid critical speed limitations or to achieve sufficient power inputs at high solids loadings, then up‐pumping impellers may be a viable option.

A study of an up‐ and a down‐pumping wide blade hydrofoil impeller: Part I. LDA measurements

AbstractA wide blade, high solidity ratio, axial flow hydrofoil impeller has been developed particularly for gas dispersion when used in the upward‐pumping mode. It has a low power number so that a relatively low torque is required when using it with a large impeller to tank diameter ratio, a commonly desired aim of retrofitting. As part of the work characterising these impellers, laser Doppler anemometry has been used to study the single phase flow both down‐ and up‐pumping, the latter for the first time. Mean and root‐mean‐square velocities were measured throughout a fully baffled, flat bottomed cylindrical tank. When down‐pumping, the discharge flow was almost axial but when up‐pumping, it had a strong radial component. Turbulence energy dissipation rates in the impeller region and the discharge stream when up‐pumping were found to be higher than those when down‐pumping. In both modes, the impeller was found to have a high circulation efficiency. In Part II, the data presented here are used for obtaining computational fluid dynamic solutions of the flow field.

The influence of impeller type in pilot scale Xanthan fermentations

The rheological complexity of Xanthan fermentations presents an interesting problem from a mixing viewpoint, because the phenomena of poor bulk blending and low oxygen mass transfer rates inherent in highly viscous fermentations (and their consequences) can be systematically investigated, even at the pilot plant scale. This study in a 150 L fermentor compares the physical and biological performance of four pairs of impellers: a standard Rushton turbine, a large diameter Rushton turbine, a Prochem Maxflo T, and a Scaba 6SRGT. Accurate in-fermentor power measurements, essential for the comparison of impellers in relation to operating costs are also reported. It is demonstrated that the agitator performance in Xanthan fermentations is very specific and the choice of which impeller to use in bioreactors to obtain enhanced performance is dependant on the applied criterion. None of the criterion favored the use of the standard Rushton turbine, therefore suggesting that there are strong grounds for retrofitting these impellers with either large diameter impellers of similar design or with novel agitators. In addition, fluid dynamic modeling of cavern formation has clearly highlighted the importance of a well mixed and oxygenated region for providing the capacity for high microbial oxygen uptake rates which govern Xanthan productivity and quality. Copyright 1998 John Wiley & Sons, Inc.

The effect of flow reversal on solids suspension in agitated vessels

AbstractFlow patterns in agitated vessels are influenced by geometry, particularly impeller diameter and impeller off‐bottom clearance. Large impellers and/or high off‐bottom clearances lead to reversed flow in which the flow at the base of the vessel is radially‐inward as opposed to radially‐outward as expected with axial‐flow impellers. Reversed flow is detrimental in solids suspension agitation because inordinately high torque and power are required to achieve suspension. This work experimentally characterizes the effect of flow reversal on solids suspension performance, including guidelines for avoiding flow reversal with straight‐blade turbines, pitched‐blade turbines, and high‐efficiency impellers.

Agitation, aeration and perfusion modules for cell culture bioreactors.

For an optimized bioreactor design which is adapted to the cultivation of sensitive animal cells different modular bioreactor components for gentle agitation, sufficient aeration and long-term perfusion were developed and investigated with respect to their suitability from laboratory to production scale. Aeration systems have been designed for both shear sensitive cells and cells which tolerate bubbles. The systems are based on either membranes for bubble-free aeration or stainless steel sparger systems. They were characterized by determination of their oxygen transfer capacity and optimized in cultivation processes of different cell lines under process conditions such as batch and perfusion mode. Different impellers for suspension cells and cells grown on carriers were investigated for their suitability to ensure homogeneous gentle mixing. A large pitch blade impeller as well as a novel 3-blade segment impeller are appropriate for homogeneous mixing at low shear rates. Especially with the 3-blade segment impeller fluid mechanical stress can be reduced at a given stirrer speed which is advantageous for the cultivation of cells attached to microcarriers or extremely shear sensitive suspension cells. However, our results indicate that shear sensitivity of animal cells has been generally overestimated. Continuous perfusion of both suspension cell cultures and cells cultivated on microcarriers could be successfully performed over extended periods of time using stainless steel spinfilters with appropriate pore sizes and systems based on microporous hydrophilic membranes. Spinfilters are suitable cell retention systems for technical scale bioreactors allowing continuous perfusion cultures of suspension cells (pore size 10 to 20 microns) as well as anchorage dependent cells grown on microcarriers (pore size 75 microns) over six weeks to 3 months. Applying the developed modules for agitation, aeration and perfusion process adapted bioreactor set-ups can be realized which ensure optimum growth and product formation conditions in order to maximize cell and product yields.

Interfacial polymerization encapsulation of a viscous pigment mix: emulsification conditions and particle size distribution.

A viscous organic phase, containing up to 65 per cent solid pigment, was dispersed into water with an emulsifier by a rotor-stator homogenizer and the droplets formed were encapsulated by interfacial polymerization. Microcapsules with volume median diameters d50 ranging from 10 to 25 microns and geometric standard deviation (GSD), from 1.25 to 1.65, were obtained depending on emulsification conditions. Larger impellers gave smaller d50 and slightly narrowed GSD; d50 decreased and GSD increased as volume fraction of dispersed phase is decreased. Higher homogenizer speed and emulsifier concentration decreased d50 but slightly increased GSD. Increasing pigment content in dispersed phase decreased d50 but had little effect on GSD. These effects were assessed quantitatively by fitting an empirical model to the data.

A model of the vertical distribution of solids during slurry mixing

A theoretical description is derived for the distribution of solids that are imperfectly mixed in a tank by an impeller. The main assumptions are that all particles settle at the same constant rate, and that the mixing effect of the impeller can be characterized by a turbulent diffusivity term. The resultant mathematical model predicts the local solids concentration at any depth within the liquid under conditions of partial mixing. The model does not indicate any lateral concentration gradients. The model was tested experimentally with pig slurry in a cylindrical tank of 10·8 m 3 capacity. The mixer was a vertical axis, concentric, disc turbine impeller of diameter equal to 16% of the tank diameter, and was operated at three speeds. The model satisfactorily predicted solids distribution at the slowest speed, but not at the faster speeds. It cannot, therefore, be recommended for wider use for slurries, but the approach may merit further consideration for solid-liquid systems with a smaller range of solids settling velocities. The results indicated that the resuspension of settled solids was probably a stepwise process and that zones of different turbulence levels resulted from the shear-thinning properties of the slurry. It was therefore concluded that cheaper slurry mixing may be achieved by using two-part mixers: a small high-speed impeller to maintain particles in suspension and a large slow impeller to initially lift the particles.

33] Mammalian cell culture

A number of the proteins that are produced by mammalian cells are now possible to produce in bacteria through recombinant DNA methodology. However, this technology can be used only after the identification and subsequent purification of a specific protein and its gene. Thus, mammalian cell culture is necessary before recombinant methods can be applied. Mammalian cells can be divided in two major groups. Suspension cells are able to grow when freely suspended in the medium while anchorage-dependent cells must be attached to a surface to grow. This basic difference is also reflected in the systems that are used for their cultivation. As mammalian cells do not have a protective cell wall, they are more sensitive to shear forces than microbial cells. The mixing system is thus usually exchanged to large impellers in fermentors used for culture of suspension cells. Another type of fermentor is equipped with four slowly turning flexible sheets that span the depth of the culture fluid. The use of hollow fibers provides another way to avoid shear forces. The cells are thus injected in the space outside the fibers while medium is circulated within the fibers.

Determining velocity gradients in laboratory and full‐scale systems

Simulation of plant flocculators can be accomplished by using jar testing equipment. Similar conditions should be maintained between laboratory and full‐scale work. In this article several G curves for jar testing are presented that allow different small‐scale geometries to be used. Power curves were also developed and correlated to power numbers available in the literature for large impellers. Subsequently, a procedure is presented for determining the full‐scale G values.

The effect of impeller/tank, configurations on fluid-particle mass transfer

Integral mass transfer coefficients for dissolving monodisperse spherical sodium chloride crystals have been measured for six-blade disc turbines, two-blade flat paddles and four-blade 45°-pitch turbines in closed vessels 0.14 and 0.28 m in diameter and of Rushton dimensions. Thirtynine different geometries were employed and in all cases the coefficient obtained at the stirrer speed at which the crystals were just fully suspended was approximately constant. The power per unit mass required to achieve this coefficient was some 30 times less in some geometries than in others. The specific power consumed in just suspending the particles was always less in the larger for geometrically similar impeller/ tank configurations. In general, the larger impellers with a small clearance above the vessel base, consumed less power than other configurations, but the 45°-pitch turbine consumed least and did not follow this rule. Photographs of particle suspension and flow patterns allowed an explanation of this apparent anomaly.

Dissolution mass transfer in a turbine agitated baffled vessel

AbstractMass transfer coefficients were measured for the dissolution of crystalline electrolytes suspended in a baffled vessel by means of disc turbines. A 3‐fold range of density differences, a 5‐fold range of diffusivities and a 50‐fold range of particle size were covered and the effect of impeller speed, size and clearance from the base was studied.The increase in coefficent with speed is both a function of the clearance and solid density and size and is difficult to quantify. However, in the most economic configuration of small clearance and large impeller, this is not of great importance since the coefficient increases but slowly with speed and the minimum suspended coefficient can be calculated. This is done by means of a slip velocity theory which has been extended to include larger particles, which allows for the effect of concentration dependent physical properties and which is based on the impeller speed at which particle suspension occurs.

Aeration‐agitation studies on the novobiocin fermentation

AbstractNovobiocin fermentations in 20 l. baffled fermentors were studied as a function of the size and speed of the dual four‐bladed flat‐blade turbines used. Power input and sulphite oxidation rate measurements were made. The courses of pH, sugar utilization, mycelial dry weight, carbon dioxide evolution and antibiotic titer were determined. Optimum antibiotic yields were achieved at a power input of 0·5 h.p./100 gal, equivalent to a sulphite oxidation rate of 110 mmoles O2/l.h, when the impeller diameters were 29 per cent or 39 per cent of the tank diameter. A power input of 0·75 h.p./100 gal, equivalent to a sulphite oxidation rate of 160 mmoles O2/l.h, was required for equivalent results with the impeller diameter 49 per cent of the tank diameter. Some explanations of the lack of equivalent results with the large impeller are discussed.