Varying Impeller Speed Research Articles

Heat transfer coefficient evaluated for a designed jacketed vessel for parameter optimization in coal water slurry preparation using dual impellers

ABSTRACT This study examines the heat transfer coefficient during coal water slurry preparation in a jacketed vessel, focusing on key parameters influencing heat transfer. A dual impeller system is utilized to enhance mixing and heat transmission capacity. Using the Ant Colony Optimization (ACO) method, the equipment is designed, comprising a cylindrical flat-bottom tank with an impeller diameter of 0.079 m, a height of 0.3 m, and four uniformly positioned detachable flat baffles around the vessel’s perimeter. Experiments are conducted using the L27 orthogonal array at three different levels, varying impeller speed, clearance height to tank diameter ratio, slurry height to tank diameter ratio, and distance between the two impellers. Two categories of dual impeller experiments, propeller and Rushton, are examined. Statistical analysis indicates that impeller speed and Z/T ratio significantly influence heat transfer coefficient, while C/T ratio and distance between impellers have a lesser impact. In both propeller and Rushton experiments, impeller speed and Z/T ratio have the most substantial effect on heat transfer coefficient, contributing 62.907% and 59.47%, respectively, in propeller and Rushton experiments. These findings underscore the importance of impeller speed and Z/T ratio in enhancing heat transfer coefficient during coal water slurry preparation in a jacketed vessel.

Use of acoustic emission in combination with machine learning: monitoring of gas\u2013liquid mixing in stirred tanks

Operations involving gas–liquid agitated vessels are common in the biochemical and chemical industry; ensuring good contact between the two phases is essential to process performance. In this work, a methodology to compute acoustic emission data, recorded using a piezoelectric sensor, to evaluate the gas–liquid mixing regime within gas–liquid and gas–solid–liquid mixtures was developed. The system was a 3L stirred tank equipped with a Rushton Turbine and a ring sparger. Whilst moving up through the vessel, gas bubbles collapse, break or coalesce generating sound waves transmitted through the wall to the acoustic transmitter. The system was operated in different flow regimes (non-gassed condition, loaded and complete dispersion) achieved by varying impeller speed and gas flow rate, with the objective to feed machine learning algorithms with the acoustic spectrum to univocally identify the different conditions. The developed method allowed to successfully recognise the operating regime with an accuracy higher than 90% both in absence and presence of suspended particles.

Hydrodynamic characterization within a spinner flask and a rotary wall vessel for stem cell culture

Stirred suspension culture is becoming a popular method for expanding human pluripotent stem cells (hPSCs). While stirring generates adequate fluid motions to lift the cells and facilitates mass transfers (of nutrients, dissolved gases, and metabolic wastes), excessive stirring could impose hydrodynamic forces deleterious for the growth of the cells. In this study, computational fluid dynamics (CFD) simulations were performed to first investigate hydrodynamic characteristics of fluid flows in a spinner flask, a common stirred suspension culture vessel used in laboratories. Flow patterns and distributions of shear stresses and the Kolmogorov length scales at varying impeller speeds were obtained. Comparison of the Kolmogorov length scales and sizes of hPSC aggregates, measured in the authors’ previous experimental study, showed a strong correlation between the two. In addition to the spinner flask which generated complex and transient turbulent flows, this study investigated a newly developed rotary wall vessel that had been designed to produce laminar, circular Couette flows in order to control shear stress. CFD simulations revealed significantly more uniform and homogeneous flows compared to those in the spinner flask, suggesting that the rotary wall vessel is a suitable culture vessel to investigate roles of shear stress on hPSCs in suspension.

Development of an advanced imaging technique for dynamic emulsion stability

Oil-water emulsions consist of droplets that are approximately spherical in shape and with a distribution in size. An image processing technique that can be used for studying emulsion droplet stability behaviour in multiphase flow has been developed. In this work, the developed technique is used to study a model system, comprising Nexbase (with and without SPAN 80 surfactants) oil and saline water emulsion in a stirred tank system. A state of the art stirred tank experimental set up was constructed that measured dynamic droplet size evolution with varying impeller speed and torque. The equilibrium droplet size measurement and torque data were used to validate with theoretical inertial (Hinze model) and viscous subrange models. Besides, this method was successfully used to obtain droplet size relaxation coefficients with varying impeller speeds.

Feasibility of bench-scale selective bioflotation of copper oxide minerals using Rhodococcus opacus

Bench-scale selective bioflotation tests were carried out in a 1L Denver cell using Rhodococcus opacus (R. opacus) cells as a collector in malachite–silica binary mixture and copper (Cu) oxide ore systems. Through the first set of tests carried out on the binary mixture, cell concentration and malachite fraction in the feed were optimized, and further the role of shear force by varying impeller speed (800, 1000, and 1200rpm) and adhesive force by varying pH (7 and 11) and ionic strength (1, 100, and 300mM) were examined. Overall, high malachite selectivity was observed under optimized conditions (both recovery and grade>90%). In addition, the recovery and grade of malachite were very sensitive to pH changes, with greater values being obtained at pH7, whereas the impeller speed and the ionic strength did not seem to play a big role over the range tested. The trend was in qualitative agreement with classical Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energy profiles, which show the relative magnitudes of the adhesive forces between the mineral and the attached cell under equal hydrodynamic conditions. The second set of tests was conducted using Cu oxide ores with two different particle sizes (74–150μm and −74μm) under the optimized conditions determined from the first set of tests. The recovery and grade of malachite were found to be much greater for finer ores (recovery=29.15% and grade=5.17%) than those for coarser ones (recovery=1.34% and grade=3.06%), which is likely due to differences in malachite liberation. Further comparison of the finer ore result with that obtained from the conventional process (i.e., sulfidization followed by xanthate adsorption) indicated that the malachite selectivity of bioflotation is much higher than that of the conventional process (grade=1.43%).

The effect of cell hydrodynamics on flotation performance

It is clear that along with gas dispersion characteristics, the energy dissipated by the impeller is process determining in flotation, and its effect on flotation kinetics has been widely studied. However, turbulent conditions inside a flotation cell have usually only been changed by varying impeller speed or air flowrate or both. Therefore, there is a need to investigate not only these variables but also how changes in impeller/stator mechanism size and design, cell aspect ratio and cell design affect turbulence. This should lead to a better understanding of the effect of cell hydrodynamics on flotation performance. The aim of this work was to evaluate the role of flotation cell hydrodynamics on flotation performance in a fully instrumented 3m3 cell. The cell was operated at a copper concentrator in Australia with different combinations of airflow rates, impeller speeds and sizes and cell aspect ratio providing a wide range of hydrodynamic conditions. An analysis of the flotation cell performance showed that the overall copper recoveries were very similar for the conditions tested. However, by decoupling pulp effects from froth effects it was possible to determine whether the changes made affected the pulp zone and/or froth zone responses. The analysis showed that the overall recovery had the potential to be up to 10% higher if not limited by froth recovery. Comparing the metallurgical performance of the cell with the different hydrodynamic conditions it was found that the collection zone flotation rate was not directly related to overall energy dissipation, as is commonly observed at laboratory scale, where energy is usually only changed by varying impeller tip speed. Results suggest that it is the size of the turbulent zone, rather than just energy input, that affects flotation recovery.

Mixing performance comparison of milliscale continuous high‐shear mixers

Mixing performance of two continuous flow millilitre‐scale reactors (volumes 9.5 mL and 2.5 mL) equipped with rotor‐stator mixers was studied. Cumulative residence time distributions (RTD) were determined experimentally using a step response method. Distributions were measured for both reactors by varying impeller speed and feed flow rate. The mixing effect was determined by measured RTDs. Computational fluid dynamics (CFD) were used to verify that the residence time distribution in the measurement outlet agreed with the outlet flow. The mixing power of both reactors was determined using a calorimetric method. The reactor inlet flow rate was found to affect mixing performance at 1–13 s residence times but the effect of impeller speed could not be noted. Both milliscale reactors are close to an ideal continuous stirred‐tank reactor (CSTR) at the studied impeller speed and flow rate ranges. The specific interfacial area was found to depend on the reactor inlet flow rate at constant impeller speed for the case of copper solvent extraction.

The effect of process parameters on audible acoustic emissions from high-shear granulation

Product quality in high-shear granulation is easily compromised by minor changes in raw material properties or process conditions. It is desired to develop a process analytical technology (PAT) that can monitor the process in real-time and provide feedback for quality control. In this work, the application of audible acoustic emissions (AAEs) as a PAT tool was investigated. A condenser microphone was placed at the top of the air exhaust on a PMA-10 high-shear granulator to collect AAEs for a design of experiment (DOE) varying impeller speed, total binder volume and spray rate. The results showed the 10 Hz total power spectral densities (TPSDs) between 20 and 250 Hz were significantly affected by the changes in process conditions. Impeller speed and spray rate were shown to have statistically significant effects on granulation wetting, and impeller speed and total binder volume were significant in terms of process end-point. The DOE results were confirmed by a multivariate PLS model of the TPSDs. The scores plot showed separation based on impeller speed in the first component and spray rate in the second component. The findings support the use of AAEs to monitor changes in process conditions in real-time and achieve consistent product quality.

Investigation of the effect of impeller speed on granules formed using a PMA-1 high shear granulator

Impeller speed was varied from 300 to 1500 rpm during the wet high shear granulation of a placebo formulation using a new vertical shaft PharmaMATRIX-1 granulator. The resulting granules were extensively analysed for differences caused by the varying impeller speed with emphasis on flowability. Microscopy showed that initial granules were formed primarily from microcrystalline cellulose at all tested impeller speeds. At low impeller speed of 300 rpm in the “bumpy” flow regime, forces from the impeller were insufficient to incorporate all the components of the formulation into the granules and to promote granule growth to a size that significantly improved flowability. The “roping” flow regime at higher impeller speeds promoted granule growth to a median particle size of at least 100 µm that improved the flowability of the mixture. Particle size distribution measurements and advanced indicators based on avalanching behavior, however, showed that an impeller speed of 700 rpm produced the largest fraction of optimal granules with the best flowability potential. This impeller speed allowed good development of “roping” flow for sufficient mixing, collision rates and kinetic energy for collisions while minimizing excessive centrifugal forces that promote buildup around the bowl perimeter.

CFD Simulation of Gas-Liquid Flow in a Large Scale Flotation Cell

The actual performance of a new flotation cell is commonly examined through industrial tests. Since the controlled environment of industrial tests differ from laboratory tests, we cannot know the hydrodynamic features in flotation cells and these features are difficult to examine in the industrial test situations. CFD simulation of flotation cells provides a tool that can predict the hydrodynamic features and analyse the influence of variations in design features and operating conditions on the performance of flotation cells. A large scale flotation cell designed by BGRIMM has been modelled by using the Eulerian two-fluid method. Complex gas-liquid flow fields within the cells and the gas volume fraction are predicted. The surface velocity of gas, the power required at varying impeller speeds and gas flow rates have been found and compared against measured values obtained from industrial tests. The effects of some boundary conditions, such as the outlet setting and the timescale, which are important to the model, have been discussed. The effects of impeller speed and gas flow rate on the flow fields in the large scale flotation cell have been investigated using computational modelling. In general, the modelling method is suitable for the large scale flotation cell, and the model can give good information for investigations on the design and operation of flotation cells.

CFD model of a self-aerating flotation cell

The effect of impeller speed on the air flow in a self-aerated Denver laboratory flotation cell was investigated using computational modelling. Air is induced into the slurry by the impeller's rotating action. The rate of air flow is determined by the suction pressure created by the impeller, the hydrostatic head of the slurry and the frictional losses along the delivery shaft from the inlet valve to the impeller. From two-phase simulations of the flotation cell at varying impeller speeds, the predicted air flow rates have been found to compare favourably against measured values reported in the literature. The effect of increasing impeller speed is to increase the air flow rate and gas holdup in the cell. Simulations with flotation kinetics showed that the gravitational force acting on the attached particles is significant. The effect is a decrease in the bubble rise velocity which in turn affects the flotation rate as predicted by the model. The effects of the local turbulence level on the local attachment rate and bubble loading have been discussed and quantified.

Nonisothermal, uncontrolled homo- and copolymerization of ethylene using selected zirconocenes

Nonisothermal, uncontrolled polymerization, conducted in varying mixing regimes, offered a facile methodology to evaluate the influence of several important process development factors such as mixing, reaction exotherm, and thermal perturbations on the catalytic activity and kinetic stability, polymerization performance, and properties of the resulting polymers. Ethylene was homo- and copolymerized with hexene-1 under varying impeller speeds (hence, thermal perturbations), using Ind2ZrCl2 and Et(Ind)2ZrCl2 and the MAO cocatalyst. With respect to the effects of the above process development factors, the following was observed: The reaction exotherm profiles, tracing the polymerization history, qualitatively represented the kinetic profile and the catalytic stability. The unbridged Ind2ZrCl2 was shown to be more stable than the bridged Et(Ind)2ZrCl2. With change in the level of stirring from a diffusion-controlled regime to a nondiffusion-controlled, external gas–liquid mass-transfer resistance-free one, the reaction exotherm and the run time-average catalytic activity increased. So far as the influence of the chiral versus the achiral zirconocene structure is concerned, the copolymer composition distribution and soluble fraction generated by chiral Et(Ind)2ZrCl2 were more sensitive to the mixing conditions and thermal perturbations than were those produced by achiral Ind2ZrCl2. Et(Ind)2ZrCl2 produced higher molecular weight backbones, incorporated more hexene-1 and chain branching, and introduced less crystallinity in the copolymers than did Ind2ZrCl2. The influence of Ind2ZrCl2 on higher-weight homopolymer backbones was opposite to that of Et(Ind)2ZrCl2. Incorporation of hexene-1 significantly decreased the average molecular weights and density and increased the run-time-dependent average catalyst activity. A positive comonomer effect took place. The bulk polymer properties did not critically depend on the mixing state. Thermal perturbations broadened the polydispersity index. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 137–147, 1998

Influence of hydrodynamic conditions on naphthalene dissolution and subsequent biodegradation.

The influence of hydrodynamic conditions on the dissolution rate of crystalline naphthalene as a model polycyclic aromatic hydrocarbon (PAH) was studied in stirred batch reactors with varying impeller speeds. Mass transfer from naphthalene melts of different surface areas to the aqueous phase was measured and results were modeled according to the film theory. Results were generalized using dimensionless numbers (Reynolds, Schmidt, and Sherwood). In combined mass transfer and biodegradation experiments, the effect of hydrodynamic conditions on the degradation rate of naphthalene by Pseudomonas 8909N was studied. Experimental results were mathematically described using mass-transfer and microbiological models. The experiments allowed determination of mass-transfer and microbiological parameters separately in a single run. The biomass formation rate under mass transfer limited conditions, which is related to the naphthalene biodegradation rate, was correlated to the dimensionless Reynolds number, indicating increased bioavailability at increased mixing in the reactor liquid. The methodology presented in which mass transfer processes are quantified under sterile conditions followed by a biodegradation experiment can also be adapted to more complex and realistic systems, such as particulate, suspended PAH solids or soils with intrapartically sorbed contaminants when the appropriate mass-transfer equations are incorporated.

Hydrodynamic behaviour and oxygen transfer rate in a pilot plant fermenter: I. Influence of viscosity

AbstractThe effects of varying impeller speed, aeration rate and viscosity on mixing time, power consumption and oxygen transfer rate were studied in the pilot plant fermenter. The rheological behaviour of erythromycin fermentation broth was simulated by colloidal starch solutions at apparent viscosities of 0.02–0.20 Pa/s. The volumetric oxygen transfer rate coefficient was determined by the sulphite and static method. The experimental results showed that at low viscosities, up to of 0.02 Pa/s, the suitable range of impeller speed is 250–300 min−1 at aeration rate 0.6–0.8 VVM from the point of view of power input (2.6–2.8 kW/m3) and sufficient coefficient KLa (160–200 h−1). At viscosities higher than 0.02 Pa/s (with pseudoplastical character) the suitable range of impeller speed is 300–350 min−1 at the same aeration rate. Then the power input for mixing is 3.0–3.5 kW/m3 and the coefficient KLa ∼ 50 h−1.

Hydrodynamic behaviour and oxygen transfer rate in a pilot plant fermenter. I. Influence of viscosity

AbstractThe effects of varying impeller speed, aeration rate and viscosity on mixing time, power consumption and oxygen transfer rate were studied in the pilot plant fermenter. The rheological behaviour of erythromycin fermentation broth was simulated by colloidal starch solutions at apparent viscosities of 0.02–0.20 Pa/s. The volumetric oxygen transfer rate coefficient was determined by the sulphite and static method. The experimental results showed that at low viscosities, up to of 0.02 Pa/s, the suitable range of impeller speed is 250–300 min−1 at aeration rate 0.6–0.8 VVM from the point of view of power input (2.6–2.8 kW/m3) and sufficient coefficient KLa (160–200 h−1). At viscosities higher than 0.02 Pa/s (with pseudoplastical character) the suitable range of impeller speed is 300–350 min−1 at the same aeration rate. Then the power input for mixing is 3.0–3.5 kW/m3 and the coefficient KLa ∼ 50 h−1.