High-speed Impeller Research Articles

The effect of impeller speeds on the nanobubbles flotation efficiency of ultrafine coal particles

Ultrafine particles exhibit poor flotation behavior due to the low collision efficiencies with conventional gas bubbles. Introducing nanobubbles are expected to improve the collision probability of ultra-fine particles with bubbles. However, it remains unclear whether nanobubbles can enhance flotation at high impeller speeds during flotation process as strong turbulence may scour away the nanobubbles from solid-liquid interface of particles. This study investigates the influence and the underlying mechanism of nanobubbles on flotation performance of ultrafine coal particles under varying impeller speeds. Specifically, the surface nanobubbles (SNBs) and bulk nanobubbles (BNBs) were utilized respectively, which were produced based on the temperature difference method and hydrodynamic cavitation, and characteristized by atomic force microscopy (AFM) and nanoparticle tracking analysis (NTA), respectively. The formation of ultrafine coal particle aggregates at different impeller speeds was analyzed through laser particle size analyzer (LPSA) and high-speed camera imaging. Result showed that as the impeller speed increases, the radius of the aggregates increases, and the number and radius of aggregates in SNBs/BNBs slurries is significantly larger than that in conventional slurry at varying impeller speeds from 1200 to 2800 rpm, which is consistent with the increased flotation recoveries and flotation rates. It is demonstrated that nanobubbles remain stable even at high impeller speeds, and thus promote aggregation and thereby enhance flotation performance. The findings of this study provide theoretical support and valuable insights for ultrafine particle separation technologies.

Optimization of bromocresol green degradation using ozone micro bubbles: response surface analysis and techno-commercial aspects of a 75 kL/day scale-up plant

Degradation of bromocresol green in alcohol-water solution using ozone micron sized bubbles is reported. A high speed impeller is used to generate micro bubbles of ozone in liquid while ozone is generated using a commercial ozone generator. A 3-level-3-factor Box–Behnken experimental design is used to statistically identify relative importance of the operating parameters studied namely impeller speed, initial concentration of dye and treatment time. Ozone dosage is kept fixed at 4 g/h. Impeller speed is found to be the most dominant factor according to Analysis of Variance (ANOVA) calculations. Kinetics for degradation of bromocresol green in solution is also reported. A second order kinetics is seen to fit the experimental (concentration–time) data, corresponding value of rate constant being 0.0153 L/mg-s. Based on the laboratory scale data a detailed techno-commercial analysis of a scaled up (75 kL/day) plant for ozone based degradation of bromocresol is presented. All relevant economic indicators pertaining to the scaled up plant are reported. A specific treatment cost of Rs. 100/m3 of treated water yields a return on investment (ROI) of 68.77 and discounted payback period of 5 years.

Visualization investigation of the motion of rags in a double blades pump

Accumulation of fibrous materials often leads to pump clogging in wastewater systems. Visualizing these clogging phenomena helps identify ways to reduce the risk. In this study, we analyze a double blades pump made of acrylic glass using a fast digital camera to record the motion of rags inside. Recordings at three rotational speeds investigate the impact of rag materials, sizes, and quantities on their passage through the pump. Results show that smaller rags pass through the pump more quickly. At the same rotational speed and with the same material, the maximum passage time increases by over 100% compared to the shortest passage time. Additionally, rags flow more rapidly at high impeller speeds. At low speeds, rags tend to linger inside the impeller or stick to the pump casing’s tongue. The passage time at high speed is about 10% less than at lower speeds. Furthermore, under the same mass concentration, larger rags with fewer quantities are more prone to clogging than smaller rags with greater quantities. The proportion of larger rags with fewer quantities smoothly leaving the pump is about 10% less than that of smaller rags with greater quantities.

Improved mixing performance of gas-liquid stirred tanks by using V-shaped punched baffles

BackgroundIt has been shown that the V-shaped punched baffle can enhance the fluid mixing in a single-phase stirred tank in our previous work, while its application in gas-liquid systems has not been investigated. MethodsExperiments on macro-mixing and computational fluid dynamics (CFD) simulation were carried out in an air-water system to compare the performance of standard baffle (SB), one-section V-shaped punched baffle (VSPB-1), two-section V-shaped punched baffle (VSPB-2) and three-section V-shaped punched baffle (VSPB-3). Significant findingsThe simulation results indicate that the V-shaped punched baffle can not only generate impinging jet streams, but also increase the homogeneity of gas dispersion and the turbulence in the baffle region, resulting in a better mixing performance. The experimental results demonstrate that the use of V-shaped baffles, especially VSPB-2 and VSPB-3, can effectively enhance fluid mixing at high impeller speeds. Moreover, VSPB-2 has the best effect with Rushton turbine (RT), and all V-shaped punched baffles have better effect with up-pumping pitched turbine (PTU) at different gas flow rates. With increasing impeller speed, the trend of decreasing mixing time for the four baffles is the same for RT and PTU. However, with increasing gas flow rate, the performance of the baffles is different.

Direct Tableting on a Continuous Manufacturing Line—Impact of Mixing Parameters, Material Densities, and Drug Load on Subsequent Process Parameters and Tablet Quality

The continuous manufacturing (CM) of solid oral dosage forms has received increased attention in recent years and has become a leading technology in the pharmaceutical industry. A model has been developed based on process data from two design of experiments (DoEs), where the impact of the mixer process parameters, throughput (THR), hold up mass (HUM), impeller speed (IMP), and the input raw material bulk density (BDi), on the continuous process and the resulting drug product has been investigated. These statistical models revealed equations, describing process parameter interactions for optimization purposes. For the exit valve opening width (EV) at the bottom of the continuous mixer (CMT), the combination of high throughput (30 kg/h) and low impeller speed (300 rpm) resulted in optimal process conditions. Apparent bulk density of the blend (BD) within the process, fill depth (FD), and tensile strength (TS) were mainly impacted by input bulk density (BDi) of the tableting mixture, emphasizing the role of material attributes on the continuous manufacturing process. The apparent bulk density itself was, other than from the input bulk density, equally dependent from THR and IMP in opposite deflections. However, process parameters (THR and IMP) revealed a minor impact on the apparent BD compared to the input bulk density. FD was impacted mainly by THR ahead of IMP and the TS by IMP and THR to a similar extend, in opposite deflections. A simplified linear model to estimate the input bulk density revealed satisfactory prediction quality when included in the derived statistical model equations.

Local volumetric mass transfer coefficients in sections of multiple-impeller stirred tank reactors: Data analysis

The analysis of local values of volumetric mass transfer coefficient, kLa, was performed to identify the effects of local hydrodynamics on the mass transfer rate in multiple-impeller stirred tank reactors. The database of kLa values measured by Dynamic Pressure Method was analysed. The database provides data for two vessel sizes (laboratory and pilot-scale reactor), a wide range of operational conditions (gas flow rate, impeller speed), various impeller geometries and sizes and four different batches including viscous media. The local kLa values in multiple impeller configurations were measured in several positions corresponding to the individual impeller regions. The overall kLa value averaged through the whole volume is considered as the best representation of the correct kLa value. Different hydrodynamic effects on mass transfer rate were obtained in the two end regions. In the bottom region, the local kLa values increase with impeller speed faster than the overall kLa. The opposite behaviour was obtained in the uppermost region corresponding to the decrease of power consumption and the mass transfer rate with increasing surface aeration under high impeller speeds. The best representation of the overall kLa value was obtained in the middle position of the vessel, where the exchange flows bring the liquid saturated to different measure from the end regions. Even if the local kLa data in individual impellers' regions are shifted to the average values due to the inter-impeller exchange flows, the differences between them are still significant. We show the local kLa values by the combination of the data measured at different liquid level heights. The results can be used in the fermenter design with various impeller combinations.

Gas Hold-Up in Vessel with Dual Impellers and Different Baffles

The influence of impellers system, baffles system and type of liquid on gas hold-up in a vessel has been presented in this paper. The analysis of gas hold-up was conducted on the basis of the data obtained in the vessel. The vessel used in the study was of inner diameter D = 0.288 m, and it was filled with liquid up to a height of H = 0.576 m. The vessel used in the study was equipped in four planar standard baffles or 24 vertical tubular baffles located on the circuit. A high-speed impellers system, consisting of two impellers located on the shaft, was used to agitate the liquid. The six gas–liquid systems were tested. The gas used in the study was air. The liquids were distilled water, aqueous solutions of NaCl (concentration c = 0.4 kmol/m3 or 0.8 kmol/m3), aqueous solution of sucrose (concentration c = 2.5% mass., 5% mass.), 5% mass. aqueous solution of sucrose and yeast suspension concentration ys = 1% mass. The obtained set of over 1600 experimental points allowed to derive the equations describing the effect of gas flow number Kg, Weber number We and parameter Y (for air–water and air–aqueous solution of NaCl) and Kg, We, c and ys (for air–water, air–aqueous solution of sucrose and air–yeast suspension–aqueous solution of sucrose) on gas hold-up. These equations do not have equivalents in the literature.

Investigation of granular dynamics in a continuous blender using the GPU-enhanced discrete element method

Continuous powder blending is an essential operation during continuous pharmaceutical manufacturing. However, the complex granular dynamics in the blender is still poorly understood. This study employs a graphic processor unit (GPU) enhanced discrete element method (DEM) to analyse the granular dynamics in a continuous blender. Numerical results indicate that only a small fraction of powder distributes in the upper region of the blender, while most of that distributes in the middle and lower regions. Besides, a higher impeller speed leads to a smaller hold-up mass and a shorter mean residence time. Interestingly, the maximum number of blade passes is achieved at an intermediate impeller speed. There are two distinct regimes during continuous blending: i) a shearing regime at low impeller speeds; and ii) a dynamic regime at high impeller speeds. This study demonstrates that the GPU-enhanced DEM can be a robust tool for analysing powder flow during continuous pharmaceutical manufacturing. • GPU-enhanced DEM analysis on the full-scale continuous blending is performed. • Impeller speed has a significant impact on the powder flow in the inclined blender. • At steady-state, powder mainly distributes in the middle and bottom zones of blender. • Maximum number of blade passes can be achieved at an intermediate impeller speed.

A Continuous Conical-Mill Operation for Dry Coating of Pharmaceutical Powders: The Role of Processing Time

Over the last decade, the conical mill has emerged as a potential piece of equipment to use for continuous dry coating pharmaceutical powders. In this work, silicon dioxide was used as a guest particle on two excipients, fast flow lactose (FFL) and grade PH200 microcrystalline cellulose (MCC), for dry coating by a conical mill with a modified screen that permitted batch and continuous mode operation. Samples were pre-processed in a V-blender. SEM images, particle size distribution, and EDS mapping were used to characterise the treated powders. Pre-processed samples showed some discrete coating of the host particle. After batch processing, the samples were covered with a complete coating. When processed at high impeller speed, coating of FFL was a mix of A200P and FFL fines generated by attrition. Continuous mode processed samples, which had a lower processing time, were coated discretely and showed a better coating than the pre-processed samples. Increasing guest/host mass ratio with FFL host particle had a positive impact on the quality of the coating. These results help to build the case that the processing time of the conical mill is a key parameter to the success of the conical mill as dry coating equipment in the pharmaceutical industry.

DEM study of mixing behaviours of cohesive particles in a U-shaped ribbon mixer

Cohesive particles handling is frequently encountered in particle-mixing processes, especially in food industries, but the mixing mechanism of cohesive particles in large-scale mixers is not well understood. In this work, the influence of different liquid properties (i.e., water and honey) on the mixing performance of cohesive particles in an industrial-scale U-shaped ribbon mixer is explored using discrete element simulation (DEM). The results show that the cohesive force performs a significant influence on the particle behaviours during the mixing process. The wet mixture presents a faster mixing rate and higher final mixing degree than the dry mixture at low impeller speeds. With the increase of impeller speeds, the mixing rates for both wet and dry mixtures increase but the overall final mixing degrees decrease. Meanwhile, the influence of cohesive force becomes more insignificant at higher impeller speeds. For industrial mixing processes, the addition of a certain liquid helps improve the final mixing quality. For both dry and wet mixtures, a low impeller speed (50– 100 rpm) is recommended to realise a high final mixing quality, but a high impeller speed can increase the processing capacity per unit time. A suitable impeller speed should be selected in practical industrial mixing processes with a promise between the product quality and handling capacity. This study sheds light on the detailed mixing behaviour of the U-shaped ribbon mixer for cohesive particle mixing and provides an operation guide for the practical food mixing process.

Enhanced Separation of Oil and Solids in Oily Sludge by Froth Flotation at Normal Temperature

Oily sludge (OS) contains a large number of hazardous materials, and froth flotation can achieve oil recovery and non-hazardous disposal of OS simultaneously. The influence of flotation parameters on OS treatment and the flotation mechanism were studied. OS samples were taken from Shengli Oilfield in May 2017 (OSS) and May 2020 (OST), respectively. Results showed that Na2SiO3 was the suitable flotation reagent treating OSS and OST, which could reduce the viscosity between oil and solids. Increasing flotation time, impeller speed and the ratio of liquid to OS could enhance the pulp shear effect, facilitate the formation of bubble and reduce pulp viscosity, respectively. Under the optimized parameters, the oil content of OST residue could be reduced to 1.2%, and that of OSS could be reduced to 0.6% because of OSS with low heavy oil components and wide solid particle size distribution. Orthogonal experimental results showed that the impeller speed was the most significant factor of all parameters for OSS and OST, and it could produce shear force to decrease the intensity of C-H bonds and destabilize the OS. The oil content of residue could be reduced effectively in the temperature range of 24–45 °C under the action of high impeller speed.

Study of Novel Punched-Bionic Impellers for High Efficiency and Homogeneity in PCM Mixing and Other Solid-Liquid Stirs

Improvement of stirring performance is one of the primary objectives in solid–liquid mixing processes, such as the preparation of phase change materials (PCMs) for energy saving in refrigeration and heat pump systems. In this paper, three novel impellers are proposed: pitched-blade punched turbine (PBPT), bionic cut blade turbine (BCBT) and bionic cut punched blade turbine (BCPBT). An experimental test was conducted to validate the stirring system model based on the Eulerian–Eulerian method with the kinetic theory of granular flow. Then the performance of the novel impellers was predicted, studied, and compared. The outcomes indicate that a novel impeller, specifically BCPBT, can effectively suspend particles and dramatically reduce power consumption. A better solid–liquid suspension quality was obtained with an aperture diameter of 8 mm and aperture ratio of 13%. Within the range of impeller speeds and liquid viscosity studied in this this paper, higher impeller speeds and more viscous liquids are more conducive to particle dispersion. One of the most important contributions of this work lies in the design of novel impellers, an extent of energy conservation to 17% and efficient mixing was achieved. These results have reference significance for improving the energy efficiency of temperature regulation systems.

Solventless-mixing layering using a high shear mixer for preparing drug pellets: A feasibility study using acetaminophen

Our aim was to investigate the feasibility of mechanical-mixing layering using a high shear mixer, which can produce drug pellets by simply mixing drug crystals and inactive seed particles without the need for solvents or binders. Acetaminophen crystals and microcrystalline cellulose spheres were mechanically mixed using various impeller speeds and the resulting composite particles were characterized. Acetaminophen particles were separated from the spheres using a low impeller speed and deposited on the surface of the spheres at a higher impeller speed. The diameter of the acetaminophen crystals in the composite particles decreased as the impeller speed increased, due to increased collision impact between the spheres. The correlation between drug content and drug particle diameter in the composites indicates that acetaminophen particles were layered on the cellulose spheres due to their pulverization during the mixing treatment. We examined additional mixing treatments to enhance drug loading: after mechanically mixing acetaminophen crystals and cellulose spheres, fresh acetaminophen crystals were added and mechanically mixed with the composite particles. Additional mixing increased the loading of acetaminophen particles without lowering the layering efficiency. In conclusion, mechanical-mixing layering can be accomplished using a high shear mixer.

Characterization of mixing performance in bioreactors using flow-following sensor devices

A quantitative description of flow and mixing in bioreactors is necessary to assess the exposure of the microorganisms to suboptimal conditions, which may negatively impact key performance parameters of a process, such as yield and productivity. The traditional approach of quantifying mixing, by means of a terminal mixing time, is inadequate because it rests on the assumption that a homogeneous state is reached, and it provides little information on the mixing process itself. In this study, it has been demonstrated how flow-following sensor devices can be used to obtain detailed knowledge on macroscopic flow in stirred bioreactors under turbulent flow conditions. The flow generated by common impeller types; the Rushton disc turbine and the pitched blade turbine, has been examined in terms of the axial flow features and circulation times under various levels of agitation. It was demonstrated that useful features to characterize the flow in the vessel, such as the axial distribution and axial velocity profile of the sensor devices could be obtained for the examined conditions. Based on these features it was observed that the flow following behavior of the sensor devices were negatively affected at higher impeller speeds. This finding was corroborated by CFD simulations of the vessel. The mean circulation times determined by the sensor devices were found to be highly correlated to the mixing times determined from homogenization of a chemical tracer (tm=2.2–2.6 t¯c), which demonstrates that flow-following sensor devices can be used to quantify macromixing in bioreactors. Furthermore, the underlying circulation time distributions could be derived, which were found to be well described by the lognormal probability distribution.

Investigation of Slip Models for High-Speed Centrifugal Compressors

Slip in centrifugal compressors arises from imperfect guidance of the flow by the impeller blades and reduces the work input delivered by the impeller. Slip models are used to predict slip in preliminary design. However, slip models are typically calibrated with data that are not representative of modern aerospace compressors (i.e., pumps, turbochargers, or industrial compressors) and do not account for the variation of slip factor with operating condition. This Paper investigates the efficacy of slip models to predict the slip factor and work input for modern, high-speed impellers at design and off-design conditions. Three slip models are used to predict the performance of four well-known impellers in the open literature. All three slip models generally overpredict the slip factor, and the largest error in the prediction of slip factor typically occurs at maximum operating speed (design speed). In addition, the analysis shows a close correlation between slip factor and two key design parameters of machine Mach number and loading coefficient over the entire compressor operating range. Finally, error propagation analysis shows that the error in impeller work input is proportional to the error in slip factor scaled by the square of machine Mach number and reveals the inherent challenge in accurate prediction of work input for high-speed machines.

Granulation of teawaste and limestone using sodium-based lignosulfonate and DEM simulation of powder mixing

Co-granulation of teawaste with limestone was investigated using sodium-based lignosulfonate as a binder. A quality by design (QbD) approach was employed to formulate models for predicting the granule mean size, granule strength, limestone and binder uniformity from the process, and formulation variables. The results showed that teawaste volume fraction and liquid to solid ratio had a greater effect than other variables on granule mean size. Increasing teawaste in a binary mixture had a negative effect on granule mean size but positive effect on granule tensile strength. Limestone and binder uniformity could be increased by using higher impeller speed. DEM simulation showed that the impeller speed had a significant effect on the mixing quality and light particle fraction could significantly affect the fluctuation of the mixing index at the equilibrium stage. X-ray micro-computer tomography was applied as a technique to see the internal structure of the granules produced.

A novel gas inducing rotor-stator impeller for gas-liquid foam generation

A novel gas inducing rotor-stator impeller was used to produce foam from viscous non-Newtonian shear thinning liquid and non-ionic surfactant Tween 80. Foam produced under various experimental conditions, was characterised based on the bubble size and foam half-life. Higher impeller speed and solution viscosity, resulted in smaller size bubbles and more stable foam. Higher gas hold-up is associated with reduction in bubble size and, longer mixing times resulted in increase in bubble size because of coalescence. Empirical models were developed to predict the foam bubble size and its half-life and a good correlation between the measured data and the predicted data was obtained (R2 > 0.94). Foam bubble size was found to be a function of Reynolds number, mixing time number and Froude number. Foam half-life was found to be dependent on the same dimensionless number along with the Weber number. It is shown that, gas inducing impeller, can produce foam with similar properties at ambient conditions and with better process control, compared to foam produced by conventional agitation-based devices.

An investigation into the impact of key process variables on the uniformity of powder blends containing a low-dose drug in a gentle-wing high shear mixer

Manufacture of low-dose tablet products has always the uniformity challenge. The objective of this work was to investigate the influence of key process variables of a gentle-wing high shear mixer on the uniformity of 1.0% w/w albuterol sulphate/microcrystalline cellulose blend. Albuterol sulphate and excipients were mixed at various impeller and chopper speeds from 0.5 to 30 min. Triplicate samples were taken from nine different positions in the mixer bowl and the albuterol content was analyzed spectrophotometrically. Long mixing time (15 min) was necessary to achieve proper blend uniformity at low speeds of impeller and chopper. Otherwise, when the high chopper speed was applied, demixing occurred after 8 min with low impeller speed and after 6 min with high impeller speed. Furthermore, ANOVA analysis indicated the significant effect (P ≤ 0.05) of impeller speed and mixing time on the uniformity of the powder blends. Finally, dry mixing of low-dose based formulations in a gentle-wing high shear mixer; improving the dispersion of drug particles and formation of stable interactive mixture upon suitable selection of process variables. This study make the process variables of gentle-wing mixer a better candidate for further investigation and optimization using quality by design approach.

Unstable Flow Structures Present at Different Rotational Velocities of the Centrifugal Compressor

Unstable flow structures cause inevitable energy losses in all power energy systems, including turbomachines. In this study, a set of analyses was conducted with the use of spectral maps on the pressure signals obtained from an industrial centrifugal compressor. The spectral maps provide one a detailed visualization of the flow conditions present in the machine along the performance curve and to distinguish the flow phenomena present prior to the surge. The method accuracy is especially useful in detecting the inlet recirculation. The study was conducted at four impeller rotational speeds with varying loads imposed by a valve at the outlet. At each speed, the machine experienced different stages of unstable flow conditions prior to the surge. Five main frequency peaks that appeared in all cases were identified and discussed. The surge was observed at all impeller speeds. At lower ones, however, it appeared at higher valve closures. At higher speeds, the surge was much more intense. The study has also shown that the inlet recirculation appears also for the closed-type industrial impeller. The phenomenon was present in all conditions. The higher impeller speed, the faster onset of the inlet recirculation was. This structure has a strong potential for an early instability warning because it appears in various types of impellers, has a very particular spectral structure and its positioning is very predictable. This study gives another example of the inlet recirculation universality and potential for efficient anti-surge protection.

Investigation of gas–liquid dispersion and mass transfer performance of wide-viscosity-range impellers in water solutions of xanthan gum

Wide-viscosity-range impellers have extensive demands and applications in process industry. The gas–liquid dispersion and mass transfer characteristics of wide-viscosity-range impellers including Large-double-blade (LDB) impeller, Fullzone (FZ) impeller and Maxblend (MB) impeller in water solutions of xanthan gum were investigated experimentally and compared. The influences of gas flow rate, impeller speed and polymer concentration of liquid on the power consumption, overall gas holdup εg and mass transfer coefficient KLa were also analyzed. On this basis, the appropriate operating parameters and impeller type were determined. The results indicate that with rising flow rate, the higher εg and KLa can be achieved with a drop in power consumption, and a relatively high flow rate is recommended on the premise of guaranteeing the complete dispersal condition in aerated vessel. Higher impeller speed provides better gas–liquid dispersion and mass transfer performance, but results in more power consumption simultaneously. The appropriate impeller speed should be just enough to meet the requirements of εg and KLa. It also is found that the increasing concentration of water solution of xanthan gum adds to the complexity of gas dispersion and mass transfer in aerated vessel. Under the same specific power consumption PV, the εg and KLa are positive and negative correlation with the polymer concentration of liquid, respectively. Specially, when the concentration of water solution of xanthan gum is relatively low, the FZ impeller exhibits the best gas dispersion and mass transfer performance under the same specific power consumption. Nevertheless, the mass transfer performance of FZ impeller deteriorates significant when the concentration of water solution of xanthan gum increases to 1.00 wt%, and the MB impeller becomes the most appropriate impeller type in this condition.