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Alessandro Paglianti - One of the best experts on this subject based on the ideXlab platform.
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spectral and wavelet analysis of the flow pattern transition with impeller Clearance variations in a stirred vessel
Chemical Engineering Science, 2003Co-Authors: Chiara Galletti, Elisabetta Brunazzi, Alessandro PagliantiAbstract:Abstract The double- to single-loop pattern transition in stirred vessels stemming from a change in the off-Bottom Clearance of a Rushton turbine has been investigated by laser doppler anemometry. Time-resolved data showed the transition occurring within a range of Clearance values and allowed the distinction of three types of flow: the double-loop regime, the single-loop regime and an unstable one termed “transitional state”. Experiments of up to 3– 4 h duration showed that both the onset and the lifetimes of these types of flow were random; however, in the transitional state, the flow varied between the two circulation patterns in a periodic manner, with a frequency linearly related to the impeller rotational speed. The results have important implications for mixing process and vessel design as well as CFD predictions of the flows which are discussed.
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Analysis of immiscible liquid-liquid mixing in stirred tanks by Electrical Resistance Tomography
Chemical Engineering Science, 2026Co-Authors: Francesco Maluta, Giuseppina Montante, Alessandro PagliantiAbstract:Abstract In this work, the dispersion of diesel fuel in water in a baffled tank stirred by a Rushton turbine is investigated. The impeller speeds corresponding to the initial drawdown, the just dispersed regime and the completely dispersed regime are identified by suitable variables obtained by means of the Electrical Resistance Tomography technique. The effect of the dispersed phase volume fraction and of the impeller off-Bottom Clearance on the different dispersion regimes is considered. The impeller Froude number, the Archimedes number and a dimensionless distance between the liquid-liquid interface position and the impeller elevation are adopted for the data interpretation. For the investigated system, the local volume fraction profiles show that the completely dispersed condition is practically achieved above the just dispersed impeller speed, with negligible gradients of the local volume fraction distribution in the axial direction.
Suzanne M Kresta - One of the best experts on this subject based on the ideXlab platform.
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impact of tank geometry on the maximum turbulence energy dissipation rate for impellers
Aiche Journal, 1996Co-Authors: Genwe Zhou, Suzanne M KrestaAbstract:The maximum turbulence energy dissipation rate per unit mass, emax, is an important variable in dispersion systems, particularly for drop breakup and coalescence, and for gas dispersion. The effect of tank geometry (number of baffles, impeller diameter, and off-Bottom Clearance) on emax for four impellers (the Rushton turbine, RT; the pitched blade turbine, PBT; the fluidfoil turbine, A310; and the high-efficiency turbine, HE3) is examined. Mean and fluctuating velocity profiles close to the impellers were measured in a cylindrical baffled tank using laser doppler velocimetry. Local and maximum turbulence energy dissipation rates in the impeller region were estimated using e = Av3/L with A = 1 and L = D/10 for all four impellers. Factorial designs were used to test for the effects of single geometric variables under widely varying conditions and interactions between variables. Several factorial designs were used to ensure that real effects were separated from effects that appeared as an artifact of the experimental design. Results show that the tank geometry has a significant effect on emax, primarily with respect to variations in impeller diameter and interactions between the off-Bottom Clearance and impeller diameter. For the same power input and tank geometry, the RT consistently produces the largest emax and/or emax scaled with N3D2.
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the effects of impeller and tank geometry of circulatory time distributions in stirred tanks
Chemical Engineering Research & Design, 1995Co-Authors: R M Roberts, M R Gray, Suzanne M KrestaAbstract:The Lagrangian experience of a fluid particle in a stirred tank is one of the factors affecting process results ranging from drop size distributions to cell growth kinetics. To examine the Lagrangian experience of a fluid particle with respect to the impeller plane, the mean circulation time (MCT) and the circulation time distribution (CTD) in a stirred tank were studied using the magnetic flow follower method. Two-level factorial designs were used to examine two impellers, a pitched-blade turbine (PBT), and a Rushton turbine (RT). Four geometric variables were varied for each impeller: the baffle width, the number of baffles, the off-Bottom Clearance of the impeller, and the impeller diameter. All four variables, and the interaction between the number of baffles and the off-Bottom Clearance, had a statistically significant effect (5%) on the MCT for the RT. The significant factors for the PBT were the off-Bottom Clearance, the impeller diameter and the interaction between the Clearance and the diameter (30%). The CTDs were mostly bi-modal. Changes in the shape of the distribution were observed for different geometric configurations. We show that the Lagrangian experience of a fluid particle depends on the type of impeller used, and on the tank geometry
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the mean flow field produced by a 45 pitched blade turbine changes in the circulation pattern due to off Bottom Clearance
Canadian Journal of Chemical Engineering, 1993Co-Authors: Suzanne M Kresta, P E WoodAbstract:On a analyse le courant de decharge et l'ecoulement principal produits par une turbine a aubes inclinee dans un reacteur agite a chicanes, en combinant des techniques de visualisation de l'ecoulement et de mesure anemometrique laser Doppler. Deux profils de circulation globale distincts ont ete observes, l'un pour des degagements dans le fond du reservoir importants, l'autre pour des petits degagements aux parois. On montre que les profils de circulation ont un effet marque sur le courant de decharge, meme a proximite des aubes de la turbine. Ces decouvertes indiquent que l'action de la turbine n'est pas independante de la geometrie du reservoir et qu'elle subit une influence non negligeable de l'ecoulement global. Cette conclusion a des implications importantes pour la modelisation et la prediction des champs d'ecoulement dans les reservoirs agites
Nenad Kuzmanic - One of the best experts on this subject based on the ideXlab platform.
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effect of impeller off Bottom Clearance on crystal growth kinetics of borax in dual impeller batch cooling crystallizer
Chemical engineering transactions, 2017Co-Authors: Antonija Kacunic, Marija Cosic, Davor Rusic, Nenad KuzmanicAbstract:The aim of this work was to experimentally investigate the influence of impeller off-Bottom Clearance on crystal growth kinetics of disodium tetraborate decahydrate (borax) in a dual-impeller batch cooling crystallizer. Examinations were carried out in a crystallizer of 15 dm3 with liquid height to crystallizer diameter ratio (H/dT) of 1.3. In this research, two pitched blade turbines (PBT) with diameter D/dT=0.33, were mounted on the same shaft with a constant spacing between impellers (s/D=1.0). Mixing was performed at the impeller speed which ensured the state of complete suspension in the system (N=NJS). The impeller off-Bottom Clearance (C/D) was varied from 0.2 to 1.3. Crystallization was carried out by a controlled cooling of the solution saturated at 30°C. Concentration of the solution was measured in line by potentiometric method, while the changes of crystal size over time were determined by optical microscope and image analysis software. Experimental results indicate that impeller off-Bottom Clearance influences the change of crystal size over process time. But, regardless of the impeller off-Bottom Clearance, crystal growth is integration limited. The value of an order of crystal growth rate changes insignificantly with C/D ratio, while the crystal growth rate constant is affected by the change in hydrodynamics. Obtained results are a consequence of the hydrodynamic conditions in the dual-impeller crystallizer. In order to analyze the overall fluid flow pattern, in which an interaction of the flows generated by each of the impellers occurred, photographs of the flows were taken and simulations by VisiMix 2000 Turbulent were made. Taking into account the values of kinetic parameters of crystal growth, characteristic of obtained crystals, power consumption and the overall fluid flow pattern, it is possible to define the impact of impeller off-Bottom Clearance on borax crystal growth kinetics.
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influence of impeller off Bottom Clearance on mechanism and crystal growth kinetics of borax in pbt sbt dual impeller crystallizer
25. hrvatski skup kemičara i kemijskih inženjera - Knjiga sažetaka, 2017Co-Authors: Antonija Celan, Marija Cosic, Davor Rusic, Nenad KuzmanicAbstract:In the last few decades, an interest in batch crystallization had increased significantly due to the numerous advantages of this separation method. Even though the number of scientific papers dedicated to the subject is continuously increasing, only a few of them deal with the impact of mixing on the process of crystallization. Additionally, the number of those researches in which mixing is performed by dual or even a multi-impeller system is almost negligible. Since mixing ensures favorable conditions for mass and energy transfer, an impact of this operation on every aspect of a crystallization process should be investigated. For this reason, the aim of this work was to find how the off-Bottom Clearance influences mechanism and crystal growth kinetics. Batch crystallization of disodium tetraborate (borax) by controlled cooling was conducted in a crystallizer of 15 dm 3 , which was equipped with four baffles. Saturated solution was prepared at 30°C and was cooled at the rate of 6 °C/h. Since liquid height to vessel diameter ratio (H/d T ) equaled 1.3, mixing was performed by using two impellers of a standard diameter (D/d T =0.33). Radial SBT impeller was mounted above an axial PBT impeller on a single shaft (PBT-SBT dual impeller system). Impeller spacing (S/D) was kept constant and equal to S/D=1 while the off-Bottom Clearance was varied in the range of 0.2 to 1.3. Impeller speed, at which crystallization was conducted, ensured the state of a complete suspension (N/N JS =1). Solution concentration was monitored continuously by Na-ion selective electrode. Linear crystal size over process time was determined by microscopic analysis of the slurry samples with Motic Images Advanced 3.2 software. In order to explain obtained results, it was also necessary to analyze the fluid flow generated by impellers. For this purpose, the overall fluid flow pattern was photographed according to the method developed by S. Ibrahim and A. W. Nienow. Results of this investigation indicate that the change of off-Bottom Clearance reflects on the values of linear crystal size over process time and consequently on the crystal growth kinetics. It was found that the overall crystal growth coefficient, k g , increases with an increase of impeller off-Bottom Clearance. A deviation from this trend is only present when the flow generated by the lower impeller is transformed due to the vicinity of the crystallizer Bottom. An analysis of numerical values of the order of overall crystal growth process, g, did not imply a regular dependence of this parameter on the off-Bottom Clearance. To determine the crystal growth mechanism, a model proposed by Mersmann was used. By this model, in all tested conditions, crystal growth was integration limited.
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suspension of floating solids with up pumping pitched blade impellers mixing time and power characteristics
Chemical Engineering Journal, 2001Co-Authors: Nenad Kuzmanic, Branimir LjubicicAbstract:Abstract Floating solids suspension in water has been studied using up-pumping pitched blade turbines (PTU). Studies were performed to determine optimum geometry and hydrodynamic characteristics of agitated systems for floating solids suspension. The impact of floating solids concentrations and particle size on the mixing time, tm, the just completely suspended impeller speeds, NJS, and the power consumption were examined. The effects of impeller diameter and its off-Bottom Clearance as well as the impeller blade angle on mentioned parameters were also observed. The mixing time of suspended system was measured by a conductivity technique using the sodium chloride solution as tracer, whereas, power consumption was measured by the torque table. The efficiency of PTU impellers was compared to that of other, more conventional impellers. Obtained results point to the fact that the PTU may be a viable option if the floating slurry process requires the state of complete solids suspension and its longer time maintenance.
Christian Larroche - One of the best experts on this subject based on the ideXlab platform.
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mixing and liquid to gas mass transfer under digester operating conditions
Chemical Engineering Science, 2017Co-Authors: Zaineb Trad, Christophe Vial, J P Fontaine, Christian LarrocheAbstract:Abstract This work deals with the analysis and the optimization of a dual-impeller design in terms of mixing, hydrodynamics, mass transfer properties and power input in a mechanically stirred digester devoted to biohydrogen production through acidogenic fermentation of lignocellulosic waste. Various mixer designs involving Rushton turbines, an Elephant Ear impeller and a marine propeller, were compared. Experimental data were successfully confronted to CFD-based simulations used to reveal the respective roles of impeller type, geometry and Clearance. The results showed that the flow pattern was strongly influenced by the off-Bottom and inter-impeller Clearances, and by the size and type of the lower impeller. Straw suspension was enhanced by a small disk turbine with a low off-Bottom Clearance and a large inter-impeller Clearance that promoted an axial flow circulation together with a small mixing time due to the interaction with the larger turbine used as the upper impeller. Conversely, kLa evolution was weakly dependent on impeller design, position, and rotation speed until a deep vortex formed on the free surface, showing that power input was too weak to enhance liquid-to-gas mass transfer. Finally, the design including an Elephant Ear turbine as the upper impeller and a smaller Rushton turbine as the lower impeller was selected as the best compromise between distributive and dispersive mixing, while the objective of a power input lower than 10 W/m3 was achieved.
Juntang Yu - One of the best experts on this subject based on the ideXlab platform.
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effects of geometrical design on hydrodynamic and mass transfer characteristics of a rectangular column airlift bioreactor
Biochemical Engineering Journal, 2007Co-Authors: Peter M Kilonzo, Argyrios Margaritis, M A Bergougnou, Juntang Yu, Qin YeAbstract:Abstract Gas holdup and gas–liquid mass transfer coefficients were measured in a 21-L rectangular-column airlift bioreactor with aspect ratio of 10 and working volumes ranging from 10 to 16 L. The effect of the Bottom and top Clearances was investigated using water and mineralized CMC solutions and covering a range of effective viscosity from 0.02 to 0.5 Pa s and surface tension from 0.065 to 0.085 N m −1 . The gas holdup and mass transfer results were successfully correlated using expressions derived via dimensional analysis. The separator gas holdup was found to be similar to the total gas holdup in the airlift bioreactor. The downcomer gas holdup ( ɛ d ) increased two-fold when the Bottom Clearance ( h b ) was increased from 0.014 to 0.094 m while the top Clearance ( h t ) had no effect. Increasing h b decreased the mass transfer by 50% compared to 31% when the top Clearance ( h t / D hr ) was increased. It was found that the gas–liquid separator diameter ratio ( D hs / D hc ) exerted the maximal influence of over 65% on mass transfer as compared to both Clearances.
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influence of the baffle Clearance design on hydrodynamics of a two riser rectangular airlift reactor with inverse internal loop and expanded gas liquid separator
Chemical Engineering Journal, 2006Co-Authors: Peter M Kilonzo, Argyrios Margaritis, M A Bergougnou, Juntang YuAbstract:Abstract The influence of baffle Clearance design on the liquid circulating velocity, gas holdup and pressure drop in a two riser rectangular airlift reactor with inverse internal loop and expanded gas–liquid separator was investigated using water and mineralized CMC solutions covering a range of effective viscosity from 0.02 to 0.5 Pa s and surface tension from 0.065 to 0.085 N/m. The gas holdup results in the riser, downcomer, and gas–liquid separator were satisfactorily derived using expressions obtained via dimensional analysis. The separator gas holdup was found to be similar to the total gas holdup in the airlift reactor. The baffle Clearances were found to influence the liquid circulation velocity to some degree, with the Bottom Clearance being the significant design parameter. An attempt was also made to correlate the liquid velocity using empirical equations of the loss coefficient in the baffle top and Bottom zones. The calculated and observed liquid circulation velocity agreed well with an error of ±29% for the air–water system.