Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer... more Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled vessels. However they may be expected to provide significant advantages in a wide range of applications (e.g. crystallization, food and pharmaceutical processes, etc) where the presence of baffles is often undesirable. In the present work solid-liquid suspension in an unbaffled stirred tank is investigated. The tank was equipped with a top-cover in order to avoid vortex formation. A novel experimental method (the "steady cone radius method", SCRM) is proposed to determine experimentally the minimum impeller speed at which solids are completely suspended. Experimental N js and power consumption data are provided over fairly wide ranges for particle size, density and concentration. Dependence of N js on particle density and concentration is similar to that observed in baffled tank. Conversely, a negligible dependence of particle diameter on N js is observed in the unbaffled tank, a difference from baffled vessels with important practical implications.
The Canadian Journal of Chemical Engineering, 1998
ABSTRACT The research on mass transfer coefficients in solid-liquid agitated systems has received... more ABSTRACT The research on mass transfer coefficients in solid-liquid agitated systems has received substantial attention in the past, due both to the interest in fundamental aspects of mass transfer between particles and turbulent fluids and to the importance of practical applications. In contrast, little information is available on solid-liquid mass transfer when a third gaseous phase is also dispersed into the system, in spite of the importance of the applications of gas-solid-liquid mass transfer coefficient in gas-solid-liquid vessels stirred by either radial or axial impellers. The mechanical power dissipated by the stirrers at various agitation speeds and gas flow rates was also measured by means of a new technique. The mass transfer data obtained were found to be well correlated to the 0.25 power of the specific power dissipation, indicating that the Kolmogorov's theory of mass transfer applies to these systems, while no clear influence of the gas hold-up was ascertained.
An accurate estimation of particle settling velocities, and/or of particle drag coefficients, is ... more An accurate estimation of particle settling velocities, and/or of particle drag coefficients, is required for modelling purposes in many industrially important multiphase processes involving the suspension of millimetre and sub-millimetre size particles in a liquid phase. It is known that the settling velocity of particles in a turbulent fluid may be significantly different from that in the still fluid, depending
The three-dimensional turbulent flow field in unbaffled tanks stirred by radial impellers was num... more The three-dimensional turbulent flow field in unbaffled tanks stirred by radial impellers was numerically simulated by a finite-volume method on body-fitted, co-located grids. Simulations were run, with no recourse to empirical input, in the rotating reference frame of the impeller. Free-surface problems were also simulated, in which the profile of the central vortex was computed as part of the solution
Potentially dangerous events involving heavy gas dispersion and their severe consequences have be... more Potentially dangerous events involving heavy gas dispersion and their severe consequences have been largely publicized by the media. Simplified models have been widely applied to describe the effects of these accidents. However, most simplified models deal with flat terrain scenarios and are based on quite crude simplifications of the complex phenomenology involved.
Suspensions of solid particles into liquids within industrial stirred tanks are frequently carrie... more Suspensions of solid particles into liquids within industrial stirred tanks are frequently carried out at an impeller speed lower than the minimum required for complete suspension conditions. This choice allows power savings which usually overcome the drawback of a smaller particle-liquid interfacial area. Despite this attractive economical perspective, only limited attention has been paid so far to the modelling of the partial suspension regime.
Industrial tanks devoted to the mixing of solid particles into liquids are often operated at an i... more Industrial tanks devoted to the mixing of solid particles into liquids are often operated at an impeller speed N less than N js (defined as the lowest speed allowing the suspension of all particles): under such conditions the distribution of solid-particles is very far from being homogeneous and very significant concentration gradients exist. The present work is devoted to assessing the capability of Computational Fluid Dynamics (CFD) in predicting the particle distribution throughout the tank.
In this paper experimental information on the retention time distribution (RTD) of solid particle... more In this paper experimental information on the retention time distribution (RTD) of solid particles in a high aspect-ratio vessel, stirred by three equally spaced Rushton turbines, is presented. The relevant data were obtained by a special technique named Twin System Approach (TSA), that greatly simplifies the handling of particle laden streams and is therefore particularly suited for investigating particle RTD in flow systems. The technique fundamentals are first summarised, together with the data analysis procedure. This last requires a numerical deconvolution operation that is easily performed with the help of Z-transforms. Two different approaches for excluding the spurious contributions of the external piping required for the experimentation are tested and discussed.
The ow and turbulence ÿelds in a fully ba ed vessel stirred by a Rushton turbine have been simula... more The ow and turbulence ÿelds in a fully ba ed vessel stirred by a Rushton turbine have been simulated by means of computational uid dynamics (CFD) techniques. The simulation techniques adopted (the "Sliding-grid" and "Inner-outer" methods) are fully predictive and require no experimental data as boundary or initial conditions. The e ect of clearance (C) on the ow patterns in stirred vessels was simulated as the ow ÿeld transition observed can be considered a particularly stringent benchmark for the testing of CFD techniques. The results are compared with previously obtained LDA data and show that the double-to single-loop transition experimentally observed when impeller clearance from the vessel bottom is suitably reduced, can be well reproduced by the CFD simulations. The single-loop ow structure present for C=T = 0:15 is compared in detail with the experimental data and good overall agreement is shown between the experiment and simulation. The periodic component of the kinetic energy is well predicted, but the random component is underestimated, a ÿnding also observed in earlier investigations. The mean ow in most of the vessel is also well predicted but the angle to the horizontal of the impeller discharge ow is overestimated. Predictions are also reported with grid reÿnements and di erent turbulence models, in an e ort to identify means of improving agreement with the measurements. The ÿndings indicate that further improvements in turbulence modelling might be necessary. ? : S 0 0 0 9 -2 5 0 9 ( 0 1 ) 0 0 0 8 9 -6
Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer... more Unbaffled stirred tanks are seldom employed in the process industry as they are considered poorer mixers than baffled vessels. However they may be expected to provide significant advantages in a wide range of applications (e.g. crystallization, food and pharmaceutical processes, etc) where the presence of baffles is often undesirable. In the present work solid-liquid suspension in an unbaffled stirred tank is investigated. The tank was equipped with a top-cover in order to avoid vortex formation. A novel experimental method (the "steady cone radius method", SCRM) is proposed to determine experimentally the minimum impeller speed at which solids are completely suspended. Experimental N js and power consumption data are provided over fairly wide ranges for particle size, density and concentration. Dependence of N js on particle density and concentration is similar to that observed in baffled tank. Conversely, a negligible dependence of particle diameter on N js is observed in the unbaffled tank, a difference from baffled vessels with important practical implications.
The Canadian Journal of Chemical Engineering, 1998
ABSTRACT The research on mass transfer coefficients in solid-liquid agitated systems has received... more ABSTRACT The research on mass transfer coefficients in solid-liquid agitated systems has received substantial attention in the past, due both to the interest in fundamental aspects of mass transfer between particles and turbulent fluids and to the importance of practical applications. In contrast, little information is available on solid-liquid mass transfer when a third gaseous phase is also dispersed into the system, in spite of the importance of the applications of gas-solid-liquid mass transfer coefficient in gas-solid-liquid vessels stirred by either radial or axial impellers. The mechanical power dissipated by the stirrers at various agitation speeds and gas flow rates was also measured by means of a new technique. The mass transfer data obtained were found to be well correlated to the 0.25 power of the specific power dissipation, indicating that the Kolmogorov's theory of mass transfer applies to these systems, while no clear influence of the gas hold-up was ascertained.
An accurate estimation of particle settling velocities, and/or of particle drag coefficients, is ... more An accurate estimation of particle settling velocities, and/or of particle drag coefficients, is required for modelling purposes in many industrially important multiphase processes involving the suspension of millimetre and sub-millimetre size particles in a liquid phase. It is known that the settling velocity of particles in a turbulent fluid may be significantly different from that in the still fluid, depending
The three-dimensional turbulent flow field in unbaffled tanks stirred by radial impellers was num... more The three-dimensional turbulent flow field in unbaffled tanks stirred by radial impellers was numerically simulated by a finite-volume method on body-fitted, co-located grids. Simulations were run, with no recourse to empirical input, in the rotating reference frame of the impeller. Free-surface problems were also simulated, in which the profile of the central vortex was computed as part of the solution
Potentially dangerous events involving heavy gas dispersion and their severe consequences have be... more Potentially dangerous events involving heavy gas dispersion and their severe consequences have been largely publicized by the media. Simplified models have been widely applied to describe the effects of these accidents. However, most simplified models deal with flat terrain scenarios and are based on quite crude simplifications of the complex phenomenology involved.
Suspensions of solid particles into liquids within industrial stirred tanks are frequently carrie... more Suspensions of solid particles into liquids within industrial stirred tanks are frequently carried out at an impeller speed lower than the minimum required for complete suspension conditions. This choice allows power savings which usually overcome the drawback of a smaller particle-liquid interfacial area. Despite this attractive economical perspective, only limited attention has been paid so far to the modelling of the partial suspension regime.
Industrial tanks devoted to the mixing of solid particles into liquids are often operated at an i... more Industrial tanks devoted to the mixing of solid particles into liquids are often operated at an impeller speed N less than N js (defined as the lowest speed allowing the suspension of all particles): under such conditions the distribution of solid-particles is very far from being homogeneous and very significant concentration gradients exist. The present work is devoted to assessing the capability of Computational Fluid Dynamics (CFD) in predicting the particle distribution throughout the tank.
In this paper experimental information on the retention time distribution (RTD) of solid particle... more In this paper experimental information on the retention time distribution (RTD) of solid particles in a high aspect-ratio vessel, stirred by three equally spaced Rushton turbines, is presented. The relevant data were obtained by a special technique named Twin System Approach (TSA), that greatly simplifies the handling of particle laden streams and is therefore particularly suited for investigating particle RTD in flow systems. The technique fundamentals are first summarised, together with the data analysis procedure. This last requires a numerical deconvolution operation that is easily performed with the help of Z-transforms. Two different approaches for excluding the spurious contributions of the external piping required for the experimentation are tested and discussed.
The ow and turbulence ÿelds in a fully ba ed vessel stirred by a Rushton turbine have been simula... more The ow and turbulence ÿelds in a fully ba ed vessel stirred by a Rushton turbine have been simulated by means of computational uid dynamics (CFD) techniques. The simulation techniques adopted (the "Sliding-grid" and "Inner-outer" methods) are fully predictive and require no experimental data as boundary or initial conditions. The e ect of clearance (C) on the ow patterns in stirred vessels was simulated as the ow ÿeld transition observed can be considered a particularly stringent benchmark for the testing of CFD techniques. The results are compared with previously obtained LDA data and show that the double-to single-loop transition experimentally observed when impeller clearance from the vessel bottom is suitably reduced, can be well reproduced by the CFD simulations. The single-loop ow structure present for C=T = 0:15 is compared in detail with the experimental data and good overall agreement is shown between the experiment and simulation. The periodic component of the kinetic energy is well predicted, but the random component is underestimated, a ÿnding also observed in earlier investigations. The mean ow in most of the vessel is also well predicted but the angle to the horizontal of the impeller discharge ow is overestimated. Predictions are also reported with grid reÿnements and di erent turbulence models, in an e ort to identify means of improving agreement with the measurements. The ÿndings indicate that further improvements in turbulence modelling might be necessary. ? : S 0 0 0 9 -2 5 0 9 ( 0 1 ) 0 0 0 8 9 -6
Uploads
Papers by A. Brucato