1,721,119 research outputs found
Simple model to evaluate loading/flooding transition in aerated vessels stirred by Rushton disc turbines
No full textPrediction of the flooding/loading transition in an aerated stirred tank is crucial for the correct design of aerated stirred tank reactors. Despite the importance of this problem, only semiempirical equations are available at the moment. This work provides a simple mechanistic model for predicting the flooding/loading transition in stirred tank reactors equipped with Rushton turbines. This model could be a useful one, since it allows the user to scale-up agitated vessels easily. It has only been tested for Rushton turbines, but because the model is based simply on mass balances, energy balances and continuity equations, it could also be used with other impellers after a few modifications
Observations on the significance of instabilities, turbulence and intermittent motions on fluid mixing processes in stirred reactors
No full textFlow instabilities of different nature have been observed inside stirred vessels and the motions associated with them may enhance mixing in a number of ways. In the present work the different kinds of flow instabilities which occur continuously for most configurations (precessional macro-instabilities, (Chem. Eng. Sci. 58 (2003) 2937) and constantly/intermittently for particular vessel/impeller configurations (Chem. Eng. Sci. 58 (2003) 2297, Chem. Eng. Sci. 58 (2003) 3859), were investigated simultaneously through two-point Laser Doppler Anemometry (LDA) measurements and their effect on mixing operations is discussed. A multi-channel LDA was used to investigate the flow instabilities in different regions of a stirred vessel for both radial and axial impellers, and velocity data were analysed with wavelet and spectral/FFT analysis. The energy associated with the different kinds of flow instabilities was estimated in many locations of the vessel. The amount of broadening of the turbulence levels that such instabilities produce through the superimposition of the instability flow variation on the mean flow pattern was quantified. The relative strength of the different instabilities in different parts of the vessel as well as the presence of intermediate frequencies that may arise from the combined effect of different instability frequencies are also discussed
Liquid mixing time and solid dissolution in slurry stirred tanks
No full textElectrical Resistance Tomography is adopted for analysing salt particles dissolution in dense solid-liquid stirred tanks. The process provides an ambitious challenge for the application of ERT in the process industry, because the presence of high inert particles loadings requires a not trivial analysis of the experimental data. For the optimization of the working conditions of the stirred tank, two characteristic times are required: the time for the liquid homogenization in the tank and the time required for the complete dissolution of the salt particles. The former time has been experimentally determined in previous investigations both in stirred tanks working with single-phase and well as with multiphase mixtures. The latter characteristic time has not been analysed so far, due to the lack of experimental procedures for distinguishing it from the former one. In this work, a novel approach for the simultaneous identification of the homogenization and the dissolution time is presented. The impact of the new procedure is significant for the production processes, because it allows to distinguish when the process is influenced by the dissolving particle size, and therefore a reduction of the particle size has an impact on the dissolution dynamics, and when the stirred tank dynamics is influenced only by the liquid homogenization, and therefore a reduction of the particle size doesn’t influence the process accomplishment
An impedance probe for the measurements of liquid hold-up and mixing time in two/three-phase stirred tank reactors
No full textA new probe based on the measurement of the electrical impedance has been developed to measure liquid hold-up in gas/liquid, solid/liquid and gas/solid/liquid stirred tank reactors. It allows measurements of liquid hold-up and mixing time to be made in stirred tanks. The main advantage of the new probe is that it is absolutely non-intrusive, because it uses the shaft and the baffles' support as electrodes, and that it can be used both for laboratory scale reactors as well as for industrial stirred tank reactors. The relation between impedance and liquid hold-up under loading conditions closely follows the predictions made by means of the Bruggeman model. Therefore, the new probe does not need any calibration, in that the liquid hold-up can be theoretically computed by the measurement of impedance
How to evaluate the mass transfer resistances in dense solid–liquid suspensions
Full text linkThe dissolution of solids in slurry stirred tanks is affected from the solid concentration distribution to different extend, depending on the agitation conditions. In this work, the relationship between solid–liquid mixing and mass transfer is investigated by estimating the characteristic times of different phenomena: liquid mixing, solid dissolution, diffusion of the dissolved species in the vessel volume. The analysis is based on Electrical Resistance Tomography data collected during the simultaneous operations of dissolution and mixing. For dilute solid–liquid systems, the relative importance of the solid dissolution and the liquid mixing depends on the particle size and the impeller speed, while the diffusion in stagnant zones is often negligible. Instead, the diffusion mechanism has an important effect on mass transfer for dense systems, where strong axial gradients of solid concentration are established and the stirred tank volume can be divided into a lower zone containing a cloud of dense mixture and an almost stagnant clear liquid layer on top. Based on original and literature experimental data, a method for the main mass transfer resistance identification depending on the agitation condition, the solid volume fraction distribution and the soluble particle size is suggested
Simultaneous measurements of liquid velocity and tracer concentration in a continuous flow stirred tank
Full text linkThis work concerns the simultaneous measurements of the velocity of a liquid and of the concentration of a passive scalar in a standard geometry stirred tank provided with a continuous feed stream. The data are collected close to the tracer inlet on a vertical plane portion located in the vessel top region by a combined Particle Image Velocimetry and Planar Laser Induced Fluorescence system. The mean and fluctuating velocity and concentration components in the axial and radial directions obtained under stirred and unstirred conditions are presented. Following the definitions provided in the realm of the gradient diffusion hypothesis for the turbulent scalar transport, the measured variables are further exploited for the calculation of the turbulent viscosity, the turbulent diffusivity and their ratio, that is the turbulent Schmidt number. For the investigated conditions, the turbulent Schmidt number values are in the range 0.7–1.0, although significant local variations are observed
2-PHASE FLOW OF BRINE IN LONG PIPELINES - ANALYSIS OF FIELD EXPERIMENTS
No full textThe design of two-phase flow lines for geothermal applications requires reliable information about main flow parameters. In the present work a one-dimensional, steady-state model for the computation of hold-up and pressure losses in horizontal and near-horizontal pipes is presented. The model is based on a mechanistic analysis of Stratified, Intermittent and Bubbly flow. The model has been implemented in the two-phase flow simulator, HORF, which has been developed in order to predict pressure losses relative to pipeline flow of geothermal brines. It is shown that the predictions are in very good agreement with field data obtained at Latera geothermal field, in Italy, relative to a flow line 2400 m long, and 18" nominal diameter. © 1994
Performance of absorption columns equipped with low pressure drops structured packings
No full textDuring the past few years the industry has demanded structured packings with higher and higher efficiency and with lower and lower pressure drops. Using these internals significant improvements were obtained in the absorption process but also in direct-contact heat transfer. In the present work a new type of packing is investigated both experimentally and theoretically to evaluate gas and liquid mass transfer coefficients, surface area available for mass transfer, and pressure drops. The experimental performances of the packing are tested in absorption and desorption systems to identify relations that allowing reliable design
Particles dissolution and liquid mixing dynamics by Electrical Resistance Tomography
Full text linkSalt particles dissolution in slurry stirred tanks provides an ambitious challenge for the application of Electrical Resistance Tomography in the process industry, because the presence of high loadings of inert particles requires a purposely developed post-processing method of the experimental data. For the optimization of the working conditions of the dissolution process, two characteristic times are required: the time for the liquid homogenization in the tank and the time required for the complete dissolution of the salt particles. The former time has been experimentally determined in previous investigations both in stirred tanks working with single-phase and with multiphase mixtures. The latter characteristic time has not been analyzed so far, due to the lack of experimental procedures for distinguishing it from the former. In this work, a novel approach for the simultaneous identification of the two characteristic times is presented. The impact of the new procedure is significant for the production processes, since it offers a tool for identifying when the soluble particle size has an impact on the dissolution dynamics, and when the stirred tank dynamics is influenced by the liquid homogenization only, and therefore a reduction of the particle size does not speed up the process accomplishment
- …
