1,720,983 research outputs found
Mechanistic Model of Amine Hydrochloride Salts Precipitation in a Confined Impinging Jet Reactor
Full text linkA mechanistic model was developed to study the industrial synthesis of the polyurethane precursor, amine hydrochloride, in a confined impinging jet reactor (CIJR). Two chemical reaction steps occur in a competitive-consecutive sequence, which results in the precipitation of two amine hydrochloride salts. The formation of the di-amine byproduct means loss of starting material and expensive reprocessing of highly insoluble salts. The predictive mechanistic model includes equations for chemical reaction kinetics, nucleation, particle growth, and the first reported mixing model for the CIJR. In our previous study [Maluta, F. et al. Comput. Chem. Eng. 2017, 106, 322], we used a full factorial design to determine physically realizable values of the 11 physical constants involved in the model. In this study, we show the importance of using a mixing model to account for imperfect mixing in the impingement zone. The mixing model treats the impingement zone as a radial jet and resolves the local mixing into 198 discrete compartments. The model was able to predict an unexpected and sudden change in the reaction product distribution as the reactant inlet concentration is increased. Without the local mixing model, it was not possible to replicate this major trend in the experimental results. The local mixing model allows us to determine the conditions under which significant byproduct formation will occur. A second industrially important question is whether fine particles or larger particles will be produced. This process outcome was also dominated by local mixing conditions in the impingement region. The model results show a strong influence of local mixing on two key process outcomes
RANS-based predictions of dense solid–liquid suspensions in turbulent stirred tanks
No full textThis work is aimed at investigating the impact of different meso-scale models and constitutive equations for the RANS-based two-fluid model simulations of a turbulent solid–liquid stirred vessel with high solids loading. The model assessment is preceded with a grid convergence study, which confirms the variability of the discretization requirements depending on the observed variable. The simulation results demonstrate that for the investigated system the high solids loading contribution modelled by the kinetic theory of granular flows is negligible, both in incomplete and complete suspension conditions. Instead, the particle concentration fluctuations contribution included in the momentum equations dramatically affect the predictions, particularly in incomplete suspension conditions. The evaluation of the models is completed by the comparison of the predicted solids concentration profiles with experimental data measured by Electrical Resistance Tomography. The computational strategy for achieving realistic predictions of the solid distribution both in complete and incomplete suspension conditions is outlined
Analysis of immiscible liquid-liquid mixing in stirred tanks by Electrical Resistance Tomography
Full text linkIn 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
Analyis of liquid mixing and solid dissolution for pharmaceutical manufacturing in stirred tanks
No full textThis work concerns solid-liquid mass transfer in stirred tanks and is focused on the analysis of experimental data collected during the dissolution of different solid materials in laboratory and industrial equipment. The times required for the solid dissolution and for the liquid mixing are considered, since they are key parameters of the operation. The adoption of literature correlations, which are proved to be applicable for the estimation of the two characteristic times in different applications, is suggested for obtaining useful indications on the dissolution operation depending on the equipment scale, coupling the approaches suggested so far for the analysis of liquid blending and solid dissolution processes. Available literature data concerning particle dissolution in pharmaceutical processes are taken for the models assessment. The applicability of the proposed method to the dissolution of powders for the home care industry is also considered. The outcome of this investigation can be adopted as a simple guideline for the selection of the operating conditions of the stirred vessels and for a proper choice of the solid size as a function of the vessel scale
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
Experimental and numerical study of a compact inline swirler for gas–liquid separation
Full text linkThe liquid mean and fluctuating velocity field, the pressure drop, the gas-phase distribution and the bubble size distribution in a compact inline swirler for gas–liquid separation were experimentally measured. The results served as validation for the proposed computational approach and the most suitable turbulence model was identified. The gas phase distributions obtained considering both a constant bubble diameter and the local bubble size distribution (BSD) through the solution of a Population Balance Equation (PBE) confirmed the lighter gas phase accumulation towards the pipe centre. The study highlights that to realistically predict the BSD a coupled solution of the PBE with the flow equations is needed and that the resulting distribution is relatively narrow. The gas accumulation zone obtained coupling the solution of the PBE is in good agreement with the experimental one. It is shown that CFD may help the design of inline swirlers for gas–liquid separation
A PBM-Based Procedure for the CFD Simulation of Gas–Liquid Mixing with Compact Inline Static Mixers in Pipelines
Full text linkA compact static mixer for gas–liquid dispersion in pipelines is studied in this paper with a Reynolds averaged two fluid model approach. A procedure based on the lumped parameter solution of a population balance model is applied to obtain the bubble Sauter mean diameter needed to model the interphase forces. The gas distribution in the pipe is analyzed in two different operative conditions and the efficiency of the static mixer is assessed in terms of the gas homogeneity in the pipe section, with low coefficients of variations being obtained. A computational model to obtain the volumetric mass transfer coefficient, kLa, developed for partially segregated systems is applied finding kLa values comparable to those typically obtained with other static mixers. The proposed computational model allows us to locally analyze the oxygen transfer rate by observing the limitations due to gas accumulation behind the body of the static mixer, which leads to the local depletion of the driving force. Geometrical optimization of the static element is proposed, based on the analysis of gas–liquid fluid dynamics and of the interphase mass transfer phenomena
Numerical simulations of a compact inline gas–liquid separator: A comparison between RANS and a hybrid LES-RANS approach
Full text linkTwo computational fluid dynamics approaches are adopted to simulate the single-phase and the gas–liquid flow field generated by a compact inline separator based on the generation of a swirling flow. The results of a detached eddy simulation, DES, combining a large eddy approach with the realizable k-ε turbulence model were compared to RANS results and experimental data from the literature. The DES better replicates the experimentally observed velocity fluctuations, pressure drop, gas distribution and gas–liquid separation profiles, with respect to the RANS approach. A population balance equation was solved in the flow fields produced by the two simulation approaches, and important differences were found in the mean bubble characteristic diameter spatial distributions, possibly due to the different predictions of the local values of turbulent dissipation rates, which affect the bubble breakup and coalescence phenomena
Towards a CFD-PBE simulation of aerated stirred tanks at high gas hold ups and different flow regimes
Full text linkIn this work we analyze the challenges of the adoption of a population balance approach in aerated stirred tanks operating at high gas volume fractions, in different flow regimes and when segregation of the phases is present, i.e. due to the formation of aerated cavities behind flat blades. A decoupled solution of the equations governing the fluid flow and of the population balance equation (PBE) is adopted to obtain preliminary information on the bubble size distribution and to identify the specific issues that need to be addressed to extend the applicability of the population balance model to different regimes. A workflow is proposed for the simulation of aerated stirred tanks with geometries for which established correlations for the estimation of bubble size are not available and a simplified approach to determine whether a coupled CFD PBE solution may be required is presented
Prediction of gas cavities size and structure and their effect on the power consumption in a gas-liquid stirred tank by means of a two-fluid RANS model
Full text linkAerated cavities behind the impeller blades in stirred tanks affect the power transferred to the liquid that in turns affects heat and mass transfer, thus the development of fully predictive simulation methods to detect the formation of cavities, their size and structures is of paramount importance for an effective simulation of aerated reactors and bioreactors. In this work, operating conditions corresponding to different cavity structures are investigated by means of a Reynolds averaged two-fluid model without adjustable parameters. Based on the comparison with previous experiments and correlations, the method proved to be reliable in the prediction of the transition between vortex-clinging and small '3-3′ cavities, cavity size and power drawn reduction. For the first time, small '3-3′ cavities with volume fractions close to unity are obtained with a steady approach. The power reduction mechanism is observed and a novel interpretation of the formation of the asymmetrical cavities is proposed
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