1,721,115 research outputs found
CFD ANALYSIS OF INDUSTRIAL INERTIAL SEPARATORS Predictions of intermittent flow regimes in a vapor-liquid separator
No full textLarge scale vessels provided with suitable internals for the inertial separation of multiphase mixtures are widely adopted in the process industries. The working principle of the operation is based on the density difference of the mixture components, therefore either centrifugal forces or gravity forces are adopted for achieving the phase separation. Although the equipment functioning is very simple, the achievement of high separation efficiency is far from being straightforward. The separation efficiency of the apparatuses strongly depends on the fluid dynamic features, which in turn are affected from the vessel internals design and the working conditions. In addition, the geometrical and operation parameters must be selected taking into account the applications and the location of the separator (off-shore, on-shore, Floating Production Storage and Offloading, etc.).
The numerical solution of appropriate multiphase flow formulation of the Navier-Stokes equations can provide detailed information on the equipment fluid dynamics in a fully predictive way. As a result, design methods based on CFD are expected to significantly improve the separator design with respect to traditional design rules, which are based on empirical correlations and might lead to oversize the apparatuses [1]. Due to the tremendous impact of the separation efficiency on the overall process performances, development and investigation in this field is very attractive from both the academic and the industrial perspective.
A review on the design rules and on the CFD studies of multiphase separators has been presented by Pourahmadi Laleh et al. [2], who addressed the potential benefits of the CFD application in this field. Although multiphase models still require further improvements, the advantages of CFD methods made their application appropriate in different scenarios of the Oil & Gas industry [3], including oilfield separators [4].
Most of the CFD studies carried out so far were aimed at evaluating the effects of the design of the internals [5], while among the uncovered issues of multiphase separators by CFD simulations, the departure from steady state operations has never being reported in literature. In this work, a vapor-liquid separator treating a two phase flow consisting of liquid water and steam for is specifically investigated considering an intermittent flow regime in the inlet pipeline. The usefulness of the CFD analysis for the mitigation of the slug entrance effects by design modification is demonstrated
Gas hold-up distribution and mixing time in gas-liquid stirred tanks
Full text linkIn this work, the gas-liquid dispersion in a stirred tank equipped with different impellers is investigated by Electrical Resistance Tomography (ERT). The main goal of the study is to provide detailed information on the spatial distribution of the gas phase and on the effect of bubbles on the liquid homogenisation dynamics. The analysis is carried out under variable gas flow rates and impeller speeds, thus covering different regimes of gas-impeller interaction, as obtained by Rushton Turbines, Pitched Blade Turbines pumping upwards and Lightnin A310. The experimental technique allows us to overcome the typical limitations of optical methods and to gain insight into the complex behaviour of sparged stirred tanks without restriction on the upper value of overall gas hold-up, that is of great interest for several chemical and biochemical processes. Besides, the experimental data can be adopted as a benchmark for advanced modelling techniques based on CFD methods, whose scant validation is often due to limited information on the local dispersion features. The analysis of experimental results allows us to suggest simple correlations for the prediction of the prevailing flow regime based on the dimensionless Froude and flow numbers. Finally, the definition of a modified Peclet number is also suggested, as a simple parameter for the interpretation of both the gas hold-up distribution and the dimensionless mixing time
Towards a robust CFD modelling approach for reliable hydrodynamics and mass transfer predictions in aerobic stirred fermenters
No full textA strategy based on the Reynolds averaged two-fluid model is proved to be effective for the simulation of a turbulent gas-liquid stirred tank in the whole range of possible flow regimes, namely complete recirculation, loading, and flooding. The modelling approach is validated through the simulation of a laboratory scale tank stirred with a Rushton turbine, with superficial gas velocities ranging from 1.5 mm/s to 4.4 mm/s and specific power consumption from 39 W/m3 to 837 W/m3, since such a system in this range of operative conditions is well-characterized and several well-established correlations are available for validation purposes. The gas segregation due to the formation of aerated cavities was accounted for in the determination of the specific interfacial area available for the mass transfer and the numerical predictions of the kLa better agree with the available correlations, especially in the loading regime when clinging and ‘3–3’ cavities develop, with respect to the traditional specific interfacial area formulation that does not consider the phase segregation. Finally, starting from grid independent results, the effect of computational grid coarsening is quantified, obtaining guidelines for the affordable simulation of bioreactors of large scale
Modelling of biohydrogen production in stirred fermenters by Computational Fluid Dynamics
No full textA bioreactor for the production of hydrogen from the dark fermentation of organics is studied by a comprehensive modelling strategy. The bioreactor is a dual impeller vortex ingesting stirred tank working under batch and attached-growth conditions. Two geometrical configurations of the reactor are investigated: one devised to ensure an effective fluid dynamics behaviour and the other proposed to increase the hydrogen productivity. The turbulent gas–liquid fluid dynamics, the production and the recovery of H 2 from the liquid phase are predicted by the numerical solution of the two-phase Reynolds averaged Navier–Stokes equations and the species mass transport equations, including a simplified kinetic model for the fermentative hydrogen production found in literature and a local interphase mass transfer model for the hydrogen stripping from the aqueous to the gas phase. A simplified model for the description of the interfacial area in the context of the two-fluid model is also proposed. This work suggests a method for the predictive simulations of a complex biological process via numerical modelling based on Computational Fluid Dynamics. The main outcome of the proposed investigation method is a detailed estimation of the different relevant variables and their interaction on a local basis, providing a viable tool for the optimization and the scale-up of bioreactors
Fluid dynamics characterization of a stirred model bio-methanation digester
No full textThis work deals with the power consumption and the fluid dynamics features of a model digester for biogas production by agricultural scraps fermentation. A combination of experimental techniques is adopted in order to obtain the full characterization of an industrial digester stirred by multiple impellers, which is scaled-down following the geometrical similarity criterion, under various impeller speeds and three liquid heights.
A number of critical fluid dynamics characteristics of the reactor are identified: the velocity field is strongly dampened far from the impellers region, thus leading to wide stagnant zones; the shape of the discharge flows of the impellers and their interaction are significantly affected by the Reynolds number; under specific conditions, flow instabilities of wide amplitude take place; the power consumption is different from that of standard configurations and depends on the liquid level. Geometrical modifications are suggested for achieving better mixing and enhancing the overall process performances
Prediction of fluid dynamic instabilities of low liquid height-to-tank diameter ratio stirred tanks
No full textIn this work, original experimental data on the fluid flow instabilities associated with the oscillation of the liquid free surface in low liquid height-to-tank diameter ratio stirred tanks are presented. The local flow features of this type of tanks, which are commonly adopted as (bio)-reactors in important industrial productions such as drugs and bio-energy, are investigated by Particle Image Velocimetry. The local mean flow field and the turbulent characteristics are found to depend critically on the periodic variations of the liquid free surface, whose features vary as a function of the fill ratio. The frequency analysis of the dynamic pressure time series collected by a Pitot system shows that the frequency of the flow instabilities is mainly affected by the geometrical characteristics of the system. Therefore, the observed instabilities do not arise from the same origin of the so-called macro-instabilities previously identified in stirred vessels. A novel interpretation based on the liquid sloshing dynamics in cylindrical vessel is suggested
Blending of miscible liquids with different densities and viscosities in static mixers
Full text linkThe homogenization of two liquids of different densities and viscosities in a pipeline equipped with a corrugated plate SMV static mixer is investigated by RANS-based CFD simulations. The blending effectiveness of the mixer is compared at different Richardson numbers and viscosity ratios for equal Reynolds numbers. The mixedness level is found to be a function of the Richardson number. As a result, depending on the pipeline scale, equal density differences require a different number of pipe diameters for the achievement of the same level of homogenization. The dynamic viscosity differences give rise to less marked effects, unless back-mixing becomes significant. Besides the coefficient of variation of the scalar concentration, which is often adopted as a measure of the intensity of segregation in turbulent static mixers, novel definitions of the scale and of the rate of change of segregation are proposed, in order to add deeper insight into the evaluation of the mixing features
A study of Reynolds stresses, triple products and turbulence states in a radially stirred tank with 3-D laser anemometry
No full textAdvanced characterization with multimodal measurement techniques for process intensification in gas–liquid tubular reactors
Full text linkIn this work we investigate turbulent gas–liquid mixing and separation in a vertical pipeline equipped with Kenics Static Elements for process intensification applications in continuous operations. The investigation is based on Electrical Resistance Tomography, digital image analysis and pressure drop measurements to provide a comprehensive characterization of the two-phase system. The gas volume fraction distribution is calculated from the voltage difference measurement using two different reconstruction algorithms: the Sensitivity Conjugate Gradient and the Linear Back Projection. The Sensitivity Conjugate Gradient algorithm provides better resolution near the pipe wall and a clear detection of the asymmetric patterns typical of the helical elements. The bubble size distributions obtained from digital image analysis allow to assess the effectiveness of the static elements in providing mixing and separation, depending on their orientation. The spatial distributions of the gas phase measured by the optical and the tomographic techniques are adopted to obtain the average gas hold-up leading to similar results. Overall, the experimental data analysis demonstrates that optimal performance can be determined by balancing energy consumption and gas dispersion. The findings provide valuable insights for the design of in-line reactors where both efficient mixing and controlled separation are required
Mixing in Biogas Fermenters: Experimental Characterization of a Scale-down Geometry
Full text linkIn this work, the fluid dynamics features of a real industrial configuration of a biogas fermenter, which consists in a cylindrical tank stirred with three top-entering shafts with multiple impellers, are investigated. The analysis is based on the experimental characterization of a laboratory model digester of 0.49 m in tank diameter obtained from the scale-down based on the geometrical similarity criterion of a full-scale digester of diameter equal to 17 m. The aim of the work is to evaluate the appropriateness of the design for the requirements of the biogas production process and to suggest possible improvements to the overall mixing operation.
The fluid dynamics investigation is carried out using either water or an aqueous solution of xanthan gum, in order to assess the impact of the variation of the rheological properties at different impeller speeds and direction of rotation of the impellers on the mixing features. To this end, Particle Image Velocimetry is adopted to obtain the velocity fields for the different liquid phases. The data analysis allows to identify possible critical fluid dynamics characteristics that may affect the fermentation, as for example the presence of stagnant zones, where sinking layers might be expected, thus explaining the failure of the biogas production often observed in the biogas production plant
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