Nabil Kharoua

Hydrocyclones for De-oiling Applications—A Review

Abstract The de-oiling hydrocyclone is a device for liquid–liquid separation and production water cleanup. Significant progress has been made in the development of such a device since its first introduction and use in the late 1970s. The present article is a literature review of development and research work performed so far on de-oiling hydrocyclones. It reviews the performance parameters affecting the de-oiling hydrocyclone operation; namely, the inlet oil concentration and drop size distribution, the turn-down ratio, the pressure drop ratio, the flow split, and the geometrical parameters. The article addresses work done to elucidate the internal flow structure and performance of de-oiling hydrocylones in both experimental and computational fluid dynamics areas. Challenges and remaining research issues are also identified.

Large eddy simulation study of turbulent flow around smooth and rough domes

Large eddy simulation of turbulent flow around smooth and rough hemispherical domes was conducted. The roughness of the rough dome was generated by a special approach using quadrilateral solid blocks placed alternately on the dome surface. It was shown that this approach is capable of generating the roughness effect with a relative success. The subgrid-scale model based on the transport of the subgrid turbulent kinetic energy was used to account for the small scales effect not resolved by large eddy simulation. The turbulent flow was simulated at a subcritical Reynolds number based on the approach free stream velocity, air properties, and dome diameter of 1.4 × 105. Profiles of mean pressure coefficient, mean velocity, and its root mean square were predicted with good accuracy. The comparison between the two domes showed different flow behavior around them. A flattened horseshoe vortex was observed to develop around the rough dome at larger distance compared with the smooth dome. The separation phenomenon occurs before the apex of the rough dome while for the smooth dome it is shifted forward. The turbulence-affected region in the wake was larger for the rough dome.

Numerical Simulation of Solid-Phase Split at Junctions in Particle Laden Pipe Flow

The present study is a part of an industrial research project and consists in simulating a gas-particle flow through junctions under different geometrical and flow conditions. The purpose is to study the effects of the size of particles, the angles of the junction and the flow rate on the flow split. The particles are usually considered as products to transport, such as in pneumatic conveying, where phase split, if necessary, is done in symmetrical Y-junctions to avoid mal-distribution issues. Thus, asymmetrical junctions were, usually, avoided in transportation networks. However, it appeared that the particles can manifest within networks for transportation of gases as contaminants to be eliminated. A typical example is that of Black Powder in gas pipelines in the oil industry. In such piping networks, different types of junctions can be used and it is worth understanding the behavior of particles for unsymmetrical configurations as well. The numerical simulation combines the k-e and the Discrete Phase DPM turbulence and multiphase models, respectively. Relatively, good agreement in the results between the model and the experiments was obtained. The simulations were extended to Black Powder particles and the corresponding results showed interesting features for different Stokes numbers. The simulation results showed that, for Stokes numbers much smaller than unity (St≤0.2), the solids phase split can be considered to follow the air flow split closely. For intermediate Stokes numbers (St≈1), the particles gain some independence from the gaseous phase. For Stokes numbers slightly higher than unity (St≥5), the orientation plays an important role.Copyright

CFD Simulation of Three-Phase Separator: Effects of Size Distribution

The gas/oil/water separation in a three-phase horizontal separator, employed by the ADCO company in Abu Dhabi, was studied previously using the Eulerian-Eulerian with the k-e model assuming mono-dispersed secondary phases (oil and water). The separator was equipped with new internals due to the increasing amounts of water and Gas-to-Oil Ratio GOR from the field. The approach allowed the description of several features of the internal flow but the prediction of the overall efficiency was largely overestimated compared to the measured value from the field. The source of the discrepancy could be traced back to the assumption of mono-dispersed secondary phases and possibly to the unknown structure of the size distribution at the inlet of the separator preventing thus a correct modeling of drag between the phases and, hence, influencing momentum and secondary phases (oil and water) dispersion. Investigations, using the Population Balance Model, for size distribution, were conducted. Normal and Skewed distributions were employed to represent, only, the secondary water phase due to the limitation of the population model used to only one secondary phase. The paper presents, in addition to the separation efficiency, the internal multiphase flow behavior in terms of overall and local phase distributions. The simulations with PBM model showed a clear improvement of the results in terms of separation efficiency compared with field tests although no experimental data related to the size distribution were available.Copyright

Study of the Pressure Drop and Flow Field in Standard Gas Cyclone Models Using the Granular Model

A particle-laden flow inside solid gas cyclones has been studied using computational fluid dynamics (CFD). The effects of high temperatures and different particle loadings have been investigated. The Reynolds stress (RSM) model-predicted results, in the case of pure gas, are within engineering accuracy even at high temperatures. Using the granular mixture model for the cases of particle-laden flow, discrepancies occurred at relatively high loadings (up to 0.5 kg/m3). Since the pressure drop is strongly related to the friction inside the cyclone body, the concept of entropy generation has been employed to detect regions of high frictional effects. Friction has been observed to be important at the vortex finder wall, the bottom of the conical-part wall, and the interface separating the outer and the core streams. The discrepancies between the present numerical simulation and the experimental results taken from the existing literature, which are caused by the mixture and turbulence models simplifying assumptions, are discussed in this paper.

CFD Simulation of Liquid-Liquid Hydrocyclone: Oil/Water Application

In the present work, the oil-water separation occurring inside a de-oiling hydrocyclone is investigated numerically using the RSM [1] and the RNG k-e [2] turbulence models combined with the multiphase mixture model of Manninen et al. [3] implemented in the commercial code FLUENT. Interesting results are obtained concerning the effects of the inlet oil concentration, the oil droplet size, and the flow rate on the separation efficiency. The results are in agreement with the experimental measurements of Colman et al., [4]. The separation efficiency is known to be unaffected for a large range of inlet oil concentrations (Colman et al. [4], Gomez et al. [5]) and this is confirmed in the present study. In addition to the overall performance parameters, remarkable results describing the flow field behavior are obtained. The radial profiles of the axial and tangential velocity components are discussed. The flow reversal on the axis and the swirling behavior are shown. Results concerning the pressure drops, the friction coefficient and the turbulent Reynolds stresses are also presented. Since detailed results on the flow field for de-oiling hydrocyclones are scarce, the present study might be useful for future studies aiming to improve liquid-liquid separation efficiency depending strongly on the inside flow field behavior.© 2009 ASME

Large Eddy Simulation of Multiple Round Impinging Jets

Large eddy simulation of flow and heat transfer of multiple turbulent round jets in an in-line array impinging on a flat plate is conducted. The full geometry is used in the simulation of the 9 jets. To capture the interactions between the jets the full geometry is meshed in this work. The Reynolds number based on the nozzle diameter of 13 mm, jet initial average velocity of 23.88 m/s and properties of air at room temperature was equal to 20,000. The computations of the mean vertical and horizontal component of the velocity vector in selected planes show very good agreement with experiments. The flow behavior of the jets agrees with experimental findings in terms of vortices surrounding the jets and the appearance of the asymmetry on and close to the flat impingement plane. The predicted mean surface Nusselt number on the flat heated plate shows also excellent agreement with experiments and a relative maximum between the jets in the region of the upwash fountain flow where the wall jets collide, not seen in the experiments, is captured by the numerics.Copyright

Large Eddy Simulation of Forced and Unforced Plane Jets Impinging on a Convex Surface

Large Eddy Simulation study of plane impinging jets with different inlet velocity profiles was conducted. The inlet velocity profile was forced at a frequency equal to 600Hz and amplitude equal to 30% of the mean inlet velocity. The Reynold number, based on the jet width W and the inlet velocity, is equal to 5600. The distance of the jet exit from the target wall was varied from 2W to 10W to cover different types of impinging jets with different flow structures. The time-averaged Nusselt Number Nu profiles, along the curved wall, are characterized by two peaks for the shortest distance 2W and only one peak, at the impingement region, for the largest distance 10W.The first peak, at the impingement region is investigated through profiles of the mean axial velocity, the rms axial velocity, the mean static pressure, and the mean static temperature plotted on the jet centerline. For the second peak of the Nu (2W case), the turbulence level and the thickness of the highly turbulent layer near the curved wall were depicted on curved lines parallel and very close to the target wall.Forcing the considered jets at 600Hz was found to reduce the Nu while a fully developed inlet velocity profile causes an important increase of the Nu at the impingement region compared with flat inlet velocity profiles.Copyright

Numerical Study of the Mean and Filtered Characteristics of Turbulent Jets Impinging on Convex and Flat Surfaces Using LES

A numerical study of a turbulent plane jet impinging on a convex surface and on a flat surface is presented, using the large eddy simulation approach and the Smagorinski-Lilly sub-grid-scale model. The effects of the wall curvature on the unsteady filtered, and the steady mean, parameters characterizing the dynamics of the wall jet are addressed in particular. In the free jet upstream of the impingement region, significant and fairly ordered velocity fluctuations, that are not turbulent in nature, are observed inside the potential core. Kelvin-Helmholtz instabilities in the shear layer between the jet and the surrounding air are detected in the form of wavy sheets of vorticity. Rolled up vortices are detached from these sheets in a more or less periodic manner, evolving into distorted three dimensional structures. Along the wall jet the Coanda effect causes a marked suction along the convex surface compared with the flat one. As a result, relatively important tangential velocities and a stretching of sporadic streamwise vortices are observed, leading to friction coefficient values on the curved wall higher than those on the flat wall.© 2012 ASME

CFD Prediction of Pressure Drop and Flow Field in Standard Gas Cyclone Models

Pressure drop is an important performance parameter for cyclone separators. A computational fluid dynamics (CFD) study of the pressure drop in cyclones using the Reynolds stress (RSM) and the granular mixture models is presented in this paper. The study includes three different cases; pure gas at ambient temperature, pure gas at different temperatures, and particle-laden flow. The first two cases were reasonably well predicted while the presence of particles with a relatively high loading (up to 1 kg/m3 of fluid) caused some discrepancies in the predicted results. The concept of entropy generation, used in this work, has permitted to detect regions of high frictional effect in the vortex finder, the bottom of the conical part, and at the interface separating the outer and the core streams. The simplifying assumptions employed in the CFD models and some numerical details are discussed.Copyright