CFD modelling of flow and solids distribution in carbon-in-leach tanks

The Carbon-in-Leach (CIL) circuit plays an important role in the economics of a gold refinery. The circuit uses multiphase stirred tanks in series, in which problems such as dead zones, short-circuiting, and presence of unsuspended solids are detrimental to its efficiency. Therefore, the hydrodynamics of such a system is critical for improving the performance. The hydrodynamics of stirred tanks can be resolved using computational fluid dynamics (CFD). While the flow generated by the impellers in the CIL tanks is complex and modelling it in the presence of high solid concentration is challenging, advances in CFD models, such as turbulence and particle-fluid interactions, have made modelling of such flows feasible. In the present study, the hydrodynamics of CIL tanks was investigated by modelling it using CFD. The models used in the simulations were validated using experimental data at high solid loading of 40 wt. % in a lab scale tank. The models were further used for examining the flow generated by pitched blade turbine and HA-715 Mixtec impellers in lab scale CIL tanks with 50 wt. % solids. The effect of design and operating parameters such as off-bottom clearance, impeller separation, impeller speed, scale-up, and multiple-impeller configuration on flow field and solid concentrations profiles was examined. For a given impeller speed, better solids suspension is observed with dual impeller and triple impeller configurations. The results presented in the paper are useful for understanding the hydrodynamics and influence of design and operating parameters on industrial CIL tanks

Bubbles in viscous liquids: Time dependent behaviour and wake characteristics

The dynamics of a bubble, initially stationary and spherical, rising in a viscous Newtonian liquid have been studied numerically using 3-D Volume-of-Fluid (VOF) method implemented in the Gerris flow solver. The study encompasses 8.7≤Eo (=ΔρgD2/σΔρgD2/σ)≤641 and Re≤151. Additionally, results published in the literature encompassing bubbles with lower values of Eo numbers were also considered, such that the overall dependencies of bubble shape, wake characteristics, and drag coefficient over a large range of Eo and Re values can be identified. While it was found that the deformation of the bubbles as predicted through the numerical study can generally replicate experimental observations presented, several limitations were identified, such as in the representation of skirt formation behind a skirted bubble and the formation of satellite bubbles behind a bubble rising at high Reynolds numbers. The dependency of the bubble aspect ratio on the Weber and Morton numbers was confirmed for cases of spherical and ellipsoidal bubbles; whilst for spherical cap and skirted bubbles the aspect ratio was found to depend largely on the Reynolds and Capillary numbers, respectively. Finally, the expansion and formation of closed/open laminar wakes behind the rising bubble were analysed and was found to correlate well with the bubble Re and Eo numbers

Hydrodynamic Simulation of Cyclone Separators

Cyclone separators are commonly used for separating dispersed solid particles from gas phase. These devices have simple construction; are relatively inexpensive to fabricate and operate with moderate pressure losses. Therefore, they are widely used in many engineering processes such as dryers, reactors, advanced coal utilization such as pressurized and circulating fluidized bed combustion and particularly for removal of catalyst from gases in petroleum refinery such as in fluid catalytic cracker (FCC). Despite its simple operation, the fluid dynamics and flow structures in a cyclone separator are very complex. The driving force for particle separation in a cyclone separator is the strong swirling turbulent flow. The gas and the solid particles enter through a tangential inlet at the upper part of the cyclone. The tangential inlet produces a swirling motion of gas, which pushes the particles to the cyclone wall and then both phases swirl down over the cyclone wall. The solid particles leave the cyclone through a duct at the base of the apex of the inverted cone while the gas swirls upward in the middle of the cone and leaves the cyclone from the vortex finder. The swirling motion provides a centrifugal force to the particles while turbulence disperses the particles in the gas phase which increases the possibility of the particle entrainment. Therefore, the performance of a cyclone separator is determined by the turbulence characteristics and particle-particle interaction.Full Tex

Dynamics of gas-liquid flows in bubble column reactors

Wall pressure fluctuations were measured in bubble columns at different locations and for different gas velocities and height to diameter ratios. Non-linear analysis of the acquired data of pressure fluctuations was carried out to quantify dynamic characteristics. A bubble-bubble interaction model was developed to simulate voidage fluctuations in bubble columns. Dynamic characteristics of the simulated voidage fluctuations were compared with those of measured pressure fluctuations. The data and analysis will be useful to validate full transient CFD simulations of gas-liquid flow in bubble columns

Modeling and optimization of Carbon in leach (CIL) circuit for gold recovery

Carbon in leach (CIL) is an important step in gold processing that involves simultaneous adsorption and leaching. While the mechanism of leaching and adsorption are well known, the effect of different operating and design parameters on the dynamics and performance of CIL circuit are not yet studied. The focus of this paper is to study the effect of operating parameters like cyanide concentration, oxygen concentration and mean particle diameter on the overall efficiency of CIL circuit. A dynamic model based on first principles is developed for the entire CIL circuit. Suitable kinetic models for both leaching and adsorption are adopted from the literature. Customizable simulator is written in MATLAB to simulate the model. Simulation results are first validated using previously published results. The validated model is then used to perform sensitivity studies on different parameters that affect the gold extraction process. The key parameters are optimized by conducting a multi-objective optimization study using elitist non-dominated sorting genetic algorithm of MATLAB. The percent recovery and net profit are maximized, while minimizing the cyanide used in the process. Four decision variables are used viz. particle diameter, oxygen concentration, cyanide concentration and ore volume fraction in tanks. A Pareto-optimal solution for the process parameters is provided such that all the objectives are improved to a value above which their improvement will result in degradation of other objectives. As a result, high recovery, high profit and low cyanide concentrations can be achieved simultaneously

Vortex shape and gas‐liquid hydrodynamics in unbaffled stirred tank

The present study investigated the effect of impeller speed and vortex ingestion on vortex shape, gas holdup, and bubble size distribution in an unbaffled stirred tank using optical probe measurements. Further, the ability of the volume of the fluid model to predict vortex shape was examined. Without vortex ingestion, an increase in impeller speed resulted in a significant variation in vortex shape, whereas it had a negligible effect on vortex shape with ingestion. This suggests that when vortex ingestion occurred, most of the energy was consumed for the dispersion of gas rather than the deformation of the gas‐liquid interface. It was observed that a large number of gas bubbles were entrained into the vortex core around the impeller region, which led to a lower gas holdup at the top axial locations. An increase in the impeller speed also resulted in the formation of larger bubbles. The absence of baffles limits shear for bubble break up, resulting in larger bubbles above the impeller plane

Modeling of rotary desiccant wheels

Rotary desiccant wheels are widely used in dehumidification and energy recovery applications. In this work, we have developed a 2D, steady state model of a rotary desiccant wheel. Mass and energy balance equations for the air streams and the desiccant wheels were developed. The hydraulic diameter and surface area for heat and mass transfer were calculated based on knowledge of the flute geometry. Appropriate correlations for the Sherwood number and Nusselt number were used to estimate heat and mass transfer coefficients. The model is capable of predicting steady state behavior of desiccant wheels having at the most three sections (process, purge, and regeneration). The mathematical model was validated using a real desiccant wheel, and the calculation results are in reasonable agreement with the experimental data. Based on this model, the temperature and humidity profiles in the wheel during both the dehumidification and the regeneration processes are analyzed. The simulated results were used to gain an insight into the operation of desiccant wheels. The model and the presented results will be useful for optimizing dehumidification and energy recovery applications

Hydrodynamics of a Fluid Catalytic Cracking Stripper Using γ-ray Densitometry

This paper reports hydrodynamics of a laboratory-scale fluid catalytic cracking (FCC) stripper. The laboratory-scale stripper was designed by geometrically and dynamically scaling down an industrial-scale FCC stripper that had a disk and donut baffle. The solids holdup was measured using a γ-ray densitometry technique with a 3-μCi-strength Cs-137 radioactive source. Measurements were taken at different elevations and chordal positions. The effect of operating conditions on the solids holdup profiles was investigated in detail. For example, the particle flow rate was varied from 0.025 kg/s to 0.042 kg/s, and the superficial air velocity between 0.74 m/s and 1.1 m/s. It was observed that the shape of baffles played an important role in the hydrodynamics of the stripper. Several dead zones were noticed under the baffle regions indicating unused areas in the stripper. The measured solid holdup radial profiles were of asymmetric nature underlying the need for three-dimensional (3D) simulations. At low superficial gas velocities, there was a widespread segregation in the solid phase, which along with the solid holdup decreased on increasing the air superficial velocity. However, the change in solid flow rates did not have any effect on the solids holdup

Hydroynamic simulation of cyclone separators

Cyclone separators are commonly used for separating dispersed solid particles from gas phase. These devices have simple construction; are relatively inexpensive to fabricate and operate with moderate pressure losses. Therefore, they are widely used in many engineering processes such as dryers, reactors, advanced coal utilization such as pressurized and circulating fluidized bed combustion and particularly for removal of catalyst from gases in petroleum refinery such as in fluid catalytic cracker (FCC). Despite its simple operation, the fluid dynamics and flow structures in a cyclone separator are very complex. The driving force for particle separation in a cyclone separator is the strong swirling turbulent flow. The gas and the solid particles enter through a tangential inlet at the upper part of the cyclone. The tangential inlet produces a swirling motion of gas, which pushes the particles to the cyclone wall and then both phases swirl down over the cyclone wall. The solid particles leave the cyclone through a duct at the base of the apex of the inverted cone while the gas swirls upward in the middle of the cone and leaves the cyclone from the vortex finder. The swirling motion provides a centrifugal force to the particles while turbulence disperses the particles in the gas phase which increases the possibility of the particle entrainment. Therefore, the performance of a cyclone separator is determined by the turbulence characteristics and particle-particle interaction

Hydrodynamic study of fluid catalytic cracker unit stripper

This paper reports a three-dimensional (3D) computational fluid dynamics (CFD) simulation of a laboratory scale fluid catalytic cracking unit (FCCU) stripper. Solid holdup and solid mixing were studied in a geometrically and dynamically scaled down cold model FCCU stripper fitted with disk and donut baffles. The solid holdup was measured using a gamma-ray densitometry technique with a 3 micro-Ci strength 137Cs radioactive source. Measurements were taken at different axial levels, for different chordal positions. An Eulerian-Eulerian approach was used to simulate the gas-solid flow in the stripper column. The CFD simulations predicted asymmetric solid holdup profiles emphasizing the importance of 3D simulations. The CFD model predictions matched well with the solid holdup data from experiments. The CFD model also clearly predicted the recirculation and dead zones as noticed in the experimental analysis. Local defluidization zones were also noticed near the donut baffle wall regions. The particle axial velocity was low near the baffle walls, indicating that the particles slide down on the baffle walls. The radial and axial variations of solid holdup are discussed in this work, giving new knowledge in the complex countercurrent operation. This work is first of its kind to report detailed 3D study on the hydrodynamics of stripper operation both in cold flow and in the CFD model