Search Results (59)

A study was conducted to characterize the performance of a HydroFloat^® coarse particle flotation (CPF) cell using rougher tailings samples from an industrial copper mining operation. The work involved measuring internal hydrodynamic variables under a wide range of operating conditions. The effect of different operational and hydrodynamic conditions on the metallurgical performance of the HydroFloat^® cell was also evaluated. Gas dispersion measurements, such as bubble size distribution, superficial gas velocity (J_g), superficial area flux (Sb), and residence time distribution (RTD), were recorded, enabling a detailed analysis of the cell’s operation. Results show that copper recovery is strongly influenced by the superficial gas velocity (J_g) and the superficial liquid velocity (J_l). It was observed that the bubble diameter (d₃₂) remained relatively constant at 0.5 mm across all operating conditions, which is well below typical bubble sizes for conventional flotation cells. This suggests that contrary to what may be expected, in this kind of machine, small bubbles are able to float coarse particles. Bubble image inspection suggests that the HydroFloat^® cell creates conditions conducive to bubble–particle aggregates, which would explain how small bubbles can float coarse particles. This study contributes to the understanding of CPF and establishes a framework for optimization in copper concentrators. Full article

During continuous casting, the flow behavior of liquid steel in the tundish directly affects the temperature distribution of liquid steel, inclusion removal, and billet quality. In tundish-related research, water model experiments remain an intuitive method for investigating the flow process in the tundish. However, water model experiments are often conducted in different seasons, and variations in experimental temperature can change fluid properties such as density and viscosity, thereby affecting flow characteristics and the comparability of experimental results. In this study, a 1:3.57 transparent bare single-strand tundish model made of acrylic was used, and the differences in tracer transport processes at 7 °C and 20 °C, as well as the influence of different tracer dosages on the experimental results, were systematically investigated through flow visualization and stimulus-response experiments. The results showed that, under the 7 °C condition, the upward transport tendency of the pure ink tracer was weakened, the overall flow remained closer to the tundish bottom, the transport speed decreased, and the time required to reach the outlet was significantly prolonged. For the saturated KCl solution tracer, a lower temperature enhanced its transport along the bottom toward the outlet and suppressed its diffusion toward the liquid surface. The RTD results showed that, after the temperature was increased, the curves shifted to the left as a whole, and both the peak time and the mean residence time were shortened. The outflow percentage of tracer results showed that the difference for the 10 mL saturated KCl solution between the 7 °C and 20 °C conditions was the most significant. At 7 °C, the total outflow percentage of the 10 mL salt solution tracer at 1500 s was 76.86%, which was 22.97% lower than that at 20 °C. As the tracer dosage increased, the differences in the transport process, RTD curves, and outflow percentage curves under different temperature conditions gradually decreased, indicating that the effect of dosage on the experimental results gradually became stronger than that of temperature. These results indicate that the combined effects of experimental temperature and tracer dosage cannot be neglected in tundish water model experiments. Full article

Chlorine contact tanks are crucial for wastewater disinfection, with performance strongly influenced by internal hydraulic characteristics. This study applies Computational Fluid Dynamics (CFD) to analyze and improve the hydraulics of the chlorination contact tank in a Wastewater Treatment Plant in the Southern Italy. A three-dimensional transient CFD model was developed using the Reynolds-Averaged Navier–Stokes (RANS) equations with the Renormalized Group (RNG) turbulence closure. The model simulated flow patterns, tracer transport, and chlorine decay kinetics under the existing configuration and two alternative configurations. Conservative tracer pulse simulations enabled the calculation of Residence Time Distributions (RTDs) and hydraulic efficiency indicators, including the Baffling Factor (θ₁₀), Morrill index (Mo), and Aral–Demirel index (AD). A typical contact tanks geometry exhibits specific hydraulic characteristics, including recirculation behind baffles and stagnant zones in sharp corners, which inevitably affects the contact time. The first alternative, namely featuring rounded corners, moderately reduced dead zones, but did not substantially mitigate recirculation. The second alternative, herein called combining rounded corners with perforated baffle walls, substantially improved hydraulic performance, yielding flow patterns closer to plug-flow. RTD peaks were higher and narrower for the modified designs, and hydraulic indices improved, with Mo decreasing by approximately 5%. These hydraulic enhancements are expected to increase disinfection efficiency by providing more uniform chlorine exposure. The results demonstrate that geometric modifications effectively optimize contact tank hydraulics and highlight the role of CFD as a design and retrofit tool for water and wastewater disinfection systems. Full article

This study investigates the impact of equipment geometry on residence time distribution (RTD) using computational fluid dynamics (CFD) methods in a wastewater treatment tank with different configurations of static mixer elements. With growing environmental concerns, optimizing wastewater treatment processes is crucial. Proper mixing in these units can be achieved by optimal placement of static mixer elements such as baffle walls to create circulation zones and increase residence time of the fluid within the control volume. A CFD model of a wastewater treatment tank was developed and validated using experimental RTD data under three distinct mixer configurations.The experimentally validated model was subsequently enhanced by investigating the degradation of methylene blue (MB) during ozonation in the system. The results of the model allowed for the analysis of how tank geometry—specifically, the number and placement of baffles—affects the flow field and MB conversion. RTD was characterized using expectancy and standard deviation of residence time, revealing a link between RTD and MB degradation efficiency. Results showed that constructional parameters significantly influence residence time and mixing efficiency, with a potential 60% increase in expectancy. The model demonstrated high predictive accuracy, ranging from 75% in the worst case to nearly 90% in the best case. Full article

This study aims to use Computational Fluid Dynamics (CFD) analysis to improve inclusion removal efficiency in tundishes used in the steelmaking industry, with the broader goal of promoting more sustainable steel production and supporting circular economy objectives by producing cleaner steel. Inclusions are non-metallic particles, such as alumina, that enter the tundish with the molten steel and travel through it; if not removed, they can exit through the nozzles and adversely affect the mechanical properties of the final product and process yield. An existing tundish design is modified using three passive techniques, including adding a vertical dam, adding a horizontal baffle, and inclining the side walls, to assess their influence on fluid flow behavior and inclusion removal. Residence time distribution (RTD) analysis is employed to evaluate flow characteristics via key metrics such as dead zone and plug flow volume fractions, as well as plug-to-dead and plug-to-mixed flow ratios. In parallel, a discrete phase model (DPM) analysis is conducted to track inclusion trajectories for particles ranging from 5 to 80 μm. Results show that temperature gradients due to heat losses significantly influence flow patterns via buoyancy-driven circulation, changing RTD characteristics. Among the tested modifications, inclining the side walls proves most effective, achieving average inclusion removal improvements of 8% (Case B1) and 19% (Case B2), albeit with increased heat loss due to greater top surface exposure. Vertical dam and horizontal baffle, despite showing favorable RTD metrics, generally reduce the inclusion removal rate, highlighting a disconnect between RTD-based predictions and DPM-based outcomes. These findings demonstrate the limitations of relying solely on RTD metrics for evaluating tundish performance and suggest that DPM analysis is essential for a more accurate assessment of inclusion removal capability. Full article

Stirred tanks are widely used in polymerization processes, where the residence time distribution (RTD) significantly affects monomer conversion and polymer quality. In this study, the RTD in the stirred tank with both constant and variable viscosity fluids was investigated numerically. To account for the viscosity evolution during polymerization, a model relating fluid viscosity to the mean age of the fluid was developed. After verifying mesh and time step independence, the effects of impeller speed, fluid space time, and viscosity varying on RTD were examined in both single-tank and two-tank configurations. Compared to the constant-viscosity fluids, the variable-viscosity fluid shows different flow behaviors such as dead zones and short-circuiting. Analysis based on the number of tanks in series showed that increasing impeller speed and extending space time can enhance mixing efficiency, where the improved mixing in the second stage of the two-tank configuration eliminated the concentration fluctuations caused by recirculating flow in the first tank, which may result in a more uniform RTD curves. Full article

Random packed beds (RPBs) of various particles are widely used in chemical reactors to enhance the contact between the reactants or the catalyst. This numerical study investigates the prospects of using a helical flow deflector spanning the whole cross-section of the reactor and the height of the random packing to control residence time distribution (RTD) in RPBs of spherical particles. The packed bed geometry is generated via sequential particle deposition, while flow equations are solved for the real geometry of the packing without additional modelling terms. The results demonstrate that in laminar conditions the flow deflector significantly increases flow tortuosity and residence time (even a few times for small helix pitches) when the effective velocity in the RPB is kept fixed. The relationship between the helix pitch and tortuosity, pressure drop, and RTD is quantified, revealing that residence time scale similarly to tortuosity while the increase in pressure drop is more pronounced. The study provides a validated framework for optimising helical deflector designs in RPBs (at least in the laminar regime), with implications for reactor efficiency. Full article

Groundwater flow systems (GFSs) and associated distribution of travel times provide critical insight into the regional subsurface hydrology, especially in arid regions experiencing intensive groundwater use. This study examines the impact of large-scale irrigation pumping on GFS patterns in the arid Kongqi River Basin, China. A three-dimensional (3D) steady-state groundwater flow model was constructed using MODFLOW, and flow paths were delineated through particle tracking to quantify travel time and residence time distributions. Two scenarios with and without pumping were compared. Results show that groundwater abstraction significantly alters GFS patterns, lowering water tables in pumping zones while raising them in irrigation areas fed by surface water. This hydrologic redistribution fragments recharge and discharge zones, particularly under the influence of evapotranspiration (ET) from shallow groundwater. Simulated travel times range up to ~506 ka, with median values decreasing from 9.7 ka (no-pumping) to 8.3 ka (pumping). Both travel time distribution (TTD) and residence time distribution (RTD) exhibit power-law characteristics, reflecting the dominance of slow flow paths in deep GFSs. While the modeling results provide valuable insight into current regional groundwater flow, it does not account for transient flow effects and hydrodynamic dispersion of solutions. Future research should incorporate groundwater isotope data to validate the model and assess time-dependent changes in GFSs. Full article

Background/Objectives: In the twin-screw wet granulation (TSWG) process, accurate measurement of residence time distribution (RTD) is critical, as it characterizes material transport kinetics and mixing behavior. It plays a critical role in evaluating the homogeneity and stability of the granulation process and optimizing process parameters. It is necessary to overcome the limitations arising from the complex and time-consuming procedures of conventional RTD determination methods. Methods: This study proposes a new RTD detection method based on image processing. It uses black dye as a tracer to obtain RTD curve data, and the effects of process parameters such as tracer dosage, screw speed, and feeding rate on the RTD were investigated. Results: The results show that the established method can accurately determine RTD and that the tracer dosage has no significant effect on the detection results. Further analysis revealed that the screw speed is negatively correlated with the mean residence time (MRT). As the speed increases, not only does the MRT shorten, but its distribution also decreases. Similarly, an increase in the feeding rate also leads to a decrease in the MRT and distribution, but it is worth noting that lower feeding rates are beneficial for achieving a state close to mixed flow, while excessively high feeding rates are not conducive to sufficient mixing of materials in the extruder. Conclusions: The RTD detection method provides a reliable parameter basis and theoretical guidance for the in-depth study of the TSWG process and the development of quality control strategies. Full article

The behavior of molten steel within a tundish plays a crucial role in achieving uniform temperature and chemical composition, enhancing the removal efficiency of non-metallic inclusions, and reducing the wear of refractory linings. These aspects are key for ensuring the production of steel with superior quality. In multi-strand delta-type tundishes, such as the six-strand configuration, flow dynamics become particularly challenging. Key considerations include strand-specific residence times, the uniform distribution of steel flow, and the mitigation of refractory degradation. This paper presents a detailed numerical analysis aimed at designing an optimally shaped impact pad. The effectiveness of each proposed design was assessed through a tracer-based visualization of flow behavior and the evaluation of residence time distribution (RTD) curves. RTD curves were created in isothermal conditions, while the calculations of the temperature fields of steel in the tundish were made in non-isothermal conditions. The results of the simulations were verified by a real plant trial test and indicate that the use of the “SPHERIC-K4” impact pad can greatly enhance the flow characteristics of liquid steel during the continuous casting process. These improvements include preventing the erosion of the tundish refractory lining, improving the distribution of residence times between individual casting strands, and adjusting the proportions of the mixing zones. Full article

Constructed wetlands (CWs) are a sustainable, nature-based solution for wastewater treatment, where pollutants are removed through contact with microorganisms attached to substrates and plant roots. Efficient hydraulic performance is critical for CWs, since poor hydraulic performance can reduce treatment efficiency by altering the actual residence time relative to the design value. Two methods to evaluate the Residence Time Distribution (RTD) within the CW system are the tracer method and numerical modelling. This study provides a comprehensive review of experimental methodologies and numerical models used to investigate hydraulic processes in CWs, outlining available techniques to assist researchers in selecting the most suitable approach based on their research needs and wetland characteristics. For experimental procedures, this review focuses on the selection of tracers, indicators for hydraulic performance assessment, and water quality responses to changing hydrological conditions. The advantages and disadvantages of existing numerical models, their suitability, and future research direction are also discussed. Understanding these methodologies and their application is crucial for advancing our knowledge of the hydraulic features of CWs and improving their design and operation. Ultimately, improving hydraulic performance through appropriate experimental and modelling techniques supports the sustainable development and operation of CW systems for long-term wastewater treatment applications. Full article

The increasing demand for critical metals essential for renewable energy technologies necessitates efficient and environmentally sustainable extraction methods. Ilmenite (FeTiO₃) and similar ore deposits serve as abundant sources of primary elements while also incorporating a suite of strategically significant trace elements, including REEs and Hf, among others. Mixer–settler units are extensively utilized in metal purification processes. It is important to develop approaches for tracking the metal’s extraction process online and optimizing flow dynamics. One widely adopted technique for evaluating the flow dynamics of the various components is the residence time distribution (RTD) measurement, which provides insights into the hydrodynamic behavior of process reactors. This study explored the application of radiotracer techniques for online monitoring of solvent extraction processes in hydrometallurgy, focusing on Hf recovery. A mixer–settler system was employed using di(2-ethylhexyl) phosphoric acid (D2EHPA) as the extractant and the 1M HNO₃ aqueous phase of Ti ore. The radiotracer ¹⁸¹Hf was synthesized via neutron activation and introduced into the system to track phase distribution and RTD. Real-time monitoring revealed over 95% extraction efficiency within 133 min (8000 s). The RTD studies validated system performance using perfect mixers in series and axial dispersion models. The calculated mean residence time of 100 min (6000 s) closely aligned with the theoretical 104 min (6240 s), confirming the model accuracy. The findings demonstrate the viability of radiotracers in monitoring solvent extraction, offering real-time insights into flow dynamics and extraction efficiency. Full article

To increase the calculation speed of the computational fluid dynamics (CFD)-based simulation for the gas–solid flow in fluidized beds, a Markov chain model (MCM) was developed to simulate the particle movement in a two-dimensional (2D) circulating fluidized bed (CFB) riser. As a statistic model, the MCM takes the results obtained from a CFD–discrete element method (DEM) as samples for calculating transition probability matrixes of particle movement. The transition probability matrixes can be directly used to describe the macroscopic regularities of particle movement and further used to simulate the particle motion combined with the Monte Carlo method. Particle distribution snapshots, residence time distribution (RTD), and mixing obtained from both MCM and CFD-DEM are compared. The results indicate that the MCM offers a computational speed that is approximately 100 times faster than that of the CFD-DEM. The discrepancy in the mean particle residence time, as computed by the two models, is under 2%. Furthermore, the MCM provides an accurate depiction of time-averaged particle motion. In sum, the MCM can well describe the time-averaged particle mixing compared to the CFD-DEM. Full article

The study focuses on the four-strand tundish as the research object, aiming at the phenomenon of fewer strand casting (stable blockage) and sudden blockage of the tundish in industrial production. Numerical simulation methods are employed to compare the velocity vectors, flow fields, residence time distribution (RTD) curves, and outflow percentage curves under stable blockage and sudden blockage of the tundishes with a double-weir structure, U-shaped weir structure, and U-shaped weir structure with holes in the front. The results indicate that, after sudden blockage of the tundish strands, the flow field transitions from an unstable four-strand flow field to a stable three-strand flow field. Both the double-weir tundish and the U-shaped weir tundish reach a stable state after 200 s, while the U-shaped weir tundish with holes in the front reaches stability after 150 s. Additionally, compared to other structures, the tundish strands of the U-shaped weir with holes in the front are less affected by blockage, showing better consistency among strands and better adaptability under non-standard casting conditions. Full article

This paper aims to provide an overview of the fundamentals, development, and evolution of residence time distribution (RTD) methodology and its applications to the flow and mixing of fluids (and solid particles) modeling in different systems. A concise literature analysis is followed by a succinct presentation of RTD methodology’s experimental and theoretical foundations and RTD-based mathematical modeling, highlighting its importance. An experimental demonstration of RTD diagnostics on a photochemical reactor is performed to identify the most practical locations for the inlet/outlet pipes (axial or radial) and the photochemical reactor’s ideal working posture (horizontal, vertical, or inclined) and to understand the level of mixing and to determine the fluid flow defects. Using the relevant RTD functions and the corresponding central moments, it was possible to show that short circuits and dead zones occurred in each of the six considered reactor configurations. Following these investigations, design solutions were proposed to achieve a convenient exposure time, proper mixing, and uniform irradiation inside the reactor. Full article