Residence Time Distribution Measurements Research Articles

Determination of Flow Structure in a Gold Leaching Tank by CFD Simulation

Experimental residence time distribution (RTD) measurement and computational fluid dynamics (CFD) simulation are the best methods to study the hydrodynamics of process flow systems. However, CFD approach leads to better understanding of the flow structure and extent of mixing in stirred tanks. In the present study, CFD models were used to simulate the flow in an industrial gold leaching tank. The objective of the investigation was to characterize the flowfield generated within the tank after process intensification. The flow was simulated using an Eulerian-Eulerian multi-fluid model where the RANS standard kmixture model and a multiple reference frame approach were used to model turbulence and impeller rotation respectively. The simulated flowfield was found to be in agreement with the flow pattern of pitched blade axial-flow impellers that was used for mixing. The leaching tank exhibited good “off-bottom suspension” which reveals minimum deposition of gold ore particles on the bottom of the leaching tanks. Simulation results were consistent with experimental results obtained from a radioactive tracer investigation. CFD approach gave a better description of the flow structure and extent of mixing in a leaching tank. Hence it could be a preferred approach for flow system analysis where the cost of experimentation is high.

The characterization of the residence time distribution in a magnetic mixer by means of the information entropy

Mixing behavior in a magnetic mixer is investigated using an experimental approach. The concept of the residence time distribution (RTD) was applied to characterize a mixing process under the action of a rotating magnetic field (RMF). The main idea of the proposed approach is the utilization of experimental RTD measurements to determine the information entropy. This paper discusses a method to compute an information degree of mixing as a measure of mixing performance for a continuous flow system. The quality of mixing in the tested equipment is analyzed by means of a proposed mixing capacity. Different operating conditions with respect to liquid flow rates and a magnetic field strength are used to study corresponding effects on a recommended parameter. It is observed that mixing capacity increases with increasing magnetic field strength. This mixing parameter is analytically described as a function of operating flow rates and the applied magnetic field.

Modeling of residence time distribution in regrinding Vertimill

The regrinding stage is necessary to achieve the particle size (liberation) for the final upgrade in the cleaning flotation process. At present, vertical stirred mills are increasingly employed at industrial scale. The regrinding process characterization requires mixing regime knowledge along with the mean residence time of the liquid and solid phases. In this work, residence time distribution (RTD) measurements were performed in two different industrial flotation plants. Both plants operate with Vertimills of 1000–1500HP in the regrinding process with different circuit arrangements according to the plant treatment capacity. The RTD was measured using the radioactive tracer technique, which allows for non-invasive tracer detection in real time. It was found that mixing was not perfect in agitated Vertimills and consequently the RTD was better modeled by the more flexible Large and Small Tanks in Series (LSTS) model which allowed the characterization of the different mixing regimes for both liquid and solids in the regrinding mills. Effective mean residence times in the range of 1.7–12.5min were obtained with RTD shapes similar to those obtained in tumbling ball mills. It was found that residence times of liquid and solid particles were similar. RTD estimations are useful for better understanding the Vertimill behavior and to identify the actual breakage rates, as well as for improving Vertimill modeling and simulation.

PEMFC Cathode Contamination Mechanisms for Several VOCs - Acetonitrile, Acetylene, Bromomethane, Iso-Propanol, Methyl Methacrylate, Naphthalene and Propene

The behavior of PEMFCs exposed to airborne contaminants was characterized in 2 stages. In the first stage, impedance spectroscopy was used to identify resistance loss types. In the second stage, other ex situ and in situ characterization methods were employed to obtain more detailed information about observed resistance loss types: rotating ring/disc electrode, membrane conductivity cell, tracer based method for residence time distribution measurements, segmented cell, and fuel cell coupled with a gas chromatograph/mass spectrograph. All 7 VOCs led to kinetic and mass transfer resistance changes. Only acetonitrile led to an ohmic resistance change. A decrease in catalyst active area (7 to >90 %) and increase in H2O2 side reaction product (18 to 1027 %) accounts for the smaller oxygen reduction current (12 to 100 %). An acetonitrile decomposition product is presumed to be responsible for the observed in situ ohmic loss because acetonitrile did not show an effect in the conductivity cell. Modest current redistributions were noted (<19 % on an absolute and dimensionless basis) but acetylene also initially showed a strong transient effect (≤100 % on an absolute and dimensionless basis) attributed to a gradual contamination along the flow field length. Only the unaffected contaminant species, CO2 and CO were detected at the cathode outlet. Contaminant conversion increased at higher cell voltages (62 to 332 % V-1) indicative of electrochemical oxidation. An acetylene contamination mechanism is proposed.

Residence time distribution measurements in a two dimensional rectangular airlift reactor by digital image processing

In this work, a novel, relatively simple and accurate, as well as economically attractive technique based on image processing was introduced for determining Residence Time Distribution (RTD) curve in an airlift reactor. For this purpose, an impulse-response technique by injecting colored tracer was used in a two dimensional rectangular glass reactor with a working volume of 2400cm3. Tap water and air were used as the operating fluids. The captured images at specified time intervals of 1.5s were analyzed by the “ImageJ” software. The RTD data obtained by this new digital image processing procedure were compared with data obtained by classical colorimetric technique. The influence of different superficial gas velocities was investigated on hydrodynamic of mixing. The results showed that this quick-inexpensive technique yields to reliable data. This technique avoids the expensive probes and data acquisition systems that are imperative in other measuring techniques.

Measurement of residence time distribution in a rotary calciner

Rotary calcination is widely used in catalyst manufacturing and many other industrial processes. In this article, the influence of operational variables and material properties on the mean residence time (MRT), hold up, and axial dispersion was investigated in a pilot plant rotary calciner. Residence time distributions (RTD) of spherical, cylindrical, and quadrilobe catalyst particles were measured and contrasted. The Saeman's model was successfully applied to predict the experimental bed depth and the MRT as particles flowed through the calciner. It was observed that increasing the feed rate did not significantly affect the MRT. Results for the different particles indicated that cylinders and quadrulobes exhibited less axial dispersion than spheres due to the decreased flowability. A reliable method was developed to provide a reasonable RTD prediction in rotary calcination systems. © 2013 American Institute of Chemical Engineers AIChE J, 59: 4068–4076, 2013

Experimental study and mathematical modeling of the residence time distribution in magnetic mixer

This study reports on research results in the field of a mixing process under the action of a transverse rotating magnetic field (TRMF). The main objective of this paper is to present the effect of this type of a magnetic field on residence time distribution (RTD) measurements. This paper evaluates the performance of a magnetic mixer by comparing the results of an experimental investigations in a pilot set-up and theoretical values obtained from mathematical model. This model consisting of the set of ideal continuous stirred tank reactors (CSTR) fitted well the experimental data.

The effect of baffle spacing on hydrodynamics and solute transport in serpentine contact tanks

The effect of baffle spacing on the hydrodynamics and transport characteristics in a scaled variable-baffle-spacing contact tank is investigated by employing large-eddy simulation. Simulated solute transport is quantitatively validated by comparing simulated and measured residence time distribution curves, for which good agreement is found. The flow in serpentine disinfection tanks is dominated by a path of high streamwise velocities, a so-called short-circuiting path, which is almost constant in width regardless of the baffle spacing. In addition, large-scale turbulent vortices are shed from baffle edges and their size and dynamics depend on the spacing between subsequent baffles. The transport in serpentine-flow contact tanks is advection dominated and transport occurs mainly along the short-circuiting path. Wider compartments feature large recirculation zones with instantaneous vortices entraining solutes into these zones and thereby increasing residence times. Contact tanks with wide compartments suffer from severe short-circuiting and internal recirculation. This is directly reflected in hydraulic efficiency indicators.

Hydrodynamics of gas–liquid micro-fixed beds – Measurement approaches and technical challenges

Despite many areas that are open to investigation in the hydrodynamic study of micro-fixed bed reactors, conducting research in this field is mostly hampered by a number of experimental challenges that have made many attempts ineffectual. This work provides a summary of the technical challenges, problems and misconstrues one might encounter in performing hydrodynamic experiments on micro-fixed bed reactors. Some of these issues will be pointed out upon comparing classical residence time distribution (RTD) measurements through electrical conductivity probes at micro-fixed bed scale with near-wall RTD obtained via visualizations of dye-tracer elution and monitoring the changes in gray level intensity of the images. Laterally-averaged gray level intensity at both upstream and downstream extremities of the wall regions of interest acted as inlet and outlet curves for the Aris method. The major outcome of this work was experimental confirmation of theoretically predicted maximum liquid velocity in the high porosity zone close to the wall. Finally, the experimental results on pressure drop and liquid holdup obtained upon following the right experimental protocols are presented.

Residence time distribution in a single‐phase rotor–stator spinning disk reactor

A reactor model for the single‐phase rotor–stator spinning disk reactor based on residence time distribution measurements is described. For the experimental validation of the model, the axial clearance between the rotor and both stators is varied from 1.0 × 10−3 to 3.0 × 10−3 m, the rotational disk speed is varied from 50 to 2000 RPM, and the volumetric flow rate is varied from 7.5 × 10−6 to 22.5 × 10−6 m3 s−1. Tracer injection experiments show that the residence time distribution can be described by a plug flow model in combination with 2–3 ideally stirred tanks‐in‐series. The resulting reactor model is explained with the effect of turbulence, the formation of Von Kármán and Bödewadt boundary layers, and the effect of the volumetric flow rate. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2686–2693, 2013

Development of an oil tracer labelled with137mBa

The extraction of barium ions by isooctane solutions of di-nonyl naphthalene sulfonic acid (HDNNS) and di-cyclohexano-18-crown-6 (DC18C6) from aqueous solutions of hydrochloric acid and sodium chloride has been investigated. The extraction and the associated back extraction results suggest that this barium complex is a suitable oil tracer for petroleum related applications, such as measurement of Residence Time Distribution (RTD) and oil carry-over to the gas outlet in multiphase separators and scrubbers.

Residence time distribution measurements and simulation of the flow pattern in a scraped surface heat exchanger during crystallisation of ice cream

Scraped surface heat exchangers are used in the food industry to process highly viscous fluids, and ice-cream in particular, but most of the time, the influence of operating conditions on product quality is poorly understood. The objective of the study is to develop simple tools to help industrials understand, and then optimise their fabrication process. Residence Time Distribution (RTD) has been characterised in an industrial pilot system during real ice cream production, after the method had been validated in an experimental set-up with a simple mixture of water and sucrose. It has been shown that in its dimensionless form, RTD depend slightly on flow rate and scraper rotational speed. A simple model of flow pattern applicable to SSHE during crystallisation was developed to reproduce the observed RTD. It distinguishes two zones: the volume of fluid near the cooling wall where ice is generated and which is swept by the blades and the volume of fluid near to the rotor. Therefore, the model considers two parallel plug flow reactors with axial dispersion, and which exchange fluid by radial mixing. After adjustment of the model parameters, a good agreement was obtained with experimental results. The flow rate is lower in the zone near the cooling wall; this can be due to a higher ice concentration leading to higher viscosity. This approach can contribute to better understand, optimise and control SSHE used for ice cream production.

Online conductivity measurement of residence time distribution of thin film flow in the spinning disc reactor

Spinning disc reactors provide effective continuous flow mixing in highly sheared liquid films flowing under the action of high centrifugal forces generated as a result of the disc rotation. Although the flow is theoretically characterised to be laminar on the basis of the low Reynolds numbers, the film surface is normally covered with numerous ripples which can change the film hydrodynamics and velocity distribution and thereby affect its residence time distribution (RTD) on the rotating disc. The aim of this study is to determine the experimental conditions for which near plug flow behaviour prevails on the spinning disc. Our findings indicate that the higher the disc speed and liquid flow rate and the lower the viscosity of the processing liquid, the closer conditions within the liquid film on the disc approximate to plug flow behaviour. These effects are attributed to the greater degree of turbulence induced in the liquid film as a result of an increased intensity of surface waves, promoting transverse mixing across the film thickness and thus a more uniform velocity profile at any given radial position. The centrifugal force directing the liquid radially to the disc periphery may also play a part in reducing radial dispersion. The texture of the disc surface is also an influential parameter in achieving plug flow on the rotating disc. With flow on a grooved disc, the number of tanks-in-series increased quite significantly under identical operating conditions as those for a smooth disc, confirming that radial dispersion is lowered. One of the explanations for this effect is that the constantly changing topography enabled by the series of concentric grooves across the disc surface offers the opportunity for repeated film detachment and re-attachment which cause induced recirculation or vortices within the film, giving better transverse mixing and eliminating velocity gradients.

Power density spectrum for the identification of residence time distribution signals

One of the most important applications of radioisotopes in industry is the residence time distribution (RTD) measurement. RTD can be used for optimizing the design of industrial systems and determining their malfunctions. The RTD signal may be subject to different sorts of noise. This leads to errors in the RTD calculations, and hence leads to wrong analysis in the determination of system malfunctions. This paper presents a proposed approach for RTD signal identification based on power density spectrum (PDS). The cepstral features are extracted from the signal or/and its PDS. The PDS is estimated using nonparametric, parametric, and eigen-analysis methods. The identification results are analyzed and compared for different estimation methods in order to select the best PDS estimation method for RTD signal identification. Neural networks are used for training and testing in the proposed approach. The proposed approach is tested using RTD signals obtained from the measurements carried out with radiotracer technique. The experimental results show that the proposed approach with features extracted from the PDS of the RTD signals calculated using eigen-analysis methods is the most robust and reliable in RTD signal identification.

Preparation and characterization of agarose–nickel nanoporous composite particles customized for liquid expanded bed adsorption

Agarose–nickel nanoporous composite matrices with a series of densities, named Ag–Ni, were prepared herein for expanded bed adsorption of nanobioproduct/bioproduct by a water-in-oil emulsification method. The optical microscope (OM), scanning electronic microscope (SEM) and particle size analyzer (PSA) were utilized in order to characterize the structure and morphology of the agarose–nickel composite. The results indicated that the matrices prepared had a spherical appearance, appropriate wet density of 1.73–2.56g/ml, water content of 32.2–58.5% and porosity of 79.4–96.37% and pore size of about 100–150nm. All the Ag–Ni beads follow logarithmic normal size distribution with the range of 60–230μm and average diameter of 133.68–148.4μm. One of the useful properties of the Ag–Ni particles is the high wet density up to 2.56g/ml, which shows a potential for the operation in an expanded bed at high flow rate. The impact of nickel powder addition on the physical and hydrodynamic properties was also investigated. In addition, the fluidization behavior of the Ag–Ni particles under various conditions was characterized by the measurement of bed expansion and axial dispersion coefficients for the liquid phase when operated in a standard fluidized bed contactor. It was observed that the expansion factors were decreased with the increasing matrix density under the same velocity. The bed expansion and fluid velocity were correlated with Richardson–Zaki equation for all particles prepared and the correlation parameters (the terminal settling velocity Ut and expansion index n) were investigated. Using measurements of residence time distributions, hydrodynamic properties in the expanded beds were investigated and were compared with reported matrices in other literatures. In addition, the impact of the flow velocity, bed expansion degree and density of adsorbent on hydrodynamic properties in the expanded beds were investigated. The results indicated that the expansion factor showed little effect on the hydrodynamic properties while the fluid velocity was the most essential factor on this regard. Furthermore, the results indicated that the heavy matrices of Ag–Ni-3, Ag–Ni-4 and Ag–Ni-5, were more suited for high operation fluid velocity. In addition, even the light matrices, i.e. Ag–Ni-1 and Ag–Ni-2, seem to be superior to other matrices in hydrodynamic properties, which made them promising adsorbents for further use in EBA processes.

Liquid phase axial mixing in solid–liquid circulating multistage fluidized bed: CFD modeling and RTD measurements

Liquid phase residence time distribution (RTD) studies have been performed in conventional solid–liquid fluidized bed (SLFB) and solid–liquid circulating multistage fluidized bed (SLCMFB). The riser column was made up of 50mm i.d. and 2m long glass pipe while the multistage down comer column (glass) consisted of seven stages of 100mm i.d. and 100mm long sections each having perforated plate as a distributor (having 480 holes of 2mm diameter). RTD experiments for SLFB were carried out in the column having the same diameter as the downcomer of SLCMFB. RTD has been estimated for both the riser column and the multistage column of SLCMFB. Computational fluid dynamic (CFD) simulations of SLFB and riser section of SLCMFB have been performed to predict the RTD. In all the above cases good agreement was found between the CFD predictions and the experimental measurements. Ion exchange resins and glass beads were used as a solid phase and water as a fluidizing medium. The dispersion characteristics of SLFB and SLCMFB have been investigated for resin particles with size range of 0.36–0.72mm and glass beads with size range of 0.1–0.7mm. It was observed that the liquid phase axial dispersion coefficient depends strongly on superficial liquid velocity, particle size and particle density. Based on the experimental data, empirical correlations have been proposed for liquid phase axial dispersion coefficient and have been found to be applicable to all the available data in the published literature.

Orifice microreactor for the production of an organic peroxide – non-reactive and reactive characterization

In this article, the transfer of a two-step, biphasic, and exothermic peroxide synthesis into a microreactor assisted process is discussed as well as the non-reactive and reactive characterization of the developed orifice microreactor. Residence time distribution measurements showed nearly ideal plug-flow behaviour. The Bodenstein number at a flow rate of 21 mL min−1 is 180 and the corresponding cell number is 90, indicating a narrow residence time distribution. The determined residence times at two different flow rates are in good agreement with the theoretical values of 3.2 s and 1.5 s. The influence of flow rate on droplet size distribution is discussed as well as the influence of orifice geometry on the resulting energy density. These measurements showed a very small droplet size distribution over a wide range of flow rates applied. The smallest mean droplet size of 7 μm was obtained for a flow rate of 75 mL min−1. It is shown that a change from baffle type to conical orifices allows increasing of the throughput by keeping the homogenizing pressure similar to the baffle type system at a lower throughput. The measurement of the temperature profile on top of the thin reaction plate, covering the reaction channel is possible due to a special design of the orifice microreactor and enables monitoring the heat production under reactive conditions. A benchmark based on the product output of an industrial semi-continuous process points out the potential of micro process technology to intensify existing processes. On the examples of four fictive production scale microreactor assisted processes, it is shown that the footprint as well as the reaction volume can significantly be reduced. Using an orifice microreactor of the size of a shoebox the calculated space–time-yield for a product output of ca. 196 kg L−1 h−1 is 905 kg L−1 h−1. This is orders of magnitude higher than for the industrial semi-continuous process.

Reduction of Tundish Skull and Yield Improvement in Steel Plants Through Physical Modeling of Steelmaking Tundish Systems

New types of furniture (also termed as flow modifiers or baffles) were incorporated in industrial scale, slab and bloom casting tundish systems with an aim to reduce residual metal loss (i.e., tundish skull) at the end of sequence casting. To this end, water model experiments were carried out in which, slag vortexing phenomena during emptying of tundish was studied embodying different types of furniture into existing tundish designs. These in general indicate that a wedge shaped bottom together with an embedded pouring box applied in conjunction have the potential to reduce tundish skull and improve yield losses significantly. In addition, limited residence time distribution measurement experiments were made to investigate metallurgical performance of modified design tundish systems. These indicate that deployment of new furniture with minor design modifications, despite contributing to a reduction in tundish capacity (10–12%), do not influence metallurgical performance of steelmaking tundish systems to any significant extent. Accordingly, designs of currently employed slab (32 and 37 tonnes capacity respectively) and bloom casting tundish (10 and a 17 tonnes capacity respectively) systems were modified in four different steel mills and plant trials conducted to assess the extent of yield improvement. Significant improvements in yield losses, to the extent of 50–60%, have been confirmed by the industry during sequence casting.

Radiotracer investigation in a rotary fluidized bioreactor

A rotary fluidized bioreactor (RFBR) designed for treatment of wastewater was required to be investigated for its hydrodynamic behaviour and validation of design. A radiotracer investigation was carried out to measure residence time distribution (RTD) of wastewater in the RFBR using 82Br as a radiotracer. The radiotracer was instantaneously injected into the inlet feed line and monitored at the inlet and outlet of the reactor using collimated scintillation detectors connected to a data acquisition system. The measured RTD data was treated and simulated using a tanks-in-series model and model parameters i.e. number of tanks describing the degree of mixing was obtained. The results of the investigation showed no flow abnormalities and the reactor behaved as an ideal continuously stirred-tank reactor at all the operating conditions. Based on the results, the design of the reactor was validated.

Radiotracer investigation in gold leaching tanks

Measurement and analysis of residence time distribution (RTD) is a classical method to investigate performance of chemical reactors. In the present investigation, the radioactive tracer technique was used to measure the RTD of aqueous phase in a series of gold leaching tanks at the Damang gold processing plant in Ghana. The objective of the investigation was to measure the effective volume of each tank and validate the design data after recent process intensification or revamping of the plant. I-131 was used as a radioactive tracer and was instantaneously injected into the feed stream of the first tank and monitored at the outlet of different tanks. Both sampling and online measurement methods were used to monitor the tracer concentration. The results of measurements indicated that both the methods provided identical RTD curves. The mean residence time (MRT) and effective volume of each tank was estimated. The tanks-in-series model with exchange between active and stagnant volume was used and found suitable to describe the flow structure of aqueous phase in the tanks. The estimated effective volume of the tanks and high degree of mixing in tanks could validate the design data and confirmed the expectation of the plant engineer after intensification of the process.