Experimental Residence Time Distribution Curves Research Articles

Effect of lifter shape and operating parameters on the flow of materials in a pilot rotary kiln: Part IV. Improvement of the hydrodynamics-related dimensionless correlations

Non-spherical particles are widely processed in industrial-scale rotary kilns. There is nonetheless a lack of simple tools that make it possible to predict their hydrodynamic behavior inside rotary kilns. This article thus focuses on the hydrodynamics of crozets flowing inside a pilot-scale rotary kiln. Thirty-eight experiments were conducted with this product. Experimental Residence Time Distribution curves were successfully obtained with a tracer pulse methodology using painted crozets. Both the experimental mean residence time and the axial dispersion coefficient were obtained by fitting the analytical expression of E(t) for the open-open boundary condition with the experimental E(t) curves. The hydrodynamic behavior observed was qualitatively in agreement with the literature. Our dimensionless models were then re-identified with all of the experimental data and simplified through statistical analysis. This study provides more robust dimensionless hydrodynamic models that can be used to reliably predict the behavior of non-spherical particle flow inside an industrial kiln.

Residence time distribution and solids mixing of sawdust in a horizontal pulsed fluidized bed

A prototype horizontal pulsed fluidized bed (HPFB) has been built for continuous biomass torrefaction in our lab. Solids residence time distribution (RTD) of hemlock sawdust in the HPFB reactor was measured to study particle transport and mixing using a pulse injection of particle tracer. Effects of weir height, gas pulsation frequency, solids feed rate and gas velocity were investigated. A modified axial dispersion model was adopted to fit the experimental RTD curves, where horizontal dispersion coefficients were obtained. It is found that horizontal dispersion coefficients were slightly higher in the deeper bed. In the shallow bed, horizontal dispersion coefficients increased with increasing solids feed rate. The solids mixing intensity also found to increase with increasing the gas velocity. Horizontal solids dispersion coefficients obtained in the HPFB were significantly smaller than values calculated by literature correlations. A correlation in the shallow HPFB was established based on our experimental data.

Modelling of flow distribution within spacer-filled channels fed by dividing manifolds as found in stacks for membrane-based technologies

Manifolds are critical to the performance of different membrane-based technologies. These stack components distribute the inlet and outlet flow with the aim of providing a homogenous flow pattern inside the spacer-filled channels. This work analyzes the role of manifolds on the flow pattern within the channels of membrane stacks used primarily for electrodialysis and acid-base flow batteries. First, residence time distribution (RTD) curves are predicted for a spacer-filled channel by means of three strategies: (a) computational fluid dynamics (CFD) simulation using a periodic unit cell model, (b) parametric models: axial dispersion model (ADM) and the plug dispersion exchange (PED) model using Danckwerts-type boundary conditions, and (c) the ADM and PED model using the segregate flow laminar model (SFLM) as flow pattern at the left-hand side (z=0-) in the Danckwerts-type inlet boundary condition. In contrast to previous studies, it is shown that the dividing manifold generates experimental RTD curves with several peaks characteristic of uneven distribution and long-tailing phenomena with small fluctuations related to the woven spacer and the combining manifold, respectively. The periodic unit cell CFD model, the ADM and PED model with convectional Danckwerts-type boundary conditions do not predict the RTD experimental curves because they do not consider the effect of RTD curves for the manifolds. On the other hand, the SFLM described these RTD curves adequately, as obtained by CFD simulations; this model used the left-hand side of Danckwerts inlet boundary condition for the PDE model, which had a good agreement to the global RTD experimental curves and their peaks. This study demonstrates that the global RTD curve of electrolytes in rectangular, spacer-filled channels must consider the effect of dividing and combining manifolds as inlet/outlet distributors. The strategy developed here can be used to design and scale-up membrane stacks for industrial and energy conversion applications.

A novel technique for residence time distribution (RTD) measurements in solids unit operations

This work presents a novel technique with fast response for Residence Time Distribution (RTD) measurements in gas-solid unit operations (e.g., fluidized bed reactors). This technique is based on an optical method which eliminates the requirement of knowing the velocity and concentration profiles at the exit section of the system. Experiments were carried out with SiC particles and a phosphorescent pigment used as a tracer. A concentration measurement system was developed to measure the tracer concentration in SiC/pigment mixtures. The corresponding pigment concentrations were evaluated at the bottom of this system using a photomultiplier. The pigment concentration was derived from the integral of the signal intensity received by the photomultiplier. Then, a calibration curve was established which provided the empirical relationship between the integral and pigment concentration. In order to validate this RTD measurement technique, a series of experiments was performed in a bubbling fluidized bed and the effect of the bed height was studied. It was shown that the experimental RTD curves were in good agreement with the theoretical RTD of bubbling fluidized beds. This solids RTD measurement technique can be used to provide a better understanding of the hydrodynamics of complex solids unit operations.

Modelling and simulation of H2-H2O bubbly flow through a stack of three cells in a pre-pilot filter press electrocoagulation reactor

Computational fluid dynamics simulations were carried out to describe the hydrodynamic characteristics of a two-phase bubbly flow in a filter-press flow reactor stack of three cells, which is typically used in electrocoagulation (EC). The hydrogen evolution reaction (HER) took place at the cathode; dissolution of aluminium occurred at the anode. The fundamental transport equations of momentum and electrical potential were simultaneously solved to simulate the H2-H2O flow. Continuous (H2O) and dispersed phase (H2) velocity fields were modelled via the Euler-Eulerian approach, using the biphasic Reynolds Averaged Navier-Stokes (RANS) equations and the standard k − ε turbulence model. The influence of volumetric flow rate (1.7 ≤ Q ≤ 15 cm3 s−1) and applied current density (–28 ≤ j ≤ –5 mA cm−2) was systematically addressed to calculate the fraction of dispersed phase and current distribution along the electrodes. The evolved H2 bubbles were transported away from the electrode by the liquid flow. The dispersion of H2 through the electrode gap showed a modest bubble curtain profile due to the liquid flow rate. A homogeneous current distribution along the electrode length was experienced due to the geometrical design of the electrochemical cell and the low degree of H2 dispersion. The velocity profiles of the H2-H2O mixture were different in each cell due to the change of flow direction. H2 bubbles increased the velocity of the liquid phase but the gas fraction of such bubbles resulted in a higher pressure drop. Good agreement between theoretical and experimental residence time distribution curves was achieved; the experimental aluminium dose released by the anode agreed well with the simulations.

CFD modeling of residence time distribution and experimental validation in a redox flow battery using free and porous flow

Abstract The successful use of clean energy sources will rest, in part, on the availability of efficient energy storage systems. Thus, the present work examines two critical factors (flow patterns and residence time distribution, RTD) affecting the performance of one of the most promising energy storage systems: redox flow batteries (RFBs). The goal is to develop a methodology for analyzing the hydrodynamic and mass transport behavior within a commercial redox flow battery (CRFB). This methodology includes modeling of the flow behavior as well as its experimental validation with RTD. The CRFB consisted of a square geometry with an interdigitated flow field design. Experimental residence time distribution curves for this device were obtained at four different flow rates using the stimulus-response technique with step signal. Theoretically, the flow behavior within the CRFB was approximated by an axial dispersion model (ADM), and a plug dispersion exchange model (PDE), as well as by solving the hydrodynamic equations (Navier–Stokes and Brinkman equations) and the mass transport equation (convection-diffusion) using COMSOL Multiphysics® 5.3. The calculated RTD curves are in good agreement with the experimental RTD curves. Our theoretical and experimental analysis resulted in better approximations of the axial dispersion, and the presence of stagnant zones, and channeling and by-pass (i.e., preferential flow) effects at low and intermediate Reynolds numbers (Re). The experimental RTD curves show that the liquid flow pattern in the CRFB deviates considerably from the axial dispersion model at low Re, conditions under which the CRFB exhibits significant channeling effects, as well as stagnant and dead zones. The PDE model was able to describe the deviation from ideal flow pattern caused by channeling, recycling, or by the presence of stagnant regions in the CRFB. The mass transport in the CRFB was determined by measuring the distribution of tracer molecules at different time points after the initial (step) tracer injection. The interdigitated flow field geometry exhibited a homogeneous flow distribution, featuring an increased mean electrolyte velocity inside the electrode. This simulation allowed locating stagnant zones that would locally affect the mass transport properties and globally lead to a reduction of the RFB conversion efficiency.

Continuous degradation of an organic pollutant using heterogeneous magnetic biocatalyst and CFD analysis of the process

Abstract GOx/Fe3O4/TiO2 on natural kissiris support was produced by consecutive preparation of TiO2/kissiris and Fe3O4/TiO2/kissiris followed by immobilization of Glucose Oxidize (GOx). This magnetic biocatalyst was used for continuous elimination of Malachite Green (MG) from aqueous solution in a packed bed reactor through bio-Fenton reaction. The biocatalyst was characterized by FT-IR, EDX and SEM analyses. Residence time distribution (RTD) of MG in the reactor was measured and parameters of the Langmuir-Hinshelwood kinetics model were obtained for the decolorization process. Hydrodynamics of the packed bed reactor were investigated via a thorough computational fluid dynamics (CFD) simulation using Eulerian approach in axisymmetric space and results were validated using experimental RTD curves and decolorization efficiencies. Ultimately, process simulation was utilized to obtain design parameters of the packed bed reactor in recirculating mode with 99.1% decolorization efficiency. The knowledge obtained through this study can be used to design and scale-up continuous and efficient bioreactors for treatment of wastewater.

Simulations of single-phase flow in an up-flow electrochemical reactor with parallel plate electrodes in a serpentine array

This paper deals with hydrodynamics simulations in an up-flow pre-pilot electrochemical reactor with a stack containing eight cells in a serpentine array. Each cell contains horizontal parallel aluminum plate electrodes, open to the atmosphere at the top of the cell. The single-phase flow was simulated solving the Reynolds-Averaged Navier-Stokes (RANS) equations with k-ε turbulence model via finite element method. The residence time distribution (RTD) simulations were obtained solving the averaged diffusion-convection equation. The influence of mean linear flow velocity, 2.4 ≤ U ≤ 12.1 cm s−1, on the velocity profiles evidenced the presence of jet flow in the first channel of the stack, which is located close to the electrolyte inlet at the bottom of the reactor. Then, the flow pattern became homogeneous in the second and third channels, while the quasi-plug flow profile was developed from the fourth channel to the exit. Streamlines graphs showed that in channel 1 there was very little recirculation of the fluid elements; however, this phenomenon disappears in the subsequent channels. The RTD curves shows the absence of low velocity zones in the reactor, because the electrolyte outlet is open to the atmosphere. The experimental RTD curves showed excellent agreement with simulations.

Simulations of a Single-Phase Flow in a Compound Parabolic Concentrator Reactor

This paper deals with the analysis and interpretation of flow visualization and residence time distribution (RTD) in a compound parabolic concentrator (CPC) reactor using computational fluid dynamics (CFD). CFD was calculated under turbulent flow conditions solving the Reynolds averaged Navier–Stokes (RANS) equation expressed in terms of turbulent viscosity and the standard k−ε turbulent model in 3D. A 3D diffusion-convection model was implemented in the CPC reactor to determine the RTD. The fluid flow visualization and RTD were validated with experimental results. The CFD showed that the magnitude of the velocity field remains almost uniform in most of the bulk reactor, although near and inside the 90° connectors and the union segments, the velocity presented low- and high-speed zones. Comparisons of theoretical and experimental RTD curves showed that the k−ε model is appropriate to simulate the nonideal flow inside the CPC reactor under turbulent flow conditions.

Study of Hydrodynamics at AsahiTM prototype electrochemical flow reactor, using computational fluid dynamics and experimental characterization techniques

Fluid pattern inside of flow electrochemical reactors is strongly affected by geometric inlets design and the use of net-like spacers (or turbulence promoters). In this report a comparison of the hydrodynamic behavior between empty and net-like spacer-filled channel of a parallel plate reactor based in CH-0 electrolyzer of Asahi Glass Co. is presented. RANS equations were solved by means of a Computational Fluid Dynamics (CFD) package (COMSOL 4.4a™). Simulations were validated through experimental flow characterization techniques such as flow visualizations and residence time distribution (RTD) tests. CFD studies evidenced that net-like spacer homogenizes flow distribution for all flow rates studied in this report (0.15–0.63ms−1), causing an increase in local Reynolds numbers in this configuration in comparison with typical empty channel. The flow behavior generated by these spacers could improve mass transport performance, nevertheless, an additional cost for electrolyte pumping must be considered. Comparisons of theoretical and experimental RTD curves and flow visualizations showed that the formulation of the numerical simulation proposed here is appropriate to reproduce the flow pattern inside the net-like spacer-filled channel of a filter press type electrolyzer and can be used for further scaling-up studies.

Optimization of flow control devices for a T-type five-strand billet caster tundish: water modeling and numerical simulation

The optimization of flow control devices (FCDs) for a T-type five-strand billet caster tundish was carried out by water modeling and numerical simulation. In water modeling experiments, flow characteristics of the bare tundish and tundish configurations with designed U-type baffles and a round turbulence inhibitor were analyzed using residence time distribution (RTD) curves. Mathematical models for liquid steel in the real plant tundish were established using the fluid dynamics software package Fluent. The flow field, the temperature field, and the RTD curves of liquid steel in the proposed tundish configurations were obtained. The results of numerical simulation and water modeling were validated with each other by the predicted and experimental RTD curves. The results of flow field and temperature field were used to reflect the actual state of a real plant tundish and to choose the optimal FCD. Finally, from the whole performance of the multi-strand tundish, the optimal scheme was determined by combining the results of water modeling and numerical simulation. With the optimal tundish equipped with U-type baffle with deflector holes and round turbulence inhibitor, not only was the flow characteristic of each strand improved, but also the difference of flow characteristics between multiple strands was smaller.

A mathematical modeling study of the influence of small amounts of KCl solution tracers on mixing in water and residence time distribution of tracers in a continuous flow reactor-metallurgical tundish

In an earlier research (Chen et al., 2015a) a mathematical model was established to simulate tracer mixing (a KCl solution). The predicted Residence Time Distribution (RTD) curves showed good agreements with experimental RTD curves for larger amounts of tracer additions. However, for smaller additions (50mL) of a KCl solution into water, the predicted RTD curves tended to deviate from the experimental RTD curves for a tundish (a continuous flow reactor). The current paper focuses on the possibilities that the predictability for smaller additions could be resolved by using a suitable turbulence model. The performance of five different turbulence models representing different modeling techniques and levels of complexity were tested in combination with using a density-coupled mixed composition fluid model to simulate the mixing, i.e. the following models: LVEL, Chen–Kim k–ε, MMK k–ε, Explicit Algebraic Reynolds Stress Model (EARSM), and Large Eddy Simulation (LES): Wall-Adapting Local Eddy-viscosity (WALE). The results indicate that models that tend to resolve turbulence structures renders better predictions of the mixing process of smaller tracer amounts. In addition, the influence of different tracer amounts on the flow in tundish was assessed. The simulation results for 75mL, 100mL, 150mL, and 250mL KCl tracer additions were compared. The results showed that in an upward flow the tracer will, sooner or later (dependent on the tracer amount), sink to the bottom. This is due to the higher density of the tracer compared to the density of water. From a physical modeling perspective, this issue is like the “butterfly effect”. It is showed that for a slight increase of the amount of tracer, the flow field might be disturbed. This, in turn, will result in a shifted RTD curve.

Study of an Annular Photoreactor with Tangential Inlet and Outlet: I. Fluid Dynamics

AbstractPhotochemical reactors tend to exhibit turbulent flow even with low Reynolds numbers. The k‐ϵ model is not always appropriate in this situation. An annular photoreactor was designed with tangential inlet and outlet tubes to investigate this. The fluid flow was characterized by residence time distribution (RTD) experiments, which were reproduced by computational fluid dynamics considering four relevant turbulence models: the k‐ϵ, the k‐ω, the shear stress transport, and the Reynolds stress models. Inlet effects induced helical flow throughout the reactor, switching to plug flow depending on the flow rate and the turbulence model. The k‐ω model properly deals with viscous effects and reproduces the experimental RTD curves with correlation coefficients greater than 0.9566, against 0.8705 from the k‐ϵ model.

Mixing and transport during pharmaceutical twin-screw wet granulation: Experimental analysis via chemical imaging

Twin-screw granulation is a promising continuous alternative for traditional batch high shear wet granulation (HSWG). The extent of HSWG in a twin screw granulator (TSG) is greatly governed by the residence time of the granulation materials in the TSG and degree of mixing. In order to determine the residence time distribution (RTD) and mixing in TSG, mostly visual observation and particle tracking methods are used, which are either inaccurate and difficult for short RTD, or provide an RTD only for a finite number of preferential tracer paths. In this study, near infrared chemical imaging, which is more accurate and provides a complete RTD, was used. The impact of changes in material throughput (10–17kg/h), screw speed (500–900rpm), number of kneading discs (2–12) and stagger angle (30–90°) on the RTD and axial mixing of the material was characterised. The experimental RTD curves were used to calculate the mean residence time, mean centred variance and the Péclet number to determine the axial mixing and predominance of convective over dispersive transport. The results showed that screw speed is the most influential parameter in terms of RTD and axial mixing in the TSG and established a significant interaction between screw design parameters (number and stagger angle of kneading discs) and the process parameters (material throughput and number of kneading discs). The results of the study will allow the development and validation of a transport model capable of predicting the RTD and macro-mixing in the TSG. These can later be coupled with a population balance model in order to predict granulation yields in a TSG more accurately.

Numerical simulation and experimental study of pressure and residence time distribution of triple‐screw extruder

Three‐dimensional flow field of triangle arrayed triple‐screw extruder (TTSE) in different feed rates, screw speeds and screw configurations were simulated by POLYFLOW. Polypropylene processing by TTSE was tested to validate the numerical simulation. The reliability of simulation was tested directly and indirectly by comparing the axial pressure difference and residence time distribution (RTD) separately. The results show that the axial pressure differences from the simulations and experiments were in high agreement. The simulated and experimental RTD curves show the same trend, and so are the simulated and experimental average residence times. These results prove the reliability of simulation and its guidance to the experiment. POLYM. ENG. SCI., 55:156–162, 2015. © 2014 Society of Plastics Engineers

Characterisation of non-ideal flow behaviour in continuous casting tundish

The effect of a turbulence inhibitor and a curtain of rising bubbles on the flow pattern inside a continuous casting tundish have been experimentally evaluated using a water model. Water experiments were conducted using a two-fifths scale model of a typical strand tundish. Unlike previous studies, residence time distribution (RTD) curves were experimentally obtained in this work by first injecting a step change of an inert tracer (an aqueous NaCl solution) into the inlet stream and then detecting the response of the tracer inside the tundish at different water depths right above the outlet nozzle. A statistical expression based on the random walk model was used to represent and interpret the resulting experimental RTD curves in terms of the mean residence time θm and the Bodenstein or dispersion number Bo.On a évalué expérimentalement l’effet d’un inhibiteur de turbulence et d’un rideau de bulles ascendantes sur la configuration de l’écoulement dans un panier de coulée continue en utilisant un modèle à l’eau. On a effectué les essais avec l’eau au moyen d’un modèle à échelle 2/5 d’un panier de coulée typique à volet. Contrairement aux études précédentes, on a obtenu expérimentalement dans ce travail les courbes de distribution du temps de séjour (RTD) en injectant d’abord un traceur inerte (une solution aqueuse de NaCl) avec changement soudain dans le courant d’entrée. On a ensuite détecté la réponse du traceur à l’intérieur du panier de coulée à différentes profondeurs juste au-dessus de la buse de sortie. On a utilisé une équation statistique basée sur le modèle de parcours aléatoire pour représenter et interpréter les courbes expérimentales de RTD par rapport au temps de séjour moyen θ et au nombre de Bodenstein ou indice de dispersion Bo.

Residence time distribution measurements and flow modeling in a supercritical water oxidation reactor: Application of transfer function concept

Residence time distribution (RTD) measurements are carried out in a flow-through tubular supercritical water oxidation (SCWO) reactor using ex situ pulse response experiments. The experiments are performed from ambient to supercritical water conditions with a volumetric flow rate fixed at 1 ml/min. The transfer function concept is used to interpret tracer information and to elucidate hydrodynamic flow patterns inside the “hot zone” of the reactor vessel. The experimental RTD curves are modeled by an axially dispersed plug flow. At supercritical conditions, the studied tubular SCWO reactor can be characterized as a mixed flow system (with Péclet numbers of 1.4–2). The results also indicate the presence of fast preferential fluid flow in the reactor below 573 K and 10 MPa.

Indepth diagnosis of a secondary clarifier by the application of radiotracer technique and numerical modeling

To make an indepth diagnosis of a full-scale rectangular secondary clarifier, an experimental and numerical study has been performed in a wastewater treatment facility. Calculation results by the numerical model with the adoption of the SIMPLE algorithm of Patankar are validated with radiotracer experiments. Emphasis is given to the prediction of residence time distribution (RTD) curves. The predicted RTD profiles are in good agreement with the experimental RTD curves at the upstream and center sections except for the withdrawal zone of the complex effluent weir structure. The simulation results predict successfully the well-known flow characteristics of each stage such as the waterfall phenomenon at the front of the clarifier, the bottom density current and the surface return flow in the settling zone, and the upward flow in the exit zone. The detailed effects of density current are thoroughly investigated in terms of high SS loading and temperature difference between influent and ambient fluid. The program developed in this study shows the high potential to assist in the design and determination of optimal operating conditions to improve effluent quality in a full-scale secondary clarifier.

Liquid phase mixing and gas hold-up in a multistage-agitated contactor with co-current upflow of air/viscous fluids

A hydrodynamic study is carried out in a multistage-agitated contactor with cocurrent upflow of air/viscous fluids. The liquid phase backmixing is characterized through a residence time distribution study in various viscous fluids (up to 20 mPa s). The liquid Reynolds number with respect to the impeller applied in the investigation ranges from 1000 to 50 000. A cascade of stirred tanks with back flow model satisfactorily fits all experimental residence time distribution curves in the present study. It is found that the increase in the liquid viscosity reduces the backmixing in the liquid phase due to its dampening effect on turbulence. The presence of the gas phase helps in reducing the backmixing by the straightening effect and entrainment effect in a co-current operation manner. The gas hold-up measured in the present study is comparable to the literature data. A new type of correlation taking into account the influence of the gas phase is proposed to predict the backmixing in a multistage-agitated contactor.

Estimation of liquid phase mixing due to solids in a turbulent bed contactor

The mixing in two-phase gas–liquid and three-phase gas–liquid–solid system (turbulent bed contactor) is evaluated through residence time distribution (RTD) studies in terms of Peclet number. RTD experiments are conducted for various gas and liquid velocities, and number of stages for two- and three-phase systems. Since the mean residence time is very short in both the systems, a mixed flow tank with exponential decay RTD is used in series. After deconvolution, the RTD of the system is obtained. The experimental RTD curves are satisfactorily compared with the axial dispersion model and Peclet numbers are evaluated for all the experiments. The axial dispersion coefficients are calculated from Peclet numbers. With this study, it is thought that liquid phase mixing may be controlled by changing the quantity of solid particles in the bed.