Residence Time Distribution Curves Research Articles

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.

Experimental and Simulation Analysis of Physical Water Model through Tracer Injection under Various Baffles and Turbulence Inhibitor Conditions for Developing the Fluid Flow Parameters and Inclusion Removal Rates in Tundish

Tundish is a critical part involved in the continuous casting process, which ensures the constant flow of liquid metal and with performing different functions. In odd-numbered multistrand tundish, the confirmation consistency of the fluid flow among all the strands is one of major concerns. In this study, five-strand tundish with a set of weirs and a baffle is utilized for investigation concerning fluid flow parameters and inclusion removal analysis. The simulation is carried out in ANSYS FLUENT 19.2 software through the defined numerical model, and the experimental analysis is performed on a scaled physical water model through tracer injection. Four different baffles are designed along with two different types of turbulence inhibiter for the tundish to enhance the fluid flow parameters and inclusion removal rates. The inclusion sizes were varied from 10 to 100 µm with a difference of 20 µm respectively for studying the inclusion removal rate concerning inclusion sizes. From the outcomes obtained like the residence time distribution (RTD) curves and others, it was concluded that the fourth baffle version with the first turbulence inhibiter, which are equipped in tundish version TV8 is the most beneficial tundish version for the enhancement in fluid flow and inclusion removal rates as compared to other tundish versions in this study.

Hydraulic Characteristics, Residence Time Distribution, and Flow Field of Electrochemical Descaling Reactor Using CFD

This paper uses computational fluid dynamics (CFD) to simulate flow field distribution inside an electrochemical descaling reactor in three dimensions. First, the reactor flow field was obtained by steady-state simulation, and the grid independence was verified. Then, the steady state of the flow field was judged to ensure the accuracy of the simulation results. Transient simulations were performed on the basis of steady-state simulations, and residence time distribution (RTD) curves were obtained by a pulse-tracing method. The effects of plate height and plate spacing on reactor hydraulic characteristics (flow state and backmixing) were investigated using RTD curves, and the results showed that increasing the plate height and decreasing the plate spacing could make the flow more similar to the plug flow and reduce the degree of backmixing in the reactor. The flow field details provided by CFD were used to analyze the reactor flow field and were further verified to obtain the distribution patterns of dead and short circuit zones. Meanwhile, information regarding pressure drops was extracted for different working conditions (490, 560, and 630 mm for pole plate height and 172.6, 129.45, and 103.56 mm for pole plate spacing), and the results showed that increasing the pole plate height and decreasing the pole plate spacing led to an increased drop in pressure. In this case, a larger pressure drop means higher energy consumption. However, increasing the pole plate height had a smaller effect on energy consumption than decreasing the pole plate spacing.

Residence Time Distribution as a Tool for Hydrodynamic Study of Electrocoagulation Reactors Design and Scaling up

An electrocoagulation reactor operating continuously is designed for the treatment of industrial effluents in Senegal. One of the main challenges for electrocoagulation reactors design is each reactor is specific for its domain. In this paper, a methodology using residence time distribution curve (RTD) is presented and can be used for any other examples. The optimization of the operating parameters such as the flow rate, the residence time, the conductivity and so on for the treatment of these effluents requires an understanding of the reactor hydrodynamics. Thus, this hydrodynamic study based on the residence time distribution curve (RTD) showed the presence of dead zone in the reactor with a theoretical residence time of 607.76 s for an average residence time obtained from the residence time distribution curve of 554.85 s. A comparison of the curve obtained experimentally with the curves for an ideal reactor of the piston type or of a perfectly stirred type, has shown that the electrocoagulation reactor behavior is like a cascade of two perfectly stirred reactors with a difference between them of 0.15%. The numerical simulation with COMSOL allowed to illustrate the velocity fields and to highlight the preferential zones during the flow in the reactor and thus to confirm the results obtained with the hydrodynamics with a relatively low absolute error of 4.6% obtained with the least squares method.

Particle movement characteristics in a gas–solid vertical single helical ribbon agitated reactor

The gas–solid vertical single helical ribbon agitated reactor (G-S-VSAR) is a gas–solid dense-phase agitated reactor, in which the interaction between the agitator and the dense granules dominates the flow field and transfer behavior. The particle movement characteristics are essential for determining the reactor space–time yield and product quality. In this work, the particle residence time distribution (RTD) was measured through magnetic-particles pulse experiment to investigate the overall movement and mixing characteristics of a G-S-VSAR, with the effects of rotation speed and gas velocity studied. The RTD curves were featured by time lag, bypass peak and exponential tail, and thus a model of ‘PFR + double parallel CSTRs’ was established. Results showed that the increase in rotation speed could eliminate the bypass, but only reduce the lag time. The lag time adversely influenced the mixing. Photography was applied to characterize the local particle movements near the reactor wall. It was found that the particles spiraled upwardly along the wall in laminar-like flow and the fully mixed state reached after one circulation. During the spiraled process, the undulating displacement in the vertical direction was induced by agitation in the ribbon-wall gap, which inhibited the radial mixing. Therefore, a narrow gap was recommended for reducing such undulating movements and promoting radial mixing.

Computational Fluid Dynamics Modelling and Analysis Approach for Estimating Internal Short-Circuiting in Clearwells

Disinfection is an effective microorganism inactivation method that has contributed historically to decreasing waterborne diseases. It is necessary to improve hydraulic efficiency for ensuring disinfection ability without creating disinfection by-products. However, many hydraulic efficiency indices, which are a type of black-box analyses based on residence time distribution curves, have been used to assess short-circuiting and mixing. We propose a novel index (internal short-circuiting index, ISI) and analysis approach (internal short-circuiting estimation method, ISEM) based on computational fluid dynamics (CFD) modelling for understanding the local hydrodynamics. Then, we implement ISEM to quantify the effect of the clearwell configuration with the different length-to-width and shape ratios on the hydraulic efficiency. As the hydraulic efficiency surrogated by T10/T converges to the maximum value, the ISI values at inlet and outlet reduce rapidly, and the recirculation and dead zones shrink in the channel zones. Thus, the ISI curve changes from a V shape to U shape. The ISEM demonstrates that it is applicable under various conditions and it enables engineers to design clearwells for optimizing the disinfection ability. Furthermore, the CFD model in this study can be combined with machine learning techniques in future studies to extract knowledge for reducing the computational cost.

Development of an Automated Tracer Testing System for UASB Laboratory-Scale Reactors

Residence time distribution (RTD) curves play an essential role in the hydraulic characterization of reactors. Current approaches for obtaining RTD curves in laboratory-scale reactors are time-consuming and subject to large errors. Thus, automated systems to obtain RTD curves in laboratory-scale reactors are of great interest for reducing experimental errors due to human interaction, minimizing experimentation costs, and continuously obtaining experimental data. An automated system for obtaining RTD curves in laboratory-scale reactors was designed, built, and tested in this work. During the tests conducted in a cylindrical upflow anaerobic sludge blanket (UASB) reactor, the system worked properly using the stimulus–response pulse technique with sodium chloride as a tracer. Four main factors were found to affect the representativeness of the RTD curves: flow stabilization time, test water conductivity, temperature, and surface tension. A discussion on these factors and the corresponding solutions is presented. The RTD curves of the UASB reactor are left-skewed with a typical tank reactor’s flow shape with channeling and dead zones. A transitory flow behavior was evidenced in the reactor, which indicates the influence of internal turbulent flow structures. The system proposed herein is expected to help study the hydraulics of reactors using laboratory-scale models more efficiently.

Residence time distribution study in a pilot-scale liquid phase catalytic exchange (LPCE) column packed with a mixture of hydrophobic and hydrophilic catalysts

Residence time distribution (RTD) measurements were carried out in a packed bed column designed for exchange of hydrogen isotopes. The main objective of the study was to characterize the liquid phase mixing under various processes and operating conditions. The packed bed was composed of a mixture of two different types of catalytic packing materials, i.e., a hydrophobic material and a hydrophilic material. Technitium-99m (99mTc) as sodium pertechnetate was used as a radiotracer for RTD measurements. From the measured RTD curves, mean residence times (MRTs), liquid holdup and degree of mixing of liquid phase were evaluated. An axial dispersion model exchange with stagnant zones was used to simulate the measured RTD curves. The results of model simulation showed that volume fraction of hydrophobic to hydrophilic packing and gas/liquid superficial velocities affect the liquid holdup, bed pressure drop and liquid phase dispersion/mixing characteristics. The results of the present study will help to screen packing, optimize the volume of the packing fractions, design and construct the catalyst and optimize the operating conditions for scale up of the isotope exchange process.

Comparison of Fluid Flow and Temperature Distribution in a Single-Strand Tundish with Different Flow Control Devices

The effects of flow control devices (FCD) in a single-strand tundish, including weir, dam, turbulence inhibitor and gas curtain, have been investigated using water model experiments and CFD simulations. A scaled-down water model was built up to visualize flow pattern and measure the residence-time distribution (RTD) of different tundish configurations. A CFD model was applied to calculate the fluid flow, heat transfer and RTD curves in the prototype tundish under the nonisothermal conditions. The Eulerian–Lagrangian approach was applied to investigate the bubble flow in the system. The results show that each FCD has its own unique function to control the flow. It is important to evaluate the combined effects of FCD based on their installations. The molten steel flow in the tundish could be improved if these flow control devices were arranged properly.

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.

Fluid Flow Characterization of Single-Strand Tundish with Flow Modifiers through Physical Water Model (PWM) and CFD Simulation

The flow behavior in the continuous casting tundish dominates the quality and cleanliness of steel production. In this research, the single strand tundish's fluid flow behavior with different flow modifiers is investigated through numerical and experimental simulation. The numerical; simulation is performed in ANSYS FLUENT 19.2 (commercial package) and experimental through physical water model technique. The flow behaviors of bare tundish and tundish with three different flow modifiers are investigated. The three different flow modifiers deployed are the dam, baffle, and turbulence inhibiter (TI). Fluid flow performance is examined through residence time distribution (RTD) curves, which are derived from the measurement of the tracer concentration at the outlet. Good agreement between the CFD simulation and physical water model experiments is discovered. The results show there is an improvement in residence time and fluid flow (also improved inclusion removal) after the deployment of flow modifiers. There is a 20% improvement in peak and minimum residence time of RTD curves due to flow modifiers application. It is also discovered that the tundish configuration in this research, the tundish with turbulence inhibiter, provides optimal flow characteristics and eventually intended to promote a better level of inclusion removal.

Residence time distribution in fluidized beds: diffusion, dispersion, and adsorption

The residence time distribution (RTD) characterizes the flow patterns of tracer particles in continuous systems to identify phenomena like channelling, dispersion, dead volume, and back-mixing. Understanding the hydrodynamics in experimental reactors is imperative to derive reaction kinetics. Here, we applied the RTD methodology to examine the relationship between the physico-chemical properties of powders and gas flow patterns in an 8 mm diameter quartz micro-fluidized bed. H2 and He tracers egressed the reactor sooner than Kr, O2, CO, CH4, CO2 demonstrating that gas diffusivity is a factor to consider when choosing a tracer gas. Intraparticle porosity is a second factor that delays the RTD curve: the residence time was 7% greater for virgin vanadium pyrophosphate catalyst (VPP) with a lower particle density than an equilibrated VPP. These results suggest that RTD experiments are sensitive enough to detect phenomena like catalyst sintering and pore blocking due to coke in situ. The RTD curve of CO and CH4 tracers with a bed of fluid catalytic cracking catalyst (FCC) had a noticeable trailing edge (tail). For the CO2 tracer, the leading edge of the RTD curve was delayed, the peak height was 50% shorter, and the tail was extended demonstrating adsorption phenomena like a chromatographic effect. The residence time of Kr, with a similar gas diffusivity as CO2 was also longer compared to CO or CH4, but the tail was shorter than for CO2, which confirms the adsorptive nature of the FCC surface with CO2. A bolus-pulse input function detected these hydrodynamic anomalies with a greater certainty than a Heaviside-step input.

Effects of Salt Tracer Volume and Concentration on Residence Time Distribution Curves for Characterization of Liquid Steel Behavior in Metallurgical Tundish

The residence time distribution (RTD) curve is widely applied to describe the fluid flow in a tundish, different tracer mass concentrations and different tracer volumes give different residence time distribution curves for the same flow field. Thus, it is necessary to have a deep insight into the effects of the mass concentration and the volume of tracer solution on the residence time distribution curve. In order to describe the interaction between the tracer and the fluid, solute buoyancy is considered in the Navier–Stokes equation. Numerical results show that, with the increase of the mass concentration and the volume of the tracer, the shape of the residence time distribution curve changes from single flat peak to single sharp peak and then to double peaks. This change comes from the stratified flow of the tracer. Furthermore, the velocity difference number is introduced to demonstrate the importance of the density difference between the tracer and the fluid.

Two dimensional modeling of sewage sludge flow in a double-axis continuous paddle dryer

A two-dimensional model based on the theory of Markov chain has been developed to determine the Residence Time Distribution (RTD) of municipal sewage sludge in a double-axis continuous paddle dryer. Based on the experiments, the paper proposes further corrections to simulate the transition. To characterize the sludge transition in the dryer, the parameter of internal recirculation based on one dimensional model is further developed. In addition, two parameters, the internal forward coefficient between two axes and the internal forward coefficient in one axis are introduced to characterize the new model. In absence of available correlation, solid hold-up of each cell in dryer and both the recirculation parameters are identified by fitting the model to experiments. Through analysis, the model demonstrates its ability to describe the sludge flow in a double-axis continuous paddle dryer by the experimental and simulation RTD curve. Finally, an analysis of influence factors highlights that recirculation coefficients are critical for the model while solid hold-up Hu of each cell controls the mean residence time and the final moisture content. In addition, the geometric residence time of sludge flow has a negligible effect on sludge flow for it has no effect on dimensionless variance. Moreover, compared with the recirculation coefficient between different axes R, the recirculation coefficient in one axis r has a negligible effect. In addition, recirculation parameters have no effect on mean residence time of sludge flow.

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.

Application of Tanks-in-Series Model to Characterize Non-Ideal Flow Regimes in Continuous Casting Tundish

This study describes a new tanks-in-series model for analyzing non-ideal flow regimes in a single-strand tundish. The tundish was divided into two interconnected tanks, namely an inlet tank and an outlet tank. A water model experiment was carried out to separately measure the residence-time distribution (RTD) of the two tanks. Drift beads were adopted in the water model experiment to simulate the non-metallic inclusions in molten steel. Dead volume fraction was evaluated by analyzing measured RTD curves. The ratio between mixed flow volume and plug flow volume was proposed as a new criterion to evaluate the inclusion removal. In the inlet tank, a higher mixed flow fraction was preferred to effectively release turbulent kinetic energy and enhance inclusion collision growth. In the outlet tank, a higher plug flow fraction was preferred to facilitate inclusion removal by flotation. The optimal positions of the weir were recommended based on the RTD analysis and the inclusion removal from the results of water model experiments. A theoretical equation was derived based on the tanks-in-series model, providing a good fitting function to analyze the experimental data. The confirmation test was performed by applying computational fluid dynamics simulations of liquid steel flow in the real tundish.

Hydraulic and heat transfer characteristics in structured packed beds with methane steam reforming reaction for energy storage

Packed beds with methane steam reforming reaction (MSR) can be used as thermochemical storage devices. The packing configuration can have a significant effect on the flow distribution which will further influence the heat/mass transfer, dispersion and reaction characteristics. In the present paper, the hydraulic, dispersion, heat/mass transfer and the MSR reaction properties are investigated in packed beds with the simple-cubic (SC) packing, the body-centered cubic (BCC) packing and face-centered cubic (FCC) packing forms. ANSYS FLUENT integrated with user defined scalars (UDS) and user defined functions (UDF) are used to perform the simulations. Results show that firstly, the residence time distribution (RTD) can be used to evaluate the dispersion property in packed beds where a narrower RTD curve reflects a more uniform velocity distribution with less dispersion. Secondly it is found that, regions with backflow are not beneficial for heat/mass transfer and chemical reactions from two aspects. One is that the heat transfer between the main fluid and the particle surfaces is weak and low temperature of the particles will reduce the reaction rates. The other is that the poor species transport will prevent the reactant to flow into the reaction region and the products cannot be taken away by the main flow timely, which will have a negative effect on the reaction. Thirdly, it is revealed that, the dimensionless variation of the residence time distribution (DVRTD) can reflect the heat/mass transfer and reaction performances, where a smaller DVRTD corresponds to better heat/mass transfer and higher reaction rates.

Numerical study of influence of casting speed on fluid flow characteristics in the four strand tundish

Steel maker companies attempt to achieve high production quality and decrease production costs due to high competition of the global steel industry. Steel cleanliness plays a significant role in the steel quality which is the key success factor for the steelmaker. In order to optimize the steel cleanliness in the continuous casting process, flow characteristics were studied to enhance the inclusion removal efficiency in four strand tundish. The purpose of this research is to study the effect of casting speed on the flow characteristics of molten steel inside a four-strand tundish. The investigation was carried out by using the commercial computational fluid dynamic (CFD) software ANSYS 18.2. A 1:3 water model tundish based on process parameters from Millcon Steel PLC were employed in this study. The simulation results were validated with the physical water model. The residence time distribution (RTD) curves were applied to analyze the flow characteristics of the tundish. The results of this study show the different casting speeds have an effect on the flow characteristics inside the tundish. The start point of RTD curve of the flow in the tundish with casting speed of 3.0 m/min is the shortest amongst all casting speeds used (1.7, 2.0, 2.3 and 3.0 m/min). A short time period of the start point of RTD curve increases the chance of inclusion contamination in the casting mould and leads to low quality products. Furthermore, the outlet positions affect the minimum residence time of RTD curves and the tracer concentration directly. The RTD start point of the middle outlet located close to the inlet position is faster than that of the far outlet located far from the inlet. The results obtained from the study contributes to the optimization of productivity and quality of the example plant.

Novel Image Analysis-Based Method for Residence Time Distribution Analysis in Steelmaking Tundish

A digital image processing-based technique has been developed for residence time distribution (RTD) analysis in a steelmaking tundish. This work presents an alternative approach to current RTD analysis techniques like tracer dispersion analysis, particle image velocimetry, numerical analysis, etc. The video analysis method has been developed, and different process conditions have been evaluated. The physical model of a steelmaking tundish having water as a working fluid similar to the one which is used for RTD analysis through other methods has been used. New incoming fluid (water) has been identified using colored water so that all properties apart from color remain the same. The proposed method breaks the video into static frames, and through application of Lucas–Kanade method, the moving colored pixels in the region of interest are identified. Further intensity analysis of the moving colored pixels is done. In the present novel method, RTD curve is obtained by plotting the intensity of blue component at the outlet vs time. The obtained RTD from VT-LK method has been validated against conventional and established RTD technique widely used for RTD analysis.

Radiotracer method: A nuclear tool for flowrate measurement in the pipeline

Experimental work has been carried out to demonstrate using the radiotracer method for flowrate measurement of water in a 3 inches diameter of the carbon-steel pipeline. The water was supplied from a water tank of a volume of approximately 5000 liters. The tank was connected to the pipeline through a flexible plastic hose. A ball valve was installed at the far end of the pipeline to adjust water discharge from the pipeline. None flow meters were installed on the pipeline. Iodine-131 (131I) radioisotope with a concentration of 1 mCi.mL−1 has been injected instantaneously through the injection point located at 15 meters from the water tank. Two radiation detectors (notified as D1 and D2) which had been set-up respectively at the distance of 4 m and 7 m from the injection point recorded residence time distribution (RTD) curves of the radiation intensities emitted from the injected radioisotope. During the experiment, the system was assumed time-invariant by keeping the tank was always containing full of water. The transit time calculated based on mean residence time (MRT) was 89.9 s, and the linear velocity of the water flow is 3.33 cm.s−1