Residence Time Distribution Results Research Articles

Modelling of powder hydrodynamics in a screw reactor

Screw reactors are largely used devices in the industry. Very few studies focus on the powder hydrodynamics in screw reactors. The overflow point, a filling degree value when the powder passes over the shaft, was determined for different conditions and a dimensionless model was fitted to predict this parameter. The residence time distribution (RTD) has been measured with a pulse injection of salty powder. The influence of operating conditions and geometry parameters on the RTD has been studied. It appeared that the Hausner ratio, the pitch length and the filling degree mostly influence the RTD shape. Two models for the RTD have been compared: the compartment model was the most accurate and has one parameter of adjustment. A dimensionless model was developed and fitted with 51 experimental RTD results, predicting the RTD shape. The dimensionless model predicts with good accuracy other experimental data from the literature.

English

In this work, the business Computational Fluid Dynamics (CFD) is used for the numerical simulations of an air-solid fluidized bed container in the solid phase involving a multi-fluid Eulerian multiphase model and the Kinetic Theory of Granular Flow (KTGF). The height of the fluidized bed setup is 1.5 m while its diameter is 0.2 m. With it, a series of experimentations were obtained using Helium tracer to determine the Residence Time Distribution (RTD) at various normalized velocities, that is, mixing of air-solids at different degrees. 2 and 3 dimensions of the fluidized bed container are simulated. The main purpose of this study is to understand the hydrodynamic behavior of air-solid fluidized bed container through a framework of Eulerian multiphase model and to analyze the hydrodynamic behavior of the air-solids mixing. As a first approach, the CFD model is validated using the experimental results of the residence time study. The numerical results of RTD corresponded well with the experimental findings. This shows that the CFD modeling might be used to indicate the performance of a fluidized bed reactor. Key words: Computational fluid dynamics, fluidized bed, residence time distribution (RTD), gas-solids mixing, turbulence.

Design of a continuous-flow single-channel reactor using optimal experimental data from batch reactor for turbidity removal by electrocoagulation

Electrocoagulation was used for turbidity removal from surface water in an internal loop airlift batch reactor. The turbidity removal efficiency of 72.05 % and energy consumption of 0.210 kW h/m3 were achieved using initial conductivity of 1,494 μS/cm, applied voltage of 5 V, treatment time of 6.5 min and inter-electrode distance of 14 mm as optimal experimental conditions. These optimal experimental data were used to design a continuous-flow single-channel (CFSC) reactor for surface water treatment by Electrocoagulation. Furthermore, experimental study under previous optimal experimental data was carried out in the CFSC reactor and the performance of two reactors was compared. Then, hydrodynamic behaviour of the CFSC reactor was studied using Residence Time Distribution (RTD). According to the results, the amount of electro-dissolved aluminum and the energy consumption were lower in the CFSC reactor than in batch system. Despite these decreases, turbidity removal efficiency was more effective in the CFSC reactor than in batch reactor. This performance is attributed to the high mixing level within the reactor as well as the reuse of the tiny flocs for particles destabilization. According to the RTD results, the hydrodynamic behaviour of the CFSC reactor shows neither short-circuiting nor dead zones, but presents small deviation from plug flow. Dispersion models were used to characterize flow within the CFSC reactor. As a result, tanks-in-series model with J = 59 and axial dispersion model with Peclet = 116 are determined as models corresponding to the hydrodynamic behaviour of the CFSC reactor.

Optimization of mixing in a Chinese dome digester for tropical regions

Optimization of mixing in a Chinese dome digester for tropical regions

Evaluation of liquid and solid phase mixing in Chinese dome digesters using residence time distribution (RTD) technique

The Residence Time Distribution (RTD) technique was applied to evaluate mixing of liquid and solid phases in laboratory scale Chinese dome digesters mixed via hydraulic variation. To achieve this purpose, six laboratory scales digesters with different mixing modes and two total solids (TS) concentrations using appropriate tracers were studied over a theoretical hydraulic retention time (HRT) of 30 days. The three different mixing modes were impeller, unmixed and hydraulic mixing, each at influent concentration of ca. 7.5 and 15% TS concentrations. The mode of mixing strongly affected the effective or actual residence time (ta) and directly influenced the percentage of dead zones. The Chinese dome digesters had more dead volumes than the impeller mixed reactors and less than the unmixed reactors. This implied that mixing was more efficient in the impeller mixed reactors followed by the hydraulic mixed reactors and then the unmixed reactors, irrespective of the TS concentration. There was a clear relation between the RTD results and anaerobic digestion performance viz. methane production. There is need to optimize the hydraulic variation in the Chinese dome digester to reduce dead zones while also optimizing the effective residence time (ta).

Effect of tracer material properties on the residence time distribution (RTD) of continuous powder blending operations. Part I of II: Experimental evaluation

Residence time distribution (RTD) models are essential to understand process dynamics and support process monitoring and control in continuous manufacturing systems. RTD models can also be used to monitor material traceability and to isolate intermediate materials or finished products when specifications are not met. However, while pharmaceutical companies are currently making extensive use of RTD approaches, standard methods for conducting, interpreting, and using RTD results in continuous pharmaceutical manufacturing have not yet been established by regulatory authorities. This paper seeks to facilitate generating such standards. We discuss in detail the assumptions and conditions that are relevant to the proper selection of tracers for RTD experiments, and demonstrate that tracer selection can have substantial impact on RTD results. We selected seven materials with a wide range of properties as tracers and a single material as our base “blend”. The experimental results led to two major conclusions: (1) materials with different mechanical properties have dissimilar mean residence times (MRT) inside the systems and (2) blend ingredients with different mechanical properties travel at different speed inside of continuous blending systems. Results further indicated there were two critical mean residence times (MRTs): that of the tracer and that of the bulk. Matching of material properties between tracers is key in order to obtain similar MRTs using a given tracer. Differences between selected tracer and bulk material properties were found to lead to differences between the bulk space time and the tracer MRT. A set of recommendations on how to select tracer materials that would help characterize accurately the RTD of a continuous flow system are presented.

High purity fructose from inulin with heterogeneous catalysis – from batch to continuous operation

AbstractBACKGROUNDInulin is a valuable source of high purity fructose. The conversion of inulin to fructose utilizing an advanced sulfonic ion‐exchange resin was previously investigated in detail in a batch reactor study, where a detailed mechanistic model for the reaction kinetics was demonstrated (J Chem Technol Biotechnol 2018;93:224–232). The same catalyst was employed in tailor‐made continuous fixed‐bed reactors in the current work in order to study the feasibility of continuous operation.RESULTSTwo types of reactors were utilized: a single‐bed reactor and a multibed reactor with sample withdrawal between catalyst beds. The results from the single‐bed reactor allowed optimal reaction conditions to be determined which were extrapolated and used for experiments in the multiple‐bed reactor. High yields of fructose (75%) were obtained with the multiple‐bed reactor without any degradation products. Detailed flow characterization was performed on the reactor system and residence time distribution results were merged with the detailed model for the chemical kinetics obtained from batch reactor studies taking into account mass transfer limitations, in order to obtain a rigorous model for the performance of the continuous reactor. The fit of the mathematical model to the experimental data was very good and this provides a useful tool for further development and scale‐up purposes.CONCLUSIONSThe results of the two related studies highlight the potential of new reactor design strategies towards the realization of more efficient, sustainable and green processes, which can easily be scaled‐up and implemented in bio‐based industry. © 2018 Society of Chemical Industry

Hydrophobic/hydrophilic powders: Practical implications of screw element type on the reduction of fines in twin screw granulation

This study focuses on the role that various screw elements play in the reduction of fines in twin screw wet granulation (TSG) when using increasing amounts of hydrophobic component in the formulation. The elements used are conveying-type elements (CE-type), kneading elements (KE-type) and tooth mixing-type elements (TME-type)). Fines are defined as <150 μm granules. Further, this work attempts to rationalize the positioning of the various screw elements along the barrel by studying the granule size, granule porosity, liquid binder distribution and residence time distribution results. The length of the screw elements was kept constant to ensure constant and comparable mixing quality.It was found that CE-type elements promoted poor mixing through concentration of activity in the intermeshing region of the twin screw, which resulted in large agglomerates being formed, together with large amounts of <150 μm granules. At increased formulation hydrophobicity, the distribution of the liquid binder droplets using the CE-type elements became unachievable, forming liquid marbles - ‘solid particles spreading around the liquid droplet’. The order of superior cross-sectional mixing behavior across the barrel channel width was observed to be CE < KE < TME, particularly with increase in formulation hydrophobicity. This was due to progressively improved liquid distribution with increase in material residence time, which resulted in less production of <150 μm granules and greater extent of granule formation with that order of elements. The axial cuttings through the flights of the CE-type elements are necessary to reduce the internal free volume of the intermeshing gap and cross-sectional area which promote material compression and higher degree of fill before the mixing zone in TSG. Whilst, the increase in internal free volume of the mixing elements provide more cross-sectional free volume for granules to be compacted.

Preparation of macroporous scaffolds with holes in pore walls and pressure driven flows through them.

Controlling the pore architecture in macroporous scaffolds has important implications for their use as reactor packings and as catalyst supports. We report the preparation of a macroporous structure, where the pore walls are perforated by holes. These materials are prepared by modification of the ice-templating protocol developed in our group. We freeze a dispersion of colloidal silica, polymer and cross-linker in a water/acetonitrile medium and allow crosslinking to proceed in the frozen state. The presence of a small fraction of acetonitrile (varying between 1.6% to 6.4%) results in the formation of holes in the pore walls. Increasing the acetonitrile concentration changes the pore size distribution, and produces smaller pores on average. This also results in an increasing fraction of the wall area being covered by small pores, of the order of a few microns in size. Perforation of the walls by pores does not change the overall porosity or modulus of the scaffolds. However, the introduction of pores leads to a drastic reduction in the pressure drop required to pump liquid through the scaffolds. The observed residence time distribution (RTD) in the scaffolds is represented by two plug flow reactors (PFRs) in parallel. The RTD results indicate that increasing the hole fraction in the pore walls results in increased channelling which explains the aforementioned decreased pressure drop during pressure driven flow.

A new statistical view to modeling of particle residence time distribution in full-scale overflow ball mill operating in closed-circuit

ABSTRACTIn the present study, residence time distribution (RTD) of an industrial ball mill operating in closed-circuit with hydrocyclones was measured. Several probability distribution functions (PDFs) constituting exponential, Weibull, Gamma, logistic, normal, and lognormal were applied to obtained RTD results and evaluated based on Anderson-Darling statistic (AD) and associated p-value indices. In addition, three most common empirical RTD models (i.e., perfect mixer, N-Mixer and Weller) were fitted to the given practical data. Aside from assessment of coefficient determination (R2) for each model, a factor of incorporating the number of model parameters was considered using Bayesian information criterion, low of iterated logarithm criterion and Akaike information criterion. It was revealed that Weibull PDF is fitted reasonably well to the measured experimental data compared with the other PDFs. Despite the relative variance (σ2) of N-Mixer model was slightly less than the corresponded value of Weller model, the goodness of fit criterion (R2) and all four information criteria (IC) showed better results for Weller model. Therefore, one large with two small tanks in series along with a dead time was selected as the best model from the statistical point of view. Finally, it was concluded that the RTD models must be evaluated not only on the basis of goodness of fit but also the number of model parameters should be taken into account.

Computational fluid dynamics modeling of viscous liquid flow characteristics and end effect in rotating packed bed

Processing viscous fluid system in industrial applications is one of the challenges in process intensification. In this work, a two-dimensional CFD model was developed to reveal viscous liquid flow characteristics in the injection zone, inner cavity, packing zone and outer cavity of rotating packed bed (RPB). The CFD results agreed well with the published experimental data. The transition of the liquid flow pattern in RPB was discussed with operating conditions. The liquid viscosity showed no significant influence on the liquid holdup and the average diameter of droplets in RPB. In addition, the liquid holdup and droplet diameter decreased as rotational speed increased, and increased as liquid flow rate increased. Based on the simulated results of residence time distribution along radial direction, the end effect zone of RPB was specified, and a novel way was proposed to quantify the thickness of end effect zone. Results showed that the viscosity had great influence on the end effect zone, which had guiding significance for practical operations and configuration optimization.

Multiple approaches to predicting oxygen and glucose consumptions by HepG2 cells on porous scaffolds in an axial-flow bioreactor.

In this study, the distribution of oxygen and glucose was evaluated along with consumption by hepatocytes using three different approaches. The methods include (i) Computational Fluid Dynamics (CFD) simulation, (ii) residence time distribution (RTD) analysis using a step-input coupled with segregation model or dispersion model, and (iii) experimentally determined consumption by HepG2 cells in an open-loop. Chitosan-gelatin (CG) scaffolds prepared by freeze-drying and polycaprolactone (PCL) scaffolds prepared by salt leaching technique were utilized for RTD analyses. The scaffold characteristics were used in CFD simulations i.e. Brinkman's equation for flow through porous medium, structural mechanics for fluid induced scaffold deformation, and advection-diffusion equation coupled with Michaelis-Menten rate equations for nutrient consumption. With the assumption that each hepatocyte behaves like a micro-batch reactor within the scaffold, segregation model was combined with RTD to determine exit concentration. A flow rate of 1 mL/min was used in the bioreactor seeded with 0.6 × 10(6) HepG2 cells/cm(3) on CG scaffolds and oxygen consumption was measured using two flow-through electrodes located at the inlet and outlet. Glucose in the spent growth medium was also analyzed. RTD results showed distribution of nutrients to depend on the surface characteristics of scaffolds. Comparisons of outlet oxygen concentrations between the simulation results, and experimental results showed good agreement with the dispersion model. Outlet oxygen concentrations from segregation model predictions were lower. Doubling the cell density showed a need for increasing the flow rate in CFD simulations. This integrated approach provide a useful strategy in designing bioreactors and monitoring tissue regeneration.

Numerical and Experimental Investigation of the Effect of Geometrical Parameters on the Performance of a Contraction-Expansion Helical Mixer

Abstract In this study, the mixing efficiency of a passive contraction-expansion helical mixer, which combines several features, such as helical pipes for induction of secondary flows and sudden expansion and contraction array for expansion vortices, was numerically and experimentally studied. We employed the method of Box–Behnken to select the appropriate design points. Then, various configurations were investigated via computational fluid dynamics (CFD) simulations. The extent of mixing was evaluated by monitoring the residence time distribution (RTD) and observing the shape of the RTD curves. A fast competitive-consecutive diazo coupling reaction is carried out to validate the RTD results. The influences of radius of curvature of the helical mixer, ratio of the length of the contraction part to expansion part, pitch of helical mixer, and the Reynolds number (Re) on mixing efficiency, and pressure drop were also investigated. As expected, the radius of curvature of the helical mixer, ratio of the length of the contraction part to expansion part, and the Reynolds number affected significantly the mixing efficiency, while the pitch of helical mixer had little influence on mixing efficiency. Quadratic models for mixing efficiency and pressure drop were then proposed and could be used for designing the optimal contraction-expansion helical mixer for the required pumping power.

Design of an Inductance Measurement System for Determination of Particle Residence Time in a Dual Circulating Fluidized Bed Cold Flow Model

A tracer impulse method is designed and implemented for determination of the residence time distribution (RTD) of particles in a cold flow model of a dual circulating fluidized bed chemical looping pilot plant. Ferromagnetic particles are used as a tracer. The method based on inductance measurements previously proved its suitability and convenience for determination of the concentration of ferromagnetic particles and the bed porosity. The current paper presents an improved system for the determination of inductance, based on an impedance circuit bridge. Injection and detection systems were designed and optimized for an extremely small amount of tracer, about 0.05% by mass. The system is entirely described, variables relevant for the attainment of an appropriate measurement are analyzed, and the first results of RTD experimentally determined are presented. Signals of high quality were obtained and successfully fitted to a simple model. The system shows a great capacity regarding the response time, quality ...

Residence Time Distribution Determination of a Continuous Stirred Tank Reactor using Computational Fluid Dynamics and its Application on the Mathematical Modeling of Styrene Polymerization

Abstract The continuous operation of a stirred tank reactor for styrene polymerization was modeled. The proposed approach consists of an iterative procedure between two modules that considers the fluid-dynamics and kinetics respectively. The kinetic module considers a complex kinetic mechanism and is used to predict the time evolution of global variables, such as conversion and species concentrations, physicochemical properties and molecular structure characteristics of the final product. In order to obtain a 3D representation of the flow field, the simulation of the hydrodynamics of the reactor was carried out with the aid of a commercial computational fluid dynamics (CFD) software package. Because CFD is capable to predict the complete velocity distribution in a tank, it provided a good alternative to carry out residence time distribution (RTD) studies. It was found that the stimulus-response tracer method is reasonably accurate to obtain a complete RTD compared to the particle tracking method. The obtained RTD results showed a good agreement when validated with experimental data and literature information.From the estimates of the kinetic module and the RTD predictions, a statistical calculus allows the determination of the average properties at the reactor outlet. The convergence of the iterative procedure was tested and reasonable predictions were achieved for an industrial reactor.

Determination of the Residence Time Distribution in Twin Screw Extruders via Free Radical Modification of PE

Abstract The main objective of the present work was to introduce an inline method for measuring the residence time distribution (RTD) of twin screw extruders on the basis of information obtained from the melt free radical modification of polyethylene in a modular intermeshing co-rotating twin screw extruder. The trend of increasing rate of the extruder total torque resulted from replacing the neat polyethylene feed by a mixture of polyethylene and reactants which are chemically capable of creating chainbranching and/or crosslinking reactions was considered as the main information source for evaluating the RTD of the extruder. The RTD results were compared with those obtained from the tracer pulse input method, and good agreement was found. It was also demonstrated that, the present in-line RTD measuring technique has reliability of following the effect of feeding rate, screw speed and reactant concentration on the RTD of the twin screw extruders. It was found that, increasing the feeding rate results in decreasing the minimum residence time, and results in a narrower RTD. At constant feeding rate increasing the screw speed decreased the minimum residence time and broadened the RTD. The results also showed that, reactant concentration and variation of the viscosity along the screw have not appreciable effect on the twin screw extruders RTD.

Effects of wetland depth and flow rate on residence time distribution characteristics

The residence time distribution (RTD) representing the hydraulics of a wetland is an important tool for modeling and designing treatment wetlands for optimal constituent removal. To correctly use RTD results, it is necessary to understand the conditions under which this distribution remains stable. Dye tracer experiments were conducted on a stormwater treatment wetland to investigate hydrologic factors affecting RTD characteristics. Dye was introduced into the inflow under normal flow conditions and during simulated storm flows, providing a range of flow rates and water levels. Dye distribution in the outlet was measured using an in situ fluorometer. Results indicate that flow rates did not have a significant effect on RTD characteristics. The RTDs normalized for volume and flow demonstrated a greater amount of short-circuiting and a larger mixing scale when water depth increased, demonstrating that water level can have a direct impact on the RTD of a wetland. This effect suggests that more than one RTD may be necessary for analyzing a wetland subject to changing water levels. For the wetland in this study, increasing the water depth elicited a decrease in hydraulic efficiency. Understanding such factors that affect hydraulic efficiency will aid in the design and management of wetlands.

Numerical and Physical Simulation of Tundish Fluid Flow Phenomena

The fluid flow in a continuous casting tundish is numerically and physically simulated by means of water models. Results of residence time distribution (RTD) measurements and laser-optical measurements (Laser Doppler Anemometry - LDA, Digital Particle Image Velocimetry - DPIV) are used to validate the numerical results for water before the numerical simulation is transferred to the steel melt. The investigations are focused on both steady-state and transient casting conditions. To reduce vortexing and turbulence in the tundish different types of turbo-stoppers are installed in the water models and their influence on the spacious flow structure is discussed. The turbo-stopper produces higher turbulence in the inlet region of the tundish, but this region is spatially more limited in relation to the flow without turbo-stopper. Thereby a more homogeneous flow is created at the outlet of the tundish with better conditions for particle separation. Basic design criteria for the geometry of a turbo-stopper are developed. Moreover, the processes of first tundish filling and ladle change are simulated at a downscaled water model and these results are compared with numerical simulations using a Volume of Fluid (VoF) model. This multiphase model is able to reproduce the motion of gas bubbles and waves at the free surface.

Hydrodynamic and mass transfer characterization of a down flow jet loop bioreactor

The effects of certain pertinent parameters such as gas and liquid flow rates and nozzle position on the behavior of a down flow jet loop bioreactor (DJB) have been studied. The mean residence times of gas and liquid phases and the gas hold-up within the bioreactor have been measured. A correlation has been presented for the gas hold-up. In addition, the overall volumetric mass transfer coefficient, and the influence of the gas flow rate and the position of the nozzle inside the draft tube on the latter has been determined and a correlation has been derived. The liquid residence time distribution (RTD) within the bioreactor has been studied by tracer analysis and the mixing time was calculated using the RTD results.

Assessment of the Design Parameters for Wastewater Treatment by Reverse Osmosis

A modelling procedure is presented to predict the fluxes and solute concentrations in different flows for reverse osmosis (RO) spiral-wound modules. Important underlying factors for this procedure are the osmotic pressure for various solutions, the hydrodynamic flow profile in the concentrate channel, and the intrinsic separation characteristics of the membrane material. Experiments were carried out using a flat sheet test cell to determine the parameters of the mass transport model. Results of residence time distribution (RTD)-measurements on an industrial spiral-wound module were used to determine macroscopic fluid flow regimes resulting in the definition of dead volume fraction, average residence time and Pe-number. The evaluation of the modelling procedure has been based on experimental data of an industrial membrane plant system.