Residence Time Distribution Behavior Research Articles

Particle residence time distributions in a vortex-based solar particle receiver-reactor: An experimental, numerical and theoretical study

We report a joint experimental, numerical and theoretical study of particle residence times in a novel vortex-based vessel for thermal processing of suspended particles. The tracer pulse-response method, in which the particle phase itself is employed as the tracer, is used to measure the particle residence time distribution (RTD) within a laboratory-scale model of a class of Solar Expanding Vortex Receiver-Reactor (SEVR). The operating parameters of particle size, gas volumetric flow rate and inlet velocity were systematically varied to assess their influence on the particle RTD and to determine the mechanisms controlling the behaviour of the two-phase flow in the SEVR. The particle RTD behaviour is also described by a compartment model consisting of a small plug flow reactor followed by a series of two interconnected continuously-stirred tank reactors (CSTRs).

Modeling Residence Time Distribution (RTD) Behavior in a Packed-Bed Electrochemical Reactor (PBER)

This paper focuses on understanding the electrolyte flow characteristics in a typical packed-bed electrochemical reactor using Residence Time Distribution (RTD) studies. RTD behavior was critically analyzed using tracer studies at various flow rates, initially under nonelectrolyzing conditions. Validation of these results using available theoretical models was carried out. Significant disparity in RTD curves under electrolyzing conditions was examined and details are recorded. Finally, a suitable mathematical model (Modified Dispersed Plug Flow Model (MDPFM)) was developed for validating these results under electrolyzing conditions.

Modeling and simulation of continuous powder blending applied to a continuous direct compression process

Continuous manufacturing techniques are increasingly being adopted in the pharmaceutical industry and powder blending is a key operation for solid-dosage tablets. A modeling methodology involving axial and radial tanks-in-series flowsheet models is developed to describe the residence time distribution (RTD) and blend uniformity of a commercial powder blending system. Process data for a six-component formulation processed in a continuous direct compression line (GEA Pharma Systems) is used to test the methodology. Impulse tests were used to generate experimental RTDs which are used along with parameter estimation to determine the number of axial tanks in the flowsheet. The weighted residual from the parameter estimation was less than the χ2 value at a 95% confidence indicating a good fit between the model and measured data. In-silico impulse tests showed the tanks-in-series modeling methodology could successfully describe the RTD behavior of the blenders along with blend uniformity through the use of radial tanks. The simulation output for both impulse weight percentage and blend uniformity were within the experimentally observed variance.

Residence Time Distribution of A Tubular Reactor

Most flow reactors are neither ideal plug flow nor continuous stirred tank reactors. This makes it difficult in accounting for actual conversion obtained from such reactors thus causing much concern to the industrialists especially Chemical Engineers. The extent of departure from the two ideal limits of flow reactors remained unclear until the concept of residence time distribution (RTD) was developed and applied to real reactors, to assess the extent of non-ideality, plus their effects on reactor performance. To study the empirically generated RTD behavior, a tubular reactor was designed and built with improved features to ensure minimum axial dispersion. It was found that the RTD functions generated were generally of the shapes expected, with long tails beyond the peak concentration exit age, but with mean residence times ( ), and distribution variance fairly large, and sensitive to flow rates. While a range of experimental conditions were tried, the run with the highest turbulent Reynold Number (Re) revealed = 46s, which differed markedly from the expected (i.e., calculated) reactor space time ( ) of 17s. This significantly higher empirical was due to transport delay ( 15s) and delay due to actual axial dispersion (31s) as obtained under the peak of the E(t) curve. Furthermore, it was also found that variance of 60s 2 obtained in this work signified that the axial dispersion was high, despite high Re of 3.3 giving a calculated Peclet Number of 69. Also, when n-CSTR theoretical model was applied to that same run, an n value of 35 was estimated, which was good compared to the theoretical value of n = However, at lower flow rates and Re, the implied n in the n-CSTR model was too optimistic and high, with n =15. However, would have tallied better with the high observed axial dispersion. But that was a weakness of n-CSTR theory, not of the empirical results. Thus, every reactor is neither a CSTR nor PFR but a continuum exhibiting features of ideality of both reactors. It is recommended that RTD be applied as a tool not only for troubleshooting existing reactors but also in test running new ones as it would give insight into designing future reactors with improved performance. Keywords: Residence Time Distribution, Plug Flow Reactor, Tubular Reactor, Axial dispersion, Peclet number.

Narrow residence time distribution in tubular reactor concept for Reynolds number range of 10–100

For chemical reactions, which require residence times of several hours, enhanced heat transfer, or narrow residence time distribution (RTD), good radial mixing combined with poor axial mixing in laminar flow regime has long been desired by industry and R&D. The main goal of this work is to obtain the narrowest RTD curve in a continuously operated reactor at Reynolds numbers smaller than 100. By using a stepwise method the most promising reactor type was chosen to meet the requirements. Design parameters of this reactor, the coiled flow inverter (CFI), were characterized and their effects on RTD were experimentally investigated. Design of CFI includes several straight helix modules, where the tubular reactor is coiled around a coil tube. After each straight helix module, the coil direction is changed by a 90°-bend. As a starting point for designing a CFI reactor for specific applications, the “best performance” design space diagram was investigated. Regarding narrowing RTD, the diagram gives the user the design space for the CFI reactor, which leads to the best performance. The most significant design parameter regarding a narrow RTD was experimentally determined as number of bends. By using a CFI design consisting of 27 bends at volume flow rate of 3mL/min, which corresponds to Reynolds number of 24 and mean residence time of 2.6h, a Bodenstein number over 500 was achieved. Beside its narrow RTD behavior, CFI is a compact and cost-efficient reactor concept, which is flexible to scale-up and implement for different processes, even for single-use applications.

Reprint of: Measuring and modeling the residence time distribution of gas flows in multichannel microreactors

The optimization of microreactor designs for applications in chemical process engineering usually requires knowledge of the residence time distribution (RTD). The applicability of established models to microstructured reactors is currently under debate [1–4]. This work presents investigations of the RTD behaviour for gas flows in microstructured devices by CFD (computational fluid dynamics) simulations and new experimental data for the RTD of different microstructured reactors (some provided by industrial partners). Influence of the in- and outlet regions of the devices and uneven flow distribution inside the microstructure on the RTD are discussed. Hereby, boundary conditions for the applicability of commonly used dispersion model and the correlation proposed by Taylor and Aris for the model parameter Bodenstein number Bo are explained. The experimental data for the RTD of the microreactors investigated are analysed, whereby a good agreement of simplified models for the RTD of the complete devices (including- in and outlet regions) is found. The determination of the model parameter may be difficult as several factors have to be considered. Here, an extended correlation based on experimental data is suggested.

Measuring and modeling the residence time distribution of gas flows in multichannel microreactors

The optimization of microreactor designs for applications in chemical process engineering usually requires knowledge of the residence time distribution (RTD). The applicability of established models to microstructured reactors is currently under debate [1–4]. This work presents investigations of the RTD behaviour for gas flows in microstructured devices by CFD (computational fluid dynamics) simulations and new experimental data for the RTD of different microstructured reactors (some provided by industrial partners). Influence of the in- and outlet regions of the devices and uneven flow distribution inside the microstructure on the RTD are discussed. Hereby, boundary conditions for the applicability of commonly used dispersion model and the correlation proposed by Taylor and Aris for the model parameter Bodenstein number Bo are explained. The experimental data for the RTD of the microreactors investigated are analysed, whereby a good agreement of simplified models for the RTD of the complete devices (including- in and outlet regions) is found. The determination of the model parameter may be difficult as several factors have to be considered. Here, an extended correlation based on experimental data is suggested.

Continuous water treatment by adsorption and electrochemical regeneration

This study describes a process for water treatment by continuous adsorption and electrochemical regeneration using an air-lift reactor. The process is based on the adsorption of dissolved organic pollutants onto an adsorbent material (a graphite intercalation compound, Nyex ®1000) and subsequent electrochemical regeneration of the adsorbent leading to oxidation of the adsorbed pollutant. Batch experiments were carried out to determine the adsorption kinetics and equilibrium isotherm for adsorption of a sample contaminant, the organic dye Acid Violet 17. The adsorbent circulation rate, the residence time distribution (RTD) of the reactor, and treatment by continuous adsorption and electrochemical regeneration were studied to investigate the process performance. The RTD behaviour could be approximated as a continuously stirred tank. It was found that greater than 98% removal could be achieved for continuous treatment by adsorption and electrochemical regeneration for feed concentrations of up to 300 mg L −1. A steady state model has been developed for the process performance, assuming full regeneration of the adsorbent in the electrochemical cell. Experimental data and modelled predictions (using parameters for the adsorbent circulation rate, adsorption kinetics and isotherm obtained experimentally) of the dye removal achieved were found to be in good agreement.

Residence Time Distribution Studies in Microfluidic Mixing Structures

AbstractThe residence time distribution (RTD) characteristics of three microreactors containing different passive mixing structures, namely, a three dimensional serpentine structure, a split‐and‐recombine structure and a staggered herringbone structure, were investigated and compared. An experimental input‐response technique was applied which required deconvolution of the measured data by modeling of the RTD. The proposed technique provides useful information on optimized application and operation of microfluidic devices. The serpentine reactor and the split‐and‐recombine reactor show improvement in RTD behaviour, i.e., narrowing of RTD curves, at Re‐numbers > 30 due to effective transversal mixing and therefore reduced axial dispersion. In the case of the staggered herringbone structure, dead volumes could be observed which considerably affect the RTD.

Development and evaluation of novel designs of continuous mesoscale oscillatory baffled reactors

Oscillatory baffled reactors (OBRs) are a form of plug flow reactor, ideal for performing long reactions in continuous mode, as the mixing is independent of net flow rate leading to more compact and practical designs. Mesoscale OBRs are currently being developed for laboratory-scale processes. The systems are designed to scale-up to industrial scale directly, or to be used as small-scale production platforms in their own right. Three different meso-reactor baffle designs (integral baffles, helical baffles and axial circular baffles (or “central”) baffles) were developed. These designs were chosen, as they are easily fabricated at “mesoscales” (here typically ∼5 mm diameter) and can be operated at low flow rates (μl/min to ml/min), whereas conventional designs of OBRs cannot. It was found that an increase in the net flow Reynolds number increased the optimum range of oscillatory Reynolds numbers over which plug flow can be achieved. The oscillation conditions had little effect on the residence time distribution behaviour at net flow Reynolds numbers above 25. These designs are effective and robust in scaled-down production of high added value products and decreasing the reagents required.

Charged droplet and particle-mixing studies in liquid–liquid systems in the presence of non-linear electrical fields

This paper is concerned with the mixing and residence time distribution behaviour of an experimental electrically enhanced column contactor in which electrically charged drops of water were fed into an organic liquid phase of sunflower oil across which a non-linear electrical field was maintained. The effects of electrical field strength upon mixing in the continuous phase were determined and a significant increase in mixing was observed as the field strength was increased. A quantitative theoretical development describing the observed behaviour is also presented. The mixing is described by simultaneous consideration of the movement of the drop phase and the continuous liquid phase. The drop motion, after formation at the charged nozzle and during the subsequent trajectory motion under the influence of the non-linear electric field, is analysed. The interaction between the drop motion and the velocity field of the continuous phase is thus described. The paper describes theoretical prediction of the interactions between charged drops and the electrodes. These interactions can exert a strong influence on the overall residence time of drops passing through the column, and potentially on the velocity profile of the continuous phase. Description of both sets of interactions is used to predict mixing patterns in the continuous phase as a function of applied field strength. The predicted mixing patterns are compared with experimentally recorded images of tracer movement in the continuous phase obtained under the same conditions. The calculated patterns compare favourably with those measured by experiment. The modelling methodology was used afterwards with success for predicting the residence time distribution in the form of the normalised concentration (proportional to the exit function) versus time.

Particle residence time distribution (RTD) in three-phase annular bubble column reactor

The residence time distribution of solids in an 18-l pilot scale three-phase annular reactor has been obtained as a function of gas and liquid flow rates in order to characterize the influence of fluid dynamics on solids mixing. Both co-current and countercurrent operations were investigated. Gas velocity in the range 0.05– 0.5 cm s −1 was employed, while the liquid velocity was varied between 0.03 and 0.13 cm s −1 . Commercial titania powder was used as a tracer. The RTD curves for the co-current operation showed a bimodal behavior whose peaks decreased with decreasing gas flow rate. As a result, the solid flow dynamics was modeled by a series of CSTRs in parallel with a plug flow reactor. The model also admitted interphase solid transport between the wake and bulk liquid phase and yielded predictions with a mean squared error between 3% and 6%. The RTD behavior for countercurrent operation was, however, unimodal and therefore modeled as a series of stirred tanks with a recycle stream. The associated mean squared error was in the range 1–4%. Although fluid flow rates were held constant during tracer experiments, there seemed to be an evolution of solids flow pattern within the reactor as evidenced by the time-dependency of the intensity function, λ.

Physical and mathematical modeling of pyrometallurgical channel reactors with bottom gas injection: Residence time distribution analysis and ideal- reactor- network model

An ideal-reactor-network model has been developed to describe the fluid flow and mixing observed in pyrometallurgical channel reactors with countercurrent liquid flow and high-strength bottom gas injection. Experiments were performed in a cold model under various operating conditions to determine the effects of liquid density, viscosity, flow rate, gas injection rates, injector diameter, and injector spacing on the residence time distribution (RTD) behavior of the channel reactor. The ideal-reactor-network model was used to represent the experimentally observed RTD behavior in both heavy- and light-liquid phases in the channel reactor. The model requires one parameter for each liquid, and all other model parameters are determined on the basis of this parameter. Correlations were developed for the parameters using dimensionless variables. There is excellent agreement between model predictions and experimental measurements for a wide range of experimental conditions. Scale-up calculations and RTD predictions have been made for a QSL leadmaking reactor as well as for proposed channel reactors for coppermaking and steelmaking.