Ideal Plug Flow Research Articles

Computational Fluid Dynamics–Discrete Element Model Simulation of Flow Characteristics and Solids’ Residence Time Distribution in a Moving Bed Air Reactor for Chemical Looping Combustion

In our previous experimental study, a novel multistage moving bed air reactor was proposed for chemical looping combustion; some operating mechanisms including the gas flow carrying capacity and gas leakage performance were investigated. In this paper, a quasi-2D computational fluid dynamics–discrete element model cold flow model is developed for the experimental air reactor. Effects of the operating parameters, geometric parameters, and internals on solids’ flow characteristics and residence time distributions are studied by using the tracer particles. Results show that with increase in the loop seal gas velocity, the solids’ mass flux increases and the average solids’ residence time decreases. The reactor angle has more influence in solids’ movements in the near-wall region, while the wedge angle has more effect on the solids’ movements in the lower part of the reactor. With decrease of the reactor angle, increase of the wedge angle, or increase of the down-comer diameter, the solids’ flow gets closer to ideal plug flow in the reactor. The internals can accelerate the particles in the near-wall region; internals in triangular arrays are better than those in square arrays to slow down the central particles.

Kinetic modeling of the photocatalytic degradation of methyl ethyl ketone in air for a continuous-flow reactor

This paper reports the study on kinetic modeling of photocatalytic degradation of methyl ethyl ketone (MEK) using TiO2 coated on silica fiber felts. Two different mathematical models, ideal plug flow model and axially dispersed plug flow model (dispersion model), were used to simulate the performance of the PCO reactor. Six kinetic rate equations on the basis of Langmuir-Hinshelwood (L-H) expression were examined to find the best model that fits the experimental data. Moreover, the light intensity distribution on the photocatalyst surface was simulated using the linear source spherical emission model (LSSE) and validated with the experimental data. The Beer-Lambert model was also applied to demonstrate the diminishment of light intensity in the PCO filter. For model prediction validation, a series of experiments was carried out in a bench-scale continuous flow reactor at different concentrations (0.1–1 ppm), relative humidities (17–67%), flow rates (10–30 L/min), and light intensities (7–23.5 W/m2). In addition, adoption tests with MEK at various concentrations (0.25–1 ppm) and relative humidities (15–50%) were conducted under dark condition. Formaldehyde, acetaldehyde, acetone, and propionaldehyde were identified (through HPLC analysis) as by-products of the MEK PCO reaction. Owing to higher concentrations of formaldehyde and acetaldehyde, aldehyde group was considered as the main by-products. The result of carbon balance indicated that 7.2% of MEK mineralized to CO2 (measured by GC-FID) and around 10.8% of that was converted to by-products (for 18.4% removal efficiency). Evaluation of different kinetic rate expressions revealed that the unimolecular L-H model considering inhibitive effect of water vapor and by-products for adsorption provided the best fit. The dispersion model combined with this rate expression had a higher accuracy at all operating conditions while the plug flow model failed to predict the PCO performance at various intensities and air velocities. Evaluation of mass transfer was also performed, and it was concluded that the mass transfer effect on the PCO process is not dominating step; consequently, the PCO reaction is the rate-limiting process.

A New Mathematical Model for the Prediction of Internal Recirculation in Impinging Streams Reactors

A mathematical model for the prediction of internal recirculation of complex impinging stream reactors has been presented. The model constitutes a repetition of a series of ideal plug flow reactors and CSTR reactors with recirculation. The simplicity of the repeating motif allows for the derivation of an algebraic relation of the whole system using the Laplace transform. An impinging streams reactor system with one axial and two tangential inlet fluid streams was constructed and considered as a case study. The model predicts satisfactorily the complex and flow rate dependent experimental residence time distribution functions obtained employing a pulse tracer method for different total flow rates of the incoming feed. The variation of the controlling parameters with changing the total inlet flow rate is discussed. The presented model can predict complex internal recirculation streams within the impinging streams reactor system.

Experimental methods in chemical engineering: Residence time distribution—RTD

AbstractReactor performance, solids‐(gas)‐mixing, flow through porous media, distillation columns, or through granulators improve as the fluid dynamics approach ideal plug flow. The residence time distribution (RTD) is a diagnostic measure of how close fluid flow approaches ideal conditions. The technique introduces a step change to the inlet concentration—a Dirac‐δ function, Heaviside step function, or a rectangular pulse (bolus)—while high frequency detectors monitor the concentration along the vessel and/or at the exit. The effluent concentration profile spreads due to the variance in the process lines leading to the vessel and at the exit, the detector response, and the system. We quantify how much each of these contributes to the overall variance in a fluidized bed with 9 g of fluid cracking catalyst in 8 mm diameter quartz tubes. The injection variance is lowest for a GC sample loop configuration, compared to a 3‐way valve or 4‐way valve geometry. RTD measurements detect bypassing due to dead zones in vessels and the axial‐dispersion model and continuous stirred‐tank model to characterize deviation from plug flow. However, when the contribution to variance from the ancillary lines and detector is large compared to the system, the uncertainty in the model parameters is high. Research on RTD fundamentals concentrate on boundary conditions while, here, we focus on experimental errors: mechanical, physicochemical mathematical, and instrumental.

Assessment of flow pattern and temperature profiles by residence time distribution in typical structured packed beds

The flow and temperature fields in packed beds are rather complex but are of great importance since they will influence macroscopic parameters and the overall performance of the packed bed system. Regions with either too high or too low velocity are not preferred since they will decrease the heat and mass transfer characteristics of packed beds. The residence time distribution (RTD) is a way to assess the characteristics of the flow maldistribution, where a narrower distribution is closer to the ideal plug flow with less dispersion. In this article, the RTD in three typical structured packed beds, namely, the simple cubic (SC) packing, the body-centered cubic (BCC) packing and the face-centered cubic (FCC) packing, are studied by using ANSYS FLUENT. Results show that, firstly, the RTD curve can be used to diagnose the flow pattern in packed beds where the skewed peak indicates the channeling effect and a long tail relates to recirculation. It is revealed secondly that, FCC packing has the narrowest RTD curve in the studied structures, BCC packing comes second and SC packing has the flattest RTD curve, which demonstrates that the flow uniformity in FCC packing is the best. For the same packing form, the flow is less maldistributed at higher velocity. Finally, it is shown that the packing form with less dispersion, which behaves more like the ideal plug flow, has a more uniform distribution of the particle-to-fluid heat transfer coefficient near the particle surface.

Residence Time Distribution of a Capillary Microreactor Used for Pharmaceutical Synthesis

AbstractThe main drivers for application of small‐scale reactors in the pharmaceutical industry are the possibility of rapid synthesis and screening of novel drugs as well as the readiness of the scale‐up. The characterization of fluid flow pattern was performed through step‐up and step‐down residence time distribution experiments using a tracer at six different flow rates. Four single‐parameter models were considered for representing deviations from ideal plug flow and ideal laminar flow in tubes. The model that provided the best results was the axial dispersion model and the Peclet and Reynolds numbers could be well correlated. Obtained Peclet values from 44 to 244 were close toPe > 100, in which axial dispersion can be neglected and the reactor can be considered as plug flow reactor.

Mixing Performance in a Distributed-Feed Plate-Type Reactor with Multinozzle Injection for Fine Chemical Production Scale

The mixing performance of a distributed-feed plate-type reactor equipped with curved channel design and microstructured multinozzle injectors was investigated. Almost ideal plug flow behavior with ...

Modeling of gas-phase heterogeneous photocatalytic oxidation reactor in the presence of mass transfer limitation and axial dispersion

Mass transfer plays a critical role in the efficiency of photocatalytic oxidation (PCO) technology for air purification applications. There has been limited work on the exploration of mass transfer in the PCO reactor with the non-ideal flow. In this work, the performance of a continuous heterogeneous ultraviolet photocatalytic oxidation (UV-PCO) reactor was investigated and addressed under mass transfer limitation and axial dispersion. First, CFD modeling was used to determine the flow distribution in the reactor at various airflow rates. The residence time distribution (RTD) analysis with a tracer gas (CO2) was carried out for the experimental validation of the simulation model and good agreement between experimental and simulation data was achieved. Further, a quick and straightforward methodology employing an axial dispersion plug flow model was used to study the RTD. The proposed model could predict the residence time distribution of CO2 with high accuracy. Results of the RTD for CO2 in the presence and absence of a PCO filter (silica fiber felts (SFF) modified with TiO2) were almost identical due to the high porosity of the filter. The model was then used for the evaluation of the axial dispersion of methyl ethyl ketone (MEK) at different flow rates. Owing to a low value of the Peclet number (Pe < 100), the flow in the reactor deviated from an ideal plug flow and dispersion could not be ignored. The photocatalysis reaction of MEK by the SFF filter under the mass-transfer-controlled regime was further addressed. A steady-state mass balance equation was implemented for the assessment of the performance of the UV-PCO reactor in the presence of axial dispersion and mass transfer limitation. The mass transfer coefficient was calculated with the use of the developed model and a correlation for the Sherwood number was proposed to relate the mass transfer coefficient to the flow rate and fiber structure. The proposed correlation was assessed and compared with the other empirical formulas available in the literature.

Scaling of a catalytic cracking fluidized bed downer reactor based on computational fluid dynamics simulations.

Circulating fluidized bed downer reactors (downer reactors) exhibit good heat and mass transfer, and the flow behavior approaches the ideal plug flow. This reactor is superior for catalytic cracking reactions in which the intermediate is the desired product. However, the hydrodynamic behavior and reactor performance have mostly been investigated in small-scale or laboratory-scale reactors. The objective of this study was to investigate the up-scaling of the catalytic cracking of heavy oil in three downer reactors with heights of 5, 15, and 30 m, using computational fluid dynamics simulations. A two-fluid model with the kinetic theory of granular flow was used to predict the hydrodynamics and performance of the chemical reactions. The kinetics of catalytic cracking of heavy oil were described by a 4-lump kinetic model. The chemical performance similarity was identified by using radial and axial distributions of heavy oil conversion, gasoline mass fraction, and gasoline selectivity. The chemical performance similarity cannot be achieved by using the hydrodynamic similarity parameter . A modified up-scaling parameter was proposed, . The chemical performance similarity of identical catalytic cracking downer reactors can be achieved with deviation in the range of ±10% and mean relative absolute error of less than 5%.

Theoretically and experimentally founded simulation of the appendix gap in regenerative machines

Regenerative machines, such as the Stirling engine, may contribute substantially to the solution of the present environmental problems, if they are designed for high efficiency. This requires a thorough understanding and an accurate modelling of the various losses, including the so-called appendix gap loss. For this purpose, some analytical models as well as a few one-dimensional numerical approaches have been developed so far. In this contribution, a comparison of the results obtained by these is performed for the reference case of a well-documented experimental machine. It reveals significant discrepancies regarding the optimum gap width as well as the magnitude of the loss. Particularly for large gap widths, increasing deviations between the analytical and the numerical models are observed. The latter even predict an unrealistic maximum. This discrepancy can be resolved by additionally considering the p,V-work done by the moving seal. Further deviations at small gap widths can be attributed to the interdependence of the loss and the actual wall temperature gradients, which is included in the numerical models. Instead, a preset value is assumed in the analytical models. This also applies to a newly developed, enhanced model, which predicts the optimum width reliably, but overestimates the magnitude of the loss throughout. This underlines a need for one-dimensional differential simulations in addition to analytical modeling. However, these require separate modeling approaches for both the radial and axial energy transport. Unfortunately, the choice of these significantly affects the results. Since none of the approaches proposed so far is theoretically or experimentally founded, there is no basis for a correct choice. To solve this problem, the aforementioned analytical model is considered, since it is theoretically based and further supported by recent experimental results presented in this contribution. It is therefore used to analytically derive correlations for the phase-shifted heat exchange with the walls and the interrelation of enthalpy and mass flow. Thus, a theoretically and experimentally founded reference model is available now. Comparing the results obtained by this reference model and the other numerical approaches, it is found that an empirical model based on the assumption of parabolic radial temperature profiles and ideal plug flow yields almost the same results as the former. Since this model is easier to use, it can therefore be recommended for general practical use. The reference model may, however, be used for countercheck purposes and for further optimization work.

Intrinsic methane steam reforming kinetics on nickel-ceria solid oxide fuel cell anodes

Direct internal reforming in solid oxide fuel cells (SOFCs) is advantageous as it enables to heat and steam from the exothermic hydrogen oxidation reaction in the endothermic steam reforming reaction. However, it may increase potentially deteriorating temperature gradients as well. The temperature and concentration profiles can be accurately simulated with adequate SOFC models and intrinsic methane steam reforming (MSR) kinetics. Therefore, this study aims to derive intrinsic MSR kinetics suitable for control-oriented dynamic SOFC models. The individual influences of the methane, steam and hydrogen partial pressures on the MSR reaction are experimentally studied on functional electrolyte supported cells with nickel-gadolinium doped cerium anodes. A non-proportional dependence of the MSR rate on the methane partial pressure and a slight negative dependence on the steam partial pressure are observed, but the effect of the hydrogen partial pressure seems insignificant. Various kinetic rate equations are parameterised with the experimental data and an ideal plug flow reactor model. An intrinsic Langmuir-Hinshelwood mechanism for a rate determining step between associatively adsorbed methane and dissociatively adsorbed steam on the catalyst surface shows good agreement with the experimental data, and is thermodynamically and physically consistent.

Hydrodynamics and dynamic capacity of cryogels produced with different monomer compositions

ABSTRACTCryogels have received great attention in the area of bioseparation due to their characteristic of very large and interconnected pores. These materials allow rapid isolation of various biomolecules and permit that chromatographic processes are carried out at high flow rates. Nevertheless, studies regarding the effect of monomer concentration on the hydrodynamics and dynamic capacity of cryogels are scarce. In this study, supermacroporous polyacrylamide cryogels with different monomer and crosslinker concentrations were produced by cryopolymerization and activated with tris(hydroxymethyl)aminomethane. Porosity and permeability were determined and it was observed that these parameters changed with different monomer concentrations, indicating changes in the structure of the cryogels. Residence time distribution experiments were conducted at different flow rates and different mobile phase viscosities. As a function of composition of the cryogels, the axial dispersion increased and the flow pattern deviated considerably from the ideal plug flow model. For cryogels with lower crosslinker content, the effect of the mobile phase viscosity was more evident. The monomer composition changed the dispersion number and height equivalent to one theoretical plate, resulting in an impact on the adsorption capacity of the cryogel. Consequently, proper monomer and crosslinker concentrations may produce cryogels with suitable performance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48507.

Explicit Residence Time Distribution of a Generalised Cascade of Continuous Stirred Tank Reactors for a Description of Short Recirculation Time (Bypassing)

The tanks-in-series model (TIS) is a popular model to describe the residence time distribution (RTD) of non-ideal continuously stirred tank reactors (CSTRs) with limited back-mixing. In this work, the TIS model was generalised to a cascade of n CSTRs with non-integer non-negative n. The resulting model describes non-ideal back-mixing with n &gt; 1. However, the most interesting feature of the n-CSTR model is the ability to describe short recirculation times (bypassing) with n &lt; 1 without the need of complex reactor networks. The n-CSTR model is the only model that connects the three fundamental RTDs occurring in reactor modelling by variation of a single shape parameter n: The unit impulse at n→0, the exponential RTD of an ideal CSTR at n = 1, and the delayed impulse of an ideal plug flow reactor at n→∞. The n-CSTR model can be used as a stand-alone model or as part of a reactor network. The bypassing material fraction for the regime n &lt; 1 was analysed. Finally, a Fourier analysis of the n-CSTR was performed to predict the ability of a unit operation to filter out upstream fluctuations and to model the response to upstream set point changes.

3D CPFD simulations of gas-solids flow in a CFB downer with cluster-based drag model

The interactions of gas and solids are important in determining the flow structures in the Circulating Fluidized Bed (CFB) downer. Proper description of the interphase drag model is thus crucial for simulating the downer. Considering that cluster with several numbers of particles can be present in the downer, an interphase drag model was developed by considering the dynamic equilibrium of the amount of particles in and out of the cluster in the downer, which was then incorporated into the Computational Particle Fluid Dynamics (CPFD) model. CPFD simulation results show that the predicted particle flow behaviors are in good agreements with experimental data under different operating conditions. Solids circulation rate and gas velocity can significantly affect particle flow behaviors especially in the acceleration region near the inlet. More amounts of particles flow faster in the center while a few part of particles flow slower near the wall, resulting that the particle residence time is shorter and uniform in the center and longer near the wall. The residence time distribution (RTD) curve of particles is narrower with a higher peak in the downer than in the riser. Simulation results indicate that less back-mixing of particles is present in the downer, and the flow structure in the downer approximates the ideal plug flow. The present method considering the cluster effect is reasonable in predicting the flow behavior and RTD of particles in the downer.

Comparative Bio-sorption of Cadmium and Nickel Ions from Aqueous Solution onto Fibers of Date Palm using Fluidized Bed Column

The aim of this study is to use the dry fibers of date palm as low-cost biosorbent for the removal of Cd(II), and Ni(II) ions from aqueous solution by fluidized bed column. The effects of many operating conditions such as superficial velocity, static bed height, and initial concentration on the removal efficiency of metal ions were investigated. FTIR analyses clarified that hydroxyl, amine and carboxyl groups could be very effective for bio-sorption of these heavy metal ions. SEM images showed that dry fibers of date palm have a high porosity and that metal ions can be trapped and sorbed into pores. The results show that a bed height of 6 cm, velocity of 1.1Umf and initial concentration for each heavy metal ions of 50 mg/L are most feasible and give high removal efficiency. The fluidized bed reactor was modeled using ideal plug flow and this model was solved numerically by utilizing the MATLAB software for fitting the measured breakthrough results. The breakthrough curves for metal ions gave the order of bio-sorption capacity as follow: Cd(II)]Ni(II).

An evaluation of effects of fuel parameters and flue gas recirculation on NOx emissions through detailed chemical kinetics simulations

In this work the effects of fuel parameters and under grate flue gas recirculation (FGR) are investigated when applying staged air combustion of solid fuel for the purpose of NOx reduction. In addition to temperatures, residence times and primary and overall excess air ratio, the fuel-N content and speciation, the relative fuel-C, fuel-H and fuel-O contents, the fuel moisture content, the fuel gas composition from the dry fuel and the use of FGR (in the primary and/or secondary combustion zone) influence the NOx reduction potential to a larger or smaller extent. To assess the influence of different parameters, a simulation matrix using one ideal plug flow reactor representing only the primary zone and a recent detailed chemical kinetics mechanism was carried out. The results show a large influence of the fuel-N content and speciation on the NOx reduction potential in the primary zone, and a significant influence of the fuel moisture content due to its fuel gas dilution effect. At the close to optimum NOx reduction conditions studied in this work (973 K, primary excess air ratio of 0.9), the influence of the fuel gas composition from the dry fuel on the achievable total fixed nitrogen reduction in the primary zone is relatively small, as well as its influence on the radical pool available for the fuel-N chemistry. The influence of the relative content of C, H and O in the fuel at a constant primary excess air ratio is also relatively small. From a kinetics point of view, there is a positive effect of applying under grate flue gas recirculation if the residence time is sufficiently long, however, the effect is relatively low compared to the other main influencing parameters. At higher temperatures the importance of the radical pool availability as well as its speciation increases, and a more significant part of the NH3 and especially HCN reduction in the primary zone results in increasing NO formation with increasing temperature.

Development of the chemical reactor V-star for continuous flow reactions

This article illustrates the new modeled reactor, that allows to change the process of the flow chemical reaction from batch tank to the continuous mode. The stage efficiency form 1 (ideal plug flow, IPF) to 50 stages in a continuous stirrer tank reactor (CSTR) was plotted and illustrated graphically. Moreover, the descriptions and possibilities of the new reactor (V-star) such as: temperature control, numbers of stages, movement-speed of the reactors platform are also shown in this paper work.

Modular Coiled Flow Inverter with Narrow Residence Time Distribution for Process Development and Production

AbstractThe ideal plug flow reactor is a common model with narrow residence time distribution in reaction engineering. This contribution explains a design strategy of a coiled flow inverter (CFI) with narrow residence time and flexible adjustable process conditions. The design space diagram illustrates the design limits of inner diameter and coil diameter on the base of dimensionless numbers. Standard coil tube diameters allow for rational design of a limited number of devices. The reactor performance diagram enables the facile scale‐up and adaption of new process conditions with the CFI with economic boundary conditions.

An evaluation of effects of operational parameters on NOx emissions through detailed chemical kinetics simulations

Abstract NOx emissions from biomass and Municipal Solid Waste (MSW) combustion is a great concern. Technologies for reducing NOx emissions based on flue gas cleaning using e.g. Selective Non-Catalytic Reduction (SNCR) or Selective Catalytic Reduction (SCR) are well known. However, limiting NOx emissions by primary measures is an attractive opportunity, avoiding or limiting the use of costly secondary measures. Many factors influence the NOx emission level from biomass and MSW combustion plants, like fuel-N content and speciation, and operating parameters as air distribution and staging, flue gas recirculation, temperature and residence time. In this work a recent detailed chemical kinetics mechanism is used to investigate the NOx reduction potential by staged air combustion at different operational conditions, using two ideal plug flow reactors connected in series. This enables revealing a kinetics limited reduction potential, as a guideline for further detailed studies at targeted conditions using stochastic reactor networks or Computational Fluid Dynamics (CFD). The results show, for demolition wood as fuel, a large NOx reduction potential, as high as above 95% total fixed nitrogen reduction at theoretically optimum conditions. Through reaction path analyses the main reaction pathways and reactions are identified. Comparison with experimental results from grate combustion units is made. Finally, a comparison with earlier and less extensive detailed chemical kinetics mechanisms is made, to check for significant differences and assess the reasons for those. The final goal would be to arrive at computationally effective reduced chemical kinetics mechanisms that could be applied in CFD simulations of real biomass and MSW combustion plants, to arrive at improved operational conditions or improved design.

Virus study for continuous low pH viral inactivation inside a coiled flow inverter.

Continuous processing for the production of monoclonal antibodies (mAb) gains more and more importance. Several solutions exist for all the necessary production steps, leading to the possibility to build fully continuous processes. Low pH viral inactivation is a part of the standard platform process for mAb production. Consequently, Klutz et al. introduced the coiled flow inverter (CFI) as a tool for continuous low pH viral inactivation. Besides theoretical calculations of viral reduction, no viral clearance study has been presented so far. In addition, the validation of continuous viral clearance is often neglected in the already existing studies for continuous processing. This study shows in detail the development and execution of a virus study for continuous low pH viral inactivation inside a CFI. The concept presented is also valid for adaptation to other continuous viral clearance steps. The development of this concept includes the technical rationale for an experimental setup, a valid spiking procedure, and finally a sampling method. The experimental results shown represent a viral study using xenotropic murine leukemia virus as a model virus. Two different protein A (ProtA) chromatography setups with varying pH levels were tested. In addition, one of these setups was tested against a batch experiment utilizing the same process material. The results show that sufficient low pH viral inactivation (decadic logarithm reduction value >4) was achieved in all experiments. Complete viral inactivation took place within the first 14.5 min for both continuous studies and the batch study, hence showing similar results. This study therefore represents a successful virus study concept and experiment for a continuous viral inactivation step. Moreover, it was shown that the transfer from batch results to the continuous process is possible. This is accomplished by the narrow residence time distribution of the CFI, showing how close the setup approaches the ideal plug flow and with that batch operation.