Plug Flow Behavior Research Articles

Shallow beds are not plug flow reactors – Analysis of kinetic data in the presence of axial dispersion effects

Laboratory-scale shallow beds filled with catalyst particles are frequently used in catalyst screening, activity tests and kinetic measurements. Often the experimental data are interpreted with plug flow or differential reactor models. Residence time measurements have shown large deviations from ideal plug flow behavior in shallow beds, indication the presence of axial dispersion effects, i.e. is backmixing and diffusion in the bed. The behavior of laboratory-scale bed reactors was investigated by theoretical considerations of the axial dispersion model and numerical simulations in the Damköhler-Péclet space. The results revealed that large errors in kinetic parameters can appear when the dispersion model is replaced by the plug flow model. This key effect was quantified using the ratio (F) of two Damköhler numbers: the real one, and the one wrongly estimated on experimental data affected by dispersion problems and interpreted with ideal plug-flow approach. Thus, the effect of back-mixing is evaluated varying a single parameter of the kinetic model, namely the rate constant. The conclusions are valid for power-law, Langmuir-Hinshelwood and Eley-Rideal kinetics.

Desulfurization of Zawia Refinery Diesel Using Adsorption Fixed Bed Process

The study focuses on the dynamic modeling of a fixed-bed adsorber for the adsorption of sulfur compounds in diesel fuel. The model considers non-ideal plug flow behavior and velocity variation along the column, providing a more realistic representation of the adsorption process. Additionally, internal mass-transfer resistances due to pore diffusion mechanisms are incorporated into the model. The study investigates adsorption performance by examining different flow rates (5 cc/min, 10 cc/min, 15 cc/min, and 20 cc/min) and inlet concentrations ranging from 586 to 100 ppm. The bed height is constant at 30 cm. The behavior of various parameters, such as bed utilization, breakpoint time, film mass transfer coefficient, and height of the adsorption zone, is analyzed. The results indicate that a sharp front of the breakthrough curve is observed, followed by the broadening of the tail of the breakthrough curve. The breakthrough curve represents the adsorbate concentration in the effluent stream over time. The investigation reveals that a high flow rate of 20 cc/min and a high inlet concentration yield better overall bed capacity utilization for the adsorption system. This means that the bed is more effectively utilized at higher flow rates and higher inlet concentrations, leading to improved adsorption performance. In conclusion, high flow rates and high inlet concentrations are favorable for enhancing the adsorption system's performance in terms of bed utilization. These results provide valuable insights for optimizing the design and operation of fixed-bed adsorbers that remove sulfur compounds from diesel fuel.

The hydraulic characteristics of a subsurface flow stone biofilter for treating polluted runoff from an informal settlement

ABSTRACT Polluted runoff from informal settlements in developing countries poses a growing challenge due to the elevated and variable nature of contaminants, particularly nutrients and pathogens, introduced to the environment. Cost-effective and scalable treatment systems with the ability to reduce nutrient and other pollutant concentrations in contaminated runoff are desirable. Biofilters are passive water treatment systems that have the potential to achieve this. The Franschhoek Water Hub, a research site for nature-based solutions, features six large biofiltration cells designed to remediate runoff from an informal settlement. Due to their large size, understanding hydraulic behaviour and validating the design proves challenging. To address this, a scaled-down version of the Water Hub's biofilters was constructed to inform design criteria for purpose-built filters. The pilot-scale subsurface flow biofilter, filled with 8–11 mm aggregate stone, had an available volume of 225 L. Pulse tracer studies conducted at various flow rates demonstrated that the system approximated plug flow behaviour. Lower flow rates resulted in deeper tracer infiltration, which is crucial for maximising the distribution of nutrients within the filter bed. This research contributes to the effective design and operation of biofiltration systems, which hold promise for addressing surface water contamination issues in resource-constrained regions.

Parameters affecting plug characteristics in dense phase pneumatic conveying of ellipsoidal particles

Even though particle shape tends to have a major influence on the bulk behaviour of a granular system, perfectly spherical particles are often used in numerical simulations. In this study, we examine the plug flow of ellipsoidal particles in dense phase horizontal pneumatic conveying within a rigid and cylindrical pipe with periodic boundaries in the axial direction. We use coupled Computational Fluid Dynamics (CFD) – Discrete Element Method (DEM) simulations to investigate the effect of particle shape on four plug characteristics: velocity, length, porosity, and particle exchange rate with the stationary bed. Both ellipsoidal and spherical particles are used, and the parameters varied are the particle size, aspect ratio, pressure gradient, particle–particle and particle–wall sliding and rolling friction coefficients, and coefficients of restitution. The steady state characteristics of plugs composed of spherical particles are distinctly different from those composed of ellipsoidal particles. Certain parameters, including the particle–wall rolling friction and restitution coefficients, have minimal impact on the steady state plug characteristics. The present study also suggests that introducing interparticle rolling friction for perfectly spherical particles can achieve the velocity and length appropriate for plugs composed of ellipsoidal particles, although differences in dynamics may persist. Overall, this comprehensive analysis offers valuable insights into plug flow behaviour with non-spherical particles, aiding in the efficient modelling and design of pneumatic conveying systems.

Comparative study of high-solid anaerobic digestion at laboratory and industrial scale – Process performance and microbial community structure

High-solid anaerobic digestion (HSD) for biogas production, compared with wet digestion, is attracting interest due to advantages such as reduced fresh water usage, improved digestate quality and potential for high organic loading rates. However, the underlying processes are not well described and evaluated for HSD. In this study, two laboratory-scale reactors (46 L) of plug-flow type were designed to simulate an industrial-scale HSD process co-digesting food waste, agricultural waste and garden residues under thermophilic conditions. Performance of the laboratory-scale HSD process under stable and disturbed conditions was compared with that in industrial-scale reactors. The results showed that the laboratory- and industrial-scale processes had similar efficiency (93 %) and VS-reduction (43 % and 41 %, respectively) and relatively similar specific methane production (339 and 366 NL CH4/kg VS, respectively). Results from tracer studies combined with chemical analyses showed no phase-separation or plug-flow behaviour along the horizontal axis in either laboratory- or industrial-scale reactors, indicating a need for further process optimisation. Analyses of microbial community structure showed high similarity between laboratory- and industrial-scale, but with some differences caused by downscaling. During the experiment, the laboratory- and industrial-scale processes both showed signs of disturbance, i.e. VFA accumulation at NH4+-N levels > 4 g/L, accompanied by a shift in microbial community structure at both scales, with significant increases in relative abundance of e.g. genera Defluviitoga and Methanothermobacter. In conclusion, this study confirmed the validity of simulating HSD at laboratory scale, thus providing valuable insights into biogas production from high-solid substrates, both in laboratory- and industrial-scale processes.

Continuous Reactive Crystallization in a Mesoscale Oscillatory Baffled Reactor

AbstractContinuous reactive crystallization was carried out in a mesoscale oscillatory baffled reactor (meso‐OBR), using calcium carbonate as a case study to demonstrate the ability of high solid loadings and to examine the fouling potential in continuous processes. Plug flow behavior was easily achieved in reactive solid‐liquid phase reaction systems, as evidenced by the rapid establishment of a steady state. Effects of the operating conditions (reactant concentration, mixing conditions, and temperature) on the yield, particle size, and particle size distribution were studied. Increasing the reactant concentration and oscillation conditions reduced the particle size whereas increasing the operating temperature led to a large particle size. The meso‐OBR produced uniformly shaped spherical crystals, with a 35 % reduction in average particle size and high purity compared to conventional stirred tanks. The meso‐OBR could be used continuously for up to 6 h with less than 1 wt % of fouling (product loss).

Investigating the Relationship between the Time Constant Ratio and Plug-Flow Behaviour in the Pneumatic Conveyance of Biomass Material

This study introduces a novel methodology to evaluate the behaviour of biomass material by examining the ratio of aeration and deaeration time constants. To this end, a series of tests were conducted on four different materials, namely, cottonseed, wood chips, wood pellets, and wheat straw, in order to investigate their aeration and deaeration behaviours. The study derives the aeration and deaeration pressure drop equations, and discusses the corresponding time constant expression. Subsequently, the four materials were conveyed in 12 m long batch-fed and continuous pneumatic conveying pipelines to examine their behaviour in longer pipelines. The results indicate that the aeration and deaeration time constants increased with an increase in air superficial velocity. However, the ratio of the aeration and deaeration time constants was identified as a unique number, where a value close to 1 indicates a higher likelihood of plug flow. On the basis of the results, cottonseed, with the lowest ratio of time constant, was more likely to form a stable plug flow in both batch-fed and continuous pneumatic conveying. Given the unique properties of biomass and the limited research on the pneumatic conveyance of biomass, this methodology represents a novel approach for predicting modes of flow in materials with complex properties.

Performance evaluation of oscillatory baffle Bunsen reactor in iodine sulfur thermochemical process for hydrogen production

<abstract> <p>Hydrogen is an environmentally attractive transportation fuel that can replace fossil fuels. The iodine-sulfur (I-S) thermo-chemical process involves three reaction steps. The Bunsen reaction is one of the main reaction steps of I-S process and plays an important role in defining overall process efficiency. Different types of reactors are being studied, including the oscillatory baffle Bunsen reactor. The reactor selection is based on process intensification and process integration, i.e., reaction and separation in the single equipment. Simulations are performed for two-dimensional cases by solving incompressible Navier-Stokes equations along with continuity equation, the geometry chosen is of single section in the column and diameter of 50 mm and baffle spacing of 50, 75,100 mm i.e. 1, 1.5 and 2 times the diameter of column and baffle free area 22%. The residence time distribution experiments are carried out in a metallic reactor with NaCl as a tracer operated at different frequencies, amplitudes and flow rates. D/uL is evaluated from experimental curves. The power per unit mass is compared for oscillatory baffled column and stirred tank for a semi-batch process. From the numerical simulations, good eddy interactions are present at higher frequencies and amplitudes and at a baffle spacing of 1.5 times the diameter of the column. Residence time distribution analysis gives a lower dispersion number at medium frequencies and higher amplitudes with constant flow rate. If the flow rates are high, then it is better to operate at higher frequencies and amplitudes to achieve plug flow behavior. Power per unit mass is less for oscillatory baffle column compared to the stirred tank of given configuration. With better mixing performance and less power per unit volume, the oscillatory baffle column is a good alternative to other columns.</p> </abstract>

A novel Static Mixer for photochemical reactions

Photochemical reactions have become a widely studied topic in the past decade, as they are employed in a wide range of processes and are recognized to be a fundamental tool in the field of sustainable chemistry. Moreover, in recent years there is an increasing interest in turning discontinuous photochemical processes into continuous ones, since continuous processes can ensure better results in terms of productivity and selectivity. In this work, a novel static mixer useful to reach a plug-flow behaviour inside a square-based reactor, while not obstructing the photon flux coming from the light sources, was designed. Through RANS based CFD simulations, the residence time distribution of molecules inside the reactor was evaluated. Moreover, the effect of both absorption and scattering phenomena, which play a major role in photochemical reactions, was assessed. Finally, a case study was considered and the photochemical degradation of 2-hydroxybenzoic acid (2-HBA) was simulated. The conversion obtained in the reactor equipped with SMs was compared to an empty reactor, finding that the degradation efficiency is doubled.

Residence time distribution in reactive and non-reactive flow systems in micro and millidevices

The present research evaluated the Residence Time Distribution (RTD) of three microdevice designs (Traditional T-shape, MTB and G1) in two flow conditions: non-reactive flow and reactive flow (biodiesel synthesis). Also, the scale-up effects on RTD were explored for the space time (τ) range between 2 and 60 s. In general, the deviation between mean residence time and ideal space time decreased with increasing τ. The axial dispersion analysis showed larger deviations from plug flow for oil streams, while ethanol streams exhibited plug flow behavior for longer space times (τ ≥ 30 s). Despite this, high oil conversions (>90%) and relatively high biodiesel throughputs (≈19 g/h, about 63 greater than T-shape microchannels) were observed for MTB millidevices and G1 devices. Despite the observed deviations from plug flow, the proposed micromixer designs provided good performance for biodiesel synthesis.

Continuous Cooling Crystallization in a Coiled Flow Inverter Crystallizer Technology—Design, Characterization, and Hurdles

Continuous small-scale production is currently of utmost interest for fine chemicals and pharmaceuticals. For this purpose, equipment and process concepts in consideration of the hurdles for solids handling are required to transfer conventional batch processing to continuous operation. Based on empirical equations, pressure loss constraints, and an expandable modular system, a coiled flow inverter (CFI) crystallizer with an inner diameter of 1.6 mm was designed. It was characterized concerning its residence time behavior, tested for operation with seed crystals or an ultrasonic seed crystal unit, and evaluated for different purging mechanisms for stable operation. The residence time behavior in the CFI corresponds to ideal plug flow behavior. Crystal growth using seed crystals was demonstrated in the CFI for two amino acids. For fewer seed crystals, higher crystal growth rates were determined, while at the same time, secondary nucleation was observed. Feasibility for the interconnection of a sonicated seeding crystal unit could be shown. However, the hurdles are also identified and discussed. Prophylactic flushing combined with a photosensor for distinguishing between solvent and suspension phase can lead to stable and resource-efficient operation. The small-scale CFI technology was investigated in detail, and the limits and opportunities of the technology are presented here.

Mixing Efficiency and Residence Time Distributions of a Side-Injection Tubular Reactor Equipped with Static Mixers

Plug flow behavior in tubular reactors is often highly desirable in industry, since it can ensure high productivity, good selectivity, and enhanced heat transfer. To achieve this, good radial mixing combined with poor axial mixing is required: these conditions are quite easy to obtain if the flow regime is turbulent, but they are much more challenging to achieve if the flow is laminar. In this work radial mixing and residence time distributions in a side-injected tubular reactor equipped with a series of Sulzer SMX static mixers were investigated using Computational Fluid Dynamics. It was found that even at low values of Reynolds number the reactor can efficiently satisfy the plug flow conditions, and operative diagrams were determined to foresee the reactor behavior.

Influence of particle residence time distribution on the biomass pyrolysis in a rotary kiln

Rotary kilns are widely used in industry to perform several different operations. When numerically modeling such systems, a common simplification is to neglect the residence time distribution of particles. The present study numerically investigates the influence of particle residence time distribution on the pyrolysis of biomass in a co-current heated rotary kiln. First, the model (balance equations, reaction scheme, flow model, and heat transfer mechanisms) is developed. The applied particle flow model (Seaman's model) is then validated by comparing the calculated mean residence time τ and bed height to the results of a discrete element method (DEM) simulation of an example case. The mean residence time (τ = 552 s) and the bed height (H(z=0) = 52 mm) of the Seaman's model was found to be in good agreement with the results of the DEM simulation (τ = 542 s, H(z=0) = 50 mm).A detailed discussion of the residence time distribution's influence on the simulated conversion for the example case is then presented. We performed 54 simulations with various Péclet numbers between 1 and 100 and showed that the product yield significantly changes for Péclet numbers below 10, that is, neglecting residence time distribution (RTD) may not be a true assumption in all operating conditions.Finally, performing a comprehensive parameter study, the RTD's influence under various operating conditions and kiln designs is investigated. The varied parameters are the kiln diameter (D = 0.1–1 m) the ratio of particle to kiln diameter (d/D = 5 × 10−3–40 × 10−3), the ratio of kiln length to kiln diameter (L/D = 1–10), the kiln's inclination angle (β = 0.1–8∘), the Froude number (Fr = 10−3–10−2), the rotational Reynolds number (Re = 102–16 × 103), and the Péclet number (Pe = 5–100). Using more than 13000 simulations, we show that in one third of the investigated cases the change of product yield is higher than 2% when changing from plug flow behavior to high axial dispersion (Pe = 5). For cases where the influence of the RTD on the final product yield is small, conditions along the kiln might still change significantly. The biggest change of the product yield from plug flow behavior to high axial dispersion was found to be 5.5 wt% for the following conditions: D = 1 m, d/D =5 × 10−3, L/D = 1, β = 0.1, Fr = 10−2 and Reω = 16,000.

Three-phase trickle-bed reactor model for industrial hydrotreating processes: CFD and experimental verification

Catalytic hydrotreating (HDT) is a process used to remove impurities from refinery streams, reacting a given feed with hydrogen at high temperature and pressure in a trickle-bed reactor. In order to optimize the operation of such reactors, one needs information about the detailed performance of the reactor. However, mathematical models of such reactors have not received due attention in last decades. Therefore, this work aimed at revisiting a plug flow HDT model to predict the performance of industrial diesel hydrotreating reactor, which considers most of the HDT reactions, gas-liquid and liquid-solid mass transfer, as well as the influence of catalyst deactivation and quenches injection on model predictions. A fourteen pseudo-component reactional network was proposed and successfully used. Computational fluid dynamics (CFD) predictions verified the plug flow behavior of the industrial HDT reactor. Both the CFD and plug flow models were verified using real process plant data, inspecting residual sulfur content and temperature profiles, with maximum relative error of 14%. Pressure and species concentration profiles as well as the effect of inlet pressure and temperature on final sulfur content were predicted and discussed. Catalyst deactivation is an important factor for accurate prediction of industrial HDT over time of campaign.

Effect of air flow, intermittent aeration time and recirculation ratio in the hydrodynamic behavior of a structured bed reactor

Among the novel system configurations that remove organic matter and nitrogen simultaneously, the structured bed reactor with recirculation and intermittent aeration (SBRRIA) can be mentioned. Studies addressing the SBRRIA have shown high efficiency of organic matter and nitrogen removal, as well as low sludge production. However, operational parameters such as the recirculation rate, intermittent aeration time, and airflow are not fully established. Therefore, this research aimed to evaluate the effect of these three parameters on the mixing characteristics of an SBRRIA adopting established hydrodynamic models, also proposing a new flow model for the reactor. Hydrodynamic tests were performed using sodium chloride as a tracer, and the variables were the recirculation rate, intermittent aeration time, and airflow. Results showed that the reactor presented a plug-flow behavior when the aeration was turned off and behaved as a completely stirred reactor when the aeration was turned on. Traditional models were not sufficient to explain the flow of the SBRRIA, resulting in the development of a simplified programming pseudo-code to adequately represent the flow in the reactor. The tested airflow and recirculation rates did not affect the mixing behavior in the SBRRIA. These observations represent a significant advance in SBRRIA studies as they show that energy consumption could be optimized, making the system even more advantageous by saving electrical energy up to 75% due to pumping.

Continuous flow synthesis of zeolite FAU in an oscillatory baffled reactor

AbstractIn this work, we report, for the first time, the continuous flow synthesis of zeolite NaX successfully accomplished in a pilot‐scale (up to 100 L/h) continuous oscillatory baffled reactor (COBR). The continuous synthesis was achieved by combining the seed‐assisted method, in‐line mixing of reactants at high temperature and COBR. The use of a COBR not only allowed the same residence time as the batch system, but also a controlled temperature profile, thanks to the plug flow behavior and effective heat transfer. A stable continuous operation was maintained during 5 hours owing to an effective mixing, preventing any hydrodynamic failure caused by the sedimentation of zeolite crystals. The results showed that zeolite NaX was obtained after 99 minutes and crystals between 0.6 and 0.7 μm were observed by scanning electron microscopy. The effect of frequency and amplitude on crystal properties was assessed. Sampling at three points along the COBR allowed the study of steady states of the zeolite concentration in temporal and spatial domains.

Once through continuous flow removal of metronidazole by dual effect of photo-Fenton and photocatalysis in a compound parabolic concentrator at pilot plant scale

The recent research for the removal of various pollutants has been triggered towards the realistic approach of the plug flow reactor system. For the first time once through plug flow approaching compound parabolic reactor was employed for the removal of Metronidazole by utilizing in-situ synergistic effect of photo-Fenton and photocatalysis. The novel concept of in-situ dual effect has been introduced through composite prepared of iron-TiO2 utilizing foundry sand and fuller’s earth (industrial waste) with a coating of TiO2. For the verification of the plug flow behavior, tanks in series model as well as plug flow model has been employed and validated. With the hydraulic retention time of 15 min, 6 parabolic reactors connected in series, pH 3.0–3.5, H2O2 dose of 450 mg L−1, has led to maximum removal of 60%. 2.19 folds synergy was perceived in case of hybrid effect as compared to individual processes. Additionally, 45–50% COD reduction accompanying the formation of nitrate, nitrite ions confirmed the mineralization. Retention of dual catalytic activity without much deterioration even after 40 recycles confirms its applications at industrial scale. Scale-up cost trials have also been studied for real time analysis of energy intensive parameters. Various characterizations including XRD, SEM-EDS, Mapping, and FT-IR confirmed the potential of the composite. This research unlocks the new horizon towards the degradation of wastewater employing large scale dual effect.

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 ...

Coil-in-Coil Reactor: Augmenting Plug Flow Performance by Combining Different Geometric Features Using 3D Printing

Additive manufacturing enables the fabrication of geometries previously unavailable via conventional means. Here, a helically coiled channel containing a secondary helical coil insert was 3D-printed and operated in a net flow with superimposed oscillation. This “coil-in-coil” geometry exhibited exceptional plug flow quality at low Reynolds numbers (Re = 10–50). High levels of plug flow were observed at oscillation amplitudes of 2–8 mm. The greatest degree of plug flow occurred with Reo/Ren = 2–7 and Sr = 1–2. Surprisingly, the coil-in-coil geometry demonstrated excellent plug flow behavior (N = 40–100) at very low oscillation frequencies (f = 0.25–0.5 Hz). Compared to various benchmark designs (“plain” coiled tube, straight tube containing a helical coil insert, and plain straight tube), the coil-in-coil geometry demonstrated enhanced plug flow quality at low frequencies (≤1 Hz) at all amplitudes and low amplitudes (1–2 mm) at all frequencies.

Gas-liquid hydrogenation in continuous flow – The effect of mass transfer and residence time in powder packed-bed and catalyst-coated reactors

Catalyst-coated tube reactors have been compared with the reactors packed with catalyst powder in alkyne semi-hydrogenation over a 5 wt% Pd/ZnO catalyst and cinnamic ester full hydrogenation over a 2.4 wt% Pd/C catalyst. The “powder packed-bed” reactors (packing with catalyst powder below 30 μm) showed irreproducible performance in time due to mobility of the catalyst layer in the bed which altered the fluidic path and therefore affected the mean liquid residence time and the dispersion. The catalyst-coated tube reactors demonstrated an ideal plug-flow behaviour (Péclet number > 120), while the powder packed-bed showed a considerable back-mixing (Péclet ~ 25). Under all conditions studied, the reaction rate in the powder packed-bed was limited by external mass transfer, while in the coated tube – by the intrinsic kinetics. The coated tubes demonstrated a much lower pressure drop, an improved alkene selectivity, and a 5 times higher throughput compared to the powder packed-bed. The dilution of the catalyst bed with glass beads improved the throughput 4-fold at the expense of 4-fold increase in the pressure drop. In full hydrogenation reaction, the catalyst-coated tube showed a 14 times higher throughput than in the powder packed-bed at the full alkyne conversion. A reactor model for the catalyst-coated tube has been proposed that takes into account the change in the fluid velocity during the reaction. The model described the reaction kinetics demonstrating that the catalyst-coated tubes can be used as a tool to obtain kinetic data in gas-liquid reactions in flow.