Plug Flow Conditions Research Articles

Pulsed Micro-reactor: An alternative to estimating kinetic parameters of non-catalytic gas–solid reactions

A Pulsed Micro-reactor (PMR) technique is demonstrated as a simple and viable alternative to generate kinetic data for non-catalytic gas–solid reactions. An experimental test rig was designed and fabricated for this purpose. Oxidation of Cu using air and reduction of CuO using methane, encountered in a novel form of combustion known as Unmixed Combustion (UMC), were chosen as representative reactions in this study. Plug flow conditions in the reactor, essential for accurate estimation of kinetic parameters, was confirmed using a Residence Time Distribution (RTD) approach. A simple generic methodology, based on a uniform reaction model is presented which allowed for easy interpretation of data from this reactor system. While the methodology has been validated for both reactions which were prima facie found to be surface reaction controlled, it can easily be extended to pore diffusion limited reactions. The estimated values of reaction orders and activation energies for both reactions and the pre-exponential factor for oxidation were found to be in excellent agreement with those obtained using Thermo Gravimetric Analysis (TGA). A lower value of pre-exponential factor for reduction however was obtained in the present investigation. While the PMR, can provide an initial insight into the reaction mechanism, it can also be used to obtain reaction engineering data related to process development.

Comparison of cutting-oil emulsion treatment by electrocoagulation–flotation in bubble column and airlift reactors

ABSTRACTSeparation of nanoscale oil droplets in the cutting-oil emulsion by electrocoagulation–flotation (ECF) was carried out in a bubble column reactor (BCR) and an external-loop airlift reactor (ALR). Under the batch operation, aluminium electrode provided the highest efficiency of 99% and required the shortest separating time compared to iron and graphite electrodes. The separation performance was also affected by the electrode gap and current density due to the amount of produced aluminium ions and turbulence by bubble motions. Additionally, the ECF efficiency obtained from the ALR was similar to that of the BCR. However, the ALR was preferable owing to its lower energy consumption, less electrode sacrifice, and less sludge production. Similar results were acquired under the continuous mode; nevertheless, the highest efficiency of only 85% was achieved from both reactors. It was found that the efficiency declined with increasing flow rates. According to the results suggested by the residence time distribution (RTD), the ALR was more effective at higher flow rates since the plug flow condition can be retained. On the other hand, an increase in flow rate also provoked the bypass flow to the down-comer of the ALR, resulting in the presence of a dead zone and reduction in the treatment efficiency.

Investigation into mean residence time and filling factor in flighted rotary torrefier

Torrefaction can upgrade the properties of biomass, making it similar to coal. This study develops an innovative indirectly heated two‐stage rotating drum‐type torrefier where biomass moves due to the rotation of inclined cylindrical reactors. The rotary reactors consist of three rectangular flights placed axially along the reactor. This paper mainly investigates the mean residence time and filling factor of reactors operated at steady and plug flow conditions. Semi‐empirical expressions using dimensionless correction constants for the mean residence time and the filling factor were proposed. Experiments keeping a constant solid flow rate at the dryer inlet were conducted in order to measure filling factor and mean residence time at different angular speeds and inclinations of reactors (dryer and torrefier). Mean residence times of 5.8 min at 8 rpm and 3°, and of 27.5 min at 4 rpm and 1° were found in the dryer (diameter: 64 mm, length: 610 mm), whereas they were 4.7 and 21.4 min in the torrefier (diameter: 100 mm, length: 762 mm) at respective operating conditions. Filling factors of 3.0 % at 8 rpm and 3°, and 14.3 % at 4 rpm and 1° were found in the dryer, whereas they were 0.8 % and 3.6 % in the torrefier at the corresponding angular speed and inclination. Measured and predicted mean residence times were also compared and validated against those of other investigators. The proposed expressions, which include the effects of different operating and geometrical parameters, have strong predictive power and may be useful in scaling‐up a rotary torrefier.

Study of Flow Pattern in Jet Clarifier for Removal of Turbidity by Residence Time Distribution Approach

This study aims to determine the performance of the jet clarifier for turbidity removal and its mechanisms for proposing the optimal operating conditions and design criteria. The experiment were performed continuously using a pilot scale jet clarifier with the volume of 243 L. Effects of liquid flow rates, types of liquid phase, and sludge blanket heights on turbidity removal efficiency were investigated. Moreover, the residence time distribution (RTD) study was carried out to investigate the flow pattern. The results indicated that the jet clarifier can effectively reduce the water with the efficiency of 80% under the optimal condition. The RTD results suggested that the flow pattern in the jet clarifier corresponded to the design as the plug flow and mixed flow conditions were found in the coagulation and the flocculation/sedimentation zones, respectively. The presence of the sludge blanket can reduce the bypass and recirculated flows. Besides, the increase of flow rate resulted in the increase recirculation in the tank. It can be suggested that the jet clarifier can be used for removing turbidity in the water treatment. The hydrodynamic in the reactor, which relates to flow pattern in the reactor, is one among the important factors in a jet clarifier.

Optimal Catalyst and Cocatalyst Precontacting in Industrial Ethylene Copolymerization Processes

In industrial-scale catalytic olefin copolymerization processes, catalyst and cocatalyst precontacting before being introduced in the polymerization reactor is of profound significance in terms of catalyst kinetics and morphology control. The precontacting process takes place under either well-mixing (e.g., static mixers) or plug-flow (e.g., pipes) conditions. The scope of this work is to study the influence of mixing on catalyst/cocatalyst precontacting for a heterogeneous Ziegler-Natta catalyst system under different polymerization conditions. Slurry ethylene homopolymerization and ethylene copolymerization experiments with 1-butene are performed in a 0.5 L reactor. In addition, the effect of several key parameters (e.g., precontacting time, and ethylene/hydrogen concentration) on catalyst activity is analyzed. Moreover, a comprehensive mass transfer model is employed to provide insight on the mass transfer process and support the experimental findings. The model is capable of assessing the external and internal mass transfer limitations during catalyst/cocatalyst precontacting process. It is shown that catalyst/cocatalyst precontacting is very important for the catalyst activation as well as for the overall catalyst kinetic behavior. The study reveals that there is an optimum precontacting time before and after which the catalyst activity decreases, while this optimum time depends on the precontacting mixing conditions.

CCD series no-19: The lowest energy prospects for SWRO through single-element modules under plug-flow and closed-circuit desalination conditions

The future targets of the SWRO desalination industry to reduce significantly energy consumption are realized already today by the newly emerging closed-circuit desalination (CCD) technology with demonstrated energy consumption of 2.12 kWh/m3 for Mediterranean water (4.0%) and 1.72 kWh/m3 for ocean (3.5%) water without the need for energy recovery with energy saving greater than 35%. The fundamental differences between CCD and the conventional plug flow desalination (PFD) techniques are evaluated in the present study by rigorous theoretical model simulations of CCD compared with multi-stage PFD of the same single-element modules under the same conditions in the flux range of 13.0–0.1 lmh. The results of this study reveal that CCD under fixed flow and variable pressure conditions behaves as near-perfect multi-stage PFD with energy dependence on recovery also manifesting increased frequency of CCD cycles and residence time. The origin of energy savings by CCD and multi-stage PFD compared with conventional single-pass through multi-element module is revealed and explained. While multi-stage SWRO designs are economically prohibited by high fabrication costs, CCD designs are simple and inexpensive and should play a major role in future design of SWRO desalination plants of low energy consumption and high energy conversion efficiency without the need of ERD.

Axial dispersion and mixing phenomena of the gas phase in a packed pebble-bed reactor

The knowledge and proper analyses of axial dispersion and the mixing phenomena of the coolant gas flow in the dynamic core of pebble-bed nuclear reactors are useful for safe design and efficient operation of these reactors. These processes can be characterized in terms of the residence time distribution and quantified in terms of the axial dispersion coefficient. Therefore, in this work, the axial dispersion coefficients of the gas phase and their residence time distributions were measured experimentally in a separate effect pilot-plant scale and cold-flow experimental setup of 0.3m in diameter, using a sophisticated and advanced gaseous tracer technique. The non-ideal flow of the gas phase in the pebble bed was described using the axial dispersion model (ADM). The effect of the gas velocity on the axial dispersion was investigated using a range of velocities from 0.01 to 2m/s, covering both the laminar and turbulent flow regimes. The effect of the bed structure (pebble size) on the axial dispersion coefficient was investigated, and the results indicate that the pebble size strongly affects axial dispersion and mixing in the packed pebble-bed reactor. The results show that the flow pattern of the gas phase does not deviate much from the idealized plug-flow condition at high flow rate, which depends on the gas flow rate and the bed structure of the pebble-bed. The present work provides insight on the extent of mixing and dispersion in the gas phase in the studied bed using an advanced gas dynamics technique and methodology that properly accounts for the external dispersion.

Impact of heat and mass dispersion and thermal effects on the scale-up of monolith reactors

Monolith reactors used in catalytic after-treatment systems (e.g. TWC, DOC, LNT and SCR) are often studied in laboratory experiments using monolith samples of the same cell density, washcoat thickness, and catalyst loading but with a smaller length compared to the full-scale reactor. The flow rate in the laboratory scale operation is selected so that the same space velocity is maintained as in the full-scale system. However, matching space velocity between the laboratory-scale and full-scale reactor leads to similar performance only under ideal conditions such as isothermal or adiabatic operation with negligible heat and mass dispersion (i.e. plug flow conditions) and small adiabatic temperature rise. In this work, it is shown that when thermal effects are significant (or adiabatic temperature rise is large) and heat and mass transfer and dispersion effects are not negligible, similarity may not exist and the ignition/extinction behavior of the two systems can be qualitatively different. Further, when laboratory-scale data is used for kinetic parameter estimation, failure to consider these effects in the reactor model can lead to false kinetic parameters and inaccurate prediction of reactor performances. Another major reason for possible discrepancy between laboratory-scale and full-scale reactors, external heat exchange with furnace (surroundings), is also investigated. We present analysis, simulations, and analytical results quantifying the impact of these effects on the scale-up of monolith reactors.

Stochastic Modeling of Polymer Microstructure From Residence Time Distribution

Stochastic modeling constitutes a powerful technique to obtain complete distributions of microstructural properties of polymer materials and may help to synthesize polymers with well‐defined microstructure, as in the case of controlled radical polymerizations (CRP). However, these techniques have been often applied to describe polymerizations performed in batch or idealized continuous plug flow conditions. The present manuscript describes a Monte Carlo technique that can be used to calculate microstructural properties in polymerization systems performed continuously with arbitrary residence time distributions (RTD), under regime of complete micromixing segregation. The technique approximates the continuous system by a set of batch reactors, operated independently, whose volumes and batch times can be related to the discretized version of the RTD. The technique is applied successfully to describe a styrene‐based CRP reaction carried out in an ideal continuous stirred tank reactor and a tubular non‐ideal tubular reactor.

Accuracies of laminar counterflow flame experiments

Counterflow configurations are useful for investigating the structures of premixed, non-premixed, and partially premixed flames. Ignition and extinction conditions also are readily measured in this configuration. There is a wide range of different possible designs of apparatus to be used in such measurements. The choices vary from opposing nozzle flows without any flow-smoothing screens to opposing flows through porous plates. It is desirable to select designs that correspond best to the conditions treated in available codes for calculating reacting flows because this facilitates comparisons of experimental and computational results. The most convenient codes to use are for steady laminar flows with one-dimensional scalar fields, and they often impose rotational plug-flow conditions at the boundaries. Accuracies of axisymmetric counterflow flame measurements in experiments intended to conform to these conditions are estimated here for designs of large aspect ratios with straight-duct feed streams that have multiple-screen flow-smoothing exits. Causes of departures from assumptions underlying computational programs are addressed by methods that involve theoretical analysis, experimental measurement, and axisymmetric computation. It is concluded that experimental results would not be expected to differ from predictions made with plug-flow boundary conditions by more than five percent for properly designed counterflow experiments of this straight-duct, multiple-screen type.

Numerical investigation of heat transfer and fluid flow behaviors of a block type graphite foam heat sink inserted in a rectangular channel

In the present study, forced convection in a rectangular channel filled by a block of graphite foam is numerically investigated. A two energy equation model with the Brinkman-Forchheimer extended Darcy model is considered in the numerical investigation. A parametric study for the ranges of Re = 813–10,582 and L/H = 1–2 is carried out to examine the thermal (Nux and Nuave) and the fluid flow (Δp) behaviors of a heat sink made of graphite foam. Additionally, the heat removal and fluid flow performances of the block type graphite foam insert are compared with that of aluminum foam with the aim of demonstrating its superiority and deficit points. It is found that the plug flow conditions are valid inside both graphite and aluminum foam heat sinks. The graphite foam heat sink has a better heat transfer performance with a larger pressure drop when it is compared with aluminum foam heat sink. A significant temperature difference between the solid and gas phases inside the graphite foam heat sink is noticed over the ranges of conditions tested. The numerical code used in this study is verified by the existing experimental data and a good agreement is observed between them.

Effect of process design and operating parameters on aerobic methane oxidation in municipal WWTPs

Methane is a potent greenhouse gas and its emission from municipal wastewater treatment plants (WWTPs) should be prevented. One way to do this is to promote the biological conversion of dissolved methane over stripping in aeration tanks. In this study, the well-established Activated Sludge Model n°1 (ASM1) and Benchmark Simulation Model n°1 (BSM1) were extended to study the influence of process design and operating parameters on biological methane oxidation. The aeration function used in BSM 1 was upgraded to more accurately describe gas–liquid transfer of oxygen and methane in aeration tanks equipped with subsurface aeration. Dissolved methane could be effectively removed in an aeration tank at an aeration rate that is in agreement with optimal effluent quality. Subsurface bubble aeration proved to be better than surface aeration, while a CSTR configuration was superior to plug flow conditions in avoiding methane emissions. The conversion of methane in the activated sludge tank benefits from higher methane concentrations in the WWTP's influent. Finally, if an activated sludge tank is aerated with methane containing off-gas, a limited amount of methane is absorbed and converted in the mixed liquor. This knowledge helps to stimulate the methane oxidizing capacity of activated sludge in order to abate methane emissions from wastewater treatment to the atmosphere.

Perchloroetylene and Dibutyltin Dichloride Removal from Packed Column by Surfactant Solution

Surfactant enhanced remediation is viewed as a potential method for removing organometallic compounds from contaminated aquifers. Dibutyltin dichloride (DBT), as a representative organometallic compound, was applied in sand packed columns to observe its solubilization behavior compared to that of perchloroetylene (PCE), a normal organic solvent. Ottawa sand was used as the porous media. A mixture of DBT and PCE was applied as the contaminant. The tracer study exhibited the plug flow condition with a retention time of 79.9 min. The surfactant solution was a mixture of 3.6 wt% SDHS and 0.4 wt% C16DPDS with various concentrations of CaCl2.The column experiments were carried out by single and gradient surfactant systems. The effluent exhibited a general solubilization pattern for PCE, governed by a rate limiting mechanism. However, the concentration of DBT in the effluent observed in every experiment was just a slice of its solubilization capacity.The adsorption of DBT on the sand was suspected to be the cause of the problem. The solubilization of DBT by a surfactant was ineffective at removing DBT from the contaminated media. It may be concluded that DBT exhibits the properties of both an organic and inorganic compound; it could be solubilized by a surfactant and absorbed strongly on sand. Nevertheless, the results indicate that trapped DBT could be removed by mobilization in the form of a PCE-DBT mixture and that adsorption could be prevented by a very low pH condition.

Characterization of the Gacka River basin karst aquifer (Croatia): Hydrochemistry, stable isotopes and tritium-based mean residence times

The Gacka River basin aquifer is a highly-developed karst system, located in the Croatian Dinarides. It is mostly composed of permeable Jurassic and Cretaceous carbonate rocks, and clastic sedimentary rocks of Paleogene age. Gacka River provides high quality water for the town of Otočac and several villages; together with the neighboring Lika River, the water is used for the Hydroelectric Power Plant at Senj on the coast. About 10 perennial and over 20 seasonal springs are located at 450 to 460ma.s.l. (above sea level). Three major springs (Pećina, Majerovo and Tonkovića) provide 57% of the mean annual river flow. Similarities between the average groundwater temperatures as well as between the average specific electrical conductivity values (9.0°C-328μS/cm, 9.6°C-350μS/cm and 8.9°C-312μS/cm) of the springs imply that they are fed from aquifers with similar mean residence times (MRTs). The mean δ18O contents of Majerovo, Tonkovića, and Pećina are around −10.1‰, −9.2‰ and −8.9‰, respectively, revealing differences in the mean recharge area elevations. Compared to the temporal amplitude of the18O signal of precipitation, the 18O signal variations of the springs are substantially attenuated because the recharges occurring at different times are well mixed within the aquifers. This indicates MRTs of more than just a few years. The average tritium contents of Pećina, Majerovo and Tonkovića are 5.48TU, 6.13TU and 6.17TU, respectively. Serially connected exponential-plug type unsteady lumped-parameter models run on an annual time scale resulted in rather satisfactory matches between the observed and calculated tritium contents for all studied springs. The models revealed similar MRTs (and corresponding reservoir volumes) for Pećina, Tonkovića and Majerovo of 12years (470Mm3), 12years (1190Mm3), and 12.2years (1210Mm3), respectively. Plug flow conditions dominate in about 90% of the total aquifer volumes.

Performance and dose validation of a coiled tube UV-C reactor for inactivation of microorganisms in absorbing liquids

The inactivation performance of a coiled tube reactor was investigated in respect to hydraulic parameters and the impact of flow conditions and absorption on the UV-C energy delivery. Data of the residence time distributions using tracer step response indicate full use of the reactor volume and flow behavior close to ideal plug flow with increasing Reynolds number. Based on the hydraulic characteristics of the coiled tube, the used reactor appears to have an effective design for inactivation. Even though the hydraulic characteristics were similar to plug flow conditions, the attenuation of the UV-C energy delivery due to the presence of absorptive compounds in the liquid remains an influencing parameter on the inactivation performance. In addition, the UV-C dose was evaluated by calculation of the theoretical maximum UV-C energy input and actinometric measurements using iodide/iodate actinometry. Both dose specifications were evaluated for their suitability to describe inactivation kinetics of Escherichia coli ATCC 8739 in clear and absorptive liquid. Since the flow influences the inactivation performance in absorptive liquids, actinometric dose is suitable to describe inactivation kinetics in absorptive liquids.

Effect of Pd and Rh promotion on Ni/Al2O3 for NO reduction by hydrogen for stationary applications

Three Ni based catalysts, Ni(10 wt.%)/Al2O3, Ni(10 wt.%)–Pd(0.5 wt.%)/Al2O3, and Ni(10 wt.%)–Rh(0.5 wt.%)/Al2O3, were comparatively tested in the catalytic reduction of NO by hydrogen. Catalytic activity tests consisted in temperature programmed reactions (TPRea) under plug flow conditions, in the 50–350 °C temperature range, with a rate of 5 °C/min. The feed stream corresponded to a reactant ratio of NO:H2 = 1:1.3 and a GHSV of 4500 h−1. Catalytic activity results proved that promotion with low concentrations of both Pd and Rh increases the catalytic performances of the Ni based catalyst, in respect to NO conversion, N2 selectivity and N2 yield. By far, the most efficient catalyst in the NO + H2 reaction proved to be Ni–Pd/Al2O3, with NO conversions greater than 95% and N2 selectivity of at least 90% at T ⩾ 100 °C. Stability tests confirmed the Pd promoted catalyst as the most efficient one in the investigated series. The catalytic performance in terms of N2 selectivity/yield for the Ni–Rh/Al2O3 catalyst proved to be only slightly superior to that corresponding to Ni/Al2O3, although both Ni and Rh are efficient in the dissociative adsorption of NO (NO-TPD).

Advancement of an amended biofiltration system for the removal of ammonia

High fluctuation of influent loading deteriorates performance of biofiltration systems. We have applied a buffer column to mitigate the effect of fluctuated loading. Three inorganic materials (Mobile Crystalline Material-41 (MCM-41), Granular Activated Carbon (GAC) and Bio-coal) as adsorbent of the buffer column were compared with one another. MCM-41 has higher ammonia adsorption capacity and desorption rate than the other materials. Due to this property, MCM-41 buffer column has high potential to reduce shock loading and stably provide substrate for microorganisms in biofiltration at the lower loading periods. Based on these results, an amended biofiltration system with the buffer column packed with MCM-41 was developed for the removal of ammonia. The results showed that MCM-41 played an important role in mitigating the fluctuations in the influent concentration and normalize the ammonia influent concentration. The performance of the amended biofiltration system was more stable than a conventional biofiltration system, i.e. without any pre-treatment. Under the long-term plug-flow test conditions implemented in this study, the minimum removal efficiency of the amended biofiltration system was over 98%, while those of conventional biofiltration systems range between 60 and 80%.

Rapid Determination of the Residence Time Distribution (RTD) Function in an Oscillatory Baffled Reactor (OBR) Using a Design of Experiments (DoE) Approach

Abstract Residence time distribution (RTD) profiles were investigated in a standard oscillatory baffled reactor (OBR) as a function of oscillatory and bulk flow components using a design of experiments (DoE) approach. A second-order, polynomial model (R2=92.1%) was fitted to N values estimated from concentration profiles and used to maximise plug flow conditions. The velocity ratio (Ψ) required to maximise plug flow was 1.9, agreeing well with the range previously identified by Stonestreet and van der Veeken (1.8 < Ψ < 2.0), suggesting that the approach used here is valid. This method could be used to rapidly quantify and maximise plug flow in various OBR designs in a simple and robust manner which could prove valuable for the operation and design of continuous processes using OBR technology.

Influence of Water on the Initial Growth Rate of Carbon Nanotubes from Ethylene over a Cobalt-Based Catalyst

Water-assisted growth of multiwalled carbon nanotubes (CNTs) was studied over a Co-based catalyst under plug-flow conditions. The influence of water concentration and temperature on the growth kine...

Static mixers: Effective means for intensifying mass transfer limited reactions

The potential of using screen-type static mixers to intensify mass transfer limited multiphase-reactions was investigated using a case of diesel desulfurization. Two reactor designs were assessed at different superficial velocities and operating temperatures. By focusing energy dissipation within a very narrow distance in the regions adjacent to the screen, it was possible to generate local energy dissipation rates as high as 40,000W/kg while maintaining the specific energy consumption rates below 0.2kWh/t of liquid processed.Depending on the operating temperature, up to 15,400-fold reduction in the residence time needed to react 95% of the sulfur present in the diesel can be achieved by replacing a standard mixing tank agitated by a Rushton-type impeller with a screen-type static mixer. About 97% of this can be credited to the higher inter-phase mass transfer achievable in screen-type static mixers, while the remaining 3% can be attributed to the higher average mass transfer driving force encountered under the plug flow conditions prevalent in this contactor. The relatively uniform flow conditions encountered across the pipe diameter minimized the formation of difficult-to-separate liquid–liquid dispersions with the ensuing dispersion separating within 20s.