Showing papers on "Laminar flow reactor published in 2015"

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

Abstract: 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 3 mL/min, which corresponds to Reynolds number of 24 and mean residence time of 2.6 h, 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.

Hydrogen oxidation at high pressure and intermediate temperatures: Experiments and kinetic modeling

Abstract: Hydrogen oxidation at 50 bar and temperatures of 700–900 K was investigated in a high pressure laminar flow reactor under highly diluted conditions. The experiments provided information about H 2 oxidation at pressures above the third explosion limit. The fuel–air equivalence ratio of the reactants was varied from very oxidizing to strongly reducing conditions. The results supplement high-pressure data from RCM (900–1100 K) and shock tubes (900–2200 K). At the reducing conditions (Φ = 12), oxidation started at 748–775 K while it was shifted to 798–823 K for stoichiometric and oxidizing conditions (Φ = 1.03 and 0.05). At very oxidizing conditions (O 2 atmosphere, Φ = 0.0009), the temperature for onset of reaction was reduced to 775–798 K. The data were interpreted in terms of a detailed chemical kinetic model, drawn mostly from work of Burke and coworkers. In the present study, the rate constants for the reactions HO 2 + OH, OH + OH, and HO 2 + HO 2 were updated based on recent determinations. The modeling predictions were in good agreement with the measurements in the flow reactor. The predicted H 2 oxidation rate was sensitive to the rate of the HO 2 + OH reaction, particularly at lean conditions, and the present data support recent values for the rate constant. In addition to the current experiments, the mechanism was evaluated against ignition delay time measurements from rapid compression machines and shock tubes. The model was used to analyze the complex dependence of the ignition delay for H 2 on temperature and pressure.

Axial Dispersion and Heat Transfer in a Milli/Microstructured Coiled Flow Inverter for Narrow Residence Time Distribution at Laminar Flow

Abstract: Helically coiled tubes offer improved residence and thermal time distributions due to the formation of Dean vortices via centrifugal forces. Design and fabrication of several milli/microstructured helically coiled tube reactors are described for processes requiring a narrow residence time distribution (RTD) and efficient heat transfer at laminar flow regime. The performance of microstructured reactor capillaries, which provide a high specific surface area, is combined with a type of helically coiled tube, namely, a coiled flow inverter allowing for the narrowest RTD in laminar flow regimes. Axial dispersion is characterized by obtaining the RTD curves from different reactor setups. Overall heat transfer coefficients of a new reactor setup are measured in order to determine the heat transfer efficiency.

Characterization of single coal particle combustion within oxygen-enriched environments using high-speed OH-PLIF

Abstract: This work presents first-of-its-kind high-speed planar laser-induced fluorescence measurements of the hydroxyl radical in the boundary layer of single coal particles. Experiments were performed in a laminar flow reactor providing an oxygen-enriched exhaust gas environment at elevated temperatures. Single coal particles in a sieve fraction of 90–125 µm and a significant amount of volatiles (36 wt%) were injected along the burner’s centerline. Coherent anti-Stokes Raman spectroscopy measurements were taken to characterize the gas-phase temperature. Time-resolved imaging of the OH distribution at 10 kHz allowed identifying reaction and post-flame zones and gave access to the temporal evolution of burning coal particles. During volatile combustion, a symmetric diffusion flame was observed around the particle starting from a distance of ~150 µm from the particle surface. For subsequent char combustion, this distance decreased and the highest OH signals appeared close to the particle surface.

Stereoscopic pyrometer for char combustion characterization.

Abstract: For many pulverized fuels, especially coal and biomass, char combustion is the time determining step. Based on intensified ICCD cameras, a novel setup has been developed to study pulverized fuel combustion, mainly in a laminar flow reactor. For char burning characterization, the typical measurement parameters are particle temperature, size, and velocity. The working principle of the camera setup is introduced and its capabilities are discussed by examination of coal particle combustion under CO2-enriched, so-called oxy-fuel atmospheres with varying O2 content.

Effects of gas flow on oxidation reaction in liquid induced by He/O2 plasma-jet irradiation

Abstract: We present here analysis of oxidation reaction in liquid by a plasma-jet irradiation under various gas flow patterns such as laminar and turbulence flows. To estimate the total amount of oxidation reaction induced by reactive oxygen species (ROS) in liquid, we employ a KI-starch solution system, where the absorbance of the KI-starch solution near 600 nm behaves linear to the total amount of oxidation reaction in liquid. The laminar flow with higher gas velocity induces an increase in the ROS distribution area on the liquid surface, which results in a large amount of oxidation reaction in liquid. However, a much faster gas flow conversely results in a reduction in the total amount of oxidation reaction in liquid under the following two conditions: first condition is that the turbulence flow is triggered in a gas flow channel at a high Reynolds number of gas flow, which leads to a marked change of the spatial distribution of the ROS concentration in gas phase. Second condition is that the dimpled liquid surface is formed by strong gas flow, which prevents the ROS from being transported in radial direction along the liquid surface.

An atmospheric pressure high-temperature laminar flow reactor for investigation of combustion and related gas phase reaction systems

Abstract: A new high-temperature flow reactor experiment utilizing the powerful molecular beam mass spectrometry (MBMS) technique for detailed observation of gas phase kinetics in reacting flows is presented. The reactor design provides a consequent extension of the experimental portfolio of validation experiments for combustion reaction kinetics. Temperatures up to 1800 K are applicable by three individually controlled temperature zones with this atmospheric pressure flow reactor. Detailed speciation data are obtained using the sensitive MBMS technique, providing in situ access to almost all chemical species involved in the combustion process, including highly reactive species such as radicals. Strategies for quantifying the experimental data are presented alongside a careful analysis of the characterization of the experimental boundary conditions to enable precise numeric reproduction of the experimental results. The general capabilities of this new analytical tool for the investigation of reacting flows are demonstrated for a selected range of conditions, fuels, and applications. A detailed dataset for the well-known gaseous fuels, methane and ethylene, is provided and used to verify the experimental approach. Furthermore, application for liquid fuels and fuel components important for technical combustors like gas turbines and engines is demonstrated. Besides the detailed investigation of novel fuels and fuel components, the wide range of operation conditions gives access to extended combustion topics, such as super rich conditions at high temperature important for gasification processes, or the peroxy chemistry governing the low temperature oxidation regime. These demonstrations are accompanied by a first kinetic modeling approach, examining the opportunities for model validation purposes.

Nucleation and Growth of Cobalt Oxide Nanoparticles in a Continuous Hydrothermal Reactor under Laminar and Turbulent Flow

Abstract: Cobalt oxide (Co3O4) nanoparticles were synthesized from aqueous solutions of cobalt(II) acetate using a laboratory scale continuous hydrothermal flow synthesis reactor incorporating a confined jet (coaxial) mixer. By changing the concentration of the precursor combined with operating under flow rate conditions expected to result in a laminar or turbulent mixing, the size of the crystallites could be controlled in the range of 6.5–16.5 nm (median). A quench stream was employed to rapidly cool down the nascent stream of nanoparticles and to elucidate the mechanisms of nucleation and growth. The results show a clear correlation between increasing precursor concentration and crystallite size, which at lower concentrations in particular, decreased in laminar flow and increased in turbulent flow. The smallest particles of 6.5 nm (median) were produced at a precursor concentration of 0.1 M (at a rate of 20 g·h–1). The materials were characterized using a range of analytical methods including powder X-ray diffra...

Flow Pattern Effects on the Oxidation Deposition Rate of Aviation Kerosene

Abstract: The thermal stability characteristics of aviation kerosene were investigated using a heated tube at a constant heat power of 1087 W, mass flow rate of 1 g/s, and supercritical pressure of 3 MPa. Deposition rates on the tube walls were measured by weight. Each test lasts for 105 min. The inner diameter of the reactor was varied at 2, 4, and 6 mm to simulate different flow patterns, including residence time, Reynolds number, and temperature. Various flow states in local sections were taken to investigate the effect of Reynolds numbers on the deposition rate, including sub- and supercritical temperatures and laminar, transition, and turbulent flow. It is shown that the influences of Reynolds numbers on the deposition mainly depend upon the thickness of sublayer, where the oxidative deposit precursors are formed. The deposition rate correlation is modified by the experimental data in the laminar flow regime. Turbulent pattern and supercritical temperature enhance heat transfer of fluid and the mass transfer o...

Investigations on the characterization of laminar and transitional flow conditions after high pressure homogenization orifices

Abstract: High pressure homogenization is a well-established technique to achieve droplets in the submicron range. However, droplet breakup mechanisms are still not completely understood, since studies to characterize the flow are limited due to very small dimensions (typically several micrometers) and very large velocity ranges (from almost stagnant flow to 300 m/s and more). Furthermore, cavitation can occur resulting in multiphase flow. So far, experiments were performed only via integral measurements of, for example, the pressure drop or the droplet size distribution at the outlet. In the current study, this gap shall be closed using Particle Image Velocimetry measurements to analyze the flow field. In addition, an overall method, the characteristic correlation between the discharge coefficient (C D ) and Re0.5 is used to distinguish between laminar, transitional and turbulent flow conditions at Reynolds numbers based on the channel width (d = 200 µm) between 250 and 22,500. The investigated orifices of this study had different positions of the constriction: coaxial and next to the wall. For both orifices, the C D measurement was applicable and showed different characteristic regions which can be associated with laminar, transitional and turbulent flow conditions. Mean velocity fields and fluctuations were measured quantitatively at the outlet and 50 diameters downstream using Micro Particle Image Velocimetry (µ-PIV) in an optically accessible orifice. Increased velocity fluctuations were found in the shear layers when the flow turns from laminar into unstable transitional conditions. The combination of both measurement techniques will help to optimize these systems for the future.

Numerical investigation of flow and heat transfer in a novel configuration multi-tubular fixed bed reactor for propylene to acrolein process

Abstract: In the present contribution, a numerical study of fluid flow and heat transfer performance in a pilot-scale multi-tubular fixed bed reactor for propylene to acrolein oxidation reaction is presented using computational fluid dynamics (CFD) method. Firstly, a two-dimensional CFD model is developed to simulate flow behaviors, catalytic oxidation reaction, heat and mass transfer adopting porous medium model on tube side to achieve the temperature distribution and investigate the effect of operation parameters on hot spot temperature. Secondly, based on the conclusions of tube-side, a novel configuration multi-tubular fixed-bed reactor comprising 790 tubes design with disk-and-doughnut baffles is proposed by comparing with segmental baffles reactor and their performance of fluid flow and heat transfer is analyzed to ensure the uniformity condition using molten salt as heat carrier medium on shell-side by three-dimensional CFD method. The results reveal that comprehensive performance of the reactor with disk-and-doughnut baffles is better than that of with segmental baffles. Finally, the effects of operating conditions to control the hot spots are investigated. The results show that the flow velocity range about 0.65 m/s is applicable and the co-current cooling system flow direction is better than counter-current flow to control the hottest temperature.

Thermal transport in laminar flow over superhydrophobic surfaces, utilizing an effective medium approach

Abstract: An analytical methodology to characterizing the effects of heat transport in internal laminar flows over ridged patterns, mimicking superhydrophobic surfaces, is indicated. The finite slip velocity on such surfaces and the thermal conductivity characteristics of the constituent material are both shown to modify the convective heat transport in the fluid. We use an effective medium approach to model the lowered thermal conductivity caused by the presence of air in the ridge interstices. The proposed analytical solutions for fully developed flow were verified through comparison with numerical simulations for a periodically ridged geometry in laminar flow. While the convective heat transport and the Nusselt Number (Nu) increase due to the modified fluid velocity profile on superhydrophobic surfaces, the decrease in the thermal conductivity of the substrate may play a larger role in determining the overall heat transfer in the channel.

Entry length effects for momentum, heat and mass transfer in circular ducts with laminar flow

Abstract: This paper shows the results of a computational study relating to the calculation of heat and mass transfer coefficients in the entry region of a circular duct. Laminar flow with temperature dependent physical properties is used. The results are compared with the classical results obtained for a fluid with constant physical properties. It is seen that a plot of the Nusselt number versus the inverse Graetz number does not yield a unique curve, but rather depends on the channel diameter, wall temperature or flux value, and the inlet fluid velocity. The deviation from the curve obtained with constant physical properties increases as the velocity decreases.

Simulations of dissolution of spherical particles in laminar shear flow

Abstract: a b s t r a c t Simulations of dense suspensions of spherical solid particles in a Newtonian liquid carrier phase under simple shear flow have been performed. The simulations include solid–liquid mass transfer and (related) dissolution of the solids phase in the liquid. The interfaces between the solid particles and the liquid are fully resolved: in terms of the flow dynamics we apply a no-slip condition there and simulate the flow of the interstitial liquid by means of the latticeBoltzmann method. In terms of mass transfer we solve a convection–diffusion equation for the solute concentration in the liquid with the saturation concentration imposed at the surface of the particles. The conditions are such that the flow is laminar (particle-bases Reynolds number significantly less than one). Peclet numbers are significant (order 100) which imposes strong demands on proper resolution of the mass transfer process. Results include dissolution times as a function of process conditions such as shear rate, solids loading, diffusivity and solubility. © 2014 The Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Annular flow microreactor: An efficient tool for kinetic studies in gas phase at very short residence times

Abstract: This study deals with the modelling of a tubular flow microreactor and an annular flow microreactor that are used for kinetic studies of thermal reactions (873–1273 K) at very short residence times (10–100 ms). The construction of a kinetic model on the basis of experimental reaction data requires the precise characterization of the thermal behaviour and the hydrodynamics of the reactor. In kinetic studies, the reactor is usually considered as an isothermal Plug Flow Reactor. In this paper, we demonstrate that an annular flow microreactor used at very short residence time not only leads to a temperature profile which is better controlled than in a tubular flow microreactor, but also to less axial dispersion for laminar flow. The comparison between both reactors is performed on the basis of temperature measurements, RTD measurements at room temperature, dimensionless numbers calculations and Computational Fluid Dynamics in typical reaction conditions.