Stirred Reactor Research Articles

Modeling of thermal and kinetic processes in non-equilibrium plasma ignition applied to a lean combustion engine

In recent years novel ignition systems have been developed to enable stable and efficient engine operations with lean mixtures. Among them, radio-frequency corona ignition systems create discharges that involve a much wider region compared to traditional spark, and produce non equilibrium plasma with high levels of active radicals and excited species. These devices considerably increase the early flame growth speed and extend stable operating limits. With the aim of expanding the knowledge on high efficiency lean-burn SI engines, this paper investigates and compares the combustion development generated by spark and corona ignitions through computational fluid dynamics, within the Reynolds–Averaged Navier–Stokes framework for turbulence modeling. In order to simultaneously take thermal and chemical effects into account, the Perfectly Stirred Reactor combustion model is used. Experimental data are also collected for validation in an optical access engine, for different mixture levels, from stoichiometric to very lean. The faster burn rate generated by the corona system in the initial stage of the combustion is well predicted by the simulations, in all the relative air–fuel ratio conditions. Remarkably, as the mixture becomes lean, simulations are able to capture the non-linear transition from fast to slow kernel growth, before a self-sustainable flame propagation is established. This correlates very well with the measured engine cyclic variability and the corresponding steep change in the duration of the flame kernel formation. Ultimately, this study highlights the important role of the atomic oxygen, as active radical, in promoting and enhancing the combustion initiated by a corona discharge, in addition to the volumetric ignition effect. By contrast, the validated simulations allow to explain that the high-temperature thermal plasma generated in a traditional spark discharge is insensitive to kinetic aspects.

Microreactor facilitated preparation and Ni-doping of MnO2 nanoparticles for supercapacitors

Clustered countercurrent-flow micro-channel reactors (C-CFMCR) with varying magnification times were applied for the preparation of Ni-doped manganese dioxide (Ni-MnO2) via co-precipitation processes. Effects of the feeding flow rate of reactants and the concentration of the doping agent on the doping processes were investigated. The C-CFMCR, which is of intensified micromixing efficiency as compared with conventional stirred reactors (STR), played a crucial role in facilitating the uniform nucleation and growth of MnO2 crystals and then the doping of Ni. As a result, the prepared Ni-MnO2 had a decreased particle size and a more uniform microporous structure. EDS analysis showed an even distribution of Ni2+ within the nanocomposites and thus enhanced electrochemical performance of the MnO2 composite materials as supercapacitor electrodes was achieved. The 1.0 at% Ni-MnO2 nanocomposites prepared under optimal conditions exhibited the highest specific capacitance of ~ 389.6 F g−1 at a current density of 1 A g−1 and showed excellent cycling stability with 79.3% retention of the initial capacitance after 5000 charge/discharge cycles in 2 M KOH aqueous solution. The as-assembled Ni-MnO2//activated carbon (AC) asymmetric supercapacitor displayed a wide operating voltage (0–1.6 V), high energy and power densities (13.3 W h kg−1 and 187.52 W kg−1 at 0.25 A g−1, respectively), and a stable cycling behavior (83.8% capacitance retention after 1000 cycles at 1 A g−1). In addition, only a weak scaling-up effect of C-CFMCR on the co-precipitation process was observed, suggesting that C-CFMCR is a prospective technique for the continuous and large-scale production of nanoparticles.

Influence of substrates concentrations on the dynamics of oxygen demand and aeration performance in ideal bioreactors

The effect of bioreactor configurations on the dynamics of aeration modelling was investigated by incorporating three different correlations from the literature to estimate α-factors into the aeration model. Estimated air flow rates using the three correlations were then validated against experimental data obtained from pilot sequencing batch reactors (SBRs). Two identical SBRs were operated in parallel; one received raw wastewater and the other received primary treated wastewater. The validated aeration model was then used to evaluate aeration dynamics in different bioreactor configurations, both for nitrification only and nitrification/denitrification, with the three different correlations. The current study is the first to investigate the validity of the aforementioned correlations using various bioreactor configurations and to establish that the bioreactor configuration not only impacts spatial and temporal biological oxygen demands as currently understood but also oxygen transfer efficiency. The first correlation based on the real-time bioreactor soluble chemical oxygen demand (sCOD) was able to predict the temporal measured air flow rate in the pilot SBRs. The second correlation based on the influent COD overestimated the air flow rates as it considered the impact of the influent loading rates on the α-factor and overlooked the improvement in α-factor due to biodegradation. The third correlation based on MLSS concentrations underestimated the air flow rates at the beginning of the aeration cycle as it ignored the impact of influent loading rates on the α-factor and considered only the insignificant change in MLSS during the aeration cycle. In terms of bioreactor configuration, the model-based analysis showed that the first correlation is suitable for designing SBR, plug flow reactor (PFR), step-feed PFR, and completely mixed stirred reactor (CSTR) systems, and the third correlation is suitable for designing CSTRs and membrane bioreactors (MBRs), while the second correlation was not accurate in any of the reactors modelled. When nitrification was targeted, the CSTR reduced aeration energy by 44 %–49 % compared to the PFR, and 41 %–43 % when both nitrification and denitrification were targeted. Compared to the plug-flow reactor, the step-feed PFR reduced aeration energy by 9% when nitrification only was targeted. However, when pre-denitrification was added, both systems showed the same aeration energy consumption. Pre-denitrification reduced organic loadings to aeration tanks and decreased aeration energy by 22 %, 11 %, 15 %, and 14 % in PFR, CSTR, PFR step feed and MBR systems, respectively.

Kinetic effects of methyl radicals on PRF lean ignition: a comparative study of skeletal mechanisms

Despite considerable achievements of combustion kinetics, the chemical kinetic impacts of trace promoting species on practical fuel combustion has been hitherto handled seperately either as ignition or extinction-related effects, overlooking the impact of the extinction process on re-ignition kinetics. We herein demonstrate the applicability of oscillatory stirred reactor configuration to quantify the combined impacts of the methyl radical on ignition/extinction of primary reference fuels (PRF) in stoichiometric and ultra-lean conditions, thereby complementing steady-state configurations for kinetic mechanism validation. A new convenient reactivity metric is proposed based on the matching of temperature Fourier spectrum of additivated fuels with corresponding PRF. Using a new skeletal mechanism obtained from calibration on a variety of ignition experiments, we characterize ignition regimes by CEMA analysis and highlight the key reactions of the C1-C3 reaction subsets that should be included in minimal skeletal mechanisms used in reactive CFD modeling to properly describe re-ignition phenomena over a variety of new combustion applications involving methyl radical injection.

CFD Simulation of Solid Suspension for a Liquid–Solid Industrial Stirred Reactor

The present study examines the possibility of using an industrial stirred chemical reactor, originally employed for liquid–liquid mixtures, for operating with two-phase liquid–solid suspensions. It is critical when obtaining a high-quality chemical product that the solid phase remains suspended in the liquid phase long enough that the chemical reaction takes place. The impeller was designed for the preparation of a chemical product with a prescribed composition. The present study aims at finding, using a numerical simulation analysis, if the performance of the original impeller is suitable for obtaining a new chemical product with a different composition. The Eulerian multiphase model was employed along with the renormalization (RNG) k-ε turbulence model to simulate liquid–solid flow with a free surface in a stirred tank. A sliding-mesh approach was used to model the impeller rotation with the commercial CFD code, FLUENT. The results obtained underline that 25% to 40% of the solid phase is sedimented on the lower part of the reactor, depending on the initial conditions. It results that the impeller does not perform as needed; hence, the suspension time of the solid phase is not long enough for the chemical reaction to be properly completed.

CFD Assessment of the Hydrodynamic Performance of Two Impellers for a Baffled Stirred Reactor

When converting a baffled stirred reactor to work with a different fluid, usually the original impeller must be replaced with a customized one. If the original impeller was designed for mixing liquids, its performance for liquid–solid suspensions may not be satisfactory. A case study is presented, where a two-blade original impeller is replaced with a new three-blade design. The new impeller shows clear improvements in mixing a liquid–solid suspension, while keeping the shaft power practically at the same level. As a result, a practically homogenous liquid–solid mixture is obtained, thus ensuring the required quality of the final product. The present numerical investigations employ the Eulerian multiphase model with renormalization (RNG) k–ε turbulence model to simulate the three-dimensional unsteady free-surface liquid–solid flow in a stirred tank. A sliding mesh approach was used to account for the impeller rotation within the expert code, FLUENT 16. The comparative quantitative analysis of the solid phase distribution and the relevant velocity profiles show that the new design of three-blade-impeller is significantly increasing the sedimentation time of the solid phase beyond the chemical reaction specific time. The necessary power to drive the new impeller has a slightly higher value than for the original impeller but it can be sustained by the existing driving system.

Large Eddy simulation of a supersonic lifted hydrogen flame with perfectly stirred reactor model

Large Eddy Simulation with a Perfectly Stirred Reactor model (LES−PSR) is developed to simulate supersonic combustion with high-enthalpy flow conditions. The PSR model considers the viscous heating and compressibility effects on the thermo-chemical state, through correcting the chemical source term for progress variable and incorporating absolute enthalpy as the control variable for the look-up table. It is firstly validated by using a priori analysis of the viscous heating and compressibility effects. Then an auto-igniting hydrogen flame stabilized in supersonic vitiated co-flowing jet is simulated with LES−PSR method. The results show that the shock wave structure, overall flame characteristics, flame−shock interaction and lift-off height are accurately captured. Good agreements of the velocity and mixture fraction statistics with the experimental data are observed. The results also show that the LES−PSR model can predict the mean temperature and mole fractions of major species quite well in both flame induction and stabilization zones. However, there are some under-predictions of temperature RMS by about 100−150 K, which may be due to the chemical non-equilibrium in the H2/O2-enriched combustion product of the co-flowing jet. The scatter plots of two probe locations respectively from induction and flame zones show that the respective flame structures in mixture fraction space are captured well. However, the flucturations of the temperature and species mole fractions are under-predicted in the flame zone. The shock-induced auto-igniting spots are captured by the PSR model. These spots are highly unsteady and play an important role in flame stabilization. It is also shown that the intense reactions are initiated at mixture fractions around the stoichiometry or fuel-lean values, corresponding to local elevated pressure (1.5−2.0 atm) due to shock compression. The results also demonstrate that the pressure elevation is shown to have significant effects on the most reactive mixture fraction and shortest ignition delay time.

OptiSMOKE++: A toolbox for optimization of chemical kinetic mechanisms

As detailed chemical mechanisms are becoming viable for large scale simulations, knowledge and control of the uncertainty correlated to the kinetic parameters are becoming crucial to ensure accurate numerical predictions. A flexible toolbox for the optimization of chemical kinetics has therefore been developed in this work. The toolbox is able to use different optimization methodologies, as well as it can handle a large amount of uncertain parameters simultaneously. It can also handle experimental targets from different sources: Batch reactors, Plug Flow Reactors, Perfectly Stirred Reactors, Rapid Compression Machines and Laminar Flame Speeds. This work presents the different features of this toolbox together with five different test cases which exemplifies these features. Program summaryProgram Title: OptiSMOKE++CPC Library link to program files:https://doi.org/10.17632/tvjky2n8md.1Licensing provisions: GPLv3Programming language: C++Nature of problem: Optimization of uncertain kinetic parameters with respect to experimental data.Solution method: Using the optimization capabilities of DAKOTA [1], and solving reacting systems with OpenSMOKE++ [2], OptiSMOKE++ determines the optimal combination of specified kinetic parameters, within their uncertainty, and with respect to the experimental data.

Analyzing lean blow-off limits of gas turbine combustors based on local and global Damköhler number of reaction zone

Lean Blow-Off (LBO) is of great significance for propulsion systems, and developing an efficient LBO prediction method can technically support combustor design. In this paper, a hybrid LBO prediction method combining Damköhler (Da) model and numerical simulation is proposed. The time scales of Damköhler number are defined based on Reaction Zone (RZ) which is obtained from the simulated flow field. RZ is divided into a number of blocks, and each block is modeled as a Perfectly Stirred Reactor (PSR). Two kinds of Damköhler numbers are calculated. Local Da (Dal) number is calculated specific to each reactor in RZ, and Dal field is generated to analyze the local flow and chemical characteristics within RZ under different operating conditions. The Dal fields under different LBO conditions show that reactions concentrate near the outlet of the swirl-cup where the flows out of the swirl-cup and the backflows caused by recirculation collide. Global Da number (Dag) is calculated based on the entire flow and chemical characteristics of RZ, and acts as an indicator of LBO. Theoretically, LBO occurs when Dag is smaller than 1. In the current study, though the inlet temperature (T3), inlet pressure (P3), and inlet mass-flow rate (ma) are varied, the Dag under LBO conditions maintains at about 1, and it is about 11 under design condition, which proves that LBO limits can be accurately predicted under wide operating conditions by the method proposed in this paper.

Oxygenated PAH Formation Chemistry Investigation in Anisole Jet Stirred Reactor Oxidation by a Thermodynamic Approach

Oxygenated poly aromatic hydrocarbons (OPAH) are widely produced in biomass combustion. Recent studies suggest significantly higher toxicity for OPAH in comparison to PAH and soot. However, the pre...

Study on the Flow Field Characteristics of a Liquid–Solid–Solid Three-Phase System and the Influence of a Draft Tube in a Stirred Reactor

The liquid–solid–solid three-phase flow field was established in a stirred reactor with a draft tube and baffles. The solid concentration distribution, mixing time, velocity distribution, flow patt...

Soot Emission Simulations of a Single Sector Model Combustor Using Incompletely Stirred Reactor Network Modeling

AbstractThe simulation of soot evolution is a problem of relevance for the development of low-emission aero-engine combustors. Apart from detailed CFD approaches, it is important to also develop models with modest computational cost so that a large number of geometries can be explored, especially in view of the need to predict engine-out soot particle size distributions (PSDs) to meet future regulations. This paper presents an approach based on Incompletely Stirred Reactor Network (ISRN) modeling that simplifies calculations, allowing for the use of very complex chemistry and soot models. The method relies on a network of incompletely stirred reactors (ISRs), which are inhomogeneous in terms of mixture fraction but characterized by homogeneous conditional averages, with the conditioning performed on the mixture fraction. The ISRN approach is demonstrated here for a single sector lean-burn model combustor operating on Jet-A1 fuel in pilot-only mode, for which detailed CFD and experimental data are available. Results show that reasonable accuracy is obtained at a significantly reduced computational cost. Real fuel chemistry and a detailed physicochemical sectional soot model are consequently employed to investigate the sensitivity of ISRN predictions to the chosen chemical mechanism and provide an estimate of the soot PSD at the combustor exit.

Nitrogen-Doped Graphene Foam as a Metal-Free Catalyst for Reduction Reactions under a High Gravity Field

Herein, we report on the effect of a high gravity field on metal-free catalytic reduction, taking the nitrobenzene (NB) reduction and methylene blue (MB) degradation as model reactions in a high-gravity rotating tube reactor packed with three-dimensional (3D) nitrogen-doped graphene foam (NGF) as a metal-free catalyst. The apparent rate constant (kapp) of the metal-free catalytic reduction of NB in the rotating tube reactor under a high gravity level of 6484g (g = 9.81 m·s−2) was six times greater than that in a conventional stirred reactor (STR) under gravity. Computational fluid dynamics (CFD) simulations indicated that the improvement of the catalytic efficiency was attributed to the much higher turbulent kinetic energy and faster surface renewal rate in the high-gravity tube reactor in comparison with those in a conventional STR. The structure of the 3D metal-free catalysts was stable during the reaction process under a high gravity field, as confirmed by X-ray photoelectron spectroscopy (XPS) and Raman spectra. In the other model reaction, the rate of MB degradation also increased as the high gravity level increased gradually, which aligns with the result for the NB catalytic reduction system. These results demonstrate the potential to use a high-gravity rotating packed tube reactor for the process intensification of metal-free catalytic reduction reactions.

Rapid methane storage via sII hydrates at ambient temperature

Mixed CH4-THF hydrate formation has been studied with the objective of advancing the operational and economic feasibility of hydrate formation. In this regard, the possibility of achieving rapid kinetics of hydrate formation at ambient temperature (298.2 K) has been explored. It was found that at a relatively moderate initial driving force of 5.4 MPa (9.5 MPa initial pressure at 298.2 K), we report mixed CH4-THF hydrate formation with a methane uptake of 40.2 (±0.9) mmol of gas/mol of water gas uptake achieved in 1.5 h of hydrate formation. Further, by synergistically combining with a benign kinetic promoter, hydrophobic amino acid L-tryptophan in low concentration (500 ppm), the kinetic performance could be rapidly enhanced with 81.3 (±3.7) mmol of gas/mol of water gas uptake obtained in 1.5 h. This gas uptake capacity equates to roughly 69% of the theoretical gas uptake limit for sII hydrates of CH4 and THF. For a lower initial driving force of 3.1 MPa (7.2 MPa initial pressure), similar kinetic performance could not be replicated. This indicates that when the temperature is fixed as a rate limiting step at a rather high value as 298.2 K in the present case, sufficient driving force is required in the form of a high initial pressure so as to achieve rapid hydrate formation. Finally, by using hybrid combinatorial approach which combines stirred (to enable fast nucleation) and unstirred reactor configuration for hydrate formation, we were able to significantly reduce the stochasticity of hydrate nucleation with the overall average nucleation time in the study being 1.4 (±0.8) minutes.

Numerical calculation with detailed chemistry of effect of ammonia co-firing on NO emissions in a coal-fired boiler

Effects of ammonia (NH3) co-firing on NO emissions in a coal-fired boiler are investigated by zero-dimensional calculation with detailed chemistry. A commercial 1000 MWe boiler system is simplified and expressed as a network of Perfectly Stirred Reactors (PSRs) to calculate emissions in flue gas. The reactor network is composed of burner zone, over-fire zone, and downstream zones. As a result of varied injection positions of NH3, lower NO emission than the coal-fired case was obtained by NH3 injection into flame zone in burner zone. As a result of analysis, NH3 injection causes NH3-rich condition and decrease of temperature in flame zone. Amount of yield of NO including thermal-NO could become relatively less. In addition, since original temperature in flame zone is higher than 2000 K, supplied NH3 is immediately dissociated and converted into NO prior to char oxidation. Since there exists abundance of NO derived from supplied NH3 and more amount of left char, effect of reduction of NO by char becomes greater than the case of coal firing. Supplied NH3 is almost completely consumed, so that it was suggested that decreasing both CO2 and NO emissions in coal-fired boilers could be realized by the NH3 co-firing in a safe way.

Application of a clustered countercurrent-flow micro-channel reactor in the preparation of KMnF3 perovskite for asymmetric supercapacitors.

A clustered countercurrent-flow micro-channel reactor (C-CFMCR) with adjustable magnification times was constructed for the preparation of KMnF3 perovskite fluoride by a co-precipitation process, in which the concentrations and feed rates of reactants were precisely controlled. Benefitting from the enhanced micromixing efficiency of the microreactor, the KMnF3 particles prepared in C-CFMCR were smaller and less aggregated than those produced with traditional stirred reactors (STR). The prepared KMnF3 was applied as the electrode material in supercapacitors, and the electrochemical measurements showed that the KMnF3 obtained under optimal conditions had a discharge specific capacitance of ∼442 F g−1 at a current density of 1 A g−1, with a decline of ∼5.4% after 5000 charge–discharge cycles in an aqueous electrolyte of 2 M KOH. It was also found that the morphologies and electrochemical performances of the prepared KMnF3 particles changed accordingly with the micromixing efficiencies of C-CFMCR, which can be adjusted by the reactor structure and operating conditions. An asymmetric supercapacitor assembled with the KMnF3 and activated carbon exhibited an energy density of 13.1 W h kg−1 at a power density of 386.3 W kg−1, with eminent capacitance retention of ∼81.2% after 5000 cycles. In addition, only a slight amplification effect of C-CFMCR on the co-precipitation process was noticed, indicating that the C-CFMCR is a promising technology for the massive and controllable production of KMnF3 particles as well as other ultrafine particles.

Modelling VOCs Emissions in Coal Fired Power Stations using Perfectly Stirred Reactor Approach

The main objective of this study is to evaluate the emissions of volatile organic compounds (VOCs) from coal-fired power stations. A quantitative understanding of the chemistry controlling the formation and destruction of these intermediate species is a prerequisite for the realistic modelling of pollutant formation in flames. Therefore, well investigated skeletal reaction mechanism has been built and introduced into perfectly stirred reactor model in order to accomplish prediction of some of these hazardous important intermediate species. This can be great help in considering lack of experimental VOCs data from fossil fuel fired power stations. Predicted results have been validated against ICSTM Furnace data where possible. The performance of the model against the large laboratory scale experimental data has resulted in considerable confidence in the use of this model for a full-scale boiler configuration. But, more confidence will be forthcoming from an increase in the amount of validation data, which is unluckily lacking at the moment. Furthermore, the model can be used in order to provide deeper insight into the formation processes of VOC species emitted from coal-fired power stations. However, this can be accomplished far better by including more elementary reactions into the skeletal reaction mechanism.

Simulation and experimental study of the NOx reduction by unburned H2 in TWC for a hydrogen engine

The unburned H2 can be used to reduce NO emission in conventional TWC (three-way catalyst) for a hydrogen internal combustion engine when it works at equivalence ratio marginally higher than the stoichiometric ratio. To explore the effects and feasibility of this reaction, a Perfectly Stirred Reactor simulation model of TWC has been built with simplified mechanisms. Experiments on a 2.3 L turbocharged hydrogen engine are used to verify the conclusion. It shows that rising initial temperature accelerates the reduction of NO and the maximum reaction rate occurs at 400 °C temperature. The conversion efficiency of NO remains approximately 0 when temperatures below 300 °C. The efficiency reaches a peak value of approximately 98% with 400 °C and declines gradually. The unburned H2 to NO mixing ratio greater than 1.5 in TWC guarantees 100% NO conversion efficiency. The experiments indicate that the NOx concentration decreases from 2056 ppm to 41 ppm at the stoichiometric ratio after the treatment of TWC and NOx reaches 0 ppm with a rich ratio. Results also demonstrate that the suitable reaction temperatures for TWC locate in the range of 400 °C–500 °C. Therefore, if the temperature and the mixing ratio are appropriate, it can achieve zero emissions with NOx reduction by unburned H2 in conventional TWC for a hydrogen engine.

Leaching of indium from waste LCD screens by oxalic acid in temperature-controlled aciduric stirred reactor

Recovery of rare metal indium from waste LCD screens is important since its content is about 500 times of the original ore. Oxalic acid has excellent ability to leach indium from waste LCD screens, and how to achieve reactor scale operation is critical to promote indium recovery. In this study, a temperature-controlled aciduric stirred reactor (TCASR) was constructed to investigate the effects of operational conditions on the leaching efficiency of indium at reactor scale. Results showed that complete leaching was achieved in 10 min under the conditions of 0.2 mol/L oxalic acid, 300 g/L of ITO glass powder dosage, 90℃ and 500 rpm of stirring speed. Compared with the previous study, the consumption of oxalic acid decreased 3/5, ITO glass powder dosage increased by 6 times and reaction time decreased 7/9. Meanwhile, the duration of indium ion concentration was maintained for 20 min instead of decreasing immediately after reaching the peak, which was conducive to actual application. The process kinetics model illustrated that temperature played the most important role in affecting the reaction rate. The potential reason of longer indium ion concentration maintaining duration was preliminarily speculated as a faster leaching of indium than calcium result in less coprecipitation caused by calcium oxalate. Therefore, the TCASR is applicable for leaching of indium from waste LCD screens by oxalic acid due to the higher leaching efficiency, less reagents consumption and stronger processing ability.

ABB 800xA Applications in Process Control of a Stirred Reactor

The ABB industrial extended automation system 800xA gives us to access and organize all plant information in a single application. The compact control builder is required to control a stirred reactor with the ability to process several liquid raw materials. This paper provides a functional design specification for designing an industrial stirred reactor and programming the AC 800M controller. The 800xA system gives a reliable, secure and control environment with all the built-in security features. The control process of the stirred reactor is simulated using ABB 800xA simulation software.