Chemical Kinetic Studies Research Articles

A pneumatic piston-released rapid compression machine for chemical kinetics studies at elevated pressure and low to intermediate temperatures

This work presents a pneumatically operated piston released rapid compression machine (PRRCM) designed at the University of Sheffield that introduces a new set of pneumatic systems to lock/release the piston from its initial position. It is pneumatically operated to compress gas mixture to an adiabatically thermodynamic state and stopped hydraulically. The range of compression ratios of the facility is between 4.4 and 20. The end of compressed gas temperature, T c and pressure, P c obtained within the facility is approximately 1000 K and 22 bar respectively. The performance of the Sheffield piston released rapid compression machine (SHEF-PRRCM) facility has been characterised to ensure a high level of fidelity of experimental data over a range of test conditions. The performance test was conducted for a nonreactive test using nitrogen and argon, the result demonstrated a repeatable pressure trace. Repeatability test for the reactive mixtures was also demonstrated. Further study on ignition characteristics of aviation fuel (Jet A-1) and Banner NP1014 known as Bannersol in SHEF-PRRCM was conducted over T c of 723 K ⩽ T c ⩽ 884 K, P c = 6 and 10 bar at ϕ = 0.75 and 1.0. The influence of pressure, temperature and equivalence ratio was seen on the oxidation of Jet A-1 and Bannersol. The results showed that Bannersol displayed a negative temperature coefficient behaviour and has higher reactivity than Jet A-1. Besides, a comparative analysis of the current facility with other facilities in literature was carried out, the result showed a fair deviation of the current data from literature and these could be attributed to the inherent non-ideality of heat transfer effects in different rapid compression machine and fairly slight differences in the condition tested. This work has demonstrated the competence of the current facility to carry out further studies on combustion and validation of the chemical kinetics of hydrocarbon fuels.

NO2添加の低温酸化・着火遅れに与える影響についての化学反応論的研究(第2報)

NO2添加の低温酸化・着火遅れに与える影響についての化学反応論的研究(第2報)

Detailed examination of the combustion of diesel and glycerol emulsions in a compression ignition engine

This study examines glycerol as an additive to diesel fuel to demonstrate it has the potential to suppress the formation of soot/PM. The investigation of a diesel/glycerol emulsion included an engine trial, high-speed imaging in an optical combustion chamber and a fundamental chemical kinetic study examining soot precursor formation. The emulsion had a longer ignition delay but higher AHRR with increasing load. There was no impact on the brake thermal efficiency. CO and THC were higher with the emulsion at the lower engine loads. The emulsion emitted a smaller number of particles with diameters greater than 25 nm, with a significant drop in the number of particles at 60 nm. The number of particles with diameters greater than 25 nm is reduced by 61% at 20 Nm, by 56% at 80 Nm, and by 11% at 140 Nm. A large peak of sub 10 nm particles, 2 orders of magnitude greater than with diesel alone, was observed, hypothesised to be semi-volatile organic compounds that have started to condense. A thermogravimetric analysis supported a larger semi-volatile content. Ignition delay time, determined from the OH* flame emission, was always longer for the emulsion at all conditions. In-flame soot was always lower with the emulsion at all conditions. Flame lift-off length decreased with increasing temperature and pressure of the ambient gas whilst soot increased. The concentration of known soot precursors, C2H2 and C2H4 was reduced but the concentrations of C3H6 and PC3H4 were not significantly affected.

Outline of Discussion: A Chemical Kinetics Study on the Combustion in Internal Combustion Engines

Outline of Discussion: A Chemical Kinetics Study on the Combustion in Internal Combustion Engines

NO 2 添加の低温酸化・着火遅れに与える影響についての化学反応論的研究

NO 2 添加の低温酸化・着火遅れに与える影響についての化学反応論的研究

Updated thermochemistry for renewable transportation fuels: New groups and group values for acetals and ethers, their radicals, and peroxy species

AbstractWe present new groups and group values for the gas‐phase thermochemistry of ethers, polyethers, and acetals suited for combustion modeling. Our investigation comprises fuel species, their primary radicals, peroxy radicals, and hydroperoxide species. In total, 45 species are used for the parameterization of 14 groups, six of which are newly introduced here. Presently, calculated thermochemistry at the DLPNO‐CCSD(T)/CBS(cc‐pVTZ, cc‐PVQZ) // B3LYP‐D3BJ/def2‐TZVP level of theory is combined with thermochemistry from the literature. Validation of the new group values against the quantum mechanical results gives deviations of 1.22 kcal/mol, 2.36 cal/(molK), and 1.20 cal/(molK) for heats of formation, standard entropies, and heat capacities, respectively. The new thermochemistry is tested with a recent OME2 and OME3 chemical kinetic model, which shows large sensitivities to the updated values in the intermediate temperature regime. This highlights the importance of using updated thermochemistry in future chemical kinetic modeling studies of oxygenated methyl ethers (OMEs) and OME‐related structures.

Batch and continuous fixed bed adsorption of heavy metals removal using activated charcoal from neem (Azadirachta indica) leaf powder

The present investigate was intended for adsorption of heavy metals i.e. Pb, Cu, Cr, Zn, Ni and Cd onto activated charcoal prepared from neem leaf powder (AC-NLP) using batch and column studies. Batch adsorption was performed using different variables like adsorbent dose, temperature and contact duration. Thermodynamic analysis of batch treatment concluded that adsorption is thermodynamically feasible and endothermic. This adsorption followed the Pseudo second-order kinetic model derived from correlation coefficient values of chemical kinetic studies. For column study, interpretation of breakthrough curves and parameters were conducted by varying flow rate, initial concentration and bed height; and reveal that optimum conditions were lower flow rate (5 mL/min) and lower initial concentration (5 mg/L) and higher bed height (20 cm). Comparisons of batch and column study through isotherm models were evaluated and column study is more preferred than batch treatment. Maximum Thomas adsorption capacity was achieved upto 205.6, 185.8, 154.5, 133.3, 120.6, 110.9 mg/g for Pb, Cu, Cd, Zn, Ni and Cr respectively. This removal pattern is elucidated by metal ionic properties. Various adsorbing agents such as acids and bases were utilized for adsorption–desorption of AC-NLP.

Influence of Torsional Anharmonicity on the Reactions of Methyl Butanoate with Hydroperoxyl Radical.

An ab initio chemical kinetics study of the reactions of methyl butanoate (MB) with hydroperoxyl radical (HO2) is presented in this paper. Particular interest is placed on determining the influences of torsional anharmonicity and addition reaction on the rate constants of hydrogen abstraction reactions. Stationary points on the potential energy surface of MB + HO2 are calculated at the level of QCISD(T)/CBS//B3LYP/6-311++G(d,p). The transition state theory (TST) is used to calculate the high-pressure limit rate constants of the hydrogen abstraction reactions over a board range of temperature (500-2000 K). Anharmonicity of low-frequency torsional modes is considered in the rate calculations by using the one-dimensional hindered rotor approximation and the internal-coordinate multistructural approximation; the latter is used as a higher-level theoretical method to examine the applicability of the former in dealing with strongly coupled torsional modes. The calculated rate constants are compared with the available data from the literature and observed discrepancies are analyzed in detail. An energetically lowest-lying addition reaction with subsequent isomerization and decomposition reactions are identified on the potential energy surface. The multiple-well Master equation analysis shows that these reactions have a secondary influence on the rate constants in the temperature range of interest.

Numerical study on the auto-ignition characteristics of methane oxy-fuel combustion highly diluted by CO2

The methane oxy-fuel combustion highly diluted by CO2 at supercritical region has the best potential for its near-zero emission characteristics and high thermal efficiency, while its auto-ignition characteristic is still not clear. In current investigation, a model with detailed chemical kinetic mechanism was introduced to research its ignition delay times (IDs). Results showed that the fold points in low-temperature and high-pressure region is due to CH3O2 which only involved in Aramco-Mech 1.3. When temperature increases, the inhibition of the termination reactions plays a dominate role at low pressure, while the promotion of the branching reactions contributes more to the acceleration of ignition at high pressure. Five dominant pathways and their variations were clarified to depict the inhibition effect of the ratio of CO2/O2. Based on the newly proposed combustion mode, it was found that temperature dependence of the nonnegligible CO2 collision effect is opposite under low and high pressure with full-oxy combustion mode. This investigation extended the research region to higher pressure and comprehensively studied the relevant factors on the auto-ignition which not only provides theoretical reference for the chemical kinetics study of methane, but also contributes to the further researches of EGR, SCF and near-zero emissions technologies.

Experimental and kinetic modeling studies on the auto-ignition of methyl crotonate at high pressures and intermediate temperatures

Biofuels, including biodiesel have the potential to partially replace the conventional diesel fuels for low-temperature combustion engine applications to reduce the CO2 emission. Due to the long chain lengths and high molecular weights of the biodiesel components, it is quite challenging to study the biodiesel combustion experimentally and computationally. Methyl crotonate, a short unsaturated fatty acid methyl ester (FAME) is chosen for this chemical kinetic study as it is considered as a model biodiesel fuel. Auto-ignition experiments were performed in a rapid compression machine (RCM) at pressures of 20 and 40 bar under diluted conditions over a temperature range between 900 and 1074 K, and at different equivalence ratios (ϕ = 0.25, 0.5 and 1.0). A chemical kinetic mechanism is chosen from literature (Gaïl et al. 2008) and is modified to incorporate the low-temperature pathways. The mechanism is validated against existing shock tube data (Bennadji et al. 2009) and the present RCM data. The updated mechanism shows satisfactory agreement with the experimental data with significant improvements in low-temperature ignition behavior. The key reactions at various combustion conditions and the improved reactivity of the modified mechanism are analyzed by performing sensitivity and path flux analysis. This study depicts the importance of low-temperature pathways in predicting the ignition behavior of methyl crotonate at intermediate and low temperatures.

Combining Mass Spectrometry of Picoliter Samples with a Multicompartment Electrodynamic Trap for Probing the Chemistry of Droplet Arrays.

Single droplet levitation provides contactless access to the microphysical and chemical properties of micrometer-sized samples. Most applications of droplet levitation to chemical and biological systems use nondestructive optical techniques to probe droplet properties. To provide improved chemical specificity, we coupled a multicompartment quadrupole electrodynamic trap (QET) with single droplet mass spectrometry. Our QET continuously traps a monodisperse droplet population (tens to hundreds of droplets) and allows for the simultaneous sizing of a single droplet using its Mie scattering pattern. Single droplets are subsequently ejected into the ionization region of an ambient pressure inlet mass spectrometer. We optimized two complementary soft ionization techniques for picoliter aqueous droplets: (1) paper spray (PS) ionization and (2) thermal desorption glow discharge (TDGD) ionization. Both techniques detect oxygenated organic acids in single droplets, with signal-to-noise ratios >100 and detection limits on the order of 10 pg. Sensitivity and reproducibility across single droplets are driven by the droplet deposition location and spray stability in PS-MS and the ionization region humidity and analyte evaporation rate in TDGD-MS. Importantly, the analyte evaporation rate can control the TDGD-MS quantitative capability because high evaporation rates result in significant ion suppression. This effect is mitigated by optimizing the vaporization temperature, droplet size range, and analyte volatility. We demonstrate quantitative and reproducible measurements with a droplet internal standard (<10% RSD) and compare the sensitivity of PS-MS and TDGD-MS. Finally, we demonstrate the application of QET-MS to the study of heterogeneous chemical kinetics with the reaction of gas phase O3 and aqueous maleic acid droplets.

Crystal structure, chemical reactivity, kinetic and thermodynamic studies of new ligand derived from 4-hydroxycoumarin: Interaction with SARS-CoV-2

Currently, Covid-19 pandemic infects staggering number of people around the globe and causes a high rate of mortality. In order to fight this disease, a new coumarin derivative ligand (4-[(pyridin-3-ylmethyl) amino]-2H-chromen-2-one) (LTA) has been synthesized and characterized by single-crystal X-ray diffraction, NMR, ATR, UV-Visible and cyclic voltammetry. Chemical reactivity, kinetic and thermodynamic studies were investigated using DFT method. The possible binding mode between LTA and Main protease (Mpro) of SARS-CoV-2 and their reactivity were studied using molecular docking simulation.Single crystal X-ray diffraction showed that LTA crystallizes in a monoclinic system with P21 space group.The reactivity descriptors such as nucleophilic index confirm that LTA is more nucleophile, inducing complexation with binding species like biomolecules. The kinetic and thermodynamic parameters showed that the mechanism of crystal formation is moderately exothermic.The binding energy of the SARS-CoV-2/Mpro-LTA complex and the calculated inhibition constant using docking simulation showed that the active LTA molecule has the ability to inhibit SARS-CoV-2.

Chemical kinetic study on ignition and flame characteristic of polyoxymethylene dimethyl ether 3 (PODE3)

Polyoxymethylene dimethyl ether 3 (PODE3) is a promising diesel additive with high cetane number (78) and oxygen mass content (48%), but its ignition and flame characteristics are rarely reported. This work provides a new fundamental understanding of the ignition delay times, adiabatic flame temperature and laminar flame speed of PODE3 by detail chemical kinetic mechanism. The ignition delay time of PODE3 is lower than n-heptane (reference fuel) due to a higher global reaction rate. The ignition delay profile of PODE3 does not exhibit negative temperature coefficient (NTC) behavior and thus sensitive to temperature variation which benefits low-temperature combustion in compression ignition engines. This study discovers that the high cetane number of PODE3 guarantees a high global reaction rate, but it lacks sufficient temperature rise and active radical accumulation to act as the chemical ignition source. The oxidation reaction pathways between PODE3 and n-heptane are similar, both compose of (i) 1st O2 addition, RO2 isomerization to produce QOOH and (ii) 2nd O2 addition, (iii) ketohydroperoxide decomposition. PODE3 produces a large amount of formaldehyde due to lack of carbon-carbon bond while the n-heptane takes place β-scission to produce olefins. PODE3 has higher premixed laminar flame speed than n-heptane due to a higher global reaction rate and flame temperature. PODE3 benefits the flame sustaining at engine low load because its laminar flame speed suffers less temperature and pressure dependence. A machine learning regression model is proposed to reproduce the non-linear relationship between laminar flame speed and equivalence ratio, temperature and pressure.

Chemical kinetic modeling study of methyl esters oxidation: Improvement on the prediction of early CO2 formation

The early CO2 formation is a characteristic for the methyl esters group of biodiesel. A new reaction pathway was added to skeletal methyl esters mechanism for improving the prediction of early CO2 formation. The methyl decanoate, methyl 9-decenoate, methyl 5-decenoate and methyl stearate sub-mechanisms with added reaction pathway were optimized by adjusting reaction rate constants for more accurate prediction. Based on decoupling methodology, a new skeletal mechanism for methyl butanoate was constructed by integrating detailed H2/CO/C1 sub-mechanism, reduced C2-C3 sub-mechanism and methyl butanoate sub-mechanism. These improved mechanisms were validated well in a shock tube for ignition delay times and in a jet-stirred reactor for major species concentrations over wide operating conditions, respectively. When compared to available mechanism in the literature, the present mechanism has good improvement for the prediction of early CO2 formation. Moreover, the effect of newly added reactions on ignition delay times was analyzed by sensitivity analysis method. Added reaction of Fuel Radical = ME2J + Short Chain Hydrocarbon mainly causes the influence on ignition delay time at high temperature, and decrease the reactivity of oxidation of fuel radical. The reaction of OCHO + M<=>H + CO2 + M dominates the early CO2 formation, and makes less contribution to production of CO2 with higher temperature. The improved mechanisms, which consist of methyl esters from a relatively short to long carbon chain, have a good performance for the prediction of early CO2 formation.

A high-repetition-rate shock tube for transient absorption and laser-induced fluorescence studies of high-temperature chemical kinetics.

A newly constructed high-repetition-rate shock tube designed for kinetic studies of high-temperature reactions using spectroscopic methods is described. The instrument operates at a 0.2-Hz cycle rate with a high reproducibility of reaction conditions that permits extensive signal averaging to improve the quality of kinetic trace data. The density and temperature of the gas behind the reflected shock wave are examined by probing the product formation from reference reactions. Two types of experimental techniques are implemented: transient absorption spectroscopy and time-resolved laser-induced fluorescence. Both methods are shown to be suitable for kinetic measurements of elementary reactions, as illustrated by their application in thermal decomposition reactions of the benzyl radicals and trifluoromethane.

One-Step Dynamic Imine Chemistry for Preparation of Chitosan-Stabilized Emulsions Using a Natural Aldehyde: Acid Trigger Mechanism and Regulation and Gastric Delivery.

Chitosan is a polysaccharide widely used as a structuring agent in foods and other materials because of its positive charge (amino groups). At present, however, it is difficult to form and stabilize emulsions using chitosan due to its high hydrophilicity. In this study, oil-in-water emulsions were prepared using a one-pot green-chemistry method. The chitosan and aldehyde molecules were in situ interfacially conjugated during homogenization, which promoted the adsorption of chitosan onto the oil droplet surfaces where they created a protective coating. The universality of this method was verified by using chitosan with different molecular weights and four kinds of natural aldehydes [cinnamaldehyde (CA), citral (CT), citronella (CN), and vanillin (VL)]. Chitosan with higher molecular weight facilitated the formation of emulsions. By harnessing the dynamic covalent nature of imine bonds, chitosan emulsions with an imine link display dynamic behavior with acid-catalyzed hydrolysis. The aldehyde structure could control the pH point of trigger for breakdown of emulsions, which was 1.0, 3.0, 4.0, and 4.0 for CA emulsion, CT emulsion, CN emulsion, and VL emulsion, respectively. At pH 6.5, aldehyde helped to decrease the interfacial tension of chitosan to about 10 mN/m, while this value would increase if the pH decreased by adding acid during the measurement. Chemical kinetics studies indicated that the hydrophobicity and conjugation effect of the aldehyde together determined the trigger points and properties of the emulsion. Finally, we used the optimized emulsions to encapsulate and control the release of curcumin. The gastric release behavior of the curcumin depended on aldehyde structure: VL > CN > CT ≈ CA. Hence, a tailor-made trigger release emulsion system can be achieved by rational selection and design of aldehyde structure to control hydrophobicity and conjugation effect of aldehydes.

A study on the effects of textural properties of γ-Al2O3 support on CO2 capture capacity of Na2CO3

Na2CO3/γ-Al2O3 is one of the most promising sorbents to capture CO2 from industrial flue gas streams due to its high efficiency in low-temperature and low cost. The textural properties of the γ-Al2O3 support, such as specific surface area and pore volume play key roles in performance of the sorbent. Therefore, the aim of this study was to improve the sorbent performance, using several γ-Al2O3 supports, prepared through a modified sol-gel process in the presence of polyethylene glycol (PEG). N2 adsorption-desorption analysis showed that the pore volume and specific surface area of the sample with the PEG/Al(OPri)3 mole ratio of 0.05 were, respectively, about 340 % and 38 % higher than the sample synthesized in the absence of PEG. In addition, improving the specific surface area and pore volume of γ-Al2O3 enhanced the total CO2 capture capacity of a Na2CO3/γ-Al2O3 sorbent by 34 %. Moreover, the effect of Na2CO3 loading on the best γ-Al2O3 sample was investigated, and the optimum loading of Na2CO3 and the corresponding total CO2 capture capacity were observed to be 50 wt.% and 131.6 mg CO2/gsorbent, respectively. Finally, the regeneration conditions of the sodium-based sorbent were determined using chemical kinetic study and temperature programmed desorption (TPD) analysis.

Numerical analysis on the effects of CO2 dilution on the laminar burning velocity of premixed methane/air flame with elevated initial temperature and pressure

Understanding the different effects of CO2 dilution on the laminar premixed flame is an effective strategy for combustion control. Thus, a chemical kinetic modeling study of laminar burning velocity (LBV) for CH4/air/CO2 mixtures coupling with a detailed chemical reaction mechanism was employed under a wide range of initial conditions. After validation and comparison, the FFCM 1.0 was selected for this study. Based on the numerical prediction of LBV, the decoupling method was carried out to separate the effects of diluent CO2, including dilution, thermal and chemical effects, whose variations show that the rising initial temperature facilitates the three effects whereas the elevated pressure shows the opposite impact. In order to explain the particular process physically and chemically, the parameters analysis, sensitivity analysis and mole fraction analysis methods were conducted, and the LBV enhancement made by per unit of fuel, relevant thermal parameters and the important active radicals were proved to be responsible for the variation of the three effects, respectively. Furthermore, the domain reactions contributing to chemical effect are reactions (R32) and (R62), respectively, as temperature and pressure vary. Through this work, it not only completes the research on the effect of CO2 dilution on the laminar premixed flame, but also expands the commonly exploring initial conditions to a wide range, which provides the reference for the research on the operation condition and intensity setting of exhaust gas recirculation (EGR).

Experimental study of CO2 chemical absorption kinetics by a thermomorphic lipophilic biphasic solvent

Chemical absorption is a frequently used post-combustion process that separates CO2 from gaseous compounds of the flue gases with the use of specific solvents for which the main drawback is their cost of regeneration. In order to overcome this problem, the new Thermomorphic Lipophilic Biphasic solvents are here studied. The limited solubility of the Lipophilic amines in water offers a new degree of freedom while optimizing the global process: it is in fact possible to carry out the absorption with an initial biphasic solvent (organic, aqueous) that evolves towards a mono-phase solvent after a certain amount of CO2 is absorbed. This work aims at assessing experimentally the CO2 absorption kinetic into a Lipophilic amine based solvent, namely 3 M 1:3 DPA (dipropylamine) + DMCA (dimetylcyclohexylamine) + water using a Lewis Cell type reactor. Activation energy values obtained by the Arrhenius law show similar values for those of benchmark amines and the selected solvent seems to require less energy to start the chemical reaction.

A Single Stroke Cylinder Rapid Compression Machine for Chemical Kinetic Study at Elevated Pressure and Temperatures

The fabrication of a rapid compression machine (RCM) is in its early phase of design. The machine is designed to enhance the study of ignition delay and validation of detailed kinetics models of fuels. The machine compresses fuel/air mixtures isentropically within 25 to 52 ms with a varying stroke. The combustion chamber design is not fixed and can be adjusted through the threaded shaft lock and within chamber slots. The originality of the facility is the inclusion of a pneumatic piston release mechanism (PPRM), which is pneumatically operated. The current test facility has been characterised by conducting a nonreactive and reactive experiment, the result showed that an obtainable compressed pressure of 21 bar and end gas temperature of approximately 1000 K was achievable within the present facility. The fidelity of the facility was performed with a non-reactive experiment, which experimental pressure profile was seen to follow each other closely showing that the data are highly repeatable within the test condition, the result was free from any form of rebound or disturbance, which would have adversely distort the result. The experiment data was simulated implementing the effective volume approach and was seen to perfectly match with the experiment at both stages of compression. The reactive experiment was demonstrated with heptane/air mixture at stoichiometric condition, TC = 625 ⩽ 689 K. The results show that the experimental pressure traces overlay each other thus signifying a repeatable pressure trace and this demonstrates that the Shef-RCM is operable and ready at its first stage of design for studying the ignition delay time of liquid fuels operating within an engine like conditions and for validating chemical kinetics models.