1,721,075 research outputs found
Preface (Towards Circular Economy: Closing the Loop with Chemical Recycling of Solid Plastic Waste; Advances in Chemical Engineering, V. 60, 2022)
No full textFirst principles assessment of solvent induced cage effects on intramolecular hydrogen transfer in the free radical polymerization of acrylates
Full text linkWe investigate the rate constant of poly-butyl acrylate backbiting between 310 and 510 K using semi-empirical metadynamics in the gas phase, bulk and solution. The simulations in the condensed phase are performed through a hybrid quantum mechanics/molecular mechanics approach. The free energy landscape associated with the reactive events under vacuum and in the condensed phase is used to correct harmonic transition state theory (TST) rate constants. The Arrhenius parameters so determined are introduced in a semi-detailed mechanistic kinetic mechanism of butyl acrylate polymerization in bulk and in solution, allowing it to test how the butyl acrylate polymerization rate is affected by solvent-induced cage effects on backbiting. The results show that the backbiting rate constant is higher in the condensed phase than in the gas phase. In addition, a twofold increase is observed in xylene compared to the bulk. These results differ significantly from previous theoretical calculations, especially at high temperatures, aligning better with experimental rate measurements. The semi-detailed model, incorporating our calculated rate coefficients, is validated against monomer concentration profiles from bulk and solution polymerizations in various reactor configurations, demonstrating good agreement with experimental data. This study paves the way for developing detailed kinetic models in the condensed phase using a priori kinetic parameters derived from molecular simulations, thus widening their range of applicability beyond the one experimentally accessible
Theoretical and kinetic study of the thermal decomposition mechanism of long chain aldehydes
No full textThe mechanism of decomposition of the butanal and pentanal long chain aldehydes was investigated using the ab initio transition state theory-based master equation approach. The rate constants of the homolytic bond scission reactions of butanal and pentanol leading to the formation of CHO+C3H7, CH2CHO+C2H5, C2H4CHO+CH3, C3H7CO+H, C2H5ĊHCHO+H and CH2CHO+C3H7, C2H4CHO+C2H5, C3H6CHO+CH3, C4H9CO+H, C3H7ĊHCHO+H, respectively, as well as of the reverse barrierless recombination channels, were determined using Variable Reaction Coordinate Transition State Theory. The potential energy surfaces were determined at the multireference CASPT2/aug-cc-pVTZ level on ωB97X-D/jun-cc-pVTZ structures, while stochastic samplings were performed at the CASPT2 level using a (4e,4o) active space. Rate laws for the alkyl+R-CHO, H+R-ĊHCHO, and H+R-ĊO classes of reactions were then determined. Rate constants of H-abstraction reactions and of the three body butanal →CO+H2+C3H6 channel were determined using ωB97X-D/jun-cc-pVTZ structures and Hessians and CCSD(T) energies, extrapolated to the complete basis set. It was found that the calculated homolytic bond scission reactions are faster by a factor of 2–3 with respect to literature previous estimates. The calculated reaction rates were inserted in the CRECK kinetic scheme and used to simulate literature speciation data of butanal and pentanal pyrolysis in shock tubes and ignition delay times, finding that their inclusion into the kinetic scheme enhances the predictive capabilities of the mechanism. The aldehyde decomposition mechanism and the rate constants determined in this study are suitable to be used to interpret and predict the reactivity of aldehydes in a wide range of temperatures and pressures
Analysis of acetic acid gas phase reactivity: Rate constant estimation and kinetic simulations
Full text linkThe gas phase reactivity of acetic acid was investigated combining first principle calculations with kinetic simulations. Rate constants for the unimolecular decomposition of acetic acid were determined integrating the 1D master equation over a Potential Energy Surface (PES) investigated at the M06-2X/aug-cc-pVTZ level. Energies were computed at the CCSD(T)/aug-cc-pVTZ level using a basis set size correction factor determined at the DF-MP2/aug-cc-pVQZ level. Three decomposition channels were considered: CO2+ CH4, CH2CO + H2O, and CH3+ COOH. Rate constants were computed in the 700-2100 K and 0.1-100 atm temperature and pressure ranges. The simulations show that the reaction is in fall off above 1200 K at pressures smaller than 10 atm. Successively, the PESs for acetic acid H-abstraction by H, OH, OOH, O2, and CH3were investigated at the same level of theory. Rate constants were computed accounting explicitly for the formation of entrance and exit van der Waals wells and their collisional stabilization. Energy barriers were determined at the CASPT2 level for H-abstraction by OH of the acidic H, since it has a strong multireference character. The calculated rate constant is in good agreement with experiments and supports the experimental finding that at low temperatures it is pressure dependent. The calculated rate constants were used to update the POLIMI kinetic model and to simulate the pyrolysis and combustion of acetic acid. It was found that acetic acid decomposition and the formation of its direct decomposition products can be reasonably predicted. The formation of secondary products, such as H2and C2hydrocarbons, is underpredicted. This suggests that reaction routes not incorporated in the model may be active. Some hypotheses are formulated on which these may be
Chemical kinetics of flameless combustion
No full textFlameless combustion, a process based on autoignition of convection transported kernels at the boundaries or even outside the lower explosive/flammability, is a promising candidate capable of simultaneously meeting emission reduction and efficiency improvement. Despite many similarities with more traditional combustion regimes, some key characteristics of flameless combustion are being untangled through experimental, theoretical, and kinetic modeling efforts that may even limit the broader industrial implementation of flameless combustion technologies. Overall, this chapter aims to review the state of the art in flameless combustion from a purely thermochemical kinetics standpoint and discuss the impact of peculiar chemical and thermal effects. Then, it highlights the hidden features and critical reaction pathways as well as identifying aspects of the kinetic mechanisms that require further revision
Unveiling Solvent Effects on β-Scissions through Metadynamics and Mean Force Integration
No full textThis study introduces a methodology that combines accelerated molecular dynamics and mean force integration to investigate solvent effects on chemical reaction kinetics. The newly developed methodology is applied to the β-scission of butyl acrylate (BA) dimer in polar (water) and nonpolar (xylene and BA monomer) solvents. The results show that solvation in both polar and nonpolar environments reduces the free energy barrier of activation by ∼4 kcal/mol and decreases the pre-exponential factor 2-fold. Employing a hybrid quantum mechanics/molecular mechanics approach with explicit solvent modeling, we compute kinetic rate constants that better match experimental measurements compared to previous gas-phase calculations. This methodology presents promising potential for accurately predicting kinetic rate constants in liquid-phase polymerization and depolymerization processes
High-temperature chemistry of HCl and Cl<sub>2</sub>
Full text linkThe high temperature chlorine chemistry was updated and the inhibition mechanisms involving HCl and Cl2 were re-examined. The thermochemistry was obtained using the Active Thermochemical Tables (ATcT) approach, resulting in improved data for chlorine-containing species of interest. The HCl/Cl2 chemistry discussed in the paper was based on reference and experimental measurements of rate constants available in the literature. By coupling the new HCl/Cl2 subset with the Politecnico di Milano (POLIMI) syngas mechanism a kinetic mechanism consisting of 25 species and 102 reactions was obtained. The validation was carried out on selected experimental data from laminar flames, shock tubes and plug flow reactors. Systems containing Cl2 showed high sensitivity to Cl2 +M⇌Cl+Cl+M; the rate constant for this reaction has a significant uncertainty and there is a need for an accurate high-temperature determination. The importance of the chain propagating steps such as Cl+H2 ⇌HCl+H and Cl2 +H⇌HCl+Cl competing with the branching reaction H+O2 ⇌OH+O and the termination reaction H+Cl+M⇌HCl+M is also pointed out by the kinetic analysis. Other relevant reactions in HCl containing systems are the chain propagation reactions HCl+O⇌Cl+OH, HCl+OH⇌Cl+H2O and Cl+HO2 ⇌ClO+OH, together with the termination reaction Cl+HO2 ⇌HCl+O2. With the present thermochemistry and rate constants, reaction cycles involving HOCl and ClCO were found not to be important under the investigated conditions
Towards a lumped approach for solid plastic waste gasification: Polyethylene and polypropylene pyrolysis
Full text link: In a circular economy perspective, solid plastic wastes (SPW) can become a valuable source of chemicals, energy vectors and fuels through pyrolysis, gasification, and partial oxidation technologies, but their modelling requires first the definition of suitable condensed phase pyrolysis mechanisms for each constituent. This work proposes a semi-detailed kinetic model for polyethylene (PE) and polypropylene (PP) pyrolysis based on the functional group approach implemented for polyvinylchloride (PVC) and biomass pyrolysis to consistently address mixture modelling. This approach distinguishes polymeric chains in High Molecular Weight species, represented through their chemical functionalities, and Low Molecular Weight species, described with accuracy comparable to literature detailed models, employing the reaction classes proposed in the scientific literature. Several validated lumping techniques are introduced to reduce the model computational cost, and the resulting liquid-phase model accounts for 74 species for PE and 126 species for PP. Model validation is carried out by an extensive comparison with experimental data proving the soundness of the approach and the model capability of predicting mass-loss and product distribution profiles with similar accuracy to more expensive detailed models from the literature. The proposed condensed phase approach can be extended to other polymers and coupled with other existing subsets in the CRECK kinetic framework (e.g., biomass, PVC) paving the way for unravelling mixture interactions and secondary cracking and/or gasification reactions. The model here proposed is a powerful tool to support design and optimization of SPW thermochemical recycling technologies
Alkyl radicals rule the low temperature oxidation of long chain aldehydes
No full textA reliable and simple approach for the characterization of the low temperature kinetics of C3-C4 and heavier aldehydes was proposed. The high reactivity of the weakly bound aldehydic H-atom lresulted in the predominant formation of carbonyl radicals. While acetyl-radical addition to oxygen was a key step in the low temperature reactivity of acetaldehyde heavier carbonyl radicals were rapidly decomposed to CO and alkyl radicals. Comprehensive kinetic analysis of the low temperature oxidation data (including cool flames of acetaldehyde oxidation of propanal n-butanal and iso-butanal in jet-stirred reactor validates and supports the kinetic model as well as the assumption of the direct decomposition of heavy carbonyl radicals
Towards a lumped approach for solid plastic waste gasification: Polystyrene pyrolysis
No full textIn a circular economy perspective, solid plastic wastes (SPW) are a valuable source of chemicals, energy vectors and fuels. Chemical recycling through pyrolysis, gasification, and partial oxidation are promising alternatives to incineration and landfill disposal, allowing efficient exploitation of SPW. Modelling the combustion and gasification both at the particle and reactor scale requires first the definition of a suitable condensed phase pyrolysis mechanisms for each constituent. This work proposes a semi-detailed kinetic model for polystyrene (PS) pyrolysis. Following our recent work on polyethylene (PE) and polypropylene (PP), the model is based on the functional group approach firstly developed for polyvinylchloride (PVC) and biomass pyrolysis aiming at a consistent description of a wide palette of SPW components. The proposed approach retains an accurate description of low molecular weight species while representing high molecular weight species through their chemical functionalities only. The resulting liquid-phase model accounts for 34 species and 440 reactions and is attached to this paper together with the complementary gas-phase model. Model validation by a comprehensive comparison with experimental data proves its capability of predicting both mass loss and product distribution profiles with comparable accuracy to more expensive detailed models from the literature. The proposed model, together with other available subsets in the CRECK kinetic framework (i.e., biomass, PVC, PE, PP), offers a powerful tool for investigating key aspects in thermochemical recycling technologies thus supporting optimal reactor design. These aspects include mixture interactions (SPW and SPW/biomass mixtures), secondary cracking, catalytic, and gasification reactions
- …
