R. Vinu

Unraveling Reaction Pathways and Specifying Reaction Kinetics for Complex Systems

Many natural and industrial processes involve a complex set of competing reactions that include several different species. Detailed kinetic modeling of such systems can shed light on the important pathways involved in various transformations and therefore can be used to optimize the process conditions for the desired product composition and properties. This review focuses on elucidating the various components involved in modeling the kinetics of pyrolysis and oxidation of polymers. The elementary free radical steps that constitute the chain reaction mechanism of gas-phase/nonpolar liquid-phase processes are outlined. Specification of the rate coefficients of the various reaction families, which is central to the theme of kinetics, is described. Construction of the reaction network on the basis of the types of end groups and reactive moieties in a polymer chain is discussed. Modeling frameworks based on the method of moments and kinetic Monte Carlo are evaluated using illustrations. Finally, the prospects and challenges in modeling biomass conversion are addressed.

Peroxide-assisted microwave activation of pyrolysis char for adsorption of dyes from wastewater.

In this study, mesoporous activated biochar with high surface area and controlled pore size was prepared from char obtained as a by-product of pyrolysis of Prosopis juliflora biomass. The activation was carried out by a simple process that involved H2O2 treatment followed by microwave pyrolysis. H2O2 impregnation time and microwave power were optimized to obtain biochar with high specific surface area and high adsorption capacity for commercial dyes such as Remazol Brilliant Blue and Methylene Blue. Adsorption parameters such as initial pH of the dye solution and adsorbent dosage were also optimized. Pore size distribution, surface morphology and elemental composition of activated biochar were thoroughly characterized. H2O2 impregnation time of 24h and microwave power of 600W produced nanostructured biochar with narrow and deep pores of 357m(2)g(-1) specific surface area. Langmuir and Langmuir-Freundlich isotherms described the adsorption equilibrium, while pseudo second order model described the kinetics of adsorption.

Fast co-pyrolysis of cellulose and polypropylene using Py-GC/MS and Py-FT-IR

In this study, the production of high quality biofuel intermediates via fast co-pyrolysis of cellulose and polypropylene (PP) is investigated. Fast co-pyrolysis experiments were performed in a Pyroprobe® reactor and the generated vapors were analyzed using a gas chromatograph-mass spectrometer for the composition of pyrolysates, and Fourier transform infrared spectrometer for the time evolution of the key functional groups. The effects of cellulose : PP mass ratio (100:0, 75:25, 50:50, 25:75, 0:100) and temperature (500–800 °C) on bio-oil composition, carbon number distribution of the products, higher heating value of the products, and temporal evolution of O–H, C–O, –CH2–, CO2 and CO groups were evaluated. Formation of long chain alcohols in the carbon number range of C8–C20 was observed as a result of the interaction of cellulose and PP. Feed composition played a decisive role in the formation of alcohols and hydrocarbons. A maximum of ca. 36% alcohols and 45% hydrocarbons were obtained from PP-rich mixture at 600 °C. The yield of char decreased and that of the aromatic hydrocarbons increased with pyrolysis temperature. Significant improvement in the heating value of the products was observed when PP was blended with cellulose. Importantly, the calculated heating values correlated well with the cumulative content of alcohols, aliphatic and aromatic hydrocarbons. The addition of PP to cellulose significantly decreased the time taken for completion of pyrolysis. Based on the product distribution, hydroxyl, hydrogen and methyl abstraction were found to be the dominant reactions involved in the transformations.

Fast pyrolysis kinetics of alkali lignin: Evaluation of apparent rate parameters and product time evolution

In this study, the apparent kinetics of fast pyrolysis of alkali lignin was evaluated by obtaining isothermal mass loss data in the timescale of 2-30s at 400-700°C in an analytical pyrolyzer. The data were analyzed using different reaction models to determine the rate constants and apparent rate parameters. First order and one dimensional diffusion models resulted in good fits with experimental data with apparent activation energy of 23kJmol-1. Kinetic compensation effect was established using a large number of kinetic parameters reported in the literature for pyrolysis of different lignins. The time evolution of the major functional groups in the pyrolysate was analyzed using in situ Fourier transform infrared spectroscopy. Maximum production of the volatiles occurred around 10-12s. A clear transformation of guaiacols to phenol, catechol and their derivatives, and aromatic hydrocarbons was observed with increasing temperature. The plausible reaction steps involved in various transformations are discussed.

Understanding the interactions between cellulose and polypropylene during fast co-pyrolysis via experiments and DFT calculations

Co-pyrolysis of lignocellulosic biomass with waste plastics is a promising option to produce high quality liquid fuels. During co-pyrolysis the molecular level interactions between the intermediates produced from biomass with polymers play a crucial role in altering the distribution of various organics in bio-oil. Recently, it was shown that interactions between cellulose and polypropylene during fast co-pyrolysis leads to the formation of C8-C20 long chain alcohols in bio-oil. The formation of alcohols was proposed to occur via reaction of hydroxyl radicals from cellulose pyrolysis with polypropylene radicals. This study is an attempt to unravel the formation mechanism of long chain alcohols during co-pyrolysis using quantum chemical calculations. The reactions of propylene trimer (2,4,6-trimethyl heptane) and its primary, secondary and tertiary radicals with hydroxyl radical (?OH) and water molecule are investigated at B3LYP/6-31G(d,p) level of theory using Gaussian 09. The reaction of ?OH with propylene trimer readily leads to the formation propylene trimer radicals with the liberation of water. The Arrhenius activation energy of this reaction is in the range of 11-14 kcal/mol. It is shown that the reaction of propylene trimer radical (primary, secondary or tertiary) with a water molecule readily leads to the formation of respective alcohols with Arrhenius activation energy of 10-14 kcal/mol. This reaction competes with the barrierless recombination of polypropylene radical with ?OH. However, the presence of ?OH is limited by the high reaction barrier for its abstraction from cellulose. Therefore, the reaction of polypropylene radicals with water molecules formed via cellulose dehydration is shown to be a plausible pathway for the formation of long chain alcohols duirng fast co-pyrolysis of cellulose and polypropylene. The mass loss profiles during fast co-pyrolysis for different cellulose:polypropylene compositions were obtained in a Pyroprobe® reactor. The first order rate constants of decomposition wereevaluated, and they follow the order: 0.044 s-1 (cellulose:polypropylene 100:0) > 0.041 s-1 (75:25) ˜ 0.042 s-1 (50:50) > 0.032 s-1 (25:75) > 0.028 s-1 (0:100).