Showing papers by "Ravikrishnan Vinu published in 2016"

Mechanistic Understanding of Thermochemical Conversion of Polymers and Lignocellulosic Biomass

Abstract: Pyrolysis is a promising thermochemical technique to convert polymers such as waste plastics and lignocellulosic biomass to liquid products that are valuable either directly as or are potentially upgraded to liquid fuels and fine chemical intermediates. Mechanistically, polymer pyrolysis involves a complex set of free radical, concerted, and/or ionic reactions that occur via numerous competing pathways. Engineering these pathways to produce the required molecules warrant a thorough understanding of kinetics of the reactions under different conditions. In this critical review, after emphasizing the need for resource and energy recovery from polymers, the elementary reactions involved in the pyrolysis of polyolefins to various products are discussed along with an elucidation of detailed kinetic modeling. The reactions involved in oxidative pyrolysis of polymers are also discussed with polystyrene autoxidation as an example case. The influence of catalysts such as zeolites in altering the product distribution from pyrolysis of synthetic polymers is discussed. As lignocellulosic biomass is more complex in structure compared to synthetic polymers, the challenges and methodology involved in modeling the degradation of its basic constituents, viz. cellulose, hemicellulose, and lignin, to various organics are discussed. After elucidating the influence of various concerted reactions involved in cellulose pyrolysis on product yields, the effect of catalysts on biomass fast pyrolysis and bio-oil upgradation are discussed. The review concludes with a note on advantages of copyrolysis of synthetic polymers and biomass to enhance the quality of bio-oil that can be easily converted to biofuel with minimal upgradation.

Effect of alkaline ultrasonic pretreatment on crystalline morphology and enzymatic hydrolysis of cellulose

Abstract: In this study, ultrasound-assisted alkaline pretreatment is developed to evaluate the morphological and structural changes that occur during pretreatment of cellulose, and its effect on glucose production via enzymatic hydrolysis. The pretreated samples were characterized using scanning electron microscopy, infrared spectroscopy, and X-ray diffraction to understand the change in surface morphology, crystallinity and the fraction of cellulose Iβ and cellulose II. The combined pretreatment led to a great disruption of cellulose particles along with the formation of large pores and partial fibrillation. The effects of ultrasound irradiation time (2, 4 h), NaOH concentration (1–10 wt%), initial particle size (20–180 μm) and initial degree of polymerization (DP) of cellulose on structural changes and glucose yields were evaluated. The alkaline ultrasonic pretreatment resulted in a significant decrease in particle size of cellulose, besides significantly reducing the treatment time and NaOH concentration required to achieve a low crystallinity of cellulose. More than 2.5 times improvement in glucose yield was observed with 10 wt% NaOH and 4 h of sonication, compared to untreated samples. The glucose yields increased with increase in initial particle size of cellulose, while DP had no effect on glucose yields. The glucose yields exhibited an increasing tendency with increase in cellulose II fraction as a result of combined pretreatment.

Production of phenolics via photocatalysis of ball milled lignin–TiO2 mixtures in aqueous suspension

Abstract: In this study, photocatalytic conversion of lignin to valuable phenolics and aromatic hydrocarbons is demonstrated by subjecting ball milled mixtures of lignin and TiO2 to ultraviolet (UV) radiation. Unlike a majority of the existing studies that reported photocatalytic degradation of lignin that is solubilized in alkaline medium, this study evaluates the decomposition of lignin under natural conditions in aqueous medium. In order to facilitate better contact between lignin and nano-TiO2, the two materials were ball milled in the presence of different media, viz. without solvent, hexane, acetone and water. The ball milled mixtures were characterized using powder XRD, FT-IR and photoluminescence spectroscopy, and scanning electron microscopy. Intimate contact between lignin and TiO2 was achieved using water and acetone as the solvents in wet milling. Photocatalysis experiments were conducted in a batch photoreactor. The aqueous phase products were analyzed using UV-visible spectroscopy, MALDI-TOF and GC mass spectrometry, while the molecular weight of solid lignin was analyzed using GPC. Ball milling resulted in the formation of phenolic compounds even during dark mixing of the mixtures prior to photocatalysis. Ball milled mixtures obtained using acetone and water resulted in a high yield of phenolic compounds after 3–4 hours of UV exposure. At long UV exposure periods, the phenolics production got saturated, possibly due to the deactivation of TiO2 active sites by the intermediates. The main organic compounds produced during photocatalysis include ethyl benzene, acetovanillone, syringaldehyde, acetosyringone, vanillin, 2,6-dimethoxy benzoquinone and diisobutyl phthalate. Free radical depolymerization reactions of lignin mediated by active hydroxyl and superoxide radicals are responsible for the observed products.

Understanding the Interactions Between Cellulose and Polypropylene During Fast Co-pyrolysis via Experiments and Dft Calculations

Abstract: 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).

Understanding electrical treeing activity in electron beam irradiated XLPE cable insulation

Abstract: Tree-like-trees are observed with electron beam irradiated XLPE cable insulation under AC voltages. Charge accumulation studies indicate that the level of charge deposited by +/− DC corona charging is almost same. However, the amount of accumulated charge increases with increase in level of irradiation of electron beam. The decay time constants are marginally high with positive charge deposition. Moreover, the decay time constant of accumulated charges increases with increase in level of irradiation of the material. The rise time of injected current signal formed during tree growth in positive and negative half cycle is about 900 and 700 ps, respectively. The UHF signal generated during tree growth in virgin and with irradiated specimen has its frequency content in the range 0.5–1.5 GHz. Weibull distribution studies indicate that the life time of electron beam irradiated specimen drastically gets reduced due to treeing. Sequential measurement of UHF signal indicates that tree growth involves high and low intensity discharges. Phase resolved partial discharge studies using spectrum analyzer indicates that discharges occur at the raising portion or near zero crossing of the applied AC voltage. FTIR spectroscopy and analytical pyrolysis-GC/MS analyses of the electron beam irradiated samples reveal the formation of alkoxy and alkyl peroxy radicals, and carbonyl and hydroxyl groups via scission of the crosslinks. The weak crosslinking sites are proposed to act as nodal points from where tree growth occurs.