Showing papers by "Ravikrishnan Vinu published in 2017"

Apparent Kinetics of Fast Pyrolysis of Four Different Microalgae and Product Analyses Using Pyrolysis-FTIR and Pyrolysis-GC/MS

Abstract: This study reports an experimental technique to obtain the isothermal mass loss profiles of four different microalgae (Chlorella vulgaris, Nannochlorpsis oculata, Schizochytrium limacinum, and Arthrospira platensis) under fast pyrolysis conditions in the temperature range of 400–700 °C at short residence times of 2–60 s using an analytical pyrolyzer. The microalgae investigated in this study vary significantly, based on elemental and biochemical composition. The apparent kinetic parameters of fast pyrolysis of the algae were evaluated using first-order and multidimensional diffusion models. The apparent activation energy and pre-exponential factor evaluated under fast heating rate conditions were lower compared to those reported under slow pyrolysis conditions, which is attributed to the diffusion effects. The rate parameters were validated by constructing a kinetic compensation plot, which showed that ln(A) = 0.19Ea + 0.43. The product time evolution at short time scales was studied using in situ Fourier...

Understanding the influence of water droplet initiated discharges on damage caused to corona-aged silicone rubber

Abstract: Corona activity on the surface of silicone rubber has high impact on variation in hydrophobicity of the material, and on corona inception voltage (CIV) of water droplet sitting on the corona degraded surface. The CIV is high under negative DC as compared to positive DC/AC voltages. Increase in conductivity of water droplet reduces CIV. Surface charge accumulation studies indicate that the charge retention capability of the insulating material gets reduced drastically with silicone rubber damaged due to corona causing degradation of surface. Irrespective of the number of cycles of corona discharge activities carried out, the time constant of charge decay, after the removal of charge injection, remains the same. The water droplet initiated discharges under AC and DC voltages radiate UHF signal in the bandwidth 0.3–1.2 GHz. With single droplet sitting on top of the corona damaged silicone rubber, the amount of charge retained by the water droplet is low when the droplet is placed and charged immediately after corona discharge activity to surface of the insulating material. When the water droplet is placed after a certain time period, the charge accumulation characteristics of silicone rubber are regained. Leakage current measured during water droplet discharge studies through recurrent plot (RP) analysis reveals that, under AC voltage, it is possible to classify corona inception, arcing and bridging of water droplet between high voltage and ground electrode. The RP obtained for the current signals generated due to water droplet discharges under positive and negative DC voltages are different. The amount of damage caused to silicone rubber due to water droplet is high under negative DC voltage as compared to positive/AC voltages. While correlating the level of damage to the insulating material due to water droplet and the RQA parameters of RP, it is observed that higher the recurrence rate along with high determinism can cause severe damage to the insulating material. Structural analysis using ATR-FTIR and pyrolysis-GC/MS studies reveal significant functional group variation on surface of silicone rubber, especially when the number of cycles of corona ageing is more.

Understanding lignin depolymerization to phenols via microwave-assisted solvolysis process

Abstract: In this study, microwave assisted degradation of lignin, a paper industry and biorefinery waste product, is investigated in presence of different organic solvents such as ethylene glycol (EG), dimethyl sulfoxide (DMSO) and dimethyl formamide (DMF) at different temperatures (100–140 °C) and treatment periods (20–80 min). Experiments performed in batch microwave reactor revealed that, in all the solvents, a major reduction in weight average molecular weight (M w ) of lignin was observed in the time period of 20–40 min. In a polar protic solvent with high loss tangent (EG), maximum reduction in M w was observed at 120 °C, while in polar aprotic solvents (DMSO and DMF), M w decreased with increase in temperature. Significant yield of phenolics (20 wt.%) containing acetosyringone, guaiacol, syringaldehyde, anisole and lignin dimers (m/z 306, 322 Da) was obtained at 100 °C in DMF and DMSO, while at 140 °C, 11 wt.% phenolics were obtained in EG. Plausible reactions for the formation of phenolic monomers and dimers include etheric C β O and C α C β bond cleavages induced by solvent interactions. The shift of molecular weight distribution to higher molecular weights suggested the involvement of recombination reactions at long treatment periods. This study shows that solvent-assisted microwave processing is a promising technique to convert lignin to phenols.

Understanding Corona discharge activity in titania nanoparticles dispersed in transformer oil under AC and DC voltages

Abstract: Titania (TiO2) nanoparticles dispersed transformer oil has high Corona Inception Voltage (CIV) and breakdown strength, and gets further enhanced on insertion of the barrier in the electrode gap. The volume of nanoparticles and the amount of surfactant have a strong influence on the dispersion of nanoparticles in transformer oil and were characterized through viscosity measurement, particle size analysis and by zeta potential measurements. It was also observed that with needle plane configuration, negative DC voltages have high breakdown voltage as compared to positive DC and AC voltages. Improvement in breakdown voltage with barrier can be obtained by placing the barrier at 20–80% of electrode gap from the tip of the high-voltage electrode. The breakdown voltage of needle plane electrode configuration was calculated based on normal distribution parameters. It was observed that the corona activity generates Ultra High Frequency (UHF) signal in the frequency range of 0.7–2 GHZ with its dominant frequency at 0.9 GHz and was characterized using a ternary plot. The discharge activity was studied by using a spectrum analyzer by operating it in zero span mode with 0.9 GHz as center frequency. It was observed that for a given voltage under AC and DC voltages, the magnitude of discharges is less with nanoparticles dispersed transformer oil. A pyrolysis study indicated no variation in the composition of the oil with the addition of titania. It was observed that the esters present in the oils were alkyl substituted oxalic acid and alkyl substituted sulfurous acid esters.

Thermal aging of cellulosic pressboard material and its surface discharge and chemical characterization

Abstract: Cellulosic pressboard is a key insulation material in oil filled transformers. Surface discharge or partial discharge degrades the insulation material and reduces the life time of the transformer. This study is aimed at understanding the effect of thermal aging of pressboard on the variation of its surface discharge and chemical characteristics. Thermal degradation of pressboard was carried out at 180 °C for 500 h in presence of ester oil under simulated aging conditions. The variation of salient electrical characteristics like surface discharge inception voltage (SDIV), charge measurement and optical emission spectra of the discharges were assessed at different aging periods. A significant reduction in SDIV was observed due to thermal aging. The charge measurement indicated that total charge deposition on pressboard increased with aging duration. Optical emission spectroscopy yielded valuable information regarding the elements evolved during discharge activity. Chemical characterization of the virgin and aged pressboard samples was carried out using analytical pyrolysis coupled with gas chromatograph-mass spectrometric technique (Py-GC/MS). The production of anhydrosugars, furan derivatives and oxygenates from the pressboard increased with aging time, while the esters in the oil were degraded to carboxylic acids with aging. Owing to the weakening of the hydrogen bonding network in oil impregnated pressboard with thermal ageing, high yield of end-chain depropagation products like anhydrosugars was observed during pyrolysis. The weak sites in cellulosic pressboard are expected to act as charge deposition sites.

Influence of ambient medium on thermal ageing of pressboard in transformer oil containing dibenzyl bisulphide (DBDS)

Abstract: Impact of thermal ageing of pressboard material in transformer oil containing dibenzyl disulphide (DBDS) under different ambience, such as air, N2, He and vacuum has been studied to understand the level of copper sulphide diffusion. The characteristic variations in electrical and thermal properties of copper sulphide diffused oil impregnated pressboard (OIP) material have been measured under different ageing conditions. Surface charge accumulation studies with AC and ±DC voltages showed that surface charge decay time was longer in air aged OIP material. Surface discharge studies by adopting ultra-high frequency (UHF) sensing indicated a reduction in the SDIV with the level of ageing although the characteristic variation in flashover voltage (FOV) under lightning impulse (LI) voltage is almost the same. Also the SDIV reduced with increase in frequency of supply voltage. Thermo-gravimetric analysis (TGA) indicated that maximum weight loss temperature (Tmax) of the cellulose constituent of OIP material decreased with ageing. Laser induced breakdown spectroscopy (LIBS) was adopted to quantify the diffused copper sulphide contaminant in OIP material and the results were compared with the inductively coupled plasma (ICP) spectroscopy based analysis. Based on these studies, deteriorating effect of ageing medium on several properties of OIP material was understood.

Chapter 3 – Gas–Solid Reactions and Reactors

Abstract: Gas–solid reactions involving porous catalysts are ubiquitous in chemical and petroleum industries, and it is vital to understand the kinetics of heterogeneous catalytic reactions to develop robust reactor models. This chapter begins by providing an insight on effective diffusivity, which accounts for various mechanisms by which the gas-phase species get transported to the active sites inside a porous catalyst. The influence of pore diffusion on reaction rate is described by considering an isothermal first-order reaction occurring inside the pores of catalyst particles of different geometries. The relative importance of reaction and pore diffusion resistance is described using dimensionless Thiele modulus, and the actual rate of reaction is described using effectiveness factor. After elucidating this treatment for reactions occurring under nonisothermal conditions, various examples involving series and parallel reactions are discussed in terms of selectivity of the catalyst toward a specific product. The external mass transfer of the reactant from the fluid stream to the catalyst pore is then integrated with the rate expression to determine the overall rate. The elementary steps, viz., adsorption, surface reaction, and desorption, involved in a catalytic reaction are discussed, and the approach to derive the overall rate expression at steady state in presence of various rate-determining steps is explained. The effect of catalyst deactivation on the reaction rate is also described. Modeling of ideal and nonideal catalytic packed bed reactors operating under both isothermal and nonisothermal conditions is elucidated using examples. Finally, an approach to model the conversion of solid particles in presence of a reactive gas stream involved in noncatalytic reactions is elucidated using the shrinking unreacted core model. Expressions for solid conversion under various controlling resistances are derived, and its extension to shrinking particles and film growth is outlined.

Gas–Liquid and Gas–Liquid–Solid Reactors

Abstract: Gas–liquid and gas–liquid–solid reactions are common in various chemical engineering operations, including separation of a gas constituent from a mixture, hydrodesulphurization, and catalytic liquid-phase oxidation and hydrogenation This chapter begins with the development of a rate equation for chemical reaction with mass transfer involved in gas–liquid reactive absorption Hatta number and enhancement factor are utilized to identify the reaction regime, viz, liquid film or bulk liquid, and whether the reaction is fast or slow The various parameters required for the design of a gas–liquid reactor are discussed The salient design aspects of packed column, agitated tank, and bubble column reactors are described along with practical examples Different types of lab-scale reactors used to understand the mechanism and kinetics of gas–liquid reactions are described The second part of the chapter addresses the kinetics and reactor design aspects of three-phase gas–liquid–solid reactions Specifically, suspended bed and fixed bed reactors are described along with examples of industrial hydrogenation and hydrodesulphurization reactions