Effect of particle size on thermal decomposition and devolatilization kinetics of melon seed shell

Thermogravimetric analysis (TGA) and devolatilization kinetics of melon seed shell (MSS) at different particle sizes (150?µm and 500?µm) and at different heating rates (10, 15, 20, and 25?°C/min) were investigated with the aid of TGA. The results of the TGA analysis show that the TGA curves corresponding to the first and third stages for 150?µm particle sizes exhibited some bumps that developed at the first and third stages of pyrolysis. It was also observed that at constant heating rate, the maximum peak temperature increases as the particle sizes increase from 150 to 500?µm, whereas 500?µm particle sizes exhibited higher peak temperatures compared to 150?µm particle sizes. The resulting TGA data were applied to the Kissinger (K), Kissinger–Akahira–Sunose (KAS) and Flynn–Wall–Ozawa (FWO) methods and kinetic parameters (activation energy, E and frequency factor, A) were determined. The E and A obtained using K method were 74.27?kJ mol^?1 and 3.84?×?10⁵?min^?1 for 150?µm particle size, whereas for 500?µm particle size were 97.12?kJ mol^?1 and 3.74?×?10⁷?min^?1, respectively. However, the average E and A obtained using KAS and FWO methods were 82.35?kJ mol^?1, 1.29?×?10⁷?min^?1, and 88.50?kJ mol^?1, 1.32?×?10⁷?min^?1 for 150?µm particle sizes. While for 500?µm particle sizes, the E and A were 108.46?kJ mol^?1, 3.14?×?10⁹?min^?1, and 113.05?kJ mol^?1, 7.56?×?10⁹?min^?1, respectively. It was observed that E and A calculated from FWO and KAS methods were very close and higher than that obtained by K method. It was observed that the minimum heat required for the cracking of MSS particles into products is reached later at higher peak temperatures since the heat transfer is less effective as they are at lower peak temperatures.     

N-甲氧酰基-N'-氨基嘧啶硫脲的热分解动力学

文章合成了N-甲氧酰基-N'-氨基嘧啶硫脲,通过X-射线单晶结构分析法测定该化合物的分子结构和晶体结构.采用DSC,TG/DTG方法研究了化合物的热分解机理,并用5种积分法和1种微分法对其进行了非等温动力学研究.结果表明:化合物为三斜系,空间群为P-1.热分解机理为随机成核和随后生长机理,热分解反应的表观活化能为142...     

纳米粒子分散状况对复合材料非等温结晶动力学行为的影响

采用低剪切应力下分散混合与普通熔融混合方法制备出聚丙烯／无机纳米粒子复合材料。使用扫描电子显微镜（SEM）观察其脆断表面发现;采用低剪切应力下分散混合方法制备的试样中纳米粒子以纳米尺度均匀分散于树脂基体中,而普通熔融共混方法制备的试样中,纳米粒子多以十几个以上的粒子团聚体形式存在。采用示差扫描量热（DSC）研究了聚丙烯及其复合材料的非等温结晶动力学,通过对t1/2、Zc、n和△E等结晶动力学参数的分析,证明了在同等含量下,纳米粒子以纳米尺寸均匀分散的聚丙烯／纳米碳酸钙复合材料更容易结晶,且结晶速率更快。     

Marked particle size and support effect of Pd catalysts upon the direct decomposition of nitric oxide

The catalytic behavior of beryllia-supported Pd catalyst for the direct decomposition of NO was compared with that of silica supported one. The TOF of NO decomposition was one order of magnitude larger in the case of Pd/BeO. Over Pd/SiO₂, the TOF was increased with the increase of the Pd particle size. On the contrary, over Pd/BeO smaller Pd particles exhibited higher TOF for NO decomposition suggesting some strong electronic or structural interaction between Pd and beryllia. TPD spectra of NO(a) over reduced catalysts indicated that NO was adsorbed on Pd/SiO₂ more strongly than on Pd/BeO, and dissociation of NO(a) was easier on the former catalyst. FT-IR spectra of adsorbed NO at room temperature followed by the evacuation at elevated temperatures confirmed this. TPD spectra of O₂ desorbed from oxidized surface indicated that adsorption strength of O(a) is one of the most important factors to determine the catalytic activity of NO decomposition over supported Pd catalysts.     

Metaettringite, a decomposition product of ettringite

Dehydroxylation of ettringite in an atmosphere of constant partial water vapour pressure (30-400 Torr) and controlled temperature (55-95 °C) yields an X-ray amorphous product containing 11-13 H₂O per ettringite formula unit. The X-ray amorphous product does, however, give electron diffraction patterns similar to those of ettringite but with a considerably reduced from ∼1.123 nm (ettringite) to 0.85 nm in the partially dehydroxylated product, termed metaettringite. The structure of metaettringite is closely related to that of fleischerite, Pb₃Ge[(OH)₆](SO₄)₂·3H₂O and the isostructural despujolsite, Ca₃Mn^IV(SO₄)₂(OH)₆·3H₂O. These minerals contain columnar units. The columnar structure is like that of ettringite but with closer packing of columns in the a direction, resembling metaettringite.The mechanism of collapse of ettringite to metaettringite, involving loss of water and motion of columns, cannot be achieved without scissoring and possibly rotation of individual columns; this and other defect-producing mechanisms result in loss of crystallinity of the metaettringite product.     

Determination of the intrinsic rate constant and activation energy of CO2 gas hydrate decomposition using in-situ particle size analysis

Experimental data on the rate of decomposition of CO₂ gas hydrates has been obtained using a semi-batch stirred tank reactor, with an in-situ particle size analyser, at temperatures ranging from 274 to and pressures ranging from 1.4 to . A method for calculating the moments of the particle size distribution has been presented. The experimental data was analysed using the kinetic model of Clarke and Bishnoi (Determination of the intrinsic kinetics of gas hydrate decomposition kinetics using particle size analysis, Presented at the Third International Conference on Gas Hydrates, Salt Lake City, Utah, July 18-22, 1999; Chem. Eng. Sci. 55 (2000) 4869) in its differential form in order to account for the slight change in temperature during the decomposition of CO₂ hydrates. The applicability of the new instrument for measuring gas hydrate decomposition kinetics was examined by conducting experiments with ethane at conditions similar to those encountered by Clarke and Bishnoi (2000). It was seen that the previously obtained rate constants for ethane hydrate decomposition were able to predict the new obtained data. A new procedure for regressing the intrinsic rate constant and activation energy has also been described and it is seen that the activation energy is and the intrinsic rate constant is for CO₂ gas hydrate decomposition.     

The effect of particle size on nitric oxide decomposition and reaction with carbon monoxide on palladium catalysts

The effect of palladium particle size on its catalytic activity was investigated by the decomposition of chemisorbed nitric oxide and the reaction of nitric oxide with carbon monoxide in flow conditions. Palladium particles (30–500 Å) were prepared on silica thin films (100 Å) which were supported on a Mo(110) surface. The reactivity of the supported palladium varied with the metal particle size. On large palladium particles, nitric oxide (NO) reacts to form nitrous oxide (N₂O), dinitrogen (N₂) and atomic oxygen during temperature-programmed reaction, whereas on small particles (< 50 Å), nitrous oxide is not formed. Similarly, reactions of NO with CO on large particles, in flow conditions produce N₂O, N₂ and CO₂, whereas N₂O is not produced on small particles. In addition, more extensive NO decomposition is observed on the smaller particles.     

Critical initiation-energy density as a function of single-crystal size in explosive decomposition of silver azide

The dependence of the critical initiation-energy density of explosive decomposition of silver azide on the sample size was studied experimentally for the first time. The dependence of the critical energy density was found to be determined by the diffusion of the reactants to the crystal surface, where their recombination rate far exceeds the bulk rate. The diffusion coefficient and the rate constant of bulk electron-hole recombination obtained for the explosive decomposition agree within the experimental error with those determined earlier in photoconductivity studies. __________ Translated from Fizika Goreniya i Vzryva, Vol. 44, No. 2, pp. 76–78, March–April, 2008.     

Colour control in fly ash as a combined function of particle size and chemical composition

This paper presents the results of an investigation on the combined effect of particle size and chemical composition on the colour of fly ash - a property that determines whether fly ash polymer composites can be engineered to have very light appearance satisfying the need for a wide range of commodity applications, particularly in the building material and computer housing industry. Four fly ash samples were collected from Tarong power plant Queensland, Australia, namely fly ash from first hopper (T59), classified fly ash from first hopper (T60), grinded and then classified fly ash from first hopper (T63) and fly ash from fourth hopper (T64). It was found that the particle size of T64 is smaller but still in the same order as T63. Colour measurement and chemical composition analysis of the different FA samples showed that there is a correlation between the particle size, chemical composition and colour of the fly ash. This information could be effectively used in fly ash recycling industry.     

Modelling of decomposition of a synthetic core of methane gas hydrate by coupling intrinsic kinetics with heat transfer rates

Decomposition of a synthetic core of methane hydrate has been modelled by Selim and Sloan (1985) and Ullerich et al. (1987) based on heat transfer considerations. In the present work, the decomposition is modelled by coupling intrinsic kinetics with heat transfer rates. Numerical simulation results are presented to show the effects of incorporating the intrinsic kinetic rate of decomposition. Simulation results indicate that by changing the system pressure we can move from a heat transfer controlled regime to a regime where both heat transfer and intrinsic kinetics have a significant effect on the global rate of decomposition.     

Particle growth kinetics of calcium fluoride in a fluidized bed reactor

Crystallization process in a fluidized bed reactor to remove fluoride from industrial wastewaters has been studied as a suitable alternative to the chemical precipitation in order to decrease the sludge formation as well as to recover fluoride as synthetic calcium fluoride.In the modeling, design and control of a fluidized bed reactor for water treatment it is necessary to study the particle growth kinetics. Removal of fluoride by crystallization process in a fluidized bed reactor using granular calcite as seed material has been carried out in a laboratory-scale fluidized bed reactor in order to study the particle growth kinetics for modeling, design, control and operation purposes.The main variables have been studied, including superficial velocity (SV, ), particle size of the seed material (L₀, m) and supersaturation (S). It has been developed a growth model based on the aggregation and molecular growth mechanisms. The kinetic model and parameters given by the equation fits well the experimental data for the studied range of variables.     

The kinetics of ettringite formation and dilatation in a blended cement with β-hemihydrate and anhydrite as calcium sulfate

The phase formation, heat of hydration and dilatation in a blended cement consisting of 50 wt.% calcium aluminate cement, 25 wt.% Portland cement and 25 wt.% calcium sulfate were studied (w/c=1). The calcium sulfate was β-hemihydrate, anhydrite and mixes of the two. Kinetic expressions describing the ettringite formation in the pastes with the pure calcium sulfates were found. Hydration reactions were suggested and the phase development was compared to the hydration heat by mass and heat balances. When the calcium sulfate was 75 and 50 wt.% β-hemihydrate, the systems behaved as a linear combination of the 100 and 0 wt.% blends. At 25 wt.%, the hydration kinetics differed from the other blends. With only β-hemihydrate, the last 50% of ettringite formation was accompanied by expansion, mainly caused by interaction of crystals growing radially on cement grains. In the paste with only anhydrite, ettringite crystals grew in solution and produced no expansion.     

Electrolytic decomposition of ammonia to nitrogen in a multi-cell-stacked electrolyzer with a self-pH-adjustment function

This paper describes a study of continuous decomposition characteristics of ammonia to nitrogen in a multi-cell-stacked electrolyzer with an anion exchange membrane. The pH change of ammonia solution in both the anodic and cathodic chambers of a divided cell due to the water splitting reactions was studied together with the electrolytic decomposition of ammonia. The electrolytic decomposition efficiency of ammonia was considerably affected by the pH change of ammonia solution caused by the water splitting reactions. In the anodic chamber with the ammonia solution, the water splitting reaction which produced protons occurred at a pH of less than 8, and in the cathodic chamber, that producing hydroxyl ions occurred at a pH of more than 11. By using the characteristics of the electrolytic water splitting reactions in a divided cell, a continuous electrolyzer with a self-pH-adjustment function was devised, wherein a portion of the ammonia solution from a pH-adjustment reservoir was circulated through the cathodic chambers of the electrolyzer. This enhanced the pH of the ammonia solution being fed from a pH-adjustment reservoir into the anodic chambers of the electrolyzer, which caused a higher ammonia decomposition yield. Based on this electrolyzer, a salt-free ammonia decomposition process was suggested. In this process, ammonia in the solution was continuously and effectively decomposed into environmentally harmless nitrogen gas.     

多马来酰亚胺改性聚苯醚树脂的热分解动力学研究

采用热重分析法对自制的3,3′-二乙基-4,4′-二苯甲烷型多马来酰亚胺(PEDM)改性烯丙基化聚苯醚(PPO)固化树脂的热分解动力学进行了研究。结果表明:在热作用下,共聚交联体系在PPO的苯氧醚键处首先断链,并按无规断链或自由基引发断链方式迅速进行,交联网络被破坏成许多线形分子链,但其他键受影响很小,热分解表现为一级反应,且失重速率很快;随温度升高,当体系的苯氧醚键基本上断裂完毕后,线形分子链上的马来酰亚胺环的羰基或其他能量相近的键也开始逐渐断裂,此时热分解也表现为一级反应,但速率比第1步慢得多。     

Micro-kinetic modeling of NH3 decomposition on Ni and its application to solid oxide fuel cells

This paper presents a detailed surface reaction mechanism for the decomposition of NH₃ to H₂ and N₂ on a Ni surface. The mechanism is validated for temperatures ranging from 700 to 1500 K and pressures from 5.3 Pa to 100 kPa. The activation energies for various elementary steps are calculated using the unity bond index-quadratic exponential potential (UBI-QEP) method. Sensitivity analysis is carried out to study the influence of various kinetic parameters on reaction rates. The NH₃ decomposition mechanism is used to simulate SOFC button cell operating on NH₃ fuel.     

Effect of micropores diffusion on kinetics of CH4 decomposition over a wood-derived carbon catalyst

In order to optimise hydrogen production from biomass gasification, catalytic conversion of methane contained in a surrogate biomass syngas (CH₄ 14%; CO 19%; CO₂ 14%; H₂ 16%; H₂O 30%; N₂ 7%) is investigated over a fixed bed of porous wood char as a function of temperature (800–1000 °C) and space time (1.6–6.2 min g L⁻¹). Determination of Thiele modulus evidences a change of kinetic regime from chemically- to diffusion-controlled when the temperature increases; this finding is particularly relevant when porous chars having an average pore width of 1 nm are used as catalysts. Mass diffusion transfers are accounted for by a model introducing an internal effectiveness factor. Knudsen diffusion in micropores is shown to limit the conversion rate of methane per unit mass of catalyst, and explains why such a rate is not proportional to the BET surface area, especially when the latter is higher than typically 300 m²/g. It is concluded that diffusion limitations in micropores should be taken into account, otherwise underestimated activation energy and intrinsic kinetic constant are obtained in some experimental conditions.     

On the Solution of Non-Isothermal Reaction-Diffusion Model Equations in a Spherical Catalyst by the Modified Adomian Method

We investigate the reactant diffusion within a porous catalytic pellet including the heat of reaction. The Lane–Emden boundary value problem with an Arrhenius reaction rate is used to model the reactant concentration. We combine the Volterra integral form with the Adomian decomposition method to solve the equivalent Fredholm–Volterra integral equation. We then estimate the reactant concentration at the center of the catalytic pellet and the iterated Shanks transform is used to improve the accuracy of the approximations. The objective error analysis formulas are used to demonstrate a high accuracy and rapid rate of convergence, which does not depend on a priori knowledge of the exact solution or comparison with an alternate approximation method. Thus low-stage approximations by the Adomian decomposition method are validated for parametric simulations of the reactant concentration profiles and the effectiveness factor profiles. Our approach demonstrates enhancements over previous investigations, and is readily extensible to more general diffusion reaction models in catalytic reactor engineering.     

Oxidized carbon as a support of copper oxide catalysts for methanol decomposition to hydrogen and carbon monoxide

Copper oxide supported on oxidized activated carbon is investigated as a catalyst for methanol decomposition to H₂ and CO. The influence of the medium of the precursor deposition on the state of the active phase is observed. The role of the chemical nature of the support in the formation of catalytic active complex is discussed.     

Growth kinetics and mechanism of wet granulation in a laboratory-scale high shear mixer: Effect of initial polydispersity of particle size

The effect of initial polydispersity of particle size (unimodal versus bimodal distribution) and binder characteristics on the growth kinetics and mechanism of wet granulation was studied. Wet granulation of pharmaceutical powders with initial bimodal particle size distribution (PSD) presented growth kinetics consisting of two stages: fast growth followed by slow growth. The fast stage is controlled by the amount of binder and high probability of coalescence due to the collisions of small and large particles. The second stage is characterized by slow agglomeration of powder mixtures with water content 13.6% v/w, and slow breakage of powder mixtures with water content of 9.9% and 11.7% v/w. The wet granulation of powders with initial unimodal PSD exhibited slow growth kinetics consisting of one stage, since similar particle sizes do not promote agglomeration. The experimental results were better described by a population balance equation using a coalescence kernel that favors growth rate by collision between small and large particles. In general, the probability of a successful collision increased with higher size difference between particles, smaller particle size, and higher binder content.