Contracting Volume Model Research Articles

菱面体状無水硫酸アルミニウム結晶粒子粉末の熱分解速度

The anhydrous aluminum sulfate powder composed of rhombohedral single-crystal grains was prepared by precipitation from a concentrated sulfuric acid solution. The average grain size and the size distribution was 3.8±0.7μm. Thermal decomposition behavior of the anhydrous aluminum sulfate was investigated by thermogravimetric measurements. The kinetics of the thermal decomposition fitted to a contracting volume model with an apparent activation energy of 360 kJ/mol. The mobility of the reaction boundary of the thermal decomposition at 760°C was estimated to be about 7.0×10-5μm/s. The temperature dependence of the mobility was expressed by an Arrhenius type equation.

Kinetics of Thermal Dissociation of Ammonium Metavanadate

The solid products of thermal decomposition of ammonium metavanadate can be used as catalysts in many important processes, and a knowledge of the dynamics of these processes is therefore essential. The thermal dissociation of ammonium metavanadate was studied under non-isothermal conditions in air atmosphere. This process occurred in three steps under the applied experimental conditions, and was associated with the elimination of ammonia and water below 330°C and with the formation of nitrogen oxides above 330°C. The kinetics of particular stages of (NH4)2OV205 decomposition was evaluated from the dynamic mass loss data by means of the integral method, with applycation of the Coats and Redfern approximation. The first stage of decomposition to ammonium hexavanadate is governed by a random nucleation model, the second step by a three-dimensional diffusion or contracting volume model, and the last stage again by a random nucleation model. The apparent activation energies found for the particular stages were 144.97, 378.31 or 184.40 and 260.65 kJ mol−1, respectively.

The effect of water vapour pressure on the kinetics of the thermal dehydration of erbium formate dihydrate

The kinetics of the thermal dehydration of erbium formate dihydrate has been studied by means of isothermal gravimetry under various water vapour pressures from 5 × 10 −4 to 8 Torr. The kinetics of the dehydration, on the whole, can be described by the contracting volume model, R 3. The rate of dehydration k exhibited an unusual dependence upon the water vapour pressure: with increasing water vapour pressure, the rates increased at first, passed through a maximum, and then decreased gradually to a constant value. These phenomena were analogous to the Smith-Topley effect, and seemed to be correlated to the crystallinity of the dehydrated products. At low water vapour pressure (≈ 10 −4 Torr), the amorphous dehydrated products cover the surface of the reaction particles and interfere with the escape of the dissociated water molecules. Therefore, the rate of dehydration is slow. The presence of a few water molecules seems to promote recrystallization of the dehydrated products. The recrystallization results in the formation of wide channels between the dehydrated particles, and the dissociated water molecules can easily escape through these channels. Therefore, the dehydration rate seems to increase. At higher vapour pressure, the rate decreased gradually because of the reverse reaction caused by atmospheric water molecules.

Kinetics of the thermal decomposition of aluminum sulfate

The kinetics of the thermal decomposition of aluminum sulfate has been studied both isothermally and dynamically in a Perkin-Elmer thermobalance. The contracting-volume model is found to be the best one for describing the kinetic results under isothermal conditions. The general phase boundary reaction models, including 1-D, 2-D and 3-D, are then used to analyze the data from dynamic measurements. Under various conditions, such as different heating rates and sample weights, the most appropriate kinetic expression changes from the 3-D phase boundary reaction model at slow heating rates and small sample weights to the 1-D model at the other extremes. The distribution of thermal flux, in terms of the external heat transport effect, is thought to be the predominant factor here. Finally, a linear compensation effect is also observed between the calculated apparent activation energies and pre-exponential factors from the dynamic studies.

Kinetics of the Decomposition of Freeze‐Dried Aluminum Sulfate and Ammonium Aluminum Sulfate

Conventional thermoanalytical techniques confirmed earlier studies on the stoichiometry of the decomposition of aluminum sulfate and ammonium aluminum sulfate (alum). The kinetics of the sulfate decomposition of freeze‐dried aluminum sulfate, of sieve fractions of reagent grade aluminum sulfate, and of freeze‐dried alum were studied isothermally at several temperatures between 635 and 770°C. The activation energy, a preexponential term, and a degree‐of‐fit parameter are presented. The calculations are based on several kinetic models for each data set. The freeze‐dried aluminum sulfate fits a contracting area model with an activation energy of 69 kcal/mol, and the reagent grade aluminum sulfate fits the contracting volume model with an activation energy of 74 kcal/mol. Both were calculated using a fraction reacted (alpha) of 0.15 to 0.90. The freeze‐dried alum was fitted with less precision to a first order model with an activation energy of 83 kcal/mol over the range of alpha 0.3 to 0.9.

Kinetics of the pyrolysis of beryllium hydride

The kinetics of the pyrolytic decompositions of beryllium hydride, BeH 2 has been studied over the temperature range 205–250°C. When the fraction decomposed is planted time the reaction exhibits a sigmond-shaped curve on the processes. The initial portion of the phase (has been correlated with power law which is the result of using a model assuming random formation of beryllium and a linear growth rate of the nuclei. The data indicate that the exponent on in the power law was 7 for 95.6% hydride and about 5 for 93.1% hydride. A contracting-volume model has been to describe the final portion of the acceleratory phase and the deceleratory phase (0.35 < α < 0.80, 0.95)). A first-order rate expression correlated the data for α > 0.80. Values of the activation energy were found to be between 23 and 27 kcal mole for the 95.6% hydride and between 23 and 30 kcal mole for the 93.1% hydride for all of the models used. Any rate equation having the general form will produce similar activation energies. The hydride pyrolysis rate was unaffected by beryllium metal addition, by washing in -hexane, ether, and benzene, or by exposure to mercury vapor. X-ray irradiation changed the pyrolysis rate in a way that was consistent with the production of more nucleation sites. Grinding the hydride also increased the decomposition rate but lowered the exponent on in the power law from 7 to about 5.