Summary The aim of this work is to develop a simplified, though rigorously based thermogravimetric analysis (TG) method to estimate intrinsic reactivity parameters (activation energy, E, and pre-exponential factor, A) for the oxidation in air of engineering carbonaceous materials. To achieve this aim, a modified Coats-Redfern method for analysing linear curves has been devised. The new method assumes first-order reaction kinetics with respect to carbon, and uses a statistical criterion to estimate an ‘optimum’ heating rate. For the oxidation in air of a model carbon, an optimum rate of 27 K min-1 was determined, at which E=125.8 kJ mol-1. This is in good agreement with activation energies obtained using established, though more limited model-free or isoconversional methods.
The reaction process of the thermal dehydration of dilithium tetraborate trihydrate, Li2B4O7
3H2O, was reinvestigated from a viewpoint of reaction kinetics. On the basis of the results of thermogravimetry, constant rate thermal analysis, powder X-ray diffractometry, infrared spectroscopy and scanning electron microscopy, it was confirmed that the reaction proceeds via three consecutive kinetic steps characterized by different activation energies. The first and second kinetic steps, accompanied by the destruction of the original crystal structure of the reactant, seem to be assigned to the surface and internal reactions, respectively. During the third kinetic step, the thermal dehydration of hydrated amorphous intermediate, produced at the second kinetic step, and crystallization of the final dehydration product, Li2B4O7, are likely to take place concurrently.
Authors:Sameh Othman, Azza Ali, Nabil Mansour, and Bahgat El-Anadouli
In this study, a new mathematical model was suggested to account for the effect of internal and external diffusion on fluid–solid
noncatalytic reactions. The special features of this model are the combination of the transport processes and the chemical
reaction kinetics with the added factor due to the structural properties of the solid reactant. The model was examined theoretically
and experimentally. A reactive cloth filter that separates radionuclides from radioactive waste solutions is used as a practical
application for the model. Analyses of the respective rate data in accordance with another two theoretical models showed that
process is controlled by the rate of the diffusion step. The external and internal mass transfer constants across the liquid
film and the resin matrix were determined from the graphical representation of the proposed model. The practical validation
with the theoretical results offered satisfactory agreement at most of the process stages.
Aspects of the theories that are conventionally and widely used for the kinetic analyses of thermal decompositions of solids,
crystolysis reactions, are discussed critically. Particular emphasis is placed on shortcomings which arise because reaction
models, originally developed for simple homogeneous reactions, have been extended, without adequate justification, to represent
heterogeneous breakdowns of crystalline reactants. A further difficulty in the mechanistic interpretation of kinetic data
obtained for solid-state reactions is that these rate measurements are often influenced by secondary controls. These include:
(i) variations of reactant properties (particle sizes, reactant imperfections, nucleation and growth steps, etc.), (ii) the effects of reaction reversibility, of self-cooling, etc. and (iii) complex reaction mechanisms (concurrent and/or consecutive reactions, melting, etc.). A consequence of the contributions
from these secondary rate controls is that the magnitudes of many reported kinetic parameters are empirical and results of
chemical significance are not necessarily obtained by the most frequently used methods of rate data interpretation. Insights
into the chemistry, controls and mechanisms of solid-state decompositions, in general, require more detailed and more extensive
kinetic observations than are usually made. The value of complementary investigations, including microscopy, diffraction,
etc., in interpreting measured rate data is also emphasized.
Three different approaches to the formulation of theory generally applicable to crystolysis reactions are distinguished in
the literature. These are: (i) acceptance that the concepts of homogeneous reaction kinetics are (approximately) applicable (assumed by many researchers),
(ii) detailed examination of all experimentally accessible aspects of reaction chemistry, but with reduced emphasis on reaction
kinetics (Boldyrev) and (iii) identification of rate control with a reactant vaporization step (L’vov). From the literature it appears that, while the
foundations of the widely used model (i) remain unsatisfactory, the alternatives, (ii) and (iii), have not yet found favour. Currently, there appears to be no interest in, or discernible effort being directed towards,
resolving this unsustainable situation in which three alternative theories remain available to account for the same phenomena.
Surely, this is an unacceptable and unsustainable situation in a scientific discipline and requires urgent resolution?
This article studies the thermokinetics and safety parameters of cumene hydroperoxide (CHP) manufactured in the first oxidation tower. Vent sizing package 2 (VSP2), an adiabatic calorimeter, was employed to determine reaction kinetics, the exothermic onset temperature (T0), reaction order (n), ignition runaway temperature (TC, I), etc. The n value and activation energy (Ea) of 15 mass% CHP were calculated to be 0.5 and 120.2 kJ mol−1, respectively. The heat generation rate (Qg) of 15 mass% CHP compared with hS (cooling rate) = 6.7 J min−1 K−1 of heat balance, the TS,E and the critical extinction temperature (TC, E) under 110 °C of ambient temperature (Ta) were calculated 111 and 207 °C, respectively. The Qg of 15 mass% CHP compared with hS = 0.3 J min−1 K−1 of heat balance was applied to determine the TC, I that was evaluated to be 116 °C. This article describes the best operating conditions when handling CHP, starting from the first oxidation tower.
A thermo magnetic analysis (TMA) apparatus is used to follow reactions under controlled conditions of temperature and pressure with the Faraday method. Relations giving the conversion degree λχ at a given timet as a function of the sample susceptibility are presented. Methods for studying the effects of the magnetic field on the reaction kinetics are considered. In particular, the kinetic curves obtained for the reaction
Authors:Shivani Suri, K. K. Bamzai, and Vishal Singh
Non-isothermal kinetic parameter of pure and cadmium-doped barium phosphate single crystal grown by room temperature solution technique have been investigated. Single crystal X-ray diffraction establishes grown crystal to be orthorhombic in nature. Scanning electron microscopy supplemented with energy dispersive X-ray analysis was used to study the surface features and to find the exact stoichiometric composition of the grown crystal. Fourier transform infrared spectroscopy studies confirm the presence of various functional groups. The effect of cadmium doping on pure barium phosphate single crystal was studied using thermogravimetry analysis. Thermogravimetry studies shows that the pure crystal was stable up to a temperature of 330 °C whereas doped crystal was stable up to a temperature of 240 °C, i.e., pure crystals were more stable than doped ones. Various solid-state reaction kinetics, i.e., activation energy (Ea), frequency factor (Z), and entropy (ΔS∗) was calculated out to find the mechanism of thermal decomposition at different stages for pure and cadmium doped barium phosphate.
Authors:H. Treutler, G. Just, M. Schubert, and H. Weiss
The groundwater at a former gasoline production site in Germany is heavily contaminated with aromatic hydrocarbons (mostly
benzene) and is currently being treated in bioreactors under anaerobic conditions. To determine the reaction kinetics it is
essential to know the mean residence time of the groundwater in these reactors. Most of the commonly used tracers (dyes and
salts) did not give reliable results because of their interaction with the mineral matrix in the reactors. In this study radon
(222Rn) dissolved in the groundwater is used as the tracer. The flow rate of groundwater through the reactors is 1 l/h. Over a
period of 8 hours the radon-spiked groundwater was injected into the natural groundwater which has a very low radon concentration.
The radon concentration of the discharged water is measured online at the reactor outlet. An increasing radon concentration
at the reactor exit indicates the shortest residence time of the water. The time-dependent progress of the radon concentration
provides detailed information about the flow behavior and residence times of water in the reactor.
Hydrogen phosphate (HPO42−) or poly(acrylic acid) (PAA) stabilized cobalt(0) nanoclusters were in situ generated from the reduction of cobalt(II) chloride during the catalytic hydrolysis of sodium borohydride (NaBH4) in the presence of stabilizers, HPO42− or PAA. Cobalt(0) nanoclusters stabilized by HPO42− or PAA were characterized by using UV–Visible spectroscopy, TEM, XPS and FT-IR techniques. They were employed as catalysts in the hydrolysis of NaBH4 to examine the effect of stabilizer type on their catalytic activity and stability. Detailed reaction kinetics of the hydrolysis of NaBH4 in the presence of both catalysts was studied depending on catalyst concentration, substrate concentration and temperature. PAA stabilized cobalt(0) nanoclusters provided higher total turnover number (TTON = 6,600) than that of HPO42− stabilized cobalt(0) nanoclusters (1,285 turnovers). However, the HPO42− stabilized cobalt(0) nanoclusters provided a lower activation energy (Ea = 53 ± 2 kJ mol−1) than the PAA stabilized cobalt(0) nanoclusters (Ea = 58 ± 2 kJ mol−1) for the hydrolysis of NaBH4. The use of two types of stabilizers in the preparation of the same metal(0) nanoclusters following the same methodology enables us to compare the electrostatic and steric stabilization in terms of the catalytic activity and stability of metal(0) nanoclusters.
Authors:A. S. Al-Hobaib, D. M. Al-Dhayan, K. M. Al-Sulaiman, and A. A. Al-Suhybani
Sand filters are used as a filter bed in many ground water treatment plants to remove the physical contaminants and oxidation
products. A build-up of radioactivity may take place on the granules, where iron and manganese oxides are deposited and form
thin films on the surface of sand filter. The oxides of iron and manganese play an important role in adsorbing radium from
ground water. The disposal of those granules makes a significant problem. A batch technique is used for solubilization of
radium from sand filters in the presence of some organic acids, which act as reducing agents. These acids are formic acid,
acetic acid, benzoic acid, succinic acid, oxalic acid, phthalic acid, and adipic acid. The data were obtained as a function
of acidity, temperature, contact time and liquid/solid ratio particle size and shaking speed. It was found that oxalic acid
was the best for radium removal. The effectiveness of these acids on radium removal was as follows: oxalic acid > phthalic
acid > adipic acid > succinic acid > formic acid > acetic acid. The maximum removal obtained was 69.9% at 1M oxalic acid at
8 ml/g ratio. Reaction kinetics and mechanism parameters of the dissolution process were studied and compared with other published