Constant Reaction Rate Research Articles

Kinetics and phase evolution during carbothermal synthesis of titanium carbide from ultrafine titania/carbon mixture

The kinetics and phase evolution of the TiC formation process by carbothermal reduction of ultrafine titania/carbon mixture were investigated using thermogravimetric analysis (TGA), X-ray diffraction (XRD) and oxygen analysis. Titania (TiO2) first lowered its oxidation state to Ti3O5 via an unidentified phase (possibly one of the Magneli phases). Then Ti3O5 was further reduced to Ti2O3, followed by the formation of titanium oxycarbide (TiCxOy) phase and its purification toward high purity TiC thereafter. Ti2O3 was the oxide phase with the lowest oxidation state before forming TiCxOy phase. In the isothermal TGA trace, the formation of Ti3O5 showed a diffusion-controlled process; possibly carbon diffusion limited the solid state reaction. The formation of Ti2O3 and TiCxOy was interpreted to be associated with CO gas-assisted reduction reaction, based on constant reaction rate for each process. The activation energy for the formation of Ti2O3 (from Ti3O5) and TiCxOy (from Ti2O3) phase were calculated to be 415.6 and 264.3 kJ mol-1, respectively. The TiC powder synthesized at 1550 °C for 4 h in flowing argon atmosphere showed fine particle size (0.3–0.6 μm) with oxygen content of 0.7 wt % and lattice parameter of 0.4328 nm while interparticle agglomeration was moderate. © 1998 Chapman & Hall.

The dehydration schemes of rare-earth chlorides

The dehydration schemes of LaCl 3·7H 2O, CeCl 3·7H 2O, PrCl 3·7H 2O, SmCl 3·6H 2O, EuCl 3·6H 2O, GdCl 3·6H 2O, HoCl 3·6H 2O, ErCl 3·6H 2O, TmCl 3·6H 2O, YbCl 3·6H 2O and YCl 3·6H 2O have been investigated by the isothermal fluidizedbed technique. This technique is based on the fact that reactions proceed at a close approach to equilibrium and thus give rise to constant reaction rate regimes at constant gas flow and temperature in the bed. By injecting a small portion of HCl(g) (∼1%) into the gas stream, hydrolysis is avoided, and dehydration to the monohydrate is recorded by both thermal analysis of the preheated inlet gas and chemical analysis of samples taken from the bed. Based on the present results, together with previous results on NdCl 3·6H 2O, TbCl 3·6H 2O and DyCl 3·6H 2O, dehydration schemes of all rare-earth chlorides except LuCl 3 and ScCl 3 are suggested.

A solvable aggregation-annihilation chain model with n species

The aggregation-annihilation processes involving n species are discussed by means of meanfield theory. We propose a sort of solvable chain-reaction models in which n kinds of species are numbered, the irreversible aggregations occur only between similar species and the annihilations occur only between dissimilar species having adjacent numbers. Under constant aggregation and annihilation reaction rates and certain symmetric condition, the exact solutions of the models are obtained, which show that the kinetic exponents describing evolution behaviors of reactants at long time depend on reaction rates and the numbers of species in the reaction processes.

Synthesis of silicon carbide whiskers from carbothermal reduction of silica gel by means of the constant rate thermal analysis (CRTA) method

New Constant Rate Thermal Analysis (CRTA) equipment has been developed that permits the synthesis of SiC from carbothermal reduction of silica under both constant reaction rate and constant partial pressure of the CO generated in the reaction. It has been suggested that this method improves the formation of whiskers.

Substrate and inhibitor specificities of the monocarboxylate transporters of single rat heart cells.

We have used the intracellular pH-sensitive fluorescent dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) to characterize the substrate and inhibitor specificity of monocarboxylate transport into isolated rat heart cells. Further evidence was obtained for the presence of two lactate carriers present in heart cells (Wang et al., Biochem. J. 290: 249-258, 1993) both distinct from the recently cloned monocarboxylate transporter isoform 1 (MCT-1) found in many other cell types. Only one isoform was potently inhibited by alpha-cyano-4-hydroxycinnamate [CHC; inhibitor constant (Ki) 190 microM] and the stilbene disulfonates 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (Ki 79 microM) and 4,4'-dinitrostilbene-2,2'-disulfonate (Ki of cis- and trans-isomers 38 and 171 microM, respectively; neither isomer inhibits MCT-1). The second carrier had a Ki of approximately 3 mM for CHC and 0.5-2 mM for the stilbene disulfonates. Thus, unlike in many other tissues, in rat heart cells these inhibitors are not effective at blocking lactate transport totally unless used at very high concentrations. Both carriers were inhibited by 3-isobutyl-1-methylxanthine (Ki 340 microM) and neither by 5-nitro-2-(3-phenylpropylamino)benzoate (a potent inhibitor of MCT-1). The overall Michaelis constant (Km) and maximum reaction rate (Vmax) for transport of a variety of substituted monocarboxylates (C2-C5) were determined, although it was not possible to elucidate the kinetic parameters of the two isoforms. Of physiological interest, the ketone bodies D-beta-hydroxybutyrate and acetoacetate had K(m) values of 10 and 5.4 mM, respectively. Vmax values were similar to those of L-lactate and pyruvate and indicate that transport could limit rates of utilization of ketone bodies. No stereoselectivity for L-over D-isomers of 2-chloro or 2-hydroxy acids was observed.

A new urea sensor based on combining the surface acoustic wave device with urease extracted from green soya bean and its application— determination of urea in human urine

The urea sensor was prepared by combining a surface acoustic wave (SAW) device, in which a SAW resonator operating at 61 MHz and a pair of parallel electrodes were used in series, with urease extracted from green soya bean. The Michaelis constant and maximum reaction rate of the urease were estimated as 2·14 mM and 27·18 kHz min −1, respectively, at pH 7·0 and 25·0°C. Influences of pH, temperature and effectors on the response properties of the SAW urea sensor were investigated. Recovery of the sensor ranged from 95 to 105% and the detection limit of urea was 1·0 μg ml −1 (1·7 × 10 −5 M). The proposed sensor has been successfully applied to the rapid determination of urea in human urine samples. The results are consistent with the reported values and also support the clinical diagnosis.

Applications of new high resolution evolved-gas analysis systems for the characterisation of catalysts using rate-controlled thermal analysis

The advantages of using rate-controlled thermal analysis (RCTA) methods in the thermal characterisation of catalysts are described. The use of these methods is demonstrated by the temperature programmed reduction of vanadium pentoxide, the measurement of the number and type of acid sites in clays by temperature programmed desorption of cyclohexylamine, and temperature programmed reaction, which is illustrated by the dehydration of propan-2-ol over an acid-activated montmorillonite clay. It is shown that the new system has advantages in the study of the kinetics of reactions and in improving resolution when used to characterise the thermal decomposition of catalyst precursors. New evolved-gas analysis systems for characterising heterogeneous catalysts are described, in which a variety of RCTA methods is used. The systems are computer-controlled and use a variety of furnaces, permitting samples from a few microgrammes to fifty milligrammes to be studied, under pressures ranging from ultra high vacuum to one atmosphere. The furnace chosen for a particular experiment is linked to one of a number of gas detectors (hygrometers, katharometers and mass spectrometers), the output of which completes the control loop. The most important component of the system is the versatile software which supports a range of RCTA modes, including conventional linear heating rate, constant reaction rate, and other control strategies developed in this work.

Nonlocal Reactive Transport with Physical and Chemical Heterogeneity: Localization Errors

The origin of nonlocality in “macroscale” models for subsurface chemical transport is illustrated. It is argued that media that are either nonperiodic (e.g., media with evolving heterogeneity) or periodic viewed on a scale wherein a unit cell is discernible must display some nonlocality in the mean. A metaphysical argument suggests that owing to the scarcity of information on natural scales of heterogeneity and on scales of observation associated with an instrument window, constitutive theories for the mean concentration should at the outset of any modeling effort always be considered nonlocal. The intuitive appeal to nonlocality is reinforced with an analytical derivation of the constitutive theory for a conservative tracer without appeal to any mathematical approximations. Deng et al. (1993) present a first‐order, nonlocal, Eulerian theory for transport of a conservative solute in an infinite nondeforming domain under steady flow conditions. Hu et al. (this issue) extended these results to account for nonequilibrium linear sorption with random partition coefficient Kd but deterministic constant reaction rate Kr. These theories are localized herein, and comparisons are made between the fully nonlocal (FNL), nonlocal in time (NLT), and fully localized (FL) theories. For conservative transport, there is little difference between the first‐order FL and FNL models for spatial moments up to and including the third. However, for conservative transport the first‐order NLT model differs significantly from the FNL model in the third spatial moments. For reactive transport, all spatial moments differ between the FNL and FL models. The second transverse‐horizontal and third longitudinal‐horizontal moments for the NLT model differ from the FNL model. These results suggest that localized first‐order transport models for conservative tracers are reasonable if only lower‐order moments are desired. However, when the chemical reacts with its environment, the localization approximation can lead to significant error in all moments, and a FNL model will in general be required for accurate simulation.

Optimisation of a new continuous UV assay for the determination of blood coagulation factor XIII activity in human plasma.

The new photometric assay described by Fickenscher et al. (Thromb. Haemostas. 65 (1991) 535-540) for the determination of factor XIII facilitates the diagnosis of factor XIII deficiency. In spite of easy handling, this test should be used critically. Patients with hyperfibrinogenaemia showed factor XIII activities of less than 20%, whereas with an optimized assay we found normal factor XIII values. Also, the use of a fixed period of incubation for the analysis is questionable, because the period of constant reaction rate occurs earlier and is shorter with high factor XIII activities and later and longer with low factor XIII activities. A linear relation between factor XIII activity and signal only exists up to 80% of activity. In some plasma samples from patients with hyperfibrinogenaemia the factor XIII determination actually shows decreased values for factor XIII. During the reaction, a fibrin clot is formed. The resulting turbidity simulates an increase in absorbance so that NADH consumption is apparently decreased. In six patients with hyperfibrinogenaemia (8.1-9.4 g/l), a factor XIII activity of 26 U/l or less was determined. Using 50 microliters instead of 100 microliters sample volume, 50% (3/6) of the patients showed a normal factor XIII activities (80-96 U/l), whereas 50% (3/6) values of 6-15 U/l were found. In our modified assay we measured normal factor XIII activities (72-151 U/l) in all 6 patients. The procedure is optimized by reducing the sample volume from 100 microliters to 50 microliters.(ABSTRACT TRUNCATED AT 250 WORDS)

Test of two numerical schemes for use in atmospheric transport-chemistry models

Two fast integration methods for chemical kinetics are tested. One is the Quasi-steady State Approximation (QSSA) method and the other is a new Euler Backward Iterative (EBI) method. The EBI method is based on iterative solution of the Euler backward approximation of a coupled system of nonlinear ordinary differential equations of chemical kinetics. The efficiency of the iteration process is increased by using analytical solutions for groups of species which are strongly coupled. The accuracy of both integration methods is evaluated by comparing the results with solutions obtained by a Gear method, the Livermore Solver for Ordinary Differential Equations (LSODE). The chemical scheme used is the Carbon-bond Mechanism IV (CBM-IV). The numerical methods are tested of three chemical scenarios: two scenarios without emissions and with constant reaction rates and one scenario with variable emissions and photodissociation rates. Using a short time step (50 s), both EBI and QSSA perform very well, even under extreme chemical conditions. For larger time steps the EBI method performs better than QSSA. In the case of more realistic chemical conditions, both methods perform well even with a time step of 900 s. The accuracy of QSSA depends highly on the iteration procedure. Without iterations the QSSA method performs poorly. The great advantage of the EBI method is that concentrations are computed using linear operators only. Because of this, the method is mass conserving and can be used in air pollution transport models where higher moments of concentration distributions also need to be evaluated. Both the QSSA and the EBI methods can be recommended for use in atmospheric transport-chemistry models, where accuracy as well as computational efficiency is important. In general, the new EBI method is, however, more efficient than QSSA with a constant number of iterations.

Nonuniversality and breakdown of scaling in two-species aggregation with annihilation

We study the kinetics of a simple model of aggregation with two distinct monomeric species in which irreversible bonding occurs only between like species and annihilation occurs between unlike species. In the mean-field approximation we find an unexpected kinetic behaviour which depends crucially on the ratio of reaction rates. For a relatively simple model of constant reaction rates, we investigate the corresponding rate equations and derive exact expressions for the exponents describing the kinetics of the model. It is observed that the kinetic behaviours of distinct species are quite different and that corresponding scaling exponents are also generally nonuniversal. Analytical solutions of the rate equations show that a modified scaling description, with different sets of exponents for light and heavy species, exists in systems with sufficiently weak annihilation; in systems with strong annihilation, the scaling for the cluster-mass disribution of the light species completely breaks down. A model with constant reaction rates of fusion and sum-kernel reaction rates of annihilation is also studied.

Development of 207Pb, 208Pb and 209Bi target wheels in the synthesis of 107Ns, 108Hs and 109Mt

The development of 207Pb, 208Pb and 209Bi target wheels and their applications in heavy-ion fusion reactions are reviewed. In both fabrication and use, the centers of the evaporator or accelerator beams are focussed at wheel radii of 155 mm to specially shaped frames which generate very homogeneous target layers and very constant reaction and counting rates in the experiment. Target areas of up to ∼98% of a wheel's circumference of 974 mm can be provided. The preparation procedures for the necessary C backings and protecting layers of C are described, and details are given for the development of high-vacuum evaporations of 207Pb, 208Pb and 209Bi with deposition yields of 35–55% from tantalum crucibles. Applications of the target wheels in heavy-ion fusion reactions with beams of 54Cr and 58Fe at energies near the Coulomb barrier and intensities of ∼10 12 particles/s are reported. The target parameters for the production runs of the new chemical elements 107Ns, 108Hs and 109Mt are included.

Improved Rearrangement of the Integrated Michaelis-Menten Equation for Calculating In Vivo Kinetics of Transport and Metabolism

A multiple regression form of the integrated Michaelis-Menten equation was developed and evaluated with simulated data having controlled error. Both multiple and traditional linear regression fit errorless data perfectly, but multiple regression is much more stable with regard to accuracy and precision of estimating the Michaelis constant and maximum rate of reaction when data contain error. Bias in determining estimators of kinetic coefficients was −4 and −3% versus −56 and −35% with 10% error in the data. Multiple regression estimates for Michaelis constant and maximum rate of reaction directly as opposed to estimating 1/Km and maximum rate of reaction/Michaelis constant by linear regression. The difference in accuracy in estimating actual Michaelis constant, for example, is 4% versus 227% error with only 10% error in the data. Precision of estimation is approximately the same as precision of the data for multiple regression. For the 800 data sets examined, R2 was always greater than .92 for multiple regression, but frequently was not significant for linear regression. The actual initial concentration was provided for linear regression but calculated by multiple regression with accuracy and precision equivalent to estimation of Michaelis constant and maximum rate of reaction. The multiple regression method has statistical power to determine treatment effects on Michaelis constant and maximum rate of reaction with a practical number of animals.

A comparative study of the resolving power of both conventional thermogravimetry and constant rate thermal analysis

According to literature data, constant rate thermal analysis (CRTA) has a higher resolving power for discriminating overlapping processes than the conventional TG method. However, taking into account that the heating rate and constant reaction rate concepts are quite different, it is of interest to propose a standardization procedure for comparing the results obtained from both methods. The present theoretical kinetic study points out that CRTA and TG have very similar discriminating power, provided that both the heating rate for the TG curve and the constant reaction rate for the CRTA curve are selected in such a way that the total time required for carrying out the reaction in the range 0.05 ⩽ α ⩽ 0.95 is the same in both cases.

Coupled Process Modeling and Waste-Package Performance

ABSTRACTThe interaction of borosilicate waste glasses with water has been studied extensively, and reasonably good models are available that describe the reaction kinetics and solution chemical effects. Unfortunately, these models have not been utilized in performance assessment analyses, except in estimating radionuclide solubilities at the waste form surface. A geochemical model has been incorporated in the AREST code to examine the coupled processes of glass dissolution and transport within the engineered barrier system. Our calculations show that the typical performance assessment analyses using fixed solubilities or constant reaction rate at the waste form surface are not always conservative or realistic predictions of radionuclide release. Varying the transport properties of the waste package materials is shown to produce counterintuitive effects on the release rates of some radionuclides. The use of noncoupled performance assessment models could lead a repository designer to an erroneous conclusion about the advantages of one waste package design or host rock setting over another.

Phenol photodegradation over titanium dioxide

AbstractThe heterogeneous photocatalytic oxidation of phenol in aqueous diluted solutions over TiO2 particles under UV‐illumination has been investigated. It has been observed that the yield of the phenol photooxidation depends strongly on the pH of the solution. Maximum yields are obtained at pH 8 and also in very alkaline media. These results are explained considering the processes that take place at the surface of the semiconductor, in which OH radicals have an important role. With respect to the initial charge transfer steps a kinetic analysis has been performed. From this analysis it was deduced that the constant rate of formation of OH radicals is about 4.0 × 104 times greater than the constant rate of the direct reaction between phenol and photogenerated holes in the TiO2 particles. Finally, from HPLC analysis, hydroquinone, paraquinone and 1,2.4‐benzenetriol have been detected as intermediate products prior to the total phenol mineralization.

The linear solution for hydriding of uranium

A set of equations describing the hydriding of solid pieces of uranium is presented, together with an approximate solution of the equations. The solution predicts a constant velocity of the spall front in the solid piece, which leads to a constant value of weight gain rate (or other measure of the extent of the hydriding reaction) for a portion of the duration of the measurement. The solution allows calculation of the spall front velocity, and thus of the constant reaction rate, from expressions that are closed-form with the exception of one simple numerical integration. Expressions are given for all the parameters needed to calculate reaction rates for well annealed samples of uranium. Calculated reaction rates are compared with published data taken at a hydrogen pressure of 1.0 atm. The calculated results match the data quite well.

The linear solution for hydriding of uranium

A set of equations describing the hydriding of solid pieces of uranium is presented, together with an approximate solution of the equations. The solution predicts a constant velocity of the spall front in the solid piece, which leads to a constant value of weight gain rate (or other measure of the extent of the hydriding reaction) for a portion of the duration of the measurement. The solution allows calculation of the spall front velocity, and thus of the constant reaction rate, from expressions that are closed-form with the exception of one simple numerical integration. Expressions are given for all the parameters needed to calculate reaction rates for well annealed samples of uranium. Calculated reaction rates are compared with published data taken at a hydrogen pressure of 1.0 atm. The calculated results match the data quite well.

Vinyl chloride suspension polymerization with constant rate. A numerical study of batch reactors

AbstractBased on the analysis of a simple mathematical model, different temperature profiles are generated in order to provide vinyl chloride (VCM) suspension polymerization with constant reaction rates in batch reactors. In order to reproduce these temperature profiles in industrial‐scale reactors, some process variables, such as coolant temperature, initiator concentration, and rate of water and monomer condensation, have to be manipulated. It is shown that those temperature practices can almost never be applied to large‐scale reactors if the jacket temperature is the only variable that can be manipulated. It is also shown that developing an initiator feed procedure or using a reflux condenser may be advantageous.

Effect of the layer of air between the calorimeter and cylindrical samples in DC experiments

When using cylindrical samples in calorimetry a thin space is left between the calorimeter and holder so that the holder may be moved. The layer of air in this space must be considered in calorimetry when working under isothermal conditions (differential calorimetry) in order to obtain calculated heat flux-time histories which can be compared with the experimental values. The model used here is a numerical method with finite differences. It takes into account not only the heat evolved from the reaction, but also heat transferred by conduction through the holder and sample and through the thin layer of air. The assumption made about the heat conduction through the layer of air is proved to be correct. This layer of air is found to be responsible for a retardation in the reaction and for lower profiles of temperature and state of cure through the sample, exhibiting an almost constant rate of reaction at any location in the sample. However, the location of the sample in the calorimeter is of great interest, as is the constancy of the thickness of the layer of air.