Reactants sample

The investigations were performed in a closed circulation reactor with a volume of 175 cm. The volume of the reactant sample was 0.3 cm. The carrier gas was helium, freed from oxygen with an Alltech Oxy-Trap. The hydrogen used in the measurements was produced by a Matheson 8326 electrolysis apparatus equipped with a Pd diffusion cell. 2-5 mg catalyst samples were used. Details on the experimental procedure were reported earlier (refs. 5,6). 

Flow Toward the Sampling Orifice. The assumption that flow toward the sampling orifice is unlikely to significantly distort the reactor flow or to be a factor in reactant sampling can be justified with compressible flow arguments. For isentropic flow of an ideal gas the temperature and pressure are related to the Mach number by Eqs. 7 and 8 [40] ... 

The study and control of a chemical process may be accomplished by measuring the concentrations of the reactants and the properties of the end-products. Another way is to measure certain quantities that characterize the conversion process, such as the quantity of heat output in a reaction vessel, the mass of a reactant sample, etc. Taking into consideration the special features of the chemical molding process (transition from liquid to solid and sometimes to an insoluble state), the calorimetric method has obvious advantages both for controlling the process variables and for obtaining quantitative data. Calorimetric measurements give a direct correlation between the transformation rates and heat release. This allows to monitor the reaction rate by observation of the heat release rate. For these purposes, both isothermal and non-isothermal calorimetry may be used. In the first case, the heat output is effectively removed, and isothermal conditions are maintained for the reaction. This method is especially successful when applied to a sample in the form of a thin film of the reactant. The temperature increase under these conditions does not exceed IK, and treatment of the experimental results obtained is simple the experimental data are compared with solutions of the differential kinetic equation. 

Using the experimental values for the width of the traveling wave front (portion be, Fig. 8), let us estimate the propagation velocity for the case of a thermal mechanism based on the Arrhenius law of heat evolution from the known relationship U = a/d, where a 10"2 cm2/s is the thermal conductivity determined by the conventional technique. We obtain 5 x 10"2 and 3 x 10-2cm/s for 77 and 4.2 K, respectively, which are below the experimental values by about 1.5-2 orders of magnitude. This result is further definite evidence for the nonthermal nature of the propagation mechanism of a low-temperature reaction initiated by brittle fracture of the irradiated reactant sample. 

Models of the isothermal mechanism can be constructed using a balance equation (1) for the area of active surface per unit volume of a solid sample, with a term added which describes the propagation of this surface into the nonfractured matrix. The term requires that a certain effective transfer coefficient (analogous to the diffusion coefficient) should be introduced. To a first approximation, it can be written as D = vl = v2r, where v is the velocity of sound in the sample, / is the length of the free run of a crack for the time r, and t is the time of mechanical unloading (or the characteristic relaxation time of stresses in the real solid matrix of a reactant sample). It seems impossible to... 

The reliability and precision of all kinetic data collected depend sensitively on the constancy of the temperature within reactant sample and the reaction vessel that contains it. The control of the temperature of the reaction vessel by the furnace must include due allowance for the heat evolved or absorbed by the sample during the decomposition process (see below). Numerous designs of furnaces have been used. 

Rates of growth of small nuclei (often at the lower limits of microscopic detection) may be different from those subsequently attained. Low rates of initial growth have been ascribed to the instability of very small nuclei, but instances of relatively more rapid early growth of small nuclei are also known [10]. However, it is generally agreed that, after initial deviations, all nuclei within a particular reactant sample grow at the same rate. 

A wide variety of spectroscopic techniques [1-4] are used to provide information about the identities and amounts of reactant, the solid product(s), or the solid phase(s) present at intermediate stages of the decomposition. The cooling of reactant samples after partial or complete decomposition may be accompanied by changes such as crystallographic transformations, solidification of any molten material and particle disintegration. Interpretations of observations must take account of these possibilities. 

MICROSCOPIC EXAMINATION OF THE REACTANT SAMPLE BEFORE, DURING AND AFTER DECOMPOSITION... 

Benito and Searcy [4] argue that when large reactant samples are used, approximately equilibrium pressures of carbon dioxide may be established at regions within the sample bulk and the rate of outward COj difiusion measured in kinetic work is determined by local pressure gradients. The apparent value of is then close to the enthalpy of dissociation. During decompositions at low pressures, however, the value of the activation energy is greater because local internal pseudoequilibrium is not attained [4]. 

The observation of compensation behaviour for the decompositions of a group of comparable reactants under similar conditions is ambiguous because such behaviour could be explained either (following the approach used in with heterogeneous catalysis [59]) as the result of relatively small crystal structural differences in the rate determining bond rupture step, or (as above) by differences in the physical conditions existing within each individual reactant sample. 

Microscopic examination of the reactant sample before, during... 

The method can only be used for product gases that are condensed quantitatively under reaction conditions these include water, carbon dioxide, and ammonia. Reactions can be studied only at low pressures and significant investigations of dehydrations using this technique have been published (23-25). This technique can alternatively use a quartz crystal with the reactant sample deposited on or otherwise attached to it and the mass loss monitored through resonance frequency changes as described earlier. 

The data collected in each thermogravimetric experiment is a set of measured (m, t, T) values resulting from the chemical changes undergone by a reactant sample heated under constant or programmed temperature conditions. These are discussed as follows ... 

Heat is initially supplied to the reactant sample to raise it to reaction temperature, at which, ideally, it should be maintained without change thereafter in isothermal... 

To validate the utility of this reactor system for quantitative measurements of reaction kinetics, we evaluated the cumulative impact of errors introduced by the various system components. For this illustration, the variabilities in the pump flow rates, the initial reactant sample concentrations, and the reactor effluent analysis are included. Uncertainties in other process parameters (such as the reaction temperature) that may affect the measured rate are assumed to be negligible. 

Reactant Sample Medium Method Tin (K) E(kJ morl) Tin/E(KmoIkJ-i)... 

Let us first define a calorimeter as an instmment devised to determine heat. In any calorimeter, we may distinguish 1) the calorimetric vessel (often called the cell, container, or calorimeter proper) at temperature Tc(t), that is usually in good contact with its contents, in which the studied transformation occurs. The contents include the reactant samples and subsidiary accessories necessary to achieve the investigated transformation (e.g. to initiate the reaction, or to mix the reagents) or to calibrate the device and 2) the surroundings at temperature To(t), of-... 

In the quartz and albite tests, only very minor precipitation of new phases was seen. Fibrous material, a sheet-like phase and some amorphous gel all formed in the albite tests and a crumpled sheet C-S-H phase was observed in one of the quartz tests (by ATEM and/or SEM). These were all in such small quantities that it was not possible to obtain fully quantitative chemical data from the products by ATEM. Both of these reactant samples were unusually clean, that is with very few fines adhering to grain surfaces. It is likely that this limited the dissolution rate (by lowering surface area) and so precipitation of secondary material may also have been inhibited. 

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