Kinetics, gas evolution

Braun, R. L., Burnham, A. K. Reynolds, J. G. 1992. Oil and gas evolution kinetics for oil shale and petroleum source rocks determined from pyrolysis TQMS data at two heating rates. Energy and Fuels, 6(4), 468 74. 

The gas evolution kinetics (Table 3.5) (with reference to a value r t)) for all compounds being studied in a general way can be approximated adequately by the expression... 

The thermolysis kinetics of 1-12 have the following features in common In all of the samples, the gas evolution rate decreases steadily with time, the considerable gas evolution was observed already on the initial stage of the heating of sample. Figure 10.8 illustrates typical gas evolution kinetics during thermolysis. 

Measurements of product gas evolution, mass loss or evolved gas analysis may all be used to study the kinetics of a solid—solid interaction provided that there is strict adherence to the condition that gas evolution occurs concurrently with the solid state process. Clearly this approach is only applicable if there is direct experimental support for a single step process. For example, carbon dioxide release is identified [410] as being... 

Although there are experimental and interpretative limitations [189, 526] in the kinetic analysis of non-isothermal data, DTA or DSC observations are particularly useful in determining the temperature range of occurrence of one or perhaps a sequence of reactions and also of phase changes including melting. This experimental approach provides, in addition, a useful route to measurements of a in the study of reactions where there is no gas evolution or mass loss. The reliability of conclusions based on non-isothermal data can be increased by quantitatively determining the... 

Carbide decompositions yield no volatile product and, therefore, many of the more convenient experimental techniques based on gas evolution or mass change cannot be applied. This is a probable reason for the relative lack of information about the kinetics of reaction of these and other compounds which are correctly classifed under this heading, such as borides, silicides, etc. 

If both steps are kinetically distinct, measurement of gas evolution (D) does not give any information concerning the rate of the subsequent interaction (formation of C). If E is produced in a finely divided state, it may be appreciably more reactive than alternative bulk preparations of the... 

The global rates of heat generation and gas evolution must be known quite accurately for inherently safe design.. These rates depend on reaction kinetics, which are functions of variables such as temperature, reactant concentrations, reaction order, addition rates, catalyst concentrations, and mass transfer. The kinetics are often determined at different scales, e.g., during product development in laboratory tests in combination with chemical analysis or during pilot plant trials. These tests provide relevant information regarding requirements... 

These experiments also show the value of NEXAFS as a technique for following the kinetics of surface processes. We have shown that experiments can be tailored so a specific reaction can be studied, even if gas evolution is not involved. This represents an advantage over thermal desorption experiments, where several steps may be required in order to desorb the products to be detected. Another advantage of NEXAFS is that rates are measured isothermally, so the kinetic parameters can be determined with accuracy. Finally, NEXAFS is not a destructive technique, so we need not to worry about modifying the surface compounds while probing the system, as would be the case with other techniques such as Auger electron spectroscopy. 

Reaction kinetics (how fast a chemical reaction will proceed, and the rate of heat production and off-gas evolution)... 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

It is potentially more of a problem if the measurement pressure is higher than the maximum accumulated pressure. In this case there is the potential for more gas to dissolve in the sample under pressure than would be the case for the reactor and this can lead to the gas evolution rate being underestimated1131. In such cases, it may be possible to correct for dissolved gas if the kinetics are well-understood. A method for doing this is given in references 14 and 15. 

Where there is direct overlap with the valence band edge, the electron transfer process may be so facile as to give rise to the Hofer-Moest reaction (.2), in which the intermediate alkyl radical is itself oxidized (while it is still adsorbed to the electrode surface) to give a carbonium ion. The reaction of this carbonium ion with the aqueous electrolyte would then yield water-soluble products such as methanol, in keeping with our observation that anodic gas evolution is suppressed under these conditions. In acidic solutions, where the Kolbe reaction is energetically allowed, its kinetic competition with the other reactions on SrTiC>3 thus depends on the absence of defect surface states which are present in some electrode crystals and not in others. 

Fig 2 Kinetic gas-evolution curves in the decomposition of Tetryl with the addition of Picric Acid at 150° 1) Tetryl alone 2) 0.62 mole of Picric Acid per mole of Tetryl 3) 1.26 moles of Picric Acid per mole of Tetryl 4) 2,57 moles of Picric Acid per mole of Tetryl... 

Short heat treatments were found to result in a loss of blowing gas. It is probable that in this case gas evolution precedes polymerization and the formation of a film to keep the gas within the particle. By contrast, for longer treatment times polymerization precedes gas evolution and microspheres are formed. Data on the reaction kinetics during microspheres formation are given in36). 

Other reports of kinetic studies deal with mechanisms of thermal oxidation of a variety of simple ketones monitored via gas evolution (CO, CO2, H2, etc.),176 alkaline oxidation of aldehydes with copper and silver tellurates,177 [Mlll(H2TcOf))2]5, and oxidation of acetals of simple aldehydes in aqueous acetic acid with (i) N-chlorobenzamide (H20C1+ is the oxidant inferred)178 and (ii) /V-chlorosaccharin.179... 

This mode of addition is a traditional way of limiting the accumulation. It is also used for practical reasons, when a reactant is delivered in drums or containers. Nevertheless, the amount of reactant, which can be added in one portion, can also be limited for safety reasons. In this case, the addition must be controlled by the conversion that is, the next portion is only added if the previous portion has been consumed by the reaction. Different criteria can be used to follow the reaction the temperature, the gas evolution (where applicable), the aspect of the reaction mass, chemical analysis, and so on. For a well designed process, where the reaction kinetics are known to a certain accuracy, the successive additions can also be performed on a time basis. 

The kinetics at 673 K were reported32 to follow Prout-Tompkins kinetics, passing through an initial period of gas evolution, short acceleratory regime, and long decay. 

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