Exothermic heat evolution

The Ceramicrete process is based on the acid-base exothermic reaction. As a result, the exothermic heat evolution and its rate depend on the size of the waste forms produced. Larger forms generate more heat, which does not dissipate rapidly. Thus, the setting slurry heats up and accelerates the acid-base reaction, and the mixture is able to set even in cold surroundings. 

By combining calorimetry with three-dimensional atom probe analysis, Starink and coworkers [25] examined the aging, at room-temperature, of Al-Cu-Mg-Mn alloys and found that the process is accompanied by a substantial exothermic heat evolution, whereas the only micro-structural change involved the formation of Cu-Mg co-clusters. Tatar and Zengin [26] studied the effects of neutron irradiation on the oxidation behaviour, microstructure and transformation temperatures of CuAlNiMn shape-memory alloy. They showed that irradiation... 

Both reaction pathways to CH, and CH3OH, formation show that most of the intermediate reaction steps are endothermic. However, both CH, and CH3OH, formation from CO + are exothermic. Heat evolution occurs from adsorption and bond disso-... 

Ozone can be destroyed thermally, by electron impact, by reaction with oxygen atoms, and by reaction with electronically and vibrationaHy excited oxygen molecules (90). Rate constants for these reactions are given ia References 11 and 93. Processes involving ions such as 0/, 0/, 0 , 0 , and 0/ are of minor importance. The reaction O3 + 0( P) — 2 O2, is exothermic and can contribute significantly to heat evolution. Efftcientiy cooled ozone generators with typical short residence times (seconds) can operate near ambient temperature where thermal decomposition is small. 

An exothermic reaction involving two reactants is run in a semi-continuous reactor. The heat evolution can be controlled by varying the feed rate of one component. This is done with feedback control with reactor temperature... 

A value for the polymerization enthalpy of 21.5 kcal/mole can be used to estimate percent conversion and rates for N-substituted maleimide/vinyl ether and maleic anhydride/vinyl ether copolymerizations. A value of 18.6 kcal/mole can be used for the enthalpy of polymerization of acrylate monomers to convert heat evolution data to percent conversion. Since the molar heats of polymerization for N-substituted maleimide vinyl ether copolymerization and acrylates vary by less than 20 percent, the exotherm data in the text are compared directly. 

Experimental Methods In Differential thermal analysis (DTA) the sample and an inert reference substance, undergoing no thermal transition in the temperature range under study are heated at the same rate. The temperature difference between sample and reference is measured and plotted as a function of sample temperature. The temperature difference is finite only when heat is being evolved or absorbed because of exothermic or endothermic activity in the sample, or when the heat capacity of the sample changes abruptly. As the temperature difference is directly proportional to the heat capacity so the curves are similar to specific heat curves, but are inverted because, by convention, heat evolution is registered as an upward peak and heat absorption as a downward peak. 

Hydrogenation of coal is a highly exothermic reaction corresponding to a heat evolution of about 15 kilojoules per cubic metre of hydrogen reacted. Means must be provided to remove this heat from the reaction zone so that the reaction temperature can be maintained in the optimum range. This is usually accomplished by injecting coal liquid as quench into various sections of the reac tor. 

The raw data obtained for each dose of probe molecule that is, the evolution of the pressure above the sample (P) and the exothermic heat evolved signal (0 as a function of time (Figure 13.5a). The study of the time constant of the heat evolution for each dose provides a description of the kinetics of the adsorption process. 

In either event, the products are ZnCl 2, CO, and A12O 3 The zinc oxide cools and whitens the smoke by consuming atomic carbon in an endothermic reaction that occurs spontaneously above 1000°C (equation 8.6). The reaction with aluminum (equation 8.4 or 8.8) is quite exothermic, and this heat evolution controls the burning rate of the smoke mixture. A minimum amount of aluminum metal will yield the best white smoke. Several "HC" smoke compositions are listed in Table 8.4. 

To a mixture of 30 g of zinc powder (Merck, analytical grade) and 30 ml of absolute ethanol is added 3.5 ml of 1.2-dibromoethane. The mixture is heated until an exothermic reaction (evolution of ethene and temporary reflux) starts. The activation is completed by heating the mixture for an additional 10 min under reflux. After cooling to about 50 C, the trimethylsilylated diyne (0.05 mol, see Chap. VI for silylation methods) is added in one portion. The introduction of N2 is started and the mixture is heated for 30 min under reflux. After cooling to room temperature, the work-up is carried out in a way similar to that in exp. 2 (no aqueous ammonia is nsed). Z-CjH- CI CHC CSiMe, b.p. 75 020 mmHg, njy(20 ) 1.4592, is obtained in a high yield. 

Whereas LiCl and CsF with disparate ionic radii dissolve exothermally in water, most halides of the alkaline metals dissolve without much heat evolution. This is a remarkable observation in view of the huge Madelung energy —1.74/(rM + rx) in atomic units (14.3 eV/A) relative to gaseous M+ and X- and it was first pointed out by Fajans183 that this is compatible with the approximate equality... 

Figure 7 shows the result of warming the reaction mixture. At approximately 10°C a rapid heat evolution occurs (adiabatic temperature rise of 44.33°C). It was reasoned that this exotherm was a composite of an endotherm resulting from dissolution of the crystallized sodium borohydride and a larger exotherm corresponding to the reduction of the ester, POX-C. 

The key problems in a polymerization CSTR are the determination and characterization of micro- and macromixing, and the possibility of multiple steady states due to the exothermic nature of the reactions. Recent studies of CSTRs for bulk or solution free-radical polymerization indicate the possibility of multiple steady states due to the large heat evolution and difficult heat transfer that are characteristic of the reactors. Furthermore, even in simple solution polymerization (for example, in methyl methacrylate polymerization in ethyl acetate solvent), autocatalytic kinetics can lead to runaway conditions even with perfect temperature control for certain combinations of solvent concentration and reactor residence time. In practice, the heat evolution can be an additional source of autocatalytic behavior. 

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