Thermal decomposition, equilibrium

The catalysts are prepared from hydroxides or carbonates by thermal decomposition. Equilibrium pressures for decomposition of carbonates and peroxides are shown in Fig. 3.4. lb obtain oxides from hydroxides or carbonates, high temperature pretreatment is required. During pretreatment, evolution of H2O, CO2, and O2 occurs. Evolution of H2O begins at about 673 K as Mg(OH)2, Ca(OH)2, and commercially available BaO are heat-treated in vacuo Carbon dioxide starts to evolve at a temperature slightly higher than that for H2O evolution. From commercially available... 

At the high temperatures found in MHD combustors, nitrogen oxides, NO, are formed primarily by gas-phase reactions, rather than from fuel-bound nitrogen. The principal constituent is nitric oxide [10102-43-9] NO, and the amount formed is generally limited by kinetics. Equilibrium values are reached only at very high temperatures. NO decomposes as the gas cools, at a rate which decreases with temperature. If the combustion gas cools too rapidly after the MHD channel the NO has insufficient time to decompose and excessive amounts can be released to the atmosphere. Below about 1800 K there is essentially no thermal decomposition of NO. 

Dihydro-2/7- 74 and -4//-l,2-oxazines and thiazines 75 are interrelated by prototropy, being enamines and imines, respectively. In the case of oxazines, the imine form 75 is favored, and there are several well established examples of this system, including the parent heterocycle 75 (X = O) [84MI2]. No tautomeric equilibrium between the 2H and 4H forms has been observed under normal conditions in solution or in the solid state. However, the formation of intermediate 2H isomers 77 was proposed both for the conversion of 3-phenyl-5,6-dihydro-4//-l,2-oxazine 76 (R = Ph, r = R = H) into 2-phenylpyrrole(89TL3471) under strong basic conditions and for thermal decomposition of cyclopentene-fused 1,2-oxazine 76... 

The fact that the equilibrium moisture content may be considerable at low humidities is of especial importance in the oven methods. Under ideal conditions no water vapor should be present in the oven, but this is impossible to attain in practice. It is difficult to maintain a dry atmosphere in an air oven, and most commercial vacuum ovens are not air-tight. Thus, the discrepancies in results of different investigators have frequently been traced to different humidities in their ovens. Any attempt to reduce the relative humidity by increasing the oven temperature introduces the danger of error from thermal decomposition. 

All the trihalides are mixed valence compounds. PtF3 is isostructural with PdF3- PtX3 (X = Cl, Br, I) cannot be made by straightforward thermal decomposition of PtX4 [22] under open conditions but by routes involving continuous decomposition and formation under closed, equilibrium conditions. 

Young s moduli were determined in tensile tests using samples of 4 mm thickness. Slow cyclic loading (frequency 0.01 Hz) with small strain amplitudes (s < 3%) was used for the tests in order to maintain the thermal equilibrium as much as possible. The temperature range was limited to 260 °C as thermal decomposition became noticeable above this temperature [11],... 

Direct thermal decomposition of methane was carried out, using a thermal plasma system which is an environmentally favorable process. For comparison, thermodynamic equilibrium compositions were calculated by software program for the steam reforming and thermal decomposition. In case of thermal decomposition, high purity of the hydrogen and solidified carbon can be achieved without any contaminant. 

In spite of their relevance to air pollution and to the combustion of sulphur-contaminated fuels, kinetic data for the homogeneous decomposition of S03 are practically non-existent. The thermal decomposition is dominated by a heterogeneous component, even in clean silica vessels390 reactions of S03 at a glass surface are evident even at room temperature380. The kinetics of approach to the equilibrium... 

The fact that the boron trifluoride t-butyl fluoride (and chloride) system and the boron trifluoride isopropyl fluoride systems on thermal decomposition yielded polymer mixtures may indicate the equilibrium in equation (1). On measuring absorption spectra of boron trifluoride—... 

Thermal decomposition of a doubly labelled azidotriazole gave rise to the open-chain triazine (117) in which the labels were not scrambled. Laser flash photolysis of the triazole leads to (117) within 20 ns with no observable intermediate. Ab initio calculations indicate that a dynamic equilibrium between the open-chain triazine (117) and a cyctic nitrene (118), as originally proposed, is highly disfavoured. 

In contrast to the detonation of gaseous materials, the detonation process of explosives composed of energetic solid materials involves phase changes from solid to liquid and to gas, which encompass thermal decomposition and diffusional processes of the oxidizer and fuel components in the gas phase. Thus, the precise details of a detonation process depend on the physicochemical properties of the explosive, such as its chemical structure and the particle sizes of the oxidizer and fuel components. The detonation phenomena are not thermal equilibrium processes and the thickness of the reachon zone of the detonation wave of an explosive is too thin to identify its detailed structure.[i- i Therefore, the detonation processes of explosives are characterized through the details of gas-phase detonation phenomena. 

An important feature of any successful process based on catalytic thermal decomposition is that the system must be kept in an "upset equilibrium" condition since the equilibrium concentration of hydrogen in the presence of sulphur and H2S is low at readily accessible temperatures. By use of a flow system and separation of the reaction products, however, the yield of hydrogen can be markedly improved by utilising the relative kinetics of the forward and reverse reactions. 

Fluorine chemistry in the stratosphere was also considered and it was concluded that ozone depletion by chlorine was > 104 more efficient than that by fluorine (Rowland and Molina, 1975 Stolarksi and Rundel, 1975). Since then, the kinetics of reaction of F atoms with 02 to form the F02 radical and its thermal decomposition have been measured (e.g., see Pagsberg et al., 1987 Lyman and Holland, 1988 Ellerman et al., 1994 and review in DeMore et al., 1997). The equilibrium constant for the F-F02 system... 

Factors which affect the oxepin-benzene oxide equilibrium positions are similarly expected to influence the thiepin-benzene episulfide distribution at equilibrium. Since however the thianorcaradiene tautomer has not to date been detected, the main evidence for this form is based upon the thermal instability and reactions of the thiepin system. Thus it is assumed that where the thianorcaradiene isomer is present, a spontaneous thermal decomposition involving extrusion of a sulfur atom will occur. Substitution at the 2,7-positions in the oxepin-arene oxide system leads to a preference for the seven-membered ring form and this effect was further enhanced by bulky substituents (e.g. Bu ). A similar effect was observed in thiepins and thus the remarkable thermal stability of (49) (2,7-r-butyl groups) and (51) (2,7-hydroxyisopropyl groups) contrasts with the behavior of thiepin (55)(2,7-isopropyl groups), which was thermally unstable even at -70 °C (78CL723). The stability of thiepin (49) results from the 2,7-steric (eclipsed) interactions which obtain in the thianorcaradiene form but which are diminished in the thiepin tautomeric form (relative to the episulfide tautomer). 

At 800° C. reaction (2) appears to proceed slowly in either direction, while (1) is very rapid. The fact that the degree of dissociation in (1) is. independent of the amounts of carbon dioxide and carbon disulphide present, shows that carbon monoxide and sulphur are primary products of the decomposition of carbonyl sulphide and are not formed secondarily from the carbon dioxide and carbon disulphide. At temperatures below 400° C. decomposition according to equation (1) is not evident, while at 900° C. it reaches a maximum (64 per cent.) reaction (2) reaches a maximum at about 600° C., at which point 43 per cent, of the carbonyl sulphide is decomposed in this way and 16 per cent, according to reaction (1). The carbon monoxide equilibrium depends upon the pressure, whilst the carbon dioxide equilibrium does not. Nearly all the reactions involved in the thermal decomposition of carbonyl sulphide depend greatly on catalytic influences. Quartz is a pronounced catalyst for reaction (2), but has little influence on reaction (1). Carbonyl sulphide is comparatively rapidly decomposed in quartz vessels, but is stable when kept in glass apparatus. The viscosity of gaseous carbonyl sulphide is as follows x... 

The high activation energies resulting from the chemisorption hypothesis appear to favor its validity. However, it is possible that such a high activation energy could be caused by an equilibrium between the undecomposed coal molecules and the diffusion species resulting from thermal decomposition. That is, for diffusion in the Z dimension only, a simplified mass balance would lead to a differential equation of the form... 

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