Sustainable exothermic reactions

The Hargreaves process (late 19th C) is a variant of the salt-cake process in which NaCl is reacted with a gaseous mixture of SO2, air and H2O (i,e, H2SO4 ) in a self-sustaining exothermic reaction ... 

When a mixture of Fe203 powder and A1 powder is ignited, a self-sustaining exothermic reaction occurs according to ... 

Reactions such as the above have been used for the large-scale commercial production of WC by Kenna-metal, Inc. in a process called the Macro Process [28]. Thermodynamics for the analogous molybdenum reaction show that the reaction is highly exothermic (Table 3). The solid reactants are metered into a carbon-lined kiln to develop a self-sustained exothermic reaction. The reaction occurs in a pool of molten metal at temperatures above 2800 K. At the end of the run the kiln contents solidify into a lower heavy layer of carbide crystals and an upper slag layer of oxides. 

Historically, combustion synthesis (both SHS and VCS) is a direct descendant of classic works on combustion and thermal explosion (e.g.. Mallard and Le Chatelier, 1883 Semenov, 1929 Zeldovich and Frank-Kamenetskii, 1938 Williams, 1965 Glassman, 1977) see Hlavacek (1991) and Merzhanov (1995) for additional comments in this regard. We discuss later in Section IV how the theory of SHS grew directly from these works. The progress in combustion science made it possible to organize self-sustained exothermic reactions in powder mixtures that were controllable and predictable, hence avoiding the uncontrollable evolution of the reaction that is commonly associated with the terms combustion, fire, and explosion. 

In general combustion takes place if a flammable material enters into contact with an energy source, e.g. an electrical spark or a hot surface, and thus receives energy. If solid or liquid materials are concerned their temperature has to be raised first to such an extent that vapour is produced by vaporization or disintegration. These vapours can form flammable mixtures with air just as flammable gases. If the energy supply is sufficient a self-sustaining exothermic reaction occurs. 

The initial exotherm temperature is the temperature at which a self-sustaining exothermic reaction is detected in the sample under the specific experimental conditions. An exotherm is indicated by a sustained increase in the differential... 

Typically, SCS involves a self-sustained exothermic reaction in a solution of metal nitrates (also known as oxidizer) and a reducing agent (fuels, e g. glycine, urea, hydrazine, glucose etc ). The reaction between fuel and oxidizer provides an optimum temperature and reaction enviromnent for synthesizing crystalline materials. The stoichiometric equiUbrium combustion reaction is usually described by the following scheme ... 

Detonation explosions are similar to combustion explosions and are exothermic reactions that proceed into the unreacted material at a velocity much greater than the speed of sound in an unreacted material and are accompanied by a flame front shock wave in the material followed closely by a combustion wave that releases the energy and sustains the shock wave at extremely high pressure [39] [40]. In hydrocarbons, the velocity can reach 6,000-9,000 ft/sec. 

More recently, Rosen (R3), Spalding (S5), and Johnson and Nachbar (J4) have considered a simplified approach using the analysis of laminar-flame propagation velocities. According to these investigators, the principal exothermic reactions occur in the gas phase. Some of the heat liberated by these reactions is then transferred back to the solid surface to sustain the endothermic surface-gasification processes. Thus, the temperature profile within the reactive zone is quite similar to that of Rice and Crawford. However, gasification of the solid surface is assumed to be endothermic, while exothermic reactions were considered in the studies discussed previously. 

The burning mechanism of composite propellants differs from that described above. There is no exothermic reaction which can lead to a self-sustaining fizz zone. Instead, the first process appears to be the softening and breakdown of the organic binder/fuel which surrounds the ammonium perchlorate particles. Particles of propellant become detached and enter the flame. The binder is pyrolysed and the ammonium perchlorate broken down, initially to ammonia and perchloric acid. The main chemical reaction is thus in the gas phase, between the initial dissociation products. 

Molecules containing two or more chlorine atoms may be produced by the reaction of chlorine atoms or molecules with products generated in the earlier stage of the process. Product yields depend on irradiation conditions and can reach as high as 105 pmol.J-1. With bromine and iodine, not all of the individual steps of the reaction are exothermic. Therefore, a sustained chain reaction is not expected, and the yields are low. 

Combustion Self-sustained, exothermic, rapid oxidation reaction of explosive substance and propellant. 

Intermetallic compound formation may be observed as the result from the diffusion across an interface between the two solids. The transient formation of a liquid phase may aid the synthesis and densification processes. A further aid to the reaction speed and completeness may come from the non-negligible volatility of the component(s). An important factor influencing the feasibility of the reactions between mixed powders is represented by the heat of formation of the desired alloy the reaction will be easier if it is more exothermic. Heat must generally be supplied to start the reaction but then an exothermic reaction can become self-sustaining. Such reactions are also known as combustion synthesis, reactive synthesis, self-propagating high-temperature synthesis. 

Fire is a self-sustaining, exothermic oxidation-reduction reaction. The fire reaction usually involves oxygen which forms the oxides of the fuel. The most important examples in petrochemical and hydrocarbon processing facilities are combustion reactions of hydrocarbons with oxygen. 

Three types of polymeric materials are used inert polymers, active polymers, and azide polymers. No exothermic heat is produced when inert polymers are decomposed thermally. On the other hand, exothermic reactions occur when active polymers and azide polymers are decomposed. Self-sustaining burning is possible when active polymers and azide polymers are ignited. 

Compatibility Group G comprises any substance which is an explosive substance because it is designed to produce an effect by heat, light, sound, gas or smoke, or a combination of these as a result of non-detonative, self-sustaining, exothermic chemical reactions, or an article containing such a substance or an article containing both a substance which is explosive because it is capable by chemical reaction in itself of producing gas at such a temperature and pressure. 

PYROTECHNIC COMPOSITION Can be a substance or a mixture of substances used to produce effects (e.g. light, sound or smoke) as a result of non-detonative, self-sustaining exothermic chemical reactions. 

Typically, ATR reactions are considered to be thermally self-sustaining and therefore do not produce or consume external thermal energy. In fact, since ATR consists of the combination of an exothermic reaction (CPO) which produces heat, with an endothermic reaction (CSR) where heat must be externally generated to the reformer, the balance of the specific heat for each reaction becomes a very distinctive characteristic of this process. This makes the whole process relatively more energy efficient since the heat produced from CPO can transfer directly to be used by CSR. However, other exothermic reactions may simultaneously occur, such as WGS and methanation reactions. 

Gadolinium is produced from both its ores, monazite and bastnasite. After the initial steps of crushing and beneficiation, rare earths in the form of oxides are attacked by sulfuric or hydrochloric acid. Insoluble rare earth oxides are converted into soluble sulfates or chlorides. When produced from monazite sand, the mixture of sand and sulfuric acid is initially heated at 150°C in cast iron vessels. Exothermic reaction sustains the temperature at about 200 to 250°C. The reaction mixture is cooled and treated with cold water to dissolve rare earth sulfates. The solution is then treated with sodium pyrophosphate to precipitate thorium. Cerium is removed next. Treatment with caustic soda solution fohowed by air drying converts the metal to cerium(lV) hydroxide. Treatment with hydrochloric or nitric acid sol-... 

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