Chain reaction acceleration

The amount of fissionable matter present determines whether a nuclear chain reaction can be sustained. In a subcritical mass, the chain reaction stops because neutrons escape the sample before causing sufficient fissions to sustain the reaction, o In a supercritical mass, the chain reaction accelerates as neutrons cause more and more fissions to occur. 

One fission reaction can lead to more fission reactions, a process called a chain reaction. A chain reaction can occur only if the starting material has enough mass to sustain a chain reaction this amount is called critical mass. With a subcrifical mass, the chain reaction stops or never begins. With a supercritical mass, the chain reaction accelerates and can lead to a violent explosion. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Above about 250°C, the vapor-phase oxidation (VPO) of many organic substances becomes self-sustaining. Such oxidations are characterized by a lengthy induction period. During this period, peroxides accumulate until they can provide a source of new radicals to sustain a chain reaction. Once a critical threshold peroxide concentration is reached, the reaction accelerates very rapidly. 

A few details have been reported on the slow reductions of 8303 by As(ni) and T1(I) . In the anaerobic reductions by As(III) the reaction is first-order in 820 and although As(III) certainly catalyses decomposition, the dependence of the rate on [As(III)] is small. Aeration leaves the rate of spontaneous decomposition of 820g unaffected, but the As(III)-catalysed route is accelerated by a factor of ten, the kinetic law remaining unchanged. The oxygen effect is interpreted in terms of the chain reaction... 

The accumulation of hydroperoxides and their subsequent decomposition to alkoxyl and peroxyl radicals can accelerate the chain reaction of polyunsaturated fatty-acid p>eroxidation leading to oxidative damage to cells and membranes as well as lipoproteins. It is well-recognized that transition metals or haem proteins, through their... 

Catalysts that decrease reaction rates are usually referred to as inhibitors. They usually act by interfering with the free radical processes involved in chain reactions, and the mechanism differs from that involved in accelerating a reaction. The most familiar example of the use of inhibitors is the addition of tetraethyl lead to gasoline to improve its antiknock properties. 

Oxidation of organic compounds by dioxygen is a phenomenon of exceptional importance in nature, technology, and life. The liquid-phase oxidation of hydrocarbons forms the basis of several efficient technological synthetic processes such as the production of phenol via cumene oxidation, cyclohexanone from cyclohexane, styrene oxide from ethylbenzene, etc. The intensive development of oxidative petrochemical processes was observed in 1950-1970. Free radicals participate in the oxidation of organic compounds. Oxidation occurs very often as a chain reaction. Hydroperoxides are formed as intermediates and accelerate oxidation. The chemistry of the liquid-phase oxidation of organic compounds is closely interwoven with free radical chemistry, chemistry of peroxides, kinetics of chain reactions, and polymer chemistry. 

Dichlorine shortens the induction period of autoxidation of paraffin wax [187] and accelerates the oxidation of hydrocarbons [109]. Difluorine is known as very active initiator of gas-phase chain reactions, for example, chlorination [188,189]. 

PINO possesses a high reactivity in the reaction with the C—H bond of the hydrocarbon. Hence, the substitution of peroxyl radicals to nitroxyl radicals accelerates the chain reaction of oxidation. The accumulation of hydroperoxide in the oxidized hydrocarbon should decrease the oxidation rate because of the equilibrium reaction. 

An experimental activation energy which seems to be too low from a theoretical point of view may be caused by an early acceleration due to a chain reaction. However, it is not clear what mechanism would be operative in this case. The possibility that the intermediate formation of hydrazine plays a role cannot be fully excluded. 

Some reactions proceed explosively. The explosion are of two types (i) thermal explosion and (ii) explosion depends on chain reaction. The basic reason for a thermal explosion is the exponential dependence of reaction rate on the temperature. In an exothermic reaction, if the evolved energy cannot escape, the temperature of the reaction system increases and this accelerates the rate of reaction. The increase in reaction rate produces heat at an even greater rate. As the heat cannot escape, hence the reaction is even faster. This process continues and an explosion occurs. 

In contrast to the polyacrylamide homopolymers typical of CE,Fujimoto et al. incorporated charged functionalities into the neutral polyacrylamide chains to accelerate the migration of neutral compounds through a capillary column [90]. Despite this improvement, nearly 100 min were required to effect the separation of acetone and acetophenone, making this approach impractical even with the use of high voltage. Alternatively, Tanaka et al. [86] alkylated commercial poly-allylamine with alkyl bromides, followed by a Michael reaction with... 

The idea is to use a particle accelerator producing neutrons by spallation to feed a fuel/moderator assembly where the neutrons multiply by fission chain reactions. 

The homologous (Me3Si)2Si(H)Me does not react spontaneously with air or molecular oxygen at room temperature. However, a reaction takes place at 80 °C when air or molecular oxygen is bubbled into the pure material or its solutions to form a major product that contains the siloxane chain (Reaction 8.5) [15]. In general, yields of siloxane are about 50%. Also in this case the reaction is accelerated by radical initiation and retarded by common radical inhibitors. 

If ki and k.i are much larger than kj, the reaction Is controlled by kj. If however, ki and k.i are larger than or comparable to kz, the reaction rate becomes controlled by the translational diffusion determining the probability of collisions which Is typical for specific diffusion control. The latter case Is operative for fast reactions like fluorescence quenching or free-radical chain reactions. The acceleration of free-radical polymerization due to the diffusion-controlled termination by recombination of macroradicals (Trommsdorff effect) can serve as an example. 

Although sulfur vulcanization has been studied since its discovery in 1839 by Goodyear, its mechanism is not well understood. Free-radical mechanisms were originally assumed but most evidence points to an ionic reaction [Bateman, 1963]. Neither radical initiators nor inhibitors affect sulfur vulcanization and radicals have not been detected by ESR spectroscopy. On the other hand, sulfur vulcanization is accelerated by organic acids and bases as well as by solvents of high dielectric constant. The ionic process can be depicted as a chain reaction involving the initial formation of a sulfonium ion (XI) by reaction of the polymer with polarized sulfur or a sulfur ion pair. The sulfonium ion reacts with a polymer molecule by hydride... 

The reaction cannot begin without initiation of See radicals, and H2 dissociation is very slow. Therefore, the few H atoms produced by the initiation steps may diffuse to the walls of the vessel and recombine before they can begin chain reactions. However, the presence of H2O accelerates the rate of the reaction because it causes formation of traces of H2O2, which easily dissociates and forms more H and -OH, which initiate the reaction by attack of H2 and O2, and these reactions rapidly produce more radicals, which strongly accelerate the process. 

Most combustion reactions involve chain branching reaction steps. Under conditions where these steps are less significant than linear chain reactions, the reaction appears to be stable, but when the chain branching steps dominate, the overall reaction rate can accelerate uncontrollably. 

We have previously encountered examples of chemical autocatalysis, where the reaction accelerates chemically such as in enzyme-promoted fermentation reactions, which we modeled as A + B 2B because the reaction generates the enzyme after we added yeast to initiate the process. The other example was the chain branching reaction such as H. -I-O2 —> OH - -0 just described in hydrogen oxidation. The enzyme reaction example was nearly isothermal, but combustion processes are both chain branching and autothermal, and therefore they combine chemical and thermal autocatalysis, a tricky combination to maintain under control and of which chemical engineers should always be wary. 

A chain reaction process is composed of a sequence of reaction steps whose rates can vary by large factors. Some of these reaction steps slow the overall process, and some accelerate it, and some of these slow steps can have alternate paths. 

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