Localized exothermic reaction zone

Thermal frontal polymerization is a mode of converting monomer into polymer via a localized exothermic reaction zone that propagates through the coupling of thermal diffusion and the Arrhenius reaction kinetics of an exothermic polymerization. We review the range of nonlinear phenomena that have been observed in frontal polymerization systems and report new results on the role of gravity in spin modes and the development of spherically-propagating fronts. 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

The transition from the liquid- to the gas-phase reaction regime is often accompanied by a marked increase in the reaction rate, because the gas phase surrounding the catalyst pellet offers less mass-transfer resistance than the liquid phase. For the case of an exothermic reaction, this may have an undesirable effect, as it gives rise to a rather narrow reaction zone with steep temperature gradients. Thus, the catalyst may be exposed to local overheating, which results in subsequent deactivation of the bed or the occurrence of a number of undesirable side reactions. Furthermore, if the heat removed is insufficient, the hot-spot temperature could occur. 

A flame may be defined as a localized reaction zone which is able to propagate itself sub-sonically through the material supporting it. Most flames are concerned with exothermic reactions of this type, in which typically reactants at near ambient temperatures are converted more or less adiabatically to combustion products at 1000 K or above. Detailed kinetic studies have principally been confined to premixed flames, in which a well-defined reactant mixture at a known initial temperature is converted into combustion products in full chemical equilibrium at the final flame temperature. Assuming adiabatic combustion, the final conditions may be calculated thermodynamically. 

Once induced, additional energy is not necessary for propagation of polymerization due to highly exothermic nature of the polymerization. Therefore, many polymers can be quickly synthesized by using FP characterized by the localized reaction zone and the fast increasing temperature. 

The product mixture which exists in a particular plane in the reaction zone behind the detonation front obeys the Chapman-Jouguet (C-J) hypothesis. In essence, the C-J or sonic plane differentiates between the part of the reaction zone where the detonative decomposition is completed (and exothermic reactions supply energy with the local speed of sound to the detonation front) and that part in which further energy release due to reactions among the products is not supplied sufficiently rapidly to maintain steady wave propagation. 

Thermal frontal polymerization is a process in which a localized reaction zone propagates from the coupling of thermal diffusion and the Arrhenius dependence of reaction rate of an exothermic polymerization. Thermal frontal polymerization was discovered at the Institute of Chemical Physics in Chernogolovka, Russia by Chechilo and Enikolopyan. They studied methyl methacrylate polymerization under 3500 atm pressure. (We will consider later why these extreme conditions were used.) The literature from that Institute was reviewed in 1984. Pojman rediscovered what he called traveling fronts of polymerization in 1991. Pojman et a . reviewed the field in 1996. There have been other focused reviews. ... 

A runaway exotherm in a flow reactor can occur locally if there are lateral irregularities and insufficient mixing of adjacent portions of the reactants. It can also occur globally, in which situation all of the fluid passes through the runaway zone before it leaves the reactor. This latter type of runaway reaction is the subject of this paper. 

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