Thermal autocatalysis

Heat generation by exothermic combustion reaction can also accelerates combustion processes (as it also does slightly for fermentation), and we call this thermal autocatalysis which will be discussed more in Chapters 5, 6, and 10. 

The preceding situation is an example of thermal autocatalysis, and its consequence is fiequently a thermal explosion. 

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. 

Because of these factors, chain reactions are inherently unpredictable. Chemical and thermal autocatalysis make the overall rate r (Cj, T) not a simple function. Chain reactions can also be very fast so that the reaction may be limited by mass transfer processes. 

Synthetic polymer systems can exhibit feedback through several mechanisms. The simplest is thermal autocatalysis, which occurs in any exothermic reaction. The reaction raises the temperature of the system, which increases the rate of reaction through the Arrhenius dependence of the rate constants. In a spatially distributed system, this mechanism allows propagation of thermal fronts. Free-radical polymerizations are highly exothermic. 

Frontal polymerization discovered in 1972 (5) could be realized in free-radical polymerization because of its nonlinear behavior. If the top of a mixture of monomer and initiator in a tube is attached to an external heat source, die initiators are locally decomposed to generate radicals. The polymerization locally initiated is autoaccelerated by the c(xnbinatipropagating front, is thus formed. Pojman et al. extensively studied the dynamics of frontal polymerization (d-P) and its applicatim in matoials syndiesis (I -I3). 

Both the Trommsdorff effect and thennal autocatalysis can lead to the autoacceleration in free-radical polymerization. The results indicated that the autoacceleration observed in the smallest test tube was mostly due to the Trommsdorff effect, while both the Trommsdorff effect and thermal autocatalysis strongly affected the onset of autoacceleration in the larger polymerization system. When an exothermic reaction is performed in a larger system, more heat tends to be accumulated in a reactor, since a sur ce-to-volume ratio is decreased as the size of the system increases. There was a critical size in the inner diameter of the test tube at which the behavior of the autoacceleration of the polymerization changes. 

Nonlinear phenomena in any system require some type of feedback. The most obvious source of feedback in polymerization reactions is thermal autocatalysis, often called thermal runaway in the engineering literature. The heat released by the reaction increases the rate of reaction, which increases the rate of heat release, and so on. This phenomenon can occur in almost any reaction and will be important when we consider thermal frontal polymerization. 

Phenomenon of a thermal autocatalysis could be one of the reasons for an unstable operation of the reactor. The simplest model to give thermokinetic oscillations in a weU-stirred continuous reactor is... 

The effect of local activation accounts for such features of PCSs as catalytic and stabilizing properties, autocatalysis, specific properties of thermal degradation, structural modification, and a number of other phenomena typical of polymers with a system of conjugated bonds. 

Explanation of Principal Application Codes 1 = screening 6 = reaction due to oxidation 2 = thermal stability 7 = runaway behavior (initial phase) 3 = sensitive thermal stability 8 = complete runaway behavior and 4 = very sensitive thermal stability simultaneous pressure measurements 5 = study autocatalysis, contaminations, 9 = time to maximum rate of reaction inhibitor depletion ... 

Induction time effects (autocatalysis) e.g., the development of thermal instability after prolonged storage... 

Autocatalysis happens when a reaction product, formed during reaction, acts as a catalyst which accelerates the progress of the reaction even at constant temperature. An example is the acid-catalysed saponification of various esters and related compounds. Autocatalytic reactions can be easily experimentally identified by means of differential thermal analysis methods. 

Mixtures of natural gas and air are similarly unpredictable, being quite stable and unreactive until a spark or a hot surface ignites reaction, and then thermal and chain branching autocatalysis takes off and so does the building. 

The best inhibitor of autocatalysis is to have a small system. Surfaces act as both sinks of fi ee radicals to increase termination and as sinks of heat to prevent thermal runaway, and small systems have large surface-to-volume ratios to prevent these runaway possibilities. 

Cook (Ref 1), in describing thermal decomposition of some HE s conducted in the quartz spring apparatus (described in Ref 1, p 175 and shown there in Figs 8.1a 8.1b), stated that PETN, RDX, Tetryl and to a small extent TNT decomposed autocatalyti-cally. EDNA followed the first-order decomposition law only until about 5% of the explosive had decomposed and then the reaction stabilized. The term autostabilization was applied here on the supposition that one of the condensed decomposition products of EDNA which accumulated in the explosive apparently tended to stabilize the bulk of expl and thus slow down the decomposition. After about 10% of the expl had decompd, however, the "autocatalysis developed. 

This chapter and chapter 5 study the prototypical thermokinetic oscillator. Thermal feedback replaces autocatalysis, and the Arrhenius temperature dependence of rate coefficients supplies non-linearity in the scheme P - A - B + heat. After careful study of this chapter the reader should be able to ... 

The CSTR is, in many ways, the easier to set up and operate, and to analyse theoretically. Figure 6.1 shows a typical CSTR, appropriate for solution-phase reactions. In the next three chapters we will look at the wide range of behaviour which chemical systems can show when operated in this type of reactor. In this chapter we concentrate on stationary-state aspects of isothermal autocatalytic reactions similar to those introduced in chapter 2. In chapter 7, we turn to non-isothermal systems similar to the model of chapter 4. There we also draw on a mathematical technique known as singularity theory to explain the many similarities (and some differences) between chemical autocatalysis and thermal feedback. Non-stationary aspects such as oscillations appear in chapter 8. 

Chapter 6 considered isothermal autocatalysis in an open system here we study a classic case of thermal feedback. A rich variety of stationary-state patterns (bifurcation diagrams) are generated and considered here alongside those of the previous isothermal example. Flow diagrams are again illuminating and singularity theory provides a systematic approach. After study a reader should be able to ... 

In chapters 2-5 two models of oscillatory reaction in closed vessels were considered one based on chemical feedback (autocatalysis), the other on thermal coupling under non-isothermal reaction conditions. To begin this chapter, we again return to non-isothermal systems, now in a well-stirred flow reactor (CSTR) such as that considered in chapter 6. 

Over the last 10-15 years, interest has grown significantly in the kinetics of combustion and explosion reactions, which are characterized by the presence of some mechanism of acceleration of the reaction. This acceleration, which leads to ignition, may be related either to the accumulation of active products which catalyse the reaction, the chain carriers (autocatalysis, chain explosion), or to an increase in the temperature of the mixture due to heat release in an exothermic chemical reaction (thermal explosion). 

One of the important limitations in the use of DSC for the study of expls is that decompn is often accompanied by, or is a consequence of, melting or sublimation. Data analysis of such systems results in kinetic orders which have no significance. The problem was examined by Rogers (Ref 32) who noted that organic expls decomp normally more rapidly in the melt and, therefore, show very high apparent activation energies and preexponential factors, and that, therefore, compds which decomp without autocatalysis decomp in a DSC at a rate which is max when the melt is complete. For this reason Rogers used only the data above the ATmax peak. He performed the decompn iso thermally and ob-... 

For the wet series, none of the models tried is correct. We attribute this to an inhibition by water at the beginning of the peak (Fig. 11.14b) followed by a reduction process exhibiting a runaway character due to autocatalysis or thermal effect. 

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