Runaway reaction with autocatalytic reactions

DSC can be used effectively in the isothermal mode as well. In this case, the container with the sample is inserted into the DSC preheated to the desired test temperature. This type of experiment should be performed to examine systems for induction periods that occur with autocatalytic reactions and with inhibitor depletion reactions. (Reactions with induction periods can give misleading results in the DSC operated with increasing temperature scans.) Autocatalytic reactions are those whose rates are proportional to the concentration of one or more of the reaction products. Some hydroperoxides and peroxy esters exhibit autocatalytic decomposition. Inhibitor depletion can be a serious problem with certain vinyl monomers, such as styrene and acrylic acid, that can initiate polymerization at ambient temperatures and then selfheat into runaways. Isothermal DSC tests can be used to determine a time to runaway that is related to the inhibitor concentration. 

If one considers the case of adiabatic runaway, these two types of reaction lead to totally different temperature versus time curves. With nth-order reactions, the temperature increase starts immediately after cooling failure, while with autocatalytic reactions the temperature remains stable during the induction period and suddenly increases very sharply, as shown in Figure 12.2. 

While subtle differences between gels synthesized with different initiators may exist, they are not easy to anticipate. The most important concern is usually the polymerization rate induced by a given initiator concentration [7], Polymerization reactions are highly exothermic, so a fast initiation rate can lead to a rapid temperature increase since the initiation and polymerization rate both increase rapidly with temperature, this process becomes autocatalytic. Poor quality, irre-producible gels result on a production scale such a runaway reaction could be-... 

Thermal runaway is an event that occurs when the battery electrode s reaction with the electrolyte becomes self-sustaining and the reactions enter an autocatalytic mode. This situation is responsible for many safety incidents and fires associated with battery operations. 

A systematic study of the validity of such a procedure was performed in collaboration with ETH-Ziirich [15], The validation of the procedure was based on numerical simulations of dynamic experiments and adiabatic runaway curves. These simulations were carried out using different rate equations nth-order, consecutive, branched, and autocatalytic reactions. Moreover, the results were compared to experimental results obtained with over 180 samples of single technical chemical compounds, reactions masses, and distillation residues [17] (Figure 11.8). Thus, they are representative for industrial applications. The line corresponding to this rule (Equation 11.5) is also represented (full line) in Figure 11.8. All experimental points lie above the line and the safety margin remains reasonable. Thus, the method is conservative, but delivers a reasonable safety margin. 

This has essential consequences for the design of emergency measures. A technical measure to prevent a runaway could be a temperature alarm set at, for example, 10 K above the process temperature. This works well with nth-order reactions, where the alarm is activated at approximately half of the TMRld. However, autocatalytic reactions are not only accelerated by temperature, but also by time. This can lead to a sharp temperature increase. In the case shown in Figure 12.2, a temperature alarm is not effective, because there is no time left to take measures in the example given, only a few minutes are left before runaway. Therefore, it is important to know if a decomposition reaction is of autocatalytic nature or not that is, the safety measures must be adapted to this type of reaction. 

Thanks to its versatility, this model has proved to describe a great number of autocatalytic reaction systems [5]. Systems with a slow initiation reaction are called strong autocatalytic. Because the rate of the initiation reaction is low, product is formed slowly, leading to a long induction time under isothermal conditions. For such systems, the initial heat release rate is low or practically zero. Consequently, the reaction may remain undetected for a relatively long period of time (Figure 12.4). When the reaction accelerates, such an acceleration appears suddenly and may lead to runaway. A strong autocatalytic reaction is formally equivalent to a Prout-Tompkins mechanism. 

The presence of excessive amounts of water in the form of a separate water phase will in many cases increase solvent-induced corrosion of metals. The presence of low molecular weight alcohols added to 1,1,1-Tri can cause increased metal corrosion [2-4]. The presence of aromatic hydrocarbons (e.g., toluene or xylene) in methylene chloride formulations in contact with aluminum can cause catastrophic release of gaseous hydrogen chloride. The trace amounts of aluminum chloride produced by the normally very slow aluminum-solvent reaction acts as a catalyst for the Friedel-Crafts reaction between the aromatic hydrocarbon and methylene chloride. The resultant reaction produces hydrogen chloride which reacts with aluminum to give more aluminum chloride and resulting in a runaway autocatalytic reaction. The addition of an acid... 

The key problems in a polymerization CSTR are the determination and characterization of micro- and macromixing, and the possibility of multiple steady states due to the exothermic nature of the reactions. Recent studies of CSTRs for bulk or solution free-radical polymerization indicate the possibility of multiple steady states due to the large heat evolution and difficult heat transfer that are characteristic of the reactors. Furthermore, even in simple solution polymerization (for example, in methyl methacrylate polymerization in ethyl acetate solvent), autocatalytic kinetics can lead to runaway conditions even with perfect temperature control for certain combinations of solvent concentration and reactor residence time. In practice, the heat evolution can be an additional source of autocatalytic behavior. 

Many acylation reactions of esters using sodimn hydride as base appear autocatalytic, with considerable potential for runaway, since the active base in solution is an alkoxide and the alcohol is a product of reaction [4]. A safe form of sodium hydride (as a solid solution in a halide) for large-scale industrial use has been claimed [3]. 

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