Adiabatic induction period

Frank-Kamenetskii also tried to express his adiabatic induction period, [19]. It is expressed as... 

Adiabatic induction time Induction period or time to an event (spontaneous ignition, explosion, etc.) under adiabatic conditions, starting at operating conditions. 

DTA examination of a 35% solution of the diazonium salt in sulfuric acid showed 3 exotherms, corresponding to hydrolysis of the nitrile group (peak at 95°), decomposition of the diazonium salt (peak at 160°) and loss of the nitro group (large peak at 240°C). Adiabatic decomposition of the solution from 50°C also showed 3 steps, with induction periods of around 30, 340 and 380 min, respectively. 

It is somewhat endothermic (AH°f (g) +87.5 kJ/mol, 1.0 kJ/g), the liquid may explode on pouring or sparking at 2°C, and the gas readily explodes on rapid heating or sparking [1,2], on adiabatic compression in a U-tube, or often towards the end of slow thermal decomposition. Kinetic data are summarised [3], The spontaneously explosive decomposition of the gas was studied at 42-86°C, and induction periods up to several hours were noted [4], Preparative precautions have been detailed [5],... 

A survey, with many references, of 14 classes of preparative reactions involving hydrogen peroxide or its derivatives emphasises safety aspects of the various procedures [11]. Following the decomposition of 100 1 of 50% aqueous hydrogen peroxide which damaged the 630 1 stainless vessel rated at 6 bar, the effect of added contaminants and variations in temperature and pH on the adiabatic decomposition was studied in a 1 1 pressure vessel, where a final temperature of 310°C and a pressure around 200 bar were attained. Rust had little effect, but addition of a little ammonia (pH increased from 1.8 to 6.0) caused the induction period to fall dramatically, effectively from infinity to a few h at 40°C and a few min at 80°C. Addition of sodium hydroxide to pH 7.5 reduced the induction period at 24°C from infinity to about 4 min [12],... 

DTA shows that the reaction mixture from sulfonation of the nitro compound in 20% oleum, containing 35 wt% of 2-chloro-5-nitrobenzenesulfonic acid, exhibits 2 exothermic stages at 100 and 220° C, respectively, the latter being violently rapid. The adiabatic reaction mixture, initially at 89° attained 285°C with boiling after 17 h. At 180° the induction period was about 20 min [1], Sulfonation of 4-chloronitrobenzene with 65% oleum at 46°C led to a runaway decomposition... 

Under these conditions, the time within which a given value of 9 is attained is proportional to the magnitude tx. Consequently, the induction period in the instance of adiabatic explosion is proportional to tx. The proportionality... 

Shock waves are an ideal way of obtaining induction periods for high-temperature—high-pressure conditions. Since a shock system is nonisentropic, a system at some initial temperature and pressure condition brought to a final pressure by the shock wave will have a higher temperature than a system in which the same mixture at the same initial conditions is brought by adiabatic compression to the same pressure. Table 7.1 compares the final temperatures for the same ratios of shock and adiabatic compression for air. 

The use of a normal adiabatic calorimeter is not ideal when the reaction studied has an induction period as in reaction 3 or when a reaction has to be initiated by breaking an ampoule as in reaction 4 or 5. Much more convenient and reliable is the use of a steady-state heat flow calorimeter. The method used in References 13 and 14 is described here. 

Integration of this equation can be readily obtained for the case where the heat conduction term can be neglected (adiabatic approximation). This is not too bad an approximation, as T — T0 is small during the induction period. With this approximation in mind, Equation 23 becomes... 

Ignition delays, investigated in adiabatic compression machines (89, 91, 92, 106, 182, 209, 212, 213), have been correlated with knock (90, 93-5, 111, 158, 160, 194, 203). Ignition occurs in two stages. Levedahl (106) examined the effects of temperature, density, and fuel type on the induction periods, ti and r2, corresponding to each ignition stage. A close correlation existed between total delay, r, and knock resistance. Sensitivity was explained in terms of the relative partial contributions of n and r2 to r. 

It is known that ignition and expln of dangerous coal mine atmosphere depends on many factors, such as compn of mine atm, wt of charge of expl, induction period (time elapsed betw expln of charge and ignition of mine atm), temp of products of expln, presence of hot solid particles in products of expln, duration of flame, heating of mine atm due to adiabatic compression created by incident and reflected shock waves, etc. 

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. 

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