Critical explosion temperature

For E/RTo in the usual range near 35, this gives — To = To/35, or about 10 C if To is about 360°K. Thus these critical explosion temperatures are not much higher than those present in the gradients (T , Fig. XIV. 1) which are normally set up in exothermic reactions. 

Rice (Zoc. cit,) has shown how to integrate this equation numerically. However, if we take advantage of the fact that y varies from 0 at < = 0 to / = 1 at the critical explosion temperature [Eq. (XIV.3.4)], we can approximate ey over the range by the linear function 1 + (e — 1)//, so that the equation becomes... 

It is, however, possible to induce explosions in these systems by the use of additives which are frequently referred to as sensitizers. Thus Ashmore has shown that the addition of 0.5 mm Ilg of NO to 50 mm Ilg of an equimolar mixture of H2 + CI2 lowers the critical explosion temperature from 400 to 270°C. The explosion in this case is still, however, a thermal explosion, and it has been shown that the lowering of the explosion temperature was produced by an increase in the concentration of Cl atoms, not by a change in the chain mechanism. This increase in concentration of Cl atoms was produced by the replacement of the slow, high-activation-energy initiation reaction, M + CI2 2C1 + M(E > 57 Real), by the much-lower-activation-energy reaction, NO + CI2 NOCl + C1(jE = 22 Real). 

Figure 5. Effect of crystal thickness on the critical explosion temperature of lead azide [21].

Figure 1. Critical explosion temperature, Tcr vs. crystal thickness. The dotted line is the theoretical curve. The boxed area is for small crystals which did not explode even up to the highest temperature tested (825 K) [17]...

The explosive decomposition of the solid has been studied in detail [6], The effect of moisture upon ignitibility and explosive behaviour under confinement was studied. A moisture content of 3% allowed slow burning only, and at 5% ignition did not occur [7], Thermal instability was studied using a pressure vessel test, ignition delay time, TGA and DSC, and decomposition products were identified [8], The presence of acyl chlorides renders dibenzoyl peroxide impact-sensitive [9], There is a further report of a violent explosion during purification of the peroxide by Soxhlet extraction with hot chloroform [10], Residual traces of the peroxide in a polythene feed pipe exploded when it was cut with a handsaw [11]. The heat of decomposition has been determined as 1.39 kJ/g. The recently calculated value of 69° C for critical ignition temperature coincides with that previously recorded. 

UOP FCC unit, 11 700-702 UOP/HYDRO MTO process, 18 568 UOP Olex olefin separation process, 17 724 Up-and-Down Method, 25 217 U/Pb decay schemes, 25 393-394 Updraft sintering, 26 565 Upflow anaerobic sludge blanket (UASB) in biological waste treatment, 25 902 Upgraded slag (UGS), 25 12, 33 Upland Cotton, U.S., 8 13 U-Polymer, 20 189 Upper critical solution temperature (UCST), 20 320, 322 Upper explosive limit (UEL), 22 840 Upper flammability limit, 23 115 Upper flammable limit (UFL), 22 840 Upper Freeport (MVB) coal... 

In practice one measures explosion limits by permitting a reaction mixture (of fixed composition) to enter a flask at a previously determined higher temperature and observing the minimum pressure at which explosion takes place. If the rate law for the reaction is known (actually this is seldom true) then any of the above equations can be tested. In this way Sagulin showed that the critical explosion pressures for H2 + CI2 mixtures followed an equation of the form... 

The most extensive test which has been made of this conduction model for thermal explosion is to be found in the work of Vanp e on the explosion of CH2O + O2 mixtures. He used a calibrated thread of 10 per cent Rh-Pt alloy of 20 m diameter (jacketed by a 50-m quartz sleeve) suspended at the center of a cylindrical vessel to measure directly his reaction temperature during the induction periods preceding explosion. By Uvsing He and Ar as additives and vessels of different diameters he was able to verify the dependence of the critical explosion limits on vessel size and on thermal conductivity of the gas mixture. In addition, he was able to check the maximum predicted temperature at the center of the vessel just prior to explosion and also the value of 8c = 2 [Eq. (XIV.3.12)], the critical explosion parameter for cylindrical vessels. Finally, with a high-speed camera, he was able to show directly that the explosions in this system do start at the center, the hottest region, " and propagate to the walls. 

The critical pre-explosion temperature rise was calculated in accordance with the equation, A To- — RT[Pg.154]

The hydroxylation theory has been criticized also by Callendar 16 on the basis of the necessity for splitting of the oxygen molecule, a step not likely to occur readily at the temperatures at which the slow oxidations are conducted. The lack of experimental evidence to support any mechanism involving the ionization of oxygen prior to or at the time of oxidation of a hydrocarbon is an additional factor in opposition to the idea that an alcohol is the primary oxidation product. At explosion temperatures, however, atomic oxygen may be present and effective as such. Actually most of the experimental work on the direct oxidation of methane with elemental oxygen lias shown that water and formaldehyde are among the first reaction products, whereas methanol is not, and several processes 17 claim this reaction to fonn formaldehyde industrially. 

The minimum concentration of hydrocarbon necessary to obtain explosions by the method of heating with oxygen in dosed bulbs increases rapidly with the molecular weight. Thus, with hexane no explosions occur below 10 per cent hydrocarbon concentration, and with octane the minimum concentration necessary is 33 per cent when a heating rate of 1° C. per minute is used. Diiso-amyl failed to ignite at this rate of heating at any concentration. Lewis observed no distinction between explosion temperature and critical inflexion temperature."... 

The critical constants (temperature, pressure, volume and compressibility factor) have been determined experimentally and are available (1-7). Additional property data such as acentric factor, enthalpy of formation, lower explosion limit in air and solubility in water are also available (8-74). The property data in the top and middle parts of the tabulation are helpfiil in process engineering. The property data in the lower part of the tabulation are helpful in safety and environmental engineering. 

Various studies of the effect of particle size on the critical temperature of azides have been made. Bowden and Yoffe [2] gave data for several explosives which exhibit a critical size for explosion at a particular temperature. For lead azide Hawkes and Winkler [13] and Bowden and Singh [14] showed that the smallest dimension of a crystal controls its explosion temperature and that the temperature decreases with increasing crystal thickness. A size effect was also found by Bowden and McLaren [15] for pellets of lead azide. 

The popularity of steady-state theories of explosion is easily understood. The neglect of reactant consumption dearly divides the solutions of the heat balance equation into two classes. Bdow a critical heat rdease rate, access temperatures tend to finite, steady values, vdmeas above this critical value temperatures become infinite in finite times. It is this topological distinction betwerai submtical and supncritical solutions whidi ensures the existenoe of critical conditions. 

In the laboratory (or outside it) iplosive and non-explosive behaviour usually appears to be easily distinguished. However, in all dosed systems the reactant temperature is always bounded by the adiabatic flame tempoature, and in the long time limit all self-heating is dissipated by heat losses so that both explosive and non-explosive temperature-time histories return to ambient as reaction reaches its end. There is no distinction between the topology of the two types of bdiaviour. To recognize, distinguish, or predict the alternative extrraoes of bdiaviour we must set aside the search for criticality and approadi the proUem differently. 

The Arrhenius equation predicts that the rate of reaction increases exponentially with an increase in temperature. Bretherick noted that an increase of 10 °C in reaction temperature can increase the reaction rate by a factor of 2. (See Chemical Connection 5.3.10.1 in Section 5.3.10 for more discussion of the Arrhenius equation and reaction rates.) Thus, it is critical that temperature be adequately controlled to prevent the reaction from accelerating to a dangerous rate. Be prepared to provide adequate control of the temperature—you will need to measure the temperature and have cooling means readily available. If you are unable to control the temperature, an explosion may occur. You should try to avoid systems that hold in heat—adiabatic systems. You can often control a reaction by controlling the rate of addition of a reagent. You should consider the best way to provide adequate mixing for the reaction. 

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