Explosions kinetic limit

GP 11] [R 19] The third explosion limit is discussed in detail in [9] as it is important from both practical and mechanistic viewpoints (230-950 °C 10-10 Pa). This limit is normally responsible for the occurrence of explosions imder ambient pressure conditions. In addition, these explosions are known to be kinetically induced by radical formation. The formation of these species is sensitive to size reduction of the processing volume owing to the impact of the wall specific surface area on radical chain termination. It turns out that the wall temperature has a noticeable, but not decisive influence on the position of the third limit The thermal explosion limit lies below the kinetic limit for all conditions specified above (Figure 3.50) [9]. 

The sensitivity of explosives has been defined by Koenen et al (Ref 10) as the minimum amount of energy that must be imparted to the explosive, within limited time and space, to initiate explosive decomposition. This definition is, accdg to Ma ek (Ref 13, p 60) meaningful and can serve as a basis of quantitative fundamental treatments provided the imparted energy is thermal and provided its initial distribution in time and space is known. The accuracy of treatments of thermal explosion described in Section IIA of Mafcek s paper is then limited mainly by the accuracy of chemical kinetic data... 

Low Detonation Pressure Explosives. Most expl materials in wide use today may be characterized by deton pressures ranging from approx 150—350 kilobars. Propint materials, on the other hand, exhibit comparatively low press typical of deflgrn reactions. The difference in pressures exhibited by these two classes of materials leaves an interesting gap, the exploration of which may yield valuable information on the propagation and kinetic limitations of detong materials... 

One way of process simplification is to make molecular complex compounds out of much simpler building blocks (e.g., by multi-component one-pot syntheses like the Ugi reaction), at best directly out of the elements. Especially in the latter case, this is often quoted as a dream reaction [14]. Typically, such routes have been realized so far with hazardous elements, easily undergoing reaction, but lacking selectivity. One example is direct fluorination starting with elemental fluorine, which has been performed both with aromatics and aliphatics. Since the heat release cannot be controlled with conventional reactors, the process is deliberately slowed down. While, for this reason, direct fluorination needs hours in a laboratory bubble column it is completed within seconds or even milliseconds when using a miniature bubble column operating close to the kinetic limit. Also, conversions with the volatile and explosive diazomethane, commonly used for methylation, have been conducted safely with microreactors in a continuous mode [14]. 

GP 11] [R 19] Based on an analysis of the thermal and kinetic explosion limits, inherent safety is ascribed to hydrogen/oxygen mixtures in the explosive regime when guided through channels of sub-millimeter dimensions under ambient-pressure conditions [9], This was confirmed by experiments in a quartz micro reactor [9],... 

Figure 3.50 Extending kinetic explosion (squares) and thermal explosion limits by using a micro reactor with 300 pm channel diameter (filled symbols). Calculated values for (circles) and 7 3 = (triangles). Comparison with 1 m...

For processes under development, the most cost-effective means of avoiding potential risk is to eliminate those materials that are inherently unsafe that is, those materials whose physical or physico-chemical properties lead to them being highly reactive or unstable. This is somewhat difficult to achieve for several reasons. First, without a full battery of tests to determine, for example, flammability, upper/lower explosivity limits and their variation with scale, minimum ignition temperatures, and so on, it is almost impossible to tell how a particular chemical will behave in a given process. Second, chemical instability may make a compound attractive to use because its inherent reactivity ensures a reaction proceeds to completion at a rapid enough rate to be useful that is, the reaction is kinetically and thermodynamically favoured. 

Belles prediction of the limits of detonability takes the following course. He deals with the hydrogen-oxygen case. Initially, the chemical kinetic conditions for branched-chain explosion in this system are defined in terms of the temperature, pressure, and mixture composition. The standard shock wave equations are used to express, for a given mixture, the temperature and pressure of the shocked gas before reaction is established (condition 1 ). The shock Mach number (M) is determined from the detonation velocity. These results are then combined with the explosion condition in terms of M and the mixture composition in order to specify the critical shock strengths for explosion. The mixtures are then examined to determine whether they can support the shock strength necessary for explosion. Some cannot, and these define the limit. 

Property parameters. The physical property parameters include state of matter, phase equilibrium, thermal, mechanical, optical, and electromagnetic properties. The chemical property parameters include preparation, reactivity, reactants and products, kinetics, flash point, and explosion limit. The biological property parameters include toxicity, physiological and pharmaceutical effects, nutrition value, odor, and taste. 

Such reactions have been used to explain the three limits found in some oxidation reactions, such as those of hydrogen or of carbon monoxide with oxygen, with an "explosion peninsula between the lower and the second limit. However, the phenomenon of the explosion limit itself is not a criterion for a choice between the critical reaction rate of the thermal theory and the critical chain-branching coefficient of the isothermal-chain-reaction theory (See Ref). For exothermic reactions, the temperature rise of the reacting system due to the heat evolved accelerates the reaction rate. In view of the subsequent modification of the Arrhenius factor during the development of the reaction, the evolution of the system is quite similar to that of the branched-chain reactions, even if the system obeys a simple kinetic law. It is necessary in each individual case to determine the reaction mechanism from the whole... 

While Gray Yang are not denying the usefulness of such ideas, they consider that too literal an application on the distinction can lead to difficulties. For this reason they tried to unify both theories and this problem is discussed in their paper. They also examined the effect of fuel consumption on thermal explosions, definition of critical conditions and the effects of vessel shapes. Finally, the relationship between thermal explosion criteria and flame theoty described by Belles (Ref 2), as well as detonability limits were pointed out. Comments on the paper of Gray Yang of Profs R.R. Baldwin R. Ben-Aim are given on p 1061 of Ref 3 Refs 1) N.N. Semenov, "Chemical Kinetics... 

Since it is beyond the scope of this article to treat the subject of target vulnerability in any detail, the interested reader (with proper security clearance) is recommended to Ref 3. This handbook as a whole does not consider chemical, biological or nuclear weapons. It is intentionally limited to kinetic energy or explosive energy type weapons. A synopsis of the contents by chapter follows ... 

Three ideal reactors—the batch reactor, the plug-flow reactor and the perfectly stirred reactor—are mathematical approximations to corresponding laboratory reactors that are used regularly to study chemical kinetics (Section 13.3.2). The batch reactor (or static reactor) is particularly useful to characterize explosion limits [241] and kinetic behavior at temperatures below 1000 K (e.g., [304,351]), while stirred reactors (e.g., [151,249,296, 367,397]) and flow reactors (e.g., [233,442]) have proved highly valuable in the study of chemical kinetics at higher temperatures. 

This is of the same form as Equation 30, but involves the mixed diffusion coefficient, Jci9, instead of the thermal conductivity of the mixture. However, as seen from the kinetic theory of gases, the thermal conductivity is proportional to the diffusion coefficient. Therefore agreement of experimental results with either Equation 30 or 53a is not an adequate criterion for distinguishing between first explosion limits in branching chain reactions and purely thermal limits. It has been reported (52), that, empirically,... 

There are many methods of obtaining explosion-limit data. In general, they involve rapid heating of a mixture, rapid mixing at a known temperature, or changing the mixture pressure at a constant temperature. The results are in most cases self-consistent but rarely agree from method to method. Jost 31) discusses most of the methods and compares results obtained with some of them. Here, only those methods most commonly used in kinetics studies are discussed and critically evaluated. 

We may rule out all methods which depend upon ignition of the gases with electrically heated wires, heated spheres, or heated rods. These are kinetically unreliable, as they depend strongly on convective heat and mass transfer, they often act catalytically, and accurate temperature measurement is difficult. The following methods have found wider use in the plotting of explosion limits. 

Over the last 20 years chemical kinetics, and especially the theory of chain reactions, have achieved major successes. A theory of ignition of heated explosive mixtures has been created. However, attempts to directly explain the propagation of flame as the diffusion of active centers, or to explain the limits of propagation by the conditions of chain breaking fail to yield positive results. 

Works published on this subject include Semenov s fundamental theory of thermal explosion [1], Todes analysis of the kinetics of thermal explosion [2], Frank-Kamenetskii s calculation of the absolute values of the limit of thermal explosion [3], the theory of ignition [4], and finally, most closely related to the first part of this paper, the theory of flame propagation by Frank-Kamenetskii and the author [5]. 

E. Neal, Jr et al, Effects of Thermal Cycling on Trinitrotoluene (TNT) and Tritonal Explosive Compositions , AFATL-TR-79-15, Rept WQEC/C 79-111 (1979) (limited dis-trib) 109) Di7. McMillen et al, Kinetics and Mechanisms of the Gas-Phase Decomposition of Nitroaromatics , ACS mtg, Honolulu (April 1979) 110) S. Bulusu, JOrgMassSpec... 

The reactions of transient silylenes are so rapid that most of the limited mechanistic information that has been obtained over the past quarter-century has been through indirect means. Direct measurements of silylene reaction rates by kinetic spectroscopy in the past decade have yielded important new insights. One can predict with some confidence an explosion of mechanistic studies of silylenes employing fast spectroscopies capable of providing more structural information than traditional electronic absorption and emission techniques. The nearly universal reversibility of silylene reactions remains to be fully exploited through kinetic studies of retro-reactions. The mechanisms of most silylene reactions remain to be fully elucidated, and this task will increase in urgency as silylenes see more use in synthesis. 

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