Explosions thermal

In the treatment of chain reactions (Section 5.6) it was clear that under suitable conditions the system could become unstable. In an exothermic reaction, the temperature rises thus accelerating the reaction. Positive feedback causes another increase in temperature. Unless depletion of the reactants or heat loss to the surroundings is sufficiently rapid, the coupling of temperature and reaction rate sets the stage for an explosion. 

There are three thermal effects thermal conduction within the reaction vessel, described by (2.27) heating due to reaction exothermicity and heat loss due to interaction with the surroundings maintained at a temperature Tq, The local temperature is the solution of the differential equation 

The second term is the chemical contribution and the last one accounts for coupling with the surroundings is the progress variable for chemical reaction, Q is the enthalpy release per mole of reaction, and y accounts for the effectiveness of thermal coupling between system and surroundings. Solution of (7.1) requires knowing how the rate of reaction, d jdty depends upon r. Assume a simple situation and consider a single, irreversible 

In general more than one chemical process contributes to (7.1) and Qf is generalized to S QSi. The resulting equations are extraordinarily complex. 

Analysis of (7.1) and (7.2) is difficult however, some limiting cases are tractable. Ignoring depletion (Aq i, f) and assuming homogene- 

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That some enhancement of local temperature is required for explosive initiation on the time scale of shock-wave compression is obvious. Micromechanical considerations are important in establishing detailed cause-effect relationships. Johnson [51] gives an analysis of how thermal conduction and pressure variation also contribute to thermal explosion times. 

Polymerization Exothermic reaction which, unless carefully controlled, can run-away and create a thermal explosion or vessel overpressurization Refer to Table 7.20 for common monomers Certain processes require polymerization of feedstock at high pressure, with associated hazards Many vinyl monomers (e.g. vinyl chloride, acrylonitrile) pose a chronic toxicity hazard Refer to Table 7.19 for basic precautions... 

The evaluation of ehemieal reaetion hazards involves establishing exothermie aetivity and/or gas evolution that eould give rise to inei-dents. Flowever, sueh evaluation eannot be earried out in isolation or by some simple sequenee of testing. The teehniques employed and the results obtained need to simulate large-seale plant behavior. Adiabatie ealorimeters ean be used to measure the temperature time eurve of selfheating and the induetion time of thermal explosions. The pertinent experimental parameters, whieh allow the data to be determined under speeified eonditions, ean be used to simulate plant situations. 

Temperature of no-return Temperature of a system at which the rate of heat generation of a reactant or decomposition slightly exceeds the rate of heat loss and possibly results in a runaway reaction or thermal explosion. 

In the search for a better approach, investigators realized that the ignition of a combustible material requires the initiation of exothermic chemical reactions such that the rate of heat generation exceeds the rate of energy loss from the ignition reaction zone. Once this condition is achieved, the reaction rates will continue to accelerate because of the exponential dependence of reaction rate on temperature. The basic problem is then one of critical reaction rates which are determined by local reactant concentrations and local temperatures. This approach is essentially an outgrowth of the bulk thermal-explosion theory reported by Fra nk-Kamenetskii (F2). 

The behavior of liquid flow in micro-tubes and channels depends not only on the absolute value of the viscosity but also on its dependence on temperature. The nonlinear character of this dependence is a source of an important phenomenon - hydrodynamic thermal explosion, which is a sharp change of flow parameters at small temperature disturbances due to viscous dissipation. This is accompanied by radical changes of flow characteristics. Bastanjian et al. (1965) showed that under certain conditions the steady-state flow cannot exist, and an oscillatory regime begins. 

A steady-state solution of Eq. (3.30) exists only for < 2. At z > 2 hydrodynamic thermal explosion occurs and oscillatory flow takes place. 

Bastaniian SA, Merzhanov AG, Xudiaev SI (1965) On hydrodynamic thermal explosion. Sov Phys Docl 163 133-136... 

Exothermic reaction which, unless carefully controlled, can run-away and create a thermal explosion or vessel overpressurization... 

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]. 

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...

Comprehensive discussions on reactor stability theories and safe engineering problems were presented by Eigenberger and Schuler (1986, 1989), Zaldivar (1991), Barton and Rogers (1993), and Grewer (1994). The very basic theory developed by Semenov (1928) for zero-order reactions is very illustrative for a physical explanation of explosion phenomena. The theory enables evaluation of conditions at which thermal explosion will occur. 

Adler, J. and Enig, J.W., 1964, The Critical Conditions in Thermal Explosion Theory with Reactant Consumption, Combustion and Flame 8, 97. 

Chain reactions can lead to thermal explosions when the energy liberated by the reaction cannot be transferred to the surroundings at a sufficiently fast rate. An explosion may also occur when chain branching processes cause a rapid increase in the number of chains being propagated. This section treats the branched chain reactions that can lead to nonthermal explosions and the physical phenomena that are responsible for both branched chain and thermal explosions. 

The high heat capacity associated with the large mass of liquid facilitates control of the reactor and provides a safety factor for exothermic reactions that might lead to thermal explosions or other runaway events. 

It is very endothermic (AH°f (g) +150.2 kJ/mol, 3.66 kJ/g). A sample exploded when heated in a sealed ampoule [1], and during redistillation at 59°C/1 bar, a drop of liquid fell back into the dry boiler flask and exploded violently [2], The explosive decomposition has been studied in detail [3], and existing data on thermal explosion parameters have been re-examined and discrepancies eliminated [4], See Ethyl isocyanide... 

The monomer is sensitive to light, and even when inhibited (with aqueous ammonia) it will polymerise exothermally at above 200° C [1]. It must never be stored uninhibited, or adjacent to acids or bases [2], Polymerisation of the monomer in a sealed tube in an oil bath at 110° C led to a violent explosion. It was calculated that the critical condition for runaway thermal explosion was exceeded by a factor of 15 [3]. Runaway polymerisation in a distillation column led to an explosion and fire [4], See other polymerisation incidents... 

C3H5N303S 12C + 10H2O + 6N2 + 3S +S02 Air-pressurisation would have prevented the water from boiling and absorbing the exotherm to moderate the thermal explosion. Decomposition of the solution may have been accelerated by the presence of impurities, or by the solid salt splashed onto the heated vessel wall [1], There is an independent account of the incident [2],... 

During investigation of elfect of 1% of added impurities on the thermal explosion temperature of TNT (297°C), it was found that fresh red lead, sodium carbonate and potassium hydroxide reduced the explosion temperatures to 192, 218, and 192°C, respectively. 

It decomposed explosively at 180-200°C [1], but there is a pressure band between ambient and 64 bar in which thermal explosion does not occur [2],... 

Thermal explosion which occurred during fast anionic polymerisation of styrene, catalysed by butyllithium, was prevented by addition of low MW polystyrene before the catalyst. 

See entry THERMAL EXPLOSIONS See other PEROXYCARBONATE ESTERS... 

See THERMAL EXPLOSIONS, COMPUTATION OF REACTIVE CHEMICAL HAZARDS... 

Verhoeff, J., Experimental Stucfy of the Thermal Explosion of Liquids, Doctoral thesis, University of Delft, 1983... 

Shortly after interruption of vacuum distillation from an oil-bath at 115°C to change a thermometer, the ester exploded violently and this was attributed to overheating [1], A commercial sample of undetermined age exploded violently just after vacuum distillation had begun [2], In an examination of thermal explosion behaviour, the title compound was used as a model compound in autoclave... 

Used industrially as a powerful reducant, it is supplied as a solution in hydrocarbon solvents. The undiluted material is of relatively low thermal stability (decomposing above 50°C) and ignites in air unless diluted to below 25% concentration [1]. A thermal explosion of the compound has been studied from a theoretical standpoint [2],... 

See entry thermal explosions See other high-nitrogen compounds... 

During manufacture of 26/16/10 N/P/K fertiliser by ammoniation of nitric-phosphoric acid mixtures, followed by concentration, addition of potassium sulfate, then granulation at up to 250°C, the possibility of a thermal explosion exists. Kinetic studies showed that 240°C appears to be a safe granulation temperature, but that changes in the composition and pH of the mixture may decrease this critical temperature. 

Experiments involving explosions of molten tin and water are described [1], and the mechanism of propagation of the thermal explosions was studied [2], The dynamics of explosive interaction between multiple drops of molten tin and water has been examined experimentally [3],... 

Thermal instability Thermal explosion following bulk self-heating and mnaway reaction... 

Chain mechanisms may be classified as linear-chain mechanisms or branched-chain mechanisms. In a linear chain, one chain carrier is produced for each chain carrier reacted in the propagation steps, as in steps (3) and (4) above. In a branched chain, more than one carrier is produced. It is the latter that is involved in one type of explosion (a thermal explosion is the other type). We treat these types of chain mechanisms in turn in the next two sections. 

Another type of explosion is a thermal explosion. Instability in a reacting system can be produced if the energy of reaction is not transferred to the surroundings at a sufficient rate to prevent T from rising rapidly. A rise in T increases the reaction rate, which reinforces the rise in X. The resulting very rapid rise in reaction rate can cause an explosion. Most explosions that occur probably involve both chain-carrier and thermal instabilities. 

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