Adiabatic sensitivity heating

The Sikarex safety calorimeter system and its application to determine the course of adiabatic self-heating processes, starting temperatures for self-heating reactions, time to explosion, kinetic data, and simulation of real processes, are discussed with examples [1], The Sedex (sensitive detection of exothermic processes) calorimeter uses a special oven to heat a variety of containers with sophisticated control and detection equipment, which permits several samples to be examined simultaneously [2]. The bench-scale heat-flow calorimeter is designed to provide data specifically oriented towards processing safety requirements, and a new computerised design... 

For the above reasons, it is not advisable to use the closed cell in the adiabatic self-heating test from a point of view that we should always endeavor to get a on the low temperature side, or on the safety side, for every chemical of the TD type. We have, nevertheless, no choice but to use some closed cell in the adiabatic self-heating test performed for a chemical of the TD type which is volatile or decomposes to evolve corrosive and/or toxic gases while tested. Conversely speaking, it follows that it is always advisable to use the thermally sensitive glass open-cup cell in the adiabatic self-heating test, unless the chemical tested is volatile or decomposes to evolve corrosive and/or toxic gases while tested. 

The adiabatic sensitivity of an explosive is determined by allowing a test weight to fall from a predetermined height onto the air-compressing piston. This causes the compression of the air between the compressing piston and the sample of the explosive. Its adiabatic heating may consequently lead to the initiation of the sample. 

Polymerization processes are characterized by extremes. Industrial products are mixtures with molecular weights of lO" to 10. In a particular polymerization of styrene the viscosity increased by a fac tor of lO " as conversion went from 0 to 60 percent. The adiabatic reaction temperature for complete polymerization of ethylene is 1,800 K (3,240 R). Heat transfer coefficients in stirred tanks with high viscosities can be as low as 25 W/(m °C) (16.2 Btu/[h fH °F]). Reaction times for butadiene-styrene rubbers are 8 to 12 h polyethylene molecules continue to grow lor 30 min whereas ethyl acrylate in 20% emulsion reacts in less than 1 min, so monomer must be added gradually to keep the temperature within hmits. Initiators of the chain reactions have concentration of 10" g mol/L so they are highly sensitive to poisons and impurities. 

A reactor is run adiabatically when no heat is exchanged between the reaction zone and the surroundings. The reaction temperature can then only be controlled by quenching with a cold stream of the reaction mixture or by inter-stage heat exchangers. For highly thermally sensitive large molecules treated in the fine chemicals sector this is a very impractical mode of operation. Therefore, adiabatic reactors will not be discussed here. 

This powerful but relatively insensitive explosive decomposes violently at 202° C, and gives lead and silver salts which are highly impact sensitive [1], Though not endothermic (AH°f —103.3 kJ/mol), as a bis-nitramine it has a rather high heat of decomposition (3.91 kJ/g) which it is calculated would attain an adiabatic decomposition temperature over 2250°C, with a 60-fold pressure increase in a closed system [2],... 

A bleach solution was being prepared by mixing solid sodium chlorite, oxalic acid, and water, in that order. As soon as water was added, chlorine dioxide was evolved and later exploded. The lower explosive limit of the latter is 10%, and the mixture is photo- and heat-sensitive [1]. It was calculated that the heat of reaction (1.88 kJ/g of dry mixture) would heat the expected products to an adiabatic temperature approaching 1500°C with an 18-fold increase in pressure in a closed vessel [2],... 

PMS stars with M < 0.35 M0 have a simple structure - they are fully convective balls of gas all the way to the ZAMS. As the star contracts along its Hayashi track the core heats up, but the temperature gradient stays very close to adiabatic except in the surface layers. Li begins to burn in p, a reactions when the core temperature, Tc reaches c 3x 106 K and, because the reaction is so temperature sensitive (oc Tc16-19 at typical PMS densities) and convective mixing so very rapid, all the Li is burned in a small fraction of the Kelvin-Helmholtz timescale (see Fig. 1). 

A flow of 0.35 kg/s of a solid is to be dried from 15 per cent to 0.5 per cent moisture on a dry basis. The mean specific heat capacity of the solids is 2.2 kJ/kgdegK. It is proposed that a co-current adiabatic dryer should be used with the solids entering at 300 K and, because of the heat sensitive nature of the solids, leaving at 325 K. Hot air is available at 400 K with a humidity of 0.01 kg/kg dry air and the maximum allowable mass velocity of the air is 0.95 kg/m2s. What diameter and length should be specified for the proposed dryer ... 

As discussed in Section 2.3.1.2, SEDEX [103,104] and SIKAREX [106] types of apparatus are also used in adiabatic calorimetric techniques. Compensation for the heat capacity of the sample containment is also a feature. Typical sensitivity of this type of equipment is 0.5 W/kg, the sample size is 10 to 30g, and the temperature range is 0 to 300°C. 

Other screening tests, including shock sensitivity and flammability tests, and thermodynamic computations raised no specific concerns. After the explosion, the material was tested in an ARC. Such testing showed that a typical batch of the compound could self-heat to full decomposition if held under adiabatic conditions at 120 to 125°C for 24 hours. These tests were run within the normal processing temperature range, and the ARC test results were hailed as demonstrating the likely cause of the accident. 

Special Studies High Sensitivity Calorimetry A- DESIREEV UNDESIRED dT/dt ATadiab Kinetics, EA, A Sample size 1- 50 ml, pW/g sensitivity Shelf life studies by accelerated aging Combine with low adiabatic to confirm solids low self-heating rate studies... 

The Accelerating Rate Calorimeter (ARC ) is another adiabatic test instrument that can be used to test small samples. The ARC with the clamshell containment design can handle explosive compounds. It is a sensitive instrument that can indicate the onset of exothermicity where the reaction mixture can be accurately simulated (HSE 2000). ARC testing results can be used in determining a time to maximum rate of decomposition, as well as in calculating a temperature of no return for a container or vessel with specific heat removal characteristics. Further information and references related to the ARC are given in CCPS (1995a) and Urben (1999). 

In the discussion of the same paper (pp 286-8) M. Norrish suggested generation of shock waves by adiabatic heating of an isolated section of a gas sample by flash irradiation. Thus a H2/O2 mixture with a trace of NO2 as a sensitizer (and no added coolant gas) could be subjected to a high- intensity flash.at one end of a quartz tube, the rest of the tube (to the extent of possibly 5/6) being blacked out. In such a detonation, Thrush (Ref 8a) has observed the emission... 

Impact produces hot spots, the temperatures of which are (frequently) determined by melting of the solid, being effectively buffered at the melting point. Hence, the mp frequently determines the hot-spot temperature, T0 in the adiabatic-decomposition equation 8.8, listed on p 174 of Cook. If T0 is below a certain critical value, the reaction will not be adiabatic and, owing to heat loss, may not undergo reaction build-up. But above this critical value it becomes effectively adiabatic and expln then always results after a time T. The failure of grit to sensitize an expl may, however, depend simply on the ratio of the mp of the expl to that of the grit particle. 

Twin differential microcalorimeters have been described by Berghausen el al. (S), by Hackerman (8), and by Whalen and Johnson (9). Hacker-man employs thermistors, whereas the other two are based on thermocouples and in addition are run adiabatically. These calorimeters appear to have about 10 times the sensitivity of simpler designs, but for many purposes the large additional diflSculties in design, construction, and operation do not seem to be warranted. Berghausen and coworkers, however, have shown that they can estimate slow heat evolutions, after the first few minutes, due to surface reactions. 

It is reported in Refs 42 67 that moderate quantities of waxy materials incorporated with such expl materials as HMX, RDX and PETN do not appreciably affect their initiation sensitivity, but do inhibit propagation to expin. Ref 67 further states that no relationship between the specific and latent heats of the desensitizers and its ability to desensitize could be found, and concludes that the desensitizer cannot be regarded merely as a thermal sink. This is somewhat in conflict with results of work in Ref 91, where it is reported that it has been established firmly that the sensitivity of the RDX compositions decreases with increasing specific heats of the additives, and a solid desensitizer functions primarily by absorbing heat from local regions of initiation including any hot spots which arise from adiabatic compression of occluded gas ... 

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