| Abstract | The threat posed to life and plant by potentially unstable substances, is illustrated in the chemical and allied industries by the many incidents which have occured. The assessment of thermal instability in materials provides a way of defining safe handling conditions, and thus avoiding potentially catastrophic scenarios. To date a number of alternative means of thermal stability appraisal have been devised, based on a variety of principles. Accelerating Rate Calorimetry has proven to be one of the most reliable methods of assessing the thermal stability of substances.
The Accelerating Rate Calorimeter ( A.R.C.T)y) is considered in detail, and an appraisal of its performance limits is drawn. Its ability to handle extremely rapid reactions, oxidative combustion processes, and thermally complex systems, is analysed. In addition a number of methods are developed for overcoming many of the encountered limitations.
A number of thermokinetic parameters may be obtained from A.R.C.TM data, based on the Townsend and Tou single-stage decomposition model. The integrity of this model is considered, with the development of a mathematically more rigorous model based on the Kooij reaction rate model. Two alternative reaction systems are also considered, the first being that of a two stage sequential decomposition ( ie. A > B > PRoDUCT ), and the second a two component reaction (ie. A + B > Propucts ). Methodologies for establishing thermokinetic and safety data ( such as, time-to-maximum-rate ) are also developed in each case, supported by detailed, industrially relevant examples.
Safe storage conditions for substances within a container are defined on the basis of a self-accelerating decomposition temperature ( S.A.D.T.), this being the maximum environmental temperature above which auto-accelerative self-heating would occur. Use of the Semenov, Frank-Kamenetskii and Thomas thermal explosion models, in conjunction with the single stage decomposition model, has resulted in a rigorous method of establishing S.A.D.T.s from A.R.C.7p data. Both safe operating conditions and evasive action limits may also be established from these models.
The analysis of A.R.C.7)y data, has always been based on user interactive "trial and error" procedures. To eliminate subjectivity and provide a numerically rigorous analysis of the data, the ARCLink software package was developed. The software analyses the data using a number of statistical methods, in conjunction with the single stage decomposition model. Further, the software supports the determination of critical heat balance temperatures ( such as S.A.D.T.s ). In addition to the data handling, the ARCLink package also provides run-time support for the A.R.C.7 xy system, and graphics / data output facilities for report writing.
In response to a number of A.R.C.7y limitations, an alternative adiabatic calorimeter was developed. The Reaction Hazards Adiabatic Device ( R.H.A.D.) was based on the A.R.C.7yy with a number of additional design criteria. This provided a more flexible system, centred on the use of state-of-the-art instrumentation and control. Further, the system was designed to support subambient temperatures, a stirring facility, and external sample loading, including sequential addition. Apart from the standard A.R.C.7m™ test options, two additional modes of operation have been included. The temperature ramp test provides a rapid screen for potential thermal instability problems, whilst a user defined option, enables the user to devise a customised testing history ( such as external fire simulation ). |
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