The identification and assessment of possible hazards and risks in chemical processes is of major practical importance. It is essential for developing and controlling chemical reactions both on the laboratory scale and in an industrial environment. Frequently chemical accidents are due to loss of control and incorrect handling. The result of this is often a so-called runaway reaction that can lead to an explosion.
The earliest possible identification of possible hazards and risks is of major importance for product development. DSC measurements are very useful for this because only small amounts of sample are needed to quickly measure the enthalpies and rates of chemical reactions.
In this connection, the maximum adiabatic temperature increase and the Time to Maximum Rate (TMR) are important quantitative criteria. They describe the conditions under which a substance or a process becomes thermally unstable or a thermal runaway, for example, an explosion, could occur.
Kinetic descriptions of chemical reactions can be used to estimate their thermal behavior under any temperature profile. This procedure can be applied for the risk analysis of chemical compounds or processes. The most important objective is to ensure safe working conditions and to minimize possible risks.
Risk assessment can be carried out to introduce methods and measures to ensure that a specified level of safety is maintained and that control strategies are implemented.
Risk profiles mostly serve as the basis for classifying acceptable scenarios versus unacceptable situations. Such profiles are generally described and are linked to severity and probability.
Thermal safety with regard to severity in connection with an exothermic runaway scenario [1, 2, 3] is on the one hand described by means of the adiabatic temperature increase ∆Tad. Alternatively, the so-called “Time to Maximum Rate under adiabatic conditions” (TMR) provides information about the probability of a thermal risk.
TMR indicates the time it takes for a reaction mass to reach the maximum rate of heat release for a given starting temperature. In an adiabatic system, no heat exchange occurs between the material under investigation and the surroundings; this means that the total heat generated remains in the system and increases its temperature.
The formula for the adiabatic temperature increase is given in equation 1 and for TMR in equation 2.
Here ∆Tad is the adiabatic temperature increase, ∆h the specific enthalpy of reaction, and cp the specific heat capacity at constant pressure.
The reaction rate is given in equation 3 and is determined from kinetic calculations.
The rate of the adiabatic temperature increase is given by equation 4
Different calorimetric methods are used to measure these values. DSC experiments are particularly suitable for determining cp, ∆h, and reaction kinetics at an early stage of product development [4].
A further advantage is that only a few milligrams of substance are needed. The reaction rate can be described by means of kinetic analysis and corresponding parameters (for example, see reference [5]).
The adiabatic course of a reaction can be calculated using data from a dynamic DSC experiment. Integration of the heat flow signal yields the reaction enthalpy, and cp is determined by means of a DSC measurement [6].
Kinetic evaluation of one or more DSC measurements allows reaction behavior to be predicted with regard to time and temperature.
The typical course of an adiabatic temperature increase is shown in Figure 1 and illustrates the concept of TMR. The upper diagram (black curve) displays the adiabatic development of an exothermic reaction for a given starting temperature, T0.
The rate of change of temperature has its maximum at the point of inflection of the curve. The lower diagram (red curve) shows the first derivative of the adiabatic temperature increase; the curve displays the rate of self-heating. The time to the maximum of the peak corresponds to TMR.
The thermal decomposition of benzoyl peroxide (synonym: dibenzoyl peroxide) dissolved in dibutyl phthalate is used as a case study to illustrate the determination of the TMR. This reaction is frequently used for comparison purposes; see reference [7] and the references listed in it.
The risk of a reaction runaway can in principle be estimated from DSC measurements of the adiabatic temperature increase, ∆Tad, and the Time to Maximum Rate (TMR).
The classification is somewhat difficult because there is no direct quantitative relationship between the measured values of ∆Tad and TMR and the probability and severity of an event. A generally accepted order of magnitude is that proposed by Stoessel [8] given in Table 1.
It is important to emphasize that the TMR must be estimated with the utmost caution (worst-case scenario).
If for any reason increased risk is expected, it is imperative to take additional measures to clarify the situation.
Determination of the Adiabatic Time to Maximum Rate by DSC for Thermal Safety Assessment | Thermal Analysis Application No. UC 406 | Application published in METTLER TOLEDO Thermal Analysis UserCom 40