Thermal risks

Protective gloves against thermal risks (heat and/or fire). Superseded BS 1651 1986 AMD 1 General requirements for gloves (AMD 8515) dated 15 February 1995. Superseded BS 1651 1986... 

Stoessel, F. (1993) What is Your Thermal Risk Chemical Engineering Progress, 89( 10), 68-75. 

Shock sensitive. The parent diol is not but shows thermal risks on calorimetry. 

Traditionally, risk is defined as the product of the severity of a potential incident by its probability of occurrence. Hence, risk assessment requires the evaluation of both the severity and the probability. Obviously, the results of such an analysis aid in designing measures for the reduction of the risk (Figure 3.1). The question that arises now is What do severity and probability mean in the case of thermal risks inherent to a particular chemical reaction or process ... 

The thermal risk linked to a chemical reaction is the risk of loss of control of the reaction and associated consequences (e.g. triggering a runaway reaction). Therefore, it is necessary to understand how a reaction can switch from its normal course to a runaway condition. In order to make this assessment, the theory of thermal explosion (see Chapter 2) needs to be understood, along with the concepts of risk assessment. This implies that an incident scenario was identified and described, with its triggering conditions and the resulting consequences, in order to assess the severity and probability of occurrence. For thermal risks, the worst case will be to lose the cooling of a reactor or in general to consider that the reaction mass or the substance to be assessed is submitted to adiabatic conditions. Hence, we consider a cooling failure scenario. 

The six key questions presented above ensure that the essential knowledge about the thermal safety of a process is addressed. In this sense, they represent a systematic way of analysing the thermal safety of a process and building the cooling failure scenario. Once the scenario is defined, the next step is the actual assessment of the thermal risks, which requires assessment criteria. The criteria used for the assessment of severity and probability are presented below. 

After loss of control of the synthesis reaction, the technical limit (MTSR < MTT) cannot be reached and the decomposition reaction cannot be triggered, since the MTSR stays below TD24. Only if the reaction mass is maintained for a long time under heat accumulation conditions, can the MTT be reached. Then the evaporative cooling may serve as an additional safety barrier. The process presents a low thermal risk. 

At first glance, the data and concepts used for the assessment of thermal risks may appear complex and difficult to overlook. In practice, however, two rules simplify the procedure and reduce the amount of work to the required minimum ... 

The six key questions described in the cooling failure scenario allow us to identify and assess the thermal risks of a chemical process. The first steps allow building a failure scenario, which is easy to understand and serves as a base for the assessment The proposed procedure (Figure 3.6) is based on the separation of severity and probability, taking into account the economic aspects of data determination in a safety laboratory. In a second step, based on the scenario, the criticality index can be determined to help in the choice and design of risk-reducing measures. 

Figu re 3.8 Graphical summary of the thermal risks linked to the performance of an industrial process. 

Assess the thermal risk linked to the performance of this process, and determine the criticality class. 

Assess the thermal risks linked to this hydrogenation reaction. 

This example shows that with only sparse thermal data it is sometimes possible to assess thermal risks. This is possible due to the low concentration used in this hydrogenation. 

Such a weak adiabatic temperature rise cannot lead to a thermal explosion. The severity is low. In case of malfunction of the reactor cooling system, the reaction, providing it is not stopped, will lead to an immediate temperature rise by 6 K reaching the MTSR of 36 °C. The thermal risk linked to this hydrogenation reaction is low. 

Evaluate the thermal risk linked with the performance of this reaction at industrial scale. 

Are measures required to cope with the thermal risk ... 

A catalytic reaction must be performed in aqueous solution at industrial scale. The reaction is initiated by addition of catalyst at 40 °C. In order to evaluate the thermal risks, the reaction was performed at laboratory scale in a Dewar flask. The charge is 150 ml solution in a Dewar of 200 ml working volume. The volume and mass of catalyst can be ignored. For calibration of the Dewar by Joule effect, a heating resistor with a power of 40 W was used in 150ml water. The resistor was switched on for 15 minutes and the temperature increase was 40 K. During the reaction, the temperature increased from 40 to 90 °C within approximately 1.5 hours. The specific heat capacity of water is 4.2 kj kg K 1. 

What do you think about the thermal risks linked with an industrial performance of this process ... 

The thermal risks of a synthesis step with an exothermal bimolecular reaction (A + B —> P) must be assessed. For this, the required thermal data have to be determined in a safety laboratory equipped with a reaction calorimeter and a DSC. 

Ubrich, O. and Lerena, P. (1996) Methodology for the assessment of the thermal risks applied to a nitration reaction, in Nitratzioni sicure in lahoratorio e in impianto industriale, Stazione sperimentalo combustibili, Milano. 

Assessing the thermal risks of the process means answering the six questions in the cooling failure scenario (see Section 3.3.1). The overall energy potential of the reaction is calculated from the molar reaction enthalpy of 200 kj moT1. The concentration to be used is that of the final reaction mass (2molkg 1), since the reactant B must be added to allow the reaction ... 

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