The formation of hazardous substances in the loss of control of chemical industrial processes: the control of major accident hazard and inherent safety

The formation and release of dangerous substances as a consequence of “out of control” conditions caused severe major accidents in the chemical industry, the most important being that of Seveso. It is well known that the anomalous operating conditions caused by an accidental event may result in the formation and the release of extremely hazardous substances, normally not present in the system. This hazard is now considered by Directive 96/82/EC (better known as the “Seveso-II” Directive) on the control of major accident hazards. Article 2 of the Directive requires to consider also the hazards due to the dangerous substances “which it is believed may be generated in the loss of control of an industrial chemical process”. With the implementation of the Seveso-II Directive in the Italian legislation (DL 334/99), the analysis of the hazards caused by the formation of dangerous substances due to unwanted or unforeseen reactions was recognized as an important element to consider in the analysis of major accident hazards caused by industrial installations. However, even if the Seveso accident took place more than 25 years ago, no well-accepted criteria or specific experimental techniques are yet available for the identification of dangerous substances that may be formed in the loss of control of a chemical system. The problem is even more important in the perspective of the application of inherent safety principles to process development. As a matter of fact, among the more important principles of inherent safety is the substitution of hazardous substances with less dangerous compounds. However, a correct evaluation of substance hazard also requires to take into consideration the possible formation of dangerous compounds following unwanted reactions. Several predictive methods were proposed for the identification of products that may be formed in the loss of control of a chemical process, based on macrocomponent lumped reaction schemes, on stochastic approaches and even on the identification structure-activity correlations. However, these approaches still need to be validated, possibly using specific experimental data sets. Moreover, even if predictive approaches to the problem will surely provide a useful first-step decision-making tool, a quantitative risk assessment requires the availability of reliable data on the expected quantities of hazardous substances formed in the scenario of interest. It must be remarked that in the absence of reliable data, the use of predictive tools may lead to extremely conservative results. Therefore, adequate and specific reference experimental techniques are needed to identify the hazardous substances that may be formed in a chemical system of interest, if no data are reported in the literature. Moreover, these techniques should also allow an estimation of the expected quantities of substances that may be formed in “out of control” conditions. The present study is focused on the development and validation of specific experimental protocols for the quantitative identification of the possible products formed in a chemical system undergoing “out of control” conditions. The qualitative and quantitative identification of products formed in a chemical systems undergoing “out of control” conditions is a complex problem. Thus, the study was aimed to the development of specific experimental techniques for the simulation of different scenarios that may lead to the formation of hazardous products in industrial processes. The main effort was devoted to the development of specific procedures and devices for the coupling between techniques for the simulation of operating conditions taking place during industrial accidents (with respect to pressure, temperature, etc.) and analytical methodologies for the identification and the analysis of the products of interest. In particular, calorimetric techniques as thermogravimetry, reaction calorimetry and adiabatic calorimetry, were coupled to analytical techniques as FTIR, GC and MS spectrometry for the identification of unwanted products formed in anomalous operating conditions. Further techniques as a high heating rate pyrolyzer (“pyroprobe”) and low-residence time fixed bed reactors were coupled to integrated analytical techniques (FTIR, GC, GC/MS, HPLC). The experimental protocols were applied to several chemical systems of industrial interest. Chemical systems containing brominated flame retardants are a potential source of hazardous substances in “out of control” conditions. Both pure brominated flame retardants, specifically tetrabromobisphenol A, and brominated epoxy resins, widely used for the manufacture of printed circuit boards, were investigated. Thermal decomposition runs in both inert and oxidizing environment were carried out at moderate heating rates using thermogravimetric analyzers and a laboratory scale fixed bed reactor. The use of combined experimental techniques allowed the qualitative and quantitative characterization of the products formed in the thermal degradation process of these materials. Hydroxylamine and its salts were studied to understand their reactivity and the mechanism of the decomposition reaction, as well as the products formed as a consequence of "out of control" conditions. Recently hydroxylamine was involved in two explosions due to its poor stability at high concentrations: the heating and the presence of metals (for example ion of metals like iron, silver, mercury or presence of carbon) resulted in the acceleration of the decomposition process [1-4]. Thus, the thermal stability and the decomposition products of hydroxylamine in water and of some hydroxylamine salts (hydrochloride, sulfate and phosphate) were investigated using TG-FTIR and TG-MS for the qualitative and quantitative characterization of product yields. The results obtained evidenced that the experimental protocols developed in the present study may be successfully applied to the identification of products formed in the loss of control of chemical industrial processes. The reference data obtained by the above defined procedures are the starting point for the assessment of the hazards due to the possible formation of dangerous compounds following unwanted reactions, that is of utmost importance in the development of inherently safer processes as well as in the control of major accident hazards due to the loss of control of a chemical system