ASSESSMENT OF THE HAZARDS CAUSED BY ACCIDENTAL DECOMPOSITION PRODUCTS

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1 ASSESSMENT OF THE HAZARDS CAUSED BY ACCIDENTAL DECOMPOSITION PRODUCTS Alessandro Tugnoli 1, Federica Barontini 2, Mauro Cordella 1, Pamela Morra 1, Ilaria Di Somma 3, Roberto Sanchirico 4, Antonino Pollio 5, Valerio Cozzani 1, and Roberto Andreozzi 3 1 Dipartimento di Ingegneria Chimica, Mineraria e delle tecnologie Ambientali, Alma Mater Studiorum - Università di Bologna, Viale Risorgimento 2, Bologna, Italy; alessandro.tugnoli@mail.ing.unibo.it, mauro.cordella@mail.ing.unibo.it, pamela.morra@mail.ing.unibo.it, valerio.cozzani@mail.ing.unibo.it 2 Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Università di Pisa, Via Diotisalvi 2, Pisa, Italy; f.barontini@ing.unipi.it 3 Dipartimento di Ingegneria Chimica, Università di Napoli Federico II, Piazzale Tecchio 80, Napoli, Italy; idisomma@unina.it, roberto.andreozzi@unina.it 4 Istituto di Ricerche sulla Combustione, CNR, P.le V. Tecchio, Napoli, Italy; r.sanchirico@irc.cnr.it 5 Dipartimento di Biologia Vegetale, Università di Napoli Federico II, Via Foria, Napoli, Italy; antonino.pollio@unina.it In the exploration of inherent safety of a substance a key issue is the possible hazard deriving from the formation of unexpected decomposition products. Nevertheless, no standardized approach is currently available to afford the problem. The present study aimed to the development of a simplified tool for the assessment of the possible hazard arising from decomposition products, based on the coupling of experimental protocols and of the evaluation of toxic hazard es. The methodology is applied to the assessment of the hazard deriving from the decomposition of some intermediates currently used in industrial chemical processes. KEYWORDS: compound hazard profile, assessment, inherent safety, thermal decomposition products INTRODUCTION The hazard of chemical substances is frequently related to the proprieties of the material itself, devoting less attention to the potential dangers that may arise from the thermal decomposition reactions in the loss of control of chemical systems or accidental events. While the problem of the possible formation of extremely hazardous compounds due to the unwanted conditions is well known, the incomplete availability of information and evaluation tools limits the practical application of these concepts in safety assessment and management, affecting deeply also the application of an inherent safety approach. The results of past accident analysis confirm that severe accidents in the manufacture of fine chemicals and pharmaceutical products were caused by the release of hazardous decomposition products. Fort these reasons, European Directive 96/82/EC (better known as Seveso-II Directive) imposes (art.2) to account for the substances which it is believed may be generated during loss of control of an industrial chemical process. Nevertheless for most of the chemicals, the section of their Material Safety Data Sheet dedicated to the decomposition products contain no information or only scant data on decomposition products; mainly concerning only the evolved gases. This is principally due to the absence of widely accepted experimental protocols for the determination of these products and to the difficulty of accounting for these aspects in the safety assessment. With respect to the first problem, some protocols are proposed and reported with some improvements in present study. The main technical problems to face in this field concern both reproducing conditions similar to the accidental ones (that may vary in dependence of different factors) and obtaining qualitative/quantitative resolution of the complex mixtures coming from thermal decomposition processes. Beside this, the data about the toxicological proprieties of the decomposition products, even if identified with previous techniques, are often incomplete, limiting the possibility of understanding the hazard embedded in the primary substance. In the present paper a comprehensive framework for the assessment of the hazard related to substance decomposition that may occur as a consequence of the loss of control of chemical industrial processes is described. Experimental protocols for the analysis of the decomposition process were defined, and an approach based on the evaluation of es for the description of the inherent hazard due to substances formed in out of control conditions is presented. The outlined procedure is applied to a case study concerning the decomposition of nitrobenzaldehyde, in order to assess the validity of the method. DESCRIPTION OF THE APPROACH The approach developed may be divided in two main phases: the experimental phase, that leads to the 1

2 identification of decomposition processes and of the formed compounds, and the hazard footprint assessment, that elaborates and integrates the data from the previous phase in order to characterise the inherent safety profile of the system. The main features of the two phases are described in the following. EXPERIMENTAL FRAMEWORK The aim of this phase of the study is to obtain experimental data on the decomposition behaviour of the analysed substances, devoting particular attention to the identification of the products potentially formed. This requires the definition of an array of experimental protocols in order to reproduce significant accidental scenarios to be representative of the large spectra of possible events (fire, runaway reaction, etc.) (Cozzani & Zanelli, 1999). The defined protocols are based on the integration of calorimetric (thermal gravimeter (TG), differential scanning calorimeter (DSC), adiabatic calorimeter (AC)) and analytic techniques (infrared spectroscopy (FTIR), chromatographic techniques (CG), mass spectroscopy (MS)). The former allows to reproduce industrial out of control conditions on a safe laboratory scale, the latter yield the identification of the formed products. The main features of the developed protocols are summarized in Table 1. These protocols have been proofed by the application to a large number of chemical systems of industrial concern in previous studies (Lunghi et al, 2002; Marsanich et al., 2004c). A first protocol is based on a combined TG-FTIR technique, that allows the on-line analysis of volatile decomposition products formed in thermal degradation or combustion conditions. The experimental system comprises a thermo-gravimeter or a simultaneous gravimeter differential scanning calorimeter, that is coupled, by a thermostated transfer line, to a FTIR spectrometer for the on-line analysis of reaction or decomposition products released to the carrier gas phase. The protocol definition required the selection of the incidental scenario to be simulated and, consequently, the identification of temperature, temperature-time profile and reaction environment to be used in the experimental test. These operative conditions are set in the thermal analyser by the use of a specific thermal program and of a proper composition of fluxing gas. The analysis of TG data yields information on the thermal stability and the decomposition kinetic of the sample, while DSC data complete the framework providing thermal effects. The FTIR analysis identifies volatile compounds; moreover the use of specific calibration techniques allows quantitative determinations. The small quantity of sample required in this kind of tests represents a considerable advantage in terms of safety, costs and experimental waste disposal. On the other side, it shall be remembered that IR analysis is not suitable to identify the homonuclear diatomic compounds (e.g. H 2,Br 2 ). As well, the protocol allows the study of primary decomposition reactions, while it is not able to detect secondary gas phase reactions. Table 1. Main features of the experimental protocols developed for the identification of the substances potentially formed in the loss of control of chemical systems TG-FTIR DTA Fixed bed reactor Adiabatic calorimeter Reproduced accidental scenarios Operative methods Thermal degradation, combustion (fire) Isotherm, constant heating rate Thermal degradation Isotherm, constant heating rate Thermal degradation, combustion (fire) Isotherm, constant heating rate Runaway heat-wait-search, open cell, closed cell Max. heating rate 1008C/min 1008C/min 1008C/min Temperature range C C C C Pressure range 1 bar bar 1 bar bar Sample quantities 1 50 mg g 0,02 2 g 0,5 50 g Formed products Gas phase products analysis Condensable phase products analysis data from on-line FTIR Non suitable data from offline FTIR data from off-line GC analysis data from on-line FTIR or absorption and titration data from offline GC analysis data from off-line FTIR data from off-line GC analysis 2

3 TG-FTIR tests are not suitable for the identification of non-gaseous products; thus the study of this compounds requires different experimental techniques such as fixed bed reactors. Fixed bed reactors allow the simulation of a wide range of thermal degradation and combustion scenarios. Also in this case, specific protocols were identified by the definition of temperature, temperature profile and reaction environment; this is realized, in the experiment, by a proper temperature program and fluxing gas. Procedures for sampling and analysis of the formed fractions (solid residue, consensables and gases) were defined. Solid residue can be studied by different techniques (e.g. elemental analysis, SEM analysis) that lead to information on the composition. The off-line qualitative characterization of condensable fraction is done by chromatographic techniques (e.g. gas-chromatography, gas-chromatography coupled with mass spectrometry); quantitative information can be obtained by calibration. Gaseous compound are identified by on-line FTIR analysis of the gas flow from the reactor. Quantitative information on specific volatile compounds can be obtained by quantitative FTIR analysis or by absorption in solution of proper reactives and subsequent titration. The identification qualitative or quantitative of the formed products, represents one of the main advantages of fixed bed protocols. The application of TG-FTIR and fixed bed protocols is limited to atmospheric pressure. Thus, further protocols were defined, based on differential thermal analysis (DTA) in closed vessel, with continuous pressure recording. This allows to monitor the pressure profile and his influence on the decomposition. The analysis of runaway reaction in chemical processes lead to the definition of a specific protocol based on adiabatic calorimetry. Adiabatic calorimetry allows the simulation of the behaviour that may occur in a vessel in runaway conditions, that is loss of control due to exothermic reactions, desired or not, that may occur. This technique was coupled with analytical techniques for the gathering of quail/quantitative data on the formed compounds. The identified protocol includes the definition of the operative conditions of the test as well as the sampling and analysis strategy for decomposition products. The data on thermal and pressure profiles provide information on maximum values and maximum increase rate for temperature and pressure, as well as on the reaction apparent kinetic. The quail/quantitative analysis of formed products can be done off-line by techniques similar to those described for fixed bed protocols (FTIR, GC, etc.). THE HAZARD FOOTPRINT ASSESSMENT Definition of the hazard proprieties for substance characterization A hazard vector was defined in order to represent the hazard related to the release of any identified substance into environmental compartments. In particular the analyzed targets are: acute and long-term effects on human health, ecosystems damage and environmental media contamination. As shown in Table 2, four categories of hazardous properties were identified to represent the hazard of a chemical substance released into the environment. The analysis of the methodologies and models typically used for human health risk assessment, ecological risk assessment and environmental media vulnerability, allowed to recognize the more significant parameters to include in each category. The first category is represented by toxicological and eco-toxicological parameters, that express dose-response relationships, as acute toxicity and chronic reference doses (inhalation, oral and dermal), and cancer slope factors (inhalation, oral and dermal). In addition toxic effects on fish, daphnia and birds are taken into account in order to define the eco-toxicological behaviour. The second category contains parameters influencing the dispersion and the environmental fate of substances (i.e. chemical and physical properties that describe the partition Table 2. Potential impact categories and selected hazardous proprieties for the characterization of substances released into the environment. Categories Proprieties Symbol Toxicological and eco-toxicological properties Dispersion and environmental fate Uptake by humans and animals Persistence in the environment Acute toxicity on humans (LC50, LD50) Acute eco-toxicity on fauna and flora (LC50, LD50) Chronic toxicity on humans (RfD) Carcinogenicity (CSF) Molecular weight Henry s law constant Boiling point Water solubility Octanol-water partition coefficient (K ow ) Overall persistence time P AT P ET P ChT P C P MW P H P Bp P S P Kow P to 3

4 of chemicals between solid, liquid and gas phases). Molecular weight, melting point, boiling point, relative density (liquid, air), vapour pressure, Henry s law constant, water solubility, lipophilicity, air and water diffusivity belong to this group. The third category is represented by the parameters influencing the uptake by humans and living organisms: octanol-water partition coefficient (K ow ), organic carbon partition coefficient, bioconcentration and bioaccumulation factors, soil-water partition coefficient, suspended solidssurface water partition coefficient, sediments-pores water partition coefficient. However, it must be remarked that reliable methodologies are available to derive all these parameters from the K ow coefficient. The fourth and last category consists of properties influencing the persistence in the environment: degradation coefficients or half-life times in air, soil and water. Although many parameters can fit in these four categories, relations among them led to a selection of a small number of independent properties, as shown in Table 2. The values of the parameters in Table 2 are at the basis of the evaluation of the hazard es defined in the following, and thus their estimation is required for the application of the present methodology. The values of these parameters may be available from databases reporting experimental data or may be estimated on the basis of the structure of the chemical of concern. The predictive methods are generally described as group contribution methods, structure activity methods (SARs) or quantitative structure activity relationships (QSARs) (Allen & Shonnard, 2002). As well specific toxicological and eco-toxicological tests can be done where necessary. Hazard properties ranking In order to evaluate and compare the different hazards associated to a substance of concern, an arbitrary hazard ranking was derived from the substance parameters listed in Table 2. A score, whole number from 0 to 3, was assigned to each property, on the basis of the value of the reference parameters. Table 3 shows an example of this assignation procedure for the acute eco-toxicity for fauna and flora (P ET ). Scoring procedures similar to those described in figure were defined for all the other parameters, and are not described in detail for the sake of brevity. The scores assigned to each range of the parameters were defined on the basis of criteria depending on the specific parameter, considering typical values for a large number of substances as well as normative references (e.g. for the acute toxicity, the criteria were based on the classification given by Directive 67/548/EEC). Where more parameters contribute to the assignation of a single parameter score, as in the case of Table 3, the overall score is conservatively assumed on the basis of the highest parameter. HAZARD PROFILE DEFINITION The values of the scores for any studied substance can be listed in an hazard vector. Nevertheless, in order to allow a more significant comparison of the hazards of different substances it is useful to define the impact profile in a more synthetic way that combines property scores in a few es. Four es were defined to characterize the impact hazard on specific targets: acute toxicity on humans, eco-toxicity, chronic toxicity and carcinogenicity. The es were homogeneously defined in order to assume a numerical value between 0 and 27 and to be the products of three factors: a hazard factor, scoring the specific hazard of the substance for the target of concern; an availability factor, addressing the availability of the substance and the aptitude to reach the target; a contact probability factor, addressing the likelihood the substance is available for reaching the target. Table 4 shows the definition and the method used for the calculation of the four es starting from the hazard proprieties defined in Table 2. An overall hazard may also be defined as the sum of the previously defined ones. Although it is useful for quick hazard comparison of different substances, its intrinsic limit due to the loss of detail in the information has to be recognized. The comparison of the hazard profile between the primary substance and the decomposition products is a starting point for the risk assessment of scenarios involving decomposition reactions following the loss control of the chemical system. The single hazard vectors, featuring the impact profile of the decomposition products, can be arranged in an hazard matrix in order to allow a quick comparison. Graphical representation such, as that of Figure 1, shows the new hazards that may arise in decomposition. THE CASE-STUDY OF NITROBENZALDEHYDES The substances investigated in the present study are the three isomers of nitrobenzaldehyde. These compounds are the intermediates of organic synthesis for the production Table 3. Example of hazard ranking: acute eco-toxicity for flora and fauna (P ET ) Fish LC50 96h (g/m 3 ) Daphnia LC50 48h (g/m 3 ) Birds LD50 oral (mg/kg) Algae LC50 72h (g/m 3 ) Score Very toxic,1,1,10,1 3 Toxic Harmful Negligible toxicity

5 Table 4. Specific and overall es used to describe the hazard profile of the substances Acute Toxicity Index I at ¼ P AT. ((P H þ P Bp )/2). P MW Eco-toxicity Index I et ¼ max( (P ETwater. P S. P to ); (P ETair. P H. P to )) Chronic Toxicity Index I cht ¼ P ChT. P Kow. P to Carcinogenicity Index I c ¼ P C. P Kow. P to Overall Index I o ¼ I at þ I et þ I cht þ I c (A) Acute toxicity (B) Acute toxicity Persistence Eco-toxicity Persistence Eco-toxicity K ow Chronic tox. K ow Chronic tox. Solubility Carcinog. Solubility Carcinog. Boiling point Molecular weight Boiling point Molecular weight Henry's law constant 3-Nitrobenzaldehyde (A) (B) Henry's law constant Acute toxicity Eco-toxicity Chronic toxicity Carcinogenicity Overall Figure 1. Hazard profile of 3-nitrobenzalehyde (black) compared to that of its decomposition products (grey) as obtained by tests in mild (A) and severe (B) conditions. Some hazard es for 3-nitrbenzaldeyde are not visible in the histogram since their values are practically zero of dyes and biologically active products (pharmaceuticals, agrochemicals, etc.). For example, 2-nitrobenzaldheyde is a feedstock for the synthetic production of indigo. These substance are reported to be thermally instable if heated (Ando et al., 1991). Nevertheless safety data sheets usually report scant data on their stability and decomposition products (generally limited to carbon and nitrogen oxides). Some of the protocols previously described have been applied for the investigation of the thermal stability of nitrobenzadehydes. The samples used in the experimental runs were obtained from Aldrich Chemical. Table 5 summarizes some results of this experimental investigation. In particular data on the heat of reaction related to the first stage of decomposition (different stages are identified for the three compounds) are reported in the table. Data are Table 5. Thermal data on the thermal stability of nitrobenzaldehydes. Heats of reaction refer to the first decomposition stage in TG-DSC tests. Condensable residue at room temperature was obtained from DTA scanning stopped at 3608C. Onset temperatures refer to adiabatic calorimetry. Heat of 1 st stage (J/g) Residue Onset (8C) 2-Nitrobenzaldehyde % Nitrobenzaldehyde % Nitrobenzaldehyde % 215 5

6 obtained by TG-DSC tests in closed crucibles; the opening of the crucible lid due to the generated pressure prevents estimation of heats of further stages, because of the evaporative phenomena. Tests conduced by DTA avoid this limitation, and allow recording the value of pressure during the experimental tests. In the Table, the final values for residues at the end of the tests are reported. Adiabatic calorimetry provides, among the other, information on the onset temperature of the three isomers. The analytical techniques integrated within the protocols allow the obtainment of information on the compounds formed in the accidental conditions simulated in the tests. For sake of brevity, only results from adiabatic calorimetric tests on 3-nitrobenzalheyde are presented and discussed in this paper. In particular, tests performed with different quantities of sample made possible to explore different experimental operative conditions, due to the influence which the initial sample mass has on the phi-factor and on the pressure profile in a cell having a fixed volume. The different behaviour recorded in the tests confirmed one more time that operative conditions are of fundamental importance in determining the decomposition behaviour of substances, implying the key role of using reference protocols in testing, as previously discussed. The analysis of the volatile compounds formed and of residue allowed the identification of several decomposition products. These appear to be very different in accordance to the severity of operative conditions reached by the different tests. The envelope of the scores for the different proprieties of the substances formed is presented in the radial graph of Figure 1. As it is clear from the graph, substances characterized by different impact vectors compared to the original one are formed: in particular more dangerous substances are formed as, for instance, nitro several substituted aromatics and polycyclic aromatics as well as dangerous gases like nitrogen oxides and hydrogen cyanide. This is more evident when comparing specific impact es (histogram in Figure 1). Substances characterized by a high toxicity, both for humans and ecosystem, and carcinogens are formed in detectable quantities in the degradation. It can be generally observed that as the more severe condition are reached (higher reaction temperatures and pressures) cyclization reactions are promoted, yielding to the formation of polycyclic compounds that frequently are carcinogen compounds. In these severe conditions, higher quantities of hazardous compounds, as nitrogen oxides and hydrogen cyanide, can be identified in the gas phase. decomposition products formed in the loss of control of a chemical process. NOMENCLATURE P AT [-] ¼ propriety score for acute toxicity on humans P ET [-] ¼ propriety score for acute eco-toxicity on fauna and flora P ChT [-] ¼ propriety score for chronic toxicity on humans P C [-] ¼ propriety score for carcinogenicity P MW [-] ¼ propriety score for molecular weight P H [-] ¼ propriety score for Henry s law constant P Bp [-] ¼ propriety score for boiling point P S [-] ¼ propriety score for water solubility P Kow [-] ¼ propriety score for octanolwater partition coefficient P to [-] ¼ propriety score for overall persistence time I at [-] ¼ acute toxicity I et [-] ¼ eco-toxicity I cht [-] ¼ chronic toxicity I c [-] ¼ carcinogenicity I o [-] ¼ overall TG [-] ¼ thermal gravimeter DSC [-] ¼ differential scanning calorimeter DTA [-] ¼ differential thermal analysis AC [-] ¼ adiabatic calorimeter FTIR [-] ¼ Fourier transform infrared spectroscopy CG [-] ¼ chromatography LC50 xh [g/m 3 ] ¼ lethal concentration for 50% of population in x hours of exposition LD50 oral [mg/kg] ¼ ingestion lethal dose for 50% of population per unit of weigh of target specie RfD [mg/(kg d)] ¼ reference dose for human non-carcinogen toxicity CSF [(mg/(kg d)) 21 ] ¼ cancer slope factor K ow [-] ¼ octanol-water partition coefficient CONCLUSIONS In present paper, a methodology to define the hazard profile of decomposition products was outlined. The approach, joined to reference methodologies for the screening of expected decomposition products, proved to be effective in the identification of the accidental scenarios that should be assessed following the emission in the environment of REFERENCES Allen, D. T. & Shonnard, D. R Green Engineering: Environmentally Conscious Design of Chemical Processes. Prentice Hall PTR, Upper Saddle River, NJ07458 Ando, T., Fujimoto, Y. & Morisaki, S Analysis of differential scanning calorimetric data for reactive chemicals. Journal of Hazardous Materials, 28, 251 6

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