Thermal oscillations in the decomposition of organic peroxides: Identification of a hazard, utilization, and suppression

The purpose of this research is to identify and characterize oscillatory thermal instability in organic peroxides that are used in vast quantities in industry and misused by terrorists. The explosive thermal decompositions of lauroyl peroxide, methyl ethyl ketone peroxide, and triacetone triperoxide are investigated computationally, using a continuous stirred tank reactor model and literature values of the kinetic and thermal parameters. Mathematical stability analysis is used to identify and track the oscillatory instability, which may be violent. In the mild oscillatory regime it is shown that, in principle, the oscillatory thermal signal may be used in microcalorimetry to detect and identify explosives. Stabilization of peroxide thermal decomposition via Endex coupling is investigated. It is usually assumed that initiation of explosive thermal decomposition occurs via classical (Semenov) ignition at a turning point or saddle-node bifurcation, but this work shows that oscillatory ignition is also characteristic of thermoreactive liquids and that Semenov theory and purely steady state analyses are inadequate for identifying a thermal hazard in such systems

Analysis of the Thermal Decomposition of Untempered Peroxide Systems

Several of the most catastrophic process safety incidents, such as Bhopal and most recently Texas West Fertilizer explosion, were initiated by runaway reactions. Consequences of such incidents include, fatalities, environmental damage, and in some instances corporate bankruptcy. To prevent conditions leading to a runaway, it is necessary to understand the kinetics, and physical and thermodynamic properties of the chemical system. In the present research, calorimetric experiments were coupled with computational chemistry calculations to characterize the runaway behavior of two organic peroxides: Dicumyl Peroxide (DCP) and Cumene Hydroperoxide (CHP). These two reactive systems are particularly challenging due to their untempered behavior and complex kinetics. To characterize the physical behavior of DCP and CHP runaways, adiabatic testing was performed in two equipment. Experimental results suggest that: Scaling up methods used to estimate temperature and self-heating rate profiles on a large-scale, from laboratory data, are inconsistent for fast self-heating rate systems under runaway conditions. Moreover, the use of low thermal inertia or phi factor equipment (more costly and difficult to operate), do not always provide better large-scale estimations. This is due to potential higher heat losses. Pressure discrepancies of up to 27 times were encountered when the phi factor was increased from 1.1 to 1.8. This finding elucidates the necessity of more efforts to scale up pressure behavior. Estimation of gas generation rate from different configuration (closed vs open cell) diverges by up to 2.3 times. Principal sources of discrepancies are: open cell gas temperature assumption, pressure influence on vaporization, and gas dissolution. Due to the complexity of the decomposition reaction of systems under study, grasping knowledge of their thermo-kinetics characteristics by experimental techniques is expensive, time consuming, and probably not possible. In this work, computational quantum chemistry, transitional state theory, and thermodynamic principles are used to achieve a deeper understanding of DCP and CHP decomposition thermos-kinetics. Networks of 12 and 18 reactions for DCP and CHP decomposition, respectively, are proposed. Products of the proposed networks match those reported by analytical techniques. Using this method provides a safe alternative while dealing with complex, highly reactive and unknown systems

Assessment of Maximum Gas Production Rate of an Untempered System Under Runaway Conditions

Runaway reactions are characterized by the exponential increase of the temperature and pressure of a chemical system that could potentially lead to the explosion of the reactor or storage vessel of concern. The consequences of a runaway reaction may be very severe in terms of life, economic and environmental losses. Emergency relief systems (ERS) are the ultimate mitigation method to prevent vessel explosion following the runaway reaction. In the case of the runaway of gas producing chemical systems, ERS sizing requires the assessment of the maximum gas production rates. Significant work was performed in the 1980’s by the Design Institute for Emergency Relief Systems to develop vent sizing methods for runaway reaction cases. While vent sizing methods developed for vapor systems provided relatively good results, those developed for gas generating systems (hybrid or gassy) tend to be oversized and still need to be improved. A very significant part of this work includes the improvement of the current methods for the measurements of the maximum gas production rate for such systems. The objective of this thesis work is to experimentally study the decomposition of a gas generating system under runaway condition using adiabatic calorimetry and assess the maximum gas production rate corresponding to the runaway. A critical analysis of the current methodologies to interpret experimental data to compute the maximum gas production rate was done. The decomposition of Cumene Hydroperoxide (CHP) in Cumene was chosen for the study

Integrating Chemical Hazard Assessment into the Design of Inherently Safer Processes

Reactive hazard associated with chemicals is a major safety issue in process industries. This kind of hazard has caused the occurrence of many accidents, leading to fatalities, injuries, property damage and environment pollution. Reactive hazards can be eliminated or minimized by applying Inherently Safer Design (ISD) principles such as "substitute" or "moderate" strategies. However, ISD would not be a feasible option for industry without an efficient methodology for chemical hazard assessment, which provides the technical basis for applying ISD during process design. In this research, a systematic chemical hazard assessment methodology was developed for assisting the implementation of ISD in the design of inherently safer process. This methodology incorporates the selection of safer chemicals and determination of safer process conditions, which correspond to "substitute" and "moderate" strategies in ISD. The application of this methodology in conjunction with ISD technique can effectively save the time and investment spent on the process design. As part of selecting safer chemicals, prediction models were developed for predicting hazardous properties of reactive chemicals. Also, a hazard index was adopted to rate chemicals according to reactive hazards. By combining the prediction models with the hazard index, this research can provide important information on how to select safer chemicals for the processes, which makes the process chemistry inherently safer. As part of determining safer process conditions, the incompatibility of Methyl Ethyl Ketone Peroxide (MEKPO) with iron oxide was investigated. It was found that iron oxide at low levels has no impact on the reactive hazards of MEKPO as well as the operational safety. However, when iron oxide is beyond 0.3 wt%, it starts to change the kinetics of MEKPO runaway reaction and even the reaction mechanism. As a result, with the presence of a certain level of iron oxide (> 0.3 wt%), iron oxide can intensify the reactive hazards of MEKPO and impose higher risk to process operations. The investigation results can help to determine appropriate materials for fabricating process equipment and safer process conditions

Theoretical and Experimental Evaluation of Chemical Reactivity

Reactive chemicals are presented widely in the chemical and petrochemical process industry. Their chemical reactivity hazards have posed a significant challenge to the industries of manufacturing, storage and transportation. The accidents due to reactive chemicals have caused tremendous loss of properties and lives, and damages to the environment. In this research, three classes of reactive chemicals (unsaturated hydrocarbons, self-reacting chemicals, energetic materials) were evaluated through theoretical and experimental methods. Methylcyclopentadiene (MCP) and Hydroxylamine (HA) are selected as representatives of unsaturated hydrocarbons and self-reacting chemicals, respectively. Chemical reactivity of MCP, including isomerization, dimerization, and oxidation, is investigated by computational chemistry methods and empirical thermodynamic–energy correlation. Density functional and ab initio methods are used to search the initial thermal decomposition steps of HA, including unimolecular and bimolecular pathways. In addition, solvent effects are also examined using water cluster methods and Polarizable Continuum Models (PCM) for aqueous solution of HA. The thermal stability of a basic energetic material, Nitroethane, is investigated through both theoretical and experimental methods. Density functional methods are employed to explore the initial decomposition pathways, followed by developing detailed reaction networks. Experiments with a batch reactor and in situ GC are designed to analyze the distribution of reaction products and verify reaction mechanisms. Overall kinetic model is also built from calorimetric experiments using an Automated Pressure Tracking Adiabatic Calorimeter (APTAC). Finally, a general evaluation approach is developed for a wide range of reactive chemicals. An index of thermal risk is proposed as a preliminary risk assessment to screen reactive chemicals. Correlations are also developed between reactivity parameters, such as onset temperature, activation energy, and adiabatic time to maximum rate based on a limited number, 37 sets, of Differential Scanning Calorimeter (DSC) data. The research shows broad applications in developing reaction mechanisms at the molecular level. The methodology of reaction modeling in combination with molecular modeling can also be used to study other reactive chemical systems

Modeling the Behavior of a Vessel under Runaway Conditions

Reactive chemicals may proceed into uncontrolled chemical reactions with significant evolutions in temperature and pressure due to vapor/gas production. This happens when there is loss of control of the temperature of the system, and self-heating occurs, thereby leading to a runaway reaction. The overpressurization of the vessel following the runaway may lead to an industrial accident, a thermal explosion, resulting in damages to people, property and the environment. Emergency relief systems (ERS) act as a last line of defense against vessel overpressure. It is therefore critical to the safe operation of chemical processes that they are adequately sized. Much effort is needed to overcome the limitations presented by the current ERS sizing method used. Also, reliance solely on experimental work can prove to be time consuming and provide difficulties during scale-up to industrial scale. Thus, there is a need to employ a comprehensive dynamic model that describes the vessel behavior throughout the reaction, during depressurization and relief action. This involves the understanding of the phenomenological links between thermodynamics, kinetic and fluid dynamics inside the vessel from the onset of the runaway until the end of the venting through an ERS. These outputs of this model could then to be used to enhance ERS sizing methods and consequence analysis. This work represents a step forward in this direction. It proposes a model that takes all these factors into account, with the exception of level swell. To achieve this, this work includes: (i) an experimental study of the reactive system using calorimetric techniques; (ii) determination of the kinetic rate expression for the reactive system; (iii) formulation of dynamic lumped model; (iv) dynamic simulations of a closed vessel and partial experimental validation; (v) a sensitivity analysis of the effects of ERS area and ERS set pressure on vessel behavior. This approach was carried out through the evaluation of the decomposition of di-tert-butyl peroxide in toluene, a potentially hazardous reactive system

Thermal Hazard Analysis of Nitroaromatic Compounds

Nitroaromatic compounds are among the largest group of industry chemicals. Due to the high bond-association energy (BDE) of the C-NO2 in nitroaromatic compounds (297 ± 17 kJ/mole), once the runaway reaction is triggered, the compounds will release massive heat and gases that accelerate the system temperature and pressure increase that lead to an explosion instantly. Mononitrotoluenes (MNT) is among most important nitroaromatic compounds used as intermediates for the synthetic pharmaceuticals, agrochemicals and precursors for TNT. However, in the past 30 years, serious incidents, owing to its thermal decomposition, have killed 88 people and injured more than 900. To help prevent future thermal runaway behavior of the nitroaromatic compounds, this work presents using both the experimental and simulation methodologies to figure out the thermochemistry and thermodynamics starting from MNT. The understanding of the thermal behaviors and mechanisms can yield safer handling and storage of the reactive chemicals. To investigate the mechanisms that cause the ortho-nitrotoluene (2-NT, isomer of MNT) decomposition reactions, the effects of different incompatible substances and surrounding conditions, such as confinement, heating rate, induction effect and sample sizes, were studied using three types of calorimetry – DSC, ARSST and APTAC. Experimental results suggest that: 2-NT is the most hazardous reactive chemical among the three isomers of MNT with the much higher pressure rise rate than the others. It is an autocatalytic reaction follows three stages: induction phase, acceleration phase and decay phase. The induction phase follows the zero order reaction with activation energy (170-174 kJ mol-1 ) and preexponential factor (1011.6 -1011.7 s -1 ). The main decomposition pathway during reduction phase is the generation of anthranil and water. The six common contaminants (NaOH, Na2SO4, CaCl2, NaCl, Na2CO3 and Fe2O3) that exist in the manufacturing process of MNT lower the thermal stability of 2-NT with the three proposed mechanisms (generation of OH- , impact of chloride ions and Iron (III) oxide catalyzed nitroarenes reduction). This work demonstrates the complexity and the multiple studies required for making MNT safer, providing suggestions to the nitroaromatics industry. It can also serve as an example for comprehensive studies on various reactive chemicals

The use of a silica based coating to reduce moisture absorption of flax fibre reinforced composites

This study deals with the synthesis of silica particles, treatment of flax fabrics with silica, and the preparation and characterization of silica coated flax fibre reinforced phenolic composites treated with silica. Silica particles were successfully prepared by means of a hydrolytic sol-gel route. Two types of silica were prepared by employing either ammonium hydroxide solution as a base catalyst and acetic acid as an acid catalyst. The silica sols were then aged from three to five days in order to determine the effects of aging on the final properties of the silica. The chemical composition of the silica particles was evaluated by fourier transform infrared spectroscopy (FTIR), thermal stability was determined by using thermogravimetric analysis (TGA), and structural and physical properties of the silica particles prepared via two catalysts and aged at different time periods was investigated by x-ray diffraction (XRD). Silica sols, prepared at different conditions were then applied to treat flax fabrics (untreated/scoured) by use of the padding technique. The effects of the silica treatments on flax fabrics were evaluated by FTIR, XRD, determination of moisture content and mechanical properties. The FTIR revealed presence of silica groups on the silica treated flax fabrics, thus resulting in low moisture content for silica treated flax fabrics. XRD analysis revealed that aging the silica sols increases the crystallinity index. Silica treated flax fabrics showed enhanced tensile properties in the weft direction. The thermal, mechanical and water sorption properties of the composites were evaluated. TGA results revealed that the decomposition temperatures of the silica treated composites shifted to higher temperatures. Thus, silica treatments lead to an improvement in thermal stability for composites. A reduction in mechanical properties was also observed for silica treated composites and some composites showed a reduction in water absorption. It was quite evident from this study that the type of catalyst system used in silica preparation has a great influence on the final properties of the silica, which to a large extent changes the thermal, mechanical and water sorption properties of the composites

The effects of nanoparticle addition on the processing, structure and properties of SiC and AlN

Silicon carbide (SiC) and aluminum nitride (AlN) exhibit a combination of thermal and mechanical properties that are relevant to applications in electronics, aerospace, defense and automotive industries. However, the successful translation of these properties into final applications lies in the net-shaping of these ceramics into fully dense microstructures. Increasing the packing density of the starting powders is one effective route to achieve high sintered density and dimensional precision. The current research presents an in-depth study on the effects of nanoparticle addition on the powder injection molding (PIM) of SiC and AlN powder-polymer mixtures. In particular, bimodal mixtures of nanoscale (n) and sub-micrometer (μ) particles were found to have significantly increased powder packing characteristics (solids loading) in the powder-polymer mixtures. The influence of nanoparticle addition on the multi-step PIM process was examined by comparing the rheological and thermal properties of the novel bimodal μ-n powder-polymer mixtures to conventional monomodal μ powder-polymer mixtures. Additionally, the effect of nanoparticle addition on the mold filling behavior of the powder-polymer mixtures was examined. Subsequently, the effects of increased powder content and reduced particle size owing to nanoparticle addition were studied in the context of the polymer removal kinetics. Finally, nanoparticle addition was found to expedite the liquid phase formation during the sintering stage. The sintered parts of bimodal μ-n mixtures exhibited higher sintered densities, lower shrinkage and better thermal properties than the corresponding monomodal powder mixtures. The above results provide new perspectives which could impact a wide range of materials, powder processing techniques and applications.Keywords: powder injection molding, aluminum nitride, silicon carbide, sinterin