PI – Dr. Luc Véchot
Abstract of the Research Projects in Chemical Reaction Hazards and Ethylene Releases
1. Experimental study of the thermal decomposition of cumene hydroperoxide under runaway conditions using isothermal and adiabatic calorimetry
In the chemical industry many manufacturing processes involve highly reactive substances that can undergo undesired exothermic reactions during their transport, storage or process. Their use may expose workers, the population and the environment to a risk of incidents that can be severe in some situations. One of the main hazards associated to the use of reactive chemicals is related to the loss of the thermal control of the system leading to a runaway reaction. Runaway reactions (or thermal explosion) can be caused by an incorrect kinetic and thermodynamic evaluation in the design and scale up phases or by an abnormal situation or deviation during the process (e. g. wrong addition of chemicals in the vessel, presence of impurities in the reactor, accumulation of intermediates, failure of the cooling system, failure of the agitation system). The consequences of thermal explosion are the initiation of undesired side reactions, the evolution of toxic and/or flammable substances, and the pressure increase inside the vessel possibly leading to its explosion. If no measures are taken on time, the runaway reaction will follow its thermally uncontrolled path and all the associated consequences could take place.
The petrochemical industry is a particular field of the chemical industry where runaway reaction hazards are presents and where the associated risks must be properly managed to avoid incident and mitigate the consequences of undesired events. The major incidents happened in the last 30 years in this context taught many important lessons to chemical engineering research community. The understanding of the behaviour of chemical systems under runaway conditions is of primary importance if we think about incidents from the past such as Seveso, Bhopal and more recently the T2 laboratory that had severe consequences in term of life, economic and environmental losses.
It is important to note that Qatar is playing an increasing role in the petrochemical industry via the development of its capacities to produce of polymers (Polyolefins) to respond to the growing global demand in the world. Many reactive substances commonly used in petrochemical industry could potentially present a thermal explosion hazards. For instance the monomer itself involved in the polymerisation reaction may undergo a runaway reaction. Another chemical of concern is the peroxides used to initiate the polymerisation reaction. These peroxides may self-decompose in an uncontrolled manner if engulfed in a fire or simply if the storage conditions lead to a heat removal rate lower than the heat generation rate at the storage temperature.
For a better understanding of the process safety issues associated to the use of reactive chemicals it is necessary to recognize systematically all the potential hazards in order to guarantee adequate prevention and protection measures to minimize the risk.
An experimental approach is generally used to study runaway reactions and to evaluate their consequences. Experimentation is mainly based on well consolidated calorimetry techniques which provide useful data to predict the behaviour of the process. The issue of a correct scale up is crucial when dealing with processes that can be very exothermic. A detailed experimental study carried on at increasing scales is, in this sense, fundamental.
The aim of this project is to perform an experimental study of a particularly exothermic chemical system (a peroxide decomposition) at laboratory scale as a preliminary study for a pilot plant scale series of tests. The students involved have been trained in all the different calorimetric techniques used to analyse a runaway reaction and are themselves performing the experimental study and data analysis of the thermal decomposition of cumene hydroperoxide. More in detail students are testing the effect of: concentration of the peroxide (20%, 30%, 40% w/w), type of solvent (cumene and 2,2,4-Trimethyl-1,3-pentanediol diisobutyrate) and the filling level of the sample holder (50%, 70%). The objective is to understand the dynamic behaviour of a chemical reactor carrying on a runaway peroxide decomposition in terms of kinetic, thermodynamic and fluid dynamic from the start of the reaction to the end of the venting step. In particular attention will is paid to the characteristics of flow developed by the runaway reaction as a basis for the calculation of the necessary venting area. For this reason a detailed model of the system is needed.
The pressure increase in the reactor vessel will be due to both the generation of permanent gases and vapor during the runaway (characteristic of hybrid systems). Depending on the nature of the solvent (mainly the boiling point Tb) the behaviour of the peroxide system during runaway may differ. The lower the boiling point of the solvent the more likely the system will approach a “tempered” behaviour (the action of a pressure relief system will have an effect on liquid temperature and therefore the reaction kinetic). For such system the reaction rate may continue to rise after the vent opening due to a change in the composition of the reacting mixture or other parameters (e.g. pH, auto-catalysis …), otherwise the reaction rate can simply be controlled just by venting. The higher the boiling point of the solvent, the more likely the system will approach an “untempered” behaviour (the action of a pressure relief system will have no effect on the reaction kinetic). For this type of system the pressure relief device should be designed to vent the maximum gas generation rate.
There is very few experimental data available on the behavior of hybrid systems. This is particularly true for untempered hybrid systems. Significant effort in the modeling of the behavior of such systems under runaway conditions is still to be done.
The work performed in the UREP will serve as a basis for collaborative work that is being developed on the same topic by international research engineering groups at the Institut National de l’EnviRonnement Industriel et des RisqueS (INERIS, France) and the Health and Safety Laboratory (HSL, United Kingdom). These two institutions own large scale experimental facilities capable of performing runaway reactions of peroxide systems; in particular on the basis of our experimental sensitivity analysis they will perform the decomposition venting of a Trigonox 21S systems in runaway conditions at respectively:
- 10 L vessel (INERIS)
- Pilot scale 340 L jacket and stirred reactor vessel (HSL).
Texas A&M University (TAMU) at Qatar has been developing research on Chemical Reaction Hazards on the particular theme of the decomposition of peroxide systems, reaction calorimetry and emergency relief sizing. With the last two years, the group and TAMU-Qatar has made a substantial financial effort to build a research laboratory equipped with “state of the art” calorimetric facilities commercially available to characterise runaway reactions: PhiTec I and PhiTec II adiabatic calorimeters, Simular Isothermal calorimeter and a Power Compensation Differential Scanning Calorimeter (DSC).
All around the world, only few research groups have the possibility of experimentally study the problem of runaway reactions and vent sizing for peroxide systems at laboratory scale as the required equipment are expensive, highly specialized and require specifically trained personnel. Besides, there is very few experimental data available on the behavior of hybrid systems during runaway and significant efforts in terms of modeling and prediction is still to be performed. This research will therefore contribute to fill the knowledge gap in this area, which will be beneficial not only to the petrochemical industry but also the pharmaceutical industry where such systems are commonly used.
The proposed project is very challenging and ambitious for the students, who will be actively participating to every aspects of the work from the design of the experiments to the interpretation of the data in a process safety context, which requires the development of critical analysis skills, fundamental for a researcher.
2. Modeling and experimental validation of a gas generating thermal explosion in a chemical reactor
Significant effort in the modeling of the behavior of untempered system systems under runaway conditions is still to be done. The deep phenomenological understanding of the links between thermodynamics, kinetic and fluid dynamics inside the vessel from the onset of the runaway until the end of the venting would be an important progress in this field of process safety.
Modeling the depressurization of a vessel requires the description of all interacting phenomena: thermodynamic equilibrium, heat transfer, fluid dynamic for a closed and open system, and, transport properties.A complete model must be able to describe the reaction kinetics, the mass and the heat transfer between the different phases, the distribution of components in the different phases and the two phase fluid dynamic behaviour and all phenomenological interconnection between these phenomena.
The correct choice and proper implementation of the different protective measures is based on a deep knowledge of the process, which is achievable by experimentation, which is manly based on calorimetric and thermometric techniques, which provide useful data to predict the behaviour of the process. In particular the similarity between a chemical vessel undergoing a runaway reaction and an adiabatic system make adiabatic calorimetry the most used technique to investigate on the causes on the possible path of a thermal explosion. Adiabatic data allows us to extrapolate simultaneously kinetic and thermodynamic. Thermodynamic data (e. g. heat of reaction, maximum temperature and pressure, maximum temperature and pressure increase rates during the reaction, onset temperature for the reaction, time to maximum rate,…) are easy to extract from experimental profiles; for what concerns kinetic parameters they are not so easy to be derived from adiabatic experiments in case of complex reaction paths. This is also due to the variation of critical keys parameters (such as temperature, concentration and heat transfer properties) that can change significantly the behaviour of the system near to thermal explosion conditions.
The aim of this research is to create a model of a chemical vessel (stirred and jacketed) undergoing a runaway reaction of an untempered system that can describe the non-steady state period from the beginning of the thermal explosion until the end of the venting.
The model will be mainly phenomenological and able to describe all the relations between kinetic, thermodynamic and fluid dynamic inside the vessel from the onset of the runaway reaction to the end of the venting. In particular attention will be paid to the characteristics of flow developed by the runaway reaction. The model will be supported by experimental data for hybrid untempered systems, for which the vent sizing methods are still poor and basically semi empirical. Data that cannot be retrieved experimentally will be deducted by molecular simulations (Computational Quantum Chemistry Methods).