Process Safety Engineering
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).
Preparation of the medium and large scale experiments at Ras Laffan Emergency and Safety College (RLESC) and design of the TP-5 LNG testing facility.
Since 2009, the BP LNG Project at TAMU-Qatar has received an outstanding technical and financial support from QP through the design and construction of TP-5 LNG testing facility and the associated adjacent facilities that will be used in this research program. With the knowledge provided by TAMU-Qatar, QP kindly invested in transforming a “simple” LNG training facility for fire fighters into a “state of the art” research facility. This facility includes three LNG burning pits of which one will be used to perform our experiments. This pit received 100 thermocouples and 13 heat flux plates embedded in the concrete at two different levels, which enables this pit to monitor temperature and heat flux profile in a concrete and which can be used to determine temperature-dependent thermal properties of concrete. Additional instrumentation used in this test facility includes capacitance-based liquid cryogen level sensor, bubbler systems with differential pressure transmitters for cryogenic liquid level measurement (redundancy in level measurement by different method), various thermocouples to monitor pool spreading and level, and to measure temperature of the liquid, the cloud and the fire; methane gas detectors for LNG gas concentration tracking, ultrasonic anemometers for air movement and turbulences measurement, ultrasonic flow meter for cryogenic liquid flow, hydrocarbon cameras and ordinary video recorders for hydrocarbon vapor mapping, radiometers for measurement of thermal radiation from fires; and two weather stations at 2 and 10 meters elevation.
The scale of prepared experiments will allow the measurement of the pool vaporization and dispersion process on concrete or water as well as the heat transfer from the concrete to the LNG.
• The studies of the effectiveness of mitigation measures for controlling LNG vaporization and dispersion.
This part of the project aims to study the use of water curtains, vapour fences and high expansion foam as mitigation methods to limit the vapour generation and dispersion of the NG vapour generated after an accidental release of LNG.
• Comprehensive modeling of LNG vapor cloud generation.
Numerous studies have been performed on the modeling of the vapor cloud dispersion after a release of LNG, and although there are still many aspects yet to be discovered and improved, many models have been developed and, more importantly, successfully validated against available experimental data. However, the phenomena associated with the vapor cloud formation and the associated source term models have received much less attention despite their critical importance in the prediction of the consequences of a spill. Indeed, the result of a LNG vapor cloud dispersion simulation is highly sensitive to and dependent on the accuracy of the vapor generation rate calculated from a source term model. The results from the source term modeling will in turn have a significant effect on the predicted exclusion zones for LNG facilities. Unfortunately, the modeling of the spill of LNG is very complex and may include various phenomena such as jet flow, flashing, droplet formation and vaporization, pool formation, spreading, boiling and evaporation, depending on the specific conditions of the release. The aim of this project is to focus on LNG liquid pool formation, spreading, boiling and evaporation.
Significant efforts are still to be done in the development and validation of a comprehensive source term model able to describe the physics behind the spreading and vaporization of a LNG pool. One of the reasons behind the surprisingly insufficient amount of knowledge on source term models is the lack of a complete set of good quality experimental data that can be used for their development and validation. This is currently the priority area of our project.
• CFD modeling of LNG vapor cloud dispersion.
A part of this project consists of describing dispersion of a generated LNG vapour in air with consequent mixing and warming in complex geometries using CFD models. All past and future experiments are planned so they result in the creation of sets of experiment data that can be used to validate CFD simulations. The dispersion modelling work is being performed using Commercial Computational Fluid Dynamics (CFD) codes, FLUENT and FLACS with associated 3D visualisation using an Immersive Visualization Facility (IVF) at TAMU-QATAR.
Lead Principal Investigator / Coordinator
Dr. M. Sam Mannan
Associate Professor of Chemical Engineering, Managing Director of the Mary Kay O’Connor Process Safety Center(MKOPSC)-Qatar
The lack of fundamental understanding and improper hazard assessment of primary and secondary dust explosions are frequently the cause of serious incidents in the chemical process industry. Very few statistics of dust explosions are available in the open literature. However, from those that can be found, the severity and catastrophic consequences of these explosions are clearly reflected.
The explosions that occur in the petrochemical industry can be mainly attributed to four different categories: (1) Pressure vessel explosions, (2) condensed phase explosion, (3) runaway reactions and (4) dust explosions.
Many organic materials encountered in the petrochemical industry have the potential of being explosive when present in dust form. The potential explosibility of the dust handled in the petrochemical industry is reflected in the numerous explosions that occurred every year inside the equipment that process, transport and storage chemical dusts. For example, in different petrochemical industries in China 13 explosions in a powder silo of Low density polyethylene (LDPE) occurred in a 11 year period; 14 explosions of a powder silo of polypropylene (PP) in a 5 year period and 12 explosion of a powder silo of High Density Polyethylene (HDPE) in a 3 year period. The most widely known incident involving Polyethylene combustible dust is the incident that occurred at the West Pharmaceuticals plant, located in North Carolina, United States. This incident occurred on June 23, 2003, when a roughing fire and explosion, completely destroyed the plant. According to the final CSB report, the explosion was caused by the deflagration of fine polyethylene powder, whit less than 63 microns in diameter, which had accumulated above a suspended ceiling in the manufacturing area of the facility. The incident killed 14 and injured 81 people. Polyethylene is the most widely used polymer in daily life, some of its uses include: plastic bottles for not carbonates drinks, automobiles’ fuel tanks, chemicals and household packing, tubing, toys, etc. Therefore, it is of paramount importance to assure the safety in this product during its entire life cycle, i.e., production, storage, transportation and consumption. Polyethylene has very law flammability. However, fine polyethylene dust dispersed in air in a sufficient concentration, and in the presence of an ignition source, may pose a potential dust explosion hazard. This has been demonstrated in the different dust explosion incidents involving this polymer described above.
As a part of its expansive economic strategy set in the National Vision 2030 and the national development strategic 2011-2016, the Qatari petrochemical industry have been rapidly growing in the latest years and this growing trend will continue, particularly in the production of polyethylene.
Although, dust explosion research has been going on for more than a century, incidents continue to occur. Some of the identified research needs are addressed below:
• Effect of Polyethylene particle size disparity;
• Study of polyethylene dust in non-traditional forms (flocculent materials, hybrid mixtures, nano-particles);
• Use of powder aerosols for suppression of dust explosions;
• Study of Secondary Polyethylene Dust Explosions.
Texas A&M and the Mary Kay O’Connor Process Safety Center have been developing research on dust explosions in the last 5 years. The expertise team is composed by industry experts, professors and PhD students with a significant background on experimental and theoretical dust explosion research. The research group at MKOPSC-Qatar has recently invested in a 20 liter explosion test sphere that can be used to determine the different explosive parameters of dust clouds, e.g., the minimum ignition concentration, maximum overpressure and Kst value. The 20 litre sphere can also be used to study the turbulence effects on dust explosion; proper venting designing; and to study nano-materials and hybrid mixtures.
The laboratory at MKOPSC-Qatar is also equipped with a Modified Hartmann Tube designed as qualitative pre-test of the explosion behavior of a dust/air mixture.
In addition Texas A&M is equipped with the suitable supercomputer and software packages to perform CFD modeling.
Risk Analysis of Hazardous Materials
Olefins, principally ethylene and propylene, represent very large volume feedstocks to the petrochemical industry. High amounts of Ethylene are used in the production of styrene, polyethylene and other plastics. In order to get the magnificence of the ethylene, Europe can be taken as an example, where approximately 90% of its olefins pipeline network is ethylene service. 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. The Qatari petrochemical industry has been rapidly growing in the latest years. As part of its expansive economic strategy set in the National Vision 2030 and the national development strategic 2011-2016, the State of Qatar is investing around USD 25 billion in its petrochemical sector (from 2012 up to 2020) in order to increase its petrochemical production from about 10 million tonnes to about 23 million tonnes.
Ethylene is a chemical intermediate needed in the production of many plastic materials (polyethylene, polystyrene, PVC, …) used in agriculture, auto industry, chemical industry (oil additives, paints, solvents,…) and everyday life; for economic, safety and environmental reasons it is transported only in pipelines. Ethylene is transported thorough pipelines at high pressures and temperatures (above its critical point), and therefore it behaves as a super critical fluid (gas and liquid phases are indistinguishable). When the depressurization occurs, very low temperatures (below -100 °C) are achieved. Due to these low temperatures, pipelines used to transport ethylene pose high risk of rupture because the carbon steel can lose its ductile strength leading to the formation of a hole, further propagation and final rupture. The lack of fundamental understanding and improper hazard assessment for pipeline transporting ethylene and other hydrocarbons are frequently the cause of serious incidents in the petrochemical process industry which have led to significant fatalities, environmental and economic losses. Due to the magnitude of the potential causes of these incidents, fines exceeding hundreds of millions of dollars are being imposed to operating industries involved in incidents causing environmental damages. The current operating Qatar’s ethylene pipeline goes from Ras Laffan to Messaid. In order to avoid potential incidents related with the transport of ethylene through pipeline, it is of paramount importance understand the hazards involved and to perform a complete Risk Assessment program along the existent and future pipeline path. Depth experimental and theoretical studies are needed in order to investigate the hazards associated with the transport operation of ethylene, the properties of ethylene when encountered as a super critical fluid, the potential and characteristics of ethylene releases and most important the methods to prevent and mitigate the consequences of these releases.
In order to fully and properly asses the hazards associated with the depressurization of a pipeline containing ethylene, it is imperative to accurately predict the outflow following rupture or puncture of a pipeline containing pressurized ethylene, the fluid and wall temperatures, efflux rate, composition, phase and fluid pressure in the pipeline. The Health and Safety Executive (HSE) recently analyzed the performance of different computer codes used to calculate hydrocarbons pipeline blowdown, based on the comparison of the modeled results with experimental data (regarding LPG releases) and concluded that current computer codes are able to model full bore pipe ruptures, but the predictions show poor or non-fit to experimental data on puncture. This clearly demonstrates that the phenomena of how pipeline releases of hydrocarbons, and in this, ethylene occurs in small breaches is not well understood.
Therefore, it is of paramount importance to make further efforts in the modeling of ethylene pipeline releases in small breaches. It is important to notice that this cannot be possible unless enough and reliable set of experimental data is available to validate the simulation results. Currently no experimental data on ethylene releases are available in literature.
The objective of this work is to construct a pilot plant station capable to acquire reliable and trustable data on pipeline outflow releases transporting ethylene in order to be able to evaluate current models used to predict ethylene pipeline releases properties and make further improvements on these models when it is needed. In particular there is a need for a better understanding of:
• modelling accurately the flow deriving from small breaches in the pipeline;
• validating existing model describing the releases of supercritical fluids for the case of ethylene;
• study the effect of corrosion of the pipeline due to aging ;
• analyse the problem of the formation of hydrates;
• study a model for the dispersion of ethylene;
• study the consequences of the thermal radiation from an ignited ethylene release;
• NaTech (Natural hazard triggered Technological accidents) influence on ethylene pipeline.