c04_storage_handling_and_processing_of_dangerous_substances_COMPLETE Flashcards
The principles of iron and steel-making have not changed much since Roman times. The process is simple and uses four major raw materials:
iron ore, coke (from coal), limestone and air.
Increasing the collision rate between particles will increase the rate of reaction. The key variables are: 5
Temperature: Increasing temperature speeds up a reaction (except for those involving enzymes). Pressure: Pressure increases will increase the rate of reactions involving gases. Concentration: Increasing concentration normally increases the rate of reaction. Surface area of reactant: Increasing the surface area of reactants increases the rate of reaction. Catalyst: The presence of a catalyst is intended to increase the rate of a reaction.
Exothermic reaction means…
In many chemical reactions, energy is given out by reactants as they form products, causing the temperature of the surroundings to rise.
Endothermic reaction means…
the opposite of exothermic reactions. In an endothermic reaction energy is taken in by the reactants to form products. The energy comes from the surroundings which lose energy and cool down, resulting in a drop in temperature.
Thermal runaway begins when
the heat produced by an exothermic reaction exceeds the heat removed. The surplus heat raises the temperature of the reaction mass, which causes the rate of reaction to increase, which in turn accelerates the rate of heat production.
At best, a runaway causes loss and disruption of production, at worst it may result in a major accident. The harmful effects could include: 6
the boiling over of the reaction mass large increases in temperature and pressure resulting in an explosion blast and missile damage secondary fires or explosions (if the materials are flammable) contamination of the workplace by hot liquors and toxic materials toxic clouds that may spread off-site.
An analysis of thermal runaways in the UK has indicated that incidents occur because of: 4
inadequate understanding of the process chemistry and thermochemistry inadequate design for heat removal inadequate control systems and safety systems inadequate operational procedures, including training.
Inherently safer methods Wherever possible hazards should be eliminated or reduced by inherently safer design. Examples include: 4
Replacing hazardous materials with safer ones. Having less unreacted material in the reactor. For example: using a continuous process instead of a batch reactor. Using a semi-batch method (in which one of the raw materials is added over time) instead of a batch process. Using a heating medium which has a maximum temperature that is too low for the reaction mixture to decompose.
Process control includes the use of
sensors, alarms, trips and other control systems that either take automatic action, or allow for manual intervention to prevent the conditions for uncontrolled reaction occurring.
Protective measures do not prevent a runaway but are intended to reduce the consequences should one occur. They are rarely used on their own as some preventive measures are normally required to reduce the demand upon them. Examples include: 5
Designing the plant to contain the maximum pressure. Fitting emergency relief vents and ensuring that the vented material goes to a safe place. Crash cooling the reaction mixture if it moves outside set limits by providing additional cooling by: - bringing a reflux condenser on-line - using a refrigerant in the reactor cooling coils or jacket - pumping the reactor contents through an external heat exchanger. Adding a reaction inhibitor to kill the reaction and prevent runaway. For example: by free-radical scavenger injection or by poisoning a catalyst. Dumping the reaction into a quenching fluid.
Exothermic chemical processes rely on efficient control of temperature and pressure to prevent the phenomenon of ‘runaway reaction’ (thermal runaway) when mixing chemicals in a reactor. Outline the methods used to control temperature and pressure in such circumstances. 10 marks
An important method to control temperature and pressure would be the installation of high integrity temperature detection linked to the cooling and reactant addition systems with pressure rise detection linked to cooling, venting or auto shut-down. Consideration could also be given to the external cooling of the reactor by chilled water jacketing, an air cooled heat exchanger or the use of material with high heat conductivity in the construction of the reactor or internal cooling by the use of water or fluid refrigerant cooled coils. A further method would involve the control of the reactant by pre-chilling it, limiting the rate of its addition to the reactor, controlling the subsequent reaction by high efficiency temperature detection loops and the depth and level of reaction by pocketed thermal probes. The prevention of reactant bulk insulation can be prevented by efficient mixing with a mechanical stirrer while it would be advisable to locate the reactor away from external sources of heat and to introduce a system of regular internal cleaning to prevent the build-up of insulating residues.
The primary legislation applying to the control of substances that can cause fires and explosions in the workplace is
the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR).
Dangerous substances may be liquids, gases, vapours or dusts and include: 3
Substances or preparations classified as explosive, oxidising, extremely flammable, highly flammable, or flammable under the current CHIP Regulations, or CLP Regulation. Any kind of dust that when spread in air to form a cloud (i.e. form an explosive atmosphere), can explode. Any other substances, or mixtures of substances which, because of their physical properties and the way in which they are present in the workplace, create a risk to safety from fires and explosions, but which may not be covered by CHIP. For example high flashpoint liquids present in the workplace at elevated temperatures.
Overview of the Dangerous Substances and Explosive Atmospheres Regulations 2002 (DSEAR)
Regulation 5: Risk assessment Regulation 6: Elimination or reduction of risks from dangerous substances Regulation 7: Places where explosive atmospheres may occur Regulation 8: Arrangements to deal with accidents, incidents and emergencies Regulation 9: Information, instruction and training Regulation 10: Identification of hazardous contents of containers and pipes Regulation 11: Duty of co-ordination
Hazard identification (determination of the presence of dangerous substances) requires consideration of the following three variables:
(1) The substance or preparation is classified under the Chemicals (Hazard Information and Packaging for Supply) Regulations (CHIP) as: explosive, oxidising, extremely flammable, highly flammable or flammable. (2) The physical and chemical properties of the substance or preparation, and the work processes involved, creates a potential for fire, explosion or similar energetic (energy releasing) event. (3) The work activity involves the creation or handling of potentially combustible or explosive dusts.
factors that should be taken into account when undertaking a dangerous substances risk assessment.
The hazardous properties of the substance Supplier safety information The circumstances of the work Particular activities, which may present a high level of riskThe likely presence of explosive atmospheres and the need for hazardous area classificationSize of release Temperature and pressure VentilationExtent of zones
Bulk storage of flammable liquids Bulk storage refers to tanks with a capacity in excess of
1000 litres.
Control measures for the storage of flammable liquids can be summarised by the acronym
VICESVentilationIgnition sources (control of)ContainmentExchange (substitution)Separation
The storage of flammable liquids in IBCs in warehouses poses particular problems, notably: 3
They are prone to early failure in a fire, allowing leaked contents to feed the fire. They degrade over time, and so are prone to leaks when used for long term storage. They are generally made of cheap, non-conductive plastic, so when non-conducting flammable contents splash around in transit the surface becomes electrostatically charged.
Outline the key safety features of a facility that is to be used for the storage of highly flammable solvents in 200 litre drums 10 marks
Key safety features include: bunding to contain spills, including a facility to collect and dispose of spillages the building to be erected on an impermeable base with an adequate separation distance from other buildings and constructed of fire resistant materials with a light weight roof or blast panels a facility for the segregation of materials adequate access and egress including a ramp to facilitate the handling of drums the provision of high/low ventilation and of sprinklers/ fire extinguishers to be used in the event of an emergency the clearance of vegetation round the storage area security features such as the provision of locks and warning signs.
A company intends to build a flammable solvent distribution facility as part of its chemical manufacturing premises. The facility will include three 40,000 litre storage tanks that are pump filled via pipelines from batch reactors. The storage tanks supply an outdoor road tanker filling system as well as a small container filling facility located inside a warehouse. Outline the design features that should be adopted to prevent or minimise leakage and spills from the proposed installation. 10 marks
Design features to be adopted include: Ensuring the pipework is of all weld construction with the minimum number of flanges with a suitably corrosion resistant material used in its construction. The pipework should be routed along a containment trench or should be double skinned and further protected by robust barriers at vulnerable points near to roads or railways. The storage tanks should be in a bunded area capable of containing 110% of the contents of all three tanks with the base and walls of the bund being impervious and free from breaches for services. The tanks should be fitted with high level detectors interlocked with a pump cut out with the pumps themselves being situated within bunds with interceptor facility. The road tanker stands should be located in an intercepted shallow bund with snap shut connections on the tanker filling lines. In the warehouse the small keg filling lance should be fitted with a dead man’s handle together with a supply cut out when the expected weight is registered on a load cell. Finally all valves in the installation should be designed to prevent leaks such as for example with a double mechanical seal with means for preventing them being left in a partially open state.
