Designing and operating safe chemical reaction processes HSG143

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Health and SafetyExecutiveHAZOP: see hazard and operability study.Heat of reaction: The total quantity of thermal energy liberated or absorbedduring a chemical reaction.Hybrid: A system type for relief sizing in which pressure is due to both vapourpressure of the reacting system and the generation of permanent gas from thereaction.Inherently safer: A system is described as inherently safer if it remains in a nonhazardoussituation after the occurrence of non-acceptable deviations from normaloperating conditions. Inherently safer is used rather than inherently safe because allhazards cannot be removed.Inhibition: A protective measure where the reaction can be stopped by addition ofanother material.Isoperibolic: see isoperibolic system.Isoperibolic system: A system in which the controlling jacket temperature iskept constant.Isothermal: A system condition in which the temperature remains constant.Jacket temperature: The temperature of the fluid contained in the reactor jacketused to indirectly heat or cool the reactor contents.Kinetic data: Data associated with the conversion rate of a reaction.Level swell: The increase in liquid level due to the presence of gas or vapourbubbles within the liquid.Loop reactor: Continuous flow reactors that are characterised by the fact that partof the effluent stream is re-supplied to the reactor, either directly or mixed with areactant supply stream.Occupational exposure limit: The legal limits of concentration in the air ofsubstances hazardous to health, averaged over a specified time: often referred toas an OEL.Onset temperature: Defined as the temperature at which the heat released bya reaction can no longer be completely removed from the reaction vessel, andconsequently, results in a detectable temperature increase. The onset temperaturedepends on detection sensitivity, reaction kinetics, vessel size and on cooling,flow and agitation characteristics. Scaling of onset temperatures and applicationof ‘rules of thumb’ concerning onset temperatures must be regarded as highlyunreliable.Permanent gas: A gas that cannot be condensed (or solidified) under the range ofconditions of interest (eg temperature or pressure).Phi-Tec TM : A type of adiabatic calorimeter.Pyrophoric: A material that will burn in air without the addition of an ignitionsource.Quenching: Severe cooling of the reaction system in a short time.Designing and operating safe chemical reaction processes Page 62 of 64
Health and SafetyExecutiveRate of reaction: The rate at which conversion of reactants takes place.Reactants: Chemicals that are converted into products during the reactionprocess.Reaction: The process in which chemicals (reactants) are converted into otherchemicals (products).Reaction calorimeter: A laboratory test apparatus for measuring thermal effectsof chemical reactions or processes. For the purpose of this guidance, types ofreaction calorimeters include isothermal, isoperibolic and adiabatic systems.Reactive systems screening tool (RSST TM ): A type of pseudo-adiabaticcalorimeter.Reasonably practicable: The degree of risk in a particular job or workplaceneeds to be balanced against the sacrifice in time, trouble, cost and physicaldifficulty of taking measures to avoid or reduce the risk. Measures must be takento eliminate or control the risks unless it is clear that the sacrifice incurred in doingso is grossly disproportionate to the level of the risk. However, the ability to payfor additional control measures is not a deciding factor as to whether they arenecessary.Reflux: A system condition in which a component in the reaction system(usually a solvent) is continually boiled off, condensed in a nearby condenser andsubsequently returned to the reaction system.Relief system sizing: the sizing of an emergency relief vent.Relief system: All parts of the pressure-relief flow path from the protected vesselto atmosphere or containment within a disposal system. This includes reliefdevice(s), piping and any containment and disposal system.Risk: The chance of something adverse happening where ‘something’ refers toa particular consequence of the manifestation of a hazard. Risk reflects both thelikelihood that harm will occur and its severity in relation to the number of peoplewho might be affected, and the consequences to them.Risk assessment: The process of identifying the hazards present in anyundertaking (whether arising from work activities or other factors) and those likelyto be affected by them, and of evaluating the extent of the risks involved, bearing inmind whatever precautions are already being taken.RSST: see reactive systems screening tool.Runaway: A reaction that is out of control because the rate of heat generation byan exothermic chemical reaction exceeds the rate of cooling available.Semi-batch: An operation in which some materials are added to the reactor at thestart, with one or more other reactants added in a controlled manner during thereaction. See also semi-batch reactor.Semi-batch reactor: A type of reactor that is characterised by the supply, in acontrolled manner, of one or more key reactants during the reaction. Productsare only taken from the reactor upon conclusion of the reaction process.Designing and operating safe chemical reaction processes Page 63 of 64
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