Process Safety–A Subject for Scientific Research

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<ul><li><p>PROCESS SAFETY A SUBJECT FORSCIENTIFIC RESEARCH</p><p>N. GIBSONBurgoyne Consultants Ltd, Ilkley, West Yorkshire, UK</p><p>T he identi cation and control of process hazards is a technical activity linked closely tothe technology of individual manufacturing processes. By consideration of incidentsand assessment procedures it is shown that safety requires ongoing experimentalresearch.</p><p>Examples are given of the use of multi-sponsored projects to fund and support the requiredresearch.</p><p>Keywords: dust explosions; process safety; safety research.</p><p>INTRODUCTION</p><p>Manufacturing in many industries (oil, chemical, petro-chemical, agrochemical, pharmaceutical, paints, food-stuffs, etc.) involves the processing of one or more ofreactive chemicals, ammable liquids, vapours, gases andpowders.</p><p>A strategy is required that ensures that the industrialoperations are carried out safely. The objective of thestrategy is to establish and maintain safe operations in amanner that is compatible with the plant design, theoperating conditions, production demands, commercialrequirements and economic factors.</p><p>In essence this is the objective of process safety toprevent uncontrolled events in industrial operations.</p><p>Process safety is not an add-on but an integral part ofprocess development and manufacturing. Furthermore,the identi cation, evaluation and control of process hazardsis a technical activity linked closely to the technology ofindividual manufacturing processes.</p><p>It is the thesis of this paper that safety guidelines mustbe based on an understanding of the scienti c and technicalprinciples that control the stability of a process and thatinnovative activity scienti c research is an essentialpart of process safety.</p><p>LESSONS FROM INCIDENTS</p><p>The use of hindsight wisdom after the event as it isde ned in the dictionary although to some extent anadmission of failure in the process safety eld, can help tomake safe future operations.</p><p>Consideration of the following two incidents indicatesthe factors in uencing process safety.</p><p>Explosion in a Drying Operation</p><p>In 1976 an explosion occurred during the drying of awater wet powder (3.5 di-nitro ortho toluamide) in a doublecone dryer. Extensive damage was caused to the plant andbuildings.</p><p>A Health and Safety Executive (HSE) investigation1</p><p>concluded that the explosion involved the detonation ofthe product which had been left inside a closed dryer vesselfor a period of 24 hours after the drying process had beencompleted.</p><p>The incident is an example of uncontrolled exothermicdecomposition. This occurs when the rate of heat generatedby the decomposition reaction exceeds the rate of heatloss from the material. It was considered that the tempera-ture of the dryer contents would have increased over theweekend at an ever increasing rate until the product ignitedand burnt to detonation. This manifested itself as anextremely rapid pressure rise probably to several hundredbar in the shock wave. The rupture of the dryer probablyoccurred at a pressure of about 50 bar g.</p><p>When materials are subjected to heat or a chemicalreaction is exothermic, establishment of a safe processrequires a knowledge of the minimum temperature atwhich uncontrolled exothermic activity could be initiated.Determination of this temperature requires an understand-ing of the heat generation and the heat loss mechanism inthe manufacturing operation.</p><p>The stability of this product had been determined usingDifferential Thermal Analysis (DTA) and this indicatedan onset temperature of 274C to 284C. These temperatureswere well above the drying temperature of 130C to 140C.However, the sample in DTA is small (milligrammes) andthe temperature detection requires heat to ow from thesample to the detector. The mass of product in the dryerwas about 1300kg so heat loss rate was signi cantly lessthan in the DTA equipment. The dryer produced a nearadiabatic situation.</p><p>Research into chemical reaction hazards has led to thedevelopment of two techniques that simulate adiabatic</p><p>149</p><p>09575820/99/$10.00+0.00 Institution of Chemical Engineers</p><p>Trans IChemE, Vol 77, Part B, May 1999</p><p> The text of this paper was rst presented at the Eleventh Vernon ClanceyMemorial Lecture given at City University, London,UK on 17 March 1999.This years lecturer was Dr Norbert Gibson, a consultant with BurgoyneConsultants Ltd. Copyright of this paper remains with the author.</p></li><li><p>conditions: the ARC adiabatic calorimeter2 developed byDow and a test cell based on Dewar vessels that simulatedthe heat ow characteristics of chemical reactors3 developedby ICI.</p><p>The development of both techniques has involvedconsiderable research effort but its value can be seen bythe fact that the techniques indicated onset temperatures of115C to 125C for this material.</p><p>Electrostatic Ignition of a Dust Cloud</p><p>This incident occurred during the simple operation ofloading powder down a chute into a chemical reactor. Theoperation had been carried out for many years when,withoutwarning, a dust explosion occurred in the feed chuteand the operator was fatally injured.</p><p>The powder being loaded down the chute was anthra-quinone, and the contents of the reactor were sulphuricacid. Thus the only ammable atmosphere was a dust cloudof anthraquinone. Static electricity was identi ed as thecause of the explosion.</p><p>The incident was caused by:</p><p> Electrostatic charge being generated on the powder as it owed over the surface of the chute. Initially the chute was fabricated from metal and earthed;this safely dissipated the static electricity. To aid powder ow, the metal chute was replaced by a rubberized chutewith a metal spiral support. The non-conducting rubber accumulated electrostaticcharge, transferred it to the metal spiral insulated fromearth and this released incendive discharges. A dust cloud of anthraquinone was found to have a lowMinimum Spark Ignition Energy (</p></li><li><p> Safety margin between operating temperatures andexotherm temperature. Monitoring and control systems to maintain temperaturein the safe region. Maintenance of temperature should agitation or coolingfail for example, stop feed reactant, use solvent that boilsat safe temperature. Control sources of risk external to process for example,addition of wrong materials. Speci cation of lower temperature limit to preventaccumulation. Consequences on two-phase systems of agitation failure.</p><p>Process Control and Reactor Venting</p><p> De nition of worst case that is, conditions leading tomaximum rate of exothermic activity. Establishment of kinetics of the runaway reaction. Nature of discharge material gas, liquids, solids. Methods for calculating reactor vent area and dischargesystem for the vented materials. Safe discharge area ammable and toxic hazardsdump tanks.</p><p>Process Control and Crash Cooling/Drown-Out</p><p> Rate of temperature rise/heat generation after runawaydetected. Time to hazardous pressure. Availability of compatible cooling medium. Relative thermal capacities of reaction mass and coolingmedium. Plant design/operation to intermix reaction mass andcooling medium and stop the temperature rising beforemaximum permissible pressure is attained.</p><p>Process Control and Reaction Inhibition</p><p> Availability of compatible reaction inhibitor. Time to hazardous pressure. Inhibitor ef ciency. Plant design and operation to intermix reaction massand inhibitor and stop the temperature rising beforemaximum permissible pressure is attained.</p><p>In addition to the control of chemical reaction hazards,consideration has also to be given to the re and explosionrisk associated with the processing of ammable gases,vapours, liquids and powders.</p><p>To eliminate such hazards each stage of manufacturemust be considered in terms of:</p><p> Identi cation and characterization of ammable mate-rials. Identi cation of potential ignition sources. Selection, design and installation of the most appropriatesafety measures.</p><p>In identifying and characterizing ammable materialsconsideration has to be given to (1) presence of amm-able atmosphere, (2) sensitivity to ignition of the ammableatmosphere and (3) the potential violence of any re orexplosion.</p><p>Consideration of ignition sources in a speci c process/plant is concerned with auto-ignition, mechanical friction,</p><p>thermite reaction, static electricity, spontaneous combus-tion, thermal decomposition, pyrophoric catalysts and anyother ignition sources intrinsic to the process and plantoperation.</p><p>Once the ignition risk has been established, attentionhas to turn to safety measures. These can be one or moreof the following:</p><p> Avoidance of ammable atmosphere. Use of inert gas oroperating outside the ammability limits. Avoidance of all ignition sources. Containment of re and explosion. Explosion venting. Explosion suppression.</p><p>Critical technical considerations for each are:</p><p>Avoidance of Flammable Atmospheres</p><p> Can fuel concentrations be maintained outside amm-ability limits at all times including start-up and shutdown? Is the material dependent on atmospheric oxygen forcombustion and/or decomposition? Can the system be sealed to prevent ingress of air? Can ingress of air be avoided when reactants are addedfor example, air entrained in powders?</p><p>Avoidance of All Ignition Sources</p><p> Can all ignition sources be identi ed? Is the sensitivity to ignition by these sources known forall the materials in the process? Can all ignition sources be eliminated under normal andabnormal conditions?</p><p>Containment of Explosion/Decomposition</p><p> Can the maximum pressure developed in explosion/decomposition be predicted? Can all interconnected components withstand the maxi-mum pressure? Can the system be mechanically separated into discretevolumes to prevent pressure piling? Can the system be sealed at high pressures? Can process operations (for example, addition of powder,sampling) be carried out with a pressure-sealed system?</p><p>Explosion Venting</p><p> Can maximum rates of pressure rise under processconditions be established? Can adequate relief areas be provided relevant to processconditions? Can a safe discharge area be provided for ammable/toxic products?</p><p>Explosion Suppression</p><p> Is pressure arising from combustion the sole source ofpressure? Suppressant systems cannot control pressureresulting from gas evolution. Are the combustion characteristics of the processmaterials such that the suppressant can effectively stop ame propagation?</p><p>151PROCESS SAFETY A SUBJECT FOR SCIENTIFIC RESEARCH</p><p>Trans IChemE, Vol 77, Part B, May 1999</p></li><li><p> Are the suppressant chemicals compatible with theprocess chemicals?</p><p>In all the above areas the technical input is critical to thereaching of correct decisions with respect to both assess-ment of risk and speci cation of effective safety measures.</p><p>Process safety assessment requires both good, up-to-dateand well researched technical information and the proce-dures to apply it. Both are important.</p><p>In the past, research in the eld of process safety wascarried out in both industry and academia. In recent years,whilst papers in hazard assessment procedures continue toproliferate, the publication of scienti c or technical papersis in rapid decline.</p><p>Every two to three years the North Western Branch ofthe Institution of Chemical Engineers organizes a majorconference on process safety that has enjoyed UK andoverseas input since the 1960s.</p><p>Up to the end of the 1980s, about 60% to 90% of thepapers were concernedwith scienti c research and technicalinnovation. In the 1990s this has declined to less than 25%,with the greater proportion of papers being concerned withregulatory or procedural matters.</p><p>In the commercial conference eld the attendance atscienti cally orientated meetings has dropped from 100120 to 3040 people whilst the non-scientic meetingscontinue to maintain their numbers.</p><p>Following a major incident, Trevor Kletz5 stated that,putting too much trust in systems is a common failingtoday. There is an epidemic of papers and books on safetymanagement but they are no substitute for knowledge andexperience. All they can do is to ensure that knowledgeand experience are applied in a systematic and ef cientway.</p><p>The knowledge that we use must be up to date. Newmanufacturing processes must be developed, and existingones improved, for industry to survive. As with the otheraspects of process development and plant design, innovativeactivity must be an essential part of process safety.</p><p>PROCESS SAFETY RESEARCH A PATTERNFOR THE FUTURE?</p><p>The dearth of papers on technical process safety re ectsa real reduction in scienti c experimental research on thistopic both in industry and academia.</p><p>An area of concern to industry is the control of dustexplosions.About 75% of all powders processed in industryare combustible and if dispersed into a dust cloud andignited they will explode in a manner akin to vapour clouds.</p><p>Dust explosions are not a new phenomenon. In 1795 aCount Morozzo described6 an explosion in a our ware-house and added some observations on spontaneousin ammations. The ignition source in this case was the ame of a lamp.</p><p>In August 1998 a major grain explosion occurred atBlaye, France. A silo installationwas badly damaged and 11people killed. The source of ignition is considered to bemechanical impacts or friction in the fan or an incipient redue to auto-heating in the dust store.</p><p>In the years between these incidents, dust explosionshave continued to occur. Statistics, albeit not the mostcomprehensive, suggest that the frequency of explosions</p><p>is one a week in the UK and one a day in continental Europe.In the late 1980s it was generally recognized in Europethat our scienti c understanding of dust explosion phenom-ena and means of controlling them were far from complete,and that there was a need for research and development,not only to minimize the possibility of industrial dustexplosions, but also to control the consequences of thosethat occur.</p><p>Dust explosion research requires large-scale test facili-ties of high cost. Individual organizations were reluctantto provide the necessary nance, and important problemareas were not currently the subject of research.</p><p>Furthermore, industry-based research in this eld tendsto be short term in its objectives and aims to provideexplosion protection for existing operations or those underimmediate development. This approach tends to lead tosafety measures that are added to the plant and notto general information that can be used to modify processesand plants in a manner that leads to more intrinsicallysafe plants, in which safety measures are an integral partand not an add-on feature.</p><p>It was considered that research was required thatwould change our knowledge base from one derived fromlimited empirical experimentation, to one that can be usedto identify potential problems and prescribe measures toeliminate or control dust explosion hazards in a morefundamental and widely applicable manner.</p><p>In 1991, under the aegis of the British MaterialsHandling Board and funded by the Department of Tradeand Industry (DTI), Department of the Environment (DoE)and Health and Safety Executive (HSE), a Europe-wides...</p></li></ul>


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