Review of Safe Systems in Runaway Reactions

Journal of Loss Prevention in the Process Industries 14 (2001) 27–42www.elsevier.com/locate/jlp

Review of the selection and design of mitigation systems forrunaway chemical reactionsR.D. McIntosh 1, P.F. Nolan *

Explosion and Fire Group, Chemical Engineering Research Centre, South Bank University, Borough Road, London SE1 0AA, UK

Abstract

The chemical and pharmaceutical industries employ many exothermic reactions, and loss of control can lead to thermal runaway.Although control systems are becoming more and more sophisticated, vent systems are still commonly employed. In future years,the increase in environmental issues is likely to outlaw direct atmospheric venting, and there remains the need for passive mitigationof runaway reactions and vented materials. Various options are discussed, including inhibition, quenching of the reactants andseparation of the liquid and gas phase before further treatment or venting. Advantages and disadvantages are given. Various currentlyavailable design methods are given for separation vessels and quench systems. 2000 Published by Elsevier Science Ltd.

Keywords: Runaway; Inhibition; Quenching; Separation vessels

1. Introduction

Many exothermic reactions are used in the chemicaland pharmaceutical industries. Control of these reactionsis of paramount importance, and loss of control can leadto thermal runaway. The consequence is an uncontrolledincrease in temperature within the reactor. If, as is nor-mally the case, the reaction generates a gas phase or isheated above its atmospheric boiling point, an additionalhazard of pressure will be present. In order to preventexplosion of the reactor vessel, vents are incorporatedinto the system. The DIERS committee and project(Fisher et al., 1992) were set up after realisation thatvent sizing methods were often inadequate, and thatmany were designed for single-phase releases whereastwo-phase releases are predominant. The DIERS projectgave substantial consideration to safely sized vents, andhas made a significant contribution to ensuring that ventsare adequately sized and that ruptured reactor vessels arevery rare.A consequence of DIERS is that further work is

necessary, as venting of two-phase releases in turn gen-erates hazards. The vented material may be:

* Corresponding author. Tel.: +44-020-7815-7901; fax: +44-020-7815-7991.

1 Current address: Hazard Evaluation Laboratory Ltd, 50 MoxonStreet, Barnet, Herts, EN50 5TS, UK.

0950-4230/01/$ - see front matter 2000 Published by Elsevier Science Ltd.PII: S0950-4230 (99)00085-6

! toxic;! corrosive;! flammable; and! at high temperature.

The venting of such materials direct to the atmosphereis undesirable, generating hazards in the vicinity of thereactor, where the bulk of the liquid will fall out fromthe atmosphere and be deposited on local plant. Aflammable mixture may result in an explosive atmos-phere, and toxic, corrosive or very hot liquids are adanger to personnel. An expensive clean-up procedurecan be expected where the liquid deposits on processplant, particularly when solidification is likely. Majorincidents involving vented reactors include Bhopal(Bowonder & Miyake, 1988) and Seveso (Kletz, 1994),where untreated materials were allowed to pass to theatmosphere, with devastating consequences for thelocal community.Although major incidents such as these are rare, les-

sons must be learned and improved safety considerationsmay be necessary. In Great Britain, in a three year periodto 1996, there were 179 incidents involving exothermicchemical reaction reported to the Health and SafetyExecutive (Fowler & Hazeldean, 1998). These involved90 injuries and one fatality. Although many of the inci-dents did not result in injury, there is always the potentialfor incidents to become serious. Their occurrence indi-

28 R.D. McIntosh, P.F. Nolan / Journal of Loss Prevention in the Process Industries 14 (2001) 27–42

Nomenclature

Al cross-sectional area occupied by liquidAs area available for flow in cyclone skirtAv cross-sectional area available for vapour/gas flowC drag coefficientCq specific heat capacity of quench fluidCr specific heat capacity of reactantsDs diameter of skirtd liquid particle diameterF empirical constantG superficial gas mass fluxg gravitational constant (9.81 m/s2)K constant used in Souders–Brown relationshipMl mass flow rate of liquidMv mass flow rate of vapour/gasQ volumetric flow rateRe Reynolds numberTop operating temperature of the reactorTqf final quench fluid temperatureTqi initial quench temperature (ambient)Tr,max maximum temperature of reactorua maximum allowable velocityud drop-out velocitymg gas viscosity (cP)rl liquid densityrv vapour or gas density

cates that dangers and unforeseen circumstances doexist, and consideration must be given to the hazardspresent.

2. Control and mitigation measures

There are many methods of improving the safe oper-ation of chemical reactors that may undergo runaway.Identification of the possibility of runaway and thedesign of mitigation procedures should be undertaken inthe early stages of plant design, and there can be littleexcuse for operation of plant without such measuresbeing fully operational. The current practices with regardto dealing with runaway reactions are:

! process control;! process control plus reaction inhibition;! process control plus containment; and! process control plus venting.

Process-control systems are becoming ever more sophis-ticated and reliability is improving. In some cases, thecontrol system may incorporate sufficient protectionagainst runaway. No control system can operate withoutpower, and a back-up supply may be necessary. Dupli-

cation of hard wiring and operation of a secondary,emergency, control system may also be incorporated intothe safety design study.Where process control alone is deemed inadequate

against the possibility of runaway, reaction inhibitioncan be used. If a substance can be identified which, invery small quantities, will halt or at least dramaticallyslow down the reaction, the reaction may be stabiliseduntil cooling returns on-line or other control systemsbecome effective. The application of inhibition systemsis discussed below.In the event of runaway, the ideal scenario is complete

containment of the reactant mass. There are severaloptions for this type of system. The design pressure ofthe reactor vessel may be increased to the maximumpressure attained by the reaction. In the case of vapour-generating reactions, where the pressure increases expo-nentially with temperature, this may not be a practicalsolution. Encasement of the reactor in concrete or in asteel or concrete bunker may provide an alternative. Aninteresting system is given by Speechly, Thornton, andWoods (1979), where the reactor is vented into a largeclosed vessel (total containment vessel). The increasedvolume reduces the pressure of the reaction system. Con-tinued reaction will still occur, but if little or no vapouris present, the increase in volume for the gas phase will

29R.D. McIntosh, P.F. Nolan / Journal of Loss Prevention in the Process Industries 14 (2001) 27–42

result in a lower ultimate pressure. Where a vapour ispresent, the cooling effect of the expansion due toreduced pressure may be used to slow the reaction sig-nificantly such that it can be contained by the system.Several reactors can be connected to the same contain-ment vessel, increasing the economy of such a system.It should, however, be noted that the cause of runawayin one reactor is likely to be present in each reactor (e.g.,loss of power or global cooling, etc.).As has been mentioned previously, venting is often

the most practical system for the relief of runaway reac-tors, and regardless of other safety systems, a vent willnormally be present on a reactor, directing any flow toa known location rather than resulting in an explodingreactor. The DIERS committee has given very good andcomprehensive recommendations regarding the sizing ofvents for two-phase flow. Clearly, simply having a venton a reactor is not enough, and some consideration mustbe given to treatment of the discharge or at least to thedirection and location of the end of the vent line. Duringventing, the discharge may be passed to:

! a vent stack;! a liquid/vapour separator;! a quench tank;! a scrubber;! an incinerator; or! a flare stack.

Often, several steps will be combined to form a relieftrain, where the liquid phase is removed from the dis-charge before the gas or vapour is passed to a vent orflare stack for discharge to atmosphere. It is importantthat a suitable system is fitted for the batch being pro-cessed. Many reactors in the pharmaceutical and special-ity chemical industries are multi-purpose, and differentmitigation systems may be relevant in each batch. It maybe necessary to have more than one system available inthese cases.

2.1. Vent stack

A vent stack is used to disperse the gas phase at anelevated position in the atmosphere. Its applicationshould prevent dangerous concentrations of the gasesand vapours at ground level in the locality of the plant.Obviously toxic or flammable mixtures should not bevented through a vent stack. Direct connection to a ventline is only applicable where it is certain that only asingle gas phase will be passed through the vent line.Most discharges will however be two-phase, and separ-ation treatment will be necessary before passing thegases to a vent stack.

2.2. Liquid/vapour separators

Liquid/vapour separators take many forms, mainlyusing gravitational or centrifugal forces to effect separ-ation. Dump tanks, catch pots and knock-out pots ordrums are synonyms given to gravitational separators.The two-phase mixture is passed through a vessel wherethe flow area increases, slowing the flow to a level whereseparation of the liquid from the gas phase will occur.Obviously, the larger droplets will separate more easilythan the small particles, and such particles may needfurther treatment (e.g., scrubbing) before they separateout. Dump tanks may be mounted either horizontally orvertically, with horizontal vessels normally having theinlet and exit lines at opposite ends. Alternative designsmay have the inlet at the centre and exits at either end,or inlets at either end and a single exit in the centre.This is particularly useful where two vents are used sim-ultaneously. The inlet pipes of the dump tank may bedirected straight downwards or with elbows to directflow against the end of the vessel. Vertical vessels havethe inlet at a level above the predicted fill level of theseparated liquid and the exit at the top. The inlet maybe directed straight into the vessel or tangentially againstthe wall to take advantage of any centrifugal effects.Larger droplets will separate much more readily, and toseparate small droplets, a mist eliminator may be incor-porated upstream of the exit nozzle of the dump tank.Cyclones use centrifugal force to remove the liquid

from the gas phase. The reactor vent line is passed tan-gentially into the cyclone, and the reactor effluent streamspirals in a downward direction until it passes beneaththe skirt. During this time, the acceleration of the flowcauses the liquid to be thrown against the outer wall bycentrifugal force. A film of liquid then flows down thewall and can be collected at the base. Only very fineparticles of liquid will remain in the gas stream as itexits at the top of the cyclone.Cyclone separators are particularly suitable where

there is a large gas flow, and the required area for flowin a suitable dump tank would necessitate an impractic-ably large vessel. There are, however, several disadvan-tages. A large pressure drop at the inlet requires thatthe vent line be redesigned to cope with significant backpressure, increasing the vent size. Larger cyclones resultin less force on the liquid droplets, as the angle of thecircumference is reduced. Smaller cyclones change thedirection of the flow more dramatically, thereforeincreasing the centrifugal force. Several smaller cycloneswill be more effective, and in some cases may be neces-sary. The cost of a cyclone is also very much higher thanthat of a dump tank, which is in effect a plain vessel.An alternative centrifugal separator is presented by

Muschelknautz and Mayinger (1990), where flowthrough the separator drives a fan which in turn gives acyclonic motion to the two-phase mixture. The liquid


droplets are thrown on to the cylinder wall and flowdownwards back to the reactor. The separator unit sitson top of the reactor, and has the advantages of takingup no more space than the reactor and of being self-operating.Fauske (1990) suggests a method of separation by

directing the vent pipe against a wall or plate. The coo-ling effect of the expanding gas can be used to condensevapours, and the largest droplets will impinge on theplate and flow downward for collection. Experimentalwork on this system resulted in a plate temperaturebelow the boiling point of the flashing liquid. Such asystem would only be applicable where the gas phasecould be immediately released to the atmosphere.

2.3. Quench systems

Passive quench tanks offer an alternative method ofseparating the liquid and gas phases. The two-phasevented mixture is sparged through a dedicated liquid,which cools the reaction preventing further runaway,condenses vapours and allows only non-condensablegases to escape or pass on to a vent stack or for furthertreatment. Quench systems are versatile in that thequench tank may be open or closed. An open system isbest suited where large volumes of non-condensablegases are present, and these are allowed to exit the ves-sel. A closed tank may be used when the gas phase iscondensable (vapour pressure systems) or where onlysmall volumes of non-condensable gases are present.The vapours are cooled and condensed by the liquid, andany pressure in the quench tank (back pressure to thereactor) is due to non-condensable gases. In addition toany gases from the reactor, a slight pressure rise will beattributable to the compression of the void space abovethe quench liquid when the reactor mass enters the tank.The major uncertainty in quench systems is the design

of the sparger for optimum mixing and subsequent heattransfer from the quench fluid to the two-phase mixture.The presence of large vapour bubbles may prevent someof the vapours from contacting the cold liquid interface.The size and number of holes need to be properlydefined.An alternative to direct sparging is given by Herman

and Rogers (1995), who used jet condensers to pass thetwo-phase mixtures into the quench fluid. The advan-tages are a more rapid mixing and consequent heat trans-fer. A smaller quench vessel is therefore required. It hasalso been stated that the presence of a stable foam hasno effect on heat transfer and mixing within the tank.Foaming systems create great problems in terms ofphase separation.

2.4. Scrubbers, incinerators and flare stacks

These systems are unsuitable for treatment of mosttwo-phase releases, where large droplets of liquid are

present. They are more likely to be applicable to furthertreatment of the gas-phase flow as part of a relief train.Separators will remove the largest droplets from the two-phase mixture, allowing only very small liquid particlesto pass in the gas stream. Scrubbers are used where thesedroplets need to be removed before the cleaned gas canbe passed to the atmosphere. An example of this wouldbe where a “clean” gas, such as oxygen or carbon diox-ide, is present with a toxic liquid.Flare stacks and incinerators are used to treat the gas

phase by combustion to remove the toxic or flammablematerials in the effluent. Flare stacks can handle lowconcentrations of liquid droplets, up to 300 µm in diam-eter. Flows containing larger droplets may need to bepassed through an incinerator where a longer residencetime allows combustion of larger particles.

3. Inhibition systems

Inhibition involves injecting small quantities (in ppmquantities) of inhibitor to the reactor at an early stage ofthe runaway. The inhibitor used can either halt the reac-tion completely or reduce the reaction rate to delayfurther runaway for a time period, allowing time forabsent ancillary services or agitation to become avail-able. The suitability of a system for inhibition is depen-dent on the reaction mechanism occurring in the reactor.Inhibition is ideally suited to free-radical-initiated reac-tion systems (such as some polymerisation reactions),pH-dependent reactions and systems where the catalystcan be removed by the inhibitor. A study (Barton &Nolan, 1987) of thermal runaway incidents gave polym-erisation processes as being responsible for the mostincidents over the period 1962–1987.In spite of their apparent ideal suitability, inhibition

systems are rarely used in industry. Problems exist withthe mixing of such small quantities within the bulk massin the reactor, as good mixing is necessary to preventhot spots, i.e., pockets of reactant remaining uninhibitedand generating the temperatures and pressures thatwould occur if the reaction was allowed to continue. Theposition of the injector and the time of mixing are theimportant factors for designing a system. Additionalconsiderations must be given to the methods for earlydetection of the onset of runaway and actuation of theinjector, in order to optimise the timing of the injectionin relation to the runaway process.The injector system must be designed to give good

distribution of the inhibitor within the reactor to providesufficient mixing, and provide the mixing in as short atime as possible. The time taken to inject and distributethe inhibitor is an important factor. Aids to mixingwithin a non-agitated system are desirable to reduce thenumber of injectors required on a large system. Con-sideration should also be given to the effect of viscosity


on the mixing, as many systems, particularly polymeris-ation reactions, will become more viscous as the reactionproceeds. The increased viscosity will impede mixing,and the system should be designed for the worst case,i.e., when the reaction is nearing completion.The method of injection of the inhibitor and its

efficiency are major unknowns in the design of an inhi-bition system. The inhibitor may be injected as (whereapplicable):

! a solid powder dispersed in pressurised gas;! a solid powder in an immiscible liquid;! a powder dissolved in solvent (liquid-phase injection);! a liquid; or! a liquid and gas two-phase mixture.

The injection method must provide efficient mixing.Thermal runaway is likely to be caused by loss of power(control system), agitation or cooling. It is thereforelikely that stirring of the reactor, to distribute the inhibi-tor, is not possible. Any agitation available will be dueto residual stirring (for a short time after power loss),convection currents or agitation provided by the injector.The injection of the inhibitor can be achieved in sev-eral ways:

! piston injection, much like a syringe where the inhibi-tor is forced into the reactor using a piston in a cylin-der. The piston can be actuated either by gas pressureor by an electric motor;

! pad gas injected, where a pressurised pad gas ispresent in the inhibitor chamber. When actuated, thepressure forces the inhibitor into the reactor. This typeof system has the additional benefit of jet mixing fromthe pressurised gas;

! pumped injection, where the inhibitor is pumped intothe reactor. This system is less practical, as the rapidflow rate of inhibitor required and pumping againstthe reactor pressure will require a high specificationpump. Also, loss of ancillary services is a likely causeof thermal runaway;

! “inhibitor eggs” have previously been used(Maddison, 1996) for protection of a reactor. Theinhibitor is contained in an egg-shaped container sus-pended above the liquid surface. Upon actuation, apiston is driven through the container wall, forcingthe inhibitor on to the surface of the liquid. In additionto the mixing caused by this, natural convection cur-rents will be set up in the liquid when the surfacestops reacting, and the lower layers continue to react.The convection currents will cause dispersion of theinhibitor through the bulk liquid.

In each case, the main criteria for design are the distri-bution of the inhibitor, the time for mixing, and the rateand point of injection. The mixing time will influence the

temperature/pressure used as the initial injection point.Passive injection, i.e., systems requiring no externalpower, should be preferred.Possible alternative injection systems involve:

! connecting impellers to the injector, where the motionof the inhibitor actuates the impellers, helping to dis-perse the inhibitor;

! passing the inhibitor on to an impingement device,similar to the system used in fire-suppression systems.The impingement should give a wider spread to theinhibitor jet;

! containing the inhibitor in glass pellets and firingthese at the impeller blades and wall of the reactor.The explosion of the pellets and the deflection off theimpeller and walls should aid the mixing of the inhibi-tor.

Detecting the onset of a runaway for actuation of theinjector can be done by use of several parameters. Theactuation can be linked to the temperature or pressure inthe reactor, the failure of agitation or the loss of coolingto the system. Using temperature as a trigger may notalways be effective, as some systems, particularly thosegenerating non-condensable gases, can give high press-ure rises at small temperature differentials. A pressure-control system would normally be used for normal oper-ation, and loss of control can happen at any point duringthe runaway. Pressure is often a better choice of para-meter for runaway detection, as over-pressurisation ofthe reactor is the main concern. Care must be taken,however, as deposits may form on the pressure sensorand obstruct its operation. This problem will be presentin many polymerisation reactors. Linking the injector tothe loss of cooling or agitation is also practical, althoughthe injector may be actuated before runaway has com-menced. Mechanically rather than electrically controlledactuators should be used in this case, as the cause offailure of services is likely to cause failure of the con-trol system.A method of evaluating and comparing the perform-

ance of inhibitors has been developed by Rowe (1996).The inhibitor was added to a small-scale polymerisationreaction undergoing runaway, and the temperature andpressure monitored. A good inhibitor will reduce the rateof reaction to an almost undetectable level. The tempera-ture will continue to rise very slowly until a runawayoccurs again. The time taken to attain a predeterminedtemperature or pressure can be used in comparative stud-ies. A measure of the efficiency of the inhibition systemcan be obtained by comparing the time taken to obtaininitial temperature stability. Exhaustive testing using thismethod, covering initial injection at various points ofreaction completion, will allow optimisation of the injec-tion point in terms of temperature and pressure. Variableviscosity effects can also be handled by this technique.


The design procedure for installing an inhibition sys-tem must include studies of:

! methods for early detection of the runaway onset andloss of control;

! data obtained from small-scale tests to define the mosteffective inhibitor, and the optimal injection time afterthe start of runaway;

! optimisation of the injector system in terms of nozzledesign, number of injectors and choice of injectionsystem; and

! full-scale systems and the effective injection inrelation to the mixing and dispersion of the inhibitorfollowing injection.

In spite of their apparent suitability, there is very littlepublished literature on the design of inhibitor systems,and partly for this reason such systems are rare in indus-try.

4. Considerations for selection and design ofmitigation systems

Once the potential for thermal runaway has beenestablished, there is the need to design and implementan adequate mitigation system. The type of system andthe individual design aspects of this will be dependenton the actual system.An incident has been reported (Chemical Reaction

Hazard Forum, 1998) where a 1.5 tonne batch of glacialacrylic acid was left in intermediate storage, and a ther-mal runaway occurred. The bursting disc on the vesselburst and the contents were sprayed from the vessel intothe car park. One person was hospitalised due to vapourinhalation, and several cars were damaged. This incident,where an intermediate was stored due to a delay in oper-ations, was the result of a slight change in operating pro-cedure and is a likely event in many plants. Mitigationmeasures would have allowed the reaction mass to becontained at least partially, and have prevented theclean-up that would be necessary in this case. (It shouldbe noted that an investigation was carried out followingthis incident, and operational changes were made to pre-vent a recurrence of the situation. An appreciation of apossible worse situation is also stated.)Another example showing the need for good compat-

ible mitigation systems is quoted by Barton and Rogers(1997), where a batch reaction involving oleum wasdumped to a quench tank containing water. Half of thebatch was vented successfully, but the remainderexploded. Water had siphoned back into the reactor andreacted with the oleum, raising the temperature abovethe decomposition temperature of the main product. Thereactor lid shattered, the vessel fell to the ground andfittings were scattered over a wide area. This incident is

partly due to the incompatibility of the reactant and thequench fluid. Whereas it is not necessarily wrong to usea quench fluid that reacts with the discharged reactants,care must be taken that sufficient quench liquid is used,such that the final temperature of the quenchliquid/reactant mixture is below any self-heating tem-perature. It must also be ensured that adequate mixingof the quench liquid and reactants occurs, preventing anyhot spots.Keiter (1989) has reported the transfer of quench tank

fluid back to a reactor. Experimentation was carried outwith a dye in the quench tank of a closed system, andfollowing runaway and cooling, the dye was found inboth the reactor and the quench tank. With reference tothe oleum incident above, it is not clear as to whetherthe initial problems leading to the dumping of the reac-tants was due to thermal runaway, or indeed how thetransfer was effected. Where thermal runaway occurs,and top venting of the reactor is employed, the initiallyvented material is the pad gas and vapour above theliquid. Once this is vented, level swell in the reactorleads to a two-phase release. The reactor is likely to bealmost completely emptied of liquid, leaving only vap-our in the volume of the reactor. With atmospheric coo-ling, the vapour will begin to condense, reducing thepressure. Liquid from the quench tank is then drawnback through the sparger and vent line. This drawingaction will be slow at first, but once the first drop ofliquid reaches the reactor, the subcooled liquid willquench the vapour, causing a vacuum. At this point, theliquid will be violently drawn into the reactor. If theinitial two-phase venting of the reactor is in the churn-turbulent regime, a minimum of one-third of the reactorvolume will remain as liquid in the reactor. In this case,the result will be the reactants remaining in the reactorbeing cooled only by a restricted volume of quench fluid,equal to that of the void space following venting. Coo-ling of the reactants may not be complete, and furtherrunaway may result. The consequences of employmentof an incompatible quench fluid will be more severe,possibly giving an increased rate of reaction. Repo-sitioning the reactor relief line from the top to the bottomor lowest point on the reactor will result in the pad gasbeing vented after all of the liquid, exposing the entireliquid contents to the quench tank and giving the com-plete cooling effect. Any vacuum caused by cooling ofthe gases and vapours in the reactor (which will eventu-ally be refilled to the initial charge level) will draw amixed and quenched liquid back into the reactor. Sincethe mass of quench fluid should have been designed tocool the reaction to a safe temperature, any further reac-tion of the mixture in the reactor should not cause a ther-mal and pressure hazard.It is clear that the design of an adequate and compat-

ible mitigation system is of vital importance. There isno simple universal system, but certain guidelines can


be given. Generally passive systems should be used andcertainly are the preferred solution, since the cause ofthe initial loss of control is often a loss of power, whichis likely to affect non-passive equipment. Passive sys-tems may include containment, inhibition, venting,gravitational or cyclonic separation and quenching. Flar-ing is not passive, since a pilot flame system is used;however, the use of flares for suitable releases is oftenacceptable.Options involving containment within the reactor

should be investigated initially, as problems and uncer-tainties in vent design are excluded, and as there is nodischarge at all, the system is environmentally friendly.Where containment is impractical, the treatment dependson the nature of the chemical system present. Ventingand discharge to atmosphere may be acceptable if thedischarge is solely in the gas phase. Separation using adump tank or cyclone may remove sufficient liquid toallow an initially two-phase discharge to be vented toatmosphere. Quenching can be used to either contain allof the reacting materials (liquid and gas/vapour) or toseparate the liquid and gas phases. Quench tanks are alsoknown as passive scrubbers. Flaring is very effective inremoving combustible materials from the discharge, butphase separation may be necessary to remove largerliquid particles from the mixture, as such droplets, thatmay not be burned efficiently in the flame. Certainmaterials, such as chlorinated compounds, are not suit-able for flaring.

5. Applications and advantages/disadvantages ofindividual mitigation systems

Possible uses, advantages and drawbacks of the differ-ent systems are given below.

5.1. Inhibition

Inhibition systems are suited to reactions involvingfree-radical initiation, pH-dependent reactions and sys-tems where a catalyst can be removed. Where possible,inhibition should be used at least as an initial protectionagainst thermal runaway. The advantages are the passivenature and complete containment within the reactorwithout the need for extreme pressure design of the reac-tor. This is particularly important where the pressuregenerated during runaway is due to the vapour pressureof the system. Such reaction systems require very highpressure ratings for complete containment. Since noventing is involved, inhibition is applicable to foamingand highly viscous systems.Disadvantages of inhibition include the lack of pub-

lished information regarding the design of injection sys-tems, uncertainties over the mixing efficiency and distri-bution of the inhibitor, and the fact that reaction may

not be altogether halted and may continue at a reducedrate before a second runaway occurs.

5.2. Containment

Containment within the reactor should always be con-sidered where practicable. As with inhibition, the systemis passive and no venting occurs, making it suitable forfoaming and highly viscous systems. Design of a con-tainment system can be based on that of a simple press-ure vessel, and the data necessary for obtaining themaximum temperatures and pressures involved in a run-away can be obtained by means of standard adiabaticcalorimetry. The downside of such systems is that manyreactive systems involve very high pressures, parti-cularly where the pressure is due to vapour generation.In the case of a vapour, as opposed to a gas where themaximum pressure can be reduced by using a lower fillratio, the pressure is temperature-dependent. Under thenear-adiabatic conditions found on large reactors, reduc-ing the charge of reactants will result in the same finalpressure after runaway. Impractical wall thicknesses (interms of economic cost as well as construction) mayresult. Clean-up following an incident may be difficultif the reactants are allowed to solidify.

5.3. Gravity separation

Gravity separators can be considered when the venteddischarge is two-phase, but the vapour/gas can be eithervented or treated further (e.g., by flaring). They are sim-ple, passive pieces of plant, providing economical con-struction. Both horizontal and vertical designs are poss-ible, with horizontal vessels preferable for flows with alarger liquid content. Notwithstanding, either orientationcan be used to fit the vessel in a restricted space. A mainadvantage is the very low pressure drop caused by flowthrough the system, giving little back pressure to thereactor. Gravity separators have a proven history of goodperformance and can handle high-viscosity liquids.Foaming systems are not normally suited to this type ofsystem, since separation of the phases is very difficult,but non-stable foams may allow adequate separation.Since the design of separation vessels is based on thethrough flow rate of the discharge, area-to-volume scaleeffects may result in the need for a very large vessel.Effective separation of liquid is restricted to dropletsgreater than approximately 300 µm, as such particleshave a very low terminal drop-out velocity. Furtherdownstream treatment may be necessary if venting ofsmall liquid particles is hazardous or toxic. Further sep-aration may be possible using a mist eliminator, basicallya wire mesh upstream of the exit nozzle from the separ-ation vessel. Continued reaction in the separation vesselis likely, and care should be taken in the design that atwo-phase flow scenario is impossible.


5.4. Cyclonic separators

Cyclonic separators have the advantage over gravityseparators of being very compact, ideal for retrofittingin confined spaces. They can again handle flows withhigh liquid content, and from use in other scenarios, canremove very fine particles. The disadvantages are anincreased pressure drop, an increased cost and a morecomplicated design. Foaming and highly viscous mix-tures are not suitable for this type of system, although,again, unstable foams may be effectively separated.

5.5. Quench systems

Quench systems are very versatile, with many appli-cations. The system can be designed to either halt thereaction completely after discharge or to allow the reac-tion to continue in a diluted liquid, where no vapourswill be generated owing to the liquid being above itsboiling point. Open and closed systems can be designed,allowing complete containment of the reactive systemor the venting/further treatment of the gas/vapour phase.Additionally, if the quench fluid is carefully chosen, thequench fluid may itself react with the discharge andinhibit or halt the reaction. This active quenching is anideal substitute to reactor inhibition where larger quan-tities of the inhibiting medium are required. Quench sys-tems are suited to most reaction systems; vapour press-ure, “gassy”, foaming and viscous mixtures can bequenched with a suitable quench liquid. It is possible tohave complete containment of the chemical system usingquenching, preventing toxic or hazardous vapours orgases being passed to the atmosphere. Use of a quenchtank may involve re-evaluation of the vent system, per-haps involving a vent line on the bottom rather than thetop of the reactor (see above). Care should be taken inensuring that the reacting mass does not solidify in thesparger when the discharge first hits the cold quenchfluid in the vent line. Reacting liquids that will becomesolid at ambient temperatures are one of the few situ-ations where quench tanks should not be used. Theremay, however, be a solvent that can be used as thequench fluid in this case. A different form of quenching,non-sparged, may be suitable. Discharging the reactor tothe void space above the quench fluid and letting thereacting mass hit the liquid will quench the reaction andat least give some benefit. Mixing of the quench anddischarge is the main problem in this case. Literature onthe design of spargers is sparse, although some guide-lines are given (Guidelines for pressure relief and efflu-ent handling systems, 1998).

5.6. Vent stacks

Vent stacks are suited to single gas-phase dischargesand two-phase releases containing very fine liquid par-

ticles. The advantage is the simplicity and the low cost.To ensure that the ground-level concentration of the dis-charged material is low, the discharge may need to bevented at a high level, requiring a tall stack. Environ-mental issues become relevant when using emergencyvent stacks, especially if their use is frequent.

5.7. Flare stacks

Flare stacks can often be used when a vent stackwould give too high a concentration of hazardousmaterials in the atmosphere. Flares are capable ofdestroying over 98% of the combustible material in thedischarge, and are very reliable. Smokeless flares areusually necessary for normal day-to-day operation, butin an emergency situation, non-smokeless operation ispermissible. Care is needed to ensure that liquids andespecially foams are not fed to the flare; a low concen-tration of very fine liquid droplets may be admissible,but larger quantities may extinguish the flame. Certainvapours are not suited to flaring, where the combustedproduct is toxic or will react with atmospheric moisture(acid rain). The main disadvantage of flare systems foremergency relief is that such systems are not passiveand require constant operation even while the plant isoperating safely. Constant monitoring of the flare isalso necessary.

6. Design methods for gravity separators

These vessels are designed to reduce the flow rate ofthe two-phase mixture such that separation will occur.The design method is therefore based on having a suf-ficient area to give the flow a low velocity. Variousmethods are given in the literature to calculate therequired vapour area in the dump tank (McIntosh, Nolan,Rogers & Lindsay, 1995a). A major unknown is thediameter of the droplets generated at the inlet of thedump tank, where in addition to the normal jet break-up, the reduced pressure will cause expansion of the gasand vaporisation will also generate a force that willaffect the break-up of the droplets. The direction of theinlet pipe can also affect jet break-up. It is common todirect the pipe against the end wall of the dump tank,but impingement of the jet will cause further break-upof the liquid droplets. In conventional liquid jets,impingement of droplets reduces their diameter by a fac-tor of four, with a complementary increase in the numberof droplets. Directing the flow on to the liquid surfaceof a horizontal dump tank will generate waves, and thearea available for flow through the vessel may bereduced. The likelihood of re-entrainment of liquid isalso increased. If the flow is directed along the dumptank, momentum will cause an increased velocity


through a relatively short length, reducing the residencetime in the vessel.

6.1. Grossel’s method

This method (Grossel, 1990) has been restated by sev-eral authors, and is based on passing the vapour throughthe vessel at a velocity sufficiently low as to prevententrainment of the liquid in the gas phase, as well asallowing the droplets to fall from the two-phase stream.The Souders–Brown equation (Souders & Brown,

1934) can be used to calculate the velocity

ua!K!rl−rvrv . (1)

The factor K is a constant relating to the drag coefficientof the liquid droplets, and is given by (Talavera, 1990;Svrcek & Monnery, 1993)

K!!4gd3C . (2)

Watkins (1967) and Evans (1974) give a method ofevaluating K (see next paragraph), involving the relativemass flow rates and densities of the two phases. In manycases, these data will not be known, and an empiricalvalue must be assigned to K. Specific values are givenas 0.082 m/s by Grossel (1990), stating that this valuehas proved adequate in the past, and 0.107 m/s by Nie-meyer (1961). This latter value assumes a separator witha wire mesh demister, and is therefore slightly higher asa higher velocity in the dump tank is permissible. Small-scale experimentation suggests that these values areadequate (McIntosh, Nolan, Rogers & Lindsay, 1995b).The work done by Souders and Brown (1934), and

later by Fair and Matthews (1958), related to the designof fractionating columns, and the effects of entrainmenton the design and efficiency of columns. In the case ofa distillation column, the efficiency of the plates is affec-ted by the amount of entrainment. Entrainment shouldbe minimised for maximum efficiency. The situation ina distillation column is that the vapour flowing up thecolumn has two sources; in addition to the vapour pro-duced on each plate, there is the vapour passing throughthe column from plates lower down. This is analogousto the situation in a dump tank, where there is the vapourphase passing from the reactor, in the space above theliquid, and the vapour generated from the liquid in thevessel.The amount of entrainment has been found to be

dependent on the vapour velocity (Peavy & Baker, 1937;Colburn, 1936), but it has been observed that it is verysmall until a critical vapour velocity is reached, where-upon the mass entrained increases rapidly with the vap-our velocity. This can be applied to dump tanks where

the mass of liquid leaving the vessel is dependent on thevapour velocity, but at low velocities, the mass ofentrained liquid will be so small that it will almost beindependent of the velocities. At a critical point, themass entrained will increase rapidly, and the dependencyon the velocity will be very pronounced.This critical point can be found by using the Souders–

Brown equation [Eq. (1)]. The velocity factor is thatgiven by Watkins (1967) and Evans (1974). The velocitycoefficient can be found from Fig. 1, which plots Kagainst the factor

Ml

Mv!rvrl . (3)

This will give a value of K for a vertical dump tank. Fora horizontal dump tank a higher velocity is permissible,and the value of K can be multiplied by a factor of 1.25(Watkins, 1967).The velocity given by Eqs. (1) and (2) is the terminal

velocity of the droplet. In a vertical tank, the flow velo-city of the two-phase discharge through the vessel mustbe below this value. At an equal velocity, the liquid par-ticle will remain steady due to equal upward and down-ward velocity forces. In a horizontal tank, the two velo-cities are perpendicular, and the droplet requires to beresident in the vessel for sufficient time that it drops tothe liquid surface. With similar velocities as before, thedrop will fall at an angle of 45° to the flow. Dump tanksnormally have an L/D ratio of 1.5 to 3, hence the dropletwill always reach the liquid surface if the velocity issufficiently low. A slightly increased coefficient K istherefore used for a horizontal tank. Re-entrainmentfrom the disturbed liquid surface is a concern in thiscase.

6.2. Vertical tanks

In a vertically mounted vessel, the inlet from the reac-tor enters the vessel above the liquid. The vessel volumerequired can be calculated from the volume of liquid

Fig. 1. Graph for calculating K.


stored in the vessel and the volume required for the vap-our flow.The maximum vapour velocity can be found using Eq.

(1) and Fig. 1. The minimum cross-sectional area is then

Av!Q/ua. (4)

In addition to the minimum area, the height of the gasflow part of the dump tank must be sufficient to allowthe mixture to maintain its velocity for long enough topermit separation. Since the vent on the dump tank issmaller than the diameter of the vessel, the velocity ofthe flowing mixture will increase in the area near thevent. Watkins (1967) gives a minimum height above theinlet of 0.92 m (36 in.). Current practice is to work inSI units and a 1 m length is advised.The volume required for liquid storage should assume

that all of the reactor contents are passed to the dumptank. Additionally, it should be noted that reaction willcontinue in the dump tank, albeit at a lower rate and atatmospheric pressure, and a two-phase mixture willagain occur, giving rise to level swell. The increase involume caused by generation of vapour/gas within theliquid can be calculated using the methods developed byDIERS, and this volume should be used to estimate theminimum volume required for stored liquid followingventing.The overall height of the vessel must include the vap-

our flow space (1 m), allowance for the stored liquid,the inlet pipe diameter and a void space between theliquid surface and the inlet. A 0.3 m gap between theinlet pipe and the liquid surface should be incorporated(Fig. 2).

6.3. Horizontal tanks

Once the vapour velocity has been calculated [Eq.(1)], the minimum flow area for gas/vapour flow can befound. As before,

Av!Q/ua. (5)The dump tank can then be sized by assuming a vesseldiameter to give a cross-sectional area sufficiently largeto allow for the vapour flow and liquid storage areas.Grossel (1990) suggests initially to assume that the vap-our area is half of the total vessel area. The liquid areacan be calculated as the difference between the total ves-sel area and the vapour area, AvAl!A"Av. (6)

The length of the vessel can then be calculated as tocontain the liquid volume in this area

L!VlAl, (7)

where Vl is the liquid volume transferred to the dumptank.

Fig. 2. Sketch of a vertical separator.

The liquid volume should be equal to the maximumworking volume of the reactor for a non-foaming sys-tem. For a foaming system, the volume will increase,and experimentation may be necessary to determine theexpanded volume.To satisfactorily design the vessel, allowing adequate

residence time for drop-out of liquid particles from thegas, the L/D ratio should fall within acceptable limits.The ratio is chosen to give economic optimisation,dependent on the design pressure. Grossel (1990) giveslimits between 2 and 3, which are applicable at lowpressures (less than 3 barg), whereas Watkins (1967)gives values of 3 and 5.If the L/D ratio falls outside the limits, a larger or

smaller vessel diameter should be assumed, and thelength recalculated.

6.4. API 521 method

This method (American Petroleum Institute rec-ommended practices 521, 1997) uses the drop-out velo-city of a liquid particle in a vapour or gas stream. Thedrop-out velocity is given by

ud!1.15!gd(rl−rv)rvC. (8)

It is generally accepted that a particle diameter of300×10"6 m is used, with smaller particles being


allowed to escape in the vent stream going to furthertreatment. Allowing for this, Eq. (8) becomes

ud!0.0324!(rl−rv)rvC

. (9)

This is widely accepted for all forms of entrainment sep-aration.The drag coefficient can be found from Fig. 3, the plot

of C against C(Re)2, where

C(Re)2!0.13×108(rv)d 3(rl−rv)

m2g. (10)

For D=300×10"6 m, this becomes

C(Re)2!3.51×10−4rv(rl−rv)

m2g. (11)

The design method used then depends on whether a hori-zontal or vertical vessel is employed. A vertical vesselis designed using the method given above, using ud asthe allowable vapour velocity.For a horizontal vessel, the design procedure is as fol-

lows.

1. Calculate the drop-out velocity, ud.2. Assume vessel dimensions (L and D).3. Calculate the cross-sectional area occupied by theliquid and the distance from the top of the vessel tothe liquid surface (h).

4. Calculate the time for a particle to fall this distanceat the drop-out velocity

td!h/ud. (12)

5. Calculate the minimum vapour velocity and residencetime. If the residence time is greater than the drop-out time, td, then the vessel is of sufficient size. If thedesign is insufficient, then the calculations should bere-worked using different vessel dimensions.

Alternatively, the required drum length for drop-out canbe calculated from

Fig. 3. Graph given by API to estimate the drag coefficient, C.

L!uv#td. (13)

If this length is less than the drum length the vessel issufficient, as the particle will fall to the liquid surfacelevel within the dump tank length. To avoid oversizingthe drum, and to design an economical vessel, several“sufficient” designs should be calculated and compared.For a vertical tank, the maximum vapour velocity is

taken as the drop-out velocity. To permit fall-out, theupward velocity of the vapour must be lower than thedownward velocity of the liquid particles. The inlet tothe dump tank should be at least 1 m from the top ofthe vessel and the volume below the inlet should permitstorage of the liquid transferred from the reactor.

6.5. CCPS method

Recently, the Center for Chemical Process Safety(CCPS) published its recommendations regarding sizingof separator vessels for emergency relief (Guidelines forpressure relief and effluent handling systems, 1998). Itis based on the use of the terminal settling and re-entrainment velocities of the fluids. The process is as fol-lows.

! Estimate the terminal settling velocity, using theassumed value of K of 0.0823 m/s (0.27 ft/s)

ut!K!rlrv −1. (14)

! Calculate the maximum axial velocity that will pre-vent re-entrainment

ue!"rlrv#srv$4#g(rl−rv)ml$2%0.1. (15)

! Assume an initial fill level for the separator, y, whichis the ratio of the liquid fill level to the vessel diam-eter (0.5 is suggested as an initial attempt).

! Calculate x, the fill ratio of the vessel with the valueof y. With a value of y of 0.5, x is also 0.5. The valueof x can be calculated from

x!1pcos−1(1"2y)"

2p(1"2y)&y−y2. (16)

! Calculate the separator diameter and length from

D!Vlut(x−1−1)(1−y)Qg

(17)

and

L!Vl

(p/4)D2x. (18)


! Calculate the settling time and residence time in theseparator

t!D(1−y)ut

(19)

and

q!pD2L(1−x)4Qg

. (20)

! Calculate the vapour velocity through the separator

ua!Qg

(p/4)D2(1−x). (21)

Checks should then be made to ensure that:

1. the settling time is less than the residence time;2. the velocity through the separator is less than the re-entrainment velocity [Eq. (15)];

3. the L/D ratio of the vessel is within economical designlimits (between 1.5 and 5).

If necessary, a different diameter and length or value ofy should be chosen and the design process repeated untila satisfactory design is achieved.A method is further given to estimate the diameter of

the droplets that will be removed in the separator, basedon Newton’s and Stoke’s laws. The equations are:

dN!3CNu2trv4$rg , (22)

dS!!18mgut$rg (23)

and

dt!#12(dnN%&d 2nN +4d 2nS )$1/n. (24)

This gives an estimation of the diameter of the dropletsthat will settle out in the separator. Smaller droplets thanthis will remain in the gas-phase flow. If this figure istoo large for downstream processing equipment, a lowervalue for K should be assumed to give a larger diam-eter vessel.These equations use an assumed value of the terminal

settling velocity to calculate the droplet diameter. If itis too big, a smaller settling velocity is assumed. Thediameter predicted is only dependent on the settlingvelocity, since the other parameters are constant physicalproperties of the fluid system.

6.6. Cyclone design

A cyclone system (Fig. 4) involves the discharge pass-ing tangentially through the top part of the annulusbetween two cylinders. The discharge flows around theannulus and in a downward direction until it passes theskirt, when it begins to flow upward and through the exitline. The design of cyclone separators is based on thediameter of the skirt, which is dependent on the super-ficial velocity of the gas/vapour flowing inside the skirt.Grossel (1997) gives a design method based on anempirical factor in evaluation of the superficial velocity.The initial step is to calculate the superficial vapourmass flux

Gv!F&rv, (25)

where rv is the gas/vapour density at the operating press-ure of the cyclone and F is the empirical factor, givena value between 5 and 8. A value of 8 is suitable for“water-like” liquids, whereas a value of 5 is more suitedto more viscous materials. A greater flow will result inre-entrainment of the liquid falling down the cyclonewalls, defeating the object of separation. Lower velo-cities may not provide sufficient centrifugal force to sep-arate the liquid.The skirt area can then be calculated from

As!Mv

Gv, (26)

where Mv is the gas/vapour mass flow rate through thecyclone.The skirt diameter is then

Fig. 4. Cyclone dimensions (subscript s refers to the skirt, p to theinlet pipe).


Ds!!4Asp. (27)

The dimensions of the cyclone follow from this, asshown in Fig. 4. The overall height given as 2.5Hs is theminimum. If the liquid is to be stored, this may beextended so that all the liquid initially in the reactor maybe stored in the cyclone. Where the liquid is drainedfrom the cyclone, a swirl breaker and false bottomshould be fitted to prevent the gas phase passing throughthe drain line.

7. Quench tank design

Quenching of the runaway discharge is a very versa-tile process, and because of this versatility, the designof quench systems may require a variety of differentaspects to be examined. The main concerns in designare the sparger, the quench fluid and the design of thevessel itself.

7.1. The sparger

The sparger is the initial contact point between thedischarge and the quench fluid. It must provide themechanism for mixing and has sufficient stability duringthe venting process. Various configurations have beenproposed (Grossel, 1997) — single dip tubes, tee piecesand four-arm cross spargers. Small-diameter holes (2–6mm) in the sparger give rise to higher velocities, givingjet mixing in the quench fluid. The small diameter alsoreduces the volume of vapour bubbles formed by thedepressurising liquid, hence reducing any pressure insta-bility. Unfortunately, with very small holes, the spargeris more likely to suffer from blockages due to foulingby the vented mixture and impurities in the quench fluid.Larger holes (10–50 mm) may be necessary for viscousor fouling discharges (such as many polymer solutions)or where the discharge contains suspended solids (e.g.,a catalyst).The total area of the holes must not be less than that

of the vent line so as not to hamper the discharge flow,and may need to be slightly larger to allow for anincrease in pressure drop. Back pressure to the reactormust be avoided. The holes should be positioned to givemaximum stability of the sparger during discharge. Sym-metry of the hole pattern should equalise the dynamicforces on the sparger. The hole pattern should allow arecommended separation between the holes of at leastthree hole diameters to prevent coalescence of bubblesnear the sparger.As an alternative to a sparger system, Herman and

Rogers (1995) have proposed using jet condensers tofacilitate mixing. They report an improved performance

over a traditional sparger design, therefore requiring lessquench fluid and a smaller tank.

7.2. The quench fluid

A fluid that is compatible with the reactor dischargeshould be selected. Ideally, this should be a solvent forthe discharge, as both mixing and dispersion of the dis-charge is aided. An immiscible mixture, however, maybe desirable in product recovery following quenching.In this instance, an efficient sparger design is vital toallow initial dispersal and cooling of the discharge.Compatibility also requires that the quench fluid doesnot react with the discharge to create a further hazard.Active quench fluids can be used to reduce or halt thereaction in the quench tank, and this is particularly usefulwhen the runaway reaction is initiated at, near or belowambient temperature. It is possible to use a quench fluidthat will react exothermally with the discharge, but thequantity of fluid must be in excess of that which willreact and sufficient to dilute the mixture such that theoverall reaction will not result in boiling of the liquid.The excess quench fluid must absorb the heat of reaction,and its temperature must still be below its boiling point.Where possible, water should be considered as the

quench fluid. It has ideal properties: high heat capacityto absorb large quantities of heat without rising too muchin temperature, low vapour pressure, reasonably highboiling temperature, and is non-flammable, non-toxicand non-corrosive. It is also cheap and very readilyavailable. Measures may need to be taken to preventfreezing in cold climates. Anti-freeze additives or heattracing the tank can be used, although heat tracingreduces the passive aspects of quenching. If failureoccurs and the heater fails off, the liquid may freeze, orif the heater fails on, the liquid will heat up, with theloss of efficient quenching and possible boiling andevaporation of the quench fluid.The mass of quench fluid required can be estimated

by a heat balance involving the discharge and the quenchfluid. Heat transfer from the jet to the quench liquid willoccur more efficiently if there is a large temperature dif-ference between the two fluids. The final temperature ofthe quench liquid (Tqf) and discharge mixture should bebelow the boiling point of the mixture or the quenchfluid, whichever is lower. A temperature difference of atleast 10°C between the final quench temperature and theboiling temperature should be incorporated. If the dis-charge contains non-condensable gases, this figureshould be increased to 20°C (Huckins, 1995). The pres-ence of non-condensables will hamper heat transferbetween the mixtures. In the case of high gas concen-trations, the temperature difference may need to beincreased further. Additionally, if the final quench mix-ture contains low-boiling-point liquids, significant vap-our may be generated and a much lower final tempera-


ture may be necessary to prevent excess vapour leavingthe quench tank in the gas stream.The initial temperature of the quench (Tqi) used in the

heat balance should be taken as the ambient temperatureat the quench tank location. This would normally be themaximum ambient temperature likely to be found duringthe course of a day, but it should be noted that theremay be residual heat from surrounding process equip-ment, and that extremes of atmospheric temperature maybe a cause of failure of adequate reactor cooling.On the discharge side of the heat balance, the heat

that is required to be removed is dependent on the objectof the quenching. If it can be ensured that the mixtureis cooled below its exotherm onset temperature, and thatthe reactor is completely vented (or that no further reac-tion can take place in the reactor, possibly due to thecooling effect of the depressurisation), the mass ofquench fluid required can be calculated by using themaximum temperature achieved by the reacting mass atthe venting pressure

mq!mrCr(Tr,max−Tqf)Cq(Tqf−Tqi)

. (28)

Here, Tr,max should make allowance for the fact that thetemperature may rise during venting when the reactioncontinues in a “gassy” system or there is an allowancefor overpressure in the vent design for a vapour pressureor hybrid system.In the case where reaction does not stop altogether,

the heat of reaction and any additional heat should beused. The reactor will usually be heated to the normalisothermal operating temperature, and an allowanceshould be made for this; that is

mq!mr$Hr+mrCr(Top−Tqf)

Cq(Tqf−Tqi), (29)

where $Hr is the heat of reaction and Top is the iso-thermal operating temperature of the reaction system.It should be noted that in the case where normal iso-

thermal operation is below the maximum temperature ofthe quench fluid, Tqf, the temperature difference Top"Tqfwill be negative, and therefore the heat loss required bythe reactants is less than the overall heat of reaction.An equation similar to Eq. (28) should normally be

used, as the heat of reaction represents all of the heatavailable for heating any liquid, vapour or non-condens-able gases present or generated. The quench fluid massobtained represents that required for adiabatic behaviourof the system, and as such will never underestimate therequirement (in the absence of external heating). Analternative design equation could treat the phases indi-vidually and use a heat balance over the intermediatesystem, which is more complicated.The heat of reaction can be estimated using the tem-

peratures obtained from small-scale closed adiabatic test

data. All of the heat generated from the reaction willthen heat the reaction mass, therefore$Hr!Cr(Tf"Ti), (30)

where Tf is the final temperature of the reaction massfollowing runaway and Ti is the exotherm initiation tem-perature. The heat capacity of the reaction mass can beestimated by heating the mixture with a known powerfor a fixed period of time

Cr!Q×tmr×$T, (31)

where Q is the heat input rate and $T is the resultanttemperature rise. The heat capacity of the mixture maychange significantly during reaction, and a good practiceis to use an average value based on the initial and finalreaction masses.If the reactor is not emptied during venting, the

quench tank temperature will be below Tqf, but the liquidin the reactor will continue to react and generate vapour,which will then be passed to the quench fluid and con-densed. If the quantity of liquid remaining in the reactorcan be confidently estimated, this part of the heat balancemay be eliminated from the design equations. Althoughjustifiable, this practice should be discouraged due touncertainties over scale effects in estimating the reactorresidue. In any case, the sensible heat of the liquid abovethe final quench temperature is likely to be small.If the quench fluid is not passive and reacts with the

discharge, any heat of reaction must be taken intoaccount. Using an excess of quench fluid, it is possibleto prevent the mixture from exceeding the maximumallowable quench temperature. A potential hazard of thiscase is if a small quantity of quench liquid is returnedto the reactor by vacuum effects. The quench fluid islikely not to be in excess, and the vent design must beable to handle a potential two-phase discharge of thesecondary reacting mass. As was previously stated, bot-tom venting of the reactor will largely prevent this scen-ario.

7.3. Quench tank design

Once the volume of quench liquid required has beenestimated, the quench tank itself can be designed. Hori-zontal or vertical drums are commonly used, althoughalternative vessel shapes may be used where conditionsdictate otherwise. Vertical drums are preferred to sim-plify design procedures and reduce the ground area(footprint) required.Quench tanks can be either open (vented), closed or

use a pressure-control device to limit the pressure in thevessel. Closed quench tanks are suited to vapour press-ure reaction systems, where all of the gas phase can becondensed by efficient quenching. There is then no needto allow subsequent venting of the vessel, and to do so


may allow condensable vapours to escape to the atmos-phere. Pressure protection is nevertheless advised. Openquench tanks are likely to be required if a non-condens-able gas phase is present. Venting the gas will preventback pressure on the system, which could prevent thereactor from venting and cause overpressure in the reac-tor. A limited pressure quench tank is fitted with a reliefvalve to maintain the pressure within certain limits. Thepressure can be maintained to increase residence time ofthe gas/vapour in the quench fluid, increasing the poten-tial for heat and mass transfer. The pressure will alsoreduce any carry-over of liquid droplets from the quenchfluid. Limiting the quench tank pressure will also preventexcessive back pressure on the reactor.The dimensions of the quench tank must be sufficient

to allow storage of the quench liquid and the dischargedreactor contents, including the gas/vapour present in theliquid. They must also allow sufficient residence time ofthe gas/vapours in the quench liquid to permit heat andmass transfer. A significant depth of liquid is thereforeneeded. Void space above the liquid will reduce backpressure on a closed tank and will reduce entrainmentof any liquid droplets in the gas phase.The superficial velocity of the gas/vapour in the

quench tank can be used to estimate the minimumacceptable diameter of the vessel. The superficial gasvelocity should be sufficiently low as to prevent excess-ive entrainment of the liquid phase in the gas. Exper-imentally obtained acceptable maximum values between0.25 and 2 m/s are given in the literature (Huckins,1995). This is a wide range, and clearly more experi-mental work is required. Entrainment of liquid will bedependent inter alia on viscosity, surface tension anddensity of the fluids, and different values may be accept-able depending on the quench liquid, reactant liquid andgas phase. The maximum superficial gas velocity dic-tates the minimum vessel diameter.Once the diameter of the vessel is known, the required

height can be estimated. The height must allow for thegas/vapour present in the quench liquid. Variousmethods are given to estimate gas hold-up within theliquid. Although these relate to a non-condensable gasin a liquid, they can be safely used since a condensablevapour is likely to give a reduced volume due to conden-sation. A void space in the quench tank is also necessaryto prevent excessive liquid. A void height of 0.92 m (3ft) or 20% of the vessel diameter is given by Scheiman(1964), whereas Grossel (1997) recommends 10% of theheight. The void space will reduce back pressure on thereactor in a closed quench tank and reduce losses of theliquid in the gas stream of a vented tank.

8. Conclusions

The use of exothermic reactions is widespread in thechemical industry, and although protection measures can

reduce the likelihood of thermal runaway, its occurrencecan never be eliminated. Venting of the reactor is oftenconsidered, and although this will prevent overpressureof the reactor, the treatment of the discharge is givenless priority. Environmental issues and ever increasinganti-pollution legislation require that all discharges areminimised. In any case two-phase flow is the likelyresult of a runaway, and some form of treatment is nearlyalways necessary for safety reasons. In view of the likelyreasons for runaway events, such as loss of power, thereis the need for passive mitigation systems. Such systemsdo exist; however, there is little published informationregarding their design and suitability for a particular situ-ation.Containment within the reactor is the ideal situation.

Unfortunately, the pressures generated by most runawayreactions will result in a very high pressure rating forthe reactor. Very thick walls cause construction diffi-culties as well as being prohibitively expensive. Inhi-bition systems are suitable for many reaction mixtures,particularly free-radical polymerisation, and an efficientdesign will allow containment in a relatively low-press-ure reactor. Inhibition is rarely used in industry, and verylittle information is available. Efficient injection andmixing of the inhibitor at an increased pressure is themain concern in this case.Gravity and centrifugal separators are used where the

two phases of the discharge can be easily separatedbefore the gas phase is either vented or passed for furthertreatment. Gravity separators are designed on the basisof removing the larger particles and allowing a very finemist to leave in the gas stream. Design methodology isavailable, based mainly on empirical equations and pastexperience. Separator systems are generally unsuitablefor foaming systems, but unstable foams can be accom-modated, albeit with a larger vessel. Continued reactionin the separator may cause a further hazard.Quench tanks are suitable for most systems and are

versatile. The design process is relatively simple, andthey can be augmented with inhibiting agents. Largermasses of inhibitors can be added than in reactor inhi-bition, which may be necessary for some reaction sys-tems. The volume of published literature on quenching,although still small, is increasing. As with inhibition,quench systems are reliant on efficient mixing for heatand mass transfer. Potential pitfalls in quenching are thechoice of quenching fluid and the return of the liquidto the reactor under a vacuum generated during coolingfollowing reaction. The fluid must be compatible withall of the reactants and products of the reaction. Quenchsystems can deal with foaming systems, although thereis little information on the effect of the foam on heatand mass transfer. Liquids that form solids or very vis-cous liquids at ambient temperature are likely to foul thesparger, and are not suitable for quenching. Non-spargedquenching (dumping into a vessel of quench fluid) may


be an option in this case, but further investigation wouldneed to be carried out to evaluate the efficiency of coo-ling.Testing on both small and larger scales is often neces-

sary to obtain the data required to assess the suitabilityof a mitigation system and to evaluate its effectiveness.Without visualisation, it is difficult to assess whether ornot a reaction mass is foaming or the level of stability ofthe foam. The pharmaceutical and speciality chemicalsindustries generate another problem in that the reactorsare often multi-purpose, processing different materials ineach batch. In this case it may be necessary to have sev-eral mitigation systems available, and connect the mostsuitable. As new processes are developed, it will benecessary to examine the reactor charge to ensure thatthe system can handle the discharge. Reducing the batchsize may be necessary to permit safe operation of theplant.

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American Petroleum Institute recommended practices 521 (1997).Guide for pressure-relieving and depressurisation systems (2nded.). Washington, DC: American Petroleum Institute (API).

Barton, J. A., & Nolan, P. F. (1987). Some lessons from thermal run-away incidents. Journal of Hazardous Materials, 14 (2), 223–239.

Barton, J. A., & Rogers, R. L. (1997). Chemical reaction hazards: aguide to safety. (2nd ed.). IChemE.

Bowonder, B., & Miyake, T. (1988). Managing hazardous facilities:lessons from the Bhopal incident. Journal of Hazardous Materials,19, 237–269.

Chemical Reaction Hazard Forum (1998). Runaway polymerisation ofglacial acetic acid. http://www.chemcall.demon.co.uk/incid11.html

Colburn, A. P. (1936). Effect of entrainment on plate efficiency indistillation. Industrial and Engineering Chemistry, 28, 526.

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Review of Safe Systems in Runaway Reactions

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