4 REACTORS 4.1 INTRODUCTION This chapter presents potential failure mechanisms for reactors and suggests design alternatives for reducing the risks associated with such failures. The types of reactors covered in this chapter include: • Batch reactors • Semi-batch reactors • Continuous-flow stirred tank reactors (CSTR) • Plug flow tubular reactors (PFR) • Packed-bed reactors (continuous) • Packed-tube reactors (continuous) • Fluid-bed reactors This chapter presents only those failure modes that are unique to reaction systems. Some of the generic failure scenarios pertaining to vessels and heat exchangers may also be applicable to reactors. Consequently, this chapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6, Heat Transfer Equipment. Unless specifically noted, the failure scenarios apply to more than one type of reactor. 4.2 PAST INCIDENTS Reactors are a major source of serious process safety incidents. Several case histories are presented to reinforce the need for safe design and operating prac- tices for reactors.
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4REACTORS
4.1 INTRODUCTION
This chapter presents potential failure mechanisms for reactors and suggestsdesign alternatives for reducing the risks associated with such failures. Thetypes of reactors covered in this chapter include:
This chapter presents only those failure modes that are unique toreaction systems. Some of the generic failure scenarios pertaining to vesselsand heat exchangers may also be applicable to reactors. Consequently, thischapter should be used in conjunction with Chapter 3, Vessels, and Chapter 6,Heat Transfer Equipment. Unless specifically noted, the failure scenariosapply to more than one type of reactor.
4.2 PAST INCIDENTS
Reactors are a major source of serious process safety incidents. Several casehistories are presented to reinforce the need for safe design and operating prac-tices for reactors.
4.2. / Seveso Runaway Reaction
On July 10, 1976 an incident occurred at a chemical plant in Seveso, Italy,which had far-reaching effects on the process safety regulations of many coun-tries, especially in Europe. An atmospheric reactor containing an uncompletedbatch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its tempera-ture was 1580C, well below the temperature at which a runaway reactioncould start (believed at the time to be 23O0C, but possibly as low as 1850C).The reaction was carried out under vacuum, and the reactor was heated bysteam in an external jacket, supplied by exhaust steam from a turbine at 19O0Cand a pressure of 12 bar gauge. The turbine was on reduced load, as variousother plants were also shutting down for the weekend (as required by Italianlaw), and the temperature of the steam rose to about 30O0C. There was a tem-perature gradient through the walls of the reactor (30O0C on the outside and16O0C on the inside) below the liquid level because the temperature of theliquid in the reactor could not exceed its boiling point. Above the liquid level,the walls were at a temperature of 30O0C throughout.
When the steam was shut off and, 15 minutes later, the agitator wasswitched off, heat transferred from the hot wall above the liquid level to thetop part of the liquid, which became hot enough for a runaway reaction tostart. This resulted in a release of TCDD (dioxin), which killed a number ofnearby animals, caused dermatitis (chloracne) in about 250 people, damagedvegetation near the site, and required the evacuation of about 600 people(Kletz 1994).
Ed. Note: The lesson learned from this incident is that provision should havebeen made to limit the vessel wall temperature from reaching the known onset tem-perature at which a runaway could occur.
4.2.2 3,4-DichloroanHine Autoclave Incident
In January 1976, a destructive runaway reaction occurred during the opera-tion of a large batch hydrogenation reactor used in the production of 3,4-dichloroaniline. The process involved the hydrogenation of 3,4-dichloronitro-benzene (DCNB) under pressure in an agitated autoclave. The autoclave wasfirst charged with DCNB and a catalyst and then purged with nitrogen toremove air. A hydrogen purge followed the nitrogen purge, after which steamwas applied to the reactor jacket and the temperature raised to within 2O0C of thereaction temperature before additional hydrogen was admitted through a sparger.The heat of reaction carried the temperature to the desired operating level.
During the early stages, the rate of reaction was limited by the heatremoval capacity of the autoclave cooling coil. This resulted in a relatively low
autoclave pressure. Later, when the hydrogenation rate fell off, the autoclavepressure was allowed to increase. Based on field evidence and subsequent labo-ratory work the following conclusions were reached as to the cause of the inci-dent (Tong 1977):
• The primary cause was a sudden pressure increase due to runaway reac-tion at about 26O0C.
• The reaction mass reached runaway temperature due to the buildup andrapid exothermic disproportionation of an intermediate (3,4-dipheny-hydroxylamine). The most likely trigger for this reaction was a 1O0Cincrease in the reactor temperature set point (operator error).
Ed. Note: The lesson learned from this incident is that a, study should have beenmade of exotherm potential and provision should have been made to limit tempera-ture setpoint or an interlock provided to address this hazard. If possible a larger oper-ating temperature margin should have been employed.
4.2.3 Continuous Sulfonation Reaction Explosion
During the startup phase of a continuous system (3 CSTRs in series) for thesulfonation of an aromatic compound, a thermal explosion occurred in apump and recirculation line. Although the incident damaged the plant andinterrupted production, no personnel were injured.
Investigation revealed that, while recirculation of the reaction mass wasstarting up, the pump and the line became plugged. This problem was cor-rected and line recirculation was restarted. Four hours later the explosionoccurred, resulting in the blow-out of the pump seal, which was immediatelyfollowed by rupture of the recirculation line.
Investigation further revealed that during pipe cleanout some insulationhad been removed, leaving a portion of the line exposed and untraced. Thiscondition apparently led to slow solidification of the reaction mass and a dead-headed pump. Calculations based on pump data indicated that a temperatureof 6O0C above the processing temperature could be reached within 5 minutesafter dead-heading occurred. Previous studies had determined that the rate ofdecomposition is considerable at this temperature and that the total heat ofdecomposition (500 kcal/kg) is large (Quinn 1984).
4.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS
Table 4 presents information on equipment failure scenarios and associateddesign solutions specific to reactors. The table heading definitions are pro-vided in Chapter 3, section 3.3.
4.4 DISCUSSION
4.4.1 Use of Potential Design Solutions Table
To arrive at the optimal design solution for a given application, use Table 4 inconjunction with the design basis selection methodology presented in Chapter2. Use of the design solutions presented in the table should be combined withsound engineering judgment and consideration of all relevant factors.
4.4.2 General Discussion
Reactors may be grouped into three main types: batch, semi-batch, and con-tinuous.
In a batch reactor, all the reactants and catalyst (if one is used) are chargedto the reactor first and agitated, and the reaction is initiated, with heat beingadded or removed as needed. In a semi-batch reactor, one of the reactants isfirst charged to the reactor, catalyst is also charged and the reactor contents areagitated, after which the other reactants and possibly additional catalyst areadded at a controlled feed rate, with heat being added or removed as needed.In a continuous reactor all the reactants and catalyst (if one is used) are fedsimultaneously to the reactor, and the products, side products, unconvertedreactants, and catalyst leave the reactor simultaneously. In some continuousreactors, the catalyst is held stationary, either in tubes or occupying the entirecross-section of the vessel.
Batch and semi-batch reactors are used primarily where reaction rates areslow and require long residence times to achieve a reasonable conversion andyield. This often means large inventories and, if the contents are flammable,there is a potential for serious fires should a leak develop. Many of these reac-tors have agitators, and if there is an agitator failure (stoppage or loss of theimpeller), some reactions can run away (Ventrone 1969; Lees 1996).
Heat removal is also a concern for batch or semi-batch reactors conduct-ing exothermic reactions. Since the external jacket may not be adequate toremove the heat of reaction, it may be necessary to install an internal coolingcoil as well, or an external heat exchanger with recirculation of the reactor con-tents. These additional items of heat transfer equipment increase the potentialfor leakage problems and may lead to a runaway if the coolant leaks into thereactants.
Continuous reactors are considered to be inherently safer than batch orsemi-batch reactors as they usually have smaller inventories of flammableand/or toxic materials. Tubular reactors are generally used for gaseous reac-tions, but are also suitable for some liquid-phase reactions. Gas phase reactorsgenerally have lower inventories than liquid-phase continuous reactors of
equal volumes, and thus are usually inherently safer. Long, thin tubular reac-tors are safer than large batch reactors as the leak rate (should a leak occur) islimited by the cross-section area of the tube, and can be stopped by closing aremotely operated emergency isolation valve in the line (Kletz 1990).Continuous-flow stirred tank reactors (CSTR) are also considered to be inher-ently safer than batch reactors as they contain smaller amounts of flammableor toxic liquids. Since they are agitated, however, they have the same agitatorfailure hazard as batch reactors, and can experience runaways if this occurs.Exhibit 4.1 is a comparison of different types of reactors from the safety per-spective (CCPS 1995).
EXHI BIT 4.1Comparison of Different Reactor Types from the Safety Perspective
Plug Flow Reactor(PFR)
Continuous-FlowStirred Tank
Reactor (CSTR) Batch Semi-Batch
ADVANTAGES
• Low inventory
• Stationarycondition (steadystate operation)
• Stationarycondition (steadystate operation)
• Agitation providessafety tool
• Streams may bediluted to slowreaction
• Agitation providessafety tool
• Controllableaddition rate
• Agitation providessafety tool
• Large exothermcontrollable
DISADVANTAGES
• Processdependency
• Potential for hotspots
• Agitation presentonly if in-linemixers areavailable
• Difficult to design
• Large inventory
• Difficult to coollarge mass
• Difficult start-upand shutdownaspects
• Precipitationproblems
• Low throughputrate
• Large exothermdifficult to control
• Large inventory
• All materialspresent
• Startingtemperature iscritical (if too low,reactants willaccumulate)
• Precipitationproblems
4.4.3 Special Considerations
Table 4 contains numerous design solutions derived from a variety of sourcesand actual situations. This section contains additional information on selecteddesign solutions. The information is organized and cross-referenced by theOperational Deviation Number in the table.
Overpressure due to Loss of Agitation (3)
Runaway reactions are often caused by loss of agitation in stirred reactors(batch, semi-batch, and CSTR) due to motor failure, coupling failure, or lossof the impeller. Agitation can be monitored by measuring the amperage orpower drawn by the agitator drive. Nevertheless, this has its drawbacks as the"measurement" of agitation takes place outside of the reactor, and sometimes,if the reactor contents are not viscous enough, the amperage or power drawwill not detect that the agitator impeller has fallen off or corroded away.Wilmot and Leong (1976) present a method of detecting agitation inside areactor, which will detect the loss of the impeller by using an internal flowswitch. The flow switch, or a similar in-vessel detection device, can be inter-locked to cut off feed or catalyst being added to a semi-batch reactor or CSTR.
If agitation is critical to the operation of a batch, semi-batch, or CSTRreactor then an independent, uninterrupted power supply backup for the agi-tator motor should be provided. Alternatively, some degree of mixing can beprovided by sparging the reactor liquid with inert gas.
Failure of mechanical seals can act as a potential high-temperature sourceinitiating vapor phase ignition. Agitator mechanical seal failure is often causedby a lack of seal fluid, and results in release of flammable or toxic vapors fromthe reactor. A dry mechanical seal is now available which can sometimes beused to replace the older type of mechanical seals which required a liquid sealfluid. Dry mechanical seals use a gas such as air or nitrogen to provide the seal-ing barrier. If a liquid seal fluid is used, monitoring of the agitator mechanicalseal fluid supply reservoir should be implemented. Monitoring can be doneautomatically, by installing a low-level switch and alarm in the seal fluid reser-voir to alert the operator, or by administrative means such as requiring theoperator to check the reservoir level on a regular schedule (e.g., once per shift)and recording the level on a log sheet.
Overpressure due to Addition of Incorrect Reactant (5)The addition of a wrong reactant can result in a runaway reaction. To mini-mize this error, the following measures can be taken:
• Provide dedicated feed tanks (for liquids) or feed hoppers (for solids)for batch reactors.
• Have two operators check the drums or bags of reactants before they areadded, and then sign off on a log sheet.
• Properly color-code and label all process lines so the operators knowwhat is in them.
If the risk of adding an incorrect reactant is still present, further protectivemeasures can be implemented, such as providing a temperature sensor tomonitor the reaction and shut off a valve in the feed line upon detection of anabnormal temperature rise or rate of temperature rise.
Overpressure due to Inactive/Semi-Active or Wrong Catalyst Addition (8)The addition of a semi-active or wrong catalyst to a reactor may result in a run-away either in the reactor or in downstream equipment. If the catalyst is fedcontinuously or at a controlled rate to a semi-batch reactor, protection can beprovided by installing a temperature sensor in the reactor, interlocked with anisolation valve in the reactant feed line, which will shut the valve when thesensor detects an abnormal temperature rise. The temperature sensor couldalso be interlocked with a valve to stop the catalyst feed. Administrative con-trols, such as procedures for verifying catalyst identity and activity, can also beapplied.
Overpressure due to Monomer Emulsion Feed Breaking during FeedLeading to a Runaway Reaction (12)In some semi-batch emulsion polymerization processes, a mixture of mono-mers emulsified in water is fed from an agitated storage tank to the reactor. Ifthe monomer emulsion feed breaks into separate oil and water phases, thepotential exists for a runaway reaction in the oil (bulk monomer) phase with-out the heat sink provided by the water. To guard against this, the monomeremulsion feed can be sampled to determine that it remains stable to separationfor a predetermined period of time without agitation before the feed is begun.
4.5 REFERENCES
CCPS 1995. Guidelines for Chemical Reactivity Evaluation and Application to Process Design. NewYork: American Institute of Chemical Engineers.
Kletz, T. A. 1990. Critical Aspects of Safety and Loss Prevention, p. 265. London :Butterworth &Co. Ltd.
Kletz, T. A. 1994. What Went Wrong: Case Histories of Process Plant Disasters. 3d ed., pp. 309-310.Houston, TX: Gulf Publishing Co.
Lees, F. P. 1996. Loss Prevention in the Process Industries. 2d ed. Woburn, MA: Butterworth Inc.Quinn, M. E., Weir, E. D., and Hoppe, T. F. 1984. IChemE Symposium Series, no. 85:31-39.Tong, W. R., Seagrave, R. L., and Wiederhorn, R. 1977. Loss Prevention Manual. 11: 71-75.
New York: American Institute of Chemical Engineers.
Ventrone, T. A. 1969. Loss Prevention Manual. Vol. 3, pp. 41-44. New York: American Instituteof Chemical Engineers.
Wilmot, D. A. and Leong, A. P. 1976. Another Way to Detect Agitation. Loss Prevention Manual.Vol. 10, pp. 19-22. New York: American Institute of Chemical Engineers.
Suggested Additional Reading
CCPS 1993. Problem Set for Kinetics, Problem 16, Prepared for SACHE. New York: AmericanInstitute of Chemical Engineers.
CCPS 1995. Guidelines for Process Safety Fundamentals in General Plant Operations. New York:American Institute of Chemical Engineers.
Benuzzi, A., and Zaldivar, J. M. (eds.). 1991. Safety of Chemical Batch Reactors and Storage Tanks.Kluwer Academic Publishers, Norwell, MA.
Burton, J. and Rogers, R. 1996. Chemical Reaction Hazards, 2ded. Institution of Chemical Engi-neers, London, UK.
DIERS 1994. Risk Considerations for Runaway Reactions. Design Institute of Emergency ReliefSystems, New York: American Institute of Chemical Engineers.
Gygax, R. W. 1988. Chemical Reaction Engineering for Safety. Chemical Engineering Science.43(8), 1759-1771.
Gygax, R. W. 1990. Scaleup principles for Assessing Thermal Runaway Risks. Chemical Engi-neering Progress, February 1990, 53-60.
International Symposium on Runaway Reactions. 1989. Cooling Capacities of Stirred Vessel,Unstirred Container, Insulated Storage Tank, Uninsulated 1 cu meter Silo, Uninsulated 25 cumeter Silo: 65. Sponsored by CCPS, IChemE and AIChE, Cambridge, MA.
Maddison, N., and Rogers, R. 1.1994. Chemical Runaways: Incidents and Their Causes. Chemi-cal Technology Europe, November/December, 28-31.
Noronha, J., Merry, J., Reid, W., and SchifFhauser, E. 1982. Deflagration Pressure Containmentfor Vessel Safety Design, Plant/Operations Progress, 1(1), 1-6.
Noronha, J., and Torres, A. 1990. Runaway Risk Approach Addressing Many Issues-Matching thePotential Consequences with Risk Reduction Methods, Proceedings of the 24th Loss PreventionSymposium, AIChE National Meeting, San Diego, CA.
Wier, E., Gravenstine, G. and Hoppe, T. 1986. Thermal Runaways—Problems with AgitatioaLoss Prevention Symposium. Paper 830: 142.
• Procedural controls on theamount or concentration ofcatalyst to be added
• Manual activation of bottomdischarge valve to drop batch intodump tank with diluent, poison,or short-stopping agent, or to anemergency containment area
• Manual addition of diluent,poison, or short-stopping agentdirectly to reactor
• Intermediate location for pre-weighed catalyst charges
• Manual addition of diluent,poison, or short-stopping agentdirectly to reactor
• Manual shutdown on high flowalarm
• Manual activation of bottomdischarge valve to drop batchinto dump tank with diluent,poison, or short-stopping agent,or to an emergency containmentarea
• Procedural controls onconcentration of reactants
• Emergency relief device
• Pressure or temperature sensorsactuating bottom discharge valveto drop batch into a dump tankwith diluent, poison or short-stopping agent, or to anemergency containment area
• Automatic addition of diluent,poison, or short-stopping agentdirectly to reactor
• Limit quantity of catalyst addedby flow totalizer
• Temperature or pressure sensorinterlocked to a shutoff valve inthe feed line
• Emergency relief device• Pressure or temperature sensors
actuating bottom discharge valveto drop batch into a dump tankwith diluent, poison or short-stopping agent, or to anemergency containment area
• Automatic addition of diluent,poison, or short-stopping agentdirectly to reactor
• High flow shutdown alarm andinterlock
• Use dedicated catalystcharge tank sized to holdonly the amount of catalystneeded
• Select feed system pressurecharacteristic so that feedcannot continue at reactoroverpressure
• Use different type ofreactor
Overcharge ofcatalyst resultingin runawayreaction
Addition of areactant toorapidly resulting inrunaway reaction
Overpressure(Batch, Semi-batch, and PlugFlow Reactors)
Overpressure(Batch andSemi-batchReactors)
1
2
Procedural
• Operators to visually checkmechanical seal fluid on regularbasis
• In-vessel agitation (velocity)sensor with alarm
• Mechanical seal fluid reservoirlow level sensor with alarm
• Speed or vibration sensor withalarm
• Manual activation of bottomdischarge valve to drop batchinto dump tank with diluent,poison, or short-stopping agent,or to an emergency containmentarea
• Manual activation of inert gassparging of reactor liquid toeffect mixing
Potential Design Solutions
Active
• Agitator power consumption orrotation indication interlockedto cutoff feed of reactants orcatalyst or activate emergencycooling
• Uninterrupted power supplybackup to motor
• Emergency relief device
• Pressure or temperature sensorsactuating bottom discharge valveto drop batch into a dump tankwith diluent, poison, or short-stopping agent, or to anemergency containment area
• Inerting of vapor space
• Provide nitrogen buffer zonearound seal using enclosurearound seal
• Automatic agitator trip on lowagitation (velocity) sensor, lowseal fluid, or low shaft speed
• Alternative agitationmethods (e.g., externalcirculation eliminates shaftseal as a source of ignitionin vapor space)
Failure Scenarios
Loss of agitationresulting in run-away reaction orhot bearing/sealscausing ignition offlammables invapor space
OperationalDeviations
Overpressure(Batch, Semi-batch andCSTRReactors)
No.
3
• Manual feed charge shutdownvia indication from feed totalizeror weight comparison in chargetank
• Manual activation of bottomdischarge valve to drop batchinto dump tank with diluent,poison, or short-stopping agent,or to an emergency containmentarea
• Procedures to shutdown feedbased on indication ofunexpected reaction progress
• Procedure for double checkingreactant identification andquality
• Dedicated storage areas/unloading facilities for reactants
• Emergency relief device
• Reactant feed charge interlockedvia feed totalizer or weightcomparison in charge tank
• Pressure or temperature sensorsactuating bottom discharge valveto drop batch into a dump tankwith diluent, poison, or short-stopping agent, or to anemergency containment area
• Automatic addition of diluent,poison, or short-stopping agentdirectly to reactor
• Manual activation of bottomdischarge valve to drop batchinto dump tank with diluent,poison, or short-stopping agent,or to an emergency containmentarea
• Manual addition of diluent,poison, or short-stopping agentdirectly to reactor
Potential Design Solutions
Active
• Low coolant flow or pressure orhigh reactor temperature toactuate secondary coolingmedium via separate supply line(e.g., city water or fire water)
• Automatic isolation of feed ondetection of loss of cooling
• Emergency relief device
• Pressure or temperature sensorsactuating bottom discharge valveto drop batch into a dump tankwith diluent, poison, or short-stopping agent, or to anemergency containment area(This approach may not beeffective for systems such aspolymerization reactions wherethere is a significant increase inviscosity.)
• Automatic addition of diluent,poison, or short-stopping agentdirectly to reactor
• Use of large inventory ofnaturally circulating,boiling coolant toaccommodate exotherm
Failure Scenarios
Loss of coolingresulting inrunaway reaction
OperationalDeviations
Overpressure
No.
6
• Passivate fresh catalyst prior touse
• Procedures for testing andverification of catalyst activityand identification
• Manual isolation of catalystand/or feed based on detectionof unexpected reaction rate
• Manual addition of diluent,poison, or short-stopping agentdirectly to reactor
• Procedures for testing andverification of catalyst activityand identification
• Manual isolation of catalystand/or feed based on detectionof unexpected reaction rate
• Manual feed isolation ondetection of low diluent addition
• Manual isolation of feed basedon detection of unexpected heatbalance
• Emergency relief device
• Automatic isolation of catalystand/or feed based on detectionof unexpected reaction rate (i.e.,abnormal heat balance)
• Pressure or temperature sensorsactuating bottom discharge valveto drop batch into dump tankwith diluent, poison, or short-stopping agent, or to anemergency containment area
• Emergency relief device
• Automatic isolation of catalystand/or feed based on detectionof unexpected reaction rate (i.e.,abnormal heat balance)
• Automatic feed isolation ondetection of low diluent addition
• Automatic isolation of feedbased on detection ofunexpected reaction rate (i.e.,abnormal heat balance)
• Manual isolation of feed basedon detection of unexpectedreaction progress
• Manual isolation of feed basedon indication of mis-sequencing
• Manual activation of fixed fireprotection
• Manual reactor dump to dumptank with diluent, poison orshort-stopping agent
• Manual injection of diluent,poison or short-stopping agentinto reactor
• Operator samples the monomeremulsion feed and observes thatsample is stable without agita-tion for a predetermined lengthof time before feed is begun
• Manual feed shut-off ordumping on change of heatbalance
Potential Design Solutions
Active
• Sequence control viaprogrammable logic controller
• Interlock shutdown of reactantaddition based on detection ofmis-sequencing
• Automatic isolation of feedbased on detection ofunexpected reaction progress(i.e, abnormal heat balance)
• Automatically activated fixed fireprotection - water spray (deluge)and/or foam systems
• Emergency relief device
• Automatic reactor dump todump tank with diluent, poison,or short stopping agent
• Automatic injection of diluentpoison or short-stopping agentinto reactor
• Emergency relief device
• Automatic feed shut-off ordumping on change of heatbalance
• Reactor or downstreamvessel design accom-modating maximumexpected pressure
Incomplete reac-tion due to insuffi-cient residencetime, low tem-perature, etc. lead-ing to unexpectedreaction in subse-quent processingsteps (in reactor ordownstreamvessel)
