REACTORS 4.1 INTRODUCTION

Transcription

1 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 practices for reactors.

2 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 countries, especially in Europe. An atmospheric reactor containing an uncompleted batch of 2,4,5-trichlorophenol (TCP) was left for the weekend. Its temperature was C, well below the temperature at which a could start (believed at the time to be 23O 0 C, but possibly as low as C). The reaction was carried out under vacuum, and the reactor was heated by steam in an external jacket, supplied by exhaust steam from a turbine at 19O 0 C and a pressure of 12 bar gauge. The turbine was on reduced load, as various other plants were also shutting down for the weekend (as required by Italian law), and the temperature of the steam rose to about 30O 0 C. There was a temperature gradient through the walls of the reactor (30O 0 C on the outside and 16O 0 C on the inside) below the liquid level because the temperature of the liquid in the reactor could not exceed its boiling point. Above the liquid level, the walls were at a temperature of 30O 0 C throughout. When the steam was shut off and, 15 minutes later, the agitator was switched off, heat transferred from the hot wall above the liquid level to the top part of the liquid, which became hot enough for a to start. This resulted in a release of TCDD (dioxin), which killed a number of nearby animals, caused dermatitis (chloracne) in about 250 people, damaged vegetation 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 have been made to limit the vessel wall temperature from reaching the known onset temperature at which a runaway could occur ,4-DichloroanHine Autoclave Incident In January 1976, a destructive occurred during the operation of a large batch hydrogenation reactor used in the production of 3,4- dichloroaniline. The process involved the hydrogenation of 3,4-dichloronitrobenzene (DCNB) under pressure in an agitated autoclave. The autoclave was first charged with DCNB and a catalyst and then purged with nitrogen to remove air. A hydrogen purge followed the nitrogen purge, after which steam was applied to the reactor jacket and the temperature raised to within 2O 0 C of the reaction 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 heat removal capacity of the autoclave cooling coil. This resulted in a relatively low

3 autoclave pressure. Later, when the hydrogenation rate fell off, the autoclave pressure was allowed to increase. Based on field evidence and subsequent laboratory work the following conclusions were reached as to the cause of the incident (Tong 1977): The primary cause was a sudden pressure increase due to at about 26O 0 C. The reaction mass reached runaway temperature due to the buildup and rapid exothermic disproportionation of an intermediate (3,4-diphenyhydroxylamine). The most likely trigger for this reaction was a 1O 0 C increase in the reactor temperature set point (operator error). Ed. Note: The lesson learned from this incident is that a, study should have been made of exotherm potential and provision should have been made to limit temperature setpoint or an interlock provided to address this hazard. If possible a larger operating temperature margin should have been employed Continuous Sulfonation Reaction Explosion During the startup phase of a continuous system (3 CSTRs in series) for the sulfonation of an aromatic compound, a thermal explosion occurred in a pump and recirculation line. Although the incident damaged the plant and interrupted production, no personnel were injured. Investigation revealed that, while recirculation of the reaction mass was starting up, the pump and the line became plugged. This problem was corrected and line recirculation was restarted. Four hours later the explosion occurred, resulting in the blow-out of the pump seal, which was immediately followed by rupture of the recirculation line. Investigation further revealed that during pipe cleanout some insulation had been removed, leaving a portion of the line exposed and untraced. This condition apparently led to slow solidification of the reaction mass and a deadheaded pump. Calculations based on pump data indicated that a temperature of 6O 0 C above the processing temperature could be reached within 5 minutes after dead-heading occurred. Previous studies had determined that the rate of decomposition is considerable at this temperature and that the total heat of decomposition (500 kcal/kg) is large (Quinn 1984). 4.3 FAILURE SCENARIOS AND DESIGN SOLUTIONS Table 4 presents information on equipment failure scenarios and associated design solutions specific to reactors. The table heading definitions are provided in Chapter 3, section 3.3.

4 4.4 DISCUSSION Use of Potential Design Solutions Table To arrive at the optimal design solution for a given application, use Table 4 in conjunction with the design basis selection methodology presented in Chapter 2. Use of the design solutions presented in the table should be combined with sound engineering judgment and consideration of all relevant factors General Discussion Reactors may be grouped into three main types: batch, semi-batch, and continuous. In a batch reactor, all the reactants and catalyst (if one is used) are charged to the reactor first and agitated, and the reaction is initiated, with heat being added or removed as needed. In a semi-batch reactor, one of the reactants is first charged to the reactor, catalyst is also charged and the reactor contents are agitated, after which the other reactants and possibly additional catalyst are added 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 fed simultaneously to the reactor, and the products, side products, unconverted reactants, and catalyst leave the reactor simultaneously. In some continuous reactors, the catalyst is held stationary, either in tubes or occupying the entire cross-section of the vessel. Batch and semi-batch reactors are used primarily where reaction rates are slow and require long residence times to achieve a reasonable conversion and yield. 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 reactors have agitators, and if there is an agitator failure (stoppage or loss of the impeller), some reactions can run away (Ventrone 1969; Lees 1996). Heat removal is also a concern for batch or semi-batch reactors conducting exothermic reactions. Since the external jacket may not be adequate to remove the heat of reaction, it may be necessary to install an internal cooling coil as well, or an external heat exchanger with recirculation of the reactor contents. These additional items of heat transfer equipment increase the potential for leakage problems and may lead to a runaway if the coolant leaks into the reactants. Continuous reactors are considered to be inherently safer than batch or semi-batch reactors as they usually have smaller inventories of flammable and/or toxic materials. Tubular reactors are generally used for gaseous reactions, but are also suitable for some liquid-phase reactions. Gas phase reactors generally have lower inventories than liquid-phase continuous reactors of

5 equal volumes, and thus are usually inherently safer. Long, thin tubular reactors are safer than large batch reactors as the leak rate (should a leak occur) is limited by the cross-section area of the tube, and can be stopped by closing a remotely operated emergency isolation valve in the line (Kletz 1990). Continuous-flow stirred tank reactors (CSTR) are also considered to be inherently safer than batch reactors as they contain smaller amounts of flammable or toxic liquids. Since they are agitated, however, they have the same agitator failure 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 perspective (CCPS 1995). EXHI BIT 4.1 Comparison of Different Reactor Types from the Safety Perspective Plug Flow Reactor (PFR) Continuous-Flow Stirred Tank Reactor (CSTR) Batch Semi-Batch ADVANTAGES Low inventory Stationary condition (steady state operation) Stationary condition (steady state operation) Agitation provides safety tool Streams may be diluted to slow reaction Agitation provides safety tool Controllable addition rate Agitation provides safety tool Large exotherm controllable DISADVANTAGES Process dependency Potential for hot spots Agitation present only if in-line mixers are available Difficult to design Large inventory Difficult to cool large mass Difficult start-up and shutdown aspects Precipitation problems Low throughput rate Large exotherm difficult to control Large inventory All materials present Starting temperature is critical (if too low, reactants will accumulate) Precipitation problems

6 4.4.3 Special Considerations Table 4 contains numerous design solutions derived from a variety of sources and actual situations. This section contains additional information on selected design solutions. The information is organized and cross-referenced by the Operational Deviation Number in the table. 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 loss of the impeller. Agitation can be monitored by measuring the amperage or power 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 draw will not detect that the agitator impeller has fallen off or corroded away. Wilmot and Leong (1976) present a method of detecting agitation inside a reactor, which will detect the loss of the impeller by using an internal flow switch. The flow switch, or a similar in-vessel detection device, can be interlocked 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 CSTR reactor then an independent, uninterrupted power supply backup for the agitator motor should be provided. Alternatively, some degree of mixing can be provided by sparging the reactor liquid with inert gas. Failure of mechanical seals can act as a potential high-temperature source initiating vapor phase ignition. Agitator mechanical seal failure is often caused by a lack of seal fluid, and results in release of flammable or toxic vapors from the reactor. A dry mechanical seal is now available which can sometimes be used to replace the older type of mechanical seals which required a liquid seal fluid. Dry mechanical seals use a gas such as air or nitrogen to provide the sealing barrier. If a liquid seal fluid is used, monitoring of the agitator mechanical seal fluid supply reservoir should be implemented. Monitoring can be done automatically, by installing a low-level switch and alarm in the seal fluid reservoir to alert the operator, or by administrative means such as requiring the operator to check the reservoir level on a regular schedule (e.g., once per shift) and recording the level on a log sheet. due to Addition of Incorrect Reactant (5) The addition of a wrong reactant can result in a. To minimize this error, the following measures can be taken: Provide dedicated feed tanks (for liquids) or feed hoppers (for solids) for batch reactors.

7 Have two operators check the drums or bags of reactants before they are added, and then sign off on a log sheet. Properly color-code and label all process lines so the operators know what is in them. If the risk of adding an incorrect reactant is still present, further protective measures can be implemented, such as providing a temperature sensor to monitor the reaction and shut off a valve in the feed line upon detection of an abnormal temperature rise or rate of temperature rise. 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 runaway either in the reactor or in downstream equipment. If the catalyst is fed continuously or at a controlled rate to a semi-batch reactor, protection can be provided by installing a temperature sensor in the reactor, interlocked with an isolation valve in the reactant feed line, which will shut the valve when the sensor detects an abnormal temperature rise. The temperature sensor could also be interlocked with a valve to stop the catalyst feed. Administrative controls, such as procedures for verifying catalyst identity and activity, can also be applied. due to Monomer Emulsion Feed Breaking during Feed Leading to a Runaway Reaction (12) In some semi-batch emulsion polymerization processes, a mixture of monomers emulsified in water is fed from an agitated storage tank to the reactor. If the monomer emulsion feed breaks into separate oil and water phases, the potential exists for a in the oil (bulk monomer) phase without the heat sink provided by the water. To guard against this, the monomer emulsion feed can be sampled to determine that it remains stable to separation for a predetermined period of time without agitation before the feed is begun. 4.5 REFERENCES CCPS Guidelines for Chemical Reactivity Evaluation and Application to Process Design. New York: American Institute of Chemical Engineers. Kletz, T. A Critical Aspects of Safety and Loss Prevention, p London :Butterworth & Co. Ltd. Kletz, T. A What Went Wrong: Case Histories of Process Plant Disasters. 3d ed., pp Houston, TX: Gulf Publishing Co. Lees, F. P Loss Prevention in the Process Industries. 2d ed. Woburn, MA: Butterworth Inc. Quinn, M. E., Weir, E. D., and Hoppe, T. F IChemE Symposium Series, no. 85: Tong, W. R., Seagrave, R. L., and Wiederhorn, R Loss Prevention Manual. 11: New York: American Institute of Chemical Engineers.

8 Ventrone, T. A Loss Prevention Manual. Vol. 3, pp New York: American Institute of Chemical Engineers. Wilmot, D. A. and Leong, A. P Another Way to Detect Agitation. Loss Prevention Manual. Vol. 10, pp New York: American Institute of Chemical Engineers. Suggested Additional Reading CCPS Problem Set for Kinetics, Problem 16, Prepared for SACHE. New York: American Institute of Chemical Engineers. CCPS Guidelines for Process Safety Fundamentals in General Plant Operations. New York: American Institute of Chemical Engineers. Benuzzi, A., and Zaldivar, J. M. (eds.) Safety of Chemical Batch Reactors and Storage Tanks. Kluwer Academic Publishers, Norwell, MA. Burton, J. and Rogers, R Chemical Reaction Hazards, 2ded. Institution of Chemical Engineers, London, UK. DIERS Risk Considerations for Runaway Reactions. Design Institute of Emergency Relief Systems, New York: American Institute of Chemical Engineers. Gygax, R. W Chemical Reaction Engineering for Safety. Chemical Engineering Science. 43(8), Gygax, R. W Scaleup principles for Assessing Thermal Runaway Risks. Chemical Engineering Progress, February 1990, International Symposium on Runaway Reactions Cooling Capacities of Stirred Vessel, Unstirred Container, Insulated Storage Tank, Uninsulated 1 cu meter Silo, Uninsulated 25 cu meter Silo: 65. Sponsored by CCPS, IChemE and AIChE, Cambridge, MA. Maddison, N., and Rogers, R Chemical Runaways: Incidents and Their Causes. Chemical Technology Europe, November/December, Noronha, J., Merry, J., Reid, W., and SchifFhauser, E Deflagration Pressure Containment for Vessel Safety Design, Plant/Operations Progress, 1(1), 1-6. Noronha, J., and Torres, A Runaway Risk Approach Addressing Many Issues-Matching the Potential Consequences with Risk Reduction Methods, Proceedings of the 24th Loss Prevention Symposium, AIChE National Meeting, San Diego, CA. Wier, E., Gravenstine, G. and Hoppe, T Thermal Runaways Problems with Agitatioa Loss Prevention Symposium. Paper 830: 142.

9 TABLE 4. FAILURE SCENARIOS FOR REACTORS No. Operational Deviations Failure Scenarios Inherently Safer/Passive Potential Design Solutions Active Procedural I 1 2 (Batch, Semibatch, and Plug Flow Reactors) (Batch and Semi-batch Reactors) Overcharge of catalyst resulting in runaway reaction Addition of a reactant too rapidly resulting in Use dedicated catalyst charge tank sized to hold only the amount of catalyst needed Use different type of reactor Limit delivery capacity of feed system to within safe feed rate limitations (e.g., screw feeder for solids or flow orifice for liquids) Select feed system pressure characteristic so that feed cannot continue at reactor overpressure Use different type of reactor Pressure or temperature sensors actuating bottom discharge valve to drop batch into a dump tank with diluent, poison or shortstopping agent, or to an emergency containment area Automatic addition of diluent, poison, or short-stopping agent directly to reactor Limit quantity of catalyst added by flow totalizer Temperature or pressure sensor interlocked to a shutoff valve in the feed line Pressure or temperature sensors actuating bottom discharge valve to drop batch into a dump tank with diluent, poison or shortstopping agent, or to an emergency containment area Automatic addition of diluent, poison, or short-stopping agent directly to reactor Highflowshutdown alarm and interlock Procedural controls on the amount or concentration of catalyst to be added Manual activation of bottom discharge valve to drop batch into dump tank with diluent, poison, or short-stopping agent, or to an emergency containment area Manual addition of diluent, poison, or short-stopping agent directly to reactor Intermediate location for preweighed catalyst charges Manual addition of diluent, poison, or short-stopping agent directly to reactor Manual shutdown on high flow alarm Manual activation of bottom discharge valve to drop batch into dump tank with diluent, poison, or short-stopping agent, or to an emergency containment area Procedural controls on concentration of reactants

10 No. Operational Deviations Failure Scenarios Inherently Safer/Passive Potential Design Solutions Active Procedural 3 (Batch, Semibatch and CSTR Reactors) Loss of agitation resulting in runaway reaction or hot bearing/seals causing ignition of flammables in vapor space Use different type of reactor (plug flow) Alternative agitation methods (e.g., external circulation eliminates shaft seal as a source of ignition in vapor space) Agitator power consumption or rotation indication interlocked to cutoff feed of reactants or catalyst or activate emergency cooling Uninterrupted power supply backup to motor Pressure or temperature sensors actuating bottom discharge valve to drop batch into a dump tank with diluent, poison, or shortstopping agent, or to an emergency containment area Inerting of vapor space Provide nitrogen buffer zone around seal using enclosure around seal Automatic agitator trip on low agitation (velocity) sensor, low sealfluid,or low shaft speed Operators to visually check mechanical sealfluidon regular basis In-vessel agitation (velocity) sensor with alarm Mechanical seal fluid reservoir low level sensor with alarm Speed or vibration sensor with alarm Manual activation of bottom discharge valve to drop batch into dump tank with diluent, poison, or short-stopping agent, or to an emergency containment area Manual activation of inert gas sparging of reactor liquid to effect mixing

11 4 (Batch and Semi -batch Reactors) Overcharge or overfeed of reactant resulting in Use of dedicated reactant charge tank sized only to hold amount of reactant needed Use of continuous reactor Reactant feed charge interlocked via feed totalizer or weight comparison in charge tank Pressure or temperature sensors actuating bottom discharge valve to drop batch into a dump tank with diluent, poison, or shortstopping agent, or to an emergency containment area Automatic addition of diluent, poison, or short-stopping agent directly to reactor Manual feed charge shutdown via indication from feed totalizer or weight comparison in charge tank Manual activation of bottom discharge valve to drop batch into dump tank with diluent, poison, or short-stopping agent, or to an emergency containment area 5 (T) Addition of incorrect reactant resulting in Use of dedicated feed tank and reactor for production of one product Elimination of crossconnections Use of dedicated hoses and incompatible couplings for reactants where hose connections are used Automatic feed shutdown based on detection of unexpected reaction progress (i.e., abnormal heat balance) Procedures to shutdown feed based on indication of unexpected reaction progress Procedure for double checking reactant identification and quality Dedicated storage areas/ unloading facilities for reactants

12 No. Operational Deviations Failure Scenarios Inherently Safer/Passive Potential Design Solutions Active Procedural 6 Loss of cooling resulting in Use of large inventory of naturally circulating, boiling coolant to accommodate exotherm Low coolantflowor pressure or high reactor temperature to actuate secondary cooling medium via separate supply line (e.g., city water or fire water) Automatic isolation of feed on detection of loss of cooling Pressure or temperature sensors actuating bottom discharge valve to drop batch into a dump tank with diluent, poison, or shortstopping agent, or to an emergency containment area (This approach may not be effective for systems such as polymerization reactions where there is a significant increase in viscosity.) Manual activation of secondary cooling system Manual activation of bottom discharge valve to drop batch into dump tank with diluent, poison, or short-stopping agent, or to an emergency containment area Manual addition of diluent, poison, or short-stopping agent directly to reactor Automatic addition of diluent, poison, or short-stopping agent directly to reactor

13 7 Overactive and/or wrong catalyst results in runaway reaction Use prediluted catalyst Automatic isolation of catalyst and/or feed based on detection of unexpected reaction rate (i.e., abnormal heat balance) Pressure or temperature sensors actuating bottom discharge valve to drop batch into dump tank with diluent, poison, or shortstopping agent, or to an emergency containment area Passivate fresh catalyst prior to use Procedures for testing and verification of catalyst activity and identification Manual isolation of catalyst and/or feed based on detection of unexpected reaction rate Manual addition of diluent, poison, or short-stopping agent directly to reactor 8 (T) Inactive and/or wrong catalyst leading to delayed in reactor or downstream vessel Reactor or downstream vessel design Automatic isolation of catalyst and/or feed based on detection of unexpected reaction rate (i.e., abnormal heat balance) Procedures for testing and verification of catalyst activity and identification Manual isolation of catalyst and/or feed based on detection of unexpected reaction rate 9 Underfeed of diluent resulting in insufficient heat sink Automatic feed isolation on detection of low diluent addition Automatic isolation of feed based on detection of unexpected reaction rate (i.e., abnormal heat balance) Manual feed isolation on detection of low diluent addition Manual isolation of feed based on detection of unexpected heat balance

14 No. Operational Deviations Failure Scenarios Inherently Safer/Passive Potential Design Solutions Active Procedural 10 (Batch & Semibatch) Reactants added in incorrect order Sequence control via programmable logic controller Interlock shutdown of reactant addition based on detection of mis-sequencing Automatic isolation of feed based on detection of unexpected reaction progress (i.e, abnormal heat balance) Manual isolation of feed based on detection of unexpected reaction progress Manual isolation of feed based on indication of mis-sequencing 11 External fire initiates runaway reaction Fireproof insulation (reduced heat input) Slope-away grading under reactor to remote spill collection Locate reactor outside of fire affected zone Automatically activated fixed fire protection - water spray (deluge) and/or foam systems Automatic reactor dump to dump tank with diluent, poison, or short stopping agent Automatic injection of diluent poison or short-stopping agent into reactor Manual activation of fixed fire protection Manual reactor dump to dump tank with diluent, poison or short-stopping agent Manual injection of diluent, poison or short-stopping agent into reactor 12 (T) Feed of monomer emulsion breaks into a separate oil phase on top of a water phase while being fed to the reactor leading to accommodating the maximum pressure arising from run-away reaction of bulk (non-emulsified) monomer phase Static mixer ahead of reactor Automatic feed shut-off or dumping on change of heat balance Operator samples the monomer emulsion feed and observes that sample is stable without agitation for a predetermined length of time before feed is begun Manual feed shut-off or dumping on change of heat balance

15 13 High reactor temperature due to failure of heating system initiates Limit temperature of heating media Automatic depressuring Automatic injection of inhibitor Automatic isolation of heating media or feed Emergency cooling Manual dumping of reactor contents Manual injection of inhibitor Manual isolation of heating media or feed 14 High Temperature (Continuous Packed Bed or Packed Tube Reactors) Hot spot develops in catalyst exposing vessel wall to high temperature and potential mechanical failure or initiation of Use alternative reactor design (e.g., fluid bed) Use multiple small diameter beds to reduce maldistribution Minimize reactor head space volume to reduce residence time (partial oxidation reactors) and mitigate autoignition High temperature sensors interlocked to shut down reactor Automatic depressuring based on detection of high bed temperatures or low flow Automatic introduction of quenchfluidinto packed bed or tubes based on detection of high local temperature Manual shutdown of reactor upon detection of high temperature in bed Monitoring of exterior wall temperature with infrared optical detection system Manual depressuring based on detection of high bed temperature Manual introduction of quench fluid into packed bed or tubes on detection of high local temperature Procedures for packing tubes to ensure uniformity of catalyst filling

16 i No. Operational Deviations Failure Scenarios Inherently Safer/Passive Potential Design Solutions Active Procedural 15 Reverse Flow Reactor contents inadvertently admitted to upstream feed vessel resulting in Provide positive displacement feed pump instead of centrifugal pump Elevate feed vessel above reactor with emergency relief device on reactor set below feed vessel minimum operating pressure Provide check valve(s) in feed line Automatic closure of isolation valve (s) in feed line on detection of low or noflow,or reverse pressure differential in feed line on feed vessel or feed line Manual closure of isolation valve(s) in feed line on detection of low or noflowin feed line 16 Wrong Composition Contamination from leakage of heating/cooling media or introduction of other foreign substances (e.g., corrosion) Use heat transfer fluid that does not react with process fluid Use jacket rather than internal coil for heat transfer Periodic testing of process fluid for contamination Procedures for leak/pressure testing of jacket, coil or heat exchanger prior to operation Procedure for testing liner with continuity meter Upgrade metallurgy or use resistant liner Heat transfer loop pressure lower than process pressure

17 17 Wrong Composition Incomplete reaction due to insufficient residence time, low temperature, etc. leading to unexpected reaction in subsequent processing steps (in reactor or downstream vessel) Reactor or downstream vessel design accommodating maximum Automatic feed isolation based on detection of low reactor temperature Automatic feed isolation based on continuous on-line reactor composition monitoring Manual feed isolation based on detection of low reactor temperature Manual feed isolation based on continuous on-line reactor composition monitoring or "grab" sampling