Reaction, runaway

Preview This section describes runaway reactions, their impact, their causes, and steps to prevent these incidents. 

Those who cannot remember the past are condemned to repeat it. 

A technician was carrying out a reaction with ethylene glycol and phosphorus pentasulfide in hexane. The reaction flask was heated by an electric mantle in a hood. The procedure called for maintaining the temperature at or below 60 °C by controlling the power to the mantle. As the reaction proceeded he realized that the temperature was rising rapidly, and at 177 °C he turned off the power to the mantle. At this point he opened the hood to remove the flask from the heating mantle. As he did this the flask exploded, causing bums to his face and eyes. Pieces of the flask were scattered over the laboratory. 

A researcher prepared a Grignard reagent using tetrahydrofliran as the solvent. Magnesium turnings were added to the solvent followed by iodine to initiate the reaction. A bromobenzene derivative was added to the reaction mixture incrementally, and the reaction was left unattended in a chemical hood. After about 20-30% of the bromobenzene derivative had been added, a sudden exothermic reaction caused the solvent to boil, blowing most of the reaction mixture out of the flask. 

What lessons can be learned from these incidents  

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Clearly, the potential hazard from runaway reactions is reduced by reducing the inventory of material in the reactor. Batch operation requires a larger inventory than the corresponding continuous reactor. Thus there may be a safety incentive to change from batch to continuous operation. Alternatively, the batch operation can be... 

The use of an unnecessarily hot utility or heating medium should be avoided. This may have been a major factor that led to the runaway reaction at Seveso in Italy in 1976, which released toxic material over a wide area. The reactor was liquid phase and operated in a stirred tank (Fig. 9.3). It was left containing an uncompleted batch at around 160 C, well below the temperature at which a runaway reaction could start. The temperature required for a runaway reaction was around 230 C. ... 

In this accident, the steam was isolated from the reactor containing the unfinished batch and the agitator was switched ofiF. The steam used to heat the reactor was the exhaust from a steam turbine at 190 C but which rose to about 300°C when the plant was shutdown. The reactor walls below the liquid level fell to the same temperature as the liquid, around 160°C. The reactor walls above the liquid level remained hotter because of the high-temperature steam at shutdown (but now isolated). Heat then passed by conduction and radiation from the walls to the top layer of the stagnant liquid, which became hot enough for a runaway reaction to start (see Fig. 9.3). Once started in the upper layer, the reaction then propagated throughout the reactor. If the steam had been cooler, say, 180 C, the runaway could not have occurred. ... 

International Symposium on Runaway Reactions American Institute of Chemical Engineers, New York, 1989 and 1995. 

R. D. Coffee, in H. H. Fawcett and W. S. Wood, Safety and Accident Prevention in Chemical Operations, 2nd ed., Wiley-Interscience, New York, 1982, p. 305 International Symposium on Runaway Reactions, Center for Chemical Process Safety, New York, 1989, pp. 140, 144,177, 234. 

Hazards from combustion and runaway reactions play a leading role in many chemical process accidents. Knowledge of these reactions is essential for control of process hazards. It is important that loss of containment be avoided. For example ... 

Understanding how sudden pressure releases can occur is important. They can happen, for example, from ruptured high-pressure tanks, runaway reactions, flammable vapor clouds, or pressure developed from external fire. The proper design of pressure rehef systems can reduce the possibility of losses from unintended overpressure. 

Many runaway reactions can be prevented by changing the order of operations, reducing the temperature, or changing another parameter. 

Vessel Filled with Reactive Gas Mixtures Most cases of damage arise not from the vessel failing at its normal operating pressure but because of an unexpected exothermic reaction occurring within the vessel. This usually is a decomposition, polymerization, deflagration, runaway reaction, or oxidation reaction. In assessing the damage... 

Runaway Reactions Runaway temperature and pressure in process vessels can occur as a resiilt of many fac tors, including loss of cooling, feed or quench failure, excessive feed rates or temperatures, contaminants, catalyst problems, and agitation failure. Of major concern is the high rate of energy release and/or formation of gaseous produc ts, whiai may cause a rapid pressure rise in the equipment. In order to properly assess these effec ts, the reaction kinetics must either be known or obtained experimentally. 

Some vent streams, such as light hydrocarbons, can be discharged directly to the atmosphere even though they are flammable and explosive. This can be done because the high-velocity discharge entrains sufficient air to lower the hydrocarbon concentration below the lower explosive limit (API RP 521, 1997). Toxic vapors must be sent to a flare or scrubber to render them harmless. Multiphase streams, such as those discharged as a result of a runaway reaction, for example, must first be routed to separation or containment equipment before final discharge to a flare or scrubber. 

Accelerating Rate Calorimeter (ARC) The ARC can provide extremely useful and valuable data. This equipment determines the self-heating rate of a chemical under near-adiabatic conditions. It usu-aUy gives a conservative estimate of the conditions for and consequences of a runaway reaction. Pressure and rate data from the ARC may sometimes be used for pressure vessel emergency relief design. Activation energy, heat of reaction, and approximate reaction order can usually be determined. For multiphase reactions, agitation can be provided. 

Reactive System Screening Tool (RSST) The RSST is a calorimeter that quickly and safely determines reactive chemical hazards. It approaches the ease of use of the DSC with the accuracy of the VSP. The apparatus measures sample temperature and pressure within a sample containment vessel. Tne RSST determines the potential for runaway reactions and measures the rate of temperature and pressure rise (for gassy reactions) to allow determinations of the energy and gas release rates. This information can be combined with simplified methods to assess reac tor safety system relief vent reqiiire-ments. It is especially useful when there is a need to screen a large number of different chemicals and processes. 

Runaway reaction or polymerization—e.g., vinyl chloride monomer (Kim-E and Reid, The Rapid Depressurization of Hot, High Pressure Liquids or Supercritic Fluids, chap. 3, in M. E. Paulaitis et al., eds.. Chemical engineering at Supercritical Fluid Conditions, Ann Arbor Science, 1983, pp. 81-100)... 

Raw materials, intermediates, products, by-products and/or undesired byproducts are subject to runaway reactions that produce extreme heat and/or extreme amounts of gaseous/vapor products. 

Evaluate methods for controlling runaway reactions (e.g., short stop, inhibitors)... 

Determine consequences of runaway reactions and ensure mitigation techniques are in place... 

Addition of incorrect reactant or unanticipated material to the reactor. Possibility for runaway reaction. 

Change in feed composition. This may happen due to change in suppliers or due to introduction of reworked material. Unwanted effect on reaction products, by-products. Varying inhibitor concentrations in monomers from different vendors. Potential for runaway reaction. 

Overcharge/overfeed of reactants Possibility of overfilling vessel, or initiating runaway reaction. 

Undercharge/ underfeed of reactants. Possibility of unreacted mixed reactants left at end of batch, leading to a subsequent runaway reaction. 

Overcharge of catalyst or initiator, too much or too fast. Possibility of runaway reaction. 

Undercharge of catalyst. Potential for accumulation of reactants and subsequent runaway reaction. Possibility of no reaction resulting in a waste disposal issue. 

Develop and install emergency system and procedures to shortstop runaway reaction. 

Reactive Systems Screening Tool (RSST ) Temperature history of runaway reaction, rates of temperature and pressure rise (for gas producing reactions)... 

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