Potential runaway reaction

Have formal process hazard analyses (PHAs) been completed for highly hazardous processes (for example, those processes involving toxic or volatile substances, highly toxic materials, severe lachrymators, flammables, explosive compounds or potential runaway reactions) If yes, please summarize status of each. 

A batch chemical reactor contains 10,000 kg of reacting liquid material. A relief device must be properly sized for a potential runaway reaction. 

The Wall Street Journal reported that several weeks before the Bhopal disaster, Union Carbide changed procedures at its Institute West VA plant, on which the Bhopal plant design was based, to prevent a similar incident. Rep. Henry Waxman (D. CA.) disclosed that a Union Carbide safety team in September had warned of a potential runaway reaction of methyl isocyanate at Institute and the EPA had reported that there were 28 small leaks of the chemical in 1980 at the facility (Winslow, 1985). 

It may be also noted that (i) between 1981 and 1984 six accidents with phosgene or MIC occurred, (ii) a 1982 audit was critical of the MIC tank and instrumentation and (iii) in 1984 a warning of a potential runaway reaction hazard was given. Therefore, clues to this accident were available before it happened. 

In reviewing the hazard associated with a chemical process, one of the hazards which should be considered is that of a potential runaway reaction. If either the desired chemical reaction or an undesired reaction (e.g., a side reaction or the unintended decomposition of a product) produces more heat than can be dissipated, the heat will accumulate in the system. This can lead to the thermal runaway. If the exothermic reaction(s) is accompanied by significant pressure generation, the runaway reaction can lead to rupture of the reaction vessel. 

The process hazards analysis for this vessel stated that either high temperature or high pressure is an indication of a potential runaway reaction that may result in an explosion. The safety requirements specification states that the SIF shall be SIL 2 and a nuisance or spurious trip should be better than one in every five years. The process will be operated for mission time of ten years after which a major overhaul and rebuild will take place. 

Note that the process upset, F, is the major contributor to the probability to a potential runaway reaction. ... 

Demineraiized water charging A oontrolled charge of water is added. An overcharge might lead to a hydraulic overfill an underoharge may cause quality problems and potential runaway reaction. Any surfactants or other additives are also introduced during this step. 

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... 

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. 

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. 

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

Hot spot develops in reaction medium. Temperature excursion outside the safe operating envelope, possibly resulting in a runaway reaction or decomposition. Potential mechanical failure of reactor wall. 

Inherent An atmospheric pressure reaction using nonvolatile solvents which is incapable of generating any pressure in the event of a runaway reaction. There is no potential for overpressure of the reactor because of the chemistry and physical properties of the materials. 

Procedural The same reactor described in Example 3 above, but without the 5 psig high pressure interlock. Instead, the operator is instructed to monitor the reactor pressure and stop the reactant feeds if the pressure exceeds 5 psig. There is a potential for human error, the operator failing to monitor the reactor pressure, or failing to stop the reactant feeds in time to prevent a runaway reaction. 

A solvent used in an exothermic reaction is nonvolatile, and moderately toxic. An alternative solvent is less toxic, but also has a much lower boiling point. There is a trade-off between toxic hazards and the potential for tempering the exotherm, but also generating pressure from boiling solvent in case of a runaway reaction. 

This chapter is not concerned with accidents on the road. Rather, it describes some of the many incidents that have occurred while tank trucks and cars (known in Europe as road and rail tankers) were being filled or emptied. Section 18.8 shows how hazard and operability studies have been used to spot potential hazards in filling systems, and Section 22.3 describes some runaway reactions in tank trucks and cars.. 

Tlie fourth case study (Section 21.5) was a haztird and risk analysis of the potential impiict of the caiastropliic release of llie chemical contents of a holdup lank because of a runaway reaction. The study traced calculations leading to a risk curve portraying the healtli impact in lenns of tlie frequency with which the number of people affected exceeded various amounts. 

Figure 7-61. Reactive system screening tool (RSST) for evaluating runaway reaction potential. By permission, Fauske and Associates, Inc.

The Fauske and Associates Reactive System. Screening Tool (RSST) w as developed as a result of the DIERS studies and allow s rapid evaluadon of the potential for runaway reactions. It measures the rate of energy and gas release during the runaway and is valuable for screening various process s)stems before commercial designs are completed (see Figure 7-61). 

Changing process or mechanical conditions to reduce the potential for runaway reactions, accelerated corrosion or erosion, or other possible causes of undesirable events... 

Clearly, the potential hazard from runaway reactions is reduced by reducing the inventory of material in the reactor. 

There is a potentially dangerous reaction of carbon tetrachloride with dimethylformamide in presence of iron. The same occurs with 1,2,3,4,5,6-hexachlorocyclohexane, but not with dichloromethane or 1,2-dichloroethane under the same conditions [1], A quantitative study of the reaction by DSC and ARC techniques shows that in a 1 1 wt. mixture with carbon tetrachloride in absence of iron, an exothermic reaction sets in below 100°C. Under adiabatic conditions, the heat release (207.6 J/g) would take a runaway reaction to over 240°C. In presence of 3% of iron powder, the same mixture shows 2 exotherms, one at 56°C (108 J/g) and the second at 94°C (275 J/g), a final adiabatic temperature exceeding 285°C being possible [2], Dimethylacetamide behaves similarly but more so. 

This section discusses how a runaway reaction occurs and lists some of the process deviations that can lead to such a runaway. Equipment for identifying potentially hazardous process steps is reviewed, and general principles for inherently safe process design are given. 

The Reactive System Screening Tool (RSST), marketed by Fauske and Associates, is a relatively new type of apparatus for process hazard calorimetry [192, 196-198]. The equipment is designed to determine the potential for runaway reactions and to determine the (quasi) adiabatic rates of temperature and pressure rise during a runaway as a function of the process, vessel, and other parameters. 

Data acquired from an RSST experiment show the potential of a runaway reaction (reactive or nonreactive), the temperature history of the runaway, and the rates of temperature and pressure rise (the latter in the case of gas-produc-... 

Advantages of the RSST are its relatively low cost and its availability to permit a quick evaluation for potential runaways. Pressurized conditions may be used. The temperature-time curve and the concurrent pressure increase (of the containment vessel) can be recorded, which are measures of the reactivity hazards of the substance or reaction under investigation. The temperature-time curve shows the lowest temperature at which a runaway can be detected in the test system (initiation temperature, To). 

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