Runaway reactions containment

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. ... 

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 ... 

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. 

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. 

Accidental release, spillage Transport incidents Overfilling of containers Equipment failure Unexpected reactions Runaway reactions... 

A runaway reaction occurs when an exothermic system becomes uncontrollable. The reaction leads to a rapid increase in the temperature and pressure, which if not relieved can rupture the containing vessel. A runaway reaction occurs because the rate of reaction, and therefore the rate of heat generation, increases exponentially with temperature. In contrast, the rate of cooling increases only linearly with temperature. Once the rate of heat generation exceeds available cooling, the rate of temperature increase becomes progressively faster. Runaway reactions nearly always result in two-phase flow reliefs. In reactor venting, reactions essentially fall into three classifications ... 

Passive A reaction capable of generating 150 psig pressure in case of a runaway, done in a 250 psig reactor. The reactor can contain the runaway reaction. However, if 150 psig pressure is generated, the reactor could fail due to a defect, corrosion, physical damage or other cause. 

Containing a runaway reaction is more practical by building a smaller but stronger reactor rated for higher pressure. 

In many cases, if it is not feasible to contain a runaway reaction within the reactor, it may be possible to pipe the emergency device effluent to a separate pressure vessel for containment and subsequent treatment. 

The reaction is highly exothermic and the reactor contains large quantities of volatile oxide. Careful control of temperature is therefore required to avoid a runaway reaction and excessive pressure generation. 

One purpose of the hold tanks is to a buffer between the batch reactors and the continuous processing units that follow. Initially it woull appear that 2 hold tanks should be specified-one for each train. Each of these would be fed by 4 reactors. In case of a power failure it would be necessary to discharge those reactors that could have runaway reactions into the hold tanks immediately. At most, 4 reactors should be at this state. If properly sequenced this is 2 for each hold tank. Since a hold tank may be expected to contain at least 1 and possibly 2 reactor loads plus wash water, the system should be designed to hold 4 reactor loads plus an equivalent amount of wash water. 

In presence of sulfide ores, specifically pyrites, explosives containing ammonium nitrate may undergo runaway reaction, leading to detonation at temperatures below 40°C if pH is less than 2. The reaction is acid catalysed. 

Several commercial calorimeters are available to characterize runaway reactions. These include the accelerating rate calorimeter (ARC), the reactive system screening tool (RSST), the automatic pressure-tracking adiabatic calorimeter (APTAC), and the vent sizing package (VSP). Each calorimeter has a different sample size, container design, data acquisition hardware, and data sensitivity. 

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

As can be seen from the above list, runaway reactions do not occur by a single mechanism. They can take place not only in reactors but also in raw material and product storage containers and vessels, purification systems, and anywhere else exothermic reactive systems and selfreacting materials (as described below) are involved. 

An estimation of the half thickness of a sample in an unstirred container, in which the heat losses to the environment are less than the retained heat. This buildup of internal temperature leads to a thermal-runaway reaction. 

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