Exothermic and Pressure-Generating Reactions

Many of the reactions used in the process industry are exothermic and therefore have to be cooled (vid. Table 3.1). If cooling is insufficient or even fails the reaction temperature rises. This rise can be accompanied by gas releases and the evaporation of the substances involved. As a consequence pressure may build up and the reactor may be destroyed (cf. [ 1,2]). The reaction experiences a runaway . In this context it is recommended to examine the possibility of secondary reactions, for example decompositions, which may occur because of the rising temperamie and may be even more destructive than the ranaway of the original reaction [1]. 

Whilst the previous chapter was concerned with the properties of materials only, a runaway implies a coupling with the equipment in which the reaction takes place. Hence, equipment properties have to be accounted for as well. They furnish the boundary conditions for modelling. Therefore reaction kinetics and reactor models are now briefly reviewed. Details can be found in [4] and [5]. 

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It is important to maintain a uniform reaction environment within the vessel chamber by using a stirrer to agitate and mix the reactants. Gaseous discharge is vented through filters to the outside environment. As the reactions may generate substantial heat (exothermic) and pressure, or may even be potentially explosive, special precautionary features are designed into the vessel. 

A reactor system is shown in Figure 2 to which the HAZOP procedure can be appHed. This reaction is exothermic, and a cooling system is provided to remove the excess energy of reaction. If the cooling flow is intermpted, the reactor temperature increases, leading to an increase in the reaction rate and the heat generation rate. The result could be a mnaway reaction with a subsequent increase in the vessel pressure possibly leading to a mpture of the vessel. 

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 ... 

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. 

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. 

Ebner, W. B. etal., Proc. 8th Power Sources Symp., 119-124, 1982 An ARC study of the thermal and pressure behaviour of actual electric batteries under various atypical conditions showed the major contributions to the exothermic behaviour as the reactions between lithium and acetonitrile, lithium and sulfur and the decomposition of lithium dithionite. The first reaction can generate enough heat to trigger other exothermic rections. The hazards associated with the various parameters are quantified. 

Glycol/water mixture leaks from jacket into blender, reacts with aluminum powder and sodium hydrosulfite, and initiates exothermic decomposition No—N2 atmosphere, confinement in blender R NOAA Worksheet indicates combining sodium hydrosulfite with ethylene glycol is "explosive due to vigorous reaction or reaction products may produce detonation," "may cause fire," and indicates "flammable gas generation" and "heat generation by chemical reaction, may cause pressurization"... 

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