Two-Phase Flow Relief Sizing for Runaway Reaction

TWO-PHASE FLOW RELIEF SIZING FOR RUNAWAY REACTION  

A seleetion of relief venting as the basis of safe operation is based on the following eonsiderations [21]  

Fig ure 12-27. Temperature versus time plot from a semi-batoh heat flow experiment. (Source Hazard Evaluation Laboratory Ltd.) 

Means of measuring the reaction parameters during the runaway reaction 

Many methods have been used to size relief systems area/volume scaling, mathematical modeling using reaction parameters and flow theory, and empirical methods by the Factory Insurance Association (FIA). The Design Institute for Emergency Relief Systems (DIERS) of the AIChE has performed studies of sizing reactors undergoing runaway reactions. Intricate laboratory instruments as described earlier have resulted in better vent sizes. 

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  

In this type of reaction, no permanent gas is generated. The pressure generated by the reaction is due to the increasing vapor pressure of the reactants, products, and/or inert solvent as the temperature rises. 

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