Characterizing the Runaway Reaction

The information required to characterize a runaway reaction or decomposition is  

The onset temperature is required when the. safety of the plant is to depend on preventing the onset of runaway — for example, process control (see Section 6.3, page 112). The rates of heat and gas production, and the maximum pressure developed, are required where safety is to be based on coping with the consequences of runaway. The problems associated with measuring heat generation and heat loss, described in Section 3.4.1 for the normal reaction, also apply to the runaway reaction. Accurate calorimetry is required in each case. 

Some indication of the rates of heat and gas production can be obtained from the results of the screening tests (Section 3.4, page 32), but getting accurate data requires the use of adiabatic calorimeters (Sections 3.6.2 and 3.6.3). A different type of calorimeter, developed specially to provide data for the design of venting systems, uses a pressure equalization system and a weak test cell with a low thermal mass (Section 3.6.3). 

The minimum temperature at which a runaway reaction will occur is not an absolute value. It is linked to the rate of heat loss from the system and depends markedly on the process conditions and scale of manufacture. Thus, the rate of heat loss due to natural cooling from a 501 reactor is of the order of 0.2 W kg K whereas a typical value for a 20 m- vessel is 0.04-0.08 W kg K . Accurate laboratory assessment of the minimum temperature for onset of runaway reaction requires equipment where the rate of heat loss is the same as it is in the full-scale process. 

Two factors are important in these measurements the heat loss from the sample to its surroundings, and the heat loss from the sample to the test cell. 

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