EFFECT OF INCIDENTAL HEAT INPUT ON THE SAFETY OF A BATCHWISE REACTION

This paper examines the effect of additional heat input on the course of a moderately exothermic batchwise reaction (Delta T-ad= 60-150 K), carried out without cooling, for two upset conditions: 1. constant hea ting rate, e.g. an insulated vessel exposed to an external fire, an el ectric heating coil kept on, stirring (vigorous) without cooling; 2. h eating with a medium of constant temperature, e.g. jacket heating oper ated with high pressure steam or thermal oil. The model reaction is th e batchwise amination of p-chloro nitrobenzene in an industrial autocl ave, according to: large excess GRAPHICS The desired reaction is of a rather low exothermicity, but the more exothermic decomposition of p-n itro aniline starts at temperatures not far above the usual maximum re action temperature. The worst upset condition during the desired react ion is considered to be 'leaving the steam heating on' in the adiabati c part of the process (175-198 degrees C) combined with 'failure to bl ow off ammonia and water vapour above 200 degrees C'. The effect of th is upset condition is evaluated using the thermo-kinetic data of the r eactions taking place. It is found that eventually a runaway will show up in the consecutive reaction, but this will take quite some time, s ufficient to take counteractions. New analytical tools are developed t o evaluate in general the effect of additional heat supply to an adiab atic batch reaction. For upset condition (1), a new definition of the final temperature, the temperature at which the conversion is almost c omplete, yields an equation for the range in which a reaction will occ ur. This range will considerably shift under the influence of variable imposed heating rates. The intriguing opposite is that in the limitin g case without an imposed heating rate any reaction seems to occur bet ween T= 0 K and T= Delta T-ad. Another analytical tool is estimating t he course of the reaction temperature by simple algebraic equations. F or upset condition (2), a dimensionless approach yields an expression for the conversion at the moment that the reaction temperature has bec ome equal to the temperature of the heating medium. (C) 1997 Elsevier Science Ltd. All rights reserved.