Chapter 22 - Reactions—Planned and Unplanned

Abstract

The incidents where reactions go out of control, particularly on batch plants, have been due to runaway reactions. The reaction becomes so rapid that the cooling system cannot prevent a rapid rise in temperature, or the relief valve or rupture disc cannot prevent a rapid rise in pressure, and the reactor ruptures. Lack of knowledge is, or was at one time, the major cause of runaway reactions. After many years of safe operation, a chemical or a reaction mixture gets a little hotter than usual or is kept warm for a little longer than usual, and a runaway occurs. Today there is little excuse for such runaways, as many methods are available for testing both pure substances and reaction mixtures. They include accelerating rate calorimetry (ARC), differential scanning calorimetry, and reaction calorimetry. There are also methods for determining the size of the relief valve, rupture disc, or vent required. Expert advice is therefore needed on the most suitable technique for each case. If process conditions are changed, then further testing may also be necessary. Many similar incidents occur when a stirrer or circulating pump stops and causes poor mixing. The most famous case of a runaway reaction caused by contamination is Bhopal. In this case, the reaction occurred in a storage vessel. It did not burst but was distorted, and the discharge of vapor was larger than the scrubbing and flare systems could have handled, even if they had been in operation.