Safety of chlorination reactions

Abstract

Chlorination reactions are part of various processes in the chemical industry, to manufacture heavy chemicals, specialty chemicals, pesticides and pharmaceuticals, in inorganic and organic chemistry. They are a valuable tool in organic synthesis. The hazard of processing chlorine involves: u - Gas phase explosion; - Runaway reaction or thermal explosion in the condensed phase. Gas phase explosion hazard with chlorine as an oxidizer is present in gas phase chlorination processes as well as in chlorinations in the condensed phase. Gas phase chlorination processes are mostly continuous processes operating in the flammable area. Gas phase explosion hazard is related to burner malfunctions. Where chlorination is made by chlorine injection in the liquid phase, gas phase explosion hazard is related to chlorine evolution in the vapour phase, giving a flammable mixture with the solvent or reaction mixture vapour. Here hazard assessment is achieved by comparing the gas phase composition with the flammable area of the gaseous mixtures. Auto-ignition is also considered because the auto-ignition temperature of gaseous mixtures containing chlorine is close to the ambient temperature. The relevant flammability data is obtained in a specially designed 20 litre sphere. The main features of this explosion vessel include: Hastelloy C 276 walls, central ignition with spark, hot wire or pyrotechnic ignition source, 200 bar pressure resistance, ambient to 300°C initial temperature, easily opened for frequent cleaning. This apparatus allows precise determination of the flammability limits, autoignition temperature, explosion overpressure, rate of pressure rise and flame speed. A review of flammability data in chlorine is given. Runaway reaction hazard in chlorination reactions is related to a series of dangerous process situations or process deviations such as: u - Delay in reaction initiation - Reaction mixture instability - Production of unstable species like chloramines, nitrogen trichloride, chloro nitroso compounds. - Demixion or segregation of unstable species in case of chlorination made in aqueous solution, because the chlorinated compounds are less soluble in water than the initial reactant. A full review in runaway reaction hazard in chlorination reactions is given with examples from the literature and from the laboratory.