Safety of chlorine production and chlorination processes

Most chlorine production is obtained by electrolysis of NaCl aqueous solution. Other processes to produce chlorine involve the electrolysis of KCl or HCl aqueous solutions, the electrolysis of molten NaCl at high temperature and the Deacon process. In most cases, chlorine is a by-product in the production of caustic soda, potassium hydroxide, sodium metal or is recovered from HCl using HCl aqueous solution electrolysis or gas–solid reaction in the Deacon process.

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 valuable tools in organic synthesis.

The hazards of chlorine production and chlorination processes involve:

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  Gas phase explosion, i.e., self-ignition, deflagration and detonation in the gas phase.

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  Runaway reaction or thermal explosion, deflagration and detonation in the condensed phase.

Gas phase explosion hazard with chlorine as an oxidizer is present in the production of chlorine by electrolysis, in gas phase chlorination processes and in chlorination reactions carried out in the condensed phase.

Gas phase chlorination processes are continuous processes operating either in the flammable range like burners or outside the flammable range in loop reactors or loop processes where chlorine is the controlling reactant.

When chlorination is carried out 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. Hazard assessment is achieved by comparing the gas phase composition with the flammable area of the gaseous mixture. Self-ignition is also considered because the self-ignition temperature of gaseous fuels in chlorine atmosphere is lower than in air or oxygen and often close to the ambient temperature.

The relevant flammability data is the flammability limits, LFL, UFL, minimum oxidizer concentration (MOC), auto-ignition temperature (AIT), of fuels in chlorine and the explosion characteristics P_max and K_G for deflagration in chlorine.

A collection of flammability data is given for the reader convenience, collected in the literature or obtained in our own experimental facility, a specially designed 20 L Hastelloy C 276 sphere with 200 bar pressure resistance, ambient to 300 °C initial temperature, easily opened for frequent cleaning. This apparatus allows precise determination of the flammability limits, self-ignition temperature, explosion overpressure, rate of pressure rise and flame speed.

Runaway reaction hazards in chlorination reactions are related to a series of dangerous process situations or process deviations such as:

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  Delay in reaction initiation.

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  Reaction mixture instability.

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  Production and accumulation of unstable species like chloramines, nitrogen tri-chloride, chloro-nitroso compounds, alcohol hypochlorites.

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  Demixing or separation of unstable species in case of chlorination reactions made in aqueous solution, because the chlorinated compounds are less soluble in water than the initial reactant.

A full review of runaway reaction hazards in chlorination reactions is given with examples from the literature and from the laboratory.