Designing and operating safe chemical reaction processes HSG143

Health and SafetyExecutiveNitroglycerin is produced by the nitration of glycerine with concentrated nitricand sulphuric acids. The reaction is very exothermic and if heat removal, bycooling and stirring, is inadequate an uncontrollable reaction is followed byexplosive decomposition of the nitroglycerin.The reaction was traditionally carried out batchwise in a large stirred potcontaining about one tonne of material. The operators had to monitor the reactortemperature closely. To make sure they did not fall asleep they sat on oneleggedstools. If they fell asleep, they fell off.On redesigning the process, the engineers asked ‘Why does the reactor have tocontain so much material?’ Although they initially thought this was because thereaction was slow, it was actually because of inadequate mixing. They thereforedesigned a small, well-mixed reactor holding about one kilogram of material.The new reactor resembled a laboratory water ejector. The rapid flow of theacids through it creates a partial vacuum that sucks in the glycerine througha sidearm. Very rapid mixing occurs, and by the time the mixture leaves thereactor the reaction is complete. The residence time in the reactor fell from twohours to two minutes and the same output as the batch reactor was achieved.The control system on the reactor is inherently safer than conventional control.If the flow of acids falls the flow of glycerine must fall in proportion without theintervention of a flow ratio controller or other instrument that may fail.They also designed a simple emergency shutdown system. Deactivation of asolenoid will cause a lead weight to fall which opens a plug valve. Opening thisvalve allows air to enter into the reactor that breaks the partial vacuum createdby the flowing acid. This causes the flow of glycerine to stop and hence thereaction.Moderate11 To moderate is to use hazardous materials under less hazardous conditions,that is: at lower temperatures or pressure; as a vapour; or dissolved in a safesolvent. For example:(a) a polypropylene process uses gaseous propylene instead of liquid propylenedissolved in a flammable solvent; or(b) in a continuous method for the production of nitro-aromatics, excess acid isused to dilute the aromatics and make a reaction runaway impossible.Simplify12 Simplify means designing to eliminate unnecessary complexity, reducing theopportunities for error and misoperation. A simpler plant is generally safer and morecost effective than a complex one. Examples include:(a) building reactors with sufficiently high design pressures may result in theelimination or reduction in size of an emergency relief system; or(b) in some cases, carrying out a multi-step batch process in several vessels eachoptimised for a single processing step may be inherently safer and far moreDesigning and operating safe chemical reaction processes Page 46 of 64