Abstract: During the operation of a 200 kt/a acetonitrile-based butadiene extraction unit, the reboilers in the extraction system frequently experienced tube bundle corrosion and leakage, which has seriously threatened the safe production of the unit. It was found through analysis that the corrosion mechanism involved the stepwise hydrolysis of hydrated acetonitrile under high-temperature conditions, generating acetamide, acetic acid and ammonia gas. The generated acetic acid may cause uniform corrosion and pitting on carbon steel equipment. Meanwhile, iron ions generated by corrosion possessed strong catalytic activity, accelerating the polymerization of butadiene and alkynes to form gums and deposits. The adhesion of deposits reduced the heat-transfer efficiency of the reboilers and caused a passive rise in the column bottom temperature, which further aggravated acetonitrile hydrolysis. Consequently, a vicious cycle of hydrolysis-corrosion-polymerization-enhanced hydrolysis was formed. The insufficient regeneration capacity of the original acetonitrile recovery column design resulted in low purity and excessive water content in the circulating solvent. Continuous accumulation of impurities, moisture and catalytic media in the circulating solvent created favorable conditions for acetonitrile hydrolysis, which constituted the root cause of the corrosion. Accordingly, modifications were carried out on the trays, cold source and supporting instrument valves of the acetonitrile recovery column. The solvent regeneration rate was increased from 1 t/h to 3 t/h, and supporting process control measures including temperature control, water content control, deoxygenation and passivation were implemented. After modification, the quality of circulating acetonitrile was significantly improved. Equipment corrosion and polymer formation were effectively restrained. Meanwhile, acetonitrile loss and steam consumption were greatly reduced, bringing remarkable annual economic benefits. This case can provide engineering references for anti-corrosion and solvent-system optimization of similar acetonitrile-based butadiene extraction units.

Key words: acetonitrile hydrolysislorganic acid corrosionliron ion catalysislsolvent regenerationlprocess modification