Extractive Distillation of Diacetyl

As described in chapter 16.6.1, extractive distillation of diacetyl requires the use of aqueous sulfuric acid as volatility modifier throughout most of the column. Although the process calls for an acid strength of only 0.3 % by weight, with a column made of stainless steel type 316 L this led to considerable corrosion within a year of operation. However, it was found possible to completely eliminate this problem by adding a trace of nitric acid. 

It was known from the literature [119] that at 145 °C (a temperature not reached anywhere in the extractive distillation of diacetyl), the corrosion rate of the German stainless steel type 1.4301 (18 % Cr, 10 % Ni) in an aqueous mixture of 

On the basis of this information, it was estimated that 0.05 % by weight of HNO3 added to the 0.3 % by weight of H2SO4 would be sufficient to prevent corrosion of 316 L stainless steel at temperatures slightly below 100 °C. 

The nitric acid was added to the reboiler since HNO3 forms a low-boiling azeotrope with diacetyl, so that the HNO3 went upwards with the diacetyl, thereby protecting the entire column. After this simple measure, the corrosion problem was a specter of the past. 

The reason for the protective action is known to lie in the fact that the nitric acid, even in a very small concentration, enforces passivation of the stainless steel. Passivation is based on the formation of a thin layer of chromium (III) oxide, which brings the attack to a virtual halt  

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