Recovery of NaOH from Wet Air Oxidized Spent Caustic Using Industrial Electrodialysis

This paper describes test trials for the recovery of NaOH from oxidized ethylene spent caustic. The recovered NaOH is of sufficient strength, quality, and quantity to be useful for reuse in the caustic tower. The original spent caustic was from an industrial source. The spent caustic was oxidized using a typical 200 °C wet air oxidation process to assure high conversion of reactive sulfides to sulfate. The oxidized material passed through an electrodialysis (ED) system built by a provider of common industrial high purity water systems (i.e. CEDI systems).

The electrodialysis system used ion selective membranes and electrical potential to transfer sodium ions from the oxidized spent caustic solution. This neutralized the pH of the oxidized spent caustic to 8, and created a product stream of 10 wt% NaOH, and in some trials up to 20 wt%. Depending on the spent caustic composition, between 30 and 50% of the sodium ions were extracted and recovered as NaOH product. The laboratory system has been operated for several hundred hours using multiple drums of the industrial oxidized spent caustic. The system was not damaged by the field quality feed and the value of the NaOH product was greater than the electrical costs to operate the ED. This paper will discuss the longevity and economic viability of this approach, as well as trouble areas to avoid.