This paper describes test trials of electrodialysis for the recovery of NaOH from ethylenespent caustic treated by the wet air oxidation process (WAO). The recovered NaOH is ofsufficient strength, quality, and quantity to be useful for reuse in the caustic tower. Theoriginal spent caustic was from an ethylene gas cracker. The spent caustic was oxidized usinga typical WAO process operating at 200°C to assure high conversion of reactive sulfides tosulfate. The oxidized material passed through an electrodialysis (ED) system built by aprovider of common industrial high purity water systems (i.e. CEDI systems).The ED system tested used ion selective membranes and electrical potential to transfersodium ions from the WAO oxidized spent caustic solution to produce a clean recoveredcaustic stream. This process created a product stream 2-20 wt% NaOH and reduced the pH ofthe oxidized spent caustic. Depending on the oxidized spent caustic composition, between 30and 50% of the sodium ions were extracted and recovered as NaOH product. The laboratorysystem has been operated for several hundred hours processing multiple drums of theoxidized spent caustic.Testing with oxidized ethylene spent caustic from a WAO system showed that the EDsystem operates effectively and can produce NaOH for reuse in a cost effective manner. Therecovery of NaOH also reduces the pH of the oxidized spent caustic, minimizing the need forfurther acid neutralization prior to discharge to an effluent treatment plant. This paper willdiscuss the longevity and economic viability of this technology.
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