Study on wet oxidation process and mechanism for ethylene spent caustic

Area of Science:

• Environmental Chemistry
• Chemical Engineering

Background:

• Spent caustic from ethylene production exhibits high alkalinity, chemical oxygen demand (COD), and toxicity.
• Direct discharge of spent caustic is environmentally unsustainable without effective pretreatment.

Purpose of the Study:

• Analyze the composition of ethylene spent caustic.
• Investigate the efficacy of oxidation processes for COD removal.
• Elucidate the oxidation mechanism of organic sulfides.

Main Methods:

• Compositional analysis of spent caustic.
• Oxidation experiments varying oxidants, temperature, pressure, and residence time.
• Mechanistic study of organic sulfide degradation using dimethyl disulfide as a model.

Main Results:

• Ethylene spent caustic contains sulfide ions (S²⁻), disulfides, ketones, and phenolic organics, with COD around 24.5 g/L.
• Optimal conditions (180 °C, 2 MPa, 15 min, oxygen oxidant) achieved 100% S²⁻ removal and >83% COD removal.
• Oxidation of dimethyl disulfide yields methanesulfonic acid, followed by breakdown into sulfuric acid, formic acid, and acetic acid.

Conclusions:

• Oxygen oxidation is a viable pretreatment method for ethylene spent caustic.
• The process effectively reduces COD and removes toxic sulfides.
• Understanding the degradation pathway of organic sulfides is crucial for optimizing treatment.