Nanobubble Flotation: A Green Paradigm for Effective Disulfides Removal from Spent Caustic

This study presents an effective method for separating various disulfide compounds (RSSR) from industrial waste caustic effluent in an LPG plant using NanoBubble (NBs) flotation. This method offers a new approach compared to traditional solvent extraction techniques. An industrial sample containing 1310 ppm disulfide and other components was tested. By employing hydrodynamic cavitation in a specific configuration, nanobubbles with a D50 of 127 nanometers were generated, which improved flotation. Bubble size was evaluated using laser diffraction, and the total sulfur content was measured for the separation efficiency. Here, the separation efficiency of multi-disulfide components in real industrial samples of spent caustic solution is studied. The methods examined included extraction with pure, sulfur-free n-Hexane and sulfur-containing raw naphtha as a benchmark for comparison. Additionally, advanced methods, such as nanobubble flotation with and without polyelectrolytes (chitosan and cationic polyacrylamide, CPAM), were employed in this study. N-Hexane achieved a separation efficiency of 99.95%, which was the highest recorded. By comparison, raw naphtha (with 300 ppm sulfur) showed an efficiency of 97.4%. Nanobubble flotation alone demonstrated a 93% separation efficiency. Incorporating chitosan with nanobubbles significantly enhanced this, elevating the efficiency to 99.6%. Similarly, CPAM combined with NBs achieved a comparable efficiency of 99.64%. This remarkable enhancement was due to the improved collision efficiency and nanobubble stabilization. These findings demonstrate that an improved nanobubble flotation system, particularly when combined with polyelectrolytes, offers a practical and environmentally friendly solution for removing disulfide compounds from spent caustics. This is particularly relevant because spent caustics represent an environmentally hazardous liquid waste stream in the oil, gas, and petrochemical industries.