Improved emulsified oil removal approach for industrial coal pyrolysis wastewater by flocculation under pressurized CO2 atmosphere

Coal pyrolysis wastewater pretreatment suffers from equipment fouling and inferior mass transfer efficiency brought by low emulsified oil removal efficiency. This paper comprehensively analyzed the properties of emulsified oil and found this inadequacies attribute to its high stability and penetrability derived from high ζ potential of −33.1 mV and tiny average particle size of 114.7 nm in initial condition. H3O+ adjustment is developed to compress diffusion layer thickness and facilitate natural coalescence of emulsified oil droplets. Results of H2SO4 acidification present the peaks of average particle size sustainably stabilized between 782.9 and 1009.0 nm and emulsified oil was mostly coagulated and subsequently settled/floated without further treatment. CO2 was selected as a suitable donor of H3O+ to improve application potential, and flocculants was coupled with pressurized dissolved CO2 to intensify emulsified oil removal whose optimal efficiency achieved 93.82 wt% by changing flocculant types and operation conditions. Response surface analysis results introduced emulsified oil removal efficiency reached 95.17 wt% at PAC concentration of 1065 mg/L, CPAM concentration of 20 mg/L and operating pressure of 0.67 MPag. For renovation projects with limitations, the optimal emulsified oil removal efficiency achieved 94.33 wt% at PAC concentration of 1032 mg/L, CPAM concentration of 21 mg/L and operating pressure of 0.60 MPag. Process simulation of sour gas and ammonia stripper signified that CO2 addition has little effect on current industrial process. By comparison, the proposed approach performs high efficiency, low impact to wastewater, strong feasibility, and great prospect in pretreatment.