Oily wastewater treatment using a CNT sorbent in a three-phase circulating fluidized bed

Abstract

Fine emulsified heavy-oil droplets in wastewater can be easily dispersed in aquatic system, making it challenging to remove them using sorbents. Carbon nanotube (CNT) sorbents were prepared using a m-cresol solvent to remove emulsified heavy oil in wastewater. The bead-shaped and density-controlled sorbents (diameter: 5.22 mm; density: 1169 kg/m³) facilitated efficient contact with emulsified oil and were suitable for use in a three-phase circulating fluidized bed (TPCFB). The sorbents feature a sorption capacity of 2.4 g oil/g sorbent which remained consistent for at least 10 cycles. The hydrodynamic characteristics of the TPCFB reactor (120 × 80 mm; height: 800 mm) were determined and it was found that the circulation of both liquid and solid within the TPCFB was driven by the density difference between the riser and downcomer (Δρ). The solid holdups in the downcomer (ε_s,d) were almost constant with gas velocity and were proportional to the amount of sorbent inventory. The optimal region for emulsified oil removal was determined to be the downcomer, where turbulent flow was generated that facilitated close contact between the particles and the liquid. The solid circulation rate (G_s) increased to a constant value with Δρ, with the maximum G_s being proportional to the sorbent inventory. The optimal conditions for wastewater treatment were determined to be a maximum ε_s,d of 0.03 with a G_s value above 10.0 kg/m²s and a sorbent inventory of 90 g. The oil removal rate reached a maximum of 94.8 % at G_s = 12.2 kg/m²s. Used CNT sorbent exhibited unique oil sorption characteristics of CNTs, featuring an even distribution of sulfur on the surface and a weight loss arising from loss of oil at 350–500°C.