Microbubbles intensification and mechanism of wet air oxidation process of MDEA-containing wastewater

In order to the intensification of gas-liquid mass transfer of MDEA-containing wastewater during wet air oxidation (WAO) process, the microbubbles and millimetre bubbles were applied by fine-pore sparger (5 and 20-30 μm) and single pore sparger (6.35 mm), respectively. Effect of the superficial gas velocity on the average microbubble size, gas holdup and oxygen mass transfer coefficient (K_La) of MDEA-containing wastewater at the ambient conditions was studied. The results showed that the microbubbles (less than 1 mm) were beneficial to enhance mass transfer process and had a higher dissolved oxygen concentration during WAO process of MDEA-containing wastewater owing to higher gas holdup and larger oxygen mass transfer coefficient. The COD removal ratio was 66% at low superficial gas velocity (u_g= 0.3 cm/s) in WAO process by microbubbles, while it achieved at high superficial gas velocity (u_g= 3.0 cm/s) by millimetre bubbles. The critical oxygen mass transfer coefficient K_La was 0.183 min^-1 of MDEA-containing wastewater by 20-30 and 5 μm fine pore sparger, which was 2∼5 times more than that of single pore sparger (<0.1 min^-1). The microbubbles could improve dissolved oxygen concentration and enhance the formation of hydroxyl radical at short time with atmospheric pressure. During the WAO process, the MDEA would be converted into intermediates including formic acid, acetic acid, ammonium, nitrite and nitrate. The WAO process with microbubbles could significantly improve the gas-liquid mass transfer performance at low superficial gas velocity and greatly reduce air consumption for MDEA-containing wastewater.