Effect of NaOH on asphaltene transformation in supercritical water

• Asphaltene was converted to gas, maltene and coke in SCW (supercritical water).
• NaOH in SCW increased the asphaltene conversion and its product yields.
• Lower coke selectivity and higher maltene selectivity were achieved in SCW + NaOH.
• High temperature favored the asphaltene transformation and the formation of gas.
• Asphaltene transformation in SCW + NaOH fit the first-order kinetic model better.

Asphaltene transformation in SCW (Supercritical Water) and SCW + NaOH experiments were conducted in autoclaves with sealed gold tube reactors to investigate the effect of NaOH, reaction time (0–120 min) and temperature (400–450 °C) on asphaltene transformation. The results showed that asphaltene transformation in SCW + NaOH gave higher asphaltene conversion, maltene yield and coke yield but less gas yield compared with those in SCW at 400 °C during 120 min. Moreover, a longer reaction time and higher temperature could enhance asphaltene reactivity but led to secondary decomposition of maltene and the formation of more gas and coke in SCW + NaOH. Kinetic analysis indicated that asphaltene transformation in SCW and SCW + NaOH both fit first-order kinetics adequately. Furthermore, a three-lump kinetic model considering parallel reactions from asphaltene to maltene + gas to coke and the consecutive reaction from maltene + gas to coke was proposed to explain the effect of NaOH and temperature on asphaltene transformation. Selectivity analysis of the products implied that asphaltene transformation in SCW + NaOH at 425 °C was better than transformation in SCW + NaOH at higher temperatures or in SCW in minimizing coke formation.

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