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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