A computational fluid dynamics model of an inner preheating transpiring wall reactor for supercritical water oxidation was proposed to optimize the structural parameters. Results showed that higher feed degradation efficiencies and lower inner surface temperatures of the porous wall were obtained at higher reactor diameters. Although the increase in reactor length promoted feed degradation, the increased inner surface temperatures of the porous wall were unsuitable for water film formation. A pyknic-type reactor with a lower length/diameter (H/D) value at a constant volume was more beneficial for water film formation and feed degradation. A higher feed concentration or feed flow required a lower H/D value to ensure an ideal water film formation and feed degradation. We proposed the use of sectional heat load as a new parameter for reactor design based on the similarity between the boiler furnace and the transpiring wall reactor. The sectional heat load in a certain range was calculated to serve as guide in the design of transpiring wall reactors. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.