A scale-up plug flow reactor was evaluated for the continuous production of biodiesel from refined palm kernel oil (PKO) with supercritical methanol and optimized by response surface methodology. The effects of the operating temperature (270-350 degrees C), pressure (15.0-20.0 MPa) and methanol:PKO molar ratio (20:1-42:1) were evaluated at a constant residence time of 20 +/- 2 min by using a central composite design. Analysis of variance demonstrated that a modified quadratic regression model gave the best coefficient of determination (R-2 = 0.9615) and adjusted coefficient of determination (Adj. R-2 = 0.9273). The interaction terms in the regression model illustrated small synergistic effects of both temperature-pressure and temperature-methanol:PKO molar ratio. The optimal conditions were 325 +/- 5 degrees C, 18.0 +/- 0.5 MPa and a methanol:PKO molar ratio of 42 +/- 2:1, attaining a maximum production rate of 18.0 1.5 g biodiesel/min with a fatty acid methyl ester content of 93.7 +/- 2.1%. The product obtained from the optimal conditions had high cetane number, and could be considered as a fuel additive for cetane number enhancement. (C) 2011 Elsevier B.V. All rights reserved.