Calculation and analysis of sub/supercritical methanol preheating tube for continuous production of biodiesel via supercritical methanol transesterification

Abstract

Biodiesel is an important renewable energy. Supercritical methanol transesterification for biodiesel has recently been concerned because of its obvious advantages. The tubular reactor is an ideal reactor for continuous preparation of biodiesel via supercritical methanol transesterification. A methanol preheating tube is necessary for the tubular reaction system because the reaction temperature for supercritical methanol transesterification is usually 520–600 K. Therefore, in the range of 298–600 K, changes of the density, isobaric capacity, viscosity and thermal conductivity of sub/supercritical methanol with temperature are first discussed. Then on the basis of these thermophysical properties, an integration method is adopted for the design of sub/supercritical methanol preheating tube when methanol is preheated from 298 K to 600 K at 16 MPa and the influencing factors on the length of the preheating tube are also studied. The computational results show that the Reynolds number Re and the local convection heat-transfer coefficient α of sub/supercritical methanol flowing in Φ6× 1.5 mm preheating tube change drastically with temperature. For the local overall heat transfer coefficient K and the average overall heat transfer coefficient Km, temperature also has an important influence on them when the inlet velocity of methanol is lower than 0.5 m/s. But when the inlet velocity of methanol is higher than 0.5 m/s, K and Km almost keep invariable with temperature. Additionally, both the outlet temperature and the inlet velocity of methanol are the key affecting factors for the length of the preheating tube, especially when the outlet temperature is over the critical temperature of methanol. At the same time, the increase of tin bath’s temperature can shorten the required length of the preheating tube. At the inlet flow rate of 0.5 m/s, the required length of the preheating tube is 2.0m when methanol is preheated from 298 K to 590 K at 16 MPa with keeping the tin bath’s temperature 620 K, which is in good agreement with the experimental results.