Depression of the cloud point of biodiesel by reaction over solid acids

Abstract

Biodiesel obtained by transesterification of soy oil with methanol was isomerized in the liquid phase at 125-275 °C using solid acid catalysts. The purpose of this work was to improve the cold flow properties of biodiesel by inhibiting the formation of crystals. Crystallization was analyzed by means of the measurement of the cloud point of the fuel. Tested catalysts were SO4 2-- ZrO2 and H-mordenite. At a temperature of reaction (Tr) lower than 125 °C, the catalytic activity was negligible. At Tr > 200 °C, the reaction rate was greatly accelerated, but the cetane number was also decreased and the amount of coke deposits was greatly increased. While reacting biodiesel at 150-200 °C, a small decrease (4-6.5 °C) of the cloud point was obtained without a meaningful decrease of the cetane index. Best results were obtained with SO4 2--ZrO2 at 125 °C. Isomerization had both positive and negative effects on fuel properties: it reduced the cloud point but also increased the viscosity and decreased the cetane index. Due to the different reactivity between the saturated fraction (estearates, palmitates) and the unsaturated one (linoleates, linolenoates, oleates), an adequate control of the degree of isomerization was only possible by performing a previous fractionation step. A good combination was obtained by reacting the saturates rich fraction at 200 °C over H-mordenite and the unsaturated rich fraction at 150 °C over SO4 2-- ZrO2. The fuel obtained by mixing both reacted fractions had a cetane index of 49.5 and a cloud point of -2 °C. However, this combined approach yields only a little benefit compared to those obtained with the single reaction over SO4 2--ZrO2 at 125 °C.