Crystallisation studies of biodiesel at extreme conditions
Item statusRestricted Access
Embargo end date31/12/2100
Whilst biodiesel has many advantages as a renewable-energy fuel and as a substitute source of petroleum diesel, it suffers from poor performance at both low temperatures and high pressures. Not only does biodiesel crystallise at low temperatures below ~0 °C, but it also crystallises under the high pressures experienced in common-rail and injector systems within diesel engines. Crystalline solids induced by temperature and pressure can clog filters and injectors in the diesel engine, thereby causing engine failure. This thesis focuses on developing an enhanced understanding of the behaviour of biodiesel using a range of spectroscopy and diffraction techniques. The crystallisation behaviour of biodiesel at high pressure (0.1 GPa to 4 GPa) or low temperature (0 °C to -40 °C) was studied in this work. Structural phase transitions of the components of biodiesel induced by both temperature and pressure were observed. On account of the complex nature of biodiesel, it proved difficult to characterise these changes in biodiesel itself. Instead, one of the main components, methyl stearate, was therefore investigated. The crystallisation behaviour of methyl stearate is temperature-sensitive. A new polymorph of form II was successfully characterised by single crystal diffraction - by growing crystals from a saturated carbon disulfide solution at room temperature while data collection was conducted at 120 K. Form III was obtained by crystallisation from melt followed by slow cooling. Structural characterisation using single crystal diffraction showed disordered packing behaviour of the molecules in this form. The crystal structure of form IV was obtained using a combination of synchrotron X-ray powder diffraction and high resolution neutron powder diffraction. It was crystallised from the melt by quench cooling at low temperature. The thermal expansion behaviour of this form was also investigated in this work. Furthermore, a phase transition from form IV to form V was observed in neutron diffraction experiments for a fully deuterated sample, but no evidence for this transition was observed in X-ray diffraction studies. Due to the complexity of methyl stearate and the limitations of the experimental data, the crystal structure of form V was not solved. In addition to the temperature studies, the crystallisation behaviour of methyl stearate under variable pressure conditions was investigated in this work. A diamond-anvil cell was employed to generate high-pressure environments. Synchrotron high-pressure X-ray powder diffraction and Raman spectroscopy showed that pressures of as little as 0.1 GPa can induce form IV of methyl stearate to convert to form II. Four phase transitions in the pressure range of 0.1 GPa to 6.3 GPa were also observed. The phase behaviour of methyl stearate induced by pressure is reversible and form II was recovered when the pressure was released. The structure of these high-pressure phases of methyl stearate have still to be determined. High-pressure neutron powder diffraction experiments have also been conducted with form IV of methyl stearate using a Paris-Edinburgh Press. Fluorinert (FC-87) was employed as pressure-transmitting medium to generate hydrostatic condition. No evidence of a phase transition was observed in the pressure range up to 3.31 GPa.