CRUDE OILS - CHARACTERIZATION OF WAXES PRECIPITATED ON COOLING BY DSCAND THERMOMICROSCOPY

Mixtures of 2 and 4 wt% of pure paraffins in a crude oil matrix and 14 crude oils were characterized by differential scanning calorimetry (d .s.c.) and thermomicroscopy at a cooling rate of 2 K min(-1) in the te mperature range from +80 to -20 degrees C. Pour points were also deter mined. In the temperature range used, a constant value of 200 J kg(-1) for the enthalpy of precipitation was determined. For crude oils whic h contain >10 wt% of n-paraffins, the pour point is reached when simil ar to 2 wt% of waxes have precipitated. By thermomicroscopy, it was ob served for mixtures of pure paraffins and crude oil matrix that the si ze of the crystals is small and depends on the length of paraffinic ch ains at the pour point. For crude oils, at the pour point the same fin al state is obtained (unlike diesel fuels). Even if the rates of preci pitation are very different, the crystal size is always small (1-3 mu m). This fact suggests that each crude contains nuclei which are respo nsible for heterogeneous germination.