CRUDE OILS - CHARACTERIZATION OF WAXES PRECIPITATED ON COOLING BY DSC AND 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 temperature range from +80 to -20 degrees C. Pour points were also determined. In the temperature range used, a constant value of 200 J kg(-1) for the enthalpy of precipitation was determined. For crude oils which contain >10 wt% of n-paraffins, the pour point is reached when similar to 2 wt% of waxes have precipitated. By thermomicroscopy, it was observed for mixtures of pure paraffins and crude oil matrix that the size of the crystals is small and depends on the length of paraffinic chains at the pour point. For crude oils, at the pour point the same final state is obtained (unlike diesel fuels). Even if the rates of precipitation are very different, the crystal size is always small (1-3 mu m). This fact suggests that each crude contains nuclei which are responsible for heterogeneous germination.