Wax precipitation from North Sea crude oils. 3. Precipitation and dissolution of wax studied by differential scanning calorimetry

Differential scanning calorimetry was used to study wax precipitation from a series of North Sea crude oils by measuring glass transition temperatures, wax precipitation and dissolution temperatures (wax appearance and disappearance, respectively), and wax precipitation and dissolution enthalpies in the temperature range from +70 to-140 °C. With respect to physical characteristics, the oils ranged from very light paraffînic condensates to heavy waxy crudes including biodegraded and asphaltenic oils. On the basis of glass transition temperatures, the oils could be sorted into groups that corresponded to their physical descriptions, whereas a similar grouping according to the precipitation or dissolution temperatures did not correspond to the physical characteristics. Glass transition temperatures ranged from-128.5 to-81.5 °C, while wax precipitation and dissolution temperatures ranged from +39.5 to -26.0 °C and-16.0 to +53.5 °C, respectively. Dissolution temperatures were generally higher than corresponding precipitation temperatures, 13 °C on the average, and nonequilibrium conditions caused by undercooling and overheating during temperature scanning were probably responsible for these temperature differences. Measured precipitation and dissolution enthalpies based on total sample amounts ranged from 0.55 to 9.79 cal/g and 0.32 to 10.63 cal/g, respectively. On the average, dissolution enthalpies were ca. 14% greater than the corresponding precipitation enthalpies; this excess of enthalpy required for dissolution of wax cannot be accounted for thermodynamically and is supposed to be related to exothermal effects taking place during heating scans and hence the way data are processed. By combining the amount of wax precipitated by cooling from +45 to-40 °C, as determined by pulsed NMR, with the accumulated enthalpy of the same temperature interval, it was possible to express the average precipitation and dissolution enthalpies based on the actual amount of wax; values ranged from 23.7 to 70.6 cal/g and from 32.3 to 71.7 cal/g for the precipitation and dissolution enthalpies, respectively. Furthermore, by combining the incremental amount of wax precipitated as function of temperature in 5 °C intervals from +45 to-40 °C, as determined by pulsed NMR, with the incremental wax enthalpies accumulated in the same temperature intervals, an almost reversed parabolic dependency of wax precipitation and dissolution enthalpies on temperature was revealed; the maxima were about-20 °C.