(224b) Yield Behavior of Wax Gel Formation in the Presence of Asphaltenes
AIChE Spring Meeting and Global Congress on Process Safety
Thursday, April 10, 2008 - 8:50am to 9:10am
High molecular weight paraffinic components have been attributed to impede the oil flow along the pipeline transport in low temperature environ by either wax deposition or wax gel formation. Wax deposition occurs even during the flow, while wax gel forms during the planned or emergency shutdown within a short period of time. It has been found that paraffinic components contribute to the evolving gel strength continuously while cooling below pour point. The contributive effect of asphaltenes to the yield stress of the gel formed was also studied. Model oils were used to determine the yield stress development in this study. Model oils were prepared by mixing mineral oil and kerosene with different amount of well-characterized waxes. The measurements of WAT and PP were performed using FT-IR and ASTM methods. As the WAT and PP are dependent on wax amount and wax quality, this study examined the WAT and PP by comparing model oils with different wax composition as well as different wax amount. Vane method was adapted to measure the yield stress at different temperatures. Constant cooling rate and consistent holding time were employed for the yield stress. The yield stress was recorded with decreasing temperatures. It was shown that the yield stress is strongly dependent on the wax amount and wax composition. Increase in yield stress values with decreasing temperature was greater for higher amount of wax in model oil indicating the gel strength is dependent upon the wax amount in the model oil. The x-intercept values obtained from yield stress versus temperature were interpreted as no-flow point, which may use as pour point alternatives. The relation between wax amount and yield stress was analyzed using FT-IR. Asphaltene used in this study is from Rangely field in northwestern Colorado. Examination of asphaltene addition was performed with 0.01 wt. % and 0.1 wt. % in model oil. Model oils were prepared with mineral oil, waxes, toluene, and asphaltene for this effort. Yield stress increases linearly decrease in temperature in asphaltene-free model oil. Asphaltene additions result in pour point reductions, 1 C in 0.01 wt%, and 4 C in 0.1 wt%. Small amount of asphaltene (0.01 wt. %) also played a significant role in yield stress reduction. A considerable reduction in yield stress was observed by asphaltene addition. A departure form the linear trend in increase of yield stress with decreasing temperature was observed at lower temperatures in the model oils with asphaltenes.