Solids Deposition during "Cold Flow" of Wax-Solvent Mixtures in a Flow-loop Apparatus with Heat Transfer

"Cold flow" refers to the pipeline flow of a "waxy" crude oil at a temperature, which is below its wax appearance temperature (WAT) and above its pour point temperature (PPT), whereby precipitated wax crystals remain suspended in the flowing crude oil. It has been suggested as an alternative technology for decreasing solids deposition (Merino-Garcia, D.; Correra, S. Pet. Sci. Technol. 2008, 26, 446). An experimental investigation was undertaken to study solids deposition under cold flow in a flow-loop apparatus, incorporating a small double-pipe heat exchanger. The experiments were performed using 3 and 6 mass % mixtures of a petroleum wax dissolved in Norpar13 (a paraffinic solvent comprising C-9-C-16) at different wax-solvent mixture temperatures, T-h, and two flow rates over a deposition time of I h. Two sets of deposition experiments were performed: cold flow with {WAT >= T-h > PPT} and "hot flow" with {T-h > WAT}. The deposit mass decreased with a decrease in wax concentration and with an increase in the coolant temperature. However, the deposit mass decreased with a decrease in the mixture temperature, under cold flow, but it increased with a decrease in the mixture temperature, under hot flow. Also, the deposit mass, under cold flow, was not affected by flow rate. Predictions from a pseudosteady-state heat-transfer model were in good agreement with experimental results, indicating the deposition process to be thermally driven. The liquid-deposit interface temperature in all cases was equal to the WAT of the liquid phase. Variations in both the wax content and the carbon number distribution in deposit samples are discussed.