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

Abstract

“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 C9−C16) at different wax−solvent mixture temperatures, T h, and two flow rates over a deposition time of 1 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.

Solids Deposition from Two-Phase Wax–Solvent–Water “Waxy” Mixtures under Turbulent Flow

“…19−22 Previous experimental investigations have established that the deposit mass is not directly related to the overall thermal driving force for heat transfer, (T h − T c ); instead, the extent of solid deposition has been shown to depend on two thermal driving forces, namely, (T h − WAT) and (WAT − T c ). 7,13,14,17,18 As mentioned previously, in the thermally controlled wax deposition approach, it is assumed that T d ≈ WAT throughout the deposition process, which has been verified through batch deposition experiments under static and sheared cooling. 25,26 Effect of T c on Deposit Mass.…”

“…The combined thermal resistance can be expressed as a sum of four individual thermal resistances in series, 7,13,14,17,18 that is,…”

“…The deposition of solids during "cold flow" is decreased because of a reduced thermal driving force and a lowering of the WAT of the remainder liquid phase together with the preferential crystallization of wax onto the solid crystals flowing in the slurry that act as nucleation sites. 6,7 The process of deposit formation from waxy mixtures or crude oils is complex, and it may involve several processes and considerations, such as crystallization kinetics, mass transfer, heat transfer, fluid dynamics, rheology, solid−liquid multiphase equilibria, and thermophysical and transport properties. A number of mechanisms have been suggested for explaining the process of wax deposition and for estimating the amount of deposition that will occur in a system under a particular set of operating conditions.…”

The Influence of Operating Temperatures on Wax Deposition During Cold Flow and Hot Flow of Crude Oil

“…When the temperature of the oil near the pipe wall is lower than the wax appearance temperature (WAT), the wax and other substances dissolved in the oil will aggregate and deposit on the pipe wall. It is generally thought by early scholars that wax deposition mechanism can be summarized as molecular diffusion, Brownian diffusion, gravity settling, or shear dispersion, and the molecular diffusion is the predominant contribution in the wax deposition process (Paso and Fogler, 2004;Creek et al, 1999;Jennings and Weispfennig, 2005;Bidmus and Mehrotra, 2009). …”

“…Some showed the deposition rate increased as the temperature differential increased (Burger et al, 1981;Creek et al, 1999;Jennings and Weispfennig, 2005;Bidmus and Mehrotra, 2009;Lashkarbolooki et al, 2010;Zhenyu et al, 2011), and some showed opposite conclusion that the deposition rate decreased as the temperature differential increased (Paso and Fogler, 2004;Bidmus and Mehrotra, 2009;Zhenyu et al, 2011). On the basis of these studies, we selected a series of experimental temperatures (including all the temperature conditions in previously mentioned studies) by using cold finger device and high-temperature gas chromatography (HTGC) to study the effect of operating temperatures on the wax deposition, including the deposit mass, wax content, and carbon number distribution in the deposit.…”