Experimental and Numerical Study of Wax Deposition in a Laboratory-Scale Pipe Section under Well-Controlled Conditions

Detailed local and averaged measurements of wax deposition in an annular pipe section were conducted and combined with a comprehensive simulation model. The objective of the study was to contribute to the understanding of the fundamental mechanisms responsible for wax deposition on surfaces. To this end, a laboratory-scale annular deposition section was designed to offer well-defined boundary and initial conditions for the deposition experiments. The test section allowed visual access to the deposit formation process and was equipped with a traversing, fine-gage temperature probe and a port for sampling of the deposit. The measurements encompassed the time evolution of the axial distribution of deposit thickness, radial temperature profiles, and deposit composition. Measurements were made for deposition times, ranging from 5 min to 7 h, for laminar flow conditions. A special model solution was employed, formed by a single-molecule solvent and paraffins with a narrow carbon number distribution. This simple model solution facilitated the simulations and comparison with experiments. The simulation model solved the equations governing conservation of mass, linear momentum, energy, and chemical species, coupled with a multisolid thermodynamic model for predicting solid-liquid equilibrium. The enthalpy-porosity approach was employed to model the deposition process, with the deposit treated as a porous medium with variable porosity depending on the solution solid fraction. An excellent agreement was obtained between the measured and predicted deposit thicknesses. Enrichment of the deposit with heavier carbon molecules with time and Reynolds number was measured, characterizing the aging process. The simulations also predicted the same trends, but deviations from the experiments were found for longer deposition times. The measured and predicted temperature profiles agreed in the region close to the deposition wall. As the deposit interface was approached, the simulations predicted slightly warmer temperatures. The time evolution of the simulated species concentration profiles displayed depletion of the solvent and enrichment of the heavy components in the deposit. An increased concentration of the heavy paraffins in the liquid region close to the deposit interface was predicted. This finding indicates that, possibly, a local elevated value of wax appearance temperature prevailed, justifying the presence of crystals flowing over the deposit surface visualized in the experiments. For the test conditions investigated, the combined observations of experimental results and simulations indicate that molecular diffusion at the deposit interface is not the dominant deposition mechanism. Rather, the observations indicate that liquid-solid phase transition governed by heat transfer, coupled to diffusive and convective fluxes of the chemical species, seems to determine the deposit formation.