(47e) Modeling Multicomponent Wax Deposition and Aging on Cold Surfaces with Application to Pipeline Fouling

To understand the aging or densification of wax deposits during transport of oil in subsea oil pipelines, which is relevant to oil pipeline deposition, a multicomponent wax deposition model is developed and applied to deposition onto a cold‑finger from a stirred wax-containing oil heated by a thermal jacket. The model involves the equilibrium equations of Coutinho for wax component concentrations in the solid precipitated wax and co-existing liquid phase in the deposit, as well as the energy and mass transport equations within the deposit and from the stirred oil to the deposit using heat and mass transfer coefficients. The model predictions are compared to previously published results for deposit thickness and composition versus time from single and multicomponent wax-deposition experiments using model oils containing a single wax component, six components, and many components in commercial waxes. For the single-component wax, the model captures well the growth of the deposit thickness, and the time-dependent composition of wax near the cold finger surface and near the interface of the deposit with the oil. The predictions for the multiple-component wax are also good for the deposit thickness versus time, except that at long times the model for multi-component waxes predicts shrinkage due to wax depletion from the bulk oil, which is not seen in the experiments, and a pore-restriction parameter to model diffusion of soluble wax components into the porous deposit must be adjusted for each wax mixture. The prediction of shrinkage can be avoided by preventing re-dissolution of deposited wax in the model. The heavier wax molecules are correctly predicted to deposit much more strongly than the lighter components, especially near the outer edge of the deposit, but quantitatively, the predictions deviate significantly from the measured wax-component concentrations. These deviations could be attributed to the prediction of a higher (WAT) by Coutinho model than was observed experimentally by cloud point measurement, and possibly the gap in the modeling of the deposition phenomena. We also found that pseudo two- and one-component models can reasonably predict the deposit thickness and aging of multi-component deposits. This work should ultimately help in the appropriate design of the pigging frequency or other remediation methods for wax deposition and to help address the decades-old challenges of flow assurance facing the oil and gas industry.