Phase transition mechanisms of paraffin in waxy crude oil in the absence and presence of pour point depressant

Abstract

Molecular dynamics simulations were employed to reveal the phase change mechanisms of paraffin wax in a multiphase system of crude oil comprised of oil, paraffin wax, bitumen, and water. Simulation results revealed that in the absence of a pour point depressant, paraffin crystals intertwined and aggregated to form “three-dimensional networks” that are difficult to dissolve in the oil phase when the temperature is below the wax-out point of paraffin. By comparing three different paraffin components, (n-octadecane [n-C18]); n-octadecane + n-octadecane (n-C18+n-C28); n-octadecane + n-hexadecane n -C18+n-C36), we found that the viscosity and specific heat capacity of the system and phase transition temperature increased with the chain length of paraffin molecules. It was further revealed that the addition of pour point depressant molecules (i.e., polyethylene-vinyl acetate polymer) can prevent the mutual eutectic between paraffin molecules from forming insoluble three-dimensional networks, which instead leads to homogeneously distributed paraffin crystals. In addition, the homogeneously distributed paraffin crystals have a smaller viscosity and lower phase transition point compared to those that are untreated. Our novel discoveries provide theoretical guidance for solving practical engineering problems facing crude oil delivery.