Effects of surface roughness on wax deposition

Abstract

Interior surfaces of crude oil pipelines are far from smooth when they enter service and tend to become rougher through their operational life. Despite this, it is common for models of wax deposition to assume the surface is smooth, or else to ignore roughness altogether. To evaluate the validity of this assumption, in this work, we experimentally explored the importance of roughness on wax deposition. We applied different surface finishes to create several cold fingers with different surface roughnesses, and then collected mass, thickness, and composition data from the deposits that formed on each cold finger at several sets of rotational rates and bulk oil temperatures. It was found that surface roughness has a significant effect on deposit mass, with smoother surfaces generating fewer deposits than rougher surfaces. The effect of roughness was largely independent of rotation rate, but showed a strong dependence on bulk temperature. In particular, the difference in deposit mass due to roughness became much larger when the bulk temperatures were higher than the wax appearance temperature (WAT). These observations suggest that surface roughness plays a significant role in the wax deposition process and should not be ignored in models aiming for high-accuracy predictions. Additionally, the complex interactions between temperature and roughness suggest that a combined mechanism of gelation and preferential crystallization may better explain incipient deposition.