Paraffin wax deposition is one of the most common flow assurance problems. By their nature, petroleum fluids usually contain straight - chain or normal - chain alkanes. N-paraffin deposits are called macrocrystalline waxes. Such deposits contain n-alkanes, branched alkanes deposited with n-alkanes, and retained precipitated asphaltenes, resins, sand, oil, and other solids that may be present, such as rust and scale.

N-alkanes crystallize at higher temperatures because they have straight molecular chains.

Molecules of different sizes crystallize when they reach their crystallization temperature. The largest n-alkanes crystallize at higher temperatures, usually determining a measurable parameter called the wax appearance temperature. This parameter is subjective and depends on the laboratory's ability to detect crystallization by light, heat, force, or pressure. Some objectivity is added because modern laboratory equipment has a relatively comparable level of sensitivity.

There are several mechanisms for paraffin deposition on pipe or tubing walls: thermal diffusion and Brownian diffusion, as well as shear dispersion and deposition. The most common one is thermal diffusion. If the temperature is higher than crystallization temperature, paraffin will not deposit. If there is no temperature difference between the pipe wall and the bulk oil, there is little deposit of paraffin on the pipe wall because there is no driving force to move the paraffin molecules towards the wall. N-alkanes are usually driven from the oil solution to the tube wall by concentration differences in laminar flow and turbulence in turbulent flow. When the molecule reaches the stratified sublayer, the diffusion mechanism begins.

There are three sublayers in turbulence: concentration sublayer, thermal sublayer and flow or dynamic quantum layer. The thermal sublayer should be used because it is at this boundary that conversion to wax crystals can take place. The concentrated sublayer is slightly smaller than the thermal sublayer due to the kinetic delay effect of wax crystallization.

Incropera et al. show the most illustrative descriptions of three sub-layers. (2007). I have updated the illustration of this work here, as shown in Figure 5.31, to reflect the relative thickness of the sublayers as described by Bird et al. (1960). Understanding the differences between the three boundary layers is important for the physical effects involved in wax deposition.