B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

 
 
Figure 4: Measured and Predicted Wax Compositions
3.6 Wax Deposition 
The wax phase boundary, traditionally determined by estimating the cloud 
points (as mentioned above), is used to delineate the wax-free zone under 
static conditions. 
The actual wax deposition tendency in pipeline is, however, affected by many 
factors, such as the fluid flow rate, the temperature profile, presence of 
nucleation sites. This requires studying the wax crystallization and deposition 
under dynamic conditions. 
Wax deposition in pipelines is a consequence of three separate mechanisms, 
namely molecular diffusion, Brownian diffusion and shear dispersion (Burger et 
al., 1981). 

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TOPIC 3: Wax Deposition 
 
 
 
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The deposition starts with molecular diffusion that is driven by the 
concentration gradient: 
•
The temperature gradient between the transporting fluid and the 
pipeline wall leads to the concentration gradient of wax forming 
compounds in the liquid phase. 
•
These compounds are transported toward the cold wall by molecular 
diffusion. 
•
The amount of wax forming materials that can be dissolved in the liquid 
phase decreases with the temperature reduction, these materials 
hence precipitate out of fluid and form solid phase of wax, when 
reaching the solid-liquid phase boundary (i.e., wax phase boundary). 
•
The wax particles suspended in the fluid are continuously collided by 
the thermally agitated fluid molecules, leading to Brownian movements. 
•
Deposition of wax particles near the wall is subjected to the shear force 
caused by the fluid flow. 
The mechanism of wax deposition, the influential factors, and the deposition 
tendency have been investigated by many researchers (Burger, et al., 1981; 
Addison, 1984; Creek, et al., 1999; Labes-Carrier et al., 2002). 
The mathematical modelling of wax deposition requires calculations of phase 
behaviour, fluid flow, and heat flow. The accuracy of each calculation plays an 
important role in determining the predicted deposition results (such as wax 
deposition rate and thickness). 
Clearly, reliable modelling of wax phase equilibrium is vital to the development 
of a robust wax deposition model. Some wax deposition models calculate the 
wax phase behaviour using the wax thermodynamic model that is tuned with 
the WAT data (Labes-Carrier et al., 2002).