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

 
 
 
 
 
 

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TOPIC 3: Wax Deposition 
 
 
 
15 
©H
ERIOT
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NIVERSITY B41OA December 2018 v3 
Lira-Galeana (1996)
 
presented a wax model as follows: 
•
A multi-pure-solid approach was used for the description of wax solids. 
•
This approach assumed wax solids consisting of multiple solid phases 
and that each solid phase was a pure compound. 
•
The PR EoS was used for calculating fugacity in the liquid and vapour 
phases. 
•
The model was validated North Sea crude oils using experimental SLE 
data for binaries and measured cloud point temperatures. 
Studies on crystal structures in recent years reveal that the miscibility of n-
paraffins in a solid state depends strongly on the difference in the molecular 
sizes (i.e., carbon number differences). 
A mixture of n-paraffins with a significant carbon number difference (e.g., nC
30
- nC
36
) appears to form eutectic solids (solid with exactly the same 
composition as the liquid it is derived from (this is similar to the behaviour of a 
liquid-vapour azeotrope). 
On the other hand, mixtures of n-paraffins with consecutive carbon number 
distribution form a single orthorhombic solid solution. Thus, the assumption of 
multi-pure-solid approach is not consistent with the results of wax crystal 
structure studies. 
It follows that the model proposed by Lira-Galeana et al. is open to serious 
questions when used to predict the wax phase boundary for systems 
consisting of compounds with similar molecular size. 
Coutinho et al
 
(1995, 1996)
 
evaluated several approaches such as Flory-
Huggins, universal functional group activity coefficient (UNIFAC), Flory free-
volume, Entropic free-volume, for the calculation of activity coefficients in SLE. 
In 1998, Coutinho presented a wax thermodynamic model as follows: 
•
They applied a combined UNIFAC, Flory free-volume approach to 
describe the liquid phase. 
•
While, at the same time, using the Universal Quasi-Chemical 
(UNIQUAC) equation to describe wax solids. 
Pauly et al. (2001) presented a wax model as follows: 
•
They modified the model proposed by Coutinho (1998) by using SRK 
EoS and Gibbs energy to describe the liquid and vapour phases. 
•
The Gibbs energy was obtained using a modified UNIFAC equation. 
•
The Poynting correction term (corrects the fugacity of a liquid, at the 
saturation point, to the actual prevailing conditions) was used for 
extending the model to high-pressure conditions. 
•