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

Partial molar volumes required for calculating Poynting correction were 
estimated in accordance with the crystallography study on n-paraffin 
solids. 
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The model was validated using experimental WDT data for n-paraffin 
mixtures. 

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TOPIC 3: Wax Deposition 
 
 
 
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The thermodynamic model needs to be tuned against experimental 
measurements of wax equilibrium temperature. The majority of existing 
wax models were tuned against experimental WAT data. 
Ji et al (2004) at Heriot-Watt University developed a new wax model called 
HWWAX (Heriot-Watt Wax Model): 
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This model uses the SRK/PR equation of state for calculating the 
fugacity in the vapour and liquid phases. 
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The fugacity of solid is calculated using the activity coefficient equation. 
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They validated it against the wax equilibrium data (in terms of WDT), 
the amount of precipitated wax and its composition. 
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Besides using WDT, instead of WAT, several modifications were 
suggested in order to improve the HWWAX model reliability. 
The suggested improvements to the HWWAX model were as follows: 
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Improved correlations for estimating fusion properties. 
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For heavy hydrocarbons, new correlations for calculating the coefficient 
of temperature dependency functions, in both SRK and PR EoS (this is 
to improve the fugacity calculation). 
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A new approach, for describing wax solids (based on UNIQUAC 
equation) is suggested. 
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A method for estimating the wax phase equilibrium, at pipeline 
conditions (where the high pressure is commonly encountered), is 
proposed. 
The reliability of HWWAX was verified, Figure 3, by comparing its (and other 
wax model) predictions against independent experimental data. Synthetic 
mixtures, consisting of n-paraffins are used for the validation (due to the vital 
role n-paraffins play in terms of fixing the wax phase boundary). 
Figure 3: Comparisons of Experimental WDT Data 
 
These comparisons, using several predictive approaches, were conducted at 
Heriot-Watt laboratories using a C
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binary system. 

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TOPIC 3: Wax Deposition 
 
 
 
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Figure 4 shows several wax composition predictive methods. These were 
conducted at Heriot-Watt laboratories using different wax models at 290.2 K 
and 0.1 MPa: