Bubble Babble: The Predicted Bubble Point Really Matters

Barry Burr

May 20, 2015

The calculation of VOC, HAP, and GHG emissions from oil and condensate tanks is based on laboratory analysis of the liquid from the high pressure separator. Since the accuracy of the emissions calculation is directly based on the accuracy of this analysis of the separator liquid, inaccuracies in the analysis of the can lead to gross under or over-prediction of the tank emissions.

As the separator is performing an equilibrium flash separation at the separator’s pressure and temperature, the sampled liquid is said to be at its Bubble Point at those conditions. The lab component analysis of the separator liquid should be used to calculate a Bubble Point in ProMax® using either the Peng-Robinson or SRK equation-of-state. The resultant Bubble Point calculated in ProMax should reasonably match the temperature and pressure at which the separator operates.

Comparisons between measured separator temperatures and Bubble Point temperatures calculated in ProMax provide an indication of the accuracy of collecting samples and analyzing pressurized liquid composition. This may vary from laboratory to laboratory and between alternative methods used within a laboratory. If the ProMax-calculated Bubble Point is greater than the Measured Separator Temperature, then tank flash emissions will be under-predicted and vice versa.

We have seen some instances where pressurized liquid compositions result in Bubble Points that do agree well with measured separator temperatures and pressures. We have also observed Bubble Point calculations which were either high or low by more than 100 F! Such deviations will cause large errors in tank emissions estimates.

We highly recommend that pressurized liquid analyses be checked for thermodynamic consistency using the Bubble Point Temperature test before proceeding to Tank Emissions calculations.

BR&E’s staff of engineers will be glad to provide further assistance in performing the Bubble Point test and checking sampled compositions for thermodynamic consistency. For questions or help, please contact us at 979-776-5220 or Contact Technical Support.

For more on this subject, please see Air Emissions Modeling Advances for Oil and Gas Production Facilities by Burr, Mach, Georgeson