Actual Volumetric Flow Rate vs Standard Liquid Volumetric Flow Rate

Michael Hlavinka

Feb 28, 2008

Why does the actual volumetric flow rate in ProMax not equal the standard liquid volumetric flow rate if the stream temperature and pressure are at standard conditions?

The only time the actual and standard liquid volumetric flow rates will be equal at standard conditions is if the system forms an ideal solution. Solutions that are not ideal can have significantly different actual and standard liquid densities.

The actual density of the liquid is computed using the user selected liquid density model in the ProMax environment. This is normally either the COSTALD or the Rackett model. The predictions from these models are mixture densities which include a volume change on mixing effect.

The standard liquid densities are computed using GPA Standard 8173. This computation is simply a molar average of the liquid densities of the individual components that comprise the mixture. No volume change on mixing is calculated. GPA Standard 8173 states that the component liquid densities needed for this averaging are to be obtained from the following sources:

  1. The current edition of GPA Standard 2145. In the current release of ProMax, GPA Standard 2145-03 is used. A revised GPA Standard 2145-08 was to be promulgated on 01 January 2008. However, due to changes made, this revision has not been formally approved. When the new standard is approved, it will be included in ProMax in its next release.
  2. If the component is not listed in GPA Standard 2145 (there are only 19 components in GPA Standard 2145-03), the next source is the current version of the GPSA Engineering Data Book. ProMax includes the density data from the 12th edition of the GPSA Engineering Data Book.
  3. If the component is not listed in either of the above two sources, the API Technical Data Book is used.

Most of the components present in the gas processing industry are available in one of the three above sources. Also note that some components are supercritical and cannot exist in the liquid phase at standard conditions (e.g., H2, CH4, N2, etc.). For these components, estimated values are provided in the tables for use in these calculations.

GPA Standard 2145 contains tables of properties in both FPS (English) and metric units. The GPSA Engineering Data Book is also available in FPS and in SI units. The properties in the respective tables of these two sources are not merely unit conversions between the two sources. The standard temperature for the FPS sources is 60°F (15.56°C) while the standard temperature for the SI sources is 15.0°C. In 1980, GPA adopted a standard temperature for SI units of 15.0°C (see 12th Edition GPSA Engineering Data Book, page 1-13). ProMax includes both the SI and the FPS data internally. However, the standard temperature must be changed to 15.0°C to use the SI data. This is done by selecting the ProMax->Project Options… menu item.

In addition to standard liquid density computation from the above sources, the gross (higher) and net (lower) heating values are computed using data from the same sources. As with standard liquid densities, the heating values are also impacted by the choice of standard temperature and pressure.

Finally, another common standard for gases is known as normal conditions (as opposed to standard conditions). These conditions are widely used in Europe and other locations. Normal conditions refer to gas volumes measured at 1 atm pressure and 0°C. ProMax provides a normal volumetric flow property in the stream that can be used for specification of normal volumetric flow rates. Note that if the ideal gas reference temperature in ProMax->Options is set to 0°C, the standard gas flow rates will equal the normal gas flow rates.

Authored by Michael Hlavinka (BR&E Technical Director)