1 reply to this topic

[dhns]

Dear Sir,
I need to calculate vapor pressure of a particular stream through hysys
but ,in the property list there are two pressures Reid and True vapor Pressure.
Please clarify me which vapor pressure i have to consider for calculating Flash point and
LEL.

Thanks in advance

dhns

[Zauberberg]

True Vapor Pressure should be used in all calculations because it reflects the actual vapor pressure of hydrocarbon. Flash point can also be automatically estimated by HYSYS through "Boling Point Curves" and "Cold Properties" utility. However use it with caution, especially for heavier petroleum cuts.

To calculate TVP at any temperature, set your stream vapor fraction as zero (the resulting bubble point pressure is your TVP).
The following quotation is from BRE website and it explains the difference between RVP and TVP in a very clear and simple manner:

True Vapor Pressure is the pressure of the vapor in equilibrium with the liquid at 100 F (it is equal to the bubble point pressure at 100 F). Reid Vapor Pressure of the stream is based on ASTM D323-99a Standard Test Method for Vapor Pressure of Petroleum Products. Reid Vapor Pressure is the vapor pressure of a chilled sample of gasoline or other fuel as measured in a test bomb at 100 F. The Reid Vapor Pressure differs from the True Vapor Pressure of the sample due to sample vaporization and the presence of water vapor and air in the confined space resulting from the Reid Vapor Pressure test method. To presaturate the sample for the Reid Vapor Pressure test, the streams are flashed at 1 atm and 33 F, and the resulting liquid product is then combined with air at the rate of 4 parts air and 1 part liquid. Next the system is flashed at constant volume at 100 F. The resulting gauge pressure is the Reid vapor pressure. Because of this procedure, the Reid Vapor Pressure may be significantly different from the "True Vapor Pressure" if the Reid vapor pressure exceeds 26 psi. The Reid vapor pressure is applicable only for gasoline, volatile crude oil, and other volatile petroleum products. It is not applicable for liquefied petroleum gases.

Attached PDF document gives the relationship between RVP and TVP for crude feedstocks (source: Environmental Protection Agency):

[attachment=1027:RVP_vs_TVP.pdf]