True Vapour Pressure with the Process Analyser

The True Vapor Pressure (TVP) is the equilibrium partial pressure exerted by a volatile organic liquid, as defined by ASTM D2879 or as obtained from standard reference texts.

The American Petroleum Institute (API) has developed different equations to determine the TVP to estimate the potential emissions which could be released to the environment from liquid storage tanks.

Two different approaches are being used for crude oil stocks and refined petroleum stocks. However, for both approaches the RVP-4 analyser, from Bartec Benke GmbH, measures the ASVP (Air Saturated Vapour Pressure) and if no air is saturated in the sample (e.g. crudes) the total vapour pressure according to the master norm ASTM D5191 and ASTM D6377 (VPCR4). The vapour pressure is typically measured at a specific and constant measuring temperature, e.g. Tm = 37.8°C±0.1°C (100°F±0.2°F). The physical measured property is then used and converted to the relevant TVP value.

The first approach of determining the TVP for crude oil stocks starts by measuring the total vapour pressure at a certain measuring temperature (Tm). The RVP-4 analyser then converts the measured total vapour pressure to the RVPE (equation defined in ASTM D6377). The determined RVPE is then used to calculate the TVPE according to the equation defined by the EPA (AP-42, Compilation of Air Pollutant Emission Factors, Chapter 7 Liquid Storage Tanks – refer to Figure 2).

The second approach of determining the TVP for refined petroleum product stocks follows the first steps i.e. measuring the total vapour pressure and calculating the RVPE accordingly. However as an addition the vapour-slope at 10% according to ASTM D86 has to be measured for the same sample. Both values (RVPE and Slope at 10%) are used to calculate the TVP (refer to Figure 3). In fact the vapor slope can be provided by the BARTEC BENKE DPA-4.5 analyser (ASTM D86 compliant) or via the local DCS.