Professionally I work at BPT and focus on creating value adding App for Process Simulation. You can find more info on those on the BPT website. Below are posts that should help HYSYS, PetroSIM and UNISIM users alike in their day to day challenges to produce accurate yet fast models efficiently.

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Wednesday, 5 October 2011

Reid Vapour Pressure Methods in HYSYS

Yesterday I was asked to explain why the Reid VP at 37.8 C in the Standard HYSYS properties was different from that same property in the Cold Property utility. Once I found the explanation I did vaguely remember that this has indeed changed at some point in time, so this explanation is only valid for more recent versions of HYSYS, I'd say that 2006.5 and higher should be like this.

Both values are calculated using the ASTM D323-73/79, BUT, the RVP in Standard property is calculated on a dry basis! The RVP in the cold properties utility is calculated using the original ASTM D323-73/79 which uses the fluid as is (including any water present). NOTE that the Cold Properties page in the Boiling Point Curve Utility also shows the RVP calculated on a dry basis!
Below is a full description of the various methods available in HYSYS:


The TVP is the bubble-point pressure of the stream (i.e. when the liquid is saturated – vapour fraction = 0). This is calculated by flashing the stream at the reference temperature and with a vapour fraction of 0.


The RVP calculation in the HYSYS Standard stream correlation and Boiling Point Curves utilities is defined as the pressure at 100°F at which the vapour volume will be 4 times as much as the liquid volume (i.e. the pressure at which 80% of the stream by volume is vapour at 100°F). This is the same as the ASTM D323-73/79 method, but as mentionned above the HYSYS RVP method is always done on a dry basis (i.e. any water content of the stream is ignored), and always uses the HYSIM Flash. (i.e. the flash method that is used if HYSIM Flash is selected on the Stab Test tab of the Fluid Package, within the Simulation Basis Environment.)

API 5B1.1 (Naphtha)

This method is useful for gasolines and finished petroleum products but not crudes or oxygenated blends. TVP is correlated against RVP, Temperature and the slope of ASTM D86 distillation curve at the 10% point. This extension solves the corrected version of the API databook equation of the correlation, for the RVP. A recognized limitation of the API Naphtha method is that the D86 10% point can have a similar gradient for vastly different streams.

This method is the default in other simulation tools and is at the other end of the RVP spectrum from the HYSYS 4:1 method (10% vapour evolution vs. 80% vapour evolution). This is why default RVP methods cannot be compared across process simulators.

API 5B1.2 (Crude)

Generally used for condensate and crude oil systems, (typically wide boiling pre-processed hydrocarbons). TVP is correlated against RVP and Temperature. This extension solves the API databook equation of the correlation for RVP.

The correlation is based on very old data from 1959 but is popular with Engineers for quick and dirty calculations.

ASTM D323-82

This is the standard and accepted procedure for RVP lab measurement. Liquid hydrocarbon is saturated with air at 33°F and 1 atm pressure. Since the lab procedure does not specify that the test chamber is dry, the air used to saturate the hydrocarbon is assumed to be saturated with water.

This air-saturated hydrocarbon is then mixed with dry air in a 4:1 volume ratio, and flashed at the RVP reference temperature such that the total volume is constant (since the experimental procedure uses a sealed bomb). The gauge pressure of the resulting mixture is then reported as the RVP.

ASTM D323-73/79

Otherwise known as P323. The pressure is adjusted at the RVP reference temperature until the vapour:liquid ratio is 4:1 by volume. This method is essentially the same as the default HYSYS method, except it is not on a dry basis and the flash method used is the same as for the rest of the flowsheet.

ASTM D4953-91

This method was developed for oxygenated gasolines. It is the same as D323-82 method except that everything is on a completely dry basis – i.e. the air is not saturated with water.

ASTM D5191-91

This was developed for gasolines and gasoline-oxygenate blends as an alternative to the D4953-91 method. In the experimental procedure, the hydrocarbon is saturated with dry air then placed in an evacuated bomb with five times its volume. The total pressure is then converted to a dry vapour pressure equivalent (DVPE) and then reported as the RVP.

In this extension the method used is to mix near vacuum air at 0.01 psia and 100°F with hydrocarbon at 1 atm and 33°F in the ratio 4:1. This is then flashed at constant volume at the RVP reference temperature. The pressure is then converted to the DVPE and reported as the RVP.

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