Calculating Reid Vapor Pressure

[Rocco8]

How do I calculate the Reid vapor pressure fom the chemical composition of a hydrocarbon mixture given in mole percents from an on-line GC,if the algorithm listed below is used.

http://www.bre.com/technicalpapers/technicalpaper.asp?articlenumber=5

Step 2 reqires that one Evaluate the density of the sample at T = 35, 60, and 100°F and then compute the liquid expansion of the sample using n = 4:

This seems to be the most difficult step.

Any ideas?

 

[25362]

Rocco8, the site shows an error signal. Is it rightly written ?

 

[Rocco8]

Try

http://www.bre.com/technicalpapers/technicalpaper.asp?articlenumber=51

Sorry about that

 

[sshep]

Reid VP is useful because it is easy to measure relative to true VP. Because RVP is a fixed volume test, the flashed mixture has a lower pressure than the original liquid sample.

The typical problem is to estimate TVP given the RVP. The API Technical Data Book gives conversion nomographs for products (fig 5B1.1, with help of ASTM D86 10%pt data) and crude oils (fig 5B1.2). In the GPSA manual there is an easy to use nomograph (fig 6-4, storage), and also cites an approx conversion for data at 100F: TVP = 1.07*RVP+0.6 (Eq 6-3). My own experience has been that this last conversion formula was not very good for the heavier products I deal with.

As you have a speciated composition, you should be able to calculate the true vapor pressure. You are in effect starting from the answer typically sought. I am not sure what sort of accuracy you are shooting for, but once you have calculated the TVP you might consider using one of the standard RVP->TVP conversions in reverse.

Just a thought- sshep

 

[Guidoo]

Rocco8,

You wrote:
"Step 2 reqires that one Evaluate the density of the sample at T = 35, 60, and 100°F and then compute the liquid expansion of the sample using n = 4."

You can evaluate the density of the sample using an equation of state (your article suggests Suave-Redlich-Kwong). Normally you would do this using a commercial simulation package such as Hysys or PRO/II. There are also some web based tools available, see:

http://www.cheresources.com/physinternetzz.shtml

 

[DickRussell]

If you read the ASTM D-323 procedure (do a Google search) on the RVP procedure, you see that the test can be simulated by flash calculations with a simulator. The API Technical Data Book has a procedure developed for doing this, and that or equivalent calcs may be found in some simulators.

 

[Rocco8]

Guys thanks,

The Algorithm is going into an online analyzer and the results will be checked for accuracy when compared to the RVP predicted by ChemCad 5.4 by Chemstations.

We have no choice but to use the rigorous method.