Reid Vapor Pressure Simulation

Hi.

I am trying to do a simulation in Hysys to establish RVP in accordance with ASTM D 323 - 99a. I use the following procedure:

 

I have a crude composition with no water at 0 °C and 1.013 bara. Volume flow 1 m3/h.

I create a sidestream with 78% N2 and 22% O2 (air) at 37.8 °C at 1.013 bara. Volume flow 4 m3/h.

The two streams are mixed and heated to 37.8 °C.

The pressure is varied until the volume flow is 5 m3/h. Temperature is fixed at 37.8 °C.

 

In my opinion the corresponding pressure should give the RVP pressure, but it is not in accordance with the value given by Hysys? 

The pressure is varied until the volume flow is 5 m3/h

Verify that it was total volume flow of vapor plus liquid in your simulation, not vapor alone.

In my opinion the corresponding pressure should give the RVP pressure

You should realise that the RVP is a gauge pressure. So if your simulation required 1.5 bar absolute then the RVP is 0.5 bar (gauge).

 

Moreover the ratio V/L ratio to be used differs for different kind of samples in ASTM D323.

 

In any case you should read the Hysys manuals about available RVP methods.

I never use Hysys but I think there are several methods in Hysys that one can select to calculate RVP.

 

What RVP did you calculate with your procedure, and what RVP did Hysys give, based on which selected method?

There are different methods simulators use to calculate RVP. The simplest calculations are based on the graphs in the API Technical Data Book and determine RVP based on the TVP and the slope of the D86 at the 10% point. Hysys may have an option for a more rigorous calculation method based on ASTM D323 as a non-default selection. This method will likely not duplicate your method exactly, since the equipment is not dried in D323 and there is a small amount of water assumed present even if you do not find any water in your sample.

In my simulation I hvave the total flow as 5 m3/h, yes. I forgot to say that I subract 1.013 bar from the simulation result since I want the gauge pressure. I have also tried water saturated air.

 

Hysys gives several options for calculation method, and they give different results.

 

What I am  trying to produce is a straigtforward routine to establish RVP based on flash calculations. The ASTM test method  seems simple to simulate, but my estimates differ by 8 - 10% from what is reported by Hysys.

AFAIK different software may include different procedures and you may observe some differences in reported values,

differences may depend from some simplifications (see for example W. E. Stewart, Predict RVP of Blends Accurate) or selection of different thermo models (different properties) etc. etc.

I have a copy of Prode Properties and I normally select the PRX model for fugacities and volumes when calculating RVP,

selecting PR or SRK can produce different values...

You should realise that the RVP is a gauge pressure.

I don't think that. In standard is said:

Note 1—Because the external atmospheric pressure is counteracted by the atmospheric pressure initially present in the vapor chamber, the Reid vapor pressure is an absolute pressure at 37.8°C (100°F) in kilopascals (pounds-force per square inch).

The RVP is measured as a gauge pressure.

 

If the RVP of a petroleum liquid is 0.5 bar then the absolute pressure inside the container is 1.5 bara.

So when doing a Hysys simulation to determine RVP one has to take that into account.

 

The partial pressure of air inside the container is then about 1.0 bara

and the vapor pressure of the petroleum liquid inside the container is then about 0.5 bara.

That is what Note 1 is really saying.

PingPong, thank you for your explanation. I've found some information about this thread.

 

Regards,

Nikolai