RVP Calculation in Commercial Simulators

The TLDR version:

It appears that the RVP units that are reported by the HYSYS/UniSim (and other simulators') algorithms (as well as the cold properties utility) are gage pressures BUT they are being tied to absolute pressure units. I have observed this in HYSYS (v 12.1) and UniSim (480). E.g., if you see the RVP reported as 14 psia, it’s actually 14 psig.

Long version

The Reid Vapor Pressure (RVP) is a measure of the volatility of a hydrocarbon liquid (crude, condensate, naphtha, gasoline, and other petroleum products). It represents the volatization in atmosphere on a hot summer’s day (or an average Malaysian day).

One of the common tests is to take a 4:1 by volume mixture of air and hydrocarbon liquid in a constant temperature bath (100 °F) and measure the corresponding pressure. This roughly describes the ASTM D-323 test. The reading on the pressure gage is oddly referred to as the "absolute pressure".

Most commercial process simulators employ an algorithm to compute the RVP of a fluid based on its composition, you can Read The Friendly Manual of your simulator to see what sort of algorithms are used.

The RVP is generally reported in the pressure units of the selected unit set. E.g., in HYSYS if you choose Field, your pressure would be reported in psia.

The RVP of straight run naphtha (taken from the HYSYS CDU sample case) was calculated. It is around 14 psia.

?If the pressure units are changed to psig, they’ll be zero/slightly negative. I thought this was somewhat odd.

To check, I replicated the ASTM D-323 RVP measurement in a HYSYS flowsheet, where I took 1 m3 of cold naphtha (33 °F) and mixed it with 4 m3 of warm humidified air, then performed an isothermal (100 °F), isochoric (5 m3) settle out. This yielded a pressure of 28.1 psia, or 13.4 psig.

If you do a quick web search for straight run naphtha you’ll find it's around 15 to 20 psi.

What’s going on?

As chemical engineers, we’re used to describing pressure in the following manner:

1.?????Gage pressure. This is the reading you get off any standard pressure gage. If you measure your tire pressure using a pressure gage, it’ll read zero before you attach it to your tire. In other words, the zero reference or datum of gage pressure is atmospheric pressure.

A “g” suffix is added to units to indicate gage pressure. E.g., psig, kPag, barg.

2.?????Absolute pressure. Also referred to as the total pressure. The zero reference of absolute pressure is an absolute vacuum. To convert from gage to absolute, one would add atmospheric pressure. Most chemical engineering computations require the pressure to be expressed in absolute pressure.

It is good practice to denote absolute pressure by adding an “a” suffix to the unit. E.g., psia, kPaa, bara.

3.?????Pressure difference. Pressure difference. In this case, reference pressures cancel out, and these units are often denoted with no suffix. E.g., psi, kPa, bar.

The typical RVP test method is ASTM D-323. If you have a look at ASTM D323, Revision 20A, November 1, 2020, you’ll find the following note:

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 Reid vapor pressure differs from the true vapor pressure of the sample due to some small sample vaporization and the presence of water vapor and air in the confined space.

Unfortunately, this is NOT what chemical engineers consider “absolute pressure”. This is a gage pressure. If you scour YouTube, you can watch how this measurement is performed.

I believe this use of the term “Absolute Pressure” in ASTM D-323 Note 1, on top of the use of “psi” has resulted in a lot of confusion amongst engineers.

I did a search and came across Dr Michael Hlavinka’s (BR&E Technical Director) attempt to get the ASTM D323 committee to address this matter: https://www.bre.com/Blog/Reid-Vapor-Pressure-RVP-Is-it-Expressed-in-Units-of-Absolute-Pressure.aspx

"The Reid Vapor Pressure (RVP) is determined according to ASTM Standard D323. Many have expressed confusion related to the pressure units reported for RVP. Since most vapor pressures are reported in absolute pressures, most observers expect the RVP to follow this norm. To make matters worse, the published standard uses the term "absolute pressure" in its description. Further, many publications and reports carelessly provide values with units such as "psia" when providing RVP values.

In reality, the value is a gauge pressure. It is measured using a pressure gauge that reports a value of zero pressure when open to the atmosphere. The gauge also indicates zero pressure when attached to the ambient air filled chamber of the apparatus. The gauge is not of the type that has vacuum measurements."

The response from Dr Rey Montemayor Chief Chemist and Solvents Lab Coordinator of Imperial Oil Canada reads:

?" Although the comments made in Mr. Hlavinka's attachment is technically correct, we have to recognize that when we speak of D323 RVP, we are not talking in the engineering sense, but in the analytical (measurement) sense. When we measure the RVP of a petroleum product using D323, we are measuring the vapor pressure of the petroleum product, and the measurement method uses a gauge to read the pressure at the specific test temperature. Hence, if one uses D323 to measure the vapor pressure of a material that has NO vapor pressure, the gauge will read zero and the RVP by D323 will be reported a zero. If we have a material that has a vapor pressure of 100 psi at the test temperature, the gauge will read 100 and the RVP of the material will be reported as 100 psi.”

Which I personally find illogical. You're reporting a gage pressure, but call it absolute, then assign pressure difference units. The last time I was this confused was after I ate a bunch of brownies at the Pink Floyd Momentary Lapse of Reason after gig party in the 80's.

In conclusion:

1. Be careful when interpreting the RVP of a fluid when using a commercial simulator or talking to a chemist! (This is not an attack on chemists: I have friends who are chemists).

2. When someone uses psi to describe something other than pressure difference, you need to ask them if they're referring to absolute or gage.